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Investigation of the influence of different bracing of automotive pouch cells on cyclic liefetime and impedance spectra

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Abstract

Lithium-Ion pouch cells are types of cells with great potential regarding energy and power densities to design of battery systems and battery modules in electromobility. These cells pose a high demand on their placement and bracing in the module to compensate their expansion and therefore guarantee a long lifetime. The challenge is to identify optimal bracing for the individual pouch cells. Thus, this work studies the cyclic aging of four identical automotive pouch cells under different bracing conditions while measuring cell-specific force, distance and impedance parameters. This measurement shows an improved mechanical integration of this type of battery cell in modules. Furthermore there is a recognizable correlation between the different conditions of strain and the impedance spectra along different states of aging. From this results a specific parameter is identified which can be used for rapid test procedures.

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... Lithium-ion pouch cells degrade significantly faster and perform worse if they are cycled fully uncompressed. [205][206][207][208] Thus, at least a certain compression in the range of 50-200 kPa improves performance and lifetime. The influence of pressure on performance and lifetime behaviour has been investigated in the literature. ...
... Herein, an increment [209] and a decrement [210] of internal resistance were observed during performance tests with increasing initial pressure. Similarly, a faster, [205,208,211] as well as a slower [206] capacity fade was reported with increasing initial pressure. Both tendencies motivate the existence of an optimal initial pressure range for prolonging battery lifetime and the necessity of a [237] LCO-C pouch 6.55 [205] LCO-C Brackets () after the arrows give the range in which the ageing rate changes accordingly. ...
... Using more rigid apparatuses, i. e. higher pressure change due to cell breathing, leads unambiguously to a faster capacity loss. [205,207,208,[211][212][213] Depending on the rigidity of the compression apparatus, the change in cell volume due to lithiation can induce pressures up to 1.6 MPa, even with an initial pressure of only a few kPa. [212] In studies using a rigid apparatus, a higher initial pressure will also increase the pressure change due to cycling. ...
Article
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For the battery industry, quick determination of the ageing behaviour of lithium‐ion batteries is important both for the evaluation of existing designs as well as for R&D on future technologies. However, the target battery lifetime is 8–10 years, which implies low ageing rates that lead to an unacceptably long ageing test duration under real operation conditions. Therefore, ageing characterisation tests need to be accelerated to obtain ageing patterns in a period ranging from a few weeks to a few months. Known strategies, such as increasing the severity of stress factors, for example, temperature, current, and taking measurements with particularly high precision, need care in application to achieve meaningful results. We observe that this challenge does not receive enough attention in typical ageing studies. Therefore, this review introduces the definition and challenge of accelerated ageing along existing methods to accelerate the characterisation of battery ageing and lifetime modelling. We systematically discuss approaches along the existing literature. In this context, several test conditions and feasible acceleration strategies are highlighted, and the underlying modelling and statistical perspective is provided. This makes the review valuable for all who set up ageing tests, interpret ageing data, or rely on ageing data to predict battery lifetime.
... During the entire life cycle, batteries and their components undergo a variety of mechanical loads with a wide range of magnitudes during the manufacturing process and time of operation [5,[18][19][20][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48], as illustrated in Fig. 2. In discussing the influence of such mechanical loads on the battery performance, it is important to specify the magnitude of stress ranges under different scenarios i.e., whether it is for liquid electrolyte, solid-state electrolyte; or whether it is during the manufacturing process and time of operation. Researchers have found that the external pressure during the material preparation and the electrode calendering is of the order of dozens or hundreds of MPa, or even several GPa [5,[33][34][35][36][37][44][45][46][47][48]. ...
... Researchers have found that the external pressure during the material preparation and the electrode calendering is of the order of dozens or hundreds of MPa, or even several GPa [5,[33][34][35][36][37][44][45][46][47][48]. While during the time of operation, it is normally less than 10 MPa [18][19][20]32,[40][41][42][43]. ...
... The magnitudes and origins of such deformation have been studied and linked to the final performance of LIBs. Bulter et al. [56] measured the in-situ physical swelling of free uncharged graphite electrodes in LP40 electrolyte through scanning [38,39]; calendering: [5,[33][34][35][36][37][44][45][46][47][48]; formation / assembly / operation [18][19][20]32,[40][41][42][43]. BOL refers to the beginning of life and EOL signifies the end of life. ...
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.
... The effect was observed to be enhanced with increasing state of charge (SoC) since internal cell pressure is highest here [16]. Another study evaluated different compression setups regarding the long-term performance of a lithium-ion pouch cell and found that spring compression has the most beneficial impact on capacity retention [17]. ...
... The former reported dilation investigations [36][37][38], however, were not performed under compressed condition, representing the use case for BEV packs. For the compression studies [11,17], mainly long-term tests were performed, and tests with various charging rates are missing, as well as a more detailed evaluation of the thickness changes and statements on CE. ...
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.
... Cannarella and Arnold [32] studied the influence of mechanical loading on battery life by monitoring stacking pressure and capacity during a cycle. Wünsch et al. [33] demonstrated that different bracing mechanical conditions affect cell aging. Müller et al. [34] reported that the flexible compression within a certain range improves the cycle performance and alleviates aging. ...
... Finally, some studies showed that stress affects the electrochemical process parameters such as charge transfer resistance. [33,34] Limited by the experimental technology and loading devices, most experimental researches have been performed under compressive stress. Recently, Xie et al. [35] introduced the design of a prestressed battery structure to achieve tensile stress loading, thereby experimentally demonstrating that tensile stress enhances the electrochemical performance of silicon composite electrodes. ...
Article
Full-text available
Lithium-ion batteries suffer from mechano–electrochemical coupling problems that directly determine the battery life. In this paper, we investigate the electrode electrochemical performance under stress conditions, where seven tensile/compressive stresses are designed and loaded on electrodes, thereby decoupling mechanics and electrochemistry through incremental stress loads. Four types of multi-group electrochemical tests under tensile/compressive stress loading and normal package loading are performed to quantitatively characterize the effects of tensile stress and compressive stress on cycle performance and the kinetic performance of a silicon composite electrode. Experiments show that a tensile stress improves the electrochemical performance of a silicon composite electrode, exhibiting increased specific capacity and capacity retention rate, reduced energy dissipation rate and impedances, enhanced reactivity, accelerated ion/electron migration and diffusion, and reduced polarization. Contrarily, a compressive stress has the opposite effect, inhibiting the electrochemical performance. The stress effect is nonlinear, and a more obvious suppression via compressive stress is observed than an enhancement via tensile stress. For example, a tensile stress of 675 kPa increases diffusion coefficient by 32.5%, while a compressive stress reduces it by 35%. Based on the experimental results, the stress regulation mechanism is analyzed. Tensile stress loads increase the pores of the electrode material microstructure, providing more deformation spaces and ion/electron transport channels. This relieves contact compressive stress, strengthens diffusion/reaction, and reduces the degree of damage and energy dissipation. Thus, the essence of stress enhancement is that it improves and optimizes diffusion, reaction and stress in the microstructure of electrode material as well as their interactions via physical morphology.
... For a coating with low adhesion, the absence of pressure can as well lead to loss of electrical contact to the current collector. Only in case the housing limits the expansion or the cell is externally compressed, the volume expansion leads to pressure on the jelly roll/stack, [8]. In some publications, certain pressure is regarded to have positive influence on cell performance [3,[8][9][10] and in others, to have negative influence [11], especially applying very high pressure [12]. ...
... Only in case the housing limits the expansion or the cell is externally compressed, the volume expansion leads to pressure on the jelly roll/stack, [8]. In some publications, certain pressure is regarded to have positive influence on cell performance [3,[8][9][10] and in others, to have negative influence [11], especially applying very high pressure [12]. In case of inhomogeneously distributed pressure on the electrode, lithium metal deposition occurs very likely due to local overcharge [13][14][15][16]. ...
Article
For this contribution, two generations of uncompressed pouch cells are investigated in cyclic aging tests. The results are analyzed for the key influence factors by opposing the results of both cell setup strategies with results of compressed cells reported in literature. The cells are evaluated with respect to pulse resistance, anode overhang, dV/dQ-analysis, capacity difference analysis, homogeneity of lithium distribution (HLD) and irreversible capacity losses. It is shown, that uncompressed cells reveal, in contrast to compressed cells, a higher HLD, which is demonstrated in the missing dV/dQ flattening over aging. The trends of peak heights in dV/dQ depend mainly on the graphite voltage and expansion characteristics and are therefore different for both cell generations due to different degrees of utilization of the anode. The trends are comparable to calendric aging test results. The irreversible losses for uncompressed cycled cells increase with cell potential while compressed cells show higher losses to lower and higher SOCs caused by low HLD. Therefore, uncompressed cells have a lower risk of developing lithium plating for inhomogeneous distributed active lithium during accelerated aging tests performing a full charge. Comparing both cell generations, the lifetimes show strong deviations and the largest impact is associated with n/p ratio of the anode to the cathode.
