LiMn₂O₄ has been prepared by means of high-energy ball-milling of Li₂CO₃ and MnCO₃ followed by one-pot sintering. Milling at 800 rpm for 4 h with sintering at 700 °C for 12 h leads the enhanced cathode performance as 123.5 mAh g⁻¹ of initial discharge capacity under 1 C charge–discharge rate, retaining 95.1 and 91.6 % of capacity after 100 and 200 cycles, respectively. Mechanochemical reaction facilitates the sintering reaction to form the favorable microstructure homogeneously without second phase. Relaxation analysis showed that, at the charging around x = 0.2 for Li_xMn₂O₄, the sample prepared by optimal milling time varies mainly lithium concentration in Li-rich phase, while excessively milled sample alter the molar ratio of Li-rich and Li-lean phases. This indicates that optimal milling time allows the sample to undergo the less structural change at the charging, which enables the improved cycle performance.