Abstract— <p>High-nickel layered oxide cathode materials, with their high specific capacity and elevated voltage platforms, are among the most promising candidates for next-generation high energy density lithium-ion and solid-state batteries. However, conventional polycrystalline high-nickel oxides often suffer from intergranular microcracks and interfacial phase transitions during cycling, leading to severe capacity fading and compromised thermal stability. In contrast, single crystal architectures, owing to their continuous lattices and superior structural integrity, can effectively mitigate intergranular stress and suppress crack propagation. Yet, their synthesis typically requires high calcination temperatures, which challenge compositional homogeneity and structural ordering. Herein, single crystal LiNi_0.88Co_0.09Mn_0.03O₂ was synthesized through a molten-salt-assisted route employing a LiOH–LiNO₃ composite molten salt. In this strategy, decomposed LiNO₃ releases reactive “internal oxygen”, promoting in situ oxidation from Ni²⁺ to Ni³⁺, while molten LiOH provides a transient liquid-phase environment that facilitates rapid ionic diffusion. This cooperative “oxidation-diffusion” mechanism enables simultaneous structural ordering and defect suppression during crystal growth. The optimized sample with 60% LiNO₃ exhibits outstanding crystallinity and electrochemical properties, showing only 1.2% cation mixing, uniform elemental distribution, and a high Ni³⁺ ratio (81.3%). Benefiting from this structural refinement, it retains over 90% of its capacity after 100 cycles at 1 C and delivers ~150 mA h g^–1 at 10 C, outperforming other compositions. This work elucidates a synergistic “oxygen-liquid-phase diffusion” mechanism enabled by the dual functionality of LiNO₃ and LiOH, providing an effective pathway for the controllable synthesis and performance optimization of single crystal high-nickel cathode materials for high-energy-density lithium-ion batteries.</p>