Gradient-morph LiCoO2 single crystals with stabilized energy density above 3400 W h L−1

Abstract

The cycling stability of LiCoO₂ 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 LiCoO₂ in the bulk smoothly transitioning to Co-free LiMn_0.75Ni_0.25O₂ at the surface. By means of initial electrochemical formation, a semi-coherent LiMn_1.5Ni_0.5O₄ spinel-like shell was established in operando with little oxygen loss to integrally wrap the LiCoO₂ bulk. Then we obtained gradient-morph LiCoO₂ 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.