Atomic structural and electrochemical impact of Fe substitution on nano porous LiMnPO₄

The atomic structural and electrochemical properties of Fe substituted nano porous LiMn_1-xFe_xPO₄ (x = 0-0.8) composites are investigated and compared. X-ray scattering method is used for atomic structural investigation. Rietveld refinement shows that all Fe substituted composites have the same olivine structure (Pnma) with lithium occupying octahedral 4a sites, Fe²⁺ replacing Mn²⁺ at the octahedral 4c sites. The a, b, c parameters and cell volume decrease with the addition of Fe²⁺. When the nano porous LiMn_1-xFe_xPO₄ composites are evaluated as cathode materials in lithium cells at room temperature, x = 0.6, and 0.8 resulted in the best overall electrochemical performance, exhibiting stable cycling and high discharge capacities of 149 and 154 mA h g^-1, respectively. The composites with above x = 0.4 show a fast lithium ions transfer with high electronic conductivity because Fe transition metal substitution reduce the partly occupation of Mn in the M1 (LiO₆) sites and thereby Mn block the lithium ion diffusion pathway. We here firstly find the antisite defect in the high Mn content in porous LiMn_1-xFe_xPO₄ composites.