... Therefore, the monitoring of mechanical signals at the cell level serves as a vital indicator of the cell's internal status, with many common methodologies focusing on alterations in cell shape. For instance, in the case of prismatic and pouch cells, the use of a Linear Variable Differential Transformer (LVDT) [33][34][35] enables the measurement of internal cell pressure within the module (Figure 4a), whereas film strain resistance sensors [36] are adept at mapping pressure distribution across the cell (Figure 4b). Additionally, variations in cell thickness can be precisely gauged using inductive coil eddy current sensors ( Figure 4c) [4]. ...
Article
Full-text available
Traditional battery management systems (BMS) encounter significant challenges, including low precision in predicting battery states and complexities in managing batteries, primarily due to the scarcity of collected signals. The advancement towards a “smart battery”, equipped with diverse sensor types, promises to mitigate these issues. This review highlights the latest developments in smart sensing technologies for batteries, encompassing electrical, thermal, mechanical, acoustic, and gas sensors. Specifically, we address how these different signals are perceived and how these varied signals could enhance our comprehension of battery aging, failure, and thermal runaway mechanisms, contributing to the creation of BMS that are safer and more reliable. Moreover, we analyze the limitations and challenges faced by different sensor applications and discuss the advantages and disadvantages of each sensing technology. Conclusively, we present a perspective on overcoming future hurdles in smart battery development, focusing on appropriate sensor design, optimized integration processes, efficient signal transmission, and advanced management systems.
... There are two common designs for applying external pressures on Li-ion pouch cells 25 : constant gap or constant pressure. The former involves fixing a cell between two plates, which restricts its outward expansion ( Supplementary Fig. 1a,d), while the latter typically adds constant spring force between two plates ( Supplementary Fig. 1b,e). ...
Article
Full-text available
Lithium (Li) metal battery technology, renowned for its high energy density, faces practical challenges, particularly concerning large volume change and cell swelling. Despite the profound impact of external pressure on cell performance, there is a notable gap in research regarding the interplay between external pressure and the electroplating behaviours of Li⁺ in large-format pouch cells. Here we delve into the impact of externally applied pressure on electroplating and stripping of Li in 350 Wh kg⁻¹ pouch cells. Employing a hybrid design, we monitor and quantify self-generated pressures, correlating them with observed charge–discharge processes. A two-stage cycling process is proposed, revealing controlled pouch cell swelling of less than 10%, comparable to state-of-the-art Li-ion batteries. The pressure distribution across the cell surface unveils a complex Li⁺ detour behaviour during electroplating, highlighting the need for innovative strategies to address uneven Li plating and enhance Li metal battery technology.
... The battery cell of an LIB can be produced in several different formats, such as cylindrical-type, prismatic-type, and pouch-type battery cells. Pouch-type battery cells are wellsuited for EV battery cells due to their volumetric advantage and high energy density (Wünsch et al. 2019). ...
Article
Full-text available
A pouch battery pack includes multi-stacked battery module structures that protect the inner pouch battery cells from external hazards and deformation that may arise due to swelling effects. Recent research has found that the stack pressure, which is the suppressing force on the battery cells inside the battery module structure, has a significant impact on the degree to which the state-of-health (SOH) degrades and amount that the mechanical properties of pouch batteries change. Consequently, it is important to optimize the battery module structure design with consideration of the SOH and the structural reliability. To identify how significantly design affect the SOH and the mechanical properties, experiments under different levels of initial stack pressure and uncertainty quantification using Gaussian process are explored in this research. Reliability-based design optimization for the pouch battery module optimize the structural design that minimizes volume while satisfying structural reliability and SOH requirements. This work suggests a data-driven approach for achieving reliability-based design using experiment. Further, this research suggests formulations to calculate the performance functions, which are significant factors for reliable design of pouch battery modules.
... SOC-dependent geometry changes are the result of the intercalation and deintercalation of lithium ions into the electrodes' active materials [17,18] and are also referred to as reversible swelling. The thickness of pouch cells, for instance, changes cyclically during charging and discharging, being in the order of up to 3% or more for NMC/graphite pouch cells [17,[19][20][21][22]. ...
Article
Full-text available
Mechanical simulation models have become crucial for understanding Li-ion battery failure and degradation mechanisms. However, existing safety assessment models lack the implementation of SOC-dependent thickness variations referred to as reversible swelling. Reversible swelling affects the applied preload force on a constrained pouch cell, potentially impacting its safety. To investigate this, a finite element RVE model was developed in LS-Dyna. Two swelling models, simplified homogenous expansion (HE) and locally resolved expansion (LE), were implemented along with a reference basis model (BM) without expansion. Six different stress-or strain-based short circuit criteria were calibrated with abuse test simulations at different SOCs and preload forces. Short circuit prognosis improved on average by 0.8% and 0.7% for the LE and HE model compared to the BM, with minimum principal stress being the most suitable criterion. The LE model exhibited a softer mechanical response than the HE model or BM, accounting for the pouch cell surface unevenness at small indentations. This study demonstrated the feasibility and usefulness of implementing an expansion model in a commercial FE solver for improved short circuit predictions. An expansion model is crucial for simulating aged battery cells with significant geometry changes strongly affecting the preload force of a constrained battery cell.
... There are two common designs for applying external pressures on Li-ion pouch cells (19): constant gap or constant pressure. The former fixes a cell between two plates which does not allow outward expansion ( Fig. S1A and 1D), while the latter typically adds constant spring force between two plates ( Fig. S1B and 1E). ...
Preprint
Externally applied pressure impacts the performance of batteries particularly in those undergoing large volume changes, such as lithium metal batteries. In particular, the Li+^+ electroplating process in large format pouch cells occurs at a larger dimension compared to those in smaller lab-scale cells. A fundamental linkage between external pressure and large format electroplating of Li+^+ remains missing but yet critically needed to understand the electrochemical behavior of Li+^+ in practical batteries. Herein, this work utilizes 350 Wh/kg lithium metal pouch cell as a model system to study the electroplating of Li+^+ ions and the impact of external pressure. The vertically applied uniaxial pressure on the batteries using liquid electrolyte profoundly affects the electroplating process of Li+^+ which is well reflected by the self-generated pressures in the cell and can be correlated to battery cycling stability. Taking advantage of both constant gap and pressure application, all Li metal pouch cells demonstrated minimum swelling of 6-8% after 300 cycles, comparable to that of state-of-the-art Li-ion batteries. Along the horizontal directions, the pressure distributed across the surface of Li metal pouch cell reveals a unique phenomenon of Li+^+ migration during the electroplating (charge) process driven by an uneven distribution of external pressure across the large electrode area, leading to a preferred Li plating in the center area of Li metal anode. This work addresses a longstanding question and provides new fundamental insights on large format electrochemical plating of Li which will inspire more innovations and lead to homogeneous deposition of Li to advance rechargeable lithium metal battery technology.
... Pouch cells are usually embedded between compression pads (e.g. soft Polyurethane) or flexible bracing to improve lifetime by accounting for thickness variations and the resulting mechanical load variations during battery lifetime [20,[27][28][29][30]. ...
Article
Full-text available
Safety of lithium-ion batteries plays an important role in the context of advancing electrification for vehicles. Pouch cells suffer from low structural strength and are often constrained within a battery module to guarantee mechanical integrity. The effect of constraints and SOC-dependent changes on the mechanical abuse behavior was not sufficiently investigated. A total number of 36 pouch cells were indented with a flat-end cylinder under different boundary conditions until mechanical failure and thermal runaway occurred. The pouch cells were constrained at 30 % SOC with a preload force of 0, 300 or 4000 N and charged to 0 %, 30 %, 60 % or 100 % SOC before indentation. The maximum indentation force, corresponding indentation, initial stiffness and failure behavior indicated a dependency on the preload force. The stiffness at greater indentation was similar for all boundary conditions indicating a pre-compression and flattening of unevenness. Internal stress within the separator resulted in earlier short circuit and mechanical failure for increasing preload force. The mechanical constraint led to increased gas pressure during thermal runaway. The results in this publication give rise to an additional consideration of preload force and boundary conditions imposed by a battery module in abuse testing and simulation approaches in the future.
... An excessive preload force should be avoided as it leads to a greater deterioration in battery life [17,18,37,45,[52][53][54][55][56]. Similarly, a minimal or total absence of preload force also reduces the battery lifetime as it is necessary to maintain contact between the cells in order to prevent their deformation and a delamination of the layers [57][58][59][60][61][62][63][64][65][66][67]. Thus, maintaining the preload force within a certain range is beneficial to the long-term performance of the battery [8,45,53,54,[68][69][70][71][72][73][74][75][76]. ...
