| Summary: | The cycling stability of LiCoO[subscript 2] under high voltages (>4.5 V) was plagued by hybrid anion- and cation-redox (HACR) induced oxygen escape and uncontrolled phase collapse. With DEMS and in situ XANES mapping at the NSLS-II, we demonstrate that oxygen escape triggers irreversible transformations into “bad” surface phases that rapidly propagate inward. Enabling HACR but stopping global oxygen migration is key to a stable high-energy cathode. Therefore, we developed ∼10 μm single crystals with LiCoO[subscript 2] in the bulk smoothly transitioning to Co-free LiMn[subscript 0.75]Ni[subscript0.25]O[subscript 2] at the surface. By means of initial electrochemical formation, a semi-coherent LiMn[subscript 1.5]Ni[subscript 0.5]O[subscript 4] spinel-like shell was established in operando with little oxygen loss to integrally wrap the LiCoO[subscript 2] bulk. Then we obtained gradient-morph LiCoO[[subscript 2] single crystals to prevent the percolating migration of oxygen out of the particle and achieved enhanced HACR reversibility at high voltages. The gradient-morph HACR cathode undergoes substantially stabilized cycling when charged to above 4.6 V, and hence a stable cyclic volumetric energy density of >3400 W h L−1 has been achieved in a pouch full-cell coupled with a commercial graphite anode and lean electrolyte (2 g A h−1), exhibiting up to 2906 W h L−1 even after 300 cycles.
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