Understanding the crystal chemistry of phosphate cathode materials

Eight phosphate-based compounds, Co₃P₂O₈, Co₁.₅Mn₁.₅P₂O₈, Mn₃P₂O₈, alpha-FePO₄, Fe₃O₃PO₄, Mn₂P₂O₇, and Mn₂P₄O₁₂, were synthesised and evaluated as cathode materials for high-temperature lithium thermal batteries. These compounds were chosen for their structural diversity, thermal stability, and potential for conversion-type electrochemical energy storage. Co-based materials had high thermal stability, with Co₃P₂O₈ showing positive thermal expansion (PTE), while Co₁.₅Mn₁.₅P₂O₈ and Mn₃P₂O₈ exhibited negative thermal expansion (NTE). Alpha-FePO₄ remained thermally stable up to 500 °C with PTE, whereas Fe₃O₃PO₄, particularly from the solution route, demonstrated NTE due to possible stoichiometric or compositional effects. Mn₂P₂O₇, showed PTE up to 780 °C, while its presence within Mn₃P₂O₈ induced NTE behaviour, resembling the alpha-FePO₄/Fe₃O₃PO₄ system. Mn₂P₄O₁₂ displayed a 3.69% unit cell contraction up to 780 °C, indicating NTE and a possible phase transition. Electrochemical tests revealed that Co₃P₂O₈ and Co₁.₅Mn₁.₅P₂O₈ delivered a stable 2.2 V plateaux, with improved capacity in the Mn-substituted compound. Mn₃P₂O₈ showed a higher capacity and performance at high current densities. Fe₃O₃PO₄ offered enhanced capacity through mixed-phase activity, while Mn₂P₂O₇ and Mn₂P₄O₁₂ demonstrated multiple voltage plateaus and promising high-rate capabilities. Post-mortem PXRD confirmed Li₃PO₄ and metal formation for these eight phosphates. This work highlights the potential of phosphate materials in thermal batteries, especially through compositional tuning, mixed-metal strategies, and solution-based synthesis.