Identifying the structure of the intermediate, Li_2/3CoPO₄, formed during electrochemical cycling of LiCoPO₄

In situ synchrotron diffraction measurements and subsequent Rietveld refinements are used to show that the high energy density cathode material LiCoPO₄ (space group Pnma) undergoes two distinct two-phase reactions upon charge and discharge, both occurring via an intermediate Li_2/3(Co²⁺)_2/3(Co³⁺)_1/3PO₄ phase. Two resonances are observed for Li_2/3CoPO₄ with intensity ratios of 2:1 and 1:1 in the ³¹P and ⁷Li NMR spectra, respectively. An ordering of Co²⁺/Co³⁺ oxidation states is proposed within a (a × 3b × c) supercell, and Li⁺/vacancy ordering is investigated using experimental NMR data in combination with first-principles solid-state DFT calculations. In the lowest energy configuration, both the Co³⁺ ions and Li vacancies are found to order along the b-axis. Two other low energy Li⁺/vacancy ordering schemes are found only 5 meV per formula unit higher in energy. All three configurations lie below the LiCoPO₄-CoPO₄ convex hull and they may be readily interconverted by Li⁺ hops along the b-direction.