锂离子动力电池高倍率充放电过程中弛豫行为的仿真

基于电化学热耦合模型研究了动力锂离子电池高倍率充放电过程中的弛豫行为, 分析对比了不同充放电机制对电池弛豫行为的影响. 研究发现: 充放电过程中, 欧姆极化是造成电压骤变的主要原因; 而恒流-恒压的充电模式能够缓慢消除欧姆极化, 避免电池电压的骤变; 利用恒流恒压对电池进行充电能够充进更多的电量, 有利于电池性能的完全发挥; 固相锂离子浓度的弛豫时间比液相锂离子浓度的弛豫时间长, 并且在放电后期, 固相扩散的特征时间与液相扩散特征时间的比值不断增大, 固相扩散造成的极化在整个放电过程不可忽略.

Relaxation behavior simulation of power lithium-ion battery in high-rate charging-discharging process

The relaxation behaviors of a power lithium-ion battery significantly affect its performance, and these properties are greatly affected by temperature. This study presents a validated electrochemical-thermal model battery model covering the conservations of charge, mass, and energy and the electrochemical reaction kinetics, and considering the effect of heat on electrochemical performance of a battery. Using this battery model, the relaxation behavior of power lithium-ion battery in high-rate charging-discharging process and the effect of difference among charge-discharge systems are investigated. It is found that ohmic polarization is the main reason for voltage change in charging-discharging process. Constant-current-constant-voltage charging mode can effectively remit ohmic polarization and then avoid changing the voltage rapidly. In the shelving process after constant-current-constant-voltage charging, voltage change is smaller and the time for it to take to reach open circuit potential is shorter than in the shelving process after constant-current charging. In charging-discharging process, the values of polarization at positive and negative electrode are almost the same. Power lithium-ion battery can be charged into more energy by constant-current-constant-voltage charging modes, meaning that it is beneficial to battery performance. Because active material particles in electrodes have certain sizes, in discharging process, there is some gradient between the surface and center of solid particle, and the electrodes each have a certain thickness, different place of electrode has a different lithium-ion concentration. In the shelving process after discharging, there is no outer current, so the gradient of lithium-ion concentration disappears due to the effect of diffusion process. The relaxation time of lithium-ion concentration in solid phase is longer than in liquid phase. The ratio between characteristic time of solid diffusion and that of liquid diffusion increases constantly near the end of the discharge, thus the polarization due to solid diffusion cannot be neglected in the whole discharging process.