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Abstract

Lithium ion battery performance at high charge/discharge rates is largely determined by the ionic resistivity of an electrode and separator which are filled with electrolyte. Key to understand and to model ohmic losses in porous battery components is porosity as well as tortuosity. In the first part, we use impedance spectroscopy measurements in a new experimental setup to obtain the tortuosities and MacMullin numbers of some commonly used separators, demonstrating experimental errors of <8%. In the second part, we present impedance measurements of electrodes in symmetric cells using a blocking electrode configuration, which is obtained by using a non-intercalating electrolyte. The effective ionic resistivity of the electrode can be fit with a transmission-line model, allowing us to quantify the porosity dependent MacMullin numbers and tortuosities of electrodes with different active materials and different conductive carbon content. Best agreement between the transmission-line model and the impedance data is found when constant-phase elements rather than simple capacitors are used.

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... Two experimental techniques have been developed for the determination of tortuosities of various separators, i.e. the impedance-based techniques to measure the conductivity or resistivity of a separator immersed with electrolyte [13][14][15][16][17][18][19][20][21] and the polarization-interrupt method to measure the diffusivity of a separator immersed with electrolyte [16]. Limited experimental evidences have demonstrated that the microstructure of a separator has a great influence on the capacity performance of LIB [13,15,19,22]. ...
... More interestingly, it was noted by the present authors that the reported values of tortuosity for the same separator vary quite significantly. For example, the reported tortuosities for the separator of Celgard 2500 are 3.2 [13], 2.16 [14], 2.5 [15], 1.58 [18] and 1.7 [19], respectively. These large discrepancies imply a large inconsistency in the determination of the separator's tortuosity, which needs to be resolved due to its important role in the homogenised battery model. ...
... In addition, due to the lack of experimental details, such as cell setup, the thickness of the separator, electrode area, electrolyte selection and the ambient temperature during testing, the accuracy of the reported tortuosities is questionable and should be carefully assessed. Although various experimental setups to determine a separator's tortuosity have been reported, e.g. a coin-cell format used in [14,15,19,20], a pouch-cell format in [16][17][18]21] and a self-designed copper block setup in [17,18], there is a lack of comparison among these different methods for the determinations of a separator's tortuosity. Therefore, it is necessary to further study the method for the determination of a separator's tortuosity. ...
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The porosities and tortuosities are commonly utilized to characterize the microstructure of a Li-ion battery's separator and are adopted as key input parameters in advanced battery models. Herein, a general classification of the tortuosity for a porous medium is introduced based on its bi-fold significance, i.e., the geometrical and physical tortuosities. Then, three different methods for the determination of separator's electrical tortuosity are introduced and compared, which include the empirical Bruggeman equation, the experimental method using Electrochemical Impedance Spectrum (EIS) testing a the numerical method using realistic 3D microstructure of the separator obtained from nanoscale X-ray Computed Tomography (XCT). In addition, the connection between the geometrical tortuosity and the electrical tortuosity of a separator is established by introducing the electrical phenomenological factor (\b{eta}_e), which can facilitate the understanding of the relationship between the microstructure characteristics and transport properties of the separators. Furthermore, to quantitively compare the values of the tortuosities determined by different methods, the corresponding effective transport coefficients ({\delta}) are compared, which was usually used as a correction for effective diffusivity and conductivity of electrolytes in porous media.
... Based on simulation studies using virtual but realistic microstructures, it was shown that the M-factor (the inverse of the MacMullin number N M . 25 can be predicted with high accuracy by the electrode porosity ε, the geodesic tortuosity τ geo , and the constriction factor β as the ratio of the effective conductivity κ eff to the intrinsic conductivity κ of the electrolyte. 26 An alternative concept of tortuosity is the electric tortuosity τ e , which is defined by the alteration of the conductivity in a porous structure. ...
... Equation 4 links the pore structure expressed by the tortuosity τ e with the ionic resistance R ion of the electrolyte, which can be derived from EIS measurements. 25,27,28 The electrode area A and the electrode thickness d are geometric factors of the measured sample. Note, that in the following, the simplified notation τ = τ e is used. ...
... This condition is achieved by using a blocking electrolyte, that hinders a faradaic reaction as it cannot intercalate into the active material. 25 A low conductive salt concentration in the electrolyte increases the electrolyte resistivity and therefore diffusion effects inside the electrode pore structure. Figure 2 shows a simulation of the equivalent circuit model (ECM), which is used in this investigation. ...
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Previous investigations on porous NCM particles with shortened diffusion paths and an enlarged interface between active material and electrolyte showed improved rate capability and cycle stability compared to compact particles. Due to the additional intragranular porosity of the active material, the pore structure of the overall electrode, and, as consequence, the ionic transport in the pore phase, is altered. In addition, the particle morphology influences the ohmic contact resistance between the current collector and electrode film. These effects are investigated using impedance spectroscopy in symmetrical cells under blocking conditions. The ionic resistance and the tortuosity of the electrodes are determined and analyzed by a transmission line model. Tortuosity is higher for porous particles and increases more during calendering. This limits the options for densifying these electrodes to the same level as with compact particles. In a further approach, the method is used to explain the drying related performance differences of these electrodes. At higher drying rates, the contact and the ionic resistance of electrodes with compact particles increases more strongly as for electrodes with porous particles. These investigations provide new insights into the ion transport behavior and enable a better understanding of the impact of the electrode processing condition.
... A combination of microstructure modeling and direct electrochemical measurements, such as EIS with blocking electrolyte, have been used to estimate the tortuosity for electrodes with high amount of CBD (8 wt% in anode and 10 wt% in cathode). The anode was found to have a Bruggeman coefficient of roughly 2.3 and cathode around 2.0, corresponding to tortuosities of 4 and 2.8, respectively, having porosities of ∼35%. 4 The 2320 separator tortuosity has been measured by the Gasteiger group to be approximately 4. 56 The tortuosities for electrodes with 4 wt% CBD were estimated using the microstructure modeling tools discussed above. The Bruggeman coefficient for the anode and cathode are predicted to be reduced to around 2 and 1.8, respectively. ...
... Tortuosity measurement on symmetric cell.-To validate the microstructure model transport predictions, tortuosity of graphite electrodes was measured using an EIS method previously reported by Landesfeind et al. 56 Two electrodes with varying CBD contents were examined, with details in the electrode library section (cf Table III). Electrolyte was 10 mM tetrabutylammonium perchlorate (TBAClO 4 , ⩾99.0%, Sigma-Aldrich) dissolved in EC/EMC (1:1, w/w) (Tomiyama Pure Chemical Industries, Ltd.), which has ionic conductivity of 0.46 mS cm −1 . ...
... Both methods show good agreement, with ∼13% Bruggeman reduction induced by the CBD loading reduction. No values are reported for the cathode as aluminum positive current collector interferes with the blocking condition needed for the measurement, while cupper negative current collector does not have this issue.56 Nevertheless, the direct measurement on the anode side enables validating the numerical methodology. ...
Article
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Battery performance is strongly correlated with electrode microstructure and weight loading of the electrode components. Among them are the carbon-black and binder additives that enhance effective conductivity and provide mechanical integrity. However, these both reduce effective ionic transport in the electrolyte phase and reduce energy density. Therefore, an optimal additive loading is required to maximize performance, especially for fast charging where ionic transport is essential. Such optimization analysis is however challenging due to the nanoscale imaging limitations that prevent characterizing this additive phase and thus quantifying its impact on performance. Herein, an additive-phase generation algorithm has been developed to remedy this limitation and identify percolation threshold used to define a minimal additive loading. Improved ionic transport coefficients from reducing additive loading has been then quantified through homogenization calculation, macroscale model fitting, and experimental symmetric cell measurement, with good agreement between the methods. Rate capability test demonstrates capacity improvement at fast charge at the beginning of life, from 37% to 55%, respectively for high and low additive loading during 6C CC charging, in agreement with macroscale model, and attributed to a combination of lower cathode impedance, reduced electrode tortuosity and cathode thickness.
... Impedance is an important index for evaluating the output power and available capacity of LIB cells under a certain current rate. Battery impedance mainly consists of three parts (Fig. 7(a)): (1) ohmic impedance which combines the ionic impedance R ion in the liquid phase and the electrical impedance R e in the solid phase; (2) charge transfer impedance R ct in the solid-liquid interface; and (3) Warburg impedance R Warburg relating to the diffusion of lithium ions in the active materials [257]. These three parts can be measured by the electrochemical impedance spectroscopy (EIS) technique, as represented in Fig. 7(b). ...
... The impedance of the battery cells is influenced by both the intrinsic and extrinsic factors. The intrinsic factors include ionic conductivity/ diffusivity of the liquid or solid electrolyte (bulk material without pores), and the electrical conductivity of active materials [257,259]. The extrinsic factors comprise the porosity and tortuosity of the separator and electrodes, and the contact states between the battery components [257,259]. ...
... The intrinsic factors include ionic conductivity/ diffusivity of the liquid or solid electrolyte (bulk material without pores), and the electrical conductivity of active materials [257,259]. The extrinsic factors comprise the porosity and tortuosity of the separator and electrodes, and the contact states between the battery components [257,259]. In other words, the intrinsic factors are related to the transferring speed of ions/electrons, while the extrinsic factors are related to the distance that ions/electrons need to travel. ...
Article
There are abundant electrochemical-mechanical coupled behaviors in lithium-ion battery (LIB) cells on the mesoscale or macroscale level, such as electrode delamination, pore closure, and gas formation. These behaviors are part of the reasons that the excellent performance of LIBs in the lab/material scale fail to transfer to the industrial scale. This paper aims to systematically review these behaviors by utilizing the ‘mechanical origins – structural changes – electrochemical changes – performance’ logic. We first introduce the mechanical origins i.e., the external pressure and internal deformation, based on the different stages of battery life cycle, i.e., manufacture and operation. The response of the batteries due to the two mechanical origins are determined by the mechanical constitutive relation of battery components. The resulting structural changes are ascribed to size and distribution of pores and particles of the battery components, the contact states between different components. The electrochemical changes are divided into ionic/electrical impedance and lifespan. We have summarized massive experimental observations and modelling efforts and the influencing factors in each section. We also clarify the range of external pressure and internal deformation under which the proposed structural and electrochemical changes are likely to take effects. Lastly, we apply the logic to the next generation lithium metal-based solid-state battery. This review will provide useful guidelines to the design and manufacture of lithium-based rechargeable batteries and promote the development of the electric vehicle industry.
... t ¼ ce Àa where c and a are two constants that depend on the manufacturing process and raw material used. For electrodes made with spherical particles by slurry casting, c and a are normally set as 1 and 0.5, respectively [73]. Thus, tortuosity decreases with increasing porosity. ...
... Even for the gradient Li 4 Ti 5 O 12 anodes, the porosity is randomly distributed within each coating layer. Although the extra porosity introduced by salt templates can potentially reduce electrode tortuosity and improve Li-ion transport to some extent based on the Bruggeman equation [73], the randomly distributed porosity still shows higher electrode tortuosity compared with straight channels whose tortuosity is close to the lowest value of 1 [116]. As for the template, NaCl is one of the most commonly used materials due to its excellent chemical inertness, thermal stability, ease of removal and low cost [117,118]. ...
