Enhancing the High-Rate Capability and Cycling Stability of LiMn0.6Fe0.4PO4/C Cathode Materials for Lithium-Ion Batteries by Na+ Doping

Abstract

The practical applications of lithium manganese iron phosphate (LMFP) are severely circumvented by the inferior electronic conductivity and electrochemical reaction kinetics. In this work, a Na⁺-doping method is adopted to prepare Li_1–xNa_xMn_0.6Fe_0.4PO₄/C (x = 0, 0.01, 0.02, 0.03) materials by spray drying combined with the carbothermal reduction method. It is found that appropriate Na⁺ doping enhances the crystallinity, reduces Li–Fe antisite defects, decreases the primary particle size, and homogenizes the size distribution of the LMFP material. Moreover, the inferior rate and cycling performance of LMFP are mainly ascribed to the slower Li⁺ diffusion kinetics of Mn redox. A combination of experiments and DFT calculations shows that Na⁺ doping can increase the Li–O bond length, widen the Li⁺ diffusion channel, and decrease Li⁺ diffusion energy barriers, which can accelerate the Li⁺ diffusion rate and Mn redox kinetics, thereby improving the high-rate capability and cycling stability of Na⁺-doped samples. Besides, doped Na⁺ can not only act as pillars to stabilize the structure but also reduce Mn³⁺ content and Mn–Mn interactions to alleviate the Jahn–Teller effect, which also helps to improve the cycling performance of Na⁺-doped samples, wherein the Li_0.98Na_0.02Mn_0.6Fe_0.4PO₄/C sample exhibits optimal rate and cycling performances. Its specific discharge capacity is 125.0 mAh g^–1 at 5 C, and the capacity retention rate reaches 96.7% after 100 cycles at 1 C. Therefore, the Na⁺-doping strategy is believed to be an effective modification means to ameliorate the high-rate and cycling capabilities of olivine-based cathode materials.