Optimizing Design Parameters of Li-Ion Batteries for Phev and EV
Authors: Xu Meng , Wang Xia 
Published: February 2020

A lithium ion battery can be designed as a power battery or an energy battery depending on the power and energy requirements when the battery is applied in plug-in hybrid electric vehicles (PHEV) and electric vehicles(EV). Design parameters of a battery cell usually play an important role in its power density and energy density. For instance, increasing the thickness of the porous electrode offers higher energy density for the battery cell with fixed geometrical dimensions. However, a thicker electrode tends to lengthen the path for lithium ions to diffuse in the electrolyte, which results in poor power capability. Some other parameters such as the particle size and porosity of the active materials in each electrode are also important to the electrochemical performance of a battery cell thus influencing the power density and energy density.

The objective of this research is to optimize the battery design parameters to obtain the desired power density and energy density for the batteries used in PHEV and EV. A one-dimensional electrochemical isothermal model is developed by coupling the mass conservation, charge conservation, as well as electrochemical kinetics for a Li-ion cell. The one dimensional cell unit is assumed to be a sandwich structure that is composed of a negative electrode, a separator and a positive electrode. The electrodes are porous solid matrix that consists of active particles with spherical shapes of uniform sizes and additives. The separator is a porous polymer membrane that constitutes a physical barrier between the two electrodes. Both electrodes and separator porous matrix are impregnated with electrolyte ensuring the transfer of lithium ions between two electrodes during charge and discharge. The three coupled governing equations, including mass, charge conservation as well as Butler-Volmer equation, along with other constituent equations and proper boundary conditions, will be solved by COMSOL Multiphysics software. The cell properties, such as geometry parameters and electrochemical properties, are obtained from the cell manufacturer. The experimental data are provided by an industrial partner. The model validation is conducted by comparing simulation results with experimental data of discharging behaviors of different testing samples at various discharge rates.

The parameters of interest include the thickness, porosity and particle size of electrode materials at a single cell level. For example, the electrode thickness study was conducted with the same porosity and the same particle size. It has been found that the cell discharge capacity faded more when the thickness of positive electrode increases from 45um to 70um at the same discharge rate (2C), which is consistent with the experimental results. Faster the cell is discharged, and more the discharge capacity fades. More results on the influence of porosity and particle size of electrode material on the cell performance will be reported in the conference.

Based on the study of each single parameter, a multi-parameter optimization can be conducted to meet a specific design objective of both power density and energy density properties for cells applied on PHEV or EV. This research reveals the effects of design parameters of a Li-ion battery electrode on the battery energy density and power density, which are very useful for battery modeling and design.