Article
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The development of next-generation electrodes is key for advancing performance parameters of lithium-ion batteries and achieving the target of net-zero emissions in the near future. Electrode architecture and design can greatly affect electrode properties and the effects are sometimes complicated. The architecture of current electrodes is designed mainly based on empirical studies by making trade-offs between battery performance parameters. Thus, a holistic understanding of the relationships between electrode architecture-property-performance is urgently needed. Additionally, the implementation of next-generation electrodes with optimised architectures also relies on manufacturing capability. Various manufacturing processes have been proposed to produce electrodes with characteristic architectures. Nevertheless, the merits and limitations of the manufacturing processes are not well understood and selecting appropriate manufacturing processes is challenging. Herein, ten manufacturing processes are illustrated, which have been classified into four categories of slurry casting, templating, additive manufacturing, laser ablation. The overall performance of all the manufacturing processes is first qualitatively compared from five different aspects of architectural controllability, scalability, sustainability, simplicity and cost, followed by a quantitative comparison using a Weighted Manufacturing Score method. This work provides a guideline for future electrode architectural design illustrating the limitations and potential advantages of different methodologies to stimulate the development of the next-generation LIB electrodes.
... We believe that this was due to the outstanding electronic conductivity of the conductive carbons and semi-ionic C-F bonding among the graphite particles in the original and PVDF-C-L electrodes, respectively. Similarly, the excellent electronic network of graphite and lithium titanate anodes in a previous study afforded a very low electronic resistance, thereby showing no semicircles in either electrode in the high-frequency range [45]. Although it seemed small enough to be difficult to discern the electronic resistances in the EIS data, the electronic resistance in the PVDF-C-L (=2.9 ± 0.1 mΩ cm −2 ), measured using an areal resistance meter, was reduced by 90%, compared with that of the original electrode (=28.5 ± 2.7 mΩ cm −2 ). ...
... We believe that this was due to the outstanding electronic conductivity of the conductive carbons and semi-ionic C-F bonding among the graphite particles in the original and PVDF-C-L electrodes, respectively. Similarly, the excellent electronic network of graphite and lithium titanate anodes in a previous study afforded a very low electronic resistance, thereby showing no semicircles in either electrode in the high-frequency range [45]. ...
Article
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Lithium-ion batteries with ultra-thick electrodes have high energy density and low manufacturing costs because of the reduction of the inactive materials in the same battery volume. However, the partial usage of the full capacity and the low rate capability are caused by poor ionic and electronic conduction. In this work, the effects of two approaches, such as electrode binder carbonization by heat treatment and 3-dimensionalization by the laser structuring of ultra-thick graphite anodes to lithium-ion batteries for high energy density, are investigated. During the heat treatment, the polyvinylidene fluoride (PVDF) binder is carbonized to form fluorinated graphitic carbons, thereby increasing the number of lithium-ion storage sites and the improvement of the electrode capacity by 14% (420 mAh g−1 and 20 mAh cm−2). Further, the carbonization improves the rate capability by 31% at 0.1 C by simultaneously reducing the ionic and electronic resistances. Furthermore, after the laser structuring of the carbonized electrode, the areal discharge capacity increases to 50% at the increasing current rates, resulting from drastically improved ionic conduction. In addition to the electrochemical characteristics, these two approaches contribute considerably to the fast wetting of the electrolyte into the ultra-thick electrode. The carbonization and laser structuring of the ultra-thick graphite anodes are practical approaches for high-energy batteries to overcome the thickness limitation.
... in the pores of the electrode bulk. [29][30][31] To get more insight into the porous ionic resistance in the electrodes, we performed symmetric cell impedance studies using negative electrodes with the Si8%/Gr83% and Si15%/Gr76% formulation depending on the calendered percentage (see Figure 6a and b, respectively). ...
... Contrary to positive electrodes such as NMC and LFP, the ionic resistance in pores progressively decreases when calendering. 29,32,33 Here, we observed the lowest ionic resistance corresponding to a specific or range of calendered percentages for the Si8% and Si15% contents, respectively. The ion resistances are the maximum for the electrode calendered at 30% for both formulations. ...
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The manufacturing process aims to optimize the parameters leading to enhanced Lithium-Ion Battery (LiB) electrode properties. Particularly, developing silicon/graphite blends could be an alternative for boosting LiB energy density while using the longstanding properties of graphite. Here, we report the manufacturing parameters impact of the mixing, coating, and calendering steps on the properties of silicon/graphite blend electrodes. The mixing process was assessed by the solid and silicon content dependency, where the viscosity increases when increasing the solid and decreasing the silicon content. Moreover, the slurry rheology directly impacts the mechanical stability of the electrode when coating using thicker comma gaps. The calendering step evidences a porosity threshold necessary for adequate ionic resistance and cycling life. We found that porosities between 45% to 56% for these silicon/graphite blends yield higher performance. Lower than 30% porosity highly impacts the electrochemical performance in a detrimental way.
... The result may be different from the experimentally derived result because, in many cases, it is difficult to quantify the diffusion coefficient values, and so arbitrary sets are used and analyzed [27]. On the other hand, such a tortuosity variation in the electrode is also observed in research based on experiments, e.g., electrochemical impedance spectroscopy (EIS) [27][28][29][30]. Impedance spectra obtained from EIS are affected by various reaction signals, including the state of the contact between particles in an electrode [31], contact between particles and a substrate [32], oxidation/reduction reactions [33], active material interface reactions, and reaction products [34]. ...
Article
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In this study, the effect of the active material geometry on the tortuosity in the ion transport path of the electrode composite of an all-solid-state lithium battery was systematically analyzed in terms of the different design and process factors of an electrode. A direct current technique (i.e., chronoamperometry) using an electron-blocking cell was used to analyze the tortuosity to minimize the experimental error. In addition, aluminum oxide was selected as a hypothetical active material in a composite electrode to exclude the possible disturbance of the ion transport signal caused by real active materials. The experimental results showed that the shape and composition of the active material had significant influences on the ion transport characteristics. In particular, when a fibrous material was applied with a high active material ratio, the degree of tortuosity was significantly increased, reaching values as high as 45, due to the insufficient filling in the micropores formed by particle aggregation. Moreover, the tortuosity degree decreased below 15 as the pressing pressure increased during electrode manufacturing, and the cause of this decrease differed with the active material’s particle shape. The analysis results confirmed that the change in tortuosity resulting from the electrode design factors of an all-solid-state battery has distinctive features compared to that for a conventional liquid electrolyte-based lithium-ion battery.
... Facilitated by low tortuosity pores and high porosity, the separator's ionic conductivity enables the ease of the Li ions to travel from one electrode to the other and needs to be increased for high power density batteries. Tortuosity is the increase in path-length due to the porous structure of the separator with respect to a straight line [3]. This poses an interesting dilemma for separators: creating a separator that has high ionic conductivity yet is still mechanically strong enough to prevent contact between the electrodes. ...
Article
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Benefited from their tunable porosity, bondability, high voltage stability, fast transport properties, and good electrolyte retention, plastic separators have been an attractive technology option for separators in Li ion batteries. Despite their invention in 1994 at Bellcore, there has been little optimization and further understanding of this plastic separator comprised of PVDF-HFP copolymer resin, a plasticizer (dibutyl phthalate), and nano filler (SiO2). This paper explores the impact of formulation of the Bellcore plastic separator on transport and pore structure to identify the optimal pore architecture for fast ion transport. Each formulation was investigated for pore size distribution by gas adsorption analysis (BET) and He ion microscopy, ionic conductivity and tortuosity by electrochemical impedance spectroscopy (EIS), and mechanical properties by tensile testing to correlate pore size distribution to transport, mechanical properties, and formulation. Bondable, high voltage stable separator formulations were created with ~ 280% greater conductivity than the baseline formulation and an industry standard polyolefin separator.
... Our data also indicates, that Gurley measurements are indeed a simple and capable method to detect a separator's upper limits for lamination conditions. Determination of ionic conductivity and tortuosity could provide more detailed insights into these relationships [18]. ...
Article
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To meet the requirements of today’s fast-growing Li-ion battery market, cell production depends on cheap, fast and reliable methods. Lamination of electrodes and separators can accelerate the time-consuming stacking step in pouch cell assembly, reduce scrap rate and enhance battery performance. However, few laminable separators are available on the market so far. This study introduces electrospinning as a well-suited technique to apply thin functional polymer layers to common battery separator types, enabling lamination. The method is shown to be particularly appropriate for temperature resistant ceramic separators, for which stable interfaces between separator and electrodes were formed and capacity fading during 600 fast charging cycles was reduced by 44%. In addition, a straightforward approach to apply the method to other types of separators is presented, including separator characterization, coating polymer selection, mechanical tests on intermediates and electrochemical validation in pouch cells. The concept was successfully used for the modification of a polyethylene separator, to which a novel fluoroelastomer was applied. The stability of the electrode/separator interface depends on the polymer mass loading, lamination temperature and lamination pressure, whereas poorly selected lamination conditions may cause damage on the separator. Appropriate adhesion force of 8.3 N/m could be achieved using a polymer loading as low as 0.25 g/m2. In case separator properties, coating polymer, morphology of the fibrous coating and lamination conditions are well adjusted to each other, the implementation of electrospinning and lamination allows for faster, more flexible and robust pouch cell production at comparable or better electrochemical cell behaviour.
... The sinus amplitude was set to 10 mV. The determination of R cath H + was based on the transmission line model [28][29][30][31][32]. ...
Article
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High temperature proton exchange membrane fuel cells (HT-PEMFCs) typically employ either acid-absorbing or hydrophobic electrode binders in their catalyst layers (CLs). A recently introduced alternative is the ionomeric binder PWN, poly(2,3,5,6-tetrafluorostyrene-4-phosphonic acid). In literature, PWN with a phosphonation degree of 70% was shown to remarkably improve HT-PEMFC performance. Here, we investigate the influence of the phosphonation degree (40–95%) of this ionomeric binder on HT-PEMFC performance. PWN is employed in the cathode CL and compared to the commonly used polytetrafluoroethylene (PTFE) binder. The electrochemical behavior is tested at 180 °C at ambient pressure under H2/air conditions using a commercial phosphoric acid (PA)-doped PBI-membrane. HT-PEMFCs with PWN generally outperform fuel cells (FCs) with PTFE after a full break-in regarding peak power density (PPD), activation overpotential (as studied by Tafel analysis), and reproducibility in the mass transport region. Further, PWN-electrodes show higher electrochemically active surface areas (ECSAs) than PTFE-electrodes after completing the break-in. We find that the phosphonation degree has a substantial impact on the PPD, with PWNs with lower phosphonation degrees (40–60%) outperforming highly phosphonated PWNs (70–95%). Taken together, PWN as an ionomeric electrode binder in HT-PEMFCs shows remarkable improvements in performance, but a precise adjustment of the phosphonation degree is required to obtain optimal results.
... Additionally, LiFSI's better thermal stability enables higher charging temperatures and so faster charging. A higher-porosity anode was also selected to reduce the tortuosity and MacMullin number based on the empirical relationship for graphite between porosity and MacMullin number 36 . Increasing the porosity from 0.26 to 0.35 increased the effective transport properties by 40% while decreasing the energy density by only 2% because of the need for more electrolyte (Supplementary Fig. 3b). ...
