磷酸铁锂电池寿命初期与末期安全性差异

Safety differences of LiFePO_(4)batteries at the beginning of life and end of life

相比新鲜电池,锂离子电池在全生命周期内的安全性研究更值得关注。以寿命初期(BOL)与寿命末期(EOL)两种状态下的大尺寸方型铝壳磷酸铁锂动力电池为研究对象,先分析了BOL和EOL电池在比热容、热导率、材料热稳定性以及直流内阻方面的差异,再详细对比了电池在过放电、过充电、外部短路、加热、针刺、挤压等安全性能的差异。结果表明,相比BOL电池,EOL电池比热容由1.088 J/(g·℃)降低为1.065 J/(g·℃);电池高度、宽度、厚度方向的热导率分别从25.84、21.21、1.05 W/(m·K)降低为22.20、18.44、1.00 W/(m·K);负极材料和电解液的放热峰向低温偏移,121℃附近出现固体电解质界面膜分解放热峰,嵌锂石墨层间化合物、电解液和黏结剂间的反应放热焓由1019 J/g减小为841 J/g。安全性方面,EOL电池过放电产气更多,厚度鼓胀大;过充电产气更多,防爆阀提前7%荷电态开启;外部短路电流无法熔断转接片,电池将持续放电至过放态,产气鼓胀严重;针刺无烟雾释放、防爆阀未开启,安全性大幅提升,加热引发热失控的温度差别不大,其他安全测试项差异较小。研究结果丰富了锂离子电池全生命周期内安全性能的研究,有助于电池单体、模组及系统热失控防护设计。

Compared with fresh batteries, the safety research of lithium-ion batteries in the whole life cycle is more worthy of attention. The large-size prismatic aluminum-shell lithium iron phosphate power batteries in two states,i.e., beginning of life(BOL) and end of life(EOL) were selected as the research objects. The specific heat capacity,thermal conductivity, material’s thermal stability and direct current internal resistance of BOL and EOL batteries were analyzed firstly. The safety differences of BOL and EOL batteries were compared in detail, such as overdischarge, over-charge, external short-circuit, heating, nail penetration, and crush, etc. The results show that compared to BOL battery, the specific heat capacity of the EOL battery decreases from 1.088 J/(g·℃) to 1.065 J/(g·℃). The thermal conductivity in the height, width, and thickness directions of the battery decreased from 25.84,21.21, 1.05 W/(m·K) to 22.20, 18.44, 1.00 W/(m·K). The exothermic peaks shifted to low temperature, and the exothermic peak of solid electrolyte interface film decomposition appeared near 121℃, and the exothermic enthalpy of the reaction between the intercalated lithium-graphite compounds, electrolyte, and binder decreased from 1019 J/g to 841 J/g. In terms of safety, the EOL battery after over-discharge generated more gas, leading to a larger thickness swelling. Over-charge produced more gas with EOL battery, and the vent opened ahead of 7% state of charge. The external short-circuit current cannot fuse the connecting structure, and the battery would discharge continually to over-discharge state, giving more gas production and serious swelling. The nail penetration did not release smoke, the vent was not broke, and the safety was greatly improved. The temperature of thermal runaway caused by heating was close, and the differences of other safety test items were little. The research results enriched the research on the safety performance of lithium-ion batteries in the whole life cycle, and contributed to the thermal runaway protection design of batteries, modules and systems.