Identifying the Structure of the Intermediate, Li(2/3)CoPO(4), Formed during Electrochemical Cycling of LiCoPO(4)

[Image: see text] In situ synchrotron diffraction measurements and subsequent Rietveld refinements are used to show that the high energy density cathode material LiCoPO(4) (space group Pnma) undergoes two distinct two-phase reactions upon charge and discharge, both occurring via an intermediate Li(2/3)(Co(2+))(2/3)(Co(3+))(1/3)PO(4) phase. Two resonances are observed for Li(2/3)CoPO(4) with intensity ratios of 2:1 and 1:1 in the (31)P and (7)Li NMR spectra, respectively. An ordering of Co(2+)/Co(3+) 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 Co(3+) 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 LiCoPO(4)–CoPO(4) convex hull and they may be readily interconverted by Li(+) hops along the b-direction.