Ultrafast Aqueous Potassium‐Ion Batteries Cathode for Stable Intermittent Grid‐Scale Energy Storage

Abstract The inherent short‐term transience of solar and wind sources cause significant challenges for the electricity grid. Energy storage systems that can simultaneously provide high power, long cycle life, and high energy efficiency are required to accommodate the fast‐changing output fluctuations. Here, an ultrafast aqueous K‐ion battery based on the potassium‐rich mesoporous nickel ferrocyanide (II) (K 2 NiFe(CN) 6 ·1.2H 2 O) is developed. This battery achieves an unprecedented rate capability up to 500 C (8214 W kg −1 ), which only takes 4.1 s for one charge or discharge. The open‐framework structure of K 2 NiFe(CN) 6 ·1.2H 2 O with small volume variation supports the capacity retention of 98.6% after 5000 cycles, and a superior round‐trip energy efficiency of 95.6% at a 5 C rate. Beyond monovalent ion storage, K 2 NiFe(CN) 6 ·1.2H 2 O can also function as a versatile high‐rate cathode for divalent‐ion batteries (Mg 2+ ), trivalent‐ion batteries (Al 3+ ), and hybrid full‐cells applications. These properties represent a significant step forward in the exploitation of ultrafast metal ions storage, and accelerate the development of intermittent grid‐scale energy storage technologies.