Non-uniform aging of cycled commercial LiFePO4//graphite cylindrical cells revealed by post-mortem analysis

A high energy X-ray diffraction technique is employed in a new way to make operando through-thickness measurements inside a large format commercial Li-ion pouch cell. The technique, which has a sub-mm in-plane spatial resolution, simultaneously determines the local temperature, the local state of charge of both electrodes (as opposed to the global average state of charge determined electrochemically), and the local in-plane elastic strain in the current collectors, all without embedding any intrusive sensors that alter battery behavior. As both thermal strain and mechanical strain develop during the charge-discharge cycling of the pouch cell, a novel approach developed herein makes it possible to separate them, allowing for measurement of the local temperature inside the battery. The operando experiment reveals that the temperature inside the cell is substantially higher than the external temperature. We propose that mechanical strain is due primarily to load transfer from the electrode to the current collector during lithiation, allowing determination of the local binder. Detailed local SOC mapping illustrates non-uniform degradation of the battery pouch cell. The possibility for 3D measurements is proposed. We believe that this new approach can provide critically needed data for validation of detailed models of processes inside commercial pouch cells. Today's electric vehicles generally use pouch cells rather than the more traditional cylindrical cells. An important advantage of pouch cells is their much higher surface-to-volume ratio for a given capacity, which permits better cooling. Batteries tend to heat up because of the hot environments that cars experience, internal electric resistance heating, and exothermic chemical reactions during operation. The performance of batteries fades over time, and high temperatures (say, above 45• C) greatly accelerate the fade rate 1,2 and may promote thermal runaway, 3 making temperature control critical. 4,5 Ideally, temperature control should be guided by the temperature inside the pouch cell, but making measurements inside an operating pouch cell has been difficult. Instead, the outside temperature has usually been taken as a surrogate for the internal temperature. [6][7][8] If pouch cells are sufficiently thin, and if the thermal conductivity is high enough, then it is reasonable to assume that the temperature measured at an x-y location on the outside of a pouch cell, with a thermocouple or with a thermal infrared (IR) camera, is close to the temperature at that x-y location all the way through its thickness. However, auto makers are motivated to make pouch cells thicker, reducing the number of expensive seals, electrical connections, and controls. Since local temperatures inside a thick cell might well be too high under some conditions, knowledge of the spatial distribution of internal temperature is of vital importance for achieving long life at low cost.A number of techniques have been developed to measure internal cell temperatures, 10-17 but measurement of loca...

Section: -54mentioning

confidence: 45%

Simultaneous Operando Measurements of the Local Temperature, State of Charge, and Strain inside a Commercial Lithium-Ion Battery Pouch Cell

Feng

Ren

et al. 2018

“…Intercalation current density [9] including temperature-dependent parameters are initially calibrated at 20…”

Section: Governing Equations Boundary Conditionsmentioning

confidence: 99%

“…6,7 Multiscale characterization methods have in fact evidenced that the non-uniform utilization of electrode surface leads to highly uneven degradation. 8,9 Furthermore, cell-to-cell variations in large battery packs (due to packaging constraints) may lead to non-uniform heat transfer in packs and induce further thermal gradients among the various cells. 10,11 This motivates the development of procedures and tools that allow for characterization and prediction of thermal gradients in large-format Lithium ion batteries, and understanding their impact on the performance and durability.…”

mentioning

confidence: 99%

A Reduced-Order Multi-Scale, Multi-Dimensional Model for Performance Prediction of Large-Format Li-Ion Cells

Fan

Pan

Storti

et al. 2016

In this paper, a reduced-order, Multi-Scale, Multi-Dimensional (MSMD) model is developed to achieve accurate and fast simulation of the 2D distribution of thermal and electrochemical properties across the surface of a large-format Li-ion battery cell. Since the proposed model aims at supporting long-term simulation, virtual design and optimization studies, minimization of the computational complexity is achieved through analytical Model Order Reduction (MOR) based on a Galerkin projection method. The model is then verified against experimental data and a high-fidelity numerical model at various input current conditions. Results show that the computational complexity of the MSMD model is significantly reduced without sacrificing the accuracy in characterizing the distribution of the electrochemical and thermal properties. Finally, the impact of the applied current and thermal boundary conditions on the distribution of transverse current density is also evaluated. Temperature and temperature gradients are some of the key factors that influence the performance and degradation of Li-ion batteries. Despite in Hybrid Electric Vehicles (HEVs) or Electric Vehicles (EVs) the average pack temperature is regulated by the thermal management systems, thermal gradients may still occur within the pack or across individual cells.1-3 For instance, when cooling plates are in contact with only one side of the cells, the heat generated inside each cell will be dissipated to the cooling plates along a preferential direction. Therefore, thermal gradients in the order of ±5• C are common, and particularly in high-power packs for HEVs or high-performance EVs. 4,5 Thermal gradients on the cell surface are particularly critical in large-format batteries because hot spots may induce localized changes in the physical and chemical properties of the electrodes and electrolyte solution, which in turn can increase the local transverse current density and ultimately cause non-uniform degradation of the electrodes.6,7 Multiscale characterization methods have in fact evidenced that the non-uniform utilization of electrode surface leads to highly uneven degradation. 8,9 Furthermore, cell-to-cell variations in large battery packs (due to packaging constraints) may lead to non-uniform heat transfer in packs and induce further thermal gradients among the various cells. 10,11 This motivates the development of procedures and tools that allow for characterization and prediction of thermal gradients in large-format Lithium ion batteries, and understanding their impact on the performance and durability.Multi-Scale, Multi-Dimensional (MSMD) modeling approaches have been proposed to simulate the distribution of surface temperature and Lithium concentration in large format, prismatic cells. As a first approximation, the rate of heat generation within the cell can be approximated as uniform across the surface.12-16 More recently, this simplification has been abandoned in favor of more rigorous approaches that accurately compute the local heat generation rate ...