Article
Full-text available
The safety of lithium-ion batteries has to be guaranteed over the complete lifetime considering geometry changes caused by reversible and irreversible swellings and degradation mechanisms. An understanding of the pressure distribution and gradients is necessary to optimize battery modules and avoid local degradation bearing the risk of safety-relevant battery changes. In this study, the pressure distribution of two fresh lithium-ion pouch cells was measured with an initial preload force of 300 or 4000 N. Four identical cells were electrochemically aged with a 300 or 4000 N preload force. The irreversible thickness change was measured during aging. After aging, the reversible swelling behavior was investigated to draw conclusions on how the pressure distribution affected the aging behavior. A novel test setup was developed to measure the local cell thickness without contact and with high precision. The results suggested that the applied preload force affected the pressure distribution and pressure gradients on the cell surface. The pressure gradients were found to affect the locality of the irreversible swelling. Positions suffering from large pressure variations and gradients increased strongly in thickness and were affected in terms of their reversible swelling behavior. In particular, the edges of the investigated cells showed a strong thickness increase caused by pressure peaks.
... In addition, system integration can have a significant impact. Examples include uneven and/or defective cell contacts, an improperly designed cooling system or other external thermal influences, and cell bracing [6,7]. Finally, there are also production-related effects that result in divergent aging behavior, especially when mismatched cells are used in the battery pack. ...
Article
Full-text available
This paper presents a detailed correlation index of health indicators for lithium‐ion batteries. Identifying potential correlations of health indicators is of high importance with regard to the cell selection process and in order to minimize the occurring cell‐to‐cell spread within the lifetime. Health indicators that are taken into account are among others impedance measurements of different pulse lengths, capacity values at different discharge procedures and checkups, weight and initial voltage. The work is based on four different aging datasets covering variations in cell chemistry (NMC, LFP, NCA), cell type (round, prismatic), as well as the size and designated application (consumer, automotive). A publicly available dataset was included to allow for an easy reproduction of the results. This article is protected by copyright. All rights reserved.
... An external mechanical pressure applied on the batteries can be beneficial as it, e. g., counteracts contact losses of electrodes with the separator. [9][10][11][12] A method to study the mechanical changes inside a LIB during cycling that has become more popular in recent years is the use of ultrasound. [13][14][15] In this method, an ultrasonic transducer is acoustically coupled to the battery and the transmitted or the reflected signal is recorded and analyzed. ...
Article
Full-text available
The interplay between the internal mechanical properties and external mechanical conditions of a battery cell, e. g., Young's modulus and thickness change, has a crucial impact on the cell performance and lifetime, and thus, needs to be fully understood. In this work, 12 Ah lithium‐ion battery pouch cells were studied during cycling and aging by non‐invasive operando ultrasonic and dilation measurements. The effective Young's modulus increases and the thickness varies the most within a single cycle during the graphite transition from stage 1L to 4, at the beginning of the 2 to 1 stage transition and at the phase transition of the nickel‐rich NCM from H2 to H3. After 1000 cycles of aging, the overall effective Young's modulus of the lithium‐ion battery decreases by ∼11 %–12 % and the cell thickness increases irreversibly by ∼3 %–4 %, which is mostly related to a thicker and possibly softer, more porous solid electrolyte interphase layer.
... [3,4] One possible way to tackle the issue of the lithium dendrite formation during cycling is to apply an external mechanical force to the cell which also results in increased pressure onto the surface of the lithium metal. Mechanical pressure, which is also known to enhance the performance of SoA LIB, [11][12][13][14] may restrict lithium dendrite formation and extend cycle life. [15][16][17][18] The use of carbonate-based electrolytes in combination with lithium metal results in a short cycle life due the poor deposition morphology of lithium in this electrolyte combined with the high chemical reactivity of the carbonate components with Li, particularly at high current rates. ...
Article
Full-text available
Lithium metal is considered as the ‘holy‐grail’ among anode materials for lithium‐ion batteries, but it also has some serious drawbacks such as the formation of dendritic and dead lithium. In this study, the interplay of external pressure and different carbonate‐ and ether‐based electrolytes on the (ir)reversible expansion of lithium metal during cycling against lithium titanate and lithium iron phosphate is studied. In carbonate‐based electrolytes without any additives, lithium metal shows tremendous irreversible expansion and significant capacity reduction at elevated current densities due to the formation of mossy and dead lithium. The addition of fluoroethylene carbonate can reduce irreversible expansion and capacity reduction, especially when a high external pressure is applied. When an ether‐based electrolyte is used, the irreversible dilation of the lithium metal is suppressed when applying increased external pressures. Overall, increased external pressure appears to reduce the formation of mossy and dead lithium and improve the performance.
... To avoid the breathing-induced delamination of electrode layers, pouch and prismatic cells are often braced. However, excessive bracing pressure must be avoided as well, as this can also reduce cell lifespan [16][17][18][19][20]. Also, in cylindrical cells, these forces play https://doi. ...
Article
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Particularly in mobile applications, 18650 lithium-ion batteries can be exposed to mechanical abuse. Deforming mechanical abuse can severely damage the battery case, but sometimes without causing instantaneous cell failure. If such cases remain undetected, the cells may remain in use and pose a potential long-term safety hazard. Due to research gaps in the scientific literature regarding the long-term implications of such damage, the influence of cell design, intrusion depth, as well as the impact on the electrical behavior, two cyclic aging studies and further electrical tests (CC-CV capacity, DVA) were conducted with mechanically deformed 18650 cells; using CT, post-mortem, and SEM for further analysis. In the cyclic aging studies, both cell types tested (with a mandrel and mandrel-free) remained electrically functional and showed no safety-critical behavior, despite intrusions of up to 6 mm. Cells with significant intrusion exhibited increased CC-CV charge capacity for one charging process, and CC-discharge capacity decreases of 3.2 % after mechanical deformation, but no or only slightly accelerated aging rates after the initial capacity drop. DVA indicated a global capacity loss of useable material rather than specific anode or cathode damage. CT analysis revealed case re-deformation after cyclic aging, likely due to jelly roll swelling. Post-mortem analysis showed imprints on all electrode components and active material debonding. SEM analysis revealed changes in cell internal pressure distribution due to external deformation.
... An external mechanical pressure applied on the batteries can be beneficial as it, e. g., counteracts contact losses of electrodes with the separator. [9][10][11][12] A method to study the mechanical changes inside a LIB during cycling that has become more popular in recent years is the use of ultrasound. [13][14][15] In this method, an ultrasonic transducer is acoustically coupled to the battery and the transmitted or the reflected signal is recorded and analyzed. ...
Article
One way to increase the sustainability of lithium-ion batteries (LIB) is to extend the cycle life, e.g., in electric vehicles. To do this, the effects within a LIB must be understood and tracked over its lifetime. In addition to electrical (e.g. IR-drop) and thermal measurements (e.g. temperature inhomogeneities), a new approach for such studies is the use of ultrasound to investigate the mechanical changes inside a LIB [1]. With this low-cost technology, it is possible to detect mechanical degradation (e.g. local thickness change due to lithium plating) or Young’s moduli changes over lifetime and possibly counteract them by the battery management system. In this work a self-built device containing a LIB between two opposing transducers is used to study the impact of state-of-charge (SOC), current rate and frequency on the ultrasound signal inside the LIB. Based on the time the ultrasound signal takes to pass through the LIB (time-of-flight (TOF)), the speed of sound and the Young’s moduli can be determined. The influence of the applied current rate and the frequency of the transducer to the TOF/ speed of sound in the LIB is relatively small. However, the SOC has an impact on the TOF/ speed of sound, where the speed of sound is around 4.5% higher in the fully charged state with ~1740 m s ⁻¹ compared to the discharged state with ~1660 m s ⁻¹ (see Figure 1 (a)). One explanation for this phenomenon is the change in the lithium staging within the graphite anode. In a charged battery, the graphite anode is filled with lithium ions, which enhances the transmission properties of ultrasound. By knowing speed of sound, the Young’s modulus of the whole battery can be estimated as ~4.2 GPa in discharged state and ~5.6 GPa in the charged state (Figure 1 (b)). This significant increase can also be explained with the lithiation stages of the graphite anode, as the graphite particles themselves exhibit a threefold increase of the Young’s modulus during lithiation [2]. Additionally, the evolution of the speed of sound and Young’s moduli is studied during aging of the LIB. References: [1] Gold, L., Bach, T., Virsik, W., Schmitt, A., Müller, J., Staab, T. E., & Sextl, G. (2017). Probing lithium-ion batteries' state-of-charge using ultrasonic transmission–Concept and laboratory testing. Journal of Power Sources, 343, 536-544. [2] Qi, Y., Guo, H., Hector Jr, L. G., & Timmons, A. (2010). Threefold increase in the Young’s modulus of graphite negative electrode during lithium intercalation. Journal of The Electrochemical Society, 157(5), A558. Figure 1
... Appendix 133 Yes No 95% 100% Willenberg et al. 24 No Yes 90% 120% Rahe et al. 78 No No 90% 160% Martinez-Laserna et al. 177 Yes No 85% 170% Klett et al. 87 No No 80% 110% Lewerenz et al. 67,126 Yes Yes 80% 110% Ecker et al. 88 Yes No 80% 150% Frisco et al. 77 No No 80% 200% Schuster et al. 17 Yes No 80% 300% Pfrang et al. 136 No Yes 75% 130% Braco et al. 31 No Yes 70% 200% Broussely et al. 14 No Yes 70% 200% ...
