Direct synthesis and characterization of mixed-valent Li0.5-dCoPO4, a Li-deficient derivative of the Cmcm polymorph of LiCoPO4

Ludwig, J. ; Alarcón-Suesca, C. ; Geprägs, S. ; Nordlund, D. ; Doeff, M.M. ; Puente Orench, I. ; Nilges, T.

Resumen: 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 Li0.5-dCoPO4. 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-dCoPO4 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 Li0.5-dCoPO4 is metastable and exhibits a complex, multi-step thermal decomposition mechanism. In the first step at 394 °C, it decomposes to a-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 Li0.5-dCoPO4 are 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.

Idioma: Inglés

DOI: 10.1039/c7ra04043a

Año: 2017

Publicado en: RSC Advances 7, 45 (2017), 28069-28081
ISSN: 2046-2069

Factor impacto JCR: 2.936 (2017)

Categ. JCR: CHEMISTRY, MULTIDISCIPLINARY rank: 71 / 171 = 0.415 (2017) - Q2 - T2

Factor impacto SCIMAGO: 0.863 - Chemistry (miscellaneous) (Q1) - Chemical Engineering (miscellaneous) (Q1)

Tipo y forma: Artículo (Versión definitiva)