Article
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Lithium-ion batteries with nickel-rich layered oxide cathodes and graphite anodes have reached specific energies of 250–300 Wh kg⁻¹ (refs. 1,2), and it is now possible to build a 90 kWh electric vehicle (EV) pack with a 300-mile cruise range. Unfortunately, using such massive batteries to alleviate range anxiety is ineffective for mainstream EV adoption owing to the limited raw resource supply and prohibitively high cost. Ten-minute fast charging enables downsizing of EV batteries for both affordability and sustainability, without causing range anxiety. However, fast charging of energy-dense batteries (more than 250 Wh kg⁻¹ or higher than 4 mAh cm⁻²) remains a great challenge3,4. Here we combine a material-agnostic approach based on asymmetric temperature modulation with a thermally stable dual-salt electrolyte to achieve charging of a 265 Wh kg⁻¹ battery to 75% (or 70%) state of charge in 12 (or 11) minutes for more than 900 (or 2,000) cycles. This is equivalent to a half million mile range in which every charge is a fast charge. Further, we build a digital twin of such a battery pack to assess its cooling and safety and demonstrate that thermally modulated 4C charging only requires air convection. This offers a compact and intrinsically safe route to cell-to-pack development. The rapid thermal modulation method to yield highly active electrochemical interfaces only during fast charging has important potential to realize both stability and fast charging of next-generation materials, including anodes like silicon and lithium metal.
... This phenomenon is generally referred to as "dispersion effect". Therefore, it is difficult to obtain a satisfactory fitting result using the capacitive element (C) in the AR-ECM, and thus the Constant Phase Element (CPE) is introduced instead [39]. ...
Article
This paper explores the major degradation characteristics of commercial lithium-ion battery cells with nickel–cobalt-aluminum-oxide (NCA) electrode during cyclic overcharging, and proposes non-destructive methods for detecting overcharging degradation failure. The experimental results show that battery capacity drops significantly with increasing overcharge depth and number of cycles especially during the first three cycles and when the charging termination voltage is set to 5 V. At the same time, the cell overcharge tolerance decreases with the cyclic overcharging. The combination of the electrochemical impedance spectroscopy and the incremental capacity and differential voltage analysis is used to diagnose cell degradation during cyclic overcharging. Three main degradation modes are identified and quantified by extracting characteristic parameters such as internal resistance and peak, valley, and curve position changes of incremental capacity curves. It is concluded that loss of lithium inventory and loss of active materials are the most dominant degradation modes during cyclic overcharging. Besides, the sharp increase of the third peak on incremental capacity curves has been identified as a unique feature of overcharging degradation, which can be used for diagnosing cyclic overcharging-induced degradation for batteries with NCA cathode.
... N M is defined as the ratio of the ionic conductivity (κ) of an electrolyte solution to the effective ionic conductivity (κ eff ) of an electrolyte-filled porous electrode, and indicates how effectively ions are transported through the electrode. 673 The N M value of SC-PCF was estimated to be 2.18 and that of PCF to be 4.47, which means that the integration of surface craters in SC-PCF helps to provide robust pathways for facile Li + transport through the electrode. PCFs are mainly composed of micropores, which provide a narrow path for the movement of Li + , resulting in high ionic resistance. ...
Article
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Batteries are a promising technology in the field of electrical energy storage and have made tremendous strides in recent few decades. In particular, lithium‐ion batteries are leading the smart device era as an essential component of portable electronic devices. From the materials aspect, new and creative solutions are required to resolve the current technical issues on advanced lithium (Li) batteries and improve their safety. Metal‐organic frameworks (MOFs) are considered as tempting candidates to satisfy the requirements of advanced energy storage technologies. In this review, we discuss the characteristics of MOFs for application in different types of Li batteries. A review of these emerging studies in which MOFs have been applied in lithium storage devices can provide an informative blueprint for future MOF research on next‐generation advanced energy storage devices. In this review, we discuss the characteristics of metal‐organic frameworks (MOFs) applied to lithium storage devices containing Li‐ion, Li‐sulfur, Li‐metal, and Li‐O2. We summarize the origin, nomenclature, and synthesis method of MOFs, and report on recent studies in which MOFs and MOF‐derived materials are applied to lithium rechargeable batteries. This provides an informative roadmap for next‐generation advanced energy storage devices.
... Taking the case at 75% DoD as an example, the three stages of potential response in the time domain(Figure 3c) match the three impedance regions in the frequency domain(Figure 3d). In the Nyquist plots(Figure 3d), the smaller arc (Z c ) is the contact impedance of the current collector/electrode interface[45][46][47] , and the bigger one (Z ct ), which increases with DoD (Figure S9a), corresponds to η ct because it depends on Li + /vacancy concentration at the surface of active particles. The tail of the impedance curves (Warburg impedance Z w ) is associated with solid diffusion, which can be clearly distinguished at lower DoDs (Figure S9b). ...
Preprint
Identifying overpotential components of electrochemical systems enables quantitative analysis of polarization contributions of kinetic processes under practical operating conditions. However, the inherently coupled kinetic processes lead to an enormous challenge in measuring individual overpotentials, particularly in composite electrodes of lithium-ion batteries. Herein, the full decomposition of electrode overpotential is realized by the collaboration of single-layer structured particle electrode (SLPE) constructions and time-resolved potential measurements, explicitly revealing the evolution of kinetic processes. Perfect prediction of the discharging profiles is achieved via potential measurements on SLPEs, even in extreme polarization conditions. By decoupling overpotentials in different electrode/cell structures and material systems, the dominant limiting processes of battery rate performance are uncovered, based on which the optimization of electrochemical kinetics can be conducted. Our study not only shades light on decoupling complex kinetics in electrochemical systems, but also provides vitally significant guidance for the rational design of high-performance batteries.
... Contrarily, Liu et al. [49] reported ionic conductivities of 10 mS.cm − 1 and 11.6 mS.cm − 1 (16% more) for the electrolytes 1 M LiPF 6 and 0.8 M LiPF 6 -0.2 M LDFP in EC/DEC/EMC 1:1:1 wt.%, respectively. Within the porous polypropylene separator (Fig. 2b), the ionic conductivity of the three electrolytes is divided almost by 10, due to tortuosity [50], and the differences when LDFP is added are very small. As in liquid configuration, the 0.8 M LiPF 6 + 0.2 M LDFP electrolyte presents slightly lower values at temperatures higher than ca. 10 • C. ...
Article
From the literature overview, lithium difluorophosphate salt, LiPO2F2, is considered a powerful electrolyte additive capable of enhancing lithium-ion batteries’ capacity retention. Lower cell impedance associated with SEI and/or CEI layers composition and texture modifications had been widely demonstrated, but without providing clear mechanisms. This paper sheds light on the reactivity of LiPO2F2 by combining electrochemical measurements and analyzes of electrolyte degradation products at the interphases and in the electrolytes by means of infrared and mass spectrometry techniques. Fluorine substitution reaction by electrochemically formed anions leads to a mitigation of electrolyte solvents degradation and to the enrichment of the SEI layer with LiF and lithium organofluorophosphates. Furthermore, a discussion on the composition of the CEI layer that enables the prevention of the transition metals dissolution from positive electrode layered oxides materials is also presented.
Article
The optimization of the electrodes manufacturing process constitutes a critical step to ensure high-quality Lithium-Ion Battery (LIB) cells, in particular for automotive applications. Because LIB electrode manufacturing is a complex process involving multiple steps and process parameters, we have shown in our previous works that 3D-resolved physics-based models constitute very useful tools to provide insights into the impact of the manufacturing process parameters on the textural and performance properties of the electrodes. However, their high-throughput application for electrode properties optimization and inverse design of manufacturing parameters is limited due to the high computational cost associated with these models. In this work, we tackle this issue by proposing a generalizable and innovative approach, supported by a deterministic machine learning (ML)-assisted pipeline for multi-objective optimization of LIB electrode properties and inverse design of its manufacturing process. Firstly, the pipeline generates a synthetic dataset from physics-based simulations with low discrepancy sequences, that allows to sufficiently represent the manufacturing parameters space. Secondly, the generated dataset is used to train deterministic ML models to implement a fast multi-objective optimization, to identify an optimal electrode and the manufacturing parameters to adopt in order to fabricate it. Lastly, this electrode was successfully fabricated experimentally, proving that our modeling pipeline prediction is physical-relevant. Here, we demonstrate our pipeline for the simultaneous minimization of the electrode tortuosity factor and maximization of the effective electronic conductivity, the active surface area, and the density, all being parameters that affect the Li⁺ (de-)intercalation kinetics, ionic, and electronic transport properties of the electrode.
Article
Despite the electrochemical benefits of laser electrode structuring, the process is not yet implemented in state-of-the-art industrial battery production due to a limited knowledge regarding its implementation into the manufacturing process chain. In this study, three process integration positions for laser structuring of graphite anodes, which are either after coating, after drying or after calendering, were experimentally evaluated. The obtained electrodes were analyzed regarding geometrical, mechanical and electrochemical characteristics. The results indicate that the material ablation process is governed by the evaporation of solvent and binder for wet and dry electrodes, respectively. As a consequence, electrodes structured in wet condition exhibited fewer particle residues on the electrode surfaces and a high coating adhesion strength. In contrast, laser structuring of dry electrodes significantly reduced the pull-off strengths of the electrode coatings. A tortuosity reduction and an increased discharge capacity at high C-rates by laser structuring were observed for all structured electrodes, but with higher performance improvements for electrodes structured in dry state. Although a partial clogging of the structures was observed in electrodes structured before calendering, laser structuring yielded a comparable electrochemical performance of electrodes which were structured in dry condition before and after calendering.
Article
To mitigate the shuttle effect and enhance the electrical conductivity in lithium battery cathode, the unique characteristics of supercritical CO2 solvent (SC–CO2) and the distinctive porous and layered microstructure of reduced graphene oxide (rGO) are exploited in the fabrication of a high-performance rGO/sulfur composite cathode. Exploiting SC-CO2 technology can realize highly efficient sulfur transfer and precise microstructure regulation of S/C composite cathodes for Li–S batteries. On exposure, due to the sudden pressure release process, the SC-CO2 expands the interlayers of rGO rendering plenty of storage space for small sulfur allotropes in carbon matrices which increases the active sulfur loading. Being a remarkable hydrophobic solvent, the wetting properties of SC-CO2 are excellent, ensuring sulfur dissolution and penetration deep into the voids and interlayers of rGO. This creates intimate contact of sulfur with rGO interlayers, guaranteeing precise sulfur content, uniform sulfur distribution, and strong interaction between sulfur and carbon leading to enhanced electrical conductivity and sulfur utilization efficiency. Another important feature is that the S/C composites can be prepared at room temperature, unlike other conventional techniques which require a higher temperature. Moreover, the product mixture can be separated simply by de-pressuring SC-CO2. Herein, the rGO/sulfur composite cathode prepared on a lab scale showed an initial discharge capacity of 1024 mAh/g at 0.1C rate with capacity retention of 92.2% and coulombic efficiency of 99% even after 200 charge-discharge cycles. The developed cells showed excellent performance (929 mAh/g at 1 C rate) with an ultralow decay of 0.04% per cycle even after 200 charge-discharge cycles. Through this work, we believe that the synergistic effect of SC-CO2 technology and rGO as sulfur host will open up a promising future for the synthesis of efficient S/C composite cathodes with ultra-high cycling stability.
