Calorimetric Study of Mixed Phosphates Na4M3(PO4)2P2O7 (M = Mn2+, Fe2+, Co2+, Ni2+) to Evaluate the Electrochemical Trends

Mixed polyanionic compounds have been studied extensively as viable cathode materials for sodium-ion batteries. Mixed phosphates, Na4M3(PO4)2P2O7 (M = Mn2+, Fe2+, Co2+, Ni2+), provide a low barrier for Na-ion diffusion, being advantageous in comparison to phosphates and pyrophosphates. The reported order of sodium extraction is ambiguous and remains unclear. Despite being structurally similar, electrochemical performance differs for all four analogues with different degrees of (de)sodiation, according to the transition element present. Here, high-temperature oxide melt solution calorimetry has been used to establish the relation between thermodynamic phase stability and observed capacity for this series of mixed phosphates. Thermodynamic phase stability largely depends on the kind of structure, type of bonding, and size of the cations present. So, according to our results, the thermodynamic phase stability follows the order Na4Mn3(PO4)2P2O7 > Na4Fe3(PO4)2P2O7 > Na4Co3(PO4)2P2O7 > Na4Ni3(PO4)2P2O7. The thermodynamic studies serve as guidelines for the selection of compositions with the potential for fabricating advanced cathode materials with maximum performance.