The Influence of Structure on the Electrochemical and Thermal Response of Li-Ion Battery Electrodes

Abstract

Abstract The continued advancement of lithium ion batteries for transportation applications requires addressing two key challenges: increasing energy density and providing fast charging capabilities. The first of these challenges can be met in part through the use of thicker electrodes, which reduce the electrochemically inactive mass of the cell. However, implementation of thick electrodes inherently presents a trade-off with respect to fast charging capabilities. As thickness is increased, transport limitations exert greater influence on battery performance and reduce the ability of the battery to meet aggressive charge conditions. This trade-off can manifest over multiple length scales. At the particle-scale, interactions between solid diffusion and reaction kinetics influence the effective storage of lithium within the active material. At the electrode scale, diffusion limitations can lead to local variations in salt concentrations and electric potential. These short-range and long-range effects can combine to influence local current and heat generation. In the present work, a pseudo-2D lithium ion battery model is applied to understand how active material particle size, porosity, and electrode thickness impact local field variables, current, heat generation, and cell capacity within a single cell stack. COMSOL Multiphysics 5.2 is used to implement the pseudo-2D model of a lithium ion battery consisting of a graphite negative electrode, polymer separator, and lithium transition metal oxide positive electrode. Lithium hexafluorophosphate (LiPF6) in 1:1 ethylene carbonate (EC) and diethylene carbonate (DEC) was used as the electrolyte. The model was built assuming that the active particles are representative spherical particles. The governing equations and boundary conditions were set following the common Newman model. Cell response under varied combinations of charge and discharge cycling is assessed for rates of 1C and 5C. Aggressive charge and discharge conditions lead to locally elevated C-rates and attendant increases in local heat generation. These variations can be impacted in part by tailoring electrode structures. To this end, results for parametric studies of active material particle size, porosity, and electrode thickness are presented and discussed.