Large Format Lithium Ion Battery Research Articles

Electrochemical and Thermal Modeling for the Fast-Charge of Lithium-Ion Batteries with Cocurrent and Countercurrent Tab Connections and the Assessment of Li Plating

Abstract Our focus is on large-format lithium-ion batteries, used in electric vehicles today and in the foreseeable future, which are charged at high rates. In order to fully charge the battery, we employ a protocol often referred to as cc-cv (constant current followed by constant voltage). We compare and contrast results for cocurrent and countercurrent tab locations. We show how the pseudo three-dimensional (P3D) model can be used to assess temperature and current distributions and determine if Li plating is expected. We demonstrate the advantages of countercurrent tab locations to (i) obtain more uniform current and temperature distributions and (ii) lower the propensity for Li plating. Sensitivity analyses include the influence of ambient temperature and cell length. The methodology laid out in this work can facilitate rational battery-cell design and robust operation, including high-rate charging.

Effects and mechanism of thermal insulation materials on thermal runaway propagation in large-format pouch lithium-ion batteries

The large heat transfer area of large-format lithium-ion batteries primarily facilitates conduction heat, which is responsible for triggering the thermal runaway of adjacent cells. Therefore, the primary consideration is to utilize thermal insulation materials between cells in order to slow down or prevent the process of thermal runaway propagation. In this study, three varieties of thermal insulation materials were employed to insulate individual cells within a module comprising three large-format pouch cells. These materials include pre-oxygenated wire aerogel (AG-ST-POF), polymer-coated pre-oxygenated wire aerogel (PC-AG-ST-POF), and silicone (SI). The study experimentally examined the thermal runaway propagation (TRP) behavior of the battery module under conditions without insulation materials and with various types of insulation materials. The impact of thermal insulation materials on the TRP among cells was analyzed. The study revealed that none of the three materials completely suppressed the propagation of thermal runaway, although they did impede its progress. The overall impact of AG-ST-POF surpassed that of SI and PC-AG-ST-POF. Furthermore, the mechanism by which thermal insulation materials influence TRP was explored. The results of the thermophysical properties testing suggest that evaluating the impact of materials on TRP solely through thermal conductivity is not a reliable approach. Alternatively, the maximum service temperature and thermal diffusion coefficient are critical parameters for the selection of thermal insulation materials. This study can provide a valuable reference for process engineering design and application in the context of lithium-ion batteries, with the aim of mitigating the risk of TRP.

Quantifying the Entropy and Enthalpy of Insertion Materials for Battery Applications Via the Multi-Species, Multi-Reaction Model

The entropy coefficient of a battery cell is the property that governs the amount of reversible heat that is generated during operation. In this work, we propose an extension of the Multi-Species, Multi-Reaction (MSMR) model to capture the entropy coefficient of a large format lithium-ion battery cell. We utilize the hybridized time-frequency domain analysis (HTFDA) method using a multi-functional calorimeter to probe the entropy coefficient of a large format pouch type lithium-ion battery with a NMC 811 cathode and a graphite anode. The measured entropy coefficient profile of the battery cell is deconvoluted into an entropy coefficient for each active material, which is then estimated using an extension of the MSMR model. Finally, we extend the entropy of a material to individual entropy for each gallery as treated by the model.

Co-Estimation of State-of-Charge and State-of-Temperature for Large-Format Lithium-ion Batteries Based on a Novel Electrothermal Model

