Enhancing\nthe High-Rate Capability and Cycling Stability\nof LiMn<sub>0.6</sub>Fe<sub>0.4</sub>PO<sub>4</sub>/C Cathode Materials\nfor Lithium-Ion Batteries by Na<sup>+</sup> Doping

The\npractical applications of lithium manganese iron phosphate\n(LMFP) are severely circumvented by the inferior electronic conductivity\nand electrochemical reaction kinetics. In this work, a Na<sup>+</sup>-doping method is adopted to prepare Li<sub>1–<i>x</i></sub>Na<sub><i>x</i></sub>Mn<sub>0.6</sub>Fe<sub>0.4</sub>PO<sub>4</sub>/C (<i>x</i> = 0, 0.01, 0.02, 0.03) materials\nby spray drying combined with the carbothermal reduction method. It\nis found that appropriate Na<sup>+</sup> doping enhances the crystallinity,\nreduces Li–Fe antisite defects, decreases the primary particle\nsize, and homogenizes the size distribution of the LMFP material.\nMoreover, the inferior rate and cycling performance of LMFP are mainly\nascribed to the slower Li<sup>+</sup> diffusion kinetics of Mn redox.\nA combination of experiments and DFT calculations shows that Na<sup>+</sup> doping can increase the Li–O bond length, widen the\nLi<sup>+</sup> diffusion channel, and decrease Li<sup>+</sup> diffusion\nenergy barriers, which can accelerate the Li<sup>+</sup> diffusion\nrate and Mn redox kinetics, thereby improving the high-rate capability\nand cycling stability of Na<sup>+</sup>-doped samples. Besides, doped\nNa<sup>+</sup> can not only act as pillars to stabilize the structure\nbut also reduce Mn<sup>3+</sup> content and Mn–Mn interactions\nto alleviate the Jahn–Teller effect, which also helps to improve\nthe cycling performance of Na<sup>+</sup>-doped samples, wherein the\nLi<sub>0.98</sub>Na<sub>0.02</sub>Mn<sub>0.6</sub>Fe<sub>0.4</sub>PO<sub>4</sub>/C sample exhibits optimal rate and cycling performances.\nIts specific discharge capacity is 125.0 mAh g<sup>–1</sup> at 5 C, and the capacity retention rate reaches 96.7% after 100\ncycles at 1 C. Therefore, the Na<sup>+</sup>-doping strategy is believed\nto be an effective modification means to ameliorate the high-rate\nand cycling capabilities of olivine-based cathode materials.