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 H 2 O 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.