Revealing the structural degradation mechanism of the Ni-rich cathode surface: How thick is the surface?

Abstract

Improving the cycling performance of Ni-rich LiNixCoyMnzO2 (NCM, 0 <= x,y,z < 1) is critical for commercializing rechargeable batteries based on Ni-rich NCM cathodes. Herein, we studied the structural degradation of Ni-rich NCM/graphite cylindrical 18650-type cells as a function of the cutoff voltage in the 4.2-4.4 V range by electrochemical impedance spectroscopy (EIS), scanning transmission electron microscopy-electron energy loss spectroscopy (STEM-EELS), and high-angle annular dark-field (HAADF) STEM, and modeled the Ni-rich NCM surface using density functional theory (DFT). We verified that the phase changes continuously rather than discretely from the surface into the bulk through cation mixing. Our results suggest that the thickness of the phase-change region at the surface causes the battery performance to suddenly degrade at a certain value. We found that the deterioration in cell performance is mainly due to increasing diffusion resistance in the positive electrode. A 10-25 nm cation mixing layer was observed at the cathode surface after 300 cycles, and this surface layer thickened with increasing charging voltage. Further, simulations revealed that the cathode surface spontaneously evolves oxygen at higher electrochemical potentials.