Abstract: To address the demand of high-performance manganese sources for Mn-based cathode materials in lithium-ion batteries, (Mn, Ni)₃O₄ solid solutions with varying Ni doping levels were prepared via a two-step solid-state method combining alloying and oxidation. The spinel-type LiMn_2–xNi_xO₄ (x = 0.20, 0.40, 0.66) cathode materials were subsequently synthesized using (Mn, Ni)₃O₄ solid solutions as manganese sources. The influence of Ni doping content on the phase structure, morphology, and electrochemical properties of (Mn, Ni)₃O₄ solid solution precursors and the resulting LiMn_2–xNi_xO₄ cathode materials was systematically investigated. The results indicate that as the Ni doping mass fraction increases from 10% to 33%, the proportion of the cubic spinel (Mn, Ni)₃O₄ phase (space group Fd–3m) in the solid solution precursor increases, accompanied by a decrease in lattice constant and a more pronounced truncated octahedral morphology. Among the obtained cathode materials, LiMn_1.80Ni_0.20O₄ exhibits the best overall electrochemical performance, delivering an initial specific discharge capacity of 120.1 mA•h/g at a current density of 100 mA/g, retaining 78.1% of its initial capacity after 500 cycles, and showing low charge transfer resistance as well as favorable rate capability. Appropriate Ni doping not only enhances structural stability but also preserves sufficient capacity contribution from Mn³⁺/Mn⁴⁺ redox, which is the primary reason for its superior electrochemical performance. This study provides an effective strategy of develop the high-performance manganese sources for cathode materials of lithium-ion batteries.