Mn-Rich Induced Alteration on Band Gap and Cycling Stability Properties of LiMn_<i>x</i>Fe_1–<i>x</i>PO₄ Cathode Materials

Olivine-type LiMn_<i>x</i>Fe_1-<i>x</i>PO₄ (LMFP) has inherited the excellent heat-stable structure of LiFePO₄ (LFP) and the high-voltage property of LiMnPO₄ (LMP), which shows great promise as a high-safety, high-energy-density cathode material. In order to combine the high energy density and excellent electrochemical performance, it is essential to consider the Mn/Fe ratio. This paper presents a theoretical investigation of the lattice structure parameters, embedded lithium voltage, local electron density, migration barrier, and lithium ion delithiation and lithiation mechanism of different LiMn_<i>x</i>Fe_1-<i>x</i>PO4 (0.5 ≤ <i>x</i> ≤ 0.8) compounds. In situ-coated LiMn_<i>x</i>Fe_1-<i>x</i>PO₄ (0.5 ≤ <i>x</i> ≤ 0.8) composite cathode materials with a size of 100-200 nm were prepared by a hydrothermal method to verify the theoretical study. LiMn_0.6Fe_0.4PO₄/C exhibited a specific capacity of 140.2 and 97.58 mA h·g^-1 at 1 and 5C, respectively, and a remarkable capacity retention rate of 88.5% after 200 cycles at 1C. When LiMn_0.6Fe_0.4PO₄/C was assembled into a flexible pouch battery and subjected to long cycle tests at different rates and squeeze and extrusion tests, it demonstrated a capacity retention rate of 99.35% for 100 cycles at 0.2C and 93.2% for 200 cycles at 0.5C. Moreover, the structural evolution of LiMn_0.6Fe_0.4PO₄/C were analyzed in situ XRD, indicating a high stability and the resulted as obtine electrochemical performance, paving the way for optimization of high-energy-density LMFP cathode materials.