MEM Charge Density Study of Olivine LiMPO4 and MPO4 (M = Mn, Fe) as Cathode Materials for Lithium-Ion Batteries

We have investigated the electron-density distributions in LiMPO4 and MPO4 (M = Mn, Fe) by analyzing high-energy synchrotron-radiation powder-diffraction data using the maximum entropy method (MEM)/Rietveld method to deepen understanding of the electrochemical properties in designing a promising cathode material. The Jahn-Teller Mn3+ ion in a high electric field induced by the adjacent (PO4)(3-) ion in MnPO4 forms anisotropic chemical bonding with the neighboring oxide ions. The net charge analysis shows that the electrons assigned to both the Mn and P atoms influence the delithiation for LiMnPO4, while the electrons of the Fe atom in LiFePO4 mainly contribute to the delithiation. The effects of the anisotropy of the d-electron density of the Mn3+ ion and the role played by the charge transfer of the P atom are discussed on the instability at high temperature and the metastability at room temperature, respectively.