Identifying the Structure of the Intermediate, Li2/3CoPO4, Formed during Electrochemical Cycling of LiCoPO4

High Resolution Image Download MS PowerPoint Slide 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 × 3 b × 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.