Electrochemical Delithiation/Relithiation of LiCoPO4: A Two-Step Reaction Mechanism Investigated by in Situ X-ray Diffraction, in Situ X-ray Absorption Spectroscopy, and ex Situ 7Li/31P NMR Spectroscopy

LiCoPO4 was synthesized by a solid-state and a supercritical solvo-thermal method. In situ X-ray absorption near-edge structure (XANES) experiments were evaluated on the basis of full-cycle experiments confirming the predominantly reversible character of the redox reaction. In situ X-ray diffraction (XRD) measurements were performed to follow structural changes during cycling indicating the existence of an intermediate phase upon cycling. The local phosphorus and lithium environments were studied by ex situ P-31 and Li-7 NMR at different states of charge proving the existence of an intermediate phase of the composition Li2/3CoPO4. On the basis of these findings, a model of the (de)lithiation process of LiCoPO4 is developed and discussed.