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Rate performance is one of the important indexes of lithium-ion batteries. However, the discharge capacity would sometimes collapse after a critical C-rate, which is related to a classic concept termed the diffusion-limited C-rate (DLC). In this work, DLC is revisited using the Pseudo-Two-Dimensional model. The DLC analytical model is improved by incorporating the concentration-dependent liquid diffusion coefficient and shows good effectiveness compared with the simulation results. Subsequently, the impact of rate-limiting factors on the rate performance is examined to determine the effective scope of the proposed DLC model. Based on the adjustment of resistance ratios and time constant ratios of different kinetic processes, we find that the rate performance below DLC is hardly limited by electronic transports and charge transfer processes while the discharge capacity below DLC may be severely decayed if the solid-state diffusion in the electroactive particles is too slow. DLC helps identify the ‘knee’ on discharge capacity curves with discharge C-rate, which is a criterion of rate performance in some scenarios.
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Compared to traditional lithium-ion batteries, solid-state batteries (SSBs) are characterized by higher safety and energy density, so the key component, solid-state electrolyte (SSE) has received considerable attention. The most common SSEs can be divided into three types: oxides, sulfides, and polymers. This paper mainly presents and discusses the latest research on unconventional solid-state electrolytes (USSEs) such as halides, zeolites, etc., focusing on their properties, structures, costs, synthesis methods, etc. Although SSEs such as halides are less well known, they are becoming a very hot topic due to their numerous advantages, such as: a) ionic conductivity at room temperature of more than 0.6 mS cm⁻¹; b) oxidation potential exceeding 4V, which means that they are more stable than most other types of SSEs; c) high stability in dry air (i.e., easier handling during the fabrication process), unlike sulfides, which decompose to produce toxic gases; d) some halides can be synthesized by the liquid phase method, which is more suitable for mass production. Then, some typical USSEs were compared with conventional SSEs such as oxides and liquid electrolytes in various aspects. The paper concludes with a discussion on future research directions for USSEs and the challenges to be overcome in practical applications.
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Identifying overpotential components of electrochemical systems enables quantitative analysis of polarization contributions of kinetic processes under practical operating conditions. However, the inherently coupled kinetic processes lead to an enormous challenge in measuring individual overpotentials, particularly in composite electrodes of lithium-ion batteries. Herein, the full decomposition of electrode overpotential is realized by the collaboration of single-layer structured particle electrode (SLPE) constructions and time-resolved potential measurements, explicitly revealing the evolution of kinetic processes. Perfect prediction of the discharging profiles is achieved via potential measurements on SLPEs, even in extreme polarization conditions. By decoupling overpotentials in different electrode/cell structures and material systems, the dominant limiting processes of battery rate performance are uncovered, based on which the optimization of electrochemical kinetics can be conducted. Our study not only shades light on decoupling complex kinetics in electrochemical systems, but also provides vitally significant guidance for the rational design of high-performance batteries.
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Lithium‐ion battery cells with high energy density and good fast charging properties are subject of current research. One approach to achieve high energy densities is the use of higher mass loadings. The challenges of these so called “thick” electrodes are transport limitations: Lithium‐ions cannot reach all layers of the electrode, which results in a drop of performance. Possible concepts to overcome these limitations are the use of different active materials (silicon oxide, graphite, hard carbon) and a two‐layer coating of the anode to create a defined pore network, which reduces the ionic resistance and ensure better fast charging capability even at higher mass loadings (8 mAh cm‐2). It could be demonstrated that by using a two‐layered anode with hard carbon in the upper layer the electrical conductivity could be increased by a factor of 10 compared to the reference anode. Furthermore, the interporous hard carbon leads to a capacity retention increase up to 20 % with no loss of capacity at moderate C‐rates and a low electrode density. This can be explained by the low tortuosity which results in additional conductive paths for the ions through the coating, reduces the ionic resistance and ultimately enables faster lithiation of the anode. This article is protected by copyright. All rights reserved.
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Lithium‐ion batteries ensuring high energy densities are the focus of ongoing research. The main challenge is the fast charging capability, which is restricted by transport limitations of the lithium‐ions. For this reason, graphite (Gr) and hard carbon (HC) blend anodes at different calendering degrees and, thus, electrode densities are investigated in terms of their structural features as well as their electrochemical performance. The motivation of blend anodes is the combination of the advantageous properties of these materials. Due to the different microstructures of Gr and HC, major differences in the lithiation process can be found. While the turbostratic structure of HC enables fast charging, its large specific surface area is associated with a low initial Coulombic efficiency and, thus, a loss of capacity. Consequently, the combination with Gr in blends is reasonable. While the lithium‐ion diffusion is enhanced using HC, the availability of the interporous structure of HC is highly dependent on the electrode density. In addition to an increase in adhesion strength and reduction in electrical resistance, a reduction in tortuosity and lithium plating can be demonstrated. Furthermore, a higher capacity retention in the charge rate test (up to 3C) at low coating densities was found for the blends.
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The increasing demand for high‐energy powers have greatly incentivized the development of lithium carbon fluoride (Li || CF x ) cells. Five kinds of non‐aqueous liquid electrolytes with various kinds of lithium salts (LiX, X = PF 6 − , TFSI − , BF 4 − , ClO 4 − , and CF 3 SO 3 − ) were comparatively studied. Intriguingly, the LiBF 4 ‐based electrolyte show relatively moderate ionic conductivities; yet, the corresponding Li || CF x cells deliver the highest discharge capacities among them. A combination of morphological and compositional analyses of the discharge CF x cathode suggest that the moderate donicity of BF 4 − anion is accountable for favoring the breakdown of C−F bonds, and subsequently forming crystalline lithium fluoride as the main discharge products. This work brings not only fresh understanding on the role of salt anions for Li || CF x cells, but also inspire the electrolyte design for other conversion‐type (sulfur and/or organosulfur) cathode materials desired for high‐energy applications.
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Commercialization of silicon anodes remains a challenge due to severe volume changes during cycling. In this study, a novel binder was synthesized via in situ crosslinking of sodium alginate (NaA) and poly(ethylene imine) (PEI) and used in micro-silicon/graphite composite anodes. Ball-milled silicon possesses an increased hydrophilic character, which leads to poor compatibility with graphite in composite anodes using a standard NaA binder. However, the addition of PEI to NaA, led to an increase in the specific capacity of ~1000 mAh/gSi, which can be traced back to the increased compatibility between silicon and graphite induced by a crosslinked binder structure.
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The increasing demand for high‐energy powers have greatly incentivized the development of lithium carbon fluoride (Li || CF x ) cells. Five kinds of non‐aqueous liquid electrolytes with various kinds of lithium salts (LiX, X = PF 6 − , TFSI − , BF 4 − , ClO 4 − , and CF 3 SO 3 − ) were comparatively studied. Intriguingly, the LiBF 4 ‐based electrolyte show relatively moderate ionic conductivities; yet, the corresponding Li || CF x cells deliver the highest discharge capacities among them. A combination of morphological and compositional analyses of the discharge CF x cathode suggest that the moderate donicity of BF 4 − anion is accountable for favoring the breakdown of C−F bonds, and subsequently forming crystalline lithium fluoride as the main discharge products. This work brings not only fresh understanding on the role of salt anions for Li || CF x cells, but also inspire the electrolyte design for other conversion‐type (sulfur and/or organosulfur) cathode materials desired for high‐energy applications.
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Ion transport limitation is a well-known problem that occurs when the energy density of a battery is increased by using thicker or higher densified electrodes. To counteract this effect, several studies are currently investigating post-processing methods such as ablative trenching or perforation. This is a promising approach as long as significant loss of active material is accepted. In this study, a pulsed nanosecond laser machining is presented, which aims at a material-preserving surface treatment. Microscopic images show the exposure of additional accesses into the microporous structure of the anodes. At the same time, there is an increase in the outer surface area and roughness. This leads to reduced cell overpotentials and an increased rate capability of the anodes. For example, it is reflected in a higher CC share of up to 50% of the CC-CV charge capacity. Reduced ionic resistance of the laser treated anode layers are measured by impedance spectroscopy. The cyclic stability of the electrodes is not affected. Anodes of 3 mAh/cm2 and layer densities of 1.4 g/cm³ to 1.7 g/cm³ were investigated. Electrochemical tests were performed with half- and full-cells using Li metal and NCM622 counter electrodes as well as symmetrical anode cells.
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An experimental method is introduced that uses microprobe Li/Cu reference electrodes to map “safe” charging regimes to avoid Li metal plating on graphite anodes at rates to 8C. With this method, a Li-ion cell closely approaches the Li plating condition but avoids descending into this regime. This “safe” approach is used to demonstrate effects of the microporous separator on Li plating in cells consisting of a layered nickel-manganese-cobalt oxide cathode and graphite anode. We argue that most of the difference in behaviors for different separators arises from time delayed Li⁺ ion percolation through the membrane. Our study suggests that explicitly treating kinetics of phase transitions in the lithiated graphite anode is essential for electrochemical models predicting “safe” lines for fast charging cells.
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The influence of industrial‐suited mixing and dispersing processes on the processability, structure and properties of suspensions and electrodes for lithium‐ion batteries (LIB) is investigated for the case of ultra‐thick NCM 622 cathodes (50 mg cm‐²). Performed with a 10 dm³ planetary mixer, two different process strategies for the preparation of the suspensions are compared in which (1) all powders are mixed initially and the solvent is added stepwise so that the process starts with very high shear stress or (2) the powders are added stepwise to a binder solution so that lower shear stress is exerted. It is shown that the process strategy and within this, the level of solid content throughout the process as a measure of shear stress strongly affect the properties of the suspensions and the microstructure, mechanical quality and electrochemical performance of the resulting electrodes. Compared to the more unfavorable processes following stragegy (1), the most beneficial process following strategy (2) leads to a strongly enhanced elasticity of ultra‐thick electrodes making them suitable for roll‐to‐roll processing and furthermore to a drastic increase of their rate capability expanding their range of outperforming state of the art electrodes regarding energy density to higher current densities. This article is protected by copyright. All rights reserved.
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To maximize the performance of energy storage systems more effectively, modern batteries/supercapacitors not only require high energy density but also need to be fully recharged within a short time or capable of high-power discharge for electric vehicles and power applications. Thus, how to improve the rate capability of batteries or supercapacitors is a very important direction of research and engineering. Making low-tortuous structures is an efficient means to boost power density without replacing materials or sacrificing energy density. In recent years, numerous manufacturing methods have been developed to prepare low-tortuous configurations for fast ion transportation, leading to impressive high-rate electrochemical performance. This review paper summarizes several smart manufacturing processes for making well-aligned 3D microstructures for batteries and supercapacitors. These techniques can also be adopted in other advanced fields that require sophisticated structural control to achieve superior properties.
