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, 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. Cells were cycled at different state of charge (SOC) ranges (0-100%, 20-80%, and 40-60%), temperatures (15, 25, and 35 °C) and discharge rates (0.5C, 1C, 2C, and 3C). Additionally, we present data from a concurrent calendar aging study at different temperatures (15, 25, and 35 °C) and SOCs (25, 50, and 90%). Our group has previously reported on trends for cycle aging down to 80% capacity ^[1]. Here, we present an update on performance of cells cycled beyond 80% capacity to an end of life (EOL) of 40% capacity and cells undergoing calendar aging.

Results so far suggest that SOC range continues to be the largest factor in capacity fade rate for both chemistries. Combining stress factors (NMC at high SOC and low temperature) causes rapid capacity fade beyond what would be expected by simple addition of degradation rates from the individual stress factors. Depending on the conditions of cycling, so called knee points do not always appear when a cell is cycled beyond 80%. When a knee point occurs varies by cycling conditions. We also consider additional metrics by which battery performance can be measured such as round-trip efficiency (RTE), total discharged energy, and internal resistance (IR). And find that IR increases generally matches well with capacity fade and appear to increase dramatically at observed knee points. For the calendar aging study, we find that holding cells at higher SOCs and temperatures generally increases the rate of fade. 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. SAND2022-5089 A

References:

¹Preger et al. “Degradation of Commercial Lithium-Ion Cells as a Function of Chemistry and Cycling Conditions” J. Electrochem. Soc., 2020, 167, 120532.