NCA三元锂离子电池分荷电状态循环的热特性和容量衰退研究

层状三元材料LiNi_0.8Co_0.15Al_0.05O₂（NCA）具有高能量密度和高比容量,在电动汽车领域占据重要地位.但是较差的容量保持率和热安全问题限制了其应用. 本文研究了18650型NCA/graphite（2.4 Ah）锂电池分区间循环容量衰退机理和热行为. 所考虑的荷电状态（state of charge,SOC）区间有0% ~ 20%（低）、20% ~ 70%（中）、70% ~ 100%（高）及0% ~ 100%（全）四个区间. 为了获得电池在不同SOC区间循环后衰减状况,以100个循环为一个周期,每个循环周期结束后,在25 ^oC下测试四个电池的基础特性,包括容量、容量增量（incremental capacity,IC）、电阻及电化学阻抗谱(electrochemical impedance spectroscopy, EIS),同时监测电池放电时的温度来讨论电池不同区间循环后的热行为. 测试结果表明,电池在全区间循环会降低电池寿命,而在非全区间循环的电池都能一定程度上减缓电池衰老的速度. 另外,全区间循环热特性最差而中端循环则表现出较好的热性能,对容量增量曲线分析发现,在高中低区间的性能衰退的主要原因是活性锂离子的损失,而在全区间还包括活性材料的损失和反应内阻的增大.

Degradation and Thermal Characteristics of LiNi0.8Co0.15Al0.05O2/Graphite Lithium Ion Battery after Different State of Charge Ranges Cycling

The LiNi_0.8Co_0.15Al_0.05O₂ (NCA) cathode exhibits high energy density and large reversible capacity, which plays an essential role in the field of electric vehicles (EVs). However, low capacity retention and poor thermal stability limit its application. Few literatures are found for the capacity degradation mechanism of NCA/graphite batteries at home and abroad. The different state of charge (SOC) ranges cycle degradation behaviors of 18650-type NCA/graphite (2.4 Ah) battery were studied in this paper. The SOC ranges considered were 0% ~ 20% (low), 20% ~ 70% (medium), 70% ~ 100% (high), and 0% ~ 100% (whole). To obtain the states of the batteries being cycled in different SOC ranges, the basic characteristics of the four batteries, including capacity, incremental capacity (IC), internal resistance, and electrochemical impedance spectroscopy (EIS), were tested at 25 ^oC before and after every 100-cycle up to 400 cycles. At the same time, the surface temperature of the batteries during discharging was monitored to analyze the thermal characteristics. A detailed analysis for the IC curve of NCA/graphite was performed, making the mechanism of capacity degradation more clear. The results show that the battery life would be shortened after the whole SOC range cycling and the battery aging rate would be reduced to a certain extent upon cycled in the partial range. In addition, the battery thermal characteristic became the worst after the whole SOC range cycling, but the battery thermal performance became the best after the medium SOC range cycling. Analyzing IC data reveals that the main reason for the performance degradation of batteries in the high, medium and low SOC ranges cycling may be the loss of active lithium ions, and that in the high SOC range cycling may also include the loss of active materials and the increase of reaction internal resistance.