Sodium Storage Performance and Mechanism of Prussian Blue Doped Cathode Materials

Prussian blue analogs (PBAs) exhibit great potential as cathode materials for sodium-ion batteries due to their open three-dimensional framework, but their applications are limited by crystal defects and structural stability issues. To address this issue, a strategy of doping lanthanum (La) into the interstitial (A) sites was proposed in this paper. LaCdFe-PBAs cathode materials were prepared via an ion-exchange method using lanthanum nitrate/cadmium nitrate as raw materials and sodium citrate as a complexing agent. The effects of La doping on the crystal structure, sodium storage performance, and electrochemical reaction mechanism of the materials were investigated by means of ex-situ X ray diffraction (XRD), inductively coupled plasma optical emission spectroscopy (ICP-OES), and X ray photoelectron spectroscopy (XPS). The results indicate that, compared to the undoped CdFe-PBAs, a specific capacity of 180 mAh·g⁻¹ is delivered by LaCdFe-PBAs at 1 A•g⁻¹, and a capacity of 35 mAh•g⁻¹ is still maintained even at 20 A·g⁻¹, demonstrating excellent rate performance. After 3 000 cycles at a high current density of 10 A•g⁻¹, a capacity retention rate of 73.5% is still achieved, exhibiting an outstanding long-term cycling life. Electrochemical kinetic analysis reveals that fast ion reaction kinetics and highly reversible charge-discharge characteristics are endowed by La doping. Mechanistic investigations reveal that during cycling, the internal structure is effectively stabilized by Fe³⁺ from the electrolyte, the continuous dissolution of La is inhibited, and a phase transformation from a hexagonal to a cubic crystal system is induced, thereby significantly enhancing the structural stability. Furthermore, an AC//LaCdFe-PBAs ASIC device is constructed with LaCdFe-PBAs as the cathode and activated carbon (AC) as the anode, for which a maximum energy density of 63.5 Wh•kg⁻¹ and a power density of 8 064 W•kg⁻¹ are achieved, and a capacity retention rate of approximately 70% is maintained after 1 000 cycles at 10 A•g⁻¹. The interstitial site La doping strategy proposed in this study is suggested to provide a new approach for solving the structural stability challenge of PBA cathode materials, and it is of great significance for the development of high-energy-density, long-life sodium-ion battery cathode materials and the promotion of low-cost, high-performance energy storage systems.