Anion Vacancy Regulated Sodium/Potassium Intercalation in Potassium Prussian Blue Analog Cathodes for Hybrid Sodium-Ion Batteries

Fe-based potassium Prussian blue analogs (K-PBAs) are commonly used as K-ion battery (KIB) cathodes. Interestingly, K-PBAs are appealing cathodes for Na-ion batteries (NIBs). In a hybrid NIB cell, where Na-ion is in the electrolyte and K-ion is in the PBA cathode, cation intercalation and electrochemical performance of the cathode can be significantly affected by [Fe(CN)6]4- anion vacancy. This work studies the effect of anion vacancy in K-PBAs on regulating K-ion/Na-ion intercalation mechanism in hybrid NIB cells, by comparing two K-PBA cathodes with different vacancy contents. The results demonstrate that introducing a level of anion vacancy can maximize the number of K-ion intercalation sites and enhance K-ion diffusion in the PBA framework. This facilitates K-ion intercalation and suppresses Na-ion intercalation, resulting in a K-ion-dominated and high-discharge-voltage ion storage process in the hybrid NIB cell. The K-PBA cathode with 20% anion vacancy delivers 128 mAh g-1 at 25 mA g-1 and 67 mAh g-1 at 1000 mA g-1, as well as retains 89% and 81% capacity after 100 and 300 cycles, respectively. It completely outperforms the counterpart with 7% anion vacancy, which exhibits increased Na-ion intercalation but overall deteriorated ion storage. Fe-based Prussian blue analogs exhibit enhanced K-ion intercalation and suppressed Na-ion intercalation in the presence of anion vacancies in the structure. When K-containing Prussian blue analogs are used as cathodes in hybrid Na-ion batteries, the enhancement results in an increase in discharge voltage, reversible capacity, and rate capability, compared to Na-containing counterparts.image

Automated summary

This study investigates the effect of anion vacancies in Fe-based potassium Prussian blue analogs on the intercalation mechanism of potassium and sodium ions in hybrid sodium-ion batteries. The results show that introducing anion vacancies can enhance potassium-ion diffusion and intercalation in the cathode, resulting in a potassium-ion-dominated high-discharge-voltage ion storage process.