A new LiCoPO4 polymorph via low temperature synthesis

Abstract

Non-olivine LiCoPO₄ microcrystals have been synthesized at low temperatures by microwave-assisted hydrothermal synthesis. The resulting octahedra-like micrometer-sized crystals exhibit a Pn2₁a symmetry and are isostructural to LiZnPO₄. The orthorhombic lattice constants are a = 10.023(8) Å, b = 6.724(7) Å, and c = 4.963(4) Å. The microcrystals are metastable and transform to olivine LiCoPO₄ upon heating above T ≈ 220 °C under an argon atmosphere. The magnetic properties confirm a phase pure material with a Co-valence of 2+, and imply long-range antiferromagnetic ordering below T_N = 11 K. Electrochemical analysis by means of cyclic voltammetry displays a reversible redox couple at around 5.1 V/4.8 V vs. Li/Li⁺.