The safe and efficient operation of the electric vehicle significantly depends on the accurate state-of-charge (SOC) and state-of-temperature (SOT) of Lithium-ion (Li-ion) batteries. Given the influence of cross-interference between the two states indicated above, this study establishs a co-estimation framework of battery SOC and SOT. This framwork is based on an innovative electrothermal model and adaptive estimation algorithms. The first-order RC electric model and an innovative thermal model are components of the electrothermal model. Specifically, the thermal model includes two lumped-mass thermal submodels for two tabs and a two-dimensional (2-D) thermal resistance network (TRN) submodel for the main battery body, capable of capturing the detailed thermodynamics of large-format Li-ion batteries. Moreover, the proposed thermal model strikes an acceptable compromise between the estimation fidelity and computational complexity by representing the heat transfer processes by the thermal resistances. Besides, the adaptive estimation algorithms are composed of an adaptive unscented Kalman filter (AUKF) and an adaptive Kalman filter (AKF), which adaptively update the state and noise covariances. Regarding the estimation results, the mean absolute errors (MAEs) of SOC and SOT estimation are controlled within 1% and 0.4 °C at two temperatures, indicating that the co-estimation method yields superior prediction performance in a wide temperature range of 5 °C to 35 °C.

Understanding the influence of the confined cabinet on thermal runaway of large format batteries with different chemistries: A comparison and safety assessment study

With the increasing development of large format lithium-ion batteries (LIBs) in automotive sectors, thermal runaway (TR) and fire hazards have become crucial challenges. A series of overheating experiments were performed on four large format LIBs with various chemistries under two conditions. To simulate the electric vehicle applications, the cabinet was employed in the confined space tests. The results showed that under the condition of stopping heating at safety venting (SV), all LiNixCoyMnzO2 (NCM) cells caught fire and TR, while no TR occurred for LiFePO4 (LFP) cells. The LFP cells cannot ignite spontaneously but only spray the electrolyte during TR. The triggering temperatures of TR for NCM cells are similar and close to the temperature of SV for LFP cells. For NCM LIBs, with the ratio of nickel increasing, the TR hazards increase, including the thermal and toxic hazards, larger than that of LFP cells. From the aspects of the TR risks and TR hazards, a safety assessment method was established to grade the fire hazards. Compared with open space tests, the LIBs with different chemistries in confined space tests have the same TR risks but lower TR hazards. The fire-induced toxicity was quantitatively evaluated by determining the fractional effective concentration and fractional effective dose. Moreover, the fire of LIBs is compared to the gas jet flame and pool fire of several typical fuels. Such a comparison analysis on large-scale LIBs with different chemistries can contribute to the selection of cells and safety design for automotive sectors.

Trifunctional composite thermal barrier mitigates the thermal runaway propagation of large-format prismatic lithium-ion batteries

With the increasing energy density and capacity of lithium ion batteries (LIB), the safety problems caused by thermal runaway propagation (TRP) has become the most predominant hazard for electric vehicles (EVs) and energy storage systems (ESSs). Mitigating the TRP is significant for large-format LIB application. This study proposed a novel thermal barrier with nano-ceramic fiber, phase change materials (PCM) and mica cross skeleton. A 3 mm thermal barrier can effectively prevent the TRP behavior of a 153 Ah Li(Ni0.5Co0.2Mn0.3) (NCM523) battery module. The enthalpy of heat absorption of PCM is 1958 J/g. Besides, the mica skeleton can resist the extrusion pressure from the batteries, and the remaining nano-ceramic fiber provides insulation capacity. Moreover, the thermal barrier still maintains desirable heat insulation under pressure and high temperature operations, exhibiting high compressive strength. Furthermore, the thermal barrier can reduce the maximum temperature of the battery by about 10 °C during 2C charging and discharging conditions. This study guides the safety design of thermal barrier for large capacity LIB systems, contributing to the potential safety protection of next-generation EVs and ESSs.

Research on Temperature Inconsistency of Large-Format Lithium-Ion Batteries Based on the Electrothermal Model