Article
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Lithium-ion batteries can last many years but sometimes exhibit rapid, nonlinear degradation that severely limits battery lifetime. Here, we review prior work on “knees” in lithium-ion battery aging trajectories. We first review definitions for knees and three classes of “internal state trajectories” (termed snowball, hidden, and threshold trajectories) that can cause a knee. We then discuss six knee “pathways”, including lithium plating, electrode saturation, resistance growth, electrolyte and additive depletion, percolation-limited connectivity, and mechanical deformation, some of which have internal state trajectories with signals that are electrochemically undetectable. We also identify key design and usage sensitivities for knees. Finally, we discuss challenges and opportunities for knee modeling and prediction. Our findings illustrate the complexity and subtlety of lithium-ion battery degradation and can aid both academic and industrial efforts to improve battery lifetime.
... Appendix 126 Yes No 95% 100% Willenberg et al. 24 No Yes 90% 120% Rahe et al. 74 No No 90% 160% Martinez-Laserna et al. 170 Yes No 85% 170% Klett et al. 83 No No 80% 110% Lewerenz et al. 63,119 Yes Yes 80% 110% Ecker et al. 84 Yes No 80% 150% Frisco et al. 73 No No 80% 200% Schuster et al. 17 Yes No 80% 300% Pfrang et al. 129 No Yes 75% 130% Braco et al. 30 No Yes 70% 200% Broussely et al. 14 No Yes 70% 200% ...
Preprint
Full-text available
Lithium-ion batteries can last many years but sometimes exhibit rapid, nonlinear degradation that severely limits battery lifetime. In this work, we review prior work on "knees" in lithium-ion battery aging trajectories. We first review definitions for knees and three classes of "internal state trajectories" (termed snowball, hidden, and threshold trajectories) that can cause a knee. We then discuss six knee "pathways", including lithium plating, electrode saturation, resistance growth, electrolyte and additive depletion, percolation-limited connectivity, and mechanical deformation -- some of which have internal state trajectories with signals that are electrochemically undetectable. We also identify key design and usage sensitivities for knees. Finally, we discuss challenges and opportunities for knee modeling and prediction. Our findings illustrate the complexity and subtlety of lithium-ion battery degradation and can aid both academic and industrial efforts to improve battery lifetime.
... Finally, the expansion displacement-SOC curves can be get at different discharge rates, as shown in Fig. 7. Figure 7. Expansion displacement curve By analysing the test data, it can be concluded that the expansion displacement curves under different discharge rates in Fig. 7 have similar regular changes. It can be also seen that the repeatability of the test-bench is good, and the magnitude of the expansion displacement measured by the test-bench is close to the existing literature [5][6][7][8][9]. Therefore, the expansion displacement data measured by the device designed in this paper is reliable. ...
Article
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In order to test the bulk force and expansion displacement of lithium-ion batteries, it is planned to develop a corresponding test-bench, which is mainly composed of a measurement-control system and a mechanical system. To improve the accuracy of the test data, the coupled thermal-structure simulation of the test system in the mechanical system of the test-bench is carried out to select an optimal mechanical structure of the test system. At the same time, for safe and convenient testing, a monitoring-testing system software was developed to ensure the reliability and safety of data collection. Finally, through the test-bench, the battery expansion displacement-SOC curve and the battery bulk force-SOC curve under different discharge rates were tested, providing a basis for the development of a battery management system coupling temperature-current-voltage-displacement-force.
... [1][2][3] Other studies additionally investigated the effect of non-rigid bracing of pouch cells. [4][5][6] The effect of excessive and/or non-uniform pressure, which this paper is concerned with, has also been studied in the literature in various ways. Local lithium plating in cylindrical cells has been suspected to have resulted from irregularities in the jelly roll that led to pressure variations. ...
Article
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Lithium-ion batteries can experience mechanical loads for a variety of reasons, including the rigidity of the cell casing itself, bracing of cell stacks in a module, which is generally due to limited space in the place of installation, or as a result of accidents or abuse. In all of these cases, and exacerbated by faulty manufacturing or assembly, the mechanical loads may be non-uniform across the cell surface. Here, we present an analysis of the effects of such non-uniform mechanical loads on the current density distribution during charging and show that they can provoke localized lithium plating. Pressure-compression relationships of individual cell components were determined experimentally and implemented into a pseudo-3D axisymmetric electrochemical-mechanical cell model of a 2.1 Ah pouch cell by Kokam, South Korea. The modeling results were successfully validated by comparison to a post-mortem evaluation of pouch cells that were cycled while being locally compressed.
... This is particularly important for automotive applications since battery packs are usually designed with several stacked cells, which means that due to irreversible expansion, the pressure on the cells can increase significantly and adversely impact the performance of the cells. 16,17 The evolution of the irreversible expansion also shows a strong dependency on the cycling conditions, [18][19][20] further reinforcing the idea of the coupling between electrochemical and mechanical aging processes. ...
Article
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Lithium-ion batteries cell thickness changes as they degrade. These changes in thickness consist of a reversible intercalation-induced expansion and an irreversible expansion. In this work, we study the cell expansion evolution under variety of conditions such as temperature, charging rate, depth of discharge, and pressure. A specialized fixture was used to keep the cells at a constant pressure during cycling, while measuring the thickness change both within a cycle and the cumulative growth over many cycles. The changes in positive and negative electrode capacity and stoichiometric range can be diagnosed from the evolution of the reversible expansion. The changes in the reversible expansion if combined with the voltage, lead to a higher-confidence estimation of cell health parameters important for lifetime prediction and adaptive battery management such as asymmetric charge/discharge power limits. This study raises the importance of monitoring the expansion for enabling advanced and more-informed health diagnostics of lithium-ion batteries.
... Third, the longer tail of the curves implied a slower diffusion of lithium atoms in the electrode [43]. Fourth, the negative value in (-Z im ) in the high frequency region, revealing the inductive information of the cell [44], also moved to the right. But we are unclear about what such inductance changes meant. ...
Article
Heterogeneous degradation is a key challenge faced in the production of large format lithium-ion battery (LIB) cells, and is difficult to evaluate non-destructively. This study demonstrates that reversible strain heterogeneity has the potential of becoming a useful non-destructive tool for local degradation analysis of large format LIB cells. A commercial 59.5 Ah LIB cell with a Li[Ni0.6Co0.2Mn0.2]O2 (NCM622) cathode and graphite anode was degraded at 1.3C current with and without external constraint. The aged unconstrained cells experienced a sudden capacity drop and abnormal expansion at certain locations during discharging at 87% SOH (state of health), which was not observed for the constrained ones. Detailed post-mortem analysis was carried out to understand the capacity drop mechanism. The abnormal expansion was ascribed to the gas bubbles produced by localized severe side reactions between the graphite particles and electrolyte, and the significant heat in certain regions with high impedance. The quick spread of the defective regions was responsible for the sudden capacity drop. This work confirms that the reversible strain distribution contains useful information inside the battery and can help monitor battery degradation and capacity drop.
... Fig. 1 clearly illustrates the designed setup where six bolts hold the plates and the cell on a Plexi-tray with an adjusted low torque of 0.4Nm. This constrained frame rigidly supports the cell which can result slightly in improved performance [32,33] and can support in terms of safety as well. For electrical attachment, dedicated low resistive metal blocks were used to ensure a strong connection. ...
Article
Battery lifetime modeling and prediction of precise capacity degradation for real-life applications are critical to understanding the complex and non-linear battery behavior. However, the application of accurate and robust aging models on dynamic on-road scenarios is still a challenge. In this work, a comprehensive aging dataset of 40 Nickel Manganese Cobalt (NMC) cells is generated for two years considering distinct relaxation phases in the function of the state of charge (SoC), temperature, and time. A qualitative analysis of the diversified aging parameters along with the sensitivity analysis of the rest criteria is conducted. Taking the discharge capacity as the pivotal predictor, a robust training dataset is built and preliminary fed to common data-driven models. Among them, the Gaussian process regression (GPR) is identified to be the best suit with which a 0.02% root-mean-squared error (RMSE) can be achieved for battery life prediction when tested with a static profile choosing an exponential kernel. Further, to demonstrate a real-life scenario, a worldwide harmonized light-duty test cycle (WLTC) is performed, and the capacity fade percentile can be predicted accurately with a 0.05% RMSE. This research shows that data-driven algorithms like GPR can be a promising online tool that can forecast the entire lifetime with high precision for dynamic profiles.
... The pressure is 3.5 times higher for the plunger with the step than for the flat plunger due to the smaller area (72 to 254 mm 2 ). The selection of the external forces was based on previous studies [29,[40][41][42] and the limitations of the spring (i.e. maximum load), as only one spring was used during the experiments. ...
