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Abstract

We report for the first time the development of irreversible compressive stresses in graphitic carbon electrodes during cycling in a Li-ion battery. The CVD grown c-axis oriented graphitic carbon thin film electrodes show that significant irreversible stresses develop in the first cycle, and then decrease with increasing number of cycles. The net irreversible compressive stress is roughly a factor of 4 higher than the actual Li-intercalation induced reversible compressive stress. A major fraction of the irreversible stress developed at potentials higher than the Li-intercalation potential, starting from ∼1.1 V and increasing in intensity from ∼0.75 V. Also, the variation of the irreversible stress with number of cycles follows very closely the variation of the irreversible capacity with cycle number. Measurements on carbon films with different thicknesses show that the irreversible stress is primarily a surface phenomenon. These stresses were also largely absent in films coated with a thin (0.5 nm) Al2O3 layer. Analysis of all of these observations indicates that SEI layer formation is a primary cause of the irreversible stress, along with some likely contribution from solvated Li-ion co-intercalation. The magnitude of these stresses is large enough to have a significant impact on the performance and cycle life of graphitic carbon electrodes.

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... In the past years, various materials have been studied as anode materials for LIBs. Among those, graphite has been commercialized as an anode material for lithium-ion batteries in early research [4]. However, with the higher demand for energy storage, the development of new anode materials still has good research value. ...
... The α phase is stable at room temperature, whereas the β phase begins to crystallize at 550 to 670 °C, and the β-phase structure has better stability. However, due to the difficulty in preparing the pure β-phase NiMoO 4 , there are few reports on the β-phase NiMoO 4 as anode materials for LIBs. ...
... Impressively, after being cycled at the high rates, the capacity of the NiMoO 4 / NiO@30%Fe 2 O 3 electrode can recover to about 1100 mAh g −1 when the current density is set back to 0.1 A g −1 , manifesting good rate reversibility. In particular, when the current density is 0.5 A g −1 , the specific capacity of NiMoO 4 ...
Article
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Different amounts of Fe2O3-coated NiMoO4/NiO composites were prepared by solvothermal method following with annealing process. The as-prepared NiMoO4/NiO@Fe2O3 composite exhibits a reversible specific capacity of 1250 mAh g−1 after 150 cycles at a current density of 100 mA g−1 as anode for lithium-ion batteries. It exhibits efficient long-term electrochemical performance and maintains the capacity at about 750 mAh g−1 at a high current density of 500 mA g−1 after 200 cycles. The good electrochemical performance of the NiMoO4/NiO@Fe2O3 composite may be due to its smaller size and the Fe2O3 coating which can restrain its volume collapse during the charge–discharge cycle. This work could provide a promising strategy for enhancing the performance of molybdate-based composite materials for practical application in lithium-ion batteries.
... 1,11,15,18,22,31,67 In addition to constraining effects from the current collector, binder and surrounding particles, irreversible surface phenomena taking place on the electrodes during the initial lithiation/delithiation cycles, viz., SEI layer formation, also contribute to stress development in the electrodes. 20,21,53,54,[80][81][82] SEI layer formation can be considered akin to electrochemical deposition of a 'lm' on a 'substrate', with the SEI layer being the 'lm' here and the electrode the 'substrate'. Accordingly, the stress development due to SEI layer formation can be comprehended in terms of the 'growth stresses' occurring during lm deposition. ...
... Accordingly, the stress development due to SEI layer formation can be comprehended in terms of the 'growth stresses' occurring during lm deposition. 21,48,83 2.2.2. Stress development prior to electrochemical cycling. ...
... 7,12,22 Since then researchers of different groups world-wide have extensively used this technique towards monitoring of the stress development in different electrodes in real-time and deciphering important information on many associated aspects, such as viscous ow during lithiation/ delithiation of Si, [5][6][7][11][12][13]17,30,31,50,61,87,101,102,108 instability caused by phase transformations 18,19,24,54 and structural changes 25 in electrodes, effects of the presence of graphene-based buffer interlayers 17 and stress development due to irreversible surface reactions (such as SEI layer formation). [20][21][22][23]54 More detailed discussions on the ndings, upon the usage of MOSSs, as well as the cantilever method, appear in Sections 4-7. ...
Article
One of the major issues associated with Li-ion batteries is the stress development in electrode materials. Such stresses arise primarily due to dimensional changes, structural/phase transformations and development of Li-concentration...
... These were deposited via electron-beam evaporation at rates of 0.7 A/s and 1A/s respectively. MOSS (Multi-beam Optical Stress Sensor) measurements were then conducted in custom-made electrochemical cells that permit optical access, using Li foil as the counter electrode [24]. This technique measures wafer curvature changes during electrochemical cycling, by monitoring the spacing between reflected laser beams. ...
... The primary purpose of this methodology was to incrementally increase the thickness of plated lithium from cycle to cycle. In general, making measurements at different thicknesses makes it possible to separate stress contributions that occur in the bulk film material from those that occur near the surface [24]. Our interpretations and analyses of stress evolution behaviors during these plating experiments are based on the type of in situ curvature data reported in Fig. 2(a). ...
... In general, stress-generating mechanisms that affect the entire film will lead to curvature values that change proportionally with the plated lithium thickness (i.e., a thicker film with the same average stress will cause more bending). In contrast to this, a response that does not vary with the film thickness is indicative of surface processes [24]. Based on this the initial compressive stress transient in each of these cycles is attributed to phenomena occurring near the surface. ...
Article
The potential advantages of lithium metal anodes have received widespread attention (highest capacity, lowest reduction potential, etc). However, the poor stability of Li metal / liquid electrolyte interfaces leads to chronic problems, such as dendrite formation and capacity loss. The possible impact of mechanical effects on interface stability and dendrite formation are difficult to probe directly. In this study, stress evolution during lithium plating and stripping was monitored with precise in situ measurements. The data obtained with different film thicknesses made it possible to separate the stresses associated with the lithium metal and the solid electrolyte interphase (SEI). The results show that significant stresses are created in the SEI films. Based on this, a basic model of wrinkling-ratcheting-delamination is also presented. This analysis indicates that plasticity in a growing Li film can enhance surface wrinkling, and thus lead to morphological destabilization of a planar growth front.
... Aside from the lithiation/delithiation within active materials, the surface reaction is another key process during charging/discharging cycling because the surface of active materials is in direct contact with the electrolyte containing many solvated Li-ions. Many studies have demonstrated that the surface reaction is also caused by stresses during electrochemical cycling [93][94][95][96]. We summarized three stages of stress development during the first cycle of LIBs when the SEI layer formation first occurs (take graphite anode as an example) (Fig. 4). ...
... Copyright # 1995 by Elsevier B.V.) and corresponding SEM image (reprinted with permission from Nie et al. [106]. Copyright # 2013 by American Chemical Society); (c) contact stresses generated due to SEI formation (reproduced with permission from Mukhopadhyay et al. [96]. Copyright # 2012 by Elsevier Ltd.). ...
Article
High-capacity anodes, such as Si, have attracted tremendous research interests from the last two decades because of the requirement for high energy density of next-generation lithium-ion batteries (LIBs). The mechanical integrity and stability of such materials during cycling are critical because their volume considerably changes. The volume changes/deformation result in mechanical stresses, which lead to mechanical failures, including cracks, fragmentation, and debonding. These phenomena accelerate capacity fading during electrochemical cycling and thus limit the application of high-capacity anodes. Experimental studies have been performed to characterize the deformation and failure behavior of these high-capacity materials directly, providing fundamental insights into the degradation processes. Modeling works have focused on elucidating the underlying mechanisms and providing design tools for next-generation battery design. This review presents an overview of the fundamental understanding and theoretical analysis of the electrochemical degradation and safety issues of LIBs where mechanics dominates. We first introduce the stress generation and failure behavior of high-capacity anodes from the experimental and computational aspects, respectively. Then, we summarize and discuss the strategies of stress mitigation and failure suppression. Finally, we conclude the significant points and outlook critical bottlenecks in further developing and spreading high-capacity materials of LIBs.
... Furthermore, in situ measurement techniques were developed to investigate the mechanical degradation stemming from the stress evolution. There are three main methods for in situ measurements of strain and stress, including digital image correlation (DIC) [18][19][20], multiple-optical-stress sensor (MOSS) [21][22][23][24][25][26][27] and curvature measurement system (CMS) [28][29][30]. Qi and Harris [18] implied that the volume expansion of graphite particles during lithium intercalation is mainly accommodated by a decrease in porosity employing DIC. ...
... Besides, MOSS was also used to investigate the development of irreversible compressive stresses in graphitic carbon thin film electrodes. Mukhopadhyay et al. found that SEI layer formation rather than Li ion intercalation is the primary reason for irreversible stress [25][26][27]. MOSS can observe stress evolution of a thin film electrode but cannot be widely applied to analyze the working mechanism of commercial composite electrodes. Recently, Li et al. conducted in situ measurements of composite graphite electrodes by curvature measurement system, establishing the relationship among the deformation, modulus, partial molar volume and state of charge [29]. ...
Article
Full-text available
The cyclic stress evolution induced by repeated volume variation causes mechanical degradation and damage to electrodes, resulting in reduced performance and lifetime of LIBs. To probe the electro-chemo-mechanical coupled degradation, we conducted in situ measurements of Young’s modulus and stress evolution of commercial used graphite electrodes during multiple cycles. A bilayer graphite electrode cantilever is cycled galvanostatically in a custom cell, while the bending deformation of the bilayer electrode is captured by a CCD optical system. Combined with a mechanical model, Li-concentration-dependent elastic modulus and stress are derived from the curvature of the cantilever electrode. The results show that modulus, stress and strain all increase with the lithium concentration, and the stress transforms from compression to tension in the thickness direction. During multiple cycles, the modulus decreases with an increase in the cycle number at the same concentration. The maximum stress/strain of each cycle is maintained at almost same level, exhibiting a threshold that results from the co-interaction of concentration and damage. These findings provide basic information for modeling the degradation of LIBs.
... In-situ monitoring of the stress development in electrode materials during electrochemical cycling provides more detailed understanding of the structural/mechanical changes taking place in electrode materials at different states of charges. In this context, reversible and irreversible stress developments in graphite film electrodes during electrochemical lithiation/delithiation (in Li 'half cell') was investigated by Mukhopadhyay et al. [143,144] using multi-beam optical stress sensor (MOSS). A custom-made electrochemical cell was used, which allows optical access to the back side of the substrate of the concerned film electrode (here CVD-grown graphite film; having constituent graphene planes oriented parallel to the substrate) and thus helps monitoring the changes in substrate curvature and, accordingly, the in-plane stress in the electrode film. ...
... The overall magnitude of the irreversible compressive stress developed just due to the irreversible surface processes was found to be higher than the Li-intercalation induced reversible stress by factor of~4. It was also observed that a thin coating of Al 2 O 3 (a few nm thick) on the surface of graphitic carbon film is capable of suppressing the SEI layer formation and, concomitantly, the irreversible stress development, as well [144]. ...
