Effective Ionic Potential Guided Dual-Gradient Structural Engineering for Spent LiCoO₂ Upcycling

Sustainable recycling of degraded LiCoO₂ (LCO) cathode is critical for minimizing the environmental footprint of lithium-ion batteries. Herein, we propose an upcycling method that converts degraded LCO into high-voltage cathodes by constructing a compositional and structural dual-gradient structure, guided by the effective ionic potential (EIP, Φ*), a descriptor for foreign dopant diffusivity in degraded LCO lattices. Specifically, low-Φ* dopants tend to exhibit high bulk diffusivity, whereas high-Φ* dopants are retained near the surface, which promotes the formation of the compositional gradient and leads to a structural transition in LCO from a fully disordered, dense surface to an ordered layered structure in the bulk. This structure endows the upcycled cathode with a stabilized surface and low-strain bulk structure, enabling its superior electrochemical performance over the commercial counterpart at cut-off potentials of 4.6 and 4.65 V. Comprehensive kinetic and thermodynamic analyses reveal the critical role of vacancies in spent LCO for this structural engineering: Bulk vacancies facilitate the formation of deeper dopant concentration gradients within particles, while vacancies near the surface promote the development of a continuous and dense surface disordered structure. Multiscale characterizations and theoretical calculations elucidate the relationship between the engineered structure and the electrochemical stability of the upcycled cathode.