Li-ion Pouch Cells Research Articles

Modeling Cascading Failure of Thermal Runaway in Stacks of Li-Ion Pouch Cells with Variation in Cooling Attributes

The propagation of a local thermal runaway event to adjacent cells will determine the severity of potential local thermal abuse events in large battery packs relevant to transportation or grid-scale storage. Propagation can occur or heat can be dissipated depending on the ratio of the thermochemical reaction rates to the heat transfer rates. Also of importance is the thermal capacity of a system relative to the thermal energy released. We demonstrate these phenomena with recently developed thermochemical models for thermal abuse scenarios. These models have been developed with greater fidelity for high temperature heat release rates that we will show are important. Predictions are compared with recent experiments where thermal runaway was initiated through thermal abuse of one cell and subsequent cascading propagation, or failure to propagate, through the remaining cells in a pack, depending on the energy stored and the thermal characteristics of the battery pack. Implications for cascading failure in larger systems are discussed. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525. SAND2017-12561 C

Practical high temperature (80 °C) storage study of industrially manufactured Li-ion batteries with varying electrolytes

A previous study is focused on high temperature cycling of industrially manufactured Li-ion pouch cells (NMC-111/Graphite) with different electrolytes at 80 °C [JPS 373 (2018) 172–183]. Within this article the same test set-up is used, with cells stored for 30 days at different open circuit potentials and various electrolytes instead of electrochemical cycling. The most pronounced cell degradation (capacity fade and resistance increase) happens at high potentials. However appropriate electrolyte formulations are able to suppress ageing conditions by forming passivating surface films on both electrodes. Compared with electrochemical cycling at 80 °C, cells with enhanced electrolytes only show a slight resistance increase during storage and the capacity fade is much lower. Additionally it is shown for the first time, that the resistance is decreasing and capacity is regained once these cells are cycled again at room temperature. This is not the case for electrolytes without additives or just vinylene carbonate (VC) as an additive. It is further shown that the resistance increase of cells with the other electrolytes is accompanied by a reduction of the cell volume during further cycling. This behaviour is likely related to the reduction of CO2 at the anode to form additional SEI layer components.

Effects of external pressure on the performance and ageing of single-layer lithium-ion pouch cells

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.

Core-Shell Microcapsules Containing Flame Retardant Tris(2-chloroethyl phosphate) for Lithium-Ion Battery Applications.

Flame retardant tris(2-chloroethyl phosphate) (TCP) is successfully encapsulated in core–shell poly(urea-formaldehyde) microcapsules by in situ polymerization. The microcapsules are electrochemically stable in lithium-ion (Li-ion) battery electrolytes and thermally stable to ca. 200 °C. Thermal triggering of these microcapsules at higher temperatures ruptures the shell wall, releasing the liquid core (flame retardant), and NMR spectroscopy confirms the presence of the flame retardant in the electrolyte solution. Li-ion pouch cell experiments demonstrate that microencapsulation of TCP and its incorporation into the battery electrolyte provide latent fire retardants that improve battery safety while maintaining inherent battery performance and cycling capability.

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

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.

Lithium ion batteries (NMC/graphite) cycling at 80 °C: Different electrolytes and related degradation mechanism

A comprehensive study on high temperature cycling (80 °C) of industrial manufactured Li-ion pouch cells (NMC-111/Graphite) filled with different electrolytes is introduced. Ageing processes such as capacity fade, resistance increase and gas generation are reduced by the choice of appropriate electrolyte formulations. However, even by using additive formulations designed for elevated temperatures a large resistance increase is observed after 200 cycles and more (which does not happen at 55 °C). Symmetrical EIS (Electrochemical Impedance Spectroscopy) shows that the cathodic charge transfer resistance is the main reason for this behaviour. Nonetheless most of the active Li is still available when cycling with suitable additives. No change of the cathode crystalline structure or a growth of the cathodic surface reconstruction layer is observed post cycling at 80 °C. Therefore a disintegration of NMC secondary particles is believed to be the main reason of the cell failure. A separation of single grains is leading to new decomposition and reconstruction layers between primary particles and an increased charge transfer resistance. Further approaches to improve the high temperature cycle stability of NMC based materials should therefore be aimed at the cathode particles morphology in combination with similar electrolyte formulations as used in this study.

