Path Dependence of Li-Ion Battery Degradation During Cycling to 80% Capacity

Li-ion batteries are complex systems whose long-term performance is still the subject of much research. Of particular interest is the degradation of electrode materials during long-term cycling under a variety of operating conditions. Several mechanisms for degradation of Li-ion batteries have been proposed in recent years. These proposed mechanisms have not been tied to performance of commercial cells during long-term cycling under a variety of operating conditions. Determining and understanding systematic trends in degradation is crucial to further improvement of Li-ion battery performance and safety.<br/>Previously, researchers at Sandia National Labs conducted a systematic study of commercially available battery cycling behavior. Dozens of commercial NCA (LiNixCoyAl1-x-y¬O2) 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. To better understand the reasons for the observed degradation trends, electrochemical characterization and materials postmortem characterization was carried out on cells in their as received condition, and 80% capacity. This allowed for the linking of degradation modes to specific cycling conditions and chemistries.<br/>Analysis of electrochemical characterization via Electrochemical Impedance spectroscopy (EIS) and Incremental Capacity Analysis (ICA) was combined with post-mortem materials characterization consisting of X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Electron Dispersive Spectroscopy (EDS), Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP), and X-ray Computed Tomography (XCT), to generate a relatively complete picture of a cell’s state of health when it was disassembled. Seven NCA and seven NMC cells (total of 14) were selected to be analyzed in this way. For both chemistries an as received cell was selected along with cells that were cycled at different ambient temperatures and between different state of charge (SOC) ranges. In total, this work correlated 164 sets of data to understand similarities and differences in cell degradation during cycling at different conditions.<br/>Two key conclusions have been drawn from this data. First, the degradation of cells to 80% capacity is path dependent based on cycling conditions and cell chemistry. Second, rapid capacity fade was correlated to cells with evidence of multiple degradation mechanisms occurring and it appears that multiple mechanisms can interact to amplify their effects on the cell.<br/>This work was funded by the DOE Office of Electricity under the direction of Dr. Imre Gyuk.<br/>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. SAND2021-13303 A