Investigation of capacity recovery during rest period at different states-of-charge after cycle life test for prismatic Li(Ni_(1/3)Mn_(1/3)Co_(1/3))O_2-graphite cells

摘要

In this publication two strategies are introduced to assess irreversible capacity loss during shallow cycling at different average SOCs. Due to superposed reversible capacity effects, a simple evaluation of capacity trend is not sufficient. Those reversible effects are related to contributions of the anode overhang (geometrical oversized anode) and to the homogeneity of lithium distribution (HLD). For both strategies the cycling test is additionally followed by a calendaric aging test to recover capacity. While the contribution of HLD can be assessed by storing the cells at the same average SOC as during cycling, the contribution of the anode overhang is evaluated for a defined low SOC. During the storage phase in all cases the extractable capacity rises supporting the reversible capacity theory. Moreover, the HLD, measured with differential capacity analysis and capacity difference analysis, rises as well; this is the case for all test conditions exhibiting the reversible nature of HLD and its influence on extractable capacity. The irreversible capacity losses are compared to an alternative method, called 'slope method', assuming that the aging is nearly linear and that the linear part at the end of test is mainly attributed to irreversible aging. While the results of both methods are in the same order of magnitude, the relaxation method can be applied, not only to static, but also to any dynamic aging profiles.