Synthesis and properties of some doped lithium transition-metal phosphates

Abstract

Four structurally related materials; LiNiPO₄, LiCoPO₄, Li₃Fe₂(PO₄)₃ and Li₃V₂(PO₄)₃, were synthesised, characterised and evaluated as potential cathode materials for lithium secondary batteries.

The materials were synthesized using mainly solid-state techniques. Structural characterisation was performed using powder XRD with Rietveld refinement and Raman spectroscopy. Results showed that the pure forms of LiNiPO₄, LiCoPO₄ and Li₃Fe₂(PO₄)₃ could be synthesised in air at various temperatures. Li₃V₂(PO₄)₃ required a reducing atmosphere of 2%H₂/98%N₂ to achieve phase purity. Attempts were made to substitute all four materials with aliovalent dopants. Li₃Fe₂(PO₄)₃ and Li₃V₂(PO₄)₃ underwent a phase change depending on dopant content to a higher ionic conducting phase.

AC impedance spectroscopy was used to determine conductivity of the materials. In general the phosphates are poor conductors. There was a significant increase in conductivity when substituting the transition metal Ti⁴⁺ for M³⁺ in Li₃Fe₂(PO₄)₃ and Li₃V₂(PO₄)₃, and V³⁺ for Co²⁺ in LiCoPO₄.

The four materials and their highest conducting doped analogues were evaluated as cathodes in coin type lithium cells to determine their viability in Li secondary batteries. LiCoPO₄ showed a first discharge capacity of 130 mAh/g at 4.6V, but could be cycled over only a limited number of charge-discharge cycles owing to electrolyte instability at the high oxidation potentials (>5V) required for full charge. LiNiPO₄ could not be charged at all to accessible voltages. Ti doped Li₃Fe₂(PO₄)₃ had a relatively low discharge capacity of 60 mAh/g. Li₃V₂(PO₄)₃ and Ti doped Li₃V₂(PO₄)₃ showed a discharge capacity of 130 and 110 mAh/g respectively, although the Ti doped Li₃V₂(PO₄)₃ showed better cycling characteristics. Therefore, although aliovalent doping could increase the total conductivity of the phosphate materials, the accessible capacities of these materials remained limited.