Article
Graphenic carbon, as the lower (or nano-) dimensional form of graphitic carbon, is expected to allow lithiation/delithiation of an electrode constituted by the same in lesser time and possess greater specific gravimetric Li-storage capacity, as compared to graphitic carbon. The aforementioned positive aspects of graphenic carbon are expected/predicted/observed primarily due to the lower dimensional scale, greater specific surface area (SSA) and presence of ‘defect’ sites. Nevertheless, the types and extents of defects cast significant influences (both, positive and negative) on the Li-storage behavior/performance. For example, lack of ordering between constituent graphene layers suppresses Li-storage in the inter-layer spaces. Furthermore, despite providing additional sites for Li-storage, the defect sites themselves, in addition to enhanced SSA, cause irreversible Li-loss, voltage hysteresis, altering of the nature of potential profile from being flatter (restricted to lower potentials) to sloping from higher potentials, and also negatively affect the thermal stability/safety aspects. The concerned structural features of graphenic carbon, in turn, depend on the preparation route/condition. Not surprisingly, the associated literature base presents different viewpoints. In these contexts, the present review article looks into the correlations between preparation routes/conditions, structural features of graphenic carbon and electrochemical Li-storage behavior/performance; more from a fundamental perspective.
... In particular, an enormous volume expansion of~80% accompanies the conversion of S to Li 2 S [2, 3,5]. Large volume expansions have resulted in mechanical degradation and corresponding capacity losses in many other electrode materials, such as LiCoO 2 [35], Si [36][37][38][39][40][41][42][43], Ge [44][45][46], graphite [47,48], and LiMn 2 O 4 [49,50], V 2 O 5 [51], among others. However, sulfur cathodes exhibit fundamentally distinct behavior in that previously studied electrodes remain in solid form throughout cycling during intercalation or conversion reactions, whereas sulfur undergoes solid-to-liquid, liquid-to-liquid, and liquid-to-solid phase transformations. ...
... Points (b), (d), (f) and (g) are chosen based on notable electrochemical features, while points (c) and (e) are chosen based on mechanical features of interest to be discussed later. This cycling process, involving a series of phase discontinuities, is substantially more complex than found in many other cathode materials, i.e., in many others, simple intercalation/de-intercalation of lithium occurs during cycling [35,[47][48][49][50]57]. To fully understand this complicated cycling process, we charged/discharged the composite sulfur cathodes to various extents while measuring the evolution of stresses and performed complementary structural characterization using SEM, EDS, and XRD. ...
Article
Owing to their enormous capacities, Li-S batteries have emerged as a prime candidate for economic and sustainable energy storage. Still, potential mechanics-based issues exist that must be addressed: lithiation of sulfur produces an enormous volume expansion (~ 80%). In other high capacity electrodes, large expansions generate considerable stresses that can lead to mechanical damage and capacity fading. However, the mechanics of electrochemical cycling of sulfur is fundamentally distinct from other systems due to solid-to-liquid, liquid-to-liquid, and liquid-to-solid phase transformations, and thus remains poorly understood. To this end, we measure the evolution of stresses in composite sulfur cathodes during electrochemical cycling and link these stresses to structural evolution. We observe that nucleation and growth of solid lithium-sulfur phases induces significant stresses, including irreversible stresses from structural rearrangements during the first cycle. However, subsequent cycles show highly reversible elastic mechanics, thereby demonstrating strong potential for extended cycling in practical applications.
... Some recent studies tried to synthesize microstructures based on Si-Ge nano structures to combine the superior rate capability of Ge with the higher capacity of Si. 13 However, without understanding the stress and mechanical property evolution in Ge due to lithiation, designing such innovative microstructures will not be efficient and may lead to premature failure. The critical role of mechanical stresses on the electrochemical performance and mechanical degradation of electrodes prompted several investigations; measurement of mechanical stresses and mechanical properties of the Si, [8][9]12 graphite, 14 and Sn 15 electrodes are some examples. However, no study exists on the stress and mechanical property measurements of Ge during lithiation/delithiation cycling. ...
Article
Full-text available
An in situ study of stress evolution and mechanical behavior of germanium as a lithium-ion battery electrode material is presented. Thin films of germanium are cycled in a half-cell configuration with lithium metal foil as counter/reference electrode, with 1M LiPF6 in ethylene carbonate, diethyl carbonate, dimethyl carbonate solution (1:1:1, wt%) as electrolyte. Real-time stress evolution in the germanium thin-film electrodes during electrochemical lithiation/ delithiation is measured by monitoring the substrate curvature using the multi-beam optical sensing method. Upon lithiation a-Ge undergoes extensive plastic deformation, with a peak compressive stress reaching as high as −0.76 ± 0.05 GPa (mean ± standard deviation). The compressive stress decreases with lithium concentration reaching a value of approximately −0.3 GPa at the end of lithiation. Upon delithiation the stress quickly became tensile and follows a trend that mirrors the behavior on compressive side; the average peak tensile stress of the lithiated Ge samples was approximately 0.83 GPa. The peak tensile stress data along with the SEM analysis was used to estimate a lower bound fracture resistance of lithiated Ge, which is approximately 5.3 J/m². It was also observed that the lithiated Ge is rate sensitive, i.e., stress depends on how fast or slow the charging is carried out.
... This might especially be the case when eco-friendly binder systems or challenging slurry formulations are used. Mukhopadhyay et al. expected compressive stress on the anodic active material particles to counteract the lithiation-induced tensile stress in the SEI [12]. Another beneficial effect is that evolving gases will be pressed out of the cell stack, making them less likely to hinder ion transport [13,14] which should improve cell performance and cycle life. ...
Article
Lithium-ion battery (LIB) cells undergo thickness changes during cycling that can be reversible and irreversible. According to the literature, cell compression can prevent various defects, such as volume-change-induced contact losses. Moreover, densely packed cells represent the battery electric vehicle (BEV) use case, where volumetric energy density can still be optimized. In this study, a uniaxial compression test bench is developed to evaluate the dynamic dilation behavior and stress susceptibility of commercial LIB pouch cells. Using a spring-compression approach, the active material can expand and contract under moderate normal stress variations while keeping the inner cell layers in permanent contact. After applying stress to the cell, the cycling behavior and thickness changes are monitored under the variation of several parameters. A rig-integrated force measurement mat is used to monitor stress distribution, providing additional insight. Our results show that varying the charging rate leads to significant differences in the thickness change of a mildly compressed cell. In this particular case, both reversible and irreversible dilation fractions are present and electro-mechanical effects were identified. As a result, gained insights can serve battery system manufacturers with information to optimize their battery pack regarding operating efficiency, sustainability and mechanical safety.
... Reversible deformation: the coupling between electrochemistry and mechanics of LIB materials is mainly reflected in the expansion of common electrode materials such as graphite and silicon and their composite electrodes with the increase of lithium concentration [244][245][246][247][248][249][250][251]. Irreversible deformation: During the TR of the battery, a series of internal side reactions will occur, some of which will produce gas and cause the battery to expand. ...
Article
Lithium-ion batteries (LIBs) are booming in the field of energy storage due to their advantages of high specific energy, long service life and so on. However, thermal runaway (TR) accidents caused by the unreasonable use or misuse of LIBs have seriously restricted the large-scale application of LIBs. Avoiding TR through advance warning has been becoming an increasing focus of research by scholars. In view of this, we provide a comprehensive review of TR warnings for LIBs. In this paper, an analysis of the existing monitoring parameters of the TR process is presented, and the sensitivity and robustness of multiple warning methods for the same characteristic parameters are compared. Subsequently, this is followed by a presentation of early warning applications in portable devices, electric vehicles and energy storage systems. Finally, combining the existing warning methods with the system's operational data, the future warning methods are envisioned.
... Lithium ions (Li + ) shuttle between the anode and cathode, realizing the conversion of electric energy and chemical energy of the battery. Li + insertion and extraction are usually accompanied by volumetric changes of the active particles, which change the stress at the cell level [10][11][12][13], and the magnitude of the volume change is material-dependent. The commonly used graphite anode has a volume change of approximately 12 % during (de)lithiation [14][15][16], while the LiNi 1/3 Mn 1/3 Co 1/3 O 2 (NMC111) cathode is approximately 0 % to 2.4 % [17][18][19], which is an order of magnitude lower than graphite. ...
... Local differential strains imposed by the SEI and adjoining interacting phases, such as neighboring electrodeposits and separators are captured in this description. [60][61][62] The mechanical equilibrium equation in the Lagrangian frame is: L v ...
Article
Full-text available
By developing a thermodynamically consistent phase field framework, including position-dependent large deformation mechanics, plasticity, electrochemistry, and electrodeposition, twelve growth mechanisms were identified. Specifically, the previously reported tip-controlled growth mechanism is resolved in to (a) flat tops, (b) rounded tops, (c) edge shielding, (d) electrical shielding, and (e) local electrochemical exchange. Similarly, the previously reported base-controlled growth mechanism is resolved in to (f) initial base controlled growth, (g) late base controlled growth, (h) merged bases, and (i) unmerged bases. Finally, then previously reported mixed growth mechanism is resolved in to (j) local mechanical equilibrium, (k) dendrite bending, and (l) stressed junctions. Longer dendrites predominantly grow through electrodeposition while shorter dendrites predominantly grow through plastic flow. Further, local electrochemical and mechanical dendrite branch interactions induce lateral dissolution and deposition that lead to microstructural changes in the dendrite morphology.
... These cracks would propagate upon electrochemical cycling and are responsible for the concomitant chemical and mechanical degradations, and hence the capacity fade of LIBs. Moreover, the electrochemical processes are usually coupled with mechanical stresses that further influence the aforementioned degradation mechanisms [6][7][8]. Various experimental investigations have demonstrated that the diffusion-induced stress can trigger the fragmentation and complete disintegration of individual electrode particles along with the debonding of active electrode material from the binder and the current collectors [9][10][11][12][13], eventually leading to capacity fade. On the contrary, several authors have reported [14][15][16][17][18][19][20] that an appropriate external pressure can benefit the lifespan and safety of both liquid-and solid-electrolyte based cells by improving the contact conditions and suppressing the growth of lithium dendrites [17,[21][22][23][24][25]. ...
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.
... Similar to the cases of graphitic and graphenic carbons [119][120][121], the formation of 'passivation' layer due to irreversible reduction of electrolyte species on the electrode surface (viz., solid electrolyte interface or SEI layer) has also been found to lead to compressive stress development 11a. Such a potential is slightly above the potential corresponding to the initiation of Li-uptake in Sn. ...