Large-format lithium-ion (Li-ion) batteries are increasingly applied in energy storage systems for electric vehicles, owing to their flexible shape design, lighter weight, higher specific energy, and compact layouts. Nevertheless, the large thermal gradient of Li-ion batteries leads to performance degradation and irreversible safety issues. The difference in the highest temperature position at various operational modes makes accurate temperature monitoring complicated. Accordingly, a full understanding of the temperature inconsistency of large-format Li-ion batteries is crucial. In this study, these inconsistent characteristics are analyzed by establishing an electrothermal model and conducting experiments based on an 8-Ah pouch-type ternary Li-ion battery with contraposition tabs. Regarding the characteristic of inhomogeneous temperature distribution, the analysis results demonstrate that it is primarily attributable to the uneven heat generation within the battery system and the effects of the two tabs. For the evolution of the highest temperature position, this study compares the maximum temperature rise of the positive tab and main battery body. The results illustrate that the operating temperature has a greater impact on the maximum temperature rise of the main battery body since its resistance strongly depends on the operating temperature compared to the positive and negative tabs. In addition, the electrothermal model is expected to be employed for the battery thermal management system (BTMS) to mitigate the battery temperature inconsistency.

Degradation and modeling of large-format commercial lithium-ion cells as a function of chemistry, design, and aging conditions

Demand for large-format (>10 Ah) lithium-ion batteries has increased substantially in recent years, due to the growth of both electric vehicle and stationary energy storage markets. The economics of these applications is sensitive to the lifetime of the batteries, and end-of-life can either be due to energy or power limitations. Despite this, there is little information from cell manufacturers on the sensitivity of cell degradation to environmental conditions or battery use. This work reports accelerated aging test data from four commercial large-format lithium-ion batteries from three manufacturers, with varying design (thickness, casings, …), chemistry (lithium‑iron-phosphate (LFP) or lithium‑nickel‑manganese‑cobalt-oxide positive electrodes (NMC), with graphite (Gr) negative electrodes), and capacity (50 to 250 Amp∙hours). The tested LFP|Gr cell is found to be relatively insensitive to cycling conditions like temperature or voltage window, while NMC|Gr cells have varying sensitivity. Degradation trends are further investigated by training predictive models: simple polynomial trend lines, a semi-empirical reduced-order model, and an empirical reduced-order model identified using machine-learning based on symbolic regression. Calendar and cycle life are simulated over a variety of conditions to directly compare the various batteries. Cell size and thickness are found to substantially impact sensitivity to temperature during cycle aging, while electrode chemistry impacts depth-of-discharge sensitivity. Real-world battery lifetime is evaluated by simulating residential energy storage and commercial frequency containment reserve systems in several U.S. climate regions. Predicted lifetime across cell types varies from 7 years to 20+ years, though all cells are predicted to have at least 10 year life in certain conditions.

Thermal and stoichiometry inhomogeneity investigation of large-format lithium-ion batteries via a three-dimensional electrochemical-thermal coupling model

Large-format lithium-ion batteries (LIBs) have attracted extensive concern due to the advantages of increasing energy density and reducing manufacturing costs. However, inhomogeneity is widely present in large-format LIBs, which may lead to local degradation or edge lithium plating, thereby reducing the battery durability and safety. In this study, an electrochemical-thermal (ECT) coupling model for large-format LIBs is developed to quantitatively investigate the battery inhomogeneity during charging and discharging. First, two different sizes of batteries with the same electrode material are disassembled and tested to obtain the battery model parameters. Second, a novel method for determining the parameters of the ECT coupling model for large-format LIBs is proposed to solve the technical challenge of difficult model parameter calibration. Specifically, the ECT coupling model parameters of large-format batteries are determined using those of small-format batteries. The experimental results show that the three-dimensional ECT coupling model exhibits high accuracy, with the root mean square error of voltage and temperature within 17.8 mV and 1.27 ˚C, respectively, at 1C discharge rate and SOC greater than 2.5 %. Finally, the temperature and stoichiometry coefficient inhomogeneity of large-format LIBs during charging and discharging under different conditions are analyzed quantitatively. The results show that the inhomogeneity of large-format LIBs increases significantly with increasing discharge rate and battery length. A large-format battery with a length of 900 mm achieves a balance between inhomogeneity and length. This study contributes to a quantitative understanding of the inhomogeneity of large-format LIBs and provides a model-based perspective for designing and optimizing large-format LIBs.