Article
The demand for higher energy densities in lithium-ion batteries leads to an increased utilization of the space within the confinements of the cell housing for the electrodes, resulting in increased electrochemical/mechanical interactions and stress inside the cells. In this study, the correlating effects of externally-induced mechanical stress and dilation of the electrodes on the performance of LIBs were investigated using an operando three-electrode dilation cell. The results demonstrate that most of the initial irreversible dilation in a graphite/LiNi0.33Co0.33Mn0.33O2 (NCM111) cell occurs during the initial lithium intercalation at the anode during the transition from stage 2 to 1, i.e. LiC12 to LiC6, due to SEI formation, particle rearrangement and graphene layer spacing. Moreover, high applied pressure, which leads to a reduction of the porosity inside the separator and therefore increased ionic transport resistance, tortuosity and overpotential, results in a hastened degradation mainly at graphite anode. These effects are more pronounced with an inhomogeneous pressure distribution and lithium plating is identified as the main cause of degradation. This dilation cell is a powerful tool for studying the electrode/active material expansion and electrochemical/mechanical effects for a homogeneous and inhomogeneous pressure distribution.
... With regard to cell ageing, EIS has been widely used to study degradation and degradation mechanisms of commercial LIBs during cycling and calendar ageing [70,75,90,98,122,129,[136][137][138][139][140][141][142][143][144][145][146][147] and to understand the impact of different cycling/storage conditions, including temperature [79,80,127,[148][149][150][151][152][153][154][155], rate of cell charge/discharge (C-rate) [81,125,[156][157][158][159][160][161][162], charging protocol [100,[163][164][165], overcharge/discharge [82,[166][167][168], SOC/depth of discharge (DOD) [169][170][171][172][173] and other storage/operating conditions [174][175][176][177][178][179][180][181][182][183][184][185][186], as well as manufacturing factors such as electrode misalignment [54], time required for cell formation [187,188], cell bracing [189], and application of external pressure [190]. The information gained from these studies is clearly relevant for several aspects, including design and development of cells and materials, battery management and control strategies, and extrapolation of accelerated ageing tests to real-world conditions. ...
Article
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Electrochemical impedance spectroscopy (EIS) is a widely applied non-destructive method of characterisation of Li-ion batteries. Despite its ease of application, there are inherent challenges in ensuring the quality and reproducibility of the measurement, as well as reliable interpretation and validation of impedance data. Here, we present a focus review summarising best metrological practice in the application of EIS to commercial Li-ion cells. State-of-the-art methods of EIS interpretation and validation are also reported and examined to high-light the benefits and drawbacks of the technique.
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High-voltage batteries used in electric vehicles use hundreds or thousands of battery cells. Because a large number of battery cells are used, installing each one into a battery pack causes many difficulties in production. Therefore, traditionally, multiple battery cells are composed of several battery modules and then assembled into a battery pack. However, recently, Cell-to-Pack (CTP) technology that configures battery cells directly into a battery pack is being developed to increase energy density of a battery pack. This is because parts needed for battery modules can be removed, which can have various advantages. Because modules are eliminated in CTP technology, the method of installing battery cells in a battery pack will also be modified and the effect battery cells have on the stiffness of a battery pack will also change. In this study, the differences in stiffness of battery packs based on CTP technology developed for various battery cell types are analyzed. In particular, battery packs with CTP technology are generated based on pouch-type battery cells and prismatic battery cells and how each type of battery cell changes the stiffness of a battery pack is analyzed.
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Lithium‐ion batteries (LIBs), as efficient electrochemical energy storage devices, have been successfully commercialized. Lithium plating at anodes has been attracting increasing attention as batteries advance toward high energy density and large size, given its pivotal role in affecting the lifespan, safety, and fast‐charging performance of LIBs. Lithium plating mostly happens during fast charging or charging at low temperatures. However, external pressure is often overlooked as an essential factor that influences lithium plating in LIBs. This review analyzes and discusses the influence of external pressure on performance for commercial LIBs, with a particular focus on lithium plating. Recent advances in this topic, including experimental results and mechanism analyses, are reviewed. Lithium plating is explored by examining the influence of pressure on the internal morphology and electrochemical behavior of batteries. It is emphasized that external pressure affects performance through ion transport, electron transport, and their heterogeneities, thereby increasing the risk of lithium plating in batteries. Subsequently, the rationale for external pressure mitigating lithium plating is elucidated from the perspective of the morphology optimization inside LIBs. Overall, this review provides valuable insights into the role of external pressure on lithium plating in commercial LIBs, practically guiding their rational design and development.
Article
Efficient cell packaging is crucial to increase the battery energy density and the driving range of modern electric vehicles. However, mechanical compression of the cells during pack assembly has a significant impact on cycle life and cell swelling of the cells. Therefore, this study follows the research question how lithium-ion (li-ion) pouch cells should be integrated, and at which pressure level, in order to optimize cycle life and to reduce irreversible swelling. A high precision compression test bench was utilized to cycle 5 Ah MC622/graphite li-ion pouch cells at 0.075MPa, 0.2MPa, 0.5MPa, 1.0MPa, and 1.75MPa flexible compression at constant pressure and rigid compression at constant cell thickness, respectively. To obtain transferable results, a high energy (HE) and a high power (HP) cell configuration were investigated. The cells were stressed with individual multi-step constant current (MSCC) charging profiles for each cell type, representing electric vehicle (EV) usage profiles with frequent fast charging. After cycling, post-mortem analysis was conducted at 80% capacity retention to evaluate individual electrode aging. In comparison between flexible compression and rigid compression, flexible compression achieved on average 19% increase in cycle life until 80% capacity retention. It was observed that compression could reduce deterioration of electrical contact in the cathode. Simultaneously, higher compression increased the overpotential on the anode during charging and thereby increased the risk of lithium plating. Ranging from 0.2MPa to 0.5MPa flexible compression, cycle life of the HE cells increased by up to 25% in comparison to the lowest compression. In addition, cells reached around double equivalent full cycles (EFC) before reaching 150% DCIR. The irreversible swelling, normalized to the uncompressed cell thickness at begin of life (BoL), was reduced from around 3.2% at the lowest compression to 2.1% at moderate compression. As a result, flexible compression between 0.2MPa to 0.5MPa is recommended for both cell types as a trade-off optimizing capacity fade, direct current internal resistance (DCIR) increase, and irreversible swelling during cycle life.
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Li-ion battery module systems, which are utilized to power electric vehicles (EVs), consist of a collection of battery cells that generate the necessary electrical energy and a structure designed to protect the battery system's interior components. However, a swelling effect, which results from internal composition changes that arise due to electrochemical reactions, causes the volume of the battery cells to change as the battery status changes (e.g., state of charge (SOC) and degradation). This volumetric change results in stack pressure evolution within the battery module, which leads to structural deformation and, eventually, failure of the module system. The objective of this research is to develop a method to reliably identify the stack pressure in the module level, considering the uncertainty in the system. A phenomenological model is employed to simply and statistically represent the complex mechanical behavior of the battery cells. An equivalent mechanical model is implemented to estimate the stack pressure within the module system. This paper presents three case studies that examine and compare the distributions of the stack pressure under different designs, utilizing the proposed method. The results of these case studies highlight the importance of uncertainty analysis in the design process to ensure robustness and reliability while maintaining fixed costs.
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Precise prediction of lithium‐ion cell level aging under various operating conditions is an imperative but challenging part of ensuring the quality performance of emerging applications such as electric vehicles and stationary energy storage systems. Accurate and real‐time battery‐aging prediction models, which require an exact understanding of the degradation mechanisms of battery components and materials, could in turn provide new insights for materials and battery basic research. Furthermore, the primary barrier to meaningful artificial intelligence/machine learning for accelerating the prediction period is the exploitation of accurate aging mechanistic descriptors. This review comprehensively summarizes the evolution of deterioration mechanisms at the material and cell level in different environments and usage scenarios, including the intricate relationships between aging mechanisms, degradation modes, and external influences, which are the cornerstones of modeling simulation and machine learning techniques. Recent advances in electrochemical models coupled with internal battery degradation mechanisms as well as identification and tracking of aging parameters are shown, with particular emphasis on electrode balance and the anticipated trend of machine learning‐assisted reliable remaining useful life prediction. Precise simulation prediction of cell level aging will continue to play an essential role in advanced smart battery research and management, enhancing its performance while shortening experimental sequences.
Preprint
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Lithium (Li) metal battery technology has attracted world-wide attention because of its high energy density, but its practical application is hindered by several challenges, with one significant issue being the large volume change and cell swelling. While external pressure is known to have a profound effect on cell performance, there are currently no reports exploring the relationship between external pressure and the electroplating of Li+ in large-format pouch cells to enhance overall performance. Here we investigate the influence of externally applied pressure on the electroplating and stripping of lithium metal in 350 Wh/kg pouch cells. A hybrid constant gap and constant pressure design is designed to apply a minimal external pressure for practical application. The self-generated pressures are monitored and quantified which are further correlated to the observed charge-discharge processes. A two-stage cycling process is observed. In the first stage, Li+ ions utilized are mainly supplied by the cathode which shuttle between the cathode and anode with minimal Li loss which minimizes cell swelling but only happens when pressure is applied appropriately. In the second stage, Li from the Li foil anode participates in the reaction and the thickness of the anode gradually increases. However, even after extensive cycling, cell swelling remains less than 10%, comparable to that of state-of-the-art Li-ion batteries. In addition, the pressure distribution along the horizontal direction across the surface of Li metal pouch cell reveals a complex behavior of Li+ migration during the electroplating (charge) process. The external pressure encourages a preferred plating process of Li in the central region, necessitating the development of new strategies to address uneven Li plating and utilization to advance Li metal battery technology.
