An ultra-high nickel cobalt-free cathode material toward high-energy and long-cycle stable Li-ion batteries: a single-crystal and surface high-entropy design strategy†

Abstract

Single-crystal cobalt-free, nickel-rich layered oxides have garnered considerable attention as cathode materials for lithium-ion batteries (LIBs), primarily due to their impressive reversible specific capacity and low cost-effectiveness. However, a notable drawback of cobalt-free materials is their susceptibility to rapid structural degradation during cycling. In this study, we introduce a Mg/Nb/Al co-doped and surface-coated single-crystal LiNi_0.9Mn_0.1O₂ (SHE-SC-LNM) cathode material that demonstrates significantly enhanced cycling stability. The formation of a high-entropy layer near the surface enhances the reversibility of lattice oxygen, effectively inhibiting oxygen evolution and release, alleviating the formation of oxygen vacancies, and stabilizing transition metal ions. During both delithiation and lithiation processes, this layer enhances the reversibility of the H2–H3 phase transition, reduces lattice-plane slippage, minimizes the accumulation and release of anisotropic lattice strain, and facilitates Li⁺ diffusion kinetics. Additionally, an Al₂O₃/LiAlO₂ interfacial layer forms on the surface, giving rise to a thin and stable cathode–electrolyte interphase (CEI) that effectively mitigates HF attack and curbs the dissolution of metal ions. Thanks to its fast charge/discharge capability and high-voltage stability, the SHE-SC-LNM cathode exhibits an initial discharge capacity of 213.61 mA h g⁻¹ at 0.1C. Furthermore, it retains an impressive 90.1% of its capacity after 300 cycles at 1.0C. These findings underscore the potential of Mg/Nb/Al co-doped and coated single-crystal LiNi_0.9Mn_0.1O₂ as a high-performance, cobalt-free cathode material for the next-generation lithium-ion batteries.