Manganese dissolution and the Jahn-Teller distortion represent the primary factors limiting the cycle life of spinel LiMn₂O₄. In this study, Nb⁵⁺-doped Li_1.05Mn_2-xNb_xO₄ (0 ≤ x ≤ 0.03) with truncated octahedral morphology was successfully synthesized via a simple high-temperature solid-state method. Nb⁵⁺ doping not only enhanced the structural stability of LiMn₂O₄ but also increased the Li ⁺ diffusion rate. SEM analysis revealed that Nb⁵⁺ doping effectively suppressed (110) plane growth, thereby mitigating Mn dissolution. Simultaneously, TEM results indicated that a thinner cathode electrolyte interphase film was formed upon doping, which contributed to enhanced cycling stability. Further DFT calculations confirmed that Nb⁵⁺ doping improved the structural stability of LiMn₂O₄ through a dual mechanism: reducing the occupancy of the Mn eg orbitals and strengthening the Mn-O bonding energy. On the other hand, Nb⁵⁺ doping expands the lattice, with CV and EIS tests showing increased Li⁺ diffusion rates. PDOS calculations revealed a narrowed band gap, which improved the electronic conductivity, thereby endowing LiMn₂O₄ with high-rate performance. Hence, Li_1.05Mn_2-xNb_xO₄ exhibits superior rate capability and extended cycle life. Specifically, Li_1.05Mn_1.99Nb_0.01O₄ delivered an initial discharge capacity of 124.61 mAh/g with 88.43 % capacity retention after 500 cycles at 1 C, and maintained 74.61 mAh/g even at 10 C.