Article
In application, lithium-ion cells undergo expansion during cycling. The mechanical behavior and the impact of external stress on lithium-ion battery are important in vehicle application. In this work, 18 Ah high power commercial cell with LiNi0.5Co0.2Mn0.3O2/Graphite electrode were adopted. A commercial compress machine was applied to monitor the mechanical characteristics under different SOC, lifetime and initial external force. The dynamic and steady force was obtained and the results show that the dynamic force increases as the SOC increasing, obviously. During the lifetime with HPDM (high power driving mode), different external force is shown to have a great impact on the long-term cell performance, with higher stresses result in higher capacity decay rates and faster impedance increases. A proper initial external force (900 N) provides lower impedance increasing. Postmortem analysis of the cells with 2000 N initial force suggests a close correlation between electrochemistry and mechanics in which higher initial force leads to higher DCIR increase rate. In addition, for the cell with higher external force, deformation of the cathode and thicker SEI film on the surface of anode and separator are observed. Porosity reduction and closure was also verified after cycles which is an obstacle to the lithium ion transferring. The largest cause of the observed capacity decline was the loss of active lithium through autopsy analysis.
Chapter
Among the various configurations available for lithium-ion cells, the pouch type has been grabbing attention because of its high energy density, design flexibility, low cost and lightweight. Such a pouch pack enables further reduction in the size and weight of portable electronic devices where they power and for the same reason the pouch cells are not only suitable for terrestrial applications but also attractive for space applications too. Lithium-ion pouch cells have been successfully used in many applications including space. However, this design has certain limitations. The poor rigidity of the pouch case makes them more susceptible to external mechanical damage and swelling under elevated temperature and overcharging. Therefore, the manufacturers are constantly striving to improve the performance of pouch cells. This chapter provides a brief overview about the different aspects of lithium ion pouch cells and the various strategies introduced in upgrading the performance of this thin design.KeywordsPouch cellComposite trilayerTab location
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Lithium metal is considered as an ideal substitute to low‐capacity carbon anodes for rechargeable lithium‐ion batteries (LIBs) given its ultra‐high theoretical specific capacity of 3860 mAh g⁻¹ and the lowest electrochemical potential. However, safety issues stem from the uncontrollable formation and growth of lithium dendrites, which severely plague the practical application of the Li anode. Here, a multi‐functional protection layer, prepared by a one‐step spin‐coating of CuCl2 N‐Methyl‐2‐Pyrrolidone (NMP) solution on a lithium metal surface is constructed. The as‐prepared protective layer has a variable porous morphology that consists of a conductive lithium‐copper alloy and electrochemically active CuCl, which proactively facilitates the homogeneous diffusion of Li‐ions, the elimination of random dendrite nuclei, and even distribution of charge at the Li anode surface, and subsequently the uniform deposition of Li⁺ ions. Under such a dynamic protection mechanism, the proposed CuCl2 modified Li electrode can stably cycle more than 1500 h at a current density of 1 mA cm⁻² paired with lithium foil. Moreover, the assembled full battery achieves capacity retention of 85.6% even after 2000 cycles at a high rate of 5 C with a LiFePO4 cathode. This as‐proposed dynamic protection mechanism via the incorporation of the electrochemically active component could provide new guidance in the preparation of the safe and high‐performance lithium metal anodes.
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To reduce the ecological footprint and to increase the lifetime of lithium‐ion batteries (LIBs), it is necessary to understand aging phenomena inside the cells during cycling. In this study, the positive effect of external pressure through bracing the cells on aging is investigated for automotive battery cells with more than 7000 cycles. After cycling, the aged cells are studied by using post‐mortem analysis. It is shown that bracing does not affect the anode and cathode in the same manner. A lack of external pressure results in lithium plating due to contact losses on the anode. Such a loss of lithium inventory plays only a small role in the braced cells. However, the structural and morphological degradation, such as particle cracking at the cathode, is significant. Half‐cell tests of aged and unaged anode samples extracted from the automotive cells confirm the post‐mortem findings, where only minimal differences can be seen for the braced cell. In contrast, the aged cathodes from braced cells demonstrate substantial capacity fade in half‐cell measurements as compared to the cathodes extracted from the unbraced cell. Finally, a new concept of the mechanical state of health (mechanical SOH) is introduced to correlate mechanical effects with electrode degradation.
Article
In application, lithium-ion pouch-format cells undergo expansion during cycling. To ensure the structure integrity of battery pack, preload force and fixed space were performed during pack assembly. In this paper, the swelling force of LiNi0.8Co0.1Mn0.1O2(NCM811)/Graphite large pouch cells is investigated as a function of the different preload force, state of charge (SOC), cycle number and graphite electrode kinds. The swelling force shows a significant dependency on the state of charge, but not follow linear characteristics due to NCM811’s volume extraction at high SOC. The higher SOC, the higher swelling force. 1500–1700 N is increasing from 50% to 100% SOC under 3000 N preload force. Three cells with different electrode design were cycled 1000 times, and each design with different preload force (1000, 3000 and 5000 N). The cell with 3000 N preload force shows better capacity retention for all electrode cells, the capacity retention of three different electrode cell is 93.7%, 90.9% and 91.2%, respectively. The corresponding swelling force increase fast at initial 300 cycles due to multi-factors. After 300 cycles, the swelling force increasing shows linear characteristics. After 1000 cycles, the cell with natural graphite (NG) has much larger swelling force than the cell with artificial graphite (AG), 9030 vs. 6370 N under 3000 N preload force. The functional binder polyacrylic acid (PAA) with -COOH unit and mixed with carboxymethylcellulose sodium (CMC) can form cross-link net at the graphite surface, supplying higher adhesive strength and restraining the electrode expansion. The adhesive strength increased from 7.2 to 20 N/m, corresponding the anode thickness expansion ratio from 25% reduced to 15%. The swelling force reduced from 9030 N to 6930 N, near to the cell with AG+SBR design. Delta voltage (∆V) and average discharge voltage evolution during cycling were also investigated of the different electrode cells. From the study, it can be useful for mechanical design, battery management of module and pack which can be beneficial for long life, safety and reliability of electric vehicle (EV) products.
Article
The effects of automotive-related lithium-ion module design, i.e. module stiffness and initial compression during module assembly on cell aging, swelling and pressure evolution are still largely unknown. This paper presents the results of a long-term aging study of 12 large-format automotive graphite/NMC 622 pouch cells, cycled for different module stiffnesses and initial compressions using design of experiments. Statistical analysis of mechanical and aging data revealed significant nonlinear (interaction) effects of both factors on pressure evolution, capacity loss and increase in internal resistance of the cells. Pressure dependent cell aging is observed over 1000 cycles, which was related to loss of active material at the cathode from differential voltage analysis. Post-mortem analysis confirmed a cathode active material loss via half- and full-cell measurements of harvested electrodes. Cross-section SEM micrographs revealed increasing NMC-particle cracking with higher pressure. Based on this, a fatigue-based aging model was developed to describe the capacity loss due to pressure dependent particle cracking. The presented approach enables both improved modeling of pressure dependent aging and lifetime optimized module design
Technical Report
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News on research activities, publications and events from the Institute of Power Electronics and Electrical Drives (ISEA) at RWTH Aachen University
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Lithium-ion pouch cells with lithium titanate (Li4Ti5O12, LTO) anode and lithium nickel cobalt aluminum oxide (LiNi0.8Co0.15Al0.05O2, NCA) cathode were investigated experimentally with respect to their electrical (0.1C…4C), thermal (5 °C…50 °C) and long-time cycling behavior. The 16 Ah cell exhibits an asymmetric charge/discharge behavior which leads to a strong capacity-rate effect, as well as a significantly temperature-dependent capacity (0.37 Ah ∙ K−1) which expresses as additional high-temperature feature in the differential voltage plot. The cell was cycled for 10,000 cycles inbetween the nominal voltage limits (1.7–2.7 V) with a symmetric 4C constant-current charge/discharge protocol, corresponding to approx. 3400 equivalent full cycles. A small (0.192 mΩ/1000 cycles) but continuous increase of internal resistance was observed. Using electrochemical impedance spectroscopy (EIS), this could be identified to be caused by the NCA cathode, while the LTO anode showed only minor changes during cycling. The temperature-corrected capacity during 4C cycling exhibited a decrease of 1.28%/1000 cycles. The 1C discharge capacity faded by only 4.0% for CC discharge and 2.3% for CCCV discharge after 10,000 cycles. The cell thus exhibits very good internal-resistance stability and excellent capacity retention even under harsh (4C continuous) cycling, demonstrating the excellent stability of LTO as anode material.
