动力电池循环膨胀力研究

Study on cyclic compression force of power battery

锂离子动力电池在使用过程中会发生电池鼓胀,既影响电池的寿命,也会由于其鼓胀超过模组框架允许极限而发生模组框架的破坏,进而引发安全事故,因此研究动力电池生命周期内膨胀力的变化规律对于提升电池性能及安全具有重要意义。总结了NCM622三元和磷酸铁锂体系动力电池使用寿命过程中机械力的变化规律,研究结果表明:(1)无论是三元体系还是磷酸铁锂体系动力电池,都会随着容量的衰减,膨胀力逐渐增加;(2)在单次充放电过程中三元电池由于正极材料晶胞体积变化很小,电池形变主要受负极材料影响,所以内部鼓胀力随着电压的增长而增加;磷酸铁锂电池由于正极材料在充放电过程中的变化影响,在充电和放电过程中电池膨胀力会分别出现波谷;(3)三元电池和磷酸铁锂电池的循环容量衰减和膨胀力增加均符合线性关系,循环过程中磷酸铁锂电池的容量衰减较三元电池慢,且磷酸铁锂电池膨胀力增长也小于三元电池。膨胀力的变化规律为防爆阀的泄爆压力和模组结构强度设计提供重要依据。研究表明释放循环过程中的膨胀力可以提升电池的使用寿命。

Lithium-ion power battery generated swelling behavior during charge-discharge cycle,which will deteriorate performance.Additionally,when the expansive force exceeded the allowable limit of the module frame,it will leads to safety accidents.Therefore,study of cell expansion force evolution during its full life cycle is meaningful for improving its performance and safety.The relation between capacity retention ratio and mechanical force evolution during their service life based on LiNi1-x-yCoxMnyO2(NCM)and lithium iron phosphate(LFP,LiFePO4)power battery was compared and summarized.The results demonstrate that(1)no matter the ternary system or the lithium iron phosphate system,the expansion force will increase with the decrease of the capacity;(2)in the charge discharge step,the expansion force of NCM cell increases accompanied by voltage going up because little change occurs for NCM materials cell structure,however,for LFP cell,expansion force experiences a valley value due to cell microscopic structure change of LFP during charge and discharge process;(3)the expansive force and capacity retention curves of both batteries are all fitted to linearity,of which capacity fading of NCM battery is faster than that of LFP cell,and expansive force of NCM battery increased at a faster rate comparable to that of LFP battery.The expansive force evolution of NCM and LFP power battery provides a guidance value for battery vent trigger pressure and module construction strength design.Additionally,it is demonstrated that releasing expansive force during cycle can improve service life.