A Study of Esters As Co-Solvents in Lithium-Ion Batteries

: Higher rate capability in Li-ion batteries is always better. Using esters as a co-solvent can significantly improve the rate capability of Li-ion batteries because of their low viscosity which leads to higher electrolyte conductivity [1, 2]. However, which ester is the best to use from a cell lifetime perspective? In this work, the four esters, methyl acetate (MA), methyl propionate (MP), ethyl acetate (EA) and methyl butyrate (MB) were compared as co-solvents in the electrolytes of Li-ion pouch cells at levels from 0% to 60 % by weight. Experiments included high temperature storage, ultra high precision coulometry, high rate charge to determine the onset of lithium plating, long term cycling and isothermal microcalorimetry. In addition, Gering’s Advanced Electrolyte Model was used to compare the conductivity and viscosity benefits associated with the use of each ester. The baseline electrolytes to which the esters were added were 1.2 M LiPF6 in EC:EMC 30:70 or 1.2 M LiPF6 in EC:EMC:DMC (25:5:70 by vol%). Figure 1 shows the results of 60oC storage tests for Li-ion pouch cells containing various amounts of each of the four esters. Cells were stored at either 4.2 V, to examine the stability of the positive electrode/electrolyte interface and at 2.5 V to examine the stability of the negative electrode SEI. Figure 1 shows that of the four esters, methyl acetate is greatly preferred based on the storage testing for this Li-ion cell chemistry. Experiments involving many Li-ion cells chemistries and the four esters will be reported. Based on this work, tradeoffs involving the use of esters have been identified and will be discussed here. [1] M. C. Smart, B. V. Ratnakumar, K. B. Chin, L. D. Whitcanack, J. Electrochem. Soc., 157, A1361 (2010). [2] H.-C. Shiao, D. Chua, Hsiu-ping. Lin, S. Slane, M. Salomon, J. Power Sources, 87, 167 (2000). Figure 1. Open circuit potential versus time of Li-ion pouch cells at 60±0.1oC with different ester contents: (a-d) cells containing MA, MP, EA and MB were precharged to 4.2V; (e-h) cells containing MA, MP, EA and MB were precharged to 2.5V. Figure 1

Three-dimensional electro-thermal model of li-ion pouch cell: Analysis and comparison of cell design factors and model assumptions

Thermal modeling is a powerful tool in optimizing design of battery thermal management systems. Among different modelling methods, parallel electrode based 2D electro-thermal model has been used in the prediction of the complex spatial non-uniformity of battery cell temperature. Here, an electro-thermal model of 20 Ah nickel-manganese-cobalt oxide pouch type lithium-ion battery cell is presented, where an advanced 2D-potential distribution model based on overpotential is bi-directionally coupled with a 3D-temperature distribution model. For state of charge estimation during input parameter determination and simulation, Coulomb counting with Extended Kalman filtering method is used. The model is capable of predicting temperature distribution under constant current and dynamic current accurately. Through simulation, the influence of heat transfer through the tabs and the extended pouch seam on the temperature distribution is analyzed. These design factors are found to increase the overall heat transfer rate and influence the spatial distribution pattern of temperature. Several models with analytical and empirical simplification of potential distribution model, 2D geometrical simplification, and uniform heat generation assumption are demonstrated and compared in terms of accuracy and simulation time. It is illustrated that a 3D model geometry of the battery cell is essential to reproduce spatial temperature non-uniformity with high accuracy. Different simplifications can reduce the simulation time with reasonable accuracy. By comparing the results obtained from different simplifications, the present study depicts a broad scenario of modelling method to serve the purpose of implementation in thermal modeling of large battery packs.

Numerical Analyses on Aluminum Foams Cooling Plate for Lithium-ion Batteries

Successful implementation of electric vehicle relies strongly on its energy storage systems. Li-ion battery is the best candidate for the energy storage systems due to its energy density, power density and low maintenance. Temperature will affect the power availability, driveability and cycle life of the battery. Hence, effective thermal management system is needed to prolong cycle life and ensure balance charging/discharging among the cell in the battery pack. In this study, a battery module comprised three pieces of Li-ion pouch cell is arranged side by side and attached to aluminum foam heat sink to extract heat generated. The performance of two different type of pores density aluminum foams with different porosity were investigated numerically. Experimental correlation of Nusselt Number, permeability and resistance loss coefficient from the literature was extracted and used in the steady state CFD simulation. Among the metal foams, 10 PPI aluminum foam with 0.918 porosity offered the highest thermal performance and lowest flow resistance. Hence, it was shown that aluminum foam can be used to replace heat sink in the battery pack.

