Synthesis and properties of some doped lithium transition-metal phosphates

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.

Name thesis:degree_name

Doctor of Philosophy (PhD)

Level thesis:degree_level

Doctoral

Grantor dc:publisher.institution

The University of Waikato

Year dc:date.issued

2000

Author dc:creator

• Butt, Garrett Phillip

Advisors dc:contributor.advisor

• Sammes, Nigel
• Swan, Janis E.

Statement dc:rights

• All items in Research Commons are provided for private study and research purposes and are protected by copyright with all rights reserved unless otherwise indicated.

Licence

• https://researchcommons.waikato.ac.nz/bitstreams/27a38715-f324-49b1-b7dc-0cf62217dd16/download

Language dc:language.iso

en

Handle dc:identifier.uri

https://hdl.handle.net/10289/14790

OAI record oai:identifier

https://researchcommons.waikato.ac.nz/server/oai/request?verb=GetRecord&metadataPrefix=dim&identifier=oai%3Aresearchcommons.waikato.ac.nz%3A10289%2F14790