Prussian White with Near-Maximum Specific Capacity in Sodium-Ion Batteries

Prussian blue (PB) and its analogues are promising materials for sodium-ion battery cathodes because of their high working potentials, high theoretical capacity, and low toxicity. Prussian white (PW), which is the fully reduced and sodiated form of PB, could significantly improve the manufacturability of commercial batteries as it circumvents the requirement of a reactive sodium-loaded anode in cell assembly. However, previous preparations of PW require a high-temperature, high-pressure, and oxygen-free environment, and their reported performance in battery cathodes have been modest so far. Here, we report a mild, low-temperature synthesis of PW, using a common sodium borohydride reducing agent in our reaction. We achieved PW with a remarkably high specific capacity of 169 ± 4 mA h/g at 1C in a sodium-ion half-cell battery, which is near the theoretical maximum capacity of 171 mA h/g. Complete full-cell batteries with near-maximum capacities were also demonstrated as proof-of-concept. The attainment of this high specific capacity is attributed to the successful preservation of Na⁺ intercalation sites in the fully sodiated material during a critical heat-vacuum-drying step in our process. These findings, and the significant advancement in performance, mark an important step toward the future deployment of sodium-ion rechargeable batteries in consumer electronic devices.