Direct synthesis and characterization of mixed-valent Li0.5−δCoPO4, a Li-deficient derivative of the Cmcm polymorph of LiCoPO4

Abstract

While the majority of research activities on LiCoPO₄ is focussed on the thermodynamically stable olivine-type Pnma polymorph, the metastable Pna2₁ and Cmcm modifications have recently attracted considerable attention due to their interesting material properties. In this study, we present the first Li-deficient structural derivative of the Cmcm modification with the nominal composition Li_0.5−δCoPO₄. As opposed to the substoichiometric olivine (Pnma) phases Li_xCoPO₄ (x = 0; 2/3), which are exclusively accessible by electrochemical or chemical Li extraction techniques, this is also the first time that a direct soft-chemical synthesis route towards a Li_xCoPO₄-type material is accomplished. X-ray and neutron diffraction studies indicate that Cmcm-type Li_0.5−δCoPO₄ shows vacancies on both the Li and Co sites, whereas X-ray absorption spectra demonstrate that the structure features heterovalent Co ions (+2/+3) to compensate for the Li deficit. Magnetic measurements reveal a long-range antiferromagnetic order below 10.5 K. A thorough investigation of the thermal stability using thermogravimetric analysis, differential scanning calorimetry, and temperature-dependent in situ X-ray powder diffraction demonstrates that Li_0.5−δCoPO₄ is metastable and exhibits a complex, multi-step thermal decomposition mechanism. In the first step at 394 °C, it decomposes to α-Co₂P₂O₇ (P2₁/c) and LiCoPO₄ (Cmcm) upon O₂ release. The LiCoPO₄ (Cmcm) intermediate is then irreversibly transformed to olivine-type LiCoPO₄ (Pnma) at 686 °C. The material properties of Li_0.5−δCoPO₄ are further compared to the fully lithiated, isostructural LiCoPO₄ (Cmcm) phase, for which an improved structure solution as well as Co L_2,3-edge X-ray absorption spectra are reported for the first time.