Structural study on the development of high-voltage Na4Co3(PO4)2P2O7 cathode materials for sodium-ion batteries by in situ and time-resolved X-ray diffraction

As the demand for energy-storage systems grows, lithium sources may become scarce and alternative materials will be required. Sodium-ion batteries (SIBs) are low cost and safe alternatives to lithium-ion batteries (LIBs). Herein, in situ and time-resolved X-ray diffraction (TR-XRD) was used to investigate the structural changes and thermal safety of Na4Co3(PO4)(2)P2O7 cathode materials for high-voltage SIBs. A range of undesirable compounds (Na2Co2P2O7, alpha-NaCoPO4, Na4Co7(PO4)(6), and beta-NaCoPO4) form during heating; therefore, an optimized heating temperature of 740 degrees C was used to obtain Na4Co3(PO4)(2)P2O7. The in situ XRD results showed the dependency of the lattice parameters of Na4Co3(PO4)(2)P2O7 on the Na content during charging and discharging. During the charging process from 4.41 to 4.47 V, lattice constant a showed a significant decrease and b showed a steep increase, whereas c showed a marginal increase. During the charging process from 4.47 to 4.7 V, lattice constant b continued to increase, whereas no significant changes were observed in a and c. The discharge process was evaluated in a reverse order. Na4Co3(PO4)(2)P2O7 exhibited a 6% volume reduction during charging and 4% volume expansion during discharging. The TR-XRD results revealed that the structure of desodiated Na4-xCo3(PO4)(2)P2O7 (x = 2) was decomposed to the Na2CoP2O7 phase at similar to 215 degrees C during heating in the presence of an electrolyte, which was lower than that without electrolyte (similar to 230 degrees C). These results provide an in-depth understanding of Na4Co3(PO4)(2)P2O7 cathode materials and may serve as a basis for the development of high-performance Na4Co3(PO4)(2)P2O7 cathode materials for SIBs.