Experimental Aging and Lifetime Prediction in Grid Applications for Large-Format Commercial Li-Ion Batteries

Due to the growth of electric vehicle and stationary energy storage markets, the production and use of lithium-ion batteries has grown exponentially in recent years. For many of these applications, large-format lithium-ion batteries are being utilized, as large cells have less inactive material relative to their energy capacity and require fewer electrical connections to assemble into packs. The economics of these battery systems is highly dependent on cell lifetime, requiring accelerated aging tests to characterize cell degradation. However, testing of large-format lithium-ion batteries is time consuming and requires high current channels and large testing chambers, so not many reports on the lifetime of commercial, large-format lithium-ion batteries exist. Here, accelerated aging test data from four commercial large-format lithium-ion batteries is reported. These batteries span both NMC-Gr and LFP-Gr cell chemistries, are from a variety of manufacturers, have pouch and prismatic formats, and show a range of cell designs with varying power capabilities.Accelerated aging test results are analyzed to examine both cell performance, in terms of efficiency and thermal response under load, as well as cell lifetime. Cell thermal response is characterized by measuring temperature during cycle aging, which is used to calculate a normalized thermal resistance value that may help estimate both cell cooling needs or to help extrapolate aging test results to different thermal environments. Cell lifetime is evaluated qualitatively, considering the average calendar and cycle life across a range of conditions, as well as quantitatively, using statistical modeling and machine-learning methods to identify predictive aging models from the accelerated aging data. These predictive aging models are then used to investigate cell sensitivities to stressors, such as cycling temperature, voltage window, and C-rate, as well as to predict cell lifetime in various stationary storage applications.Results from this work show that cell lifetime and sensitivity to aging conditions varies substantially across the cell types, necessitating testing for each specific cell to make quantitative lifetime predictions. That being said, all commercial cells tested here are predicted to reach at least 10-year lifetimes for stationary storage applications. Based on the aging test results and modeling, some cells are expected to be relatively insensitive to temperature and use-case, making them suited for simple use cases with little or no thermal management and simple controls, while the lifetime of other cells could be extended to 20+ years if operated with thermal management and degradation-aware controls. Figure 1

Modeling and Analysis of Temperature-Dependent Capacity Degradation in Large Format Lithium-Ion Batteries

The large-format lithium-ion batteries, widely used for electric vehicle applications, are generally characterized by spatial non-uniformities in temperature. The presence of thermal gradients and temperature-dependence of cell parameters are responsible for non-uniform degradation within the battery that leads to poor performance of the battery packs and also poses safety risks. In this work, we aim to develop a detailed physics-based simulation framework to model non-uniform, temperature-dependent capacity degradation within these large format batteries based on the classical pseudo two-dimensional (p2D) battery model. The electrochemical model will be coupled with degradation mechanisms to capture solid-electrolyte interface layer growth, reversible lithium plating, structural disintegration, and volume change within the electrodes on the cell level signatures and usable capacity over charge/discharge cycles. The effects of temperature inhomogeneities will be accounted in the multi-cell simulations at different operating temperatures for individual cells with dynamic currents and temperature and concentration dependent battery parameters as the model inputs.Overall, the primary focus of this study is to capture the effect of spatial temperature variation on battery capacity over cycles and help identify an improved design for the battery module. The model is implemented in an efficient equation-based form for the p2D model and degradation mechanisms with dynamic cell inputs using the mathematical modules in the finite element based solver, COMSOL Multiphysics. This approach enables an easier model development and analysis of the detailed physics and allows an easier implementation of any additional side reaction. The relative contributions of various fade modes have also been compared to understand their competitive interplay over cell cycles. The model predictions are validated with the available experimental data and in-house codes for each case. We believe that this simple yet detailed model can be further developed to be computationally feasible for adoption and implementation in a battery thermal management system.

