Nb⁵⁺ concentration-gradient driven lattice and charge coupling for structural and interfacial stability in cobalt-free high-nickel cathodes

Abstract

Nickel-rich layered oxides are key cathodes for high-energy-density lithium-ion batteries, but cobalt removal exacerbates lattice and interfacial degradation, especially under high-voltage operation. This study introduces a Nb⁵⁺ concentration-gradient doping strategy based on a “lattice-charge” coupling concept to achieve multiscale stabilization of cobalt-free LiNi₀.₈Mn₀.₂O₂ (NM82). A decreasing Nb gradient from surface to bulk forms a “surface-reinforced, bulk-stabilized” structure. The Nb-rich surface preferentially occupies Ni sites, forming a robust Nb-O covalent network that strengthens transition metal-oxygen bonding, suppresses oxygen release, and mitigates interfacial side reactions. Concurrently, the Nb-lean bulk maintains efficient Li⁺ transport and electronic conductivity. High-valence Nb⁵⁺ also modulates Ni valence via charge compensation, reducing Ni³⁺ content to suppress Jahn-Teller distortion and stabilize the NiO₆ octahedral framework. This enhances reversibility of the H2→H3 phase transition. Electrochemical and structural analyses, supported by first-principles calculations, confirm a “lattice reinforcement–charge regulation” mechanism that strengthens the oxygen framework and improves thermal and cycling stability. The optimized NM82-1.0 cathode retains 95.4% capacity after 100 cycles at 4.3 V and 89.2% at 4.5 V, shows a >25 °C increase in exothermic peak temperature, and reduces precursor costs by over 13% versus commercial NCM. This work highlights the critical role of high-valence gradient doping in lattice–charge co-stabilization and provides a viable route for developing robust, cobalt-free nickel-rich cathodes.