Critical analysis of open circuit voltage and its effect on estimation of irreversible heat for Li-ion pouch cells

Abstract Battery polarisation is not only responsible for reducing battery available capacity but also for controlling heat generation characteristics of batteries. This phenomenon was therefore carefully studied and modelled by Newman, Tiedemann and Gu (NTG). The NTG model is now widely used for simulating battery thermal behaviour and has even been adopted by CD-adapco for their proprietary battery modelling software - Battery Design Studio. The model however revolves around an idealised battery cell. It may thus not be applicable to commercial battery cells. This paper scrutinises the effect of open circuit voltage (OCV), a key parameter in the NTG model, on the irreversible heat generation of a commercial cell under a controlled environment by differentiating the OCV recorded immediately after the current stops flowing through the cell and the OCV corresponding to the equilibrium state of the cell. It is noticed that the NTG model underestimates the irreversible heat generation rates for a 20 Ah Li-ion pouch cell by approximately 0.15 W and 0.22 W for discharge currents at 0.33C and 0.5C at an operating temperature of 27 °C, respectively. It is also observed that the accuracy of the NTG model is significantly improved in simulating thermal behaviour of commercial battery cells when the OCV is representative of the cell equilibrium voltage.

Mechanical abuse simulation and thermal runaway risks of large-format Li-ion batteries

Abstract Internal short circuit of large-format Li-ion pouch cells induced by mechanical abuse was simulated using a modified mechanical pinch test. A torsion force was added manually at ∼40% maximum compressive loading force during the pinch test. The cell was twisted about 5° to the side by horizontally pulling a wire attached to the anode tab. The combined torsion-compression force created small failure at the separator yet allowed testing of fully charged large format Li-ion cells without triggering thermal runaway. Two types of commercial cells were tested using 4–6 cells at each state-of-charge (SOC). Commercially available 18 Ahr LiFePO4 (LFP) and 25 Ahr Li(NiMnCo)1/3O2 (NMC) cells were tested, and a thermal runaway risk (TRR) score system was used to evaluate the safety of the cells under the same testing conditions. The aim was to provide the cell manufacturers and end users with a tool to compare different designs and safety features.

An extended polarization model to study the influence of current collector geometry of large-format lithium-ion pouch cells

Abstract In this work, depth-of-discharge and temperature distribution of a large-format lithium-ion pouch cell are examined by means of a two-dimensional electro-thermal polarization model. A method of improving the dynamic behavior of the model while maintaining its accuracy under constant current loads by applying intermittent charge and discharge data is given. The model is validated with the aid of experimental data gained from dynamic and constant current discharge profiles applied to a commercial 40 Ah Li-ion pouch cell. Two major design studies are carried out focusing on a variation of geometrical parameters, namely the size and the positioning of the cell tabs. For each design, the influence of current collector thickness on the uniformity of the temperature and depth-of-discharge distribution is investigated during a 4C constant current discharge operation. Simulation results show that reducing the current collector thickness results in a moderate increase of 3 °C in maximum temperature and 1.5% in depth-of-discharge imbalance if the tab size is increased. In consequence, lowering the share of inactive components within a lithium-ion cell by optimizing the thickness of the current collector foils should be further considered to enhance the performance of typical lithium-ion cell designs.

Volume, Pressure and Thickness Evolution of Li-Ion Pouch Cells with Silicon-Composite Negative Electrodes

In-situ volume, pressure and thickness measurements were performed on Li-ion pouch cells with various silicon-composite negative electrodes to quantify electrode volume expansion. Li(Ni1-x-yCoxAly)O2/SiO-graphite, LiCoO2/Si Alloy-graphite and Li(Ni1-x-yCoxAly)O2/nano Si-C pouch cells were tested in this work. Archimedes-type in-situ volume measurements and in-situ thickness measurements showed cell expansion during charge and contraction during discharge due to electrode lithiation and de-lithiation. An in-situ pressure measurement was used to measure the effect of electrode volume expansion on volumetrically-constrained pouch cells. The volume expansion and contraction profiles measured exhibit a non-linear, asymmetric behavior as a function of cell state of charge for all cell types. To explain this, calculations of the volume expansion contribution of each electrode component were performed. Based on the results of the calculations, conclusions about the mechanisms contributing to the measured expansion profiles can be made.