Experimental assessment and comparison of single-phase versus two-phase liquid cooling battery thermal management systems

Liquid-based cooling systems are becoming the dominant approach for thermal management of lithium-ion batteries due to the favorable specific heat capacity and heat transfer coefficient. In this study, single-phase and two-phase liquid cooling (SPLC and TPLC) systems are experimentally evaluated and compared in two indirect-contact modes for a large-format lithium-ion battery. Two commonly used liquid coolants, e. g. Novec 7000 and deionized water, are utilized as heat transfer fluids. The experimental results reveal that both SPLC and TPLC systems with coolant flow exhibit excellent ability to control temperature spikes, and the SPLC system using deionized water outperforms the TPLC system using Novec 7000 in terms of cooling capability. Additionally, the cooling effect of copper foam configuration surpasses that of straight fin configuration. The SPLC system equipped with copper foam holds the best cooling performance, with a maximum surface temperature of 32.1 °C and a largest temperature difference of 1.8 °C observed over the 3C discharge process at a flow rate of 10 mL/min. These findings are valuable for designers seeking to develop efficient and practical thermal management systems for large-format lithium-ion batteries.

How to enable large format 4680 cylindrical lithium-ion batteries

The demand for large format lithium-ion batteries is increasing, because they can be integrated and controlled easier at a system level. However, increasing the size leads to increased heat generation risking overheating. 1865 and 2170 cylindrical cells can be both base cooled or side cooled with reasonable efficiency. Large format 4680 cylindrical cells have become popular after Tesla filed a patent. If these cells are to become widely used, then understanding how to thermally manage them is essential. In this work, we create a model of a 4680 cylindrical cell, and use it to study different thermal management options. Our work elucidates the comprehensive mechanisms how the hot topic ‘tabless design’ improves the performance of 4680 cell and makes any larger format cell possible while current commercial cylindrical cells cannot be simply scaled up to satisfy power and thermal performance. As a consequence, the model identifies the reason for the tabless cell's release: the thermal performance of the 4680 tabless cell can be no worse than that of the 2170 cell, while the 4680 tabless tab cell boasts 5.4 times the energy and 6.9 times the power. Finally, via the model, a procedure is proposed for choosing the thermal management for large format cylindrical cell for maximum performance. As an example, we demonstrate that the best cooling approach for the 4680 tabless cell is base cooling, while for the 2170 LG M50T cell it is side cooling. We conclude that any viable large format cylindrical cell must include a continuous tab (or ‘tabless’) design and be cooled through its base when in a pack. The results are of immediate interest to both cell manufacturers and battery pack designers, while the developed modelling and parameterization framework is of wider use for all energy storage system design.

Comparative study on the influence of incident heat flux on thermal runaway fire development of large-format lithium-ion batteries

In response to the growing demand for higher energy density in various applications, large-format batteries have gained popularity. However, the larger size of these batteries also increases their exposed area, which may lead to more severe thermal events. This study aims to understand the influence of incident heat flux on the occurrence and progression patterns of fires in large-format lithium-ion batteries. Two representative large-format lithium-ion batteries, 38 Ah LiNi0.3Co0.3Mn0.3O2 prismatic cell and 78 Ah LiNi0.8Co0.1Mn0.1O2 pouch cell, were investigated. To evaluate the thermal hazard to the environment, the heat release rates (HRR) of the lithium-ion battery fire were measured under incident heat fluxes ranging from 10 to 50 kW∙m−2, using the oxygen consumption principle. The results highlight significant differences in thermal runaway and fire development between the 38 Ah and 78 Ah cells, attributable to variations in structure and materials, and scale effect. The 38 Ah cell exhibited an evident venting stage, characterized by an unignited jet on the verge of ignition, followed by a transition from momentum-driven jet fire to buoyancy-driven jet fire. In contrast, the 78 Ah cell experienced direct ignition, accompanied by strong air entrainment, overflow flame, and flame sinking phenomena as the fire progressed. Notably, unlike the 38 Ah cell, the peak HRR, THR, and burning time of the 78 Ah cell were minimally sensitivity to the incident heat flux. Furthermore, a generalized relationship model between the incident heat flux and the ignition time was established and validated based on the energy conservation equation during the ignition stage of the cell. This model enhances our understanding of the interplay between incident heat flux and ignition time, contributing to the development of strategies for fire prevention and mitigation in large-format lithium-ion batteries.