Article
Enhancement of energy density and safety aspects of Li-ion cells necessitate the usage of 'alloying reaction' based anode materials in lieu of the presently used intercalation-based graphitic carbon. This becomes even more important for the upcoming Na-ion battery system since graphitic carbon does not intercalate sufficient Na-ions to qualify as an anode material. Among the potential 'alloying reaction' based anode materials for Li-ion batteries and beyond (viz., Na-ion, K-ion battery systems), Si and Sn have received the major focus; with the inherently ductile nature of Sn (as against the brittleness of Si) and the considerably better performance in the context of electrochemical Na-/K-storage, of late, tilting the balance somewhat in favor of Sn. Nevertheless, similar to Si and most other 'alloying reaction' based anode materials, Sn also undergoes volume expansion/contraction and phase transformations during alkali metal-ion insertion/removal. These cause stress-induced cracking, pulverization, delamination from current collector, accrued polarization and, thus, rapid capacity fade upon electrochemical cycling. Unlike Si, the aforementioned loss in mechanical integrity is believed to be primarily caused by some of the deleterious 1st order phase transformations and concomitant formation of brittle intermetallic phases during the alloying/de-alloying process. Against this backdrop, this review article focuses on aspects related to deformation, stress development and associated failure mechanisms of Sn-based electrodes for alkali-metal ion batteries; eventually establishing correlations between phase assemblage/transformation, stress development, mechanical integrity, electrode composition/architecture and electrochemical behavior.
... Furthermore, graphite exhibits a short cycling life and severe safety issues arising from the formation of unstable SEI layers and lithium dendrites during cycling. [4][5][6] Over the last decade, transition metal oxides and suldes have been considered as promising alternative anode materials for LIBs on account of their large theoretical capacity, high electrochemical reactivity and abundant natural resources. 7-10 Nevertheless, they suffer from inferior rate capability and rapid capacity fading because of their large volume change and poor electronic conductivity. ...
Article
Full-text available
Transition metal carbides have been studied extensively as anode materials for lithium-ion batteries (LIBs), but they suffer from sluggish lithium reaction kinetics and large volume expansion. Herein, a hierarchical Mo2C/C nanosheet composite has been synthesized through a rational pyrolysis strategy, and evaluated as an anode material with enhanced lithium storage properties for LIBs. In the hierarchical Mo2C/C nanosheet composite, large numbers of Mo2C nanosheets with a thickness of 40-100 nm are uniformly anchored onto/into carbon nanosheet matrices. This unique hierarchical architecture can provide favorable ion and electron transport pathways and alleviate the volume change of Mo2C during cycling. As a consequence, the hierarchical Mo2C/C nanosheet composite exhibits high-performance lithium storage with a reversible capacity of up to 868.6 mA h g-1 after 300 cycles at a current density of 0.2 A g-1, as well as a high rate capacity of 541.8 mA h g-1 even at 5.0 A g-1. More importantly, this hierarchical composite demonstrates impressive cyclability with a capacity retention efficiency of 122.1% over 5000 successive cycles at 5.0 A g-1, which surpasses the cycling properties of most other Mo2C-based materials reported to date.
... Especially in the case of uneven lithium embedding, the active substance in different lithium embedding states will produce different strains, resulting in great stress. [7][8][9][10][11] In addition, when LIBs are constrained by a rigid shell or module frame, significant stress also occurs. [12] The separator of LIBs is a porous polymer membrane that deforms under stress. ...
Article
Full-text available
The separator is the weakest mechanical part of a lithium‐ion battery. The displacement load formed by the expansion of an electrode induces the microstructure evolution of the separator, such as a decreasing porosity and an increasing tortuosity, which affects its ability to transport Li+ and degrades battery performance. In this paper, an in situ mechanical loading device combined with focused ion beam‐scanning electron microscopy (FIB‐SEM) is designed to reveal the real microstructure evolution of a separator under displacement loading. An image‐based finite element model is tailored to investigate the microstructure evolution and its nonuniformities of the separator at different deformation levels. The quantitative relationship between the separator porosity and external displacement load is presented based on the experimental and simulation results. This paper provides new insight into the degradation mechanisms of commercial lithium‐ion batteries. This article is protected by copyright. All rights reserved.
... Beyond the elastic stresses, a-Si sometimes shows plastic deformation at room temperature and C-rate typically used in LiBs 140 . MOSS technique allowed then to show that irreversible compressive stress appearing during the first cycle were associated with the electrochemical deposition of the SEI for both graphite and silicon electrodes [141][142][143] . On the other hand, a tensile strain is measured in the SEI due to active material expansion 144 . ...
Thesis
The rise of silicon and nickel manganese cobalt layered oxides (NMC) as new negative and positive electrode materials for Li-ion batteries appear the question of their integration in full cells. Indeed, silicon lithiation induces a high material expansion, which leads to significant swelling and mechanical stresses at the anode and cell level. Additionally, interphases formed at the surface of the electrodes during the first cycle (SEI and CEI) play an essential role in the operation and ageing of the cell. Recent studies showed the occurrence of a SEI-CEI crosstalk influenced by the nature of the positive electrode. Thus, this study aims at understanding the role of mechanical constraints and nature of the positive electrode on the capacity retention of a high performance silicon carbon graphite composite electrode (Si-C/G) in a Li-ion cell. In case of cylindrical 18650 cells, strain gauges confirmed the rigidity of the casing and measured a maximum pressure of 4.3 MPa; in addition, internal cell components thickness change was captured at 8 different states of charge (SOC) during a cycle by in situ 3D imaging with X-Ray micro computed-tomography (voxel size 1.6μm) combined with a specific image treatment. For bi-layer pouch cells, operando swelling was measured using an in-house high precision (< 0.1μm) compression set-up with simultaneous pressure and thickness recording as well as dynamic pressure regulation system. Combining these unique experimental techniques and the modelling of Si-C/G active material swelling in function of the SOC we were able to provide insights in porosity changes of anodes for the two cell formats. Then three cathode materials were compared (NMC622, NMC811 and LCO) leading to a better capacity retention with NMC811 and then NMC622. NMC811 contributes firstly to minimize the maximum swelling in contrary to LCO. Post-mortem analyses of positive and negative electrodes harvested from pouch cells were carried out by combining Electrochemical Impedance Spectroscopy (EIS), X-ray Photoelectron Spectroscopy (XPS), Time of Flight Secondary-Ion Mass Spectrometry (Tof-SIMS) and Inductively Coupled Plasma (ICP) mass spectroscopy. For all type of cells, the same fading mechanism occurs. A continuous growth of the SEI has been highlighted at the anode side, trapping lithium ion but without charge transfer increase. At the cathode side, an increase of the charge transfer was observed for all cells correlated to CEI thickening upon cycling. The better capacity retention in NMC811 cells was found to be related to a lower amount of SEI formed upon cycling. In another hand, faster degradation of LCO cells originates from a more significant thickening of both the CEI and SEI. Various approaches used in this work could easily be applied to develop other Si-C composite or anode formulation with limited swelling, to optimize cell design and the integration of this new generation of cells which will finally promote higher energy density of Li-ion battery. The study of the mechanical stresses at particle level might also provide significant insight to understand and limit the swelling at electrode level. A direct correlation between the mechanical stress and the SEI evolution is still under debate.
... Through the stress growth of different thickness electrodes, Mukhopadhyay et al. believed that the irreversible stress of graphite electrodes was mainly related to the formation of the SEI layer in the first cycle, which was also confirmed by chemical vapor deposition. [60,67] ...
Article
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Lithium‐ion batteries (LIBs) have the advantages of high energy density, stable working voltage, long cycling life, and no memory effect. They have been widely used as power sources for electric vehicles (EVs) and hybrid electric vehicles (HEVs). However, during periods of usage or storage, some unfavorable factors such as thermal runaway, volumetric expansion, and growth of lithium dendrites can severely reduce the reliability and safety of LIBs. Therefore, an accurate health estimation system and a reliable life expectancy strategy toward LIBs have been proved significantly important. However, due to the large volume and high price, most characterization methods in the laboratory cannot be applied directly to commercial electric devices. Therefore, small portable monitor methods are more practical. In this review, we first systematically reviewed the recent progress of modeling methods towards LIBs. Some typical modeling strategies (e. g., thermal models, electrical models and aging models) and advanced sensors which can be imbedded in LIBs are emphatically introduced and compared. At last, some promising directions of development on portable in‐situ monitor strategies are also predicted and supposed.
... Li et al. measured the curvature change in silicon electrode film/copper substrate and in graphite electrode film/copper substrate, analyzed the chemical stress and the variation in elastic modulus of these electrodes, and found that the elastic modulus and stress of electrodes were dependent on the Li concentration [18,19]. Kumar et al. and other researchers monitored the curvature changes in Si- [20][21][22][23], SiO 2 - [24], graphite- [25,26], and germanium- [27] based thin electrode films/substrate systems and obtained the lithiation-induced stresses in these electrodes. Different from the measurement of stress, the measurement of chemical strain during (de)lithiation is generally conducted on freestanding thin film electrodes using DIC. ...
Article
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Electrochemical lithiation/delithiation of electrodes induces chemical strain cycling that causes fatigue and other harmful influences on lithium-ion batteries. In this work, a homemade in situ measurement device was used to characterize simultaneously chemical strain and nominal state of charge, especially residual chemical strain and residual nominal state of charge, in graphite-based electrodes at various temperatures. The measurements indicate that raising the testing temperature from 20°C to 60°C decreases the chemical strain at the same nominal state of charge during cycling, while residual chemical strain and residual nominal state of charge increase with the increase of temperature. Furthermore, a novel electrochemical-mechanical model is developed to evaluate quantitatively the chemical strain caused by a solid electrolyte interface (SEI) and the partial molar volume of Li in the SEI at different temperatures. The present study will definitely stimulate future investigations on the electro-chemo-mechanics coupling behaviors in lithium-ion batteries.
... Cross-sectional TEM revealed that the carbon structure at the surface had been disrupted, creating an amorphous carbon layer at the carbonelectrolyte interface. 153 This was attributed to the insertion of solvated ions above lithiation potentials. 63 The formation of the amorphous layer was found to soften the impact of carbon expansion and to help stabilize the SEI by creating a slightly compressive stress in the SEI before lithiation which helped offset the tensile stress that develops in the SEI during lithiation. ...
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A stable solid electrolyte interphase (SEI) layer is key to high performing lithium ion batteries for metrics such as calendar and cycle life. The SEI must be mechanically robust to withstand large volumetric changes in anode materials such as lithium and silicon, so understanding the mechanical properties and behavior of the SEI is essential for the rational design of artificial SEI and anode form factors. The mechanical properties and mechanical failure of the SEI are challenging to study, because the SEI is thin at only ~ 10 - 200 nm thick and is air sensitive. Furthermore, the SEI changes as a function of electrode material, electrolyte and additives, temperature, potential, and formation protocols. A variety of in situ and ex situ techniques have been used to study the mechanics of the SEI on a variety of lithium ion battery anode candidates; however, there hasn't been a succinct review of the findings thus far. Because of the difficultly of isolating the true SEI and its mechanical properties, there have been a limited number of studies that can fully de-convolute the SEI from the anode it forms on. A review of past research will be helpful for culminating current knowledge and helping to inspire new innovations to better quantify and understand the mechanical behavior of the SEI. This review will summarize the different experimental and theoretical techniques used to study the mechanics of SEI on common lithium ion battery anodes and their strengths and weaknesses.
