Scaling Analysis of a Li-Ion Pouch Cell Via a Simplified Electrochemical/Thermal Model

Abstract

Owing to their merits of high energy density, low self-discharge and absence of memory effects, Li-ion batteries have become the leading energy storage devices for consumer electronics and electric vehicles. Thermal management of large-format Li-ion cells is crucial because of their small surface-to-volume ratios. High temperature and temperature inhomogeneity can lead to locally irregular degradation of active electrode materials, or even catastrophic thermal runaway [1]. Spatially variable internal temperatures are commonly observed in large-format batteries, and have been shown to impact performance and cycle life significantly [2-4]. Since cell temperature can strongly affect the physical properties and reaction kinetics within a battery, it is vital to understand the heat generation and transient temperature within a cell during charging/discharging. An experimentally validated electrochemical/thermal cell model is therefore necessary to predict and optimize operational parameters. Currently, thermal simulations are primarily based on equivalent circuit models (ECM) and electrochemical models [5]. Due to their simplicity, ECMs can be solved quickly, favoring their use in battery management systems (BMSs). In contrast, electrochemical models describe the basic electrochemical and physical processes through detailed local descriptions accounting for the conservation of charge, mass and energy, etc. The most popular electrochemical Li-ion battery model was developed by Fuller, Doyle, and Newman [6, 7]. In this study, we demonstrate a simplified 3D electrochemical model for large format Li-ion prismatic pouch cell, which achieves computational efficiency by reducing the P2D model to the simpler structure originally developed by Newman and Tobias [8]. The model has advantages of both ECM and continuum models, that is, it is fast enough for performance prediction and thermal management while still retaining physical information that can be ascribed to microscopic charge and mass transport phenomena. A scaling analysis is carried out via non-dimensionalization of the cell model. Several key dimensionless numbers are shown to govern the dynamic performance of the battery during charging and discharging. The complexity of the model, such as the number of cell layers and mesh scheme, can be reduced by asymptotic analysis to make the model more time-efficient. The non-uniformity of both state of charge (SOC) and temperature within the cell are attributed to non-uniformity of the 3D current-density distribution, and also to the anisotropic nature of the thermal conductivity tensor. The relative importance of the fundamental dimensionless quantities in controlling local heat generation and thermal evolution will be discussed.