Experimental research on flammability characteristics and ignition conditions of hybrid mixture emissions venting from a large format thermal failure lithium-ion battery

When the concentration of gas is below its lower flammable limit and the content of the particles is below its minimum explosible concentration, it can be explosible. Thermal abuse tests for 50 Ah lithium-ion batteries were conducted under an inert atmosphere. The battery failure venting emissions were collected and their composition was identified previously. It still needs further research for the flammability characteristics and ignition conditions for hybrid mixture emissions venting from a large format lithium-ion battery thermal failure. In this study, the likelihood, severity, and kinetics of the batteries' hybrid mixture emissions flammability have been researched. To analyze battery emission flammability characteristics, five indexes have been investigated; they are ignition temperature (Tig), ignition time (τig), maximum explosion pressure (Pmax), pressure rise maximum rate ((dP/dτ)max), and size-normalized maximum rate pressure rise (KSt). Based on quantitative analysis for the hybrid mixture flammability characteristics, it indicates that particle size and gas content can impact hybrid mixture emission flammability characteristics. As for the mixture with finer particles, the optimum combustion condition of the particle and gas concentration pairs were 0.3 and 0.7. The ignition temperature for the optimum combustion conditions was 495 °C. The pressure rising velocity was 23 Mpa/s during ignition. High solvent content and low particle concentration are the conditions for optimum combustion. This indicates that the gas admixture solvent plays a major role in the combustion kinetics of the mixture emission. This research offers a fundamental perspective and provides preventive and protective thermal safety designs that are more suitable for large format commercial battery thermal systems.

Toward high-accuracy and high-efficiency battery electrothermal modeling: A general approach to tackling modeling errors

An accurate control-oriented electrothermal model is a prerequisite for efficient thermal management of battery-powered applications. Despite the great efforts made to pursue high-accuracy modeling of the heat generation and transfer processes inside lithium-ion batteries for control purposes, little work has shed light on the error sources in such electrothermal modeling for obtaining an optimal model. In this article, we propose a practical and general framework for investigating the potential error sources in computationally efficient electrothermal modeling of large-format lithium-ion batteries. The results demonstrate that the high-accuracy modeling of the thermal interplay between two tabs and the cell body and the optimization of the temperature sensor placements should be given high priority. Further, the overpotential-based method is preferred to calculate the local generation rates of the battery body for accuracy and generalizability. This study provides important insights into accurate and computationally efficient electrothermal modeling for large-format energy storage batteries.

Toward the flexible production of large-format lithium-ion batteries using laser-based cell-internal contacting

Lithium-ion battery cells are used for energy storage in many industrial sectors, such as consumer electronics or electromobility. Due to the diversity of these applications, the demand for tailored battery cells is increasing. Consequently, the technical development of the cells leads to numerous coexisting cell variants. Examples of such variants are altering cell materials, formats, and capacities. Different target capacities can be realized by changing the geometrical dimensions of the individual electrodes or the number of electrodes in the cell cores. The increasing quantity of variants poses challenges within battery cell manufacturing, such as the need to adapt the process parameters for the cell-internal contacting of a higher number of electrode sheets in the cell stack. Each new cell variant currently requires elaborate experimental parameter studies for its manufacture. An approach for selecting suitable process parameters for laser-based cell-internal contacting in terms of a modification of the cell properties is presented in this paper. A model was built to determine the weld depth in copper sheets using a millisecond pulsed laser welding strategy. The process parameters for welding stacks of electrode sheets to an arrester tab were calculated on the basis of this model. The necessary weld depth in the arrester tab for achieving suitable mechanical properties of the cell-internal joint was considered. The presented approach was validated by welding different numbers of foils to an arrester tab and varying the thickness of the foils. It was shown that the experimental effort for the selection of the process parameters for laser-based contacting can be reduced significantly.