... As has been studied in the literature, volumetric expansions of active particles can induce large mechanical stresses on the SEI layer, causing it to fracture and fail. [61][62][63] The double-walled aSi nanotubes proposed by Wu et al. 45 consist of an aSi hollow nanotube, whose exterior is oxidized to form a mechanically stiff SiO 2 layer, thus forming a double-walled structure. Due to the high relative mechanical stiffness of the exterior SiO 2 layer when compared to the softer aSi core, volumetric expansions are accommodated primarily through expansion into the hollow internal cavity and minimal strains and stresses are incurred by the SEI layer forming on the exterior surface. ...
Article
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Modeling of the chemo-mechanical interactions between active particles in battery electrodes remains a largely unexplored research avenue. Of particular importance is modeling the local current densities which may vary across the surface of active particles under galvanostatic charging conditions. These depend on the local, stress-coupled electrochemical potential and may also be affected by mechanical degradation. In this work, we formulate and numerically implement a constitutive framework, which captures the complex chemo- mechanical multi-particle interactions in electrode microstructures, including the potential for mechanical degradation. A novel chemo-mechanical surface element is developed to capture the local non-linear reaction kinetics and concurrent potential for mechanical degradation. We specialize the proposed element to model the electrochemical behavior of two electrode designs of engineering relevance. First, we model a traditional liquid Li-ion battery electrode with a focus on chemical interactions. Second, we model a next generation all- solid-state composite cathode where mechanical interactions are particularly important. In modeling these electrodes, we demonstrate the manner in which the proposed simulation capability may be used to determine optimized electro-chemical and mechanical properties as well as the layout of the electrode microstructure, with a focus on minimizing mechanical degradation and improving electrochemical performance.
... 20,21 However, these properties are difficult to evaluate due to the complex and varied chemical composition of the SEI. [22][23][24] Researchers have found that the SEI layer can be formed in single, double, multilayer, 25,26 porous, 27 and sandwich structures. 28 Experimental findings reported that the SEI layer on the carbon electrode could be brittle 29 or elastic. ...
Article
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Understanding of degradation mechanisms in batteries is essential for the widespread use of eco‐friendly vehicles. Degradation mechanisms affect battery performance not only individually but also in a coupled manner. Solid electrolyte interface (SEI) formation deteriorates battery capacity through consuming available lithium ions. On the other hand, as the SEI layer grows over multiple cycles, the level of mechanical constraints is changed, which can affect the fracture behavior of the active particles. We investigate the effect of the SEI layer growth on the fracture probability of the electrode particles. The simulations show that as the SEI layer grows, tensile stress inside the active particles turns into compressive stress, reducing the probability of particle fracture. Once the SEI layer is fractured, the particle fracture is sequentially more likely to happen because the SEI constraint is removed. The study emphasizes that the stability of SEI layers is important because it helps in alleviating electrochemical performance fade as well as mechanical failure probability. In addition, the SEI layer on small particles tends to be more fractured than that on large particles, suggesting that the particle size uniformity is essential for reducing the fracture probability of the SEI layers at the electrode.
... Since the quartz substrate deforms elastically, the stress in the active film(s) is proportional to the induced change in wafer curvature, which can be determined quantitatively with established relationships. 47 Material Characterization. Structural characterization of composite electrodes before and after electrochemical cycling, were characterized using high-resolution X-ray diffraction (XRD, Bruker D8 Discovery) with 2θ range from 5°to 80°. ...
... They used the extended finite element method to investigate the effect of current density, particle size, and particle aspect ratio on defect propagation. A larger current density and particle size promoted defect propagation, with a particle aspect ratio of 1. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 A c c e p t e d M a n u s c r i p t 33 State-of-charge heterogeneity on cathode accelerates crack formation. State-ofcharge heterogeneity is defined as heterogeneous distribution of oxidation state of a transition metal element on cathode at multiple length scales, ranging from secondary particles to electrodes 9, 16, 89-91 . ...
Article
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The fracture of battery materials is one of the main causes of battery degradation. This issue is further amplified in emerging solid-state batteries, where the more robust interface between the liquid electrolyte and solid electrode in conventional batteries is replaced by a brittle solid–solid interface. In this review, we summarize the observed fracture behavior in battery materials, the origin of fracture initiation and propagation, as well as the factors that affect the fracture processes of battery materials. Both experimental and modeling analyses are presented. Finally, future developments regarding the quantification of fracture, the interplay of chemo-mechanical factors, and battery lifespan design are discussed along with a proposed theoretical framework, in analogy to fatigue damage, to better understand battery material fracture upon extended cycling.
... The macroscopic strain generated in the carbon cathode is closely related to changes in the graphite interlayer spacing at atomic scales. In addition, the formation of intercalation compounds in this stage exacerbates the development of stresses and strains in the carbon cathodes [13,14]. ...
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Sodium expansion plays an important role in cathode deterioration during aluminum electrolysis. In this work, the sodium expansion of semigraphitic cathode material has been measured at various cathodic current densities using a modified Rapoport apparatus. We have studied the microstructural changes of carbon cathodes after aluminum electrolysis using high-resolution transmission electron microscopy (HRTEM). Because of an increasing trend toward higher amperage in retrofitted aluminum reduction cells, an investigation is conducted both at a representative cathode current density (0.45 A/cm2) and at a high cathodic current density (0.7 A/cm2). The results indicate that the microstructures of carbon cathodes can be modified by Joule heating and electrostatic charging with higher current densities during aluminum electrolysis. With the penetration of the sodium and melt, zigzag and armchair edges, disordered carbon, and exfoliation of the surface layers may appear in the interior of the carbon cathode. The penetration of the sodium and melt causes remarkable stresses and strains in the carbon cathodes, that gradually result in performance degradation. This shows that increasing the amperage in aluminum reduction cells may exacerbate the material deterioration of the cathodes.
... In the context of SEI formation, considerable irreversible stress during the initial charging of graphitic carbon electrodes has been reported [3,4]. However, the typical SEI layer displays a thickness on the nanometer scale, and therefore, its growth on the graphite surface does not correlate with the much larger magnitude of the irreversible swelling, measured during the first lithiation of graphite composite electrodes [5]. ...
Article
The volumetric expansion of graphite composite electrodes for Li ion battery is investigated by electrochemical dilatometry in electrolytes with different concentration of vinylene carbonate (VC). While the reversible dilation of the anode coatings is not influenced by the VC concentration the irreversible part displays a strong dependence. With the increase of VC amount in the electrolyte the irreversible dilation decreases significantly, showing that the addition of VC has a positive effect on the mechanical performance of the battery. The observed behavior is associated with differences in the decomposition mechanism of the electrolyte components and their reaction kinetics, influenced by the presence of VC. In contrast to the VC containing electrolyte, the passivation layer formed on the anode in the absence of VC cannot effectively terminate the electroactivity of the graphite surface and the electron charge transfer. This leads to a continuous incorporation of decomposition products in the composite layer during the subsequent cycles and related to this additional irreversible volume expansion. The analysis performed by means of combined application of dilatometric, impedance and XPS techniques reveals the positive role of VC for improving the electrochemical-mechanical properties of the graphite porous anode.
... A part of the overall compressive stress developed in the first half-cycle is also likely to be associated with irreversible surface reactions including SEI layer formation. 6,9,43,66 Following that, the flattening and gradual stress release are most likely due to combinations of (i) incremental plastic flow of a-Si film 1−6 (with some contribution from reduction in stiffness of a-Si not being ruled out 45−49 ) and (ii) pseudoelastic deformation of the NiTi layer "underneath", which has been "observed" to undergo the B2→B19′ phase transformation (see section 3.3). In this regard, when compared to the corresponding in-situ ("corrected") stress profile for a-Si, sans NiTi (as reported in our previous publication 6 as Figure 4a), even though the overall profile does not look too different for the first lithiation half-cycle, a closer look does indicate a few differences. ...
Article
Combination of in-situ synchrotron X-ray diffraction and in-situ stress measurements performed during electrochemical Li-alloying/de-alloying of amorphous Si (a-Si) films, having ~150 nm thick NiTi film as interlayer between it and current collector, have confirmed the occurrence of reversible B2↔B19’ phase transformation and associated pseudoelastic deformation of the nanoscaled NiTi film in response to dimensional changes of a-Si. The in-situ stress results also indicate that pseudoelastic deformation of the NiTi interlayer helps ‘buffer’ the stress development in a-Si during lithiation/delithiation. Top-view and cross-section imaging at different stages of electrochemical cycling confirm the concomitant improvement in integrity of a-Si film electrode over multiple lithiation/delithiation cycles, in the presence of NiTi interlayer. This, in turn, contributes towards significant improvement in the cyclic stability of a-Si. In more specific terms, the presence of NiTi interlayer improved the capacity retention of a-Si to ~78% after 50 cycles, as compared to just ~10% retention in the absence of any ‘buffer’ interlayer. This is expected to allow moving ahead from the usually investigated material as ‘buffer’ interlayer, viz., graphenic carbon; as also confirmed by our comparative studies. However, the synchrotron X-ray diffraction results also indicate the presence of some retained B19’ phase in the NiTi interlayer after each lithiation-delithiation ‘full’ cycle, presumably due to straining of the NiTi interlayer beyond the 8-10% limit for pseudoelasticity. This reduces the effectiveness of the interlayer with cycling, as also evidenced with the in-situ stress results. Hence, sustaining the effectiveness of NiTi interlayer over multiple cycles necessitates addressing this issue via careful designing/engineering of the electrode and/or the electrochemical cycling parameters.
... Ref [227] Micron and Nano Si -Anode. substrate of the electrode in comparison to the active region via an optical laser system [277,278]. But, the spatial resolution of MOSS is still much lower than TEM-based techniques and the measured values correspond to stresses averaged over a large electrode surface area. ...
... The main problem of these measurements is that all of them, without exception, are applied under slow charging rates or stationary conditions, 10,11,14,15 hence, lacking the short-time resolution required to monitor the stresses appearing at high rates of charging which are transient in nature. When these stresses exceed a critical level, ...
Article
Quick charging of Li-ion batteries is often accompanied by rapid expansion of composite battery electrodes resulting in appearance of transient stresses inside the electrodes bulk. Although predicted theoretically, they have never been tracked by direct in situ measurements. Herein, using multi-harmonic electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D), acoustic images of strong transient deformations in LiFePO4 electrodes were obtained in the form of giant resonance frequency and resonance width shifts. The formation of cracks was verified by scanning electron microscopy. The effects of charging rate, stiffness of the polymeric binder and solution concentration have been identified. The attractive feature of EQCM-D is its high sensitivity for selective probing of average mechanical characteristics of the operated electrodes, especially of the particle-binder interactions, directly linked to the electrode cycling performance. Using EQCM-D, an inexpensive, simple, and fast method of structural health monitoring for battery electrodes can be intelligently designed.