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Sodium-ion batteries (SIBs) are considered as a promising candidate to replace lithium-ion batteries (LIBs) in large-scale energy storage applications. Abundant sodium resources and similar working principles make this technology attractive to be implemented in the near future. However, the development of high-performance carbon anodes is a focal point to the upcoming success of SIBs in terms of power density, cycling stability, and lifespan. Fundamental knowledge in electrochemical and physicochemical techniques is required to properly evaluate the anode performance and move it in the right direction. This review aims at providing a comprehensive guideline to help researchers from different backgrounds (e.g., nanomaterials and thermochemistry) to delve into this topic. The main components, lab configurations, procedures, and working principles of SIBs are summarized. Moreover, a detailed description of the most used electrochemical and physicochemical techniques to characterize electrochemically active materials is provided.
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The thick electrode design is preferential in high-energy lithium-ion batteries (LIBs) systems. However, the sluggish ionic transport in homogeneous porous thick electrodes severely limits the areal capacity at high charging/discharging rates. The hierarchical porous design is a promising approach to mitigate kinetic limitations because it can distribute mass effectively in natural systems. In this study, the effects of bimodal microscale pores are fully investigated in thick electrodes from both architectural and electrochemical perspectives. Notably, by introduction of the bimodal microscale porous structure, the rate capability improves remarkably in thick electrodes with a low porosity (39%). By combining experimental results with simulations, this work presents a rational design guideline for preparing thick electrodes with a porosity at the commercial level, as well as simultaneous high energy and power densities, which brings new insights into the advanced electrode architecture design in scalable high-energy and high-power energy storage systems for practical applications.
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Growth in the Li-ion battery market continues to accelerate, driven by increasing need for economic energy storage in the electric vehicle market. Electrode manufacture is the first main step in production and in an industry dominated by slurry casting, much of the manufacturing process is based on trial and error, know-how and individual expertise. Advancing manufacturing science that underpins Li-ion battery electrode production is critical to adding value to the electrode manufacturing value chain. Overcome the current barriers in the electrode manufacturing requires advances in material innovation, manufacturing technology, in-line process metrology and data analytics to improve cell performance, quality, safety and process sustainability. In this roadmap we present where fundamental research can impact advances in each stage of the electrode manufacturing process from materials synthesis to electrode calendering. We also highlight the role of new process technology such as dry processing and advanced electrode design supported through electrode level, physics-based modelling. To compliment this, the progresses in data driven models of full manufacturing processes is reviewed. For all the processes we describe, there is a growing need process metrology, not only to aid fundamental understanding but also to enable true feedback control of the manufacturing process. It is our hope this roadmap will contribute to this rapidly growing space and provide guidance and inspiration to academia and industry.
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Mathematical models for porous electrode impedance have been widely used in energy conversion and storage. They are also utilized for obtaining the physicochemical dynamics, resulting in theoretical understanding and prediction in practical energy devices. The existing mathematical models are limited in their explanations. This limitation can be attributed to the separate consideration of simple (planar electrodes) and complex (porous electrodes) systems and the complexity of parameter distribution with non-uniform processes. Here, to address these limitations, we propose a mathematical model based on a staircase structure that calculates the individual interfacial impedance at each step in the depth direction, which helps not only in describing complex and straightforward systems but also in uniform and non-uniform processes in the form of a simple, seamless general equation. Our study includes mathematical derivations, interpretations of porous electrode impedance, and validation of the experimental data.
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Editorial for the Nature Communications collection entitled " Operando methods for rechargeable batteries" (https://www.nature.com/collections/afedjedfif)
Article
The performance of bulk-type all-solid-state Li batteries (ASSBs) depends critically on the contacts between cathode active material (CAM) particles and solid electrolyte (SE) particles inside the composite cathodes. These contacts determine the Li+ exchange current density at the CAM | SE interfaces. Nevertheless, there is a lack of experimental studies on Li+ exchange current densities, which may be caused by the poor understanding of the impedance spectra of ASSBs. We have carried out a comparative case study using two different active materials, namely, single-crystalline LiCoO2 particles and single-crystalline LiNi0.83Mn0.06Co0.11O2 particles. Amorphous 0.67 Li3PS4 + 0.33 LiI particles act as a solid electrolyte within the cathode and separator, and lithiated indium acts as the anode. The determination of the cathode exchange current density is based on (i) impedance measurements on In-Li | SE | In-Li symmetric cells in order to determine the anode impedance together with the anode | separator interfacial impedance and (ii) variation in the composite cathode thickness in order to differentiate between the ion transport resistance and the charge transfer resistance of the composite cathode. We show that under the application of stack pressures in the range of 400 MPa, the Li+ exchange current densities can compete with or even exceed those obtained for CAM | liquid electrolyte interfaces.
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The quantification of microstructural properties to optimize battery design and performance, to maintain product quality, or to track the degradation of LIBs remains expensive and slow when performed through currently used characterization approaches. In this paper, a convolution neural network-based deep learning approach (CNN) is reported to infer electrode microstructural properties from the inexpensive, easy to measure cell voltage versus capacity data. The developed framework combines two CNN models to balance the bias and variance of the overall predictions. As an example application, the method was demonstrated against porous electrode theory-generated voltage versus capacity plots. For the graphite|LiMn2O4 chemistry, each voltage curve was parameterized as a function of the cathode microstructure tortuosity and area density, delivering CNN predictions of Bruggeman’s exponent and shape factor with 0.97 R2 score within 2 s each, enabling to distinguish between different types of particle morphologies, anisotropies, and particle alignments. The developed neural network model can readily accelerate the processing-properties-performance and degradation characteristics of the existing and emerging LIB chemistries.
Thesis
Pour améliorer les performances des batteries lithium-ion actuelles avant la transition vers d’autres technologies de batteries (Li-S, Li-Air), des progrès sont fortement désirables sur l’ensemble de ses composants et notamment le séparateur. L’objectif de cette thèse multidisciplinaire porte sur l’amélioration des propriétés intrinsèques du séparateur poreux utilisé dans les batteries lithium-ion. Afin de proposer une alternative aux séparateurs des batteries lithium-ion, généralement constituées d’une membrane poreuse polyoléfine saturée d’un mélange de solvants organiques contenant un sel de lithium, de nouveaux copolymères statistiques sont synthétisés par polymérisation radicalaire classique. Ces copolymères, de formulations variables, possèdent tous un motif répétitif fournisseur de cations Li+ et un motif azoture photo-réticulable. Par la suite, les copolymères sont déposés en surface de la porosité d’un séparateur commercial par un procédé de «dip-coating» et le groupement azoture permet de réticuler le copolymère à la suite du dépôt à l’aide d’une irradiation sous ultraviolet. Les dépôts sont caractérisés par gravimétrie et leurs homogénéités sur l’ensemble de la porosité sont confirmées par microscopie électronique à balayage couplée à une analyse dispersive en énergie des rayons X. Dans un premier temps, la présence de copolymère en surface des pores a permis d’améliorer les interactions entre le séparateur et le solvant organique contenant un sel de lithium. Cette amélioration se traduit par une diminution des angles de contact entre les séparateurs modifiés et des liquides sondes. La modification des séparateurs entraine la diminution de leur tortuosité et donc l’augmentation de la conductivité effective associée à l’électrolyte. Les séparateurs sont ensuite assemblés dans des configurations symétriques Li/Li et dans des batteries Li/NMC 532 de type pile bouton pour des caractérisations électrochimiques. La cyclabilité et les performances en puissance de ces batteries et cellules symétriques sont sensiblement améliorées. Dans un deuxième temps, un électrolyte liquide pur et sans sel de lithium est utilisé comme un media de transport unipolaire des cations Li+ issus du revêtement copolymère tapissant la surface des pores du séparateur. Contrairement aux électrolytes liquides conventionnels, l’anion du sel de lithium est fixé, ce qui permet théoriquement d’atteindre des densités d’énergies et de puissance inaccessibles, et d’inhiber la croissance dendritique du lithium. En présence de propylène carbonate, la conductivité effective du séparateur modifié atteint 9 10-6 S cm-1 à 25 °C pour une concentration en lithium 0.14 mol L-1 au sein du volume poreux et un nombre de transport cationique (t+) moyen de 0.7. Des cellules symétriques Li / Li, incorporant les séparateurs modifiés saturés par du propylène carbonate, sont capables de cycler sans provoquer de court-circuit et permettent d’atteindre des densités de courants jusqu’à 300 µA cm-2. Les séparateurs modifiés assurent aussi le fonctionnement d’une batterie avec un couple d’électrodes à fort potentiel (Li/NMC 532) en préservant une capacité de décharge élevée et en diminuant fortement la polarisation lors de son fonctionnement. Ces travaux de thèses ont donc permis la synthèse de nouveaux matériaux polymères facilement utilisables pour fonctionnaliser de manière permanente la surface de la porosité d’un séparateur commercial grâce à un motif dont la réticulation est contrôlée. Les séparateurs modifiés améliorent les performances des batteries avec un électrolyte liquide contenant du sel de lithium. Ils peuvent aussi être utilisé comme un composant actif et mettent en évidence le fonctionnement d’une batterie composé d’un électrolyte liquide single-ion combinant les avantages des électrolytes liquides standards avec ceux des électrolytes polymères single-ion
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Notwithstanding the rapid advances in electric vehicles for supplanting internal combustion engines, the widespread adoption of lithium-ion battery-powered electric vehicles remains hindered owing to their relatively low power density, which correlates with their charging time. In this study, the ionic and electronic conduction properties of electrodes were balanced by rationally designing the physical properties of the electrodes. The optimized NCA/conductive additive composite electrode required only 8.2 min for charging to 80% of the state of charge. This work paves the way to achieve extreme fast charging, thus helping to usher in a new era of electric vehicles.
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Composites consisted of ceramic paper, nanoparticles as well as molten salt electrolyte, are developed as the new electrolyte separators to substitute the traditional ones in thermal batteries, due to their flexible manufacture and good exhibition in electrochemical performance. However, the existed methods for fabricating the composites by roughly dip-coating with aqueous solutions, are suffering from drawbacks such as inefficiency, low consistence, or high temperature risk. Therefore, a new approach based on a melting-absorption-diffusion process, by using precursors without solvent, is developed. Firstly, the salts are accurately weighted. Then they melt on surfaces of ceramic papers and diffuse inside. After being treated at appropriate temperature for hours, high quality electrolyte separators based on ceramic papers are achieved. Compared to the traditional approaches, the new method can obviously increase and accurately control the amount of loaded nanoparticles and molten salts for each time. The achieved electrolyte separators also exhibit good electrochemical performance in single cells of thermal batteries compared with the traditional ones. With advantages like easy operation, high efficiency, and improved performance, this melting and in situ loading method holds broad prospect in preparing electrolyte separators at large scale.
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Lithium-ion batteries with superior capacities and rate performance are needed due to the soaring demands for higher energy and power device requirements. However, the main hurdle on achieving this predominately results from the poor rate performance of electrode, which is related to thermodynamic limitations and slow kinetics. To determine the rate-limiting electrode in NMC622 vs graphite cells, a methodology based upon the galvanic intermittent titration technique, for investigating the diffusion and reaction kinetics from the observed overpotential at each electrode has been developed. Variable current densities have been used to simultaneously extract the thermodynamic and kinetic properties of each electrode with increasing mass loading. Graphite is observed to reach its thermodynamic limits quicker than NMC, due to the flat plateaus and overpotentials observed from the charge transfer kinetics and mass transport. At high rates and high mass loadings, the graphite electrode is responsible for limiting both Li⁺ diffusion and reaction rates in full cells. Slow diffusion kinetics are caused by the transport of the electrolyte in the porous electrode, which limits the availability of Li⁺ for reaction at the surface of graphite. This methodology is proposed as a fast single technique for comprehensively parameterizing the rate limitations observed in a full cell configuration.
