Correlating Materials Degradation Trends to Electrochemical Indicators in Systematic Long-Term Cycling of Commercial Li-Ion Batteries

Abstract

In order to improve the operation of Li-ion batteries the long-term performance is the subject of much research. Of particular importance is understanding systematic trends in degradation through a broad range of operating conditions. Over the course of the last few years, researchers at Sandia National Labs have conducted a systematic study of battery cycling behavior. In this study, dozens of commercial NCA (LiNixCoyAl1-x-yO2) and NMC (LiNixMnyCo1-x-yO2) cells were cycled to 80% capacity and end of life across a range of temperatures, discharge rates, and depth of discharge windows. Analysis of both in situelectrochemical data and postmortem materials degradation was done.During cycling, cell resistance (IR), Electrochemical Impedance Spectroscopy (EIS), and differential capacity (dQ/dV) experiments were conducted periodically. The data from these experiments were used to understand the state of health (SoH) of cycling cells, including SEI formation and Li plating, as well as general mass transport and kinetic properties.For the postmortem analysis cells in pristine condition, 80% capacity, and end of life were disassembled, and their electrodes were analyzed with X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Differential Scanning Calorimetry (DSC), and Brunauer-Emmett-Teller Isotherms (BET). These techniques analyzed different aspects of the electrodes’ physical and chemical properties, including Li plating, micro-cracking, SEI layer formation, and phase changes in cathode materials. BET and DSC measurements also provided insight into changes in the degree to which aged electrodes undergo thermal runaway events.Correlation of the postmortem materials analysis with the in situ electrochemical analysis gave an in-depth understanding of the origins of performance fade during cycling. The scale of this electrochemical and materials evaluation fills in gaps in the literature about battery performance and degradation beyond 80% capacity.Sandia National Laboratories is a multimission 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.