Lithium Diffusion in Cation-Mixing-Free LiMn1-xFexPO4 Synthesized by Hydrothermal Process

Lithium diffusion properties of LiMn1-xFexPO4 synthesized by hydrothermal methods were characterized from electrochemical and structural analysis. Structural defects such as cation mixing in LiMn1-xFexPO4 impede the lithium-ion diffusion. Therefore, it is important to understand effects of cation mixing in LiMn1-xFexPO4 on lithium diffusivity of LiMn1-xFexPO4. The amount of cation mixing of lithium and Mn/Fe in LiMn1-xFexPO4 was obtained by means of neutron diffraction measurements. Rietveld refinements indicated that cation mixing in the prepared LiMn1-xFexPO4 was negligible. Apparent chemical diffusion coefficients (D-app) of lithium of LiMn1-xFexPO4 dependent on the Mn/Fe ratio were obtained by potential-step chronoamperometry (PSCA) measurements of thin composite electrodes. D-app of lithium insertion or extraction reaction of LiMn1-xFexPO4 for Mn2+/Mn3+ increased with Fe content. D-app for Fe2+/Fe3+ slightly increased with decreasing Fe content. The difference of Dapp value was attributed to the activation energies (E-a) of lithium diffusion process of LiMn1-xFexPO4. The E-a of LiMn1-xFexPO4 for Mn2+/Mn3+ and Fe2+/Fe3+ was determined by the energy barrier of lithium hopping at the interphase between Li-rich and Li-poor phase under electrochemical kinetic conditions. (C) 2015 The Electrochemical Society. All rights reserved.