Identifying the Structure of the Intermediate, Li_2/3CoPO₄, 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.