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+)(1/3)PO4 phase. Two resonances are observed for Li2/3CoPO4 with intensity ratios of 2:1 and 1:1 in the P-31 and 7Li NMR spectra, respectively. An ordering of Co2+/Co3+ oxidation states is proposed within a (a X 3b X 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 LiCoPO4CoPO4 convex hull and they may be readily interconverted by Li+ hops along the b-direction.