Surface-dependent stress-corrosion cracking in Ni-rich layered oxide cathodes

Structural degradation is the principal driving force for rapid voltage decay and capacity fading of Ni-rich layered oxide (NLO) cathode materials upon cycling, but its working mechanism is not yet fully elucidated. In this work, multi-scale electron microscopy/spectroscopy techniques and theoretical calculations are applied on both polycrystalline and single-crystal NLOs. We discover that both the intergranular and intragranular cracks initiate along polar (001) basal plane, while surface structure evolution and transition metal dissolution occur on nonpolar (104) fresh surface. A new chain stress corrosion mechanism from anisotropic elastic (001) tensile deformation, microcrack generation, nonpolar surface reconstruction, HF attack to metal dissolution is proposed to paint the full picture of the structural degradation of NLOs. This surface-dependent stress-corrosion coupling effect indicates that severe intergranular cracking that accumulates within the polycrystalline NLO aggregates accounts mostly for the fast voltage decay and capacity fading, whereas minor intragranular cracking and less surface damage lead to substantial improvements on cyclability and reversible capacity of single-crystal NLOs. The surface-dependent stress-corrosion cracking in both polycrystalline and single-crystal NLOs provides grain-boundary engineering clues on designing new cathode materials with high energy density and long cycle life.