Mitigating Temperature Heterogeneity of Battery Modules Under Fast Charging

Abstract

Temperature control is essential for fast-charging performance of Li-ion battery. Cold temperature leads to sluggish ion transportation of electrolyte, brittle polymeric cell components, changes in solid electrolyte interface (SEI) properties and associated resistance build-up, and Li plating and dendrite growth [1]. High temperature boosts fast charging but can also accelerate battery degradation and increase the rise of battery thermal failure. This is caused by the increased rate of side-reactions, resulting in loss of cyclable lithium and higher rate of attrition of active materials at higher temperatures.Temperature and its gradient in a battery unit strongly affect performance and life of battery units [2]. It is recommended that pack temperature uniformity of a li-ion battery pack in electric vehicles shall be less than 3 ºC [3]. Battery thermal management systems (BTMS) are important for controlling battery pack temperature and minimizing temperature gradients to prevent thermal-related issues in Battery Energy Storage Systems (BESS). This thermal management goal is more critical for fast charging of battery modules made of large format, high-energy-density cells. Current BTMS in battery electric vehicles (BEVs) are inadequate in limiting the maximum temperature rise of the battery during extreme fast charge (i.e., 6C charge). To achieve fast-charge, the size of the battery thermal management system needs to increase from today’s BEV average size of 1–5 kW to around 15–25 kW [4].A combined experimental and modelling approach is employed to access thermal and electrochemical heterogeneities of a battery module under extreme fast charge conditions and develop corresponding mitigation approaches. The electrochemical-thermal model was built based on electrical characterization of 32 mAh pouch cells, including constant-current 1C to 9C rates of charging at varying temperatures. Predictive performance of the model in heat generation was validated by comparing results against measurements conducted using a microcalorimeter. Thereafter, the validated model is used to predict performance of a battery module consisting of six large format pouch cells. The large pouch cell has a capacity of 25 Ah and the identical electrode design of the 32 mAh cells. 3D simulation results suggest significant temperature and charge differences can be produced. The heterogeneous behaviour was enlarged along charging. As shown in Fig.1, it was found that electrodes close to the tabs were preferentially charged.Cell electrochemical heterogeneity can be reduced by reducing cell temperature difference. Two potential solutions are investigated using the developed 3D model, including the enhancement of heat transfer within cells, such as increasing cell thermal conductivities with thicker current collectors, and the optimal design of thermal management systems. The feasibility of state-of-the-art thermal management strategies for fast charging is evaluated, including liquid cooling using cold plate devices and direct liquid cooling.