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. Here we apply multi-scale electron microscopy/spectroscopy techniques and theoretical calculations on both polycrystalline and single-crystal NLOs, and describe their structural evolution upon cycling. We discover that both the intergranular and intragranular cracks initiate along polar (001) basal plane due to its large elastic anisotropy upon cycling and surface structure evolution and transition metal dissolution occur on nonpolar (104) fresh surface. With this surface-dependent stress-corrosion coupling effect, 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. A universal understanding of the surface-dependent degradation in both polycrystalline and single-crystal NLOs provides clues on designing new cathode materials with high energy density and long cycle life through grain-boundary engineering.