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Title: Direct synthesis and characterization of mixed-valent Li 0.5$-$δCoPO 4, a Li-deficient derivative of the Cmcm polymorph of LiCoPO 4

While the majority of research activities on LiCoPO 4 is focussed on the thermodynamically stable olivine-type Pnma polymorph, the metastable Pna2 1 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–δCoPO 4. As opposed to the substoichiometric olivine ( Pnma) phases Li xCoPO 4 (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 xCoPO 4-type material is accomplished. X-ray and neutron diffraction studies indicate that Cmcm-type Li 0.5–δCoPO 4 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–δCoPO 4 is metastable and exhibits a complex, multi-step thermal decomposition mechanism. In the first step at 394 °C, it decomposes to α-Co 2Pmore » 2O 7 ( P2 1/c) and LiCoPO 4 ( Cmcm) upon O 2 release. The LiCoPO 4 ( Cmcm) intermediate is then irreversibly transformed to olivine-type LiCoPO 4 (Pnma) at 686 °C. Furthermore, the material properties of Li 0.5–δCoPO 4 are further compared to the fully lithiated, isostructural LiCoPO 4 ( 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.« less
Authors:
 [1] ;  [1] ;  [2] ;  [3] ;  [4] ;  [5] ; ORCiD logo [1]
  1. Technical Univ. of Munich, Garching (Germany)
  2. Bavarian Academy of Sciences and Humanities, Garching (Germany)
  3. SLAC National Accelerator Lab., Menlo Park, CA (United States)
  4. Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
  5. Instituto de Ciencia de Materiales de Aragon, Zaragoza (Spain); Institut Laue-Langevin, Grenoble Cedex (France)
Publication Date:
Grant/Contract Number:
AC02-76SF00515
Type:
Accepted Manuscript
Journal Name:
RSC Advances
Additional Journal Information:
Journal Volume: 7; Journal Issue: 45; Journal ID: ISSN 2046-2069
Publisher:
Royal Society of Chemistry
Research Org:
SLAC National Accelerator Lab., Menlo Park, CA (United States)
Sponsoring Org:
USDOE
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; 36 MATERIALS SCIENCE
OSTI Identifier:
1369476