From Na to K-Based Prussian Blue: A Path Toward Cathode Materials for Extreme Environment

Abstract

Prussian blue (PB) is regarded as a promising host for Na or K storage because of its sustainable precursor elements (e.g., Mn, Fe) and open framework structure. However, unstable structure, high crystal H₂O content, and risky HCN generation restrain its practical applications. In this work, after systematical investigation of structural evolution from Na-based to K-based PB and its relationship with electrochemical properties, it is clarified that low crystal water content, high K content, and trace Na doping are essential for a robust structure and stable cycling of PB. It is found that a trace Na-doped K-based PB exhibits comprehensive properties of low crystal water content (3.2 wt%), high thermal stability (over 340 °C), and superior cycling stability (84.3% after 6300 cycles at 5 C). Besides, the PB can also present stable cycling under harsh conditions, such as with intermittent-overcharge/overdischarge steps (4.8 V/1.2 V, 93.3% after 2100 cycles at 5 C), in a wide voltage range (93.2% after 1000 cycles at 1.5‒4.5 V/5 C), under a high rate (83.7% after 4350 cycles at 10 C), and at a high temperature (92.0% after 1650 cycles at 45 °C/1 C). The superior electrochemical properties are attributed to its structural robustness even under harsh conditions.