“…The performance decrease on the cell level is mainly based on chemical degradation reactions on the material and on the electrode level (see Figure 9). 3,9,[15][16][17]25,28,[33][34][35][36][37][38][39][40][41] In this sense, a thorough understanding of the degradation mechanisms happening inside the cells is crucial to improve their life-time.In order to conclude on aging mechanisms, it is mandatory to disassemble the cells and analyze the relevant cell components. For a deep z E-mail: waldmann@zsw-bw.de understanding of a battery's aging process, a homogenized procedure including cell opening, disassembly, sample processing, and analysis is important to avoid damage, contamination, and modification of the cell components and to yield interpretable data.…”

mentioning

confidence: 99%

“…157 A general review about SEI analysis including XPS has been provided by Verma et al 158 Commercial LiFePO 4 / graphite cells investigated by Klett et al showed uneven aging of electrodes for cycled cells, whereas the electrodes of calendar aged cells were uniform 17 which is related to temperature 77,[159][160][161] and pressure gradients evolving during cycling. Lu et al discussed aging focusing on LiCoO 2 / graphite cells.…”

mentioning

confidence: 99%

Review—Post-Mortem Analysis of Aged Lithium-Ion Batteries: Disassembly Methodology and Physico-Chemical Analysis Techniques

Waldmann

Iturrondobeitia

Kasper

et al. 2016

214

149

Improvement of life-time is an important issue in the development of Li-ion batteries. Aging mechanisms limiting the life-time can efficiently be characterized by physico-chemical analysis of aged cells with a variety of complementary methods. This study reviews the state-of-the-art literature on Post-Mortem analysis of Li-ion cells, including disassembly methodology as well as physicochemical characterization methods for battery materials. A detailed scheme for Post-Mortem analysis is deduced from literature, including pre-inspection, conditions and safe environment for disassembly of cells, as well as separation and post-processing of components. Special attention is paid to the characterization of aged materials including anodes, cathodes, separators, and electrolyte. More specifically, microscopy, chemical methods sensitive to electrode surfaces or to electrode bulk, and electrolyte analysis are reviewed in detail. The techniques are complemented by electrochemical measurements using reconstruction methods for electrodes built into half and full cells with reference electrode. The changes happening to the materials during aging as well as abilities of the reviewed analysis methods to observe them are critically discussed. Li-ion batteries are currently used in everyday objects such as smart-phones, power tools and tablet computers as well as in the growing fields of light electric vehicles (LEVs), unmanned aerial vehicles (UAVs), battery electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs). [1][2][3][4] Furthermore, the rise of renewable energy sources like wind and solar power, which are only intermittently available, demands reliable and highly flexible stationary energy storage solutions, which provide high capacities and predictable life-times. 2,5Aging of Li-ion batteries is a general problem for manufacturers as they have to guarantee long-term reliability of their products. For state-of-the-art cells, degradation effects on the material level lead to capacity fade and resistance increase on the cell level. The aging state of a battery is often characterized by the state-of-health (SOH) in % according to 3,16,22,[29][30][31] SO H(t) = discharge capacity (t) discharge capacity (t = 0)[1]where t represents the aging time. In general, one has to differentiate between cycling 7,16,18,21,[23][24][25]32 and calendar aging. 7,19,[21][22][23][24]27 Since commercial Li-ion cells can be subject to calendar aging in the time between manufacturing and delivery, it is good practice to measure the discharge capacity at t = 0 for each cell that undergoes an aging test. Since the discharge capacity depends mainly on temperature, depth-of-discharge (DOD), and discharge current, the SOH is usually monitored by regular check-ups with defined parameter sets, 7,16,21,23,24 which can vary depending on the application. Typically, a temperature of 25• C, 16,22,24 DOD of 100%, 16,21 and discharge rates of 1C 7,16,21,22,24 or lower 23 are used in check-ups. The performance dec...