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Sintered electrode lithium-ion batteries contain only electroactive material which has undergone a mild thermal treatment to improve particle connectivity and thus electrode mechanical strength. These electrodes can be made very thick, and due to the lack of inactive binders or conductive additives there are ion transport advantages relative to equivalent thickness and porosity composite electrodes. In this study, the discharge of sintered electrode full cells was simulated at the cell level for three different form factors: CR2032, CR2025, and CR2016. Simulations also include relatively high and low rates of discharge and two different electrolytes. Using the simulations, overpotential distributions enable analysis of the contributors to resistance in the cell analyzed for the different regions of the anode, cathode, and separator. Overpotential sources include electronic overpotential, ionic overpotential, interfacial overpotential, and OCV overpotential. As the total thickness increases, the main overpotential contribution shifts from interfacial to ionic, with both originating from the electrodes. In all cases a higher conductivity electrolyte dramatically increases total capacity delivered at higher discharge rates.
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Increasing the thickness of composite electrodes in lithium‐ion batteries from about 80–100 μm in state‐of‐the‐art commercial cells to several 100 μm would enhance the energy density, however at the expense of the power density. Despite this common knowledge, quantitative studies on the impedance and rate capability of composite electrode in dependence of the electrode thickness are scarce. Therefore, we have carried out a case study on LiCoO2 composite electrodes with thicknesses up to about 250 μm. We first demonstrate that conventional composite electrode preparation leads to ion transport tortuosities in the electrolyte‐filled pores, which are virtually independent of the thickness. The thickness‐dependent impedance of these electrodes decreases with increasing thickness and follows the predictions of a generalized transmission line (GTLM) model. We use the GTLM model for analyzing in what way cycling of ultrathick electrodes (250–300 μm) with a rate of 1 C should be achievable. Thick electrodes: The impedance of composite electrodes was found to decrease considerably with increasing thickness and to follow the predictions of a generalized transmission line model (GTLM). A good agreement was observed between the resistances Rtot determined from charge‐discharge curves and the low‐frequency impedance , while the resistance of the high‐frequency semicircle due to ion transport and charge transfer was considerably lower.
Chapter
Lithium-sulfur is a promising beyond Li-ion battery (LIB) chemistry with an order of magnitude higher theoretical energy density. However, practical limitations make them inferior to the present-day LIBs. While different mechanisms have been proposed to explain the bottlenecks in chemistry, many open questions remain. In this context, a physics-based continuum-scale analysis helps quantify the importance of these mechanisms as operating conditions, electrode composition, or porous microstructure are varied. We discuss the modeling literature that examines mechanisms such as morphology-dependent microstructure evolution, nucleation dynamics of the precipitate phase, electrolyte transport limitation, and polysulfide shuttle effect in this fashion. Each of these mechanisms dominates the macroscopically measured electrochemical signatures for a certain subset of cell specifications and operation. Accordingly, the strategies for performance improvement differ. We close by discussing the opportunities for future investigations to elucidate the poorly understood interactions to bring us a step closer to the rational design of such cells.
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Lithium‐ion batteries (LIBs) are the main driving force behind the proliferation of mobile devices and electric vehicles. The production technologies of LIBs have been developed with the aim of lowering the energy cost (US$ kWh−1) and environmental impact while increasing the production efficiency. Here, we report dry‐processed Ni‐rich oxide cathodes coated with carbon nanotubes (CNTs) for LIBs. Specifically, LiNi0.8Co0.15Al0.05O2 (NCA) particles coated with multi‐walled CNTs (MWCNTs) were used to fabricate the cathodes by employing dry‐type electrode processing. In addition, the amount of polytetrafluoroethylene binder in the electrode was minimized and the inclusion of conventional carbon black (CB) conductive additives was eliminated. This approach enabled the fabrication of CB‐free, dense cathodes with an extremely high NCA content (99.6 wt%) and high electrode density of ~4.0 g cm−3. The NCA cathode had a high volumetric capacity of ~821 mAh cm−3 at a current rate of 0.5 C (~4.6 mA cm−2) and delivered good full‐cell cycling performance over 300 cycles (~60% capacity retention). Our results offer a viable way to lower the energy cost and the environmental damage toward next‐generation LIBs. Ni‐rich particles with a CNT‐coating were used for dry‐processed cathode preparation. The CNTs replace conventional carbon black conductive additives. The CNT‐coated Ni cathodes were fabricated with minimal binder content (0.4 wt%). The cathode has good rate and cycling performance with high volumetric capacities. Schematic of the dry electrode process.
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To expedite the large‐scale adoption of electric vehicles (EVs), increasing the gravimetric energy density of batteries to at least 250 Wh kg−1 while sustaining a maximum cost of $120 kWh−1 is of utmost importance. Solid‐state lithium batteries are broadly accepted as promising candidates for application in the next generation of EVs as they promise safer and higher‐energy‐density batteries. Nonetheless, their development is impeded by many challenges, including the resistive electrode–electrolyte interface originating from the removal of the liquid electrolyte that normally permeates through the porous cathode and insures efficient ionic conductivity through the cell. One way to tackle this challenge is by formulating composite cathodes (CCs) that employ solid ionic conductors as “catholytes” in their structure. Herein, it is attempted to shed light on this less studied and poorly understood approach. The different classes of catholytes that have been reported in literature alongside the most common fabrication techniques used to prepare CCs are presented. Next, the interplay between the microstructure and design parameters of CCs with the electrochemical performance of solid‐state batteries (SSBs) and the techniques used to measure their transport properties is well documented. Finally, general guidelines surrounding CC research are outlined. The advancements in solid‐state battery technology are hurdled mainly by the interfacial resistance at the cathode/electrolyte interface. Among the different techniques used to address this challenge, using a solid‐state conductor, a “catholyte,” within the cathode microstructure has shown great potential. Herein, existing literature and the details surrounding this technique are explained.
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Next generation, multifunctional separators can enhance energy storage, power, and safety performance of lithium ion batteries but must be simple to fabricate and incorporate with existing roll-to-roll manufacturing. This study presents a strategy to facilely prepare these separators using UV-initiated polymerization-induced phase separation (PIPS), wherein microporous polymer separators are fabricated directly from constituent monomers and ethylene carbonate (EC) porogen. This enables a wide compositional design space as co-monomers with specific chemical functionality can be readily incorporated into the PIPS precursor mixture. Herein, 1,4-butanediol diacrylate (BDDA) was copolymerized with poly(ethylene glycol) diacrylate (PEGDA) to increase the acrylate conversion in the photopolymerization and improve mechanical properties. By tuning the ratio of PEGDA and EC, separators with high porosity (41.3%) and effective ionic conductivity (2.09 mS cm⁻¹) were prepared. Inclusion of PEGDA was essential to increasing the elastic modulus to > 345 MPa, which is required for cell assembly by roll-to-roll manufacturing. All separators prepared were shown to enable reversible cycling of lithium metal/LiNi0.5Mn0.3Co0.2O2 half-cells for 100 cycles. Unlike conventional polyolefin separators, which were shown to melt at 160°C and shrink by up to 29.8% at elevated temperatures, the PIPS separators possess exceptional, safety-enhancing thermomechanical properties, undergoing no phase transitions or thermal shrinkage.
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The widely used Bruggeman equations correlate tortuosity factors of porous media with their porosity. Finding diverse application from optics to bubble formation, it received considerable attention in fuel cell and battery research, recently. The ability to estimate tortuous mass transport resistance based on porosity alone is attractive, because direct access to the tortuosity factors is notoriously difficult. The correlation, however, has limitations, which are not widely appreciated owing to the limited accessibility of the original manuscript. We retrace Bruggeman's derivation, together with its initial assumptions, and comment on validity and limitations apparent from the original work to offer some guidance on its use.
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We present an open source software application “BruggemanEstimator” that allows a user to estimate the tortuosity of a porous electrode. BruggemanEstimator determines the Bruggeman exponent based on the Differential Effective Medium approximation and as input requires only two microscope images: one of the top and one of a cross section through an electrode. These images, which can be easily acquired with a scanning electron or optical microscope, are used to extract a sampling of active particle shapes as well as the orientation of the particles within the electrode. We validate the accuracy of BruggemanEstimator by comparing the estimated Bruggeman exponents to values calculated by performing numerical diffusion simulations on three-dimensional microstructures obtained from tomographic techniques.
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The ability to engineer electrode microstructures to increase power and energy densities is critical to the development of high-energy density lithium-ion batteries. Because high tortuosities in porous electrodes are linked to lower delivered energy and power densities, in this paper, we experimentally and computationally study tortuosity and consider possible approaches to decrease it. We investigate the effect of electrode processing on the tortuosity of in-house fabricated porous electrodes, using three-dimensionally reconstructed microstructures obtained by synchrotron x-ray tomography. Computer-generated electrodes are used to understand the experimental findings and assess the impact of particle size distribution and particle packing on tortuosity and reactive area density. We highlight the limitations and tradeoffs of reducing tortuosity and develop a practical set of guidelines for active material manufacture and electrode preparation.
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Battery and fuel cell simulations commonly assume that electrodes are macro-homogeneous and isotropic. These simulations have been used to successfully model performance, but give little insight into predicting failure. In Li-ion battery electrodes, it is understood that local tortuosity impacts charging rates, which may cause increased degradation. This report describes a novel approach to quantifying tortuosity based on a heat transfer analogy applied to X-ray microscopy data of a commercially available LiFePO4 electrode. This combination of X-ray imaging and image-based simulation reveals the microscopic performance of the electrode; notably, the tortuosity was observed to vary significantly depending on the direction considered, which suggests that tortuosity might best be quantified using vectors rather than scalars.
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This paper is part of the special publication Developments in petrophysics (eds M.A. Lovell and P.K. Harvey). Despite its widespread use in petrophysics, tortuosity remains a poorly understood concept. Tortuosity can have various meanings when used by physicists, engineers or geologists to describe different transport processes taking place in a porous material. Values for geometrical, electrical, diffusional and hydraulic tortuosity are in general different from one another. Electrical tortuosity is defined in terms of conductivity whereas hydraulic tortuosity is usually defined geometrically, and diffusional tortuosity is typically computed from temporal changes in concentration. A better approach may be to define tortuosity in terms of the underlying flux of material or electrical current with respect to the forces which drive this flow. Unsteady transport processes, including diffusion, can be described only by a population of tortuosities corresponding to the different flow paths taken by particles traversing the medium. In measurements of steady flow (e.g., those normally used to obtain resistivity or permeability) information about particle travel times is lost, and so the multiple values of tortuosity are homogenised. It can be shown that the maximum amount of information about pore structure is embedded in transport processes that combine advective and diffusive elements. Most existing formulations of tortuosity are model-dependent, and cannot be correlated with independently measurable pore-structure properties. Nevertheless, tortuosity underpins the rigorous relationships, between transport processes in rocks, and ties them with the underlying geometry and topology of their pore spaces. Tortuosity can be redefined in terms of the energetic efficiency of a flow process. The efficiency is related to the rate of entropy dissipation (or isothermally, energy dissipation) with respect to a simple, non-tortuous model medium using the postulates of non-equilibrium thermodynamics. Through Onsager's reciprocity relation for coupled flows it is possible to inter-relate efficiency for pairs of transport processes, and so go some way towards unifying tortuosity measures. In this way we can approach the goal of predicting the value of one transport parameter from measurements of another.