Effects of Biphenyl Polymerization on Lithium Deposition in Commercial Graphite/NMC Lithium-Ion Pouch-Cells during Calendar Aging at High Temperature

Metallic lithium deposition is a typical aging mechanism observed in lithium-ion cells at low temperature and/or at high charge rate. Lithium dendrite growth not only leads to strong capacity fading, it also causes safety concerns such as short-circuits in the cell. In applications such as electric vehicles, the use of lithium-ion batteries combines discharging, long rest time and charging phases. It is foremost a matter of lifetime and safety from the perspective of the consumer or the investor.This study presents the post-mortem analyses of commercial 16 Ah Graphite/NMC (Nickel Manganese Cobalt layered oxide) Li-ion pouch cells. The cells were degraded by calendar aging at high temperature with or without periodic capacity tests. Unexpected local depositions of metallic lithium were confirmed on graphite electrodes by Nuclear Magnetic Resonance (NMR). Biphenyl, a monomer additive present in the liquid electrolyte, generates gas during its polymerization reaction occurring at high temperature and at high state of charge. As a result, dry-out areas are present between the electrodes leading to high impedance regions and no charge transfer between the electrodes. It is at the border of these areas that lithium metal is deposited.

Comparison of Single Crystal and Polycrystalline LiNi0.5Mn0.3Co0.2O2 Positive Electrode Materials for High Voltage Li-Ion Cells

Single-crystal LiNi0.5Mn0.3Co0.2O2 (NMC532) with a grain size of 2–3 μm was compared to conventional polycrystalline un-coated NMC532 and polycrystalline Al2O3-coated materials in this work. Studies were made to determine how single crystal NMC532 material with large grain size could be synthesized. Ultra high precision coulometry (UHPC), in-situ gas measurements and isothermal microcalorimetry were used to make comparative studies of the three materials in Li-ion pouch cells. All the diagnostic measurements suggested that the single crystal material should yield Li-ion cells with longer lifetime. Long-term cycling tests verified these predictions and showed that cells with single crystal NMC532 exhibited much better capacity retention than cells with the polycrystalline materials at both 40°C and 55°C when tested to an upper cutoff potential of 4.4 V. The reasons for the superior performance of the single crystal cells were explored using thermogravimetric analysis/mass spectrometry experiments on the charged electrode materials. The single crystal materials were extremely resistant to oxygen loss below 100°C compared to the polycrystalline materials. The major drawback of the single crystal material is its slightly lower specific capacity compared to the polycrystalline materials. However, this may not be an issue for Li-ion cells designed for long lifetime applications.

Degradation Diagnostics for Commercial Lithium-Ion Cells Tested at − 10°C

Degradation of lithium ion (Li-ion) cells affects both performance and safety of Li-ion batteries. In order to avoid potential safety hazards, it is crucial to detect the onset and extent of critical degradation modes in commercial Li-ion cells. This work demonstrates the application of a diagnostic algorithm to identify and quantify degradation modes of commercial Li-ion pouch cells cycled at − 10°C and a C-rate of 2 C. Rapid loss of active negative electrode material was successfully identified and results were validated using 3-electrode cells, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). The positive electrode material was less strongly affected by the tests, as found by the diagnostic algorithm and confirmed with EDX and SEM results.

Surface Area Increase of Silicon Alloys in Li-Ion Full Cells Measured by Isothermal Heat Flow Calorimetry

Li-ion pouch cells utilizing a negative electrode formulated with 15 wt% of an engineered Si alloy in a graphite composite electrode were cycled in an isothermal heat flow calorimeter against a LiCoO2 positive electrode. Two different electrolytes were investigated: a blend of ethylene carbonate and ethyl methyl carbonate (3EC:7EMC) and a blend of ethylene carbonate, ethyl methyl carbonate and 1-fluoro ethylene carbonate (27EC:63EMC:10FEC). Both electrolytes were 1 M in LiPF6 salt. The parasitic thermal power and coulombic efficiency was derived from isothermal heat flow measurements and high precision current-source meters. Cells without FEC showed high parasitic thermal power which increased with cycle number indicative of a surface area increase which was confirmed by post-cycling scanning electron micrographs and surface area measurements. Cells with FEC showed relatively stable parasitic thermal power. These measurements demonstrate the surprising function of FEC in controlling or attenuating the evolution of surface area in Si alloys. Vinylene carbonate was also found to be effective at controlling the increase in alloy surface area.

Impact of electrolyte solvent and additive choices on high voltage Li-ion pouch cells

