Abstract
Attempts to introduce vanadium doping into the olivine-structured LiMnPO4 cathode material for the Li-ion batteries resulted in the (y + z)PO43− → yVO43− + 4zOH− (y < 0.17, z ≤ 0.1) co-substitution and the formation of the hydrotriphylite-type Li1−xMn1+x(PO4)1−y−z(VO4)y(OH)4z solid solutions. Their crystal structure and chemical composition were studied with Rietveld structure refinement from synchrotron X-ray powder diffraction data, energy-dispersive X-ray analysis, infrared and Raman spectroscopy, electron energy loss analysis and mass spectrometry. Li0.96Mn1.04(PO4)0.74(VO4)0.16(OH)0.4 crystallizes in the space group Pnma and a = 10.5150(2) Å, b = 6.11773(9) Å and c = 4.78555(9) Å with up to 5% of Mn in the Li position. Charge compensation is achieved through partial oxidation of the Mn cations to the oxidation state above +2. The heterovalent substitution in anionic sublattice and defectiveness of the cationic one cause blocking of Li-ion diffusion channels that drastically impede the electrochemical performance resulting in electrochemical capacity not exceeding 35 mAh g−1.
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