<p>The ubiquitous Li₃PO₄ impurity phase and sluggish Li⁺ diffusion kinetics severely limit the electrochemical performance of LiMn_0.6Fe_0.4PO₄ (LMFP) synthesized via the solvothermal method. Herein, a synergistic strategy combining Li-site Na⁺ doping with process optimization is proposed. By reducing the Li/(Mn + Fe) molar ratio to 2.7:1 and extending the reaction time to 15&#xa0;h at 180&#xa0;°C, the Li₃PO₄ impurity is completely eliminated. Rietveld refinement reveals that Na⁺ substitution at Li sites induces an anisotropic lattice expansion. Specifically, the a-axis is elongated by 0.100%, thereby widening the cross section of the one-dimensional Li⁺ diffusion channel along the [010] direction. X-ray photoelectron spectroscopy confirms a decreased Mn3⁺ fraction on the particle surface, alleviating the Jahn–Teller distortion. At the optimal doping level of 1 at%, the material delivers a discharge specific capacity of 141.8 mAh g⁻¹ at 0.1 C (21.0% higher than that of the undoped sample) and 72.9 mAh g⁻¹ at 5 C, along with a capacity retention of 95.3% after 200 cycles at 1 C. Equivalent circuit modeling shows that 1% Na doping reduces the charge-transfer resistance from 149.3 to 84.4&#xa0;Ω and increases the apparent Li⁺ diffusion coefficient by approximately 7.1 times. This work provides a reproducible process window for synthesizing phase-pure LMFP and establishes a clear structure–performance relationship for Li-site doping.</p>