Role of Acid in Tailoring Prussian Blue as Cathode for High-Performance Sodium-Ion Battery

Prussian blue (PB) with concave centers is synthesized successfully through a hydrothermal method with the assistance of acid. In this study, the role of the acid is investigated systematically by adjusting the reaction temperature and time, using different kinds and amounts of acid, and changing the amount of PVP surfactant added. It is found that the acid can not only trigger the chemical reaction to form cubic PB, but also act as an etching reagent to tailor the morphology. The as-obtained cubic PB with concave centers demonstrates a superior cycle stability and rate performance, which can achieve a capacity of 107 mA hg(-1) at 0.2 Ag-1. The corresponding capacity retention is 74% after 500 cycles relative to the second cycle. Even at a current density of 5 Ag-1, the specific capacity remains at 82 mA hg(-1). Furthermore, the full cell, using PB as the cathode and hard carbon as the anode, exhibits a high capacity of 70 mA hg(-1) at 0.1 Ag-1, and can power an LED light successfully. This work provides new insights into the role of acid in tailoring the morphology of PB, and opens a new avenue toward the design of unique structures to improve sodium storage.