The effects that various electrolyte solvents and electrolyte additives had on both LaPO4-coated LiNi0.4Mn0.4Co0.2O2 and uncoated LiNi0.4Mn0.4Co0.2O2/graphite pouch cells were studied using automated storage, electrochemical impedance spectroscopy, gas production and long-term cycling experiments. Storage experiments showed that the voltage drop during storage at 4.3 or 4.4 V for both coated and uncoated cells was very similar for the same electrolyte choice. At 4.5 V or above, the LaPO4-coated cells had a significantly smaller voltage drop than the uncoated cells except when fluorinated electrolytes were used. Automated charge discharge cycling/impedance spectroscopy testing of cells held at 4.5 V for 24 h every cycle showed that all cells containing ethylene carbonate:ethyl methyl carbonate electrolyte or sulfolane:ethyl methyl carbonate electrolyte exhibited severe capacity fade. By contrast, cells containing fluorinated electrolytes had the best capacity retention and smallest impedance growth during these aggressive cycling/hold tests. Long-term cycling experiments to 4.5 V confirmed that cells containing fluorinated electrolyte had the best cycling performance in the uncoated LiNi0.4Mn0.4Co0.2O2/graphite cells while cells containing sulfolane:ethyl methyl carbonate electrolyte had the best cycling performance in coated LiNi0.4Mn0.4Co0.2O2/graphite cells.

Post-Mortem Analysis of Commercial NMC/Graphite Li-Ion Pouch Cells within Mat4Bat European Project

Li-ion batteries are good candidates for electric vehicles and stationary applications due to their high power and energy densities. It is well known that the lifetime of lithium ion batteries is limited by undesirable side reactions. Moreover, these side reactions can affect different parts of the battery including the electrolyte, the active material, binder, conducting agents, current collectors and the separator. Over time, this fact leads to a gradual capacity fade and the end of life of the battery. Thus, the understanding of degradation mechanisms in lithium ion batteries is considered to be of great importance. In this work, the “post mortem analyses“ of NMC/graphite commercial pouch cells has been performed. For this, a standard procedure for cell opening and preparation of the samples has been defined. As safety plays an important role in the dissemination of batteries, full discharge of the investigated cells has been carried out [1,2,3]. Furthermore, a controlled environment such as an Ar-filled glove box has been used during cell opening in order to avoid material decomposition [4,5]. Once the electrodes have been separated, the conductive salt has been removed to prevent corrosion. Diagnostic evaluation of the first degradation mechanisms has been studied by ex-situ destructive physico-chemical and electrochemical analyses of the cell components. The crystallographic and morphological studies of the electrodes have been carried out by X-ray diffraction (XRD) and electron microscopy (SEM). Additionally, elemental analysis of the electrode composition has been determined by EDS and ICP techniques. In order to evaluate the electrochemical characterization of the harvested electrodes, galvanostatic testing in half-cell configuration has been performed. The research leading to these results has been performed within the MAT4BAT project (http://www.mat4bat.eu) [1] D. Aurbach, B. Markovsky, A. Rodkin, M. Cojocaru, E. Levi, H.-J. Kim, Electrochimica Acta 47 (2002) 1899. [2] Y. Kobayashi, T. Kobayashi, K. Shono, Y. Ohno, Y. Mita, H. Miyashiro, Journal of the Electrochemical Society 160 (2013) A1181. [3] T. Waldmann, M. Wilka, M. Kasper, M. Fleischhammer, M. Wohlfahrt-Mehrens, Journal of Power Sources 262 (2014) 129. [4] H.-Y. Amanieu, D. Rosato, M. Sebastiani, F. Massimi, D.C. Lupascu, Materials Science and Engineering: A 593 (2014) 92. [5] C. Wu, Y. Bai, F. Wu, Journal of Power Sources 189 (2009) 89.

Electrochemical and Structural Investigations of Titanate Based Anodes for High Power and Long Cycle Life Li-Ion Battery

Increasing demands for utilization of intermittent renewable energy resources, cost effective, safe and long cycle life electrochemical energy storage is required. Currently, lithium-ion batteries are identified as strong contender to fulfill the future energy requirements for electric vehicle as well as stationary energy storage applications due to high voltage, energy density, and long cycle life. However, state-of-the-art lithium ion battery technology still needs improvements in terms of cost, safety, power and cycle life. For stationary application, energy density is not as critical as that for vehicle application but long cycle life and safety is required. The spinel-type lithium titanium oxide, Li4Ti5O12 is attractive as an alternative to graphite anode for advanced lithium-ion batteries, due to the reversible Li insertion and extraction property at around 1.55 V (versus Li/Li+) with so called zero strain behavior. Even though this redox potential is far higher than that of graphite or carbonaceous materials, the good cyclability of Li cells has favored its commercialization. Although the structural restriction of the spinel structure limits the theoretical capacity to 175 mAh/g, various forms of titanate compounds exists such as H2Ti6O12, H2Ti12O25 and TiO2(B) with a higher theoretical capacity up to 335 mAh/g. In our presentation, electrochemical performance of Li-ion pouch cell using NMC cathode and AlF3 coated Li4Ti5O12 anode combination will be presented. Furthermore, structural investigations and electrochemical performances of other titanate based anodes will be discussed.