Systematic Long-Term Cycling of 18650 Batteries Beyond 80% Capacity

Abstract

The true useful life of secondary Li-ion batteries is currently not well characterized. 80% of initial capacity, a decades old metric for vehicle applications, is a common endpoint in literature cycling studies and commercial cell specification sheets. As such, much is not known about cell performance beyond this metric. The 80% cutoff may not be applicable for non-vehicle applications, like grid energy storage, where there are different requirements for performance. Degradation data beyond the 80% capacity threshold is needed to make these evaluations.To address this, we have been conducting a multi-year study of both cycling and calendar aging of 18650 cells. During this study, LiFePO4/LFP, LiNixCoyAl1-x-yO2/NCA, and LiNixMnyCo1-x-yO2/NMC cells have been cycled at systematically varied temperature, state of charge (SOC) ranges, and cycling rates. Our group has previously reported on trends in cells as they were cycled to 80% capacity [1]. In this study it was generally found that SOC range impacted the capacity fade rates most significantly. Here, we present an update on performance of cells cycled beyond 80% capacity. We aim to identify so called knee points, and determine what conditions extend useful life of the cells.Results so far suggest that SOC range continues to be the largest factor in capacity fade rate. Also, combination of stress factors (high SOC with low temperature) causes rapid capacity fade beyond what would be expected by simple addition of degradation rates from the individual stress factors. Some cells do not appear to have a knee point as they have been in rapid degradation modes from the start of cycling. Cells that do display a knee have onset points that vary by chemistry and conditions of cycling. This study represents the broadest public report of post 80% capacity cycling across multiple chemistries. It will inform cell lifetime prediction and allow for better utilization of battery systems.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. SAND2021-5032 A