Identifying the Structure of the Intermediate, Li2/3CoPO4, 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.