... First, the CuO nanowires suffer from volume expansion/contraction during periodic lithiation/delithiation leading to the formation of fractures, which may allow the electrolyte to permeate through the cracks inside the nanowires and form fresh solid-electrolyte-interphase (SEI) layers. The SEI may further induce more stress 49,50 , thereby facilitating further facture of the CuO nanowires. Second, owing to the incomplete oxidation reaction of Cu during the delithiation process, lower capacity Cu 2 O gradually forms in replace of high capacity CuO 14 . ...
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Abstract We demonstrate significant improvement of CuO nanowire arrays as anode materials for lithium ion batteries by coating with thin NiO nanosheets conformally. The NiO nanosheets were designed two kinds of morphologies, which are porous and non-porous. By the NiO nanosheets coating, the major active CuO nanowires were protected from direct contact with the electrolyte to improve the surface chemical stability. Simultaneously, through the observation and comparison of TEM results of crystalline non-porous NiO nanosheets, before and after lithiation process, we clearly prove the effect of expected protection of CuO, and clarify the differences of phase transition, crystallinity change, ionic conduction and the mechanisms of the capacity decay further. Subsequently, the electrochemical performances exhibit lithiation and delithiation differences of the porous and non-porous NiO nanosheets, and confirm that the presence of the non-porous NiO coating can still effectively assist the diffusion of Li+ ions into the CuO nanowires, maintaining the advantage of high surface area, and improves the cycle performance of CuO nanowires, leading to enhanced battery capacity. Optimally, the best structure is validated to be non-porous NiO nanosheets, in contrary to the anticipated porous NiO nanosheets. In addition, considering the low cost and facile fabrication process can be realized further for practical applications.
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Transition metal oxides represent a promising anode material for rechargeable lithium-ion batteries (LIBs) because of their high theoretical specific capacity. Li2MoO4 (LMO) has a high specific capacity due to its...
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Cathode degradation of Li-ion batteries (Li⁺) continues to be a crucial issue for higher energy density. A main cause of this degradation is strain due to stress induced by structural changes according to the state-of-charge (SOC). Moreover, in solid-state batteries, a mismatch between incompatible cathode/electrolyte interfaces also generates a strain effect. In this respect, understanding the effects of the mechanical/elastic phenomena associated with SOC on the cathode performance, such as voltage and Li⁺ diffusion, is essential. In this work, we focused on LiCoO2 (LCO), a representative LIB cathode material, and investigated the effects of biaxial strain and hydrostatic pressure on its layered structure and Li⁺ transport properties through first-principles calculations. With the nudged elastic band technique and molecular dynamics, we demonstrated that in Li-deficient LCO, compressive biaxial strain increases the Li⁺ diffusivity, whereas tensile biaxial strain and hydrostatic pressure tend to suppress it. Structural parameter analysis revealed the key correlation of “Co layer distances” with Li⁺ diffusion instead of “Li layer distances”, as ordinarily expected. Structural analysis further revealed the interplay between the Li–Li Coulomb interaction, SOC, and Li⁺ diffusion in LCO. The activation volume of LCO under hydrostatic pressure was reported for the first time. Moreover, vacancy formation energy calculations showed that the Li intercalation potential could be decreased under compressive biaxial strain due to the weakening of the Li–O bond interaction. The present findings may serve to improve the control of the energy density performance of layered cathode materials.
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Ni-containing "layered"/cation-ordered LiTMO2s (TM = transition metal) suffer from Ni-migration to the Li-layer at the unit cell level, concomitant transformation to a spinel/rock salt structure, hindrance toward Li-transport, and, thus, fading in Li-storage capacity during electrochemical cycling (i.e., repeated delithiation/lithiation), especially upon deep delithiation (i.e., going to high states-of-charge). Against this backdrop, our previously reported work [ACS Appl. Mater. Interfaces 2021, 13, 25836-25849] revealed a new concept toward blocking the Ni-migration pathway by placing Zn2+ (which lacks octahedral site preference) in the tetrahedral site of the Li-layer, which, otherwise, serves as an intermediate site for the Ni-migration to the Li-layer. This, nearly completely, suppressed the Ni-migration, despite being deep delithiated up to a potential of 4.7 V (vs Li/Li+) and, thus, resulted in significant improvement in the high-voltage cyclic stability. In this regard, by way of conducting operando synchrotron X-ray diffraction, operando stress measurements, and 3D atom probe tomography, the present work throws deeper insights into the effects of such Zn-doping toward enhancing the structural-mechanical-compositional integrity of Li-NMCs upon being subjected to deep delithiation. These studies, as reported here, have provided direct lines of evidence toward notable suppression of the variations of lattice parameters of Li-NMCs, including complete prevention of the detrimental "c-axis collapse" at high states-of-charges and concomitant slower-cum-lower electrode stress development, in the presence of the Zn-dopant. Furthermore, the Zn-dopant has been found to also prevent the formation of Ni-enriched regions at the nanoscaled levels in Li-NMCs (i.e., Li/Ni-segregation or "structural densification") even upon being subjected to 100 charge/discharge cycles involving deep delithiation (i.e., up to 4.7 V). Such detailed insights based on direct/real-time lines of evidence, which reveal important correlations between the suppression of Ni-migration and high-voltage compositional-structural-mechanical stability, hold immense significance toward the development of high capacity and stable "layered" Li-TM-oxide based cathode materials for the next-generation Li-ion batteries.
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Sulfide‐based all‐solid‐state batteries (ASSBs) are one of the most promising energy storage devices due to their high energy density and good safety. However, due to the volume (stress) changes of the solid active materials during the charging and discharging process, the generation and evolution of electrochemomechanical stresses are becoming serious and unavoidable problems during the operation of all‐solid‐state batteries due to the lack of a liquid electrolyte to partially buffer the stress generated in the electrodes. To understand these electrochemo‐mechanical effects, including the origins and evolution of mechanical or internal stresses, it is necessary to develop some highly sensitive probing techniques to measure them precisely and bridge the relationship between the electrochemical reaction process and internal stress evolution. Herein, recent progress on uncovering the origins of the internal stresses, the working principle and experimental devices for stress measurement, and the application of those stress‐measuring techniques in the study of electrochemical reactions in sulfide‐based ASSBs are briefly summarized and overviewed. The investigation of precise and operando monitoring techniques and strategies for suppressing or relaxing these electrochemomechanical stresses will be an important direction in future solid‐state batteries.
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This study proposes a novel method for managing the compressive pressure imposed on a lithium-ion battery (LIB) using a phase transition actuator under constrained conditions considering the influence of compressive pressure on the performance and lifespan of LIBs. Specifically, an active pressure management strategy is proposed to maintain the optimal pressure and reduce the equivalent impedance during operation. A closed-loop control scheme is used to maintain compressive pressure via a phase transition actuator considering the dynamic characteristics of the actuator. This configuration allows managing both reversible pressure due to phase transitions at lithium intercalation/deintercalation and irreversible pressure evolution due to solid–electrolyte interface formation and growth. The analysis on experiments indicates that the equivalent impedance and capacity can be managed through active pressure management under stationary and stochastic operational conditions, demonstrating the effectiveness of the proposed method. Specifically, the accumulated stress in an LIB caused by pressure variation is reduced by 56.17% under a stochastic load condition when activating the proposed pressure management strategy, resulting in a 1.47% increase in discharge capacity immediately after operation compared to that under a passive pressurized condition. The proposed method is simple, effective, and economically feasible for battery management systems in terms of compression pressure control.
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Lithium metal is considered a holy grail anode material for high theoretical energy batteries. However, lithium dendrite associated with interface instability has severely derailed efforts to commercialize safe and high-capacity lithium batteries. Here, we propose an electrochemical-mechanical phase field model by incorporating the elastic energy into the Gibbs free energy to reveal the role of stress in lithium dendrites. It is found that the compressive stress associated with lithium electroplating gradually concentrates near the nucleation site, acting as a driving force for dendrite formation. The surface energy plays a critical role in determining the dendrite morphology, the higher the surface energy the higher the curvature of the dendrite. A phase diagram of four types of morphologies is identified in terms of the interface energy density and charging rate. Our analysis suggests that a low charging rate or improving the interfacial Li⁺ diffusion ability is beneficial to maintaining interface stability.
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Background Mechano-electro-chemical coupling during the ion diffusion process is a core factor to determine the electrochemical performance of electrodes. However, relationship between the mechanics and the electrochemistry has not been clarified by experiments.Objective In this work, we conduct an in situ, visual, comprehensive characterization of strain field and Li concentration distribution to further explore the mechano-electro-chemical relationship. Methods The digital image correlation characterized by fluorescent speckle and active optical imaging is developed. Combined with electrochromic-based Li concentration detection, the spatiotemporal evolution of in-plane strain and Li concentration of a graphite electrode during the lithiation and delithiation processes are measured and displayed visually via a dual optical path acquisition system.ResultsThe visual results show that in-plane strain and Li concentration possess a spatially non-uniform gradient distribution along the radial direction (i.e., diffusion path) with large values outside and small values inside, and that both present obvious temporal segmentation. And mechano-electro-chemical coupling analysis reveals that the in-plane strain is not always linearly related to the concentration and infers that a high strain limits the diffusion and lithiation. The strain–concentration evolution exhibits obvious asymmetric differences between lithiation and delithiation, wherein three equations are fitted to approximately represent the evolution process between in-plane strain and concentration during the lithiation and delithiation processesConclusions This work overcomes the difficulties of fine strain measurements and collaborative concentration characterization during the electrochemical process, and provides an effective experimental method and data support for further exploration of mechano-electro-chemical coupling.
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A film/substrate system is a common structural form. In its fabrication and/or operation process, diffusion is a basic and key procedure. However, there still exist unclear points in the diffusion process, i.e. the effects of stress, creep, and interface properties. To clarify these unclear points, in this paper, a coupled diffusion model including stress, creep and interface property is established. The obtained results indicate that compressive stress retards the diffusion of guest atoms. Meanwhile, creep reduces the retardation of diffusion through releasing the induced compressive stress, and then the concentration of the guest atoms can reach the prescribed value, which overcomes the much lower concentration predicted by the previous models without creep. In addition, interfacial diffusivity affects diffusion and the maximum stress in the film.
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Lithium-ion batteries (LIBs) often suffer from capacity fading and poor cyclic performance due to mechanical degradation of the solid-electrolyte interphase (SEI). Here we perform numerical simulations and theoretical analysis to elucidate the role of plasticity in wrinkling and ratcheting behaviors of an SEI/electrode system. A coupled diffusion and finite deformation framework is formulated and numerically implemented as a user-element subroutine (UEL) to describe transient lithium diffusion and accompanying elastic–viscoplastic deformation of the electrode. It is found that concentration dependent properties and plastic deformation facilitate wrinkling in such a system. A wrinkled morphology may further lead to ratcheting and related failure under cycling. A phase diagram of four types of cyclic behaviors is identified in terms of the charging rate and time. Our analysis suggests several potential strategies to avoid wrinkling and ratcheting instabilities, such as charging/discharging the electrode at a sufficiently slow rate, and/or introducing a thick artificial SEI with a pre-tension.
