Structural study on the development of high-voltage NaCo(PO)PO cathode materials for sodium-ion batteries by 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 Na 4 Co 3 (PO 4 ) 2 P 2 O 7 cathode materials for high-voltage SIBs. A range of undesirable compounds (Na 2 Co 2 P 2 O 7 , alpha-NaCoPO 4 , Na 4 Co 7 (PO 4 ) 6 , and beta-NaCoPO 4 ) form during heating; therefore, an optimized heating temperature of 740 °C was used to obtain Na 4 Co 3 (PO 4 ) 2 P 2 O 7 . The in situ XRD results showed the dependency of the lattice parameters of Na 4 Co 3 (PO 4 ) 2 P 2 O 7 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. Na 4 Co 3 (PO 4 ) 2 P 2 O 7 exhibited a 6% volume reduction during charging and 4% volume expansion during discharging. The TR-XRD results revealed that the structure of desodiated Na 4− x Co 3 (PO 4 ) 2 P 2 O 7 ( x = 2) was decomposed to the Na 2 CoP 2 O 7 phase at ∼215 °C during heating in the presence of an electrolyte, which was lower than that without electrolyte (∼230 °C). These results provide an in-depth understanding of Na 4 Co 3 (PO 4 ) 2 P 2 O 7 cathode materials and may serve as a basis for the development of high-performance Na 4 Co 3 (PO 4 ) 2 P 2 O 7 cathode materials for SIBs. Electrochemical performance of various sodium cobalt phosphates synthesized in the temperature range of 700-770 °C and their schematic crystal structure.