Thermal performance evaluation of boiling cooling system for the high-rate large-format lithium-ion battery under coolant starvations

The boiling cooling system exhibits superior cooling capacity and temperature homogenization ability, which has great potential in managing the undesirable temperature spike and temperature gradient of the large-format lithium-ion batteries. However, in-depth experimental investigation to reveal the effects of harsh operation conditions and parameters on the cooling characteristics are still lacking. In this study, thermal control ability of the boiling cooling system using Novec 7000 as coolant is evaluated for a large-format 20-Ah lithium-ion battery, with special attentions to the cooling performance under harsh operation conditions such as high-rate cyclic and coolant starvation. The experimental results show that the boiling-cooling system with uninterrupted/small coolant flow exhibits superior cooling capacity and the battery temperature can be controlled around 35 ± 2 °C over the high-rate cycles. However, when the coolant starvation issue occurs in the static mode, the maximum temperature and temperature difference rise drastically up to 53.45 °C and 18.56 °C in the fourth discharge process, which are abnormally higher than the system without injecting liquid coolant. The incorporation of 20PPI copper foam filler can alleviate the coolant starvation issue to some extent, which reduces the peak values of battery temperature and temperature difference by 13.28 % and 39.33 % under the same condition.

Modeling the inhomogeneous lithium plating in lithium-ion batteries induced by non-uniform temperature distribution

Under fast-charging or low-temperature charging conditions, large-format lithium-ion batteries are prone to inhomogeneous lithium plating, resulting in localized degradation and even internal short circuit, thus requiring a thorough investigation. This study establishes a three-dimensional electrochemical model to analyze the inhomogeneous lithium plating behavior in lithium-ion batteries. The model is validated with the voltage profiles and temperature distributions of a 24Ah pouch lithium-ion battery during low-temperature charging. The inhomogeneous lithium plating induced by non-uniform temperature distribution and the underlying mechanism are investigated using the model. The local current density and overpotential turn out to be the key factors affecting the spatial distribution of plated lithium. Moreover, the effects of temperature gradients on the lithium plating and stripping behaviors are explored. In-plane temperature gradient shows little effect on the total amount of plated lithium, but leads to apparent inhomogeneity in the distribution of plated lithium, with the differences between the concentrations of plated lithium in the upper and lower half of battery reach 57.61%, 103.06% and 159.68% of the average concentration in the whole battery during 1C charging at -5, 5 and 10°C, respectively. The temperature gradient also significantly influences the battery voltage relaxation profile related to the lithium stripping process. The characteristic plateau in the battery voltage relaxation profile disappears when the temperature gradient increases to larger than 6°C, leading to difficulty in detecting inhomogeneous lithium plating in large-format lithium batteries. The results presented in this study can provide helpful guidance for the design and management of large-format lithium-ion batteries.

Inhomogeneous degradation induced by lithium plating in a large-format lithium-ion battery

With the demand for high-energy-density power sources for electric vehicles, large-format lithium-ion batteries are widely applied, considering their advantages in reducing the weight of inactive materials. However, large-format cells suffer from internal inhomogeneities, which become the bottleneck limiting their performance. Here, the inhomogeneous degradation in a large-format pouch cell is comprehensively investigated using a series of (non-)destructive techniques. Spatial-resolved deformation detection and ultrasonic diagnostics are utilized to study the evolution of inhomogeneity inside the large-format battery. Localized deformation and deposits are found to firstly appear at the tab-near regions and then propagate into central regions, which is identified by characterization tests to be induced by lithium plating. The inhomogeneous degradation mechanism inside the battery is summarized as an initiation-propagation process. During the process, lithium deposit is initiated by a local high current density, resulting in the local separator pore closure. Pore closure in the separator in return creates a high current density and overpotential in the adjacent area, leading to a continuous propagation of the lithium deposition area. Finally, an effective indicator—peak height in the differential voltage curve is proposed to detect the inhomogeneity in the battery, and thus offers guidance to the design and management of large-format cells.