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Advances in lithium-ion batteries (LIBs) have enabled the realization of light-weight power sources with high energy density, specific capacity, and cyclic stability. As LIBs are inherently subjected to thermomechanical stress during operation, their volume change can be indicative of their electrochemical reactions and safety status. In this study, a carbon nanotube (CNT)-based dilatometer that is stretchable and can be conformally mounted on the surface of LIBs has been developed for sensitive and in-situ measurements of the LIB swelling. The CNTs form a percolation network on top of a thin elastomer, and exhibit a positive gauge factor of ~50 and a negative temperature coefficient of resistance of -0.075% K−1, enabling quantitative extraction of the extent of swelling. As a result, both regular (~50 μm swelling by lithiation/delithiation cycles) and irregular (a few mm swelling by abnormal gas evolution due to increased temperature) reactions of LIBs are successfully detected in real-time. Unlike the conventional dilatometers that are complex, expensive, and bulky, the CNT-sensor features simplicity, portability, and sensitivity, which is useful for understanding electrochemical reactions and preventing serious failures of portable LIBs, without disassembling them from other components of the device.
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In this paper, a method composed of state of health (SOH) testing experiments and artificial intelligence simulation is proposed to carry out the study on the change of battery characteristic during its operation and generate mathematical models for the prediction of aging behaviour of battery. An experiment comprising of multidisciplinary parameters-based SOH detection is conducted to study the battery aging characteristics from several aspects (ie, electrochemistry, electric, thermal behaviour and mechanics). In total, 200 sets of data (corresponding 200 charging/discharging cycles) are collected from the experiment. The data obtained from the first 150 cycles are employed in generation of the models. The result of sensitivity analysis based on the obtained genetic programming models shows that it is better to apply voltage value at the end of charging step, charging time and cycle number to predict the operational performance of the battery. The average predicted accuracy of model (without stress) is 94.52%, whereas the average predicted accuracy of model (with stress effect) is 99.42%. The proposed models could be useful for defining the optimised charging strategy, fault diagnosis and spent batteries disposal strategies.
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Lithium–ion batteries (LIBs) in consumer electronics, electric cars and large-scale energy storage systems are often susceptible to capacity fading due to mechanical degradation of the solid-electrolyte interphase (SEI) layer on the electrodes. Here we present an analytical model to describe SEI wrinkling and ratcheting behaviors during cyclic lithiation and delithiation of LIBs. The SEI-electrode system is modeled as a bilayer structure consisting of a thin film resting on a plastic substrate. Surface instability is found in such a system under cyclic plastic deformation induced by lithiation and delithiation. A linear perturbation analysis is performed to determine the critical wrinkling strain and wavenumber. The interfacial shear traction induced by surface wrinkling can further lead to plastic ratcheting, and the wrinkling amplitude increases with each lithiation/delithiation cycle. A phase diagram is plotted to characterize and predict different system behaviors, e.g., elastic, elastic wrinkling, shakedown without wrinkling, shakedown with wrinkling, and ratcheting. A series of finite element simulations are performed to validate the theoretical predictions. The analysis suggests that the mechanical instabilities of the SEI, including wrinkling and ratcheting, can be prevented by several strategies, such as introducing an artificial SEI with a sufficiently large stiffness and thickness, and/or with a tensile pre-stress in the SEI.
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We examined the surfaces of graphite anodes extracted from Li-ion cells with Fourier transform infrared spectroscopy using attenuated total reflection geometry. The cells were of the 18650-type and subjected to calender aging (60% state of charge) at 55 degreesC. The composition of the film on an anode from a control cell (not aged) is composed of lithium oxalate (Li2C2O4), lithium carboxylate (RCOOLi), and lithium methoxide (LiOCH3). After aging, there is also lithium hydroxide (LiOH) and methanol (CH3OH), and in some cases lithium hydrogen carbonate (LiHCO3), probably due to the reaction of water with the methoxide and oxalate. There is substantial variation in the relative amounts of the five compounds over the surfaces of the electrodes. Alkyl carbonates may form early on, but decompose to more "inorganic'' compounds with aging. The multicomponent composition reflects the complex chemistry of passive film formation in real Li-ion cells. (C) 2003 The Electrochemical Society.
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Using ac impedance, we studied the formation process of solid electrolyte interface (SEI) film on graphite electrode during initial cycles. Results show that the SEI formation takes place through two major stages. The first stage takes place at voltages above 0.25 V (before lithiation of graphite), during which a loose and highly resistive film is formed. The second stage occurs at a narrow voltage range of 0.25-0.04 V, which proceeds simultaneously with lithiation of graphite electrode. In the second stage, a stable, compact, and highly conductive SEI film is produced.
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Recent studies show that the SEI on lithium and on LiâCâ anodes in liquid nonaqueous solutions consists of many different materials including LiâO, LiF, LiCl, LiâCOâ, LiCOâ-R, alkoxides, and nonconducting polymers. The equivalent circuit for such a mosaic-type SEI electrode is extremely complex. It is shown that near room temperature the grain-boundary resistance (R{sub gb}) for polyparticle solid electrolytes is larger than the bulk ionic resistance. Up to now, all models of SEI electrodes ignored the contribution of R{sub gb} to the overall SEI resistance. The authors show here that this neglect has no justification. On the basis of recent results, the authors propose here for SEI electrodes equivalent circuits which take into account the contribution of grain-boundary and other interfacial impedance terms. This model accounts for a variety of different types of Nyquist plots reported for lithium and LiâCâ electrodes in liquid nonaqueous and polymer electrolytes.
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Subsurface defects and local compositional changes that occurred in graphite anodes subjected to cyclic voltammetry tests (vs. Li/Li + , using an electrolyte consisting of 1 M LiClO4 in a 1:1 volumetric mixture of ethylene carbonate and 1,2-dimethoxy ethane) were investigated using high-resolution transmission electron microscopy (HR-TEM). Cross-sections of anodes prepared by focused ion beam (FIB) milling indicated that graphite layers adjacent to solid electrolyte (SEI)/graphite interface exhibited partial delamination due to the formation of interlayer cracks. The SEI layer formed on the graphite surface con-sisted of Li2CO3 that was identified by {1 1 0} and {0 0 2} crystallographic planes. Lithium compounds, LiC6 , Li2CO3 and Li2O, were observed on the surfaces of separated graphite layers. Deposition of these co-intercalation compounds near the crack tip caused partial closure of propagating graphite cracks during electrochemical cycling, and possibly reduced the crack growth rate. Graphite fibres that were observed to bridge crack faces likely provided an additional mechanism for the retardation of crack propagation.
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The passivating solid electrolyte interphase (SEI) layer forms at the surface of the negative-electrode active material in lithium-ion cells. A continuum-scale mathematical model has been developed to simulate the growth of the SEI and transport of lithium and electrons through the film. The model is used to estimate the film growth rate, film resistance, and irreversible capacity loss due to film formation. We show that film growth at the negative electrode is faster for charged batteries than for uncharged batteries and that higher electron mobility in the film leads to faster growth. If electron mobility is low, the rate of film growth is limited by transport of electrons through the film, and the rate decreases as the thickness increases. We examine the dependence of film resistance upon both film thickness and defect concentration in the film. We also show that the concentration polarization in the film increases as it grows at open circuit, even though the concentration gradient may decrease. © 2004 The Electrochemical Society. All rights reserved.
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The rate capability of various lithium-ion half-cells was investigated. Our study focuses on the performance of the carbon negative electrode, which is composed of TIMREX SFG synthetic graphite material of varying particle size distribution. All cells showed high discharge and comparatively low charge rate capability. Up to the 20 C rate, discharge capacity retention of more than 96% was found for SFG6. The rate capability of the half-cells is a function of both the particle size distribution of the graphite material and the preparation method of the electrode. A transport limitation model is proposed to explain the restrictions of the high current performance of graphite electrodes. The key parameters found to influence the performance of a graphite negative electrode were the loading, the thickness, and the porosity of the electrode.
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Density functional theory (DFT) is used to reveal that the polycrystalline Young’s modulus of graphite triples as it is lithiated to . This behavior is captured in a linear relationship between and lithium concentration suitable for continuum-scale models aimed at predicting diffusion-induced deformation in battery electrode materials. Alternatively, Poisson’s ratio is concentration-independent. Charge-transfer analyses suggest simultaneous weakening of carbon–carbon bonds within graphite basal planes and strengthening of interlayer bonding during lithiation. The variation in bond strength is shown to be responsible for the differences between elasticity tensor components, , of lithium–graphite intercalation (Li-GIC) phases. Strain accumulation during Li intercalation and deintercalation is examined with a core–shell model of a Li-GIC particle assuming two coexisting phases. The requisite force equilibrium uses different Young’s moduli computed with DFT. Lithium-poor phases develop tensile strains, whereas Li-rich phases develop compressive strains. Results from the core–shell model suggest that elastic strain should be defined relative to the newest phase that forms during lithiation of graphite, and Li concentration-dependent mechanical properties should be considered in continuum level models.
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First principles calculations were integrated with cohesive zone and growth chemistry models to demonstrate that adsorbed species can significantly alter stresses associated with grain boundary formation during polycrystalline film growth. Using diamond growth as an example, the results show that lower substrate temperatures increase the hydrogen content at the surface, which reduces tensile stress, widens the grain boundary separations, and permits additional atom insertions that can induce compressive stress. More generally, this work demonstrates that surface heteroatoms can lead to behavior which is not readily described by existing models of intrinsic stress evolution.
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Technological improvements in rechargeable solid-state batteries are being driven by an ever-increasing demand for portable electronic devices. Lithium-ion batteries are the systems of choice, offering high energy density, flexible and lightweight design, and longer lifespan than comparable battery technologies. We present a brief historical review of the development of lithium-based rechargeable batteries, highlight ongoing research strategies, and discuss the challenges that remain regarding the synthesis, characterization, electrochemical performance and safety of these systems.
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A new model describing the interactions between the graphite anode in Li-ion batteries and the liquid electrolyte reduction products [the solid electrolyte interface (SEI)] is presented. According to this model, certain solvents and additives produce efficient passive films which mimic a double-layer capacitor, wherein local, fixed positive charges in the SEI serve to counteract the negatively charged graphite anode. The model also suggests that internal passive layer interactions are of a multilayered capacitor type. Previously published experimental data in support of this model is presented.
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The irreversible capacity of carbon electrodes for lithium is a critical parameter which must be minimized in practical Li ion cells. This paper reports studies of the origin of the irreversible capacity in high-capacity carbons derived from sugar, and for comparison, in graphitic carbons made from mesocarbon microbeads. Tablet electrodes (similar to 0.4 mm thick) of sugar carbon or of mesocarbon microbeads (mixed with 10% pitch) were prepared by pyrolysis of the precursors at 1050 degrees C under a vacuum of about 10 mTorr. Tablets exposed to different gases were used in carbon/Li coin cells, in order to study the effect of different gas exposures on irreversible capacities, C-irr, for ii insertion. Carbon electrodes exposed only to argon or nitrogen demonstrated a dramatic reduction in irreversible capacity: C-irr approximate to 50 and 10 mAh/g for sugar carbon and mesocarbon microbead tablet cells, respectively, compared to C-irr approximate to 180 and 30 mAh/g for the corresponding conventional doctor-blade spread electrode cells where the electrodes had been exposed to laboratory air for several days. Tablet electrodes were also exposed to CO2, O-2, water, steam, and air, respectively, for different periods of time. Tablets exposed to each of these reactive gases showed a dramatic increase in irreversible capacity. The irreversible capacity for hard carbons results from two sources: (i) electrolyte decomposition on nominally clean carbon surfaces and (ii) reactions with surface groups which form on carbons exposed to reactive gases. The amount of irreversible capacity from the latter source depends on the gas exposure time and involves reactions with species such as hydroxyl, carboxyl functional groups, or adsorbed water.