Article
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The power capability of a lithium ion battery is governed by its resistance, which changes with battery state such as temperature, state of charge, and state of health. Characterizing resistance, therefore, is integral in defining battery operational boundaries, estimating its performance and tracking its state of health. There are many techniques that have been employed for estimating the resistance of a battery, these include: using DC pulse current signals such as pulse power tests or Hybrid Pulse Power Characterization (HPPC) tests; using AC current signals, i.e., electrochemical impedance spectroscopy (EIS) and using pulse-multisine measurements. From existing literature, these techniques are perceived to yield differing values of resistance. In this work, we apply these techniques to 20 Ah LiFePO4/C6 pouch cells and use the results to compare the techniques. The results indicate that the computed resistance is strongly dependent on the timescales of the technique employed and that when timescales match, the resistances derived via different techniques align. Furthermore, given that EIS is a perturbative characterisation technique, employing a spectrum of perturbation frequencies, we show that the resistance estimated from any technique can be identified - to a high level of confidence - from EIS by matching their timescales.
Article
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The lifetime and safety of lithium-ion batteries are key requirements for successful market introduction of electro mobility as well as for the application in stationary use. Lithium plating, leading both to ageing as well as safety risks, is known to play a crucial role in system design of the application. To gain knowledge of different influence factors on lithium plating, low temperature ageing tests are performed in this work. Lithium-ion batteries of various types are tested under various operation conditions such as temperature, current, state of charge, charging strategy as well as state of health. To analyse the ageing behaviour capacity fade and resistance increase are tracked over lifetime. To further investigate the underlying degradation mechanisms, differential voltage curves and impedance spectra are analysed as well as a post-mortem analysis of anode degradation is performed for a selected technology. The results confirm the deposition of metallic lithium or lithium compounds in the porous structure and suggest a strongly inhomogeneous deposition over the electrode thickness with a dense deposition layer close to the separator for the considered cell. The plurality of the investigated technologies demonstrates large differences between different technologies concerning low temperature behaviour and gives insight to the impact of cell properties. For the investigated technologies no correlation is observed between the ageing behaviour and cell size or cathode material. However, cells rated to provide high power are found to be subject to faster degradation at low temperatures compared to high energy cells. For application this result shows that cells designed for high current rates are not necessarily providing a good low temperature performance. 2
Article
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This paper describes the use of a frequency domain, finite-difference scheme to simulate the impedance spectra of diffusion in porous microstructures. Both open and closed systems are investigated for a range of ideal geometries, as well as some randomly generated synthetic volumes and tomographically derived microstructural data. In many cases, the spectra deviate significantly from the conventional Warburg-type elements typically used to represent diffusion in equivalent circuit analysis. A key finding is that certain microstructures show multiple peaks in the complex plane, which may be misinterpreted as separate electrochemical processes in real impedance data. This is relevant to battery electrode design as the techniques for nano-scale fabrication become more widespread. This simulation tool is provided as an open-source MatLab application and is freely available online as part of the TauFactor platform.
Article
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Degradation of Lithium-ion batteries is a complex process that is caused by a variety of mechanisms. For simplicity, ageing mechanisms are often grouped into three degradation modes (DMs): conductivity loss (CL), loss of active material (LAM) and loss of lithium inventory (LLI). State of Health (SoH) is typically the parameter used by the Battery Management System (BMS) to quantify battery degradation based on the decrease in capacity and the increase in resistance. However, the definition of SoH within a BMS does not currently include an indication of the underlying DMs causing the degradation. Previous studies have analysed the effects of the DMs using incremental capacity and differential voltage (IC-DV) and electrochemical impedance spectroscopy (EIS). The aim of this study is to compare IC-DV and EIS on the same data set to evaluate if both techniques provide similar insights into the causes of battery degradation. For an experimental case of parallelized cells aged differently, the effects due to LAM and LLI were found to be the most pertinent, outlining that both techniques are correlated. This approach can be further implemented within a BMS to quantify the causes of battery ageing which would support battery lifetime control strategies and future battery designs.
Article
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The evaluation of floating currents is a powerful method to characterize capacity fade induced by calendaric aging and enables a highly resolved representation of the Arrhenius relation. The test arrangement is simple and could constitute a cheap alternative to state-of-the-art calendaric aging tests including checkup tests. Therefore the currents to maintain a constant voltage are evaluated. This method is validated by analyzing nine cylindrical 8 Ah LiFePO 4 |Graphite battery cells during calendaric aging at 25 °C, 40 °C and 60 °C at 3.6 V (100% SOC). The 3.6 V are kept by applying constant voltage while the floating currents are logged. The floating currents correlate with the rate of capacity loss measured during capacity tests. The floating currents reveal to be rather constant at 25 °C, linearly increasing at 40 °C and decreasing from a higher level at 60 °C. Additional tests with three test cells, with the temperature rising from 40 to 60 °C in steps of 5 K, exhibit non-constant currents starting from 50 °C on with high variations amongst the tested cells. Once stored above 50 °C, the cells exhibit increased floating currents compared to the measurement at the same temperature before exceeding 50 °C.
Article
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The formation of surface films on lithium ion electrodes is a crucial factor for the performance and durability of the respective battery. Especially the formation of the solid electrolyte interphase (SEI) on the anodes of these cells widely determines the stability and functionality of the electrode. Therefore, a precise insight into the formation process of this layer is required. Based on temperature-dependent electrochemical impedance spectroscopy a new approach to monitor the formation of the SEI was developed. This way the kinetics of the interphase could be described using its activation energy. Comparison of these values with the respective resistances regarding the charge turnover during the initial charging of the cell provided additional information about the course of the SEI formation. It could be shown that these findings are in good agreement with the descriptions of the mechanism of the SEI formation provided by the literature.
Chapter
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Electrochemical impedance spectroscopy has become a mature and well-understood technique. It is now possible to acquire, validate, and quantitatively interpret the experimental impedances. This chapter has been addressed to understanding the fundamental processes of diffusion and faradaic reaction at electrodes. However, the most difficult problem in EIS is modeling the electrode processes, which is where most of the problems and errors arise. There is an almost infinite variety of different reactions and interfaces that can be studied (corrosion, coatings, conducting polymers, batteries and fuel cells, semiconductors, electrocatalytic reactions, chemical reactions coupled with faradaic processes, etc.) and the main effort is now being applied to understanding and analyzing these processes. These applications will be the subject of a second review in a forthcoming volume in this series.
Article
Single lithium-ion cells within electric vehicles’ battery packs generally show variations in capacity and impedance due to the manufacturing process as well as operational conditions. Therefore, cells connected in parallel experience different dynamic loads during vehicle operation, which may potentially result in uneven and accelerated aging behavior. However, in literature only little is mentioned about the different reasons for parameter variations within single cells of parallel connections as well as their magnitude in real-life conditions. In this work, capacity and impedance variations within parallel-connected cells are investigated theoretically and are quantified exemplary by a batch of new cylindrical 18650 cells as well as an retired BEV battery pack with a 2p96s configuration of prismatic cells. Furthermore, the development of existing parameter variations along cycling are analyzed for two modules of the battery pack. It is demonstrated, that the aged cells show a strong increased parameter spread compared to the new cells. During further aging, the existing capacities spread of the block and especially the state of inhomogeneity of parallel couples increases. Hence, the widespread theory of a self-balancing effect inside a parallel connection, which leads to a convergence of the cells’ SOH, is disproved.
Article
The effects of extended charge/discharge cycling on the morphology of the jelly roll of commercial 18650 lithium-ion battery cells (Sanyo UR18650E) are shown and discussed. Using micro X-ray computed tomography combined with post-mortem analysis it is shown that the jelly roll exhibits significant deformations after charge/discharge cycling. This effect appears despite the presence of a solid inner pin, which had been suggested in literature as prohibitive for jelly roll deformation. The effect is related to an nhomogeneous architecture of the cells caused mainly by the cathode tab in the vicinity of which deformations were observed most often. While it is shown here that such deformations eventually lead to delamination of the otherwise rather stable cathode coating and cause a rapid capacity fade, jelly roll deformation has also been observed after more than 1700 equivalent full cycles on cells which show a typical, non-accelerated ageing behavior for this cell type. A correlation between occurrence of delamination and occurrence of deformation was clearly identified by combining post-mortem and computed tomography analysis. It is discussed, if the well-documented deformation is caused by thickness variations of the anode and cathode during charge/discharge cycles. Furthermore, an in-depth characterization of the cell design is documented.
Article
The effects of external compression on the performance and ageing of NMC(1/3)/Graphite single-layer Li-ion pouch cells are investigated using a spring-loaded fixture. The influence of pressure (0.66, 0.99, 1.32, and 1.98 MPa) on impedance is characterized in fresh cells that are subsequently cycled at the given pressure levels. The aged cells are analyzed for capacity fade and impedance rise at the cell and electrode level. The effect of pressure distribution that may occur in large-format cells or in a battery pack is simulated using parallel connected cells. The results show that the kinetic and mass transport resistance increases with pressure in a fresh cell. An optimum pressure around 1.3 MPa is shown to be beneficial to reduce cyclable-lithium loss during cycling. The minor active mass losses observed in the electrodes are independent of the ageing pressure, whereas ageing pressure affects the charge transfer resistance of both NMC and graphite electrodes and the ohmic resistance of the cell. Pressure distribution induces current distribution but the enhanced current throughput at lower pressures cell does not accelerate its ageing. Conclusions from this work can explain some of the discrepancies in non-uniform ageing reported in the literature and indicate coupling between electrochemistry and mechanics.
