LiFePO_4锂离子动力电池内阻与放电倍率关系研究

The relationship between internal resistance and discharge rate of LiFePO_4 batteries

研究锂离子动力电池内阻与放电倍率的关系可以改善电池管理系统(BMS)内阻模型的准确度和适应性,对提高电池状态如荷电状态(SOC)的估算精度具有巨大的意义和市场价值。本文采用二阶RC网络等效电路模型,通过不同倍率恒流放电和脉冲放电对25 A·h LiFePO_4锂离子动力电池进行直流内阻(DCIR)和脉冲内阻(PDIR)表征,对不同荷电状态(SOC)下DCIR、PDIR_0、PDIR_1、PDIR_2随放电倍率的变化规律进行拟合,得到DCIR、PDIR_1、PDIR_2、PDIR_(tot)都非常符合双指数关系,PDIR_0符合线性关系且几乎不变,并通过对比分析排除因温度造成内阻变化的可能。从固态电解质界面(SEI)生成速率与分解速率的化学平衡角度解释了DCIR、PDIR_1、PDIR_2、PDIR_(tot)在低放电倍率时大可能是由于SEI分解速率小于生成速率,SEI与静置时的相似,电阻较大;反之,高放电倍率时小可能是由于SEI分解速率大于生成速率,SEI分解变薄并重新达到新的速率平衡,从而表现出较低的内阻。

This paper concerns the internal resistance as a function of discharge rate of Lithium-ion batteries. The aim of the work is to improve the accuracy and adaptiveness of internal resistance model for battery management system（BMS）, which is of significance to the accurate prediction of the status of batteries such as the state of charge（SOC）. We used a second order RC equivalent circuit model to analyze the direct current internal resistance（DCIR） and pulse discharge internal resistance（PDIR） with 25 A·h LiFePO4 batteries under constant discharge and pulse discharge at various discharge rates. Data fittings were done on the DCIR, PDIR1, PDIR2 and PDIR（tot） at various states of charge as a function of discharge rate and a good agreement was obtained with a double-exponential relationship, whereas a linear relationship held for the PDIR0. The analyses also suggested independence of the internal resistance change to the temperature effect. Based on the chemical equilibrium between the formation and decomposition of the solid electrolyte interface（SEI）, we concluded that large DCIR, PDIR1, PDIR2 and PDIRtot observed at low discharge rates was likely to be due to low decomposition rate than the formation rate of the SEI with a high resistance, similar to the standby situation. On the other hand, a lower resistance at a higher discharge rate could be attributed to higher decomposition rates than the formation rates of the SEI, leading to a thinner SEI until a new equilibrium status was reached, and hence a reduced internal resistance.