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Since the birth of the lithium ion battery in the early 1990s, its development has been very rapid and it has been widely applied as power source for a lot of light and high value electronics due to its significant advantages over traditional rechargeable battery systems. Recent research demonstrates the importance of surface structural features of electrode materials for their electrochemical performance, and in this paper the latest progress on this aspect is reviewed. Electrode materials are either anodic or cathodic ones. The former mainly include graphitic carbons, whose surfaces can be modified by mild oxidation, deposition of metals and metal oxides, coating with polymers and other kinds of carbons. Through these modifications, the surface structures of the graphitic carbon anodes are improved, and these improvements include: (1) smoothing the active edge surfaces by removing some reactive sites and/or defects on the graphite surface, (2) forming a dense oxide layer on the graphite surface, and (3) covering active edge structures on the graphite surface. Meanwhile, other accompanying changes occur: (1) production of nanochannels/micropores, (2) an increase in the electronic conductivity, (3) an inhibition of structural changes during cycling, (4) a reduction of the thickness of the SEI (solid-electrolyte-interface) layer, and (5) an increase in the number of host sites for lithium storage. As a result, the direct contact of graphite with the electrolyte solution is prevented, its surface reactivity with electrolytes, the decomposition of electrolytes, the co-intercalation of the solvated lithium ions and the charge-transfer resistance are decreased, and the movement of graphene sheets is inhibited. When the surfaces of cathode materials, mainly including LiCoO2, LiNiO2 and LiMn2O4, are coated with oxides such as MgO, Al2O3, ZnO, SnO2, ZrO2, Li2O⋅2B2O3 glass and other electroactive oxides, the coating can prevent their direct contact with the electrolyte solution, suppress the phase transitions, improve the structural stability, and decrease the disorder of cations in the crystal sites. As a result, side reactions and the amount of the heat production during cycling are decreased. Simultaneously, other effects are observed such as: (1) suppression of the dissolution of Mn2+, (2) higher conductivity, and (3) removal of HF from the electrolyte solution. Consequently, after the above-mentioned effective coating, marked improvements in the electrochemical performance of the electrode materials including the reversible capacity, the coulombic efficiency in the first cycle, the cycling behavior, and the high rate capability have been achieved. However, many surface science issues are still remaining open, e.g., mechanisms of these coatings and different actions of different coatings, and some further directions are suggested for the surface modification of the electrode materials.
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In situ differential electrochemical mass spectrometry (DEMS) was used to study the SEI film formation on highly crystalline TIMREX® SLX50 graphite negative electrodes during the first electrochemical lithium insertion using either 1 M LiPF6 in ethylene carbonate (EC) with either dimethyl carbonate (DMC) or propylene carbonate (PC) as co-solvent. In the case of the propylene and ethylene carbonate containing electrolyte, DEMS measurements indicate strong formation of ethylene and propylene gas below 0.75 V versus Li/Li+, which does not decrease at lower cell potential and in the subsequent charge/discharge cycles. Whereas for the dimethyl carbonate containing electrolyte, ethylene gas formation could be observed already above 1 V versus Li/Li+. Post mortem scanning electron microscopy (SEM) studies of the electrodes show strong exfoliation of the graphite electrode when they are discharged in the ethylene/propylene carbonate electrolyte, indicating the formation of an unstable SEI layer. The addition of vinylene carbonate (VC) as a film forming additive significantly decreases the gas formation at the graphite electrode in the propylene carbonate containing electrolyte. The exfoliation was suppressed by the vinylene carbonate additive. We show that the combination of different in situ and ex situ methods can provide new useful information about the passivation process of graphite, as well as the solid electrolyte interphase layer formed, during the first electrochemical insertion of lithium into graphite negative electrode materials.
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The internal stress of silver, copper and gold films was measured as a function of film thickness during deposition and as a function of time after deposition under ultrahigh vacuum conditions. A model for the origin of the internal stress was used to interpret each stress curve in terms of the corresponding film structure. During the deposition of the three metals onto MgF2 substrate films tensile as well as compressive stresses are found as the film thickness increases and the maximum tensile stress occurs at the thickness at which the metal films become completely continuous. A compressive strain, built up at the metal substrate interface during the coalescence stage, is assumed to be the origin of the compressive stress in the continuous metal film. The tensile stress change after deposition of these metals is attributed to recrystallization and annealing processes that occur in the films. The compressive interface strain, which is maintained during this recrystallization, again determines the compressive stress measured during the deposition of a second metal layer. The growth of this second metal film is a continuation of the growth of the first film without renewed nucleation.
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We present herein Raman spectroscopy and SEM characterizations of composite graphite electrodes in conjunction with classical electroanalytical characterizations (SSCV and EIS) during prolonged cycling. During cycling, graphite particles crack into smaller pieces that are less oriented than the original platelets, with the possible filling of the cracks thus formed by the reduction products of the electrolyte solution. In addition, the average crystalline size (estimated by Raman spectroscopy) decreases as cycling progresses. The borders between the crystallites may possess dangling bonds and generally contain low-energy (or hollow) sites for irreversible interaction with Li-ions and solution species. The redistribution between the hollow and the shallow sites (i.e. the site for reversible Li-ion storage) occurring during electrode cycling is responsible for the moderate decrease of the reversible capacity of graphite electrodes observed during prolonged cycling.
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Ethylmethylcarbonate (EMC) has been found to be a promising solvent for rechargeable Li-ion batteries. Graphite electrodes, which are usually sensitive to the composition of the electrolyte solution, can be successfully cycled at high reversible capacities in several Li salt solutions in this solvent (LiAsFâ, LiPFâ, etc.). These results are interesting because lithium ions cannot intercalate into graphite in diethyl carbonate solutions and cycle poorly in dimethyl carbonate solutions. To understand the high compatibility of EMC for Li-ion battery systems as compared with the other two open-chain alkyl carbonates mentioned above, the surface chemistry developed in both Li and carbon electrodes in EMC solution was studied and compared with that developed on these electrodes in other alkyl carbonate solutions. Basically, the major surface species formed on both electrodes in EMC include ROLi, ROCOâLi, and LiâCOâ species. The uniqueness of EMC as a battery solvent is discussed in light of these studies.
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Li insertion–deinsertion into composite graphite electrodes, comprising synthetic graphite flakes (6 μm average size), polyvinylidene difluoride binder (PVdF), and copper current collectors, in commonly used alkyl carbonate solutions were studied by in situ atomic force microscopy (AFM). In this study, we were able to probe by in situ AFM the behavior of practical, composite graphite electrodes in ethylene carbonate–dimethyl carbonate (EC–DMC) solutions containing salts such as LiAsF6 and LiPF6 during entire lithiation–delithiation cycles. These in situ micro/nanomorphological studies could probe surface film formation on the graphite particles, as well as periodic volume changes in the graphite flakes during Li insertion–deinsertion cycles. These cyclic volume changes can explain the capacity fading of graphite electrodes upon prolonged cycling, in Li-ion batteries. While the overall morphology of these electrodes remains steady upon cycling in the appropriate solutions (in which the Li–C electrodes are efficiently passivated), there is a continuous problem in the extent of accommodation of the small volume changes in the graphite particles upon lithiation–delithiation, by the surface films. It is suggested that graphite electrodes fail during prolonged cycling due to small scale, continuous reactions of the active mass with solution species, which gradually increase their impedance and decrease the content of the lithium stored in the electrodes.
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We have studied the formation and growth of solid-electrolyte interphase (SEI) for the case of ethylene carbonate (EC), dimethyl carbonate (DMC) and mixtures of these electrolytes using molecular dynamics simulations. We have considered SEI growth on both Li metal surfaces and using a simulation framework that allows us to vary the Li surface density on the anode surface. Using our simulations we have obtained the detailed structure and distribution of different constituents in the SEI as a function of the distance from the anode surfaces. We find that SEI films formed in the presence of EC are rich in Li2CO3 and Li2O, while LiOCH3 is the primary constituent of DMC films. We find that dilithium ethylene dicarbonate, LiEDC, is formed in the presence of EC at low Li surface densities, but it quickly decomposes to inorganic salts during subsequent growth in Li rich environments. The surface films formed in our simulations have a multilayer structure with regions rich in inorganic and organic salts located near the anode surface and the electrolyte interface, respectively, in agreement with depth profiling experiments. Our computed formation potentials 1.0V vs. Li/Li+ is also in excellent accord with experimental measurements. We have also calculated the elastic stiffness of the SEI films; we find that they are significantly stiffer than Li metal, but are somewhat more compliant compared to the graphite anode.
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13C-carbon black substituted composite LiNi0.8Co0.15Al0.05O2 cathodes were tested in model electrochemical cells to monitor qualitatively and quantitatively carbon additive(s) distribution changes within tested cells and establish possible links with other detrimental phenomena. Raman qualitative and semi-quantitative analysis of 13C in the cell components was carried out to trace the possible carbon rearrangement/movement in the cell. Small amount of cathode carbon additives were found trapped in the separator, at the surface of Li-foil anode, and in the electrolyte. The structure of the carried away carbon particles was highly amorphous unlike the original 12C-graphite and 13C-carbon black additives. The role of the carbon additive, the mechanism of carbon retreat in composite cathodes and its correlation with the increase of the cathode interfacial charge-transfer impedance, which accounts for the observed cell power and capacity loss is investigated and discussed.
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In order to understand the properties of high-rate capability and cycleability for a disordered carbon negative electrode in LiPF6/PC based electrolyte solution, the cell performance tests with various rates and depth of discharges (DODs) has been studied by spectroscopic and electrochemical analyses. From the charge–discharge measurements, a surface carbon-edge redox reaction occurring between a carbonyl (CedgeO) and a lithium alkoxide (Cedge–OLi) that delivers a large capacity was found fast and high cycleability at only shallow DOD (2.0–0.4V). The limited or shallow charge–discharge cycling utilizing such facile and reversible action of the CedgeO/Cedge–OLi of the disordered carbon is suited to an application for an negative electrode of asymmetric hybrid capacitors. A deep DOD discharge (2.0–0.0V) revealed the existence of some complex processes involving a lithium cluster deposition at pores or microvoids as well as a lithium ion intercalation at graphene layers. The cluster deposition at pores was found to be relatively fast and reproducible. The lithium ion intercalation at graphenes and the subsequent cluster deposition at microvoids were found to be slow and degrade the cycleability after 100 cycles because of the accumulation of a thick and low-ion-conductive solid electrolyte interface (SEI) film on surface.
