Electrochemical Two-Step Delithiation/Re-Lithiation of LiCoPO4 Observed By In Situ XRD, In Situ XAS, and Ex Situ 7-Li/31-P NMR Spectroscopy

LiCoPO₄ was synthesized by solid-state and supercritical-solvothermal routes. Electrochemical tests were performed by galvanostatic cycling against Li metal at different rates and cyclic voltammetry. The electrochemical delithiation/re-lithiation mechanism was investigated by in situ x-ray diffraction (XRD), in situ x-ray absorption spectroscopy (XAS), and ex situ ⁷Li/³¹P nuclear magnetic resonance (NMR) spectroscopy. In situ XRD experiments reveal a two-step reaction with two subsequent two-phase steps and an intermediate phase of approximate composition Li_0.7CoPO₄ [1]. In order to gain insight into the oxidation/reduction process of Co^2+/3+ during cycling in situ XAS measurements were performed which indicate a highly reversible process. ⁷Li and ³¹P NMR spectra of samples with different Li contents obtained during delithiation and re-lithiation again reveal a highly reversible two-step mechanisms with an intermediate phase. For this intermediate phase, two non-equivalent Li environments (peak ratio of 1:1) and two non-equivalent P environments (2:1) could be evidenced. These results are discussed in terms of arrangement of Li ions, Li vacancies, and Co²⁺/Co³⁺ and they hint at an intermediate phase of composition Li_2/3CoPO₄.

[1] N. N. Bramnik, K. Nikolowski, C. Baehtz, K. G. Bramnik, H. Ehrenberg, Chem. Mater. 19 (2007), 908.