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 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.