Article
‘Non-graphitizable’ or ‘hard’ carbon anode materials for Li-ion batteries have many advantages and disadvantages when compared to graphitic materials such as mesocarbon micro-beads (MCMB). The advantages include higher capacity (per unit mass) [Yamada et al., United States Patent No. 5,834,138 (1998); Buiel et al., J. Electrochem. Soc. 147(2) (1988) 2252–2257; Buiel and Dahn, J. Electrochem. Soc. 145(6) (1998) 1977–1981], higher cycle life [Omaru et al., United States Patent No. 5,451,477 (1995)], good rate capabilities [Rakotondrainibe et al., In: Proceedings of the 194th Meeting of the Electrochemical Society. Abstract No. 83. 1–6 November 1998] and lower cost of production. The disadvantages that must be resolved before a successful material can be commercialized are the low density, incompatibility with current coating technologies, larger irreversible capacity and hysteresis in the voltage profile [Buiel and Dahn, J. Electrochem. Soc. 145(6) (1998) 1977–1981]. Some reports have suggested that the problem of low density can be solved using composite anode materials consisting of a mixture of hard carbon and MCMB. These composites also boast higher rate capabilities and longer cycle life when compared to pure MCMB. In this paper, reducing the hysteresis in the voltage profile and reducing the irreversible capacity of hard carbons is the primary focus. In order to achieve this goal, a study of the electrochemistry and structure of promising hard carbon materials is presented and correlated to various parameters that can be adjusted during synthesis.
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Lithium titanate (Li4Ti5O12, or LTO) is a promising anode for lithium-ion batteries due to its rate capability and cycling stability. However, its structural instability and solid electrolyte interphase formation at low potential limits its application within a high cut-off voltage above 1V, which significantly sacrifices the cell voltage and energy density. This work demonstrates how a few atomic layers of Al2O3 deposited on LTO electrode improved its cycling performance (no capacity degradation after 100cycles) and provided a higher Coulombic efficiency compared with the standard uncoated LTO electrode when they are cycled at low potential down to 1mV. It has been found that the ultrathin oxide layers served as a passivation film not only stabilized the LTO structure but also surprisingly suppressed some undesirable chemical reactions.
Article
Lithium can be inserted reversibly within most carbonaceous materials. The physical mechanism for this insertion depends on the carbon type. Lithium intercalates in layered carbons such as graphite, and it adsorbs on the surfaces of single carbon layers in nongraphitizable hard carbons. Lithium also appears to reversibly bind near hydrogen atoms in carbonaceous materials containing substantial hydrogen, which are made by heating organic precursors to temperatures near 700°C. Each of these three classes of materials appears suitable for use in advanced lithium batteries.
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It is suggested that in practical nonaqueous battery systems the alkali and alkaline earth metals are always covered by a surface layer which is instantly formed by the reaction of the metal with the electrolyte. This layer, which acts as an interphase between the metal and the solution, has the properties of a solid electrolyte. The corrosion rate of the metal, the mechanism of the deposition‐dissolution process, the kinetic parameters, the quality of the metal deposit, and the half‐cell potential depend on the character of the solid electrolyte interphase (SEI).
Article
Undesired reactions in Li-ion batteries, which lead to capacity loss, can consume or produce charge at either the positive or negative electrode. For example, the formation and repair of the solid electrolyte interphase consumes and at the negative electrode. High purity electrolytes, elimination of water, various electrolyte additives, electrode coatings, and special electrode materials are known to improve cycle life and therefore must impact coulombic efficiency. Careful measurements of coulombic efficiency are needed to quantify the impact of trace impurities, additives, coatings, etc., in only a few charge–discharge cycles and in a relatively short time. The effects of cycle-induced and time-related capacity loss could be probed by using experiments carried out at different C-rates. In order to make an impact on Li-ion cells for automotive and energy storage applications, where thousands of charge–discharge cycles are required, coulombic efficiency must be measured on the order of 0.01%. In this paper, we describe an instrument designed to make high precision coulombic efficiency measurements and give examples of its use on commercial Li-ion cells and Li half-cells. High precision coulombic efficiency measurements can detect problems occurring in half-cells that do not lead to capacity loss, but would in full cells, and can measure the impact of electrolyte additives and electrode coatings.
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In situ atomic force microscopic (AFM) observation of the basal plane of highly oriented pyrolytic graphite was performed during cyclic voltammetry at a slow scan rate of 0.5 mV in 1 mol dissolved in a mixture of ethylene carbonate and diethyl carbonate. In the potential range 1.0-0.8 V, atomically flat areas of 1 or 2 nm height (hill-like structures) and large swellings of 15-20 nm height (blisters) appeared on the surface. These two features were formed by the intercalation of solvated lithium ions and their decomposition beneath the surface, respectively, and may have a role in suppressing further solvent cointercalation. At potentials more negative than 0.65 V, particle-like precipitates appeared on the basal plane surface. After the first cycle, the thickness of the precipitate layer was 40 nm, and increased to 70 nm after the second cycle. The precipitates were considered to be mainly organic compounds that are formed by the decomposition of solvent molecules, and they have an important role in suppressing further solvent decomposition on the basal plane. © 2001 The Electrochemical Society. All rights reserved.
Article
Lithium-ion cells with 5-Ah capacity were fabricated using a spinel as a cathode active material, graphitized carbon as an anode active material, and 1 M as an electrolyte. In order to improve the calendar life of the cell, we investigated the degradation mechanism by measuring the thickness of the solid electrolyte interphase (SEI) on anode active material. The SEI thickness was measured by focused ion beam, scanning electron microscope, and X-ray photoelectron spectroscopy. The thickness of the SEI was initially , and after storage for 392 days at 25 and 40°C, the thickness was 0.15 and , respectively. The capacity decreased with increase in the thickness of SEI, because Li in the cell is consumed by forming SEI. The amount of Li consumption was estimated theoretically assuming that SEI is formed by a reaction between intercalated Li and the electrolyte in SEI on the negative carbon surface, and a diffusion of the electrolyte in the SEI is the rate-determining step of the reaction. The theoretical equation showed a good agreement with experimental capacity fade at 25, 40, and 60°C for the storing days up to 380 days. A voltage decrease of the cell after 1-s at 20 A of discharge current was measured to estimate roughly the increase of the cell internal resistance during storage. The increase of SEI resistance was estimated by the theoretical equation and compared with the experimental voltage drop data after 1-s discharge. However, the theoretical data was not in a good agreement with the experimental data. The reason is that the charge-transfer resistance on the anode also increases during storage. Another reason is the resistance change of the cathode during the storage.
Article
Amorphous silicon thin films deposited on copper foil have been observed to exhibit near theoretical capacity for a limited number of cycles. The films, however, eventually delaminate, leading to failure of the anode. In order to better understand the mechanism of capacity retention and the ultimate failure mode of a model brittle active:elastic/plastic inactive anode system, the films were subjected to in situ adhesion tests while observing the film surface using scanning electron microscopy. Atomic force and transmission electron microscopy, and electrochemical cycling were conducted to analyze the emerging morphology of the films during cycling. The adhesion of the as-deposited Si film to the Cu substrate was measured to , reflecting a weak interface adhesion strength. Plastic deformation of the underlying Cu substrate combined with a ratcheting mechanism is proposed to occur in the system, with delamination failure mode occurring only after the formation of an interface imperfection. From the analysis of slow rate cycling experiments, nucleation of a lithium compound based on the interdiffusion of Si and Cu is identified as the most probable cause of the ultimate delamination failure of the deposited film.
Article
Li/graphite and Li/petroleum coke cells using a in a 50:50 mixture of propylene carbonate (PC) and ethylene carbonate (EC) electrolyte exhibit irreversible reactions only on the first discharge. These irreversible reactions are associated with electrolyte decomposition and cause the formation of a passivating film or solid electrolyte interphase on the surface of the carbon. The amount of electrolyte decomposition is proportional to the specific surface area of the carbon electrode. When all the available surface area is coated with the film of decomposition products, further decomposition reactions stop. In subsequent cycles, these cells exhibit excellent reversibility and can be cycled without capacity loss.
Article
Wafer curvature measurements are a simple yet sensitive way to measure stress in thin films. A multibeam optical system designed for in situ monitoring makes it possible to monitor the evolution of thin film stress in real time in a variety of deposition and processing environments. Examples of stress measurements in epitaxial systems, polycrystalline films and hard coatings are discussed.
Article
This paper discusses the interrelated phenomena of solid electrolyte interphase (SEI) formation and the irreversible charge consumption which occurs during the first cycle of a graphite electrode, as well as their relevance to the cycling stability of lithium-ion batteries. Thus, results from relevant characterization methods, namely, in situ mass spectrometry, in situ infrared spectroscopy, in situ Raman and video microscopy, in situ scanning probe microscopy, in situ quartz crystal microbalance, and differential scanning calorimetry were combined for a more thorough understanding of observations made in cycling experiments. From electrochemical cycling tests, we have learned that a high specific charge (∼360Ah/kg of carbon), satisfactory cycle life of the graphite electrodes (1000 deep cycles), and an irreversible charge of
Article
The demand for graphites with specific properties and for an improvement in the economics of graphite production in recent years has led to an increased interest in catalytic graphitisation. At the present time, however, there is no review available of published studies and this paper attempts to make good this deficiency. Mechanisms whereby inorganic additives can catalyse graphitisation are considered, followed by a discussion of the mechanisms by which inorganic additives promote the graphitisation process. A comprehensive survey of the relevant literature to date is presented which describes in more detail the type of carbon used, the experimental techniques employed and the improvement obtained in graphite quality.
Article
The lithiation of thin film Si was investigated in an electrochemical cell, using in situ wafer curvature to monitor the evolution of in-plane stresses. Increasing the initial film thickness from 50 to 250 nm led to decreases in both the nominal flow stress and the Li capacity. These observations are consistent with relatively slow Li diffusion. The corresponding concentration gradients should have a substantial effect on the deformation and viscous flow that occur in lithiated Si. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Article
The composition and morphology of the solid electrolyte interphase (SEI) formation on highly ordered pyrolytic graphite (HOPG), SLX20-graphite and disordered-carbon electrodes was studied by the XPS and TOFSIMS methods. The experimental data show good evidence for compositional and morphological distinctions between the SEI formed on the basal and cross-sectional planes of graphite. The basal plane film was found to be enriched in organic compounds, but was relatively thinner than that of the cross-section, which contained predominantly salt-reduction products. The SEI formed in LiAsF6 and LiTFSI electrolytes is thinner than that in LiPF6 electrolyte on all the carbon substrates. The SEI on hard carbon is thicker than on soft in all the electrolytes studied. The SEI composition on the hard carbon with high content of salt-reduction products, is similar to that formed on the cross-section of HOPG. The SEI composition on soft carbon has some similarity to that formed on the basal plane of HOPG.