Hydration Enables Air‐Stable and High‐Performance Layered Cathode Materials for both Organic and Aqueous Potassium‐Ion Batteries

Abstract Potassium (K)‐based layered oxides are potential candidates for K‐ion storage but they suffer from chemical instability under ambient conditions that deteriorate their performance in rate‐capability and cycle life. To tackle this issue, a facile hydration strategy is employed, in which H₂O molecules are introduced into the K ion layers of P3‐type K_0.4Fe_0.1Mn_0.8Ti_0.1O₂, which induces a phase transition from the hexagonal to monoclinic symmetry accompanied by layer spacing expansion. The hydrated K_0.4Fe_0.1Mn_0.8Ti_0.1O₂ ⋅ 0.16H₂O has a strong tolerance to air and can be stored in lab air ambient for 60 days without a change in crystal structure or chemical composition. The K_0.4Fe_0.1Mn_0.8Ti_0.1O₂ ⋅ 0.16H₂O electrode shows improved K⁺ mobility and less volume change during potassiation/de‐potassiation. Owing to these merits, K_0.4Fe_0.1Mn_0.8Ti_0.1O₂ ⋅ 0.16H₂O as the cathodes for both organic and aqueous potassium‐ion full batteries attain outstanding rate capability and cycling stability (for example, capacity retention of 90% after 1000 cycles). This simple and potent hydration strategy has also been applied to improve the air stability of other K‐based layered oxides, including P3‐K_0.4MnO₂ and P2‐K_0.5Cu_0.1Fe_0.1Mn_0.8O₂, illustrating its usefulness in boosting layered oxides for durable potassium‐ion storage.