SURFACE RECONSTRUCTION AND CHEMICAL EVOLUTION OF STOICHIOMETRIC LAYERED CATHODE MATERIALS FOR LITHIUM-ION BATTERIES

摘要

Stoichiometric LiNi_xMn_xCo_(1-2x)O2 (NMC) represents a family of prominent cathode materials with potential to improve energy densities, reduce costs and enhance safety for plug-in hybrid electric vehicles and electric vehicles. The challenge of achieving these goals is attributed to undesirable modifications of LiNi_xMn_xCo_(1-2x)O2 materials during high-voltage cycling, which leads to gradual buildup of cell impedance and simultaneous capacity fading. The purpose of this study is to correlate surface and bulk structural characteristics of NMC materials with the electrochemical performance. Aided with the state-of-the-art atomic-scale annular dark-field scanning transmission electron microscopy (ADF/STEM) and electron energy loss spectroscopy (EELS) as well as high-throughput ensemble-averaged synchrotron X-ray spectroscopy, the present study provides insights into the surface reconstruction (R-3m to Fm-3m transition) and chemical evolution (surface reaction layer) in NMC materials and directly correlates with the origin(s) of long-standing challenges in NMC materials, including high-voltage capacity fading, impedance buildup and first-cycle coulombic inefficiency. Our results represent an important pathway towards understanding layered cathode materials using combined diagnostic tools at complementary length scales and the methodology herein provides guidance to advance knowledge for battery materials in general.