Mechanism of the Delithiation/Lithiation Process in LiFe0.4Mn0.6PO4: in Situ and ex Situ Investigations on Long-Range and Local Structures

LiFe_0.4Mn_0.6PO₄ olivine was prepared by a sol–gel route, using citric acid as a chelating agent and NH₄H₂PO₄ as a phosphorus source. Sucrose was used as the source for the carbon-coating of the particles. The correlation between the physicochemical and the electrochemical properties of this positive electrode material was explored. The electrochemical tests showed an initial discharge capacity of 121 mAh/g at a C/20 rate with a good reversibility of the lithiation/delithiation reactions. In situ XRD on LiFe_0.4Mn_0.6PO₄ reveals the occurrence of new phases upon cycling, which disappeared again at the end of discharge. The single phase observed after one complete cycle is identical to the pristine one. In situ XAS spectroscopy in combination with ⁵⁷Fe Mössbauer and ⁷Li NMR spectroscopy were used to investigate the changes in the local structure and the oxidation states of the transition metals and thus to complete the overall characterization of the lithiation/delithiation mechanism. All findings reveal a high reversibility of the reactions in this electrode material.