Enhancing high-voltage structural stability of single-crystalline Ni-rich LiNi0.9Mn0.05Co0.05O2 cathode material by ultrathin Li-rich oxide layer for lithium-ion batteries

Ni-rich single-crystalline LiNixCoyMn1-x-yO2 (SNCM; x ≥ 0.8) cathodes outperform conventional polycrystalline Ni-rich NCM in terms of mechanical and electrochemical stabilities. However, SNCM cathodes exhibit interfacial instability and rapid capacity decay due to NiO formation at the surface and gradual structural collapse from the surface to the bulk region. We applied an electrochemically-inert Li-rich Li2MnO3 (LMO) coating on SNCM (x = 0.9) to mitigate its structural deterioration during cycling and prevent undesired side reactions at the cathode/electrolyte interfaces. An ultrathin LMO coating layer synthesized from amorphous MnO2 with residual Li compounds suppressed c-axis lattice shrinkage/expansion of SNCM during H2–H3 phase transition, thus minimizing anisotropic lattice strain and intragranular cracking. The monoclinic LMO surface prevented the unfavorable layered to spinel and/or rock-salt phase transition of SNCM while maintaining its layered structure after repetitive high-voltage operations. Benefiting from its enhanced structural stability, the LMO-coated SNCM cathode showed high capacity retention (76.2%) after 100 cycles under high cut-off voltage (4.5 V) compared to pristine SNCM (61.5%) and enabled fast Li-ion diffusion kinetics. This study provides a simple and efficient coating strategy for improving the high-voltage stability of Ni-rich SNCM cathodes and offers insights into the performance-enhancement and degradation mechanisms of Ni-rich layered oxides.