Characterizing Materials and Electrochemical Changes in a Range of 18650 Li-Ion Cells Cycled to 80% Initial Capacity

Abstract

Understanding systematic trends in degradation of Li-ion batteries through a broad range of operating conditions is critical to improving their performance. Over the course of the last few years, researchers at Sandia National Labs have been conducting 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 across a range of temperatures, discharge rates, and depth of discharge windows. Analysis of both in situ electrochemical data and postmortem materials degradation was done and correlated to determine root causes of rapid capacity fade.During cycling, Electrochemical Impedance Spectroscopy (EIS) and Incremental Capacity Analysis (ICA) 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.X-ray Computed Tomography (XCT) scans completed before cell disassembly showed large scale deformation in the cells’ jelly rolls. For the postmortem analysis, cells in pristine condition and 80% capacity were disassembled, and their electrodes were analyzed with X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and Differential Scanning Calorimetry (DSC). 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. 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 showed that cells with the most rapid rate of capacity fade had multiple degradation mechanisms active during cycling. The scale of this electrochemical and materials evaluation fills in gaps in the literature about commercial Li-ion battery performance.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.