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The three-dimensional microstructures of a lab-scale and a high-power LiFePO4 cathode for lithium-ion cells are analyzed by combined focused ion beam (FIB) / scanning electron microscopy (SEM) tomography. The spatial distributions of (a) carbon black as electronic conductor (b) LiFePO4 as active material and (c) pore volume are reconstructed by appropriate image processing methods. The global threshold segmentation procedure is replaced by a refined local threshold method, which accounts for gradients in luminosity even within very large imaged volumes. The precise analysis of the high-power cathode demands for reconstructing a very large volume of 18.15 x 17.75 x 27.8 mu m(3), caused by the dual length-scale design of LiFePO4, carbon black and pore phase. The microstructure features, (a) electrochemically active surface area and particle size distribution of LiFePO4, (b) shape and particle size distribution of carbon black and (c) porosity and tortuosity of the pore phase are compared between lab-scale and high-power cathode.
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Polarization losses within electrodes of solid oxide fuel cells (SOFCs) are determined both by material composition and microstructure. Improvement in performance can be supported by a detailed characterization and modeling of the electrode microstructure. Focused ion beam (FIB) and scanning electron microscopy (SEM) combined with image processing have already proven potential for the reconstruction of porous electrodes. In this contribution the serialized reconstruction procedure of a high-performance, mixed ionic-electronic conducting La0.58Sr0.4Co0.2Fe0.8O3−δ (LSCF)-cathode will be illustrated in detail. Based on corrected reconstruction data sets and by the evaluation of qualified algorithms discriminating between porosity and electrode material, a sensitivity analysis of the grayscale threshold value on the essential parameters (i) surface area, (ii) volume/porosity fraction and (iii) tortuosity is performed.
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Electrochemical impedance spectroscopy has become a mature and well-understood technique. It is now possible to acquire, validate, and quantitatively interpret the experimental impedances. This chapter has been addressed to understanding the fundamental processes of diffusion and faradaic reaction at electrodes. However, the most difficult problem in EIS is modeling the electrode processes, which is where most of the problems and errors arise. There is an almost infinite variety of different reactions and interfaces that can be studied (corrosion, coatings, conducting polymers, batteries and fuel cells, semiconductors, electrocatalytic reactions, chemical reactions coupled with faradaic processes, etc.) and the main effort is now being applied to understanding and analyzing these processes. These applications will be the subject of a second review in a forthcoming volume in this series.
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The aim of the present investigation is to define microstructure parameters, which control the effective transport properties in porous materials for energy technology. Recent improvements in 3D-imaging (FIB-nanotomography, synchrotron X-ray tomography) and image analysis (skeletonization and graph analysis, transport simulations) open new possibilities for the study of microstructure effects. In this study, we describe novel procedures for a quantitative analysis of constrictivity, which characterizes the so-called bottleneck effect. In a first experimental part, methodological tests are performed using a porous (La,Sr)CoO3 material (SOFC cathode). The tests indicate that the proposed procedure for quantitative analysis of constrictivity gives reproducible results even for samples with inhomogeneous microstructures (cracks, gradient of porosity). In the second part, 3D analyses are combined with measurements of ionic conductivity by impedance spectroscopy. The investigations are preformed on membranes of electrolysis cells with porosities between 0.27 and 0.8. Surprisingly, the tortuosities remain nearly constant (1.6) for the entire range of porosity. In contrast, the constrictivities vary strongly and correlate well with the measured transport resistances. Hence, constrictivity represents the dominant microstructure parameter, which controls the effective transport properties in the analysed membrane materials. An empirical relationship is then derived for the calculation of effective transport properties based on phase volume fraction, tortuosity, and constrictivity.
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Li insertion electrodes are made by pressing a mixture of active material and additives on a metallic substrate. Here we estimate how various interphase contacts affect the electrode kinetics. We apply variable external mechanical pressure onto different cathodes and measure their impedance response. Similar experiments are performed on dry composites in contact with: Al or Cu foil, or Ag paste. Most surprisingly, we find that the high-frequency impedance arc is due to the contact impedance between the metal and the electrode material. This is in fundamental contradiction with previous interpretations. We propose an equivalent circuit explaining the observed phenomena. (C) 2008 The Electrochemical Society.
Chapter
Uncompensated Impedance ZOHM Bulk-Media Impedance–RSOL' RBULK and CBULK Electrochemical Double-Layer Capacitance CDL Electrochemical Charge-Transfer Resistance RCT Electrochemical Sorption Impedance ZSORP Mass-Transport Impedance Mixed Charge-Transfer, Homogeneous, and Diffusion-Controlled Kinetics
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This paper reports methods to measure porosity and tortuosity of Li-ion battery electrodes. A gas transport resistance measurement method adapted from [Z. Yu, R. N. Carter, J. Power Sour., 195, 1079 (2010)] is used to characterize an electrode MacMullin number (i.e. the ratio of tortuosity and porosity). Porosity is measured independently, and the tortuosity is obtained from the measured MacMullin number. The measurements were carried out for graphite and Li-metal oxide electrodes taken from an automotive Li-ion battery. Tortuosities of 5.95 +/- 0.51 at 28.5% +/- 1.3% porosity and 3.74 +/- 0.38 at a porosity of 21.5% +/- 0.25% were measured for the graphite electrode and the Li metal oxide electrode respectively. Additionally, the tortuosity values for both electrodes were found to be significantly higher than those predicted by the Bruggeman correlation, suggesting that use of the Bruggeman correlation for Li-ion battery electrode modeling is not applicable and tortuosity should be measured for the electrodes of interest.
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Detailed battery models require mass-transport resistance parameters such as ionic conductivity and salt diffusivity. Effective transport properties can be related to the tortuosities of the porous layers containing the electrolyte. Nevertheless, relatively few direct tortuosity measurements have been performed for cathodes and for anodes. Tortuosities of several Li-ion cathode and anode films were determined using a previously developed polarization-interrupt method. Also, a new and more robust procedure using liquid gallium to delaminate electrodes from aluminum current collectors was developed and validated. This method was shown to be superior to the previous mechanical removal procedure, especially with regard to repeatability. Multiple experiments were performed to assess the effect of the carbon and binder amounts and porosity on electrode tortuosity. Results are well fit with a modified Bruggeman-type function, showing as expected that tortuosity is inversely related to porosity. Additionally, increasing the amount of carbon and binder increases the electrode tortuosity due to small particles plugging the pores.
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We review work from our laboratory that suggests to us that most Li-ion battery failure can be ascribed to the presence of nano- and microscale inhomogeneities that interact at the mesoscale, as is the case with almost every material, and that these inhomogeneities act by hindering Li transport. (Li does not get to the right place at the right time.) For this purpose, we define inhomogeneities as regions with sharply varying properties—which includes interfaces—whether present by “accident” or design. We have used digital image correlation, X-ray tomography, FIB-SEM serial sectioning, and isotope tracer techniques with TOF-SIMS to observe and quantify these inhomogeneities. We propose new research approaches to make more durable, high energy density lithium-ion batteries.
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X-ray tomography allows the active-material domain (LiCoO2) of Li-ion battery cathodes to be imaged, but it is unable to resolve the carbon-binder domain (CBD). Here, a new method for creating a complete 3D representation (virtual design) of all three phases of a cathode is provided; this includes the active-material domain, the CBD, and the electrolyte-filled pore space. It combines X-ray tomographic data of active material with a statistically modeled CBD. Two different statistical CBD morphology models are compared as examples: i) a random cluster model representing a standard mixture of carbon black and polyvenylidene fluoride (PVDF) and ii) a fiber model. The transport parameters are compared in a charged and a discharged cathode. The results demonstrate that the CBD content and morphology changes the ionic and electronic transport parameters dramatically and thus cannot be neglected. Calculations yield that the fiber model shows up to three times higher electrical conductivity at the same CBD content (discharged case) and better ionic diffusion conditions for all CBD contents. In the charged case, the morphology impact on electrical conduction is small. This effective method to generate transport parameters for different CBDs can be transferred to other CBD morphologies and electrodes.
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There are growing concerns over the environmental, climate, and health impacts caused by using non-renewable fossil fuels. The utilization of green energy, including solar and wind power, is believed to be one of the most promising alternatives to support more sustainable economic growth. In this regard, lithium-ion batteries (LIBs) can play a critically important role. To further increase the energy and power densities of LIBs, silicon anodes have been intensively explored due to their high capacity, low operation potential, environmental friendliness, and high abundance. The main challenges for the practical implementation of silicon anodes, however, are the huge volume variation during lithiation and delithiation processes and the unstable solid-electrolyte interphase (SEI) films. Recently, significant breakthroughs have been achieved utilizing advanced nanotechnologies in terms of increasing cycle life and enhancing charging rate performance due partially to the excellent mechanical properties of nanomaterials, high surface area, and fast lithium and electron transportation. Here, the most recent advance in the applications of 0D (nanoparticles), 1D (nanowires and nanotubes), and 2D (thin film) silicon nanomaterials in LIBs are summarized. The synthetic routes and electrochemical performance of these Si nanomaterials, and the underlying reaction mechanisms are systematically described.
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A systematic experimental study of lithium-ion battery porous electrode microstructures using synchrotron X-ray tomographic microscopy finds particle shape and fabrication-induced alignment to cause tortuosity anisotropy, which can impact battery performance. Tortuosity anisotropy is demonstrated to be easily predicted based on simple electron microscopy assessment of particle shapes and the differential effective medium approximation.
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The performance of a porous electrode is strongly related to its electrical properties, such as the effective conductivity of the coating and the contact resistance between the coating and the current collector. This work presents a new method to measure both the effective conductivity and the contact resistance with a single measurement. No preparation is necessary for this, other than cutting a disk shaped electrode and measuring the thickness of the coating. The method is applied to three different cathodes and an anode as a proof of concept.
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We use AC impedance methods to investigate the effect of mechanical deformation on ion transport in commercial separator membranes and lithium-ion cells as a whole. A Bruggeman type power law relationship is found to provide an accurate correlation between porosity and tortuosity of deformed separators, which allows the impedance of a separator membrane to be predicted as a function of deformation. By using mechanical compression to vary the porosity of the separator membranes during impedance measurements it is possible to determine both the α and γ parameters from the modified Bruggeman relation for individual separator membranes. From impedance testing of compressed pouch cells it is found that separator deformation accounts for the majority of the transport restrictions arising from compressive stress in a lithium-ion cell. Finally, a charge state dependent increase in the impedance associated with charge transfer is observed with increasing cell compression.
