In situ X-ray absorption and diffraction studies of carbon coated LiFe_1/4Mn_1/4Co_1/4Ni_1/4PO₄ cathode during first charge

Synchrotron based in situ X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) techniques are used to study electronic and crystal structure changes of the carbon coated LiFe_1/4Mn_1/4Co_1/4Ni_1/4PO₄ (LiFe_1/4Mn_1/4Co_1/4Ni_1/4PO₄/C) cathode material for Li-ion batteries during the first charge. In situ Fe, Mn, Co and Ni K-edge XAS results revealed that the three voltage plateaus at ∼3.6, 4.2 and 4.7 V vs. Li/Li⁺ are attributed to the redox reactions of Fe²⁺/Fe³⁺, Mn²⁺/Mn³⁺ and Co²⁺/Co³⁺, respectively, while the apparent capacities above 4.9 V is not originated from the Ni²⁺/Ni³⁺ redox, but very likely from the electrolyte decomposition. Interesting phase transition behaviors of LiFe_1/4Mn_1/4Co_1/4Ni_1/4PO₄/C were observed with the formation of an intermediate phase and the solid solution regions. Combined in situ XAS and XRD techniques indicate fast electronic structural changes and slow bulk crystal structural changes.