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

  • Author(s): Ludwig, J
  • Alarcón-Suesca, C
  • Geprägs, S
  • Nordlund, D
  • Doeff, MM
  • Orench, IP
  • Nilges, T
  • et al.
Abstract

© 2017 The Royal Society of Chemistry. While the majority of research activities on LiCoPO4is focussed on the thermodynamically stable olivine-type Pnma polymorph, the metastable Pna21and 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 Li0.5-δCoPO4. As opposed to the substoichiometric olivine (Pnma) phases LixCoPO4(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 LixCoPO4-type material is accomplished. X-ray and neutron diffraction studies indicate that Cmcm-type Li0.5-δCoPO4shows 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 Li0.5-δCoPO4is 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 O2release. The LiCoPO4(Cmcm) intermediate is then irreversibly transformed to olivine-type LiCoPO4(Pnma) at 686 °C. The material properties of Li0.5-δCoPO4are further compared to the fully lithiated, isostructural LiCoPO4(Cmcm) phase, for which an improved structure solution as well as Co L2,3-edge X-ray absorption spectra are reported for the first time.

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