Diagnostic studies of components harvested from high-power LiNi0.8Co0.15Al0.05O2/Graphite lithium-ion cells were conducted to determine the causes of capacity fade and impedance rise exhibited by the cells during accelerated aging. The data show that the positive electrode is the dominant contributor to impedance rise. The electrode impedance rise appears to result from at least two distinct and maybe independent processes that become prominent in different stages of the cell life – degradation of the electrode-electrolyte interface in early- and mid- life cells, and degradation of lithium-ion diffusion characteristics within the composite electrode in late-life cells.
The various mechanisms postulated to explain cell performance degradation behavior focus on the positive electrode and include (1) polymer formation and viscosity changes in the electrode pores, (2) retreat or redistribution of carbon away from the oxide particles, (3) oxide particle shrinking resulting from oxygen loss, and (4) oxide particle surface changes as a consequence of oxygen loss. A phenomenological model, based on concentrated solution theory, has indicated that a general degradation of the ionic pathway is probably the most significant contributor to positive electrode interfacial impedance increase.
More details can be found in the following report (and references therein):
D.P. Abraham, ed., “Diagnostic Examination of Generation 2 Lithium-Ion cells and Assessment of Performance Degradation Mechanisms”, Argonne National Laboratory Report, ANL-05/21 (2005).
This report is intended to provide battery developers an insight into the process for diagnostic evaluation of a generic lithium ion battery technology. The data presented herein indicate the type of information that can be obtained by the various diagnostic techniques. The level of understanding required by the developers will dictate the type of tools used for the evaluation – this report can be a guide for appropriate tool selection.