Degradation of Commercial Lithium-Ion Cells Beyond 80% Capacity

To defray the upfront cost of lithium-ion electric vehicle (EV) batteries and to minimize waste, there is substantial interest in repurposing them after their EV end-of-life for grid-related applications (battery second life). The reliability of any economic or technical feasibility analysis for second life batteries strongly depends on the battery degradation behavior, but insight into this degradation in the region of interest remains limited. Most cycling studies in the open literature only age batteries to about 80% capacity (the typical cutoff for marking end-of-life). A few studies that have cycled batteries beyond 80% capacity have suggested the existence of a 'tipping point' upon which rapid degradation is initiated, but a limited set of cycling conditions and chemistries were explored.

We have previously reported on our large-scale, multi-year aging study of commercial LiFePO₄/LFP, LiNi_xCo_yAl_1-x-y­O₂/NCA, and LiNi_xMn_yCo_1-x-yO₂/NMC cells, varying the discharge rate, depth of discharge (DoD), and environment temperature. Now that many of those cells have reached 80% capacity, we have initiated a second life aging study. The advantage of studying these cells compared to those from second-hand EV battery suppliers is that the cycling history is known. Thus, it is more feasible to determine the extent to which prior cycling conditions influence later degradation and the early markers that could foreshadow the possibility of rapid degradation beyond 80% capacity. Diverse cycling conditions were studied to determine what protocols, if any, can mitigate rapid degradation.

Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA0003525. SAND2019-4631 A