Abstract
Cycling and storage are two typical tests in laboratory to evaluate the degradation behavior of lithium-ion cells (LIC). In real world application, however, LIC is always under the stress of multi-factors with no predefined sequence. This work addresses some of the intriguing issues in the study of the degradation behavior of LIC under multiple-stressors: the effect of different stresses on degradation rate, boundary demarcation between accelerated stress test and abuse, life prediction, path dependence, and lithium plating detection techniques, etc. The techniques developed and the insight gained are crucial to the degradation mechanism identification, charging protocol optimization, and life prediction. (1) A generic cycling profile, for which the storage is just a special case, was devised to study the main effects of the seven key stress factors, using L18(2×37) orthogonal matrix in design of experiment (DOE). Orders of the key stress factor' impacts on capacity degradation were obtained. (2) L27(313) was employed to study the effects of the levels of three key stress factors. Close examination of the test results led to a method demarking the boundary between accelerated stress test and abuse that may lead to lithium plating. (3) Within the obtained non-abuse boundary, orthogonal regression design method with 28 tests was used to build an empirical aging model, which was validated with four independent cycling conditions. (4) Storage tests using different thermal excursion profiles were devised to evaluate path dependence characteristics of the cell capacity degradation and resistance variations. (5) Based on the degradation behaviors, three techniques were explored to evaluate whether there has been lithium plating during the cycling. The prediction was validated with the post-mortem analysis.