Article
This paper presents a novel lithium-ion cell model, which simulates the current voltage characteristic as a function of state of charge (0%–100%) and temperature (0–30 °C). It predicts the cell voltage at each operating point by calculating the total overvoltage from the individual contributions of (i) the ohmic loss η0, (ii) the charge transfer loss of the cathode ηCT,C, (iii) the charge transfer loss and the solid electrolyte interface loss of the anode ηSEI/CT,A, and (iv) the solid state and electrolyte diffusion loss ηDiff,A/C/E. This approach is based on a physically meaningful equivalent circuit model, which is parametrized by electrochemical impedance spectroscopy and time domain measurements, covering a wide frequency range from MHz to μHz. The model is exemplarily parametrized to a commercial, high-power 350 mAh graphite/LiNiCoAlO2-LiCoO2 pouch cell and validated by continuous discharge and charge curves at varying temperature. For the first time, the physical background of the model allows the operator to draw conclusions about the performance-limiting factor at various operating conditions. Not only can the model help to choose application-optimized cell characteristics, but it can also support the battery management system when taking corrective actions during operation.
Article
Lithium-ion cells can unintentionally be exposed to temperatures outside manufacturers recommended limits without triggering a full thermal runaway event. The question addressed in this paper is: Are these cells still safe to use? In this study, externally applied compression has been employed to prevent lithium ion battery failure during such events. Commercially available cells with Nickel Cobalt Manganese (NCM) cathodes were exposed to temperatures at 80 °C, 90 °C and 100 °C for 10 h, and electrochemically characterised before and after heating. The electrode stack structures were also examined using x-ray computed tomography (CT), and post-mortems were conducted to examine the electrode stack structure and surface changes. The results show that compression reduces capacity loss by −0.07%, 4.95% and 13.10% respectively, measured immediately after the thermal testing. The uncompressed cells at 80 °C showed no swelling, whilst 90 °C and 100 °C showed significant swelling. The X-ray CT showed that the uncompressed cell at 100 °C suffered de-lamination at multiple locations after test, and precipitations were found on the electrode surface. The post-mortem results indicates the compressed cell at 100 °C was kept tightly packed, and the electrode surface was uniform. The conclusion is that externally applied compression reduces delamination due to gas generation during high temperature excursions.
Article
The successful development of electrified vehicles is a key factor in the transition to a more environmentally friendly transportation sector. Li-ion batteries, which are today’s choice to power electrified vehicles, have to fulfill more stringent requirements in terms of ageing and need advanced tools to study the interfaces evolution upon cycling. This work is thus focused on understanding the impedance behavior of a commercial graphite-based negative electrode, which is used in a Li-ion battery designed for such vehicles. 3-electrode pouch cells were assembled with such negative electrode, a LMO-layered oxide-based positive electrode, a Celgard® type separator soaked with a carbonate solvents-LiPF6 mixture electrolyte and a LTO-based electrode as reference. Electrochemical Impedance Spectroscopy measurements were performed at different cell states of charge and ageing times. The impedance of the graphite-based anode is analyzed for first time with de Levie’s equation for porous electrodes. The analysis is supported by designed SEI layer formation experiments with vinylene carbonate and vinylene ethyl carbonate additives. The high frequency domain of the interfacial kinetic loop reflects porosity effects and the graphite particles − composite matrix electric tranfer. The SEI layer and charge transfer phenomena are reflected in the medium and medium to low frequency domains respectively, and their impedance contributions depend on the Li content of the graphite particles. Upon ageing, the interfacial impedance of the graphite-based electrode should increase due to SEI layer growing. However, from 100% to 80% of battery capacity retention, the impedance decreases. Our analysis backed by post-mortem characterizations allows to assign this unexpected behavior to porosity rise and slight Mn-contamination of the SEI layer.
Article
This paper presents an overview on charging strategies for lithium-ion batteries. Moreover, a detailed assessment of charging strategies is performed, based on an extensive experimental study with three different cell types.The experimental results reveal that the impact of charging currents and charging voltages on cycle life can vary markedly among different lithium-ion batteries. In general, the cycle life is influenced more by high charging currents than by high discharging currents. Different boost charging protocols have disclosed that high charging currents can deteriorate cycle life not only at high state of charge (SoC), but also at very low SoC. Our investigations on pulse charging show that lithium-ion cells withstand charging pulses of high current or high voltage without any deterioration in cycle life, when the duration of the pulses remains short and the mean current and voltage values are considerably lower. For pulses of less than 1 s, cycle life has been similar for pulsed and continuous charging with the same mean charging currents and identical cycle depths. This paper also presents the impact of charging currents and charging voltages on capacity utilization, charging time, and efficiency to support the development process of optimized charging protocols for practical applications.
Article
In this book, a new procedure to analyze lithium-ion cells is introduced. The cells are disassembled to analyze their components in experimental cell housings. Then, Electrochemical Impedance Spectroscopy, time domain measurements and the Distribution function of Relaxation Times are applied to obtain a deep understanding of the relevant loss processes. This procedure yields a notable surplus of information about the electrode contributions to the overall internal resistance of the cell. © 2014 Karlsruher Institut fur Technologie (KIT). All rights reserved.
Article
Lithium iron phosphate is a promising candidate material for Li-Ion batteries. In this study, the rate determining processes are assessed in more detail in order to separate performance limiting factors. Electrochemical impedance spectroscopy (EIS) data of experimental LiFePO4/Lithium-cells are deconvoluted by the method of distribution of relaxation times (DRT), what necessitates a pre-processing of the capacitive branch. This results in a separation into cathode and anode polarization processes and in a proposition of a physically motivated equivalent circuit model. We identify three different polarization processes of the LiFePO4-cathode (i) solid state diffusion, (ii) charge transfer (cathode/electrolyte) and (iii) contact resistance (cathode/current collector). Our model is then applied to EIS data sets covering varied temperature (0 degrees to 30 degrees C) and state of charge (10% to 100%). Activation energy, polarization resistance and frequency range are determined separately for all cathode processes involved. Finally, the tape-casted LiFePO4-cathode sheet is modified in porosity, thickness and contact area between cathode/electrolyte and cathode/current collector by a calendering process. Charge transfer resistance and contact resistance respond readily in polarization and relaxation frequency.
Article
The effects of mechanical stress on lithium-ion battery life are investigated by monitoring the stack pressure and capacity of constrained commercial lithium-ion pouch cells during cycling. Stack stress is found to be a dynamic quantity, fluctuating with charge/discharge and gradually increasing irreversibly over long times with cycling. Variations in initial stack pressure, an important controllable manufacturing parameter, are shown to produce different stress evolution characteristics over the lifetime of the cells. Cells manufactured with higher levels of stack pressure are found to exhibit shorter cycle lives, although small amounts of stack pressure lead to increased capacity retention over unconstrained cells. Postmortem analysis of these cells suggests a coupling between mechanics and electrochemistry in which higher levels of mechanical stress lead to higher rates of chemical degradation, while layer delamination is responsible for the capacity fade in unconstrained cells. Localized separator deformation resulting in nonuniform lithium transport is also observed in all cells.
Article
An extensive set of accelerated aging tests has been carried out employing a Li-ion high energy 18650 system (2.05 Ah), negative electrode: carbon, positive electrode: Li(NiMnCo)O2). It is manufactured by Sanyo, labeled UR18650E, and is a commercial off-the-shelf product. The tests comprise both calendar life tests at different ambient temperatures and constant cell voltages and cycle life tests operating the cells within several voltage ranges and levels using standard test profiles. In total, 73 cells have been tested. The calendar life test matrix especially investigates the influence of SOC on aging in detail, whereas the cycle life matrix focuses on a detailed analysis of the influence of cycle depth. The study shows significant impact of the staging behavior of the carbon electrode on cycle life. Furthermore a strong influence of the carbon potential on calendar aging has been detected. Observed relations between aging and the different influence factors as well as possible degradation mechanisms are discussed. Analysis of C/4 discharge voltage curves suggests that cycle aging results in different aging processes and changes in material properties compared to calendar aging. Cycling, especially with cycles crossing transitions between voltage plateaus of the carbon electrode seems to destroy the carbon structure.
Untersuchung der Alterung von Lithium-Ionen-Batterien mittels Elektroanalytik und elektrochemischer Impedanzspektroskopie
  • S R Käbitz
S.R. Käbitz, Untersuchung der Alterung von Lithium-Ionen-Batterien mittels Elektroanalytik und elektrochemischer Impedanzspektroskopie, (2016).