Investigation of the Difference in Charge/Discharge Resistance for Cathode Materials after Cycle Test Combined with STEM-EELS and XAFS Analysis

Abstract

The coin cells with a lithium nickel cobalt manganese oxide (LiNi0.33Co0.33Mn0.33O2) and lithium metal as a cathode and anode, respectively, were degraded by the cycle tests. Electrochemical analysis and characterization were performed on the degraded coin cells and the cathode materials taken out after decomposition. The DC resistance and AC impedance measurements showed that the difference in charge/discharge resistance increased with degradation and correlated with the resistance component derived from lithium insertion into the lattice. The multiple curve resolution (MCR) analysis using scanning transmission electron microscopy—electron energy loss spectroscopy (STEM-EELS) data and X-ray absorption near edge structure (XANES) analysis revealed that cation mixing occurred at 10 nm region from the surface of cathode after cycle degradation. These analysis clarified that the cation mixing region did not expand inside even if the degradation progressed, and the degree of cation mixing on the surface increased. Based on our results, the coulomb repulsion to Li+ and the reduction of the oxygen interlayer distance by the transition metal transferred into the Li layer were presumed to be predominant factors of an increase in the difference in the resistance between charge and discharge process.