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

Mixedpolyanionic compounds have been studied extensively as viablecathode materials for sodium-ion batteries. Mixed phosphates, Na4M3(PO4)(2)P2O7 (M = Mn2+, Fe2+, Co2+, Ni2+), provide a low barrier for Na-ion diffusion, being advantageousin comparison to phosphates and pyrophosphates. The reported orderof sodium extraction is ambiguous and remains unclear. Despite beingstructurally similar, electrochemical performance differs for allfour analogues with different degrees of (de)sodiation, accordingto the transition element present. Here, high-temperature oxide meltsolution calorimetry has been used to establish the relation betweenthermodynamic phase stability and observed capacity for this seriesof mixed phosphates. Thermodynamic phase stability largely dependson the kind of structure, type of bonding, and size of the cationspresent. So, according to our results, the thermodynamic phase stabilityfollows the order Na4Mn3(PO4)(2)P2O7 > Na4Fe3(PO4)(2)P2O7 > Na4Co3(PO4)(2)P2O7 > Na4Ni3(PO4)(2)P2O7. The thermodynamic studies serve as guidelinesfor the selectionof compositions with the potential for fabricating advanced cathodematerials with maximum performance.