Eliminating crystal water enables enhanced sodium storage performance in an oxalate-phosphate cathode material

The oxalate-phosphate polyanion-mixed cathode materials are promising for sodium-ion batteries (SIBs) due to their unique open-framework structures and high voltage property. However, materials of this type generally contain crystal water molecules in the lattice frameworks, which may affect their energy storage properties. This work aims to disclose the impacts of crystal water on physiochemical and electrochemical properties of Na2(VO)2(HPO4)2(C2O4)·2H2O (NVPC-W). It shows that the water molecules can be eliminated by vacuum drying at 150 °C. The elimination of water molecules does not change the crystal phase of the material, while the obtained Na2(VO)2(HPO4)2(C2O4) (NVPC) exhibits significant improvements in cycling stability, Coulombic efficiency, as well as rate performances. Kinetics analysis indicates that the existence of lattice water molecules hinders sodium-ion diffusion and promotes the degradation of electrodes. We believe the findings can help to develop high-performance cathode materials.