关键词: 锂离子电池, 零和脉冲, 加速循环, 弛豫阻抗, 直流内阻, 极化电压

Abstract:

The zero-sum pulse method was used to perform the accelerated cycle tests on the cylindrical 21700 battery with a nominal capacity of 4.8 Ah. The experimental batteries, composed of three positive and negative materials, were evaluated in an accelerated cycle for 7 days, with 10%SOC chosen as the best test interval. In comparison with the test findings of the reference battery, the cycle performance of the battery was investigated and evaluated by incorporating different characteristics. The assessment result was similar with the traditional cycle test when the capacity retention rate of the battery was compared before and after the accelerated cycle. The negative binary battery had the greatest discharge capacity retention rate of 99.75%, followed by the reference battery with 99.43%, while the positive binary battery had the lowest discharge capacity retention rate of 96.33%. Additionally, according to the examination of the DC internal resistance and polarization voltage growth rate of the battery during the accelerated cycle, the growth rate of the positive binary battery was significantly larger than that of the other two types of batteries. Further examination of the battery's instantaneous and relaxation impedance revealed that a considerable number of side reactions had happened on both the positive and negative electrodes, which was the main reason for the positive binary battery's poor cycle. The solid phase diffusion impedance increased due to the interface film's thickening and the growth of the deposits. Consequently, the relaxation impedance growth rate reached 40%. Through the EIS test and physical property analysis of the positive and negative electrodes of the battery after cycling, it was revealed that the diffusion impedance of the positive binary battery was noticeably higher, and the cracking degree of the secondary particles of the positive electrode was higher. This finding offers a basic explanation for the phenomenon whereby the positive electrode cracking side reaction causes the positive binary battery's solid diffusion impedance to increase quickly during accelerated cycling. The accelerated cycle test method is based on the real cycle operating parameters without adding extraneous variables like temperature and rate. Through comprehensive analysis of numerous test parameters, the objective of qualitatively evaluating the cycle performance of experimental batteries can be accomplished, which can not only considerably minimize the cycle test length but also offer reference data for the investigation of battery cycle failure.

Key words: Li-ion batteries, zero-sum pulse, accelerated cycle, relaxation impedance, DC internal resistance, polarization voltage