Abstract: In this experiment, the cobalt-free lithium-rich manganese precursor Ni_0.3Mn_0.7(OH)₂ was successfully synthesized by small-scale co-precipitation and solid-phase reaction, and the precursor was mechanically crushed with or without the precursor to obtain two different particle sizes of Ni_0.3Mn_0.7(OH)₂ (D₅₀=1.626 µm and 0.710 µm) precursors. The precursors with different particle sizes were mixed with LiOH in a molar ratio of 1:1.55 and sintered at high temperature to synthesize two Li Mn-rich cathode materials with different particle sizes, Li_1.2Ni_0.24Mn_0.56O₂ (D₅₀=1.667 µm and 1.148 µm). The experimental results show that the particle size also affects its physicochemical properties and electrochemical performance. Li_1.2Ni_0.24Mn_0.56O₂ (D₅₀=1.148 µm) cathode material with smaller particle size shows excellent electrochemical performance (capacity of 190.7 mAh/g after 100 cycles at 0.5 C and cycle retention rate of 91.2%). Our study reveals an easy way to change the particle size and demonstrates the importance of particle size for electrochemistry.