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

Abstract

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)₂P₂O₇ 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 °C was used to obtain Na₄Co₃(PO₄)₂P₂O₇. The in situ XRD results showed the dependency of the lattice parameters of Na₄Co₃(PO₄)₂P₂O₇ 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)₂P₂O₇ 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−xCo₃(PO₄)₂P₂O₇ (x = 2) was decomposed to the Na₂CoP₂O₇ 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 Na₄Co₃(PO₄)₂P₂O₇ cathode materials and may serve as a basis for the development of high-performance Na₄Co₃(PO₄)₂P₂O₇ cathode materials for SIBs.