Book
Compiling the cumulative research of the last two decades on theoretical considerations and practical applications of impedance spectroscopy, this book covers all of the topics that will help readers quickly determine whether this technique is an appropriate method of analysis for their own research problems, and how to apply it. This includes understanding how to correctly make impedance measurements, interpret the results, compare these results with previously published information, and use appropriate mathematical formulas to verify data accuracy. Unique to this monograph is an emphasis on practical applications of impedance spectroscopy. Impedance Spectroscopy is developed around a representative catalogue of the most commonly encountered impedance data examples for a large variety of established, emerging, and non-conventional experimental and applied systems. The book also presents theoretical considerations for dealing with impedance data modeling, equivalent circuits, relevant complex domain mathematical equations, and physical and chemical interpretation of the experimental results for many problems encountered in research and industrial settings. A review of impedance instrumentation, selection of best measurement methods for particular systems, and analysis of potential sources of error is also included. Many helpful references to scientific literature for further information on particular topics and current research are offered, along with an overview of impedance spectroscopy modifications and related techniques. Impedance Spectroscopy is primarily addressed to industrial scientists, engineers, researchers, and graduate students working in electrochemistry, chemical engineering, biomedical sciences, advanced materials, renewable energy, sensors, electronics, and other related fields
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Introduction The use of electrical well logs for the quantitative determination of suchreservoir parameters as connate water saturation, formation permeability andconnate water salinity has recently been attracting the attention of a numberof workers. While the theory of the determination of connate water salinity from theself potential S.P. log has received rather detailed treatment, relativelylittle attention has been paid to the theoretical aspects involved in thequantitative interpretation of resistivity data. It is clear that if electricalwell logs can be used for the quantitative evaluation of physicalcharacteristics of reservoir rock, they will provide a valuable tool tosupplement cheaply information obtained by more laborious core analysis. Incertain cases it is conceivable that the coring program could be considerablycurtailed if the electric log could be relied upon to give reasonably accuratequantitative information. It is our object in this paper to examine the theoretical basis ofquantitative log interpretation as expressed in such well-established loggingconcepts as formation factor and cementation factor. It is also our object toinvestigate the physical aspects of the relationship which is presumed to existbetween resistivity index and brine saturation in reservoir rock. Inparticular, we will endeavor to draw attention to the fact that it is possibleto express these logging concepts in terms of capillary pressure-saturationrelationships, permeability and tortuosity parameters which we will consider inthis paper to be fundamentally indicative of rock texture. The probability ofbeing able to obtain from log recordings alone the data theoretically essentialto permit quantitative log interpretation will be examined, and considerationwill also be given to the problem of formulating simple semiempiricalrelationships for use in the field. Theoretical Considerations The concept of formation resistivity factor, or as it is now commonlycalled, formation factor, appears to have been introduced by G. E. Archie.Formation resistivity factor as defined by Archie is the resistivity of a rock100 per cent brine-saturated divided by the resistivity of the brine. Thisrelationship had previously been used by physical chemists and the concept is,for example, implicit in an early treatment by Fricke of the conductivity of aqueous slurries. T.P. 2852
Article
The electronic structure of LiFePO4 and delithiated FePO4 is revisited in the light of the previous calculations taking into account the coulomb correlation potential for d-electrons. The nature of the optical transitions across the energy gap is investigated. In LiFePO4, these are intra-atomic Fe2+−Fe3+ transitions suffering a strong Franck−Condon effect due to the local distortion of the lattice in FePO4, which is indirect evidence of the formation of a small polaron. This situation contrasts with that met in the much more covalent delithiated phase, where the optical transition across the energy gap is associated with a transfer of an electron from the p-states of the oxygen to the d-states of iron ions. The small polarons in LiFePO4 are associated with the presence of Fe3+ ions introduced by native defects in relative concentration [Fe3+]/[Fe2++Fe3+] = 3 × 10-3 in the samples known to be optimized with respect to their electrochemical properties. The nearest iron neighbors around the central polaron site are spin-polarized by the indirect exchange mediated by the electronic charge in excess. These small magnetic polarons are responsible for the interplay between electronic and magnetic properties that are quantitatively and self-consistently analyzed.
Article
The performance of commercial separators at high charge rates was evaluated using Li4Ti5O12 and LiMn2O4 as negative and positive electrodes, respectively. Most of the porous separators tested induced a sharp decrease in the conductivity of the liquid electrolyte. The conductivity decrease was related to the amount of porosity, polymer/electrolyte affinity, and the size of the pores and their interconnection. The decrease in conductivity induced by the separator incorporation and the separator thickness seems to be relevant indicators for optimizing a separator dedicated to high charge rate lithium-ion batteries.
Article
Experiments on suspensions of glass beads in electrolytes indicate that Bruggemann's approximation represents the dependence of effective conductance on volume fraction very satisfactorily when the dispersed phase contains a broad range of particle sizes. Data on narrow size ranges fall in between values predicted by the Maxwell and Bruggemann equations. These findings are consistent with the physical assumptions implicit in both theoretical developments.
Article
Capacitance dispersion on the fractal carbon electrode with edge and basal orientations was investigated using atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and a.c.-impedance spectroscopy. For this purpose, four types of as-received pyrolytic graphite electrode, as-received, mechanically polished, and as-activated glassy carbon electrodes were prepared with different surface irregularities and amounts of edge orientations. The apparent self-similar fractal dimensions of the carbon electrodes were determined from the analyses of AFM images based upon triangulation method. The amounts of edge orientations on the surface of the carbon electrodes were qualitatively estimated from the XPS analysis of surface acidic functional groups that were preferably formed on the edge planes by the heat treatment of the carbon electrodes. The values of the constant phase element exponent α determined from the apparent self-similar fractal dimensions did not accord with those α determined from the measured impedance spectra. Instead, they were closely related to the amounts of the edge orientations. From the results, it is indicated that the contribution of surface inhomogeneity is much higher than the contribution of the surface irregularity to the capacitance dispersion on the carbon electrode.
Article
Various types of fractal electrode/electrolyte interfaces are investigated in the presence of an electrode reaction, the rate of which is described by the Butler-Volmer equation in a wide range of overpotentials. Applying scaling analysis, an interesting connection between the Tafel current at a fractal electrode, both in the limit of high and small overpotentials on the one hand and the admittance behaviour of the same electrode on the other, reveals itself. In particular, from Tafel plots in the limit of high overpotentials the effective transfer coefficients are seen to be modified by a geometrical factor α which is equal to the exponent appearing in the description of the CPE frequency dependence found in admittance measurements. Furthermore, at small overpotentials the current is shown to contain information on the exchange current density in a way which applies to all electrode geometries which give rise to CPE behaviour, i.e. not specifically to fractal electrodes.
Article
We performed experimental studies to determine electronic properties of multilayered LiFePO4 cathodes in order to quantify reductions in LiFePO4 matrix resistivity and/or contact resistances between matrices and current collectors by addition of carbon black and graphite. In order to extract these layerwise and interlayer properties, we extended the Schumann-Gardner approach to analysis of a four-point probe experiment and solved the resulting coupled nonlinear equations numerically. We studied five cathodes with varying amounts (3-12 wt %) and types (carbon black, graphite) of conductive additives. LiFePO4 particles within the electrodes were precoated with carbon before mixing with additives and binder. Experimental results showed reductions of similar to 62% in electrical resistivities of LiFePO4 matrix with addition of carbon black from 3 to 10 wt %; addition of graphite additives produced only small reductions. For concentrations above 6 wt % of conductive additives, homogeneous electronic resistivities were observed. Contact resistances at interfaces between LiFePO4 matrix and carbon coating of current collector and between carbon coating and current collector were similar in all cases, indicating consistency in manufacturing. Future work will focus on combining models for capacitive loss with models for conductive properties, along with experimental verifications. (c) 2005 The Electrochemical Society.
Article
Synchrotron radiation X-ray tomographic microscopy is performed on transition metal oxide-based porous electrodes to obtain statistically significant volume 3D reconstructions of the microstructure. A segmentation algorithm that allows identification of individual particles for electrochemical simulations is developed and implemented. The tomographic data (raw and processed) and the corresponding electrochemical data for 16 different cathodes is provided open source.
Article
The ion conductivity of two series of porous ceramic diaphragms impregnated with caustic potash was investigated by electrochemical impedance spectroscopy. To understand the impact of the pore structure on ion conductivity, the three-dimensional (3-D) pore geometry of the diaphragms was characterized with synchrotron x-ray absorption tomography. Ion migration was calculated based on an extended pore structure model, which includes the electrolyte conductivity and geometric pore parameters, for example, tortuosity (τ) and constriction factor (β), but no fitting parameters. The calculated ion conductivities are in agreement with the data obtained from electrochemical measurements on the diaphragms. The geometric tortuosity was found to be nearly independent of porosity. Pore path constrictions diminish with increasing porosity. The lower constrictivity provides more pore space that can effectively be used for mass transport. Direct measurements from tomographs of tortuosity and constrictivity opens new possibilities to study pore structures and transport properties of porous materials.
Article
The electrode in a proton exchange membrane (PEM) fuel cell is composed of a carbon-supported Pt catalyst coated with a thin layer of ionomer. At the cathode, where the oxygen reduction reaction occurs, protons arrive at the catalyst sites through the thin ionomer layer. The resistance to this protonic conduction through the entire thickness of the electrode can cause significant voltage losses, especially under dry conditions. The in the cathode with various ionomer/carbon weight ratios (I/C ratios) was characterized in a cell using ac impedance under various operating conditions. AC impedance data were analyzed by fitting , cathode capacitance , and high frequency resistance to a simplified transmission-line model with the assumption that the proton resistance and the pseudocapacitance are distributed uniformly throughout the electrode. The proton conductivity in the given types of electrode starts to drop at I/C ratios of approximately or an ionomer volume fraction of in the electrode. The comparison to fuel cell performance shows that the ohmic loss in the electrode can be quantified by this technique. The cell voltage corrected for ohmic losses is independent of relative humidity (RH) and the electrode’s I/C ratio, which indicates that electrode proton resistivity (ratio of over cathode thickness) is indeed an intrinsic RH-dependent electrode property. The effect of RH on the ORR kinetics was further identified to be rather small for the range of RH studied ( RH).
Article
The three-dimensional microstructure of a porous composite cathode for lithium-ion cells has been analyzed by a combined focused ion beam (FIB)/scanning electron microscopy (SEM) approach. The spatial distributions of 1) carbon black as electronic conductor, 2) LiFePO4 as active material and 3) pore volume were reconstructed by appropriate image processing methods. The representative volume element with a size of 5×5×15μm3=375μm3 (24million volumetric pixels) delivered critical microstructural features as volume fractions, volume-specific surface areas and tortuosity of the three individual phases.
Article
Es werden verschiedene physikalische Konstanten heterogener Körper aus den Konstanten ihrer homogenen Bestandteile nach einer einheitlichen Methode berechnet. In dieser ersten Arbeit wird die Berechnung der Dielektrizitätskonstanten und der Leitfähigkeiten für Elektrizität und Wärme der Mischkörper aus isotropen Bestandteilen behandelt. Die Genauigkeit der älteren Formeln wird untersucht und die bis jetzt unbekannten Konstanten dieser Formeln werden berechnet. Sodann wird die Theorie geprüft an Messungen der Leitfähigkeit bei heterogenen Metallegierungen und an den DK. von gepreßten Pulvern und Emulsionen; die verschiedenen Formeln werden bestätigt. Bei dieser Anwendung werden einige Widersprüche zwischen früheren Untersuchungen aufgehoben und es wird versucht, einige ungenau bekannte DK. genauer zu bestimmen.