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

Abstract

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 31P and 7Li 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.