Durability and Reliability of Commercial Lithium-Ion Cells As a Function of Chemistry and Cycling Conditions

Energy storage systems (ESS) utilizing Li-ion batteries are increasingly deployed to maintain a robust and resilient grid and facilitate the integration of renewable energy resources. For this application, ESS are expected to last for a decade or even longer. To ensure long-term system safety and reliability, batteries must be selected based on application specific requirements and performance characteristics. However, suitable cell selection is made difficult by the dearth of data comparing popular commercial chemistries under comparable operating conditions. To address this gap, we initiated a large-scale, multi-year aging study of commercial LiFePO4/LFP, LiNixCoyAl1-x-y­O2/NCA, and LiNi0.80Mn0.15Co0.05O2/NMC cells, varying the discharge rate, depth of discharge (DoD), and environment temperature. Though all cells were cycled within manufacturer specifications, the time to reach 80% capacity varied by thousands of hours among cells of different chemistries and cycling conditions. Analysis of variance was performed to determine what variable levels or combinations of variables contribute most to degradation. Notable results included the resilience of LFP cells to full discharge (in contrast to the sensitivity of NCA and NMC cells), and the inverse temperature dependence of LFP and NMC cells. The source of differences in aging behavior was investigated through electrochemical impedance spectroscopy and differential capacity analysis. Efforts were made to identify markers of degradation that could be translated beyond high-precision lab equipment to field instrumentation. The next phase of this study involves monitoring the abuse response of aged cells to determine if the aging pathway influences the severity of the failure. 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. SAND2018-13934 A