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

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 K0.4Fe0.1Mn0.8Ti0.1O2, which induces a phase transition from the hexagonal to monoclinic symmetry accompanied by layer spacing expansion. The hydrated K0.4Fe0.1Mn0.8Ti0.1O2 ⋅ 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 K0.4Fe0.1Mn0.8Ti0.1O2 ⋅ 0.16H2O electrode shows improved K+ mobility and less volume change during potassiation/de‐potassiation. Owing to these merits, K0.4Fe0.1Mn0.8Ti0.1O2 ⋅ 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‐K0.4MnO2 and P2‐K0.5Cu0.1Fe0.1Mn0.8O2, illustrating its usefulness in boosting layered oxides for durable potassium‐ion storage. A facile hydration engineering strategy is innovatively proposed here, exhibiting multiple functions: an air‐stable K‐based layered material is achieved, compatible with both organic and aqueous potassium‐ion batteries; the expanded interlayer space boosts the pseudocapacitive effect and reduces the activation energy barrier; the crystal water shows pillar effect and improves long‐term cycling stability.