4.2 V O3‐Layered Cathodes in Sodium‐Ion Pouch Cells Enabled by an Intermolecular‐Reinforced Ether Electrolyte

Abstract To fulfill the requirements for practical applications, it is urgent to boost the gravimetric energy density of sodium‐ion batteries. An effective way is to increase the charging voltage of O3‐type layered cathodes preferably to 4.2 V versus Na/Na⁺ (V_Na). Nevertheless, it is extremely challenging to achieve stable cycling of the cathodes at such a high cut‐off voltage. Here a novel electrolyte strategy to design an intermolecular‐reinforced electrolyte (IRE) is presented, utilizing meticulously protected ether molecules, which facilitates stable high‐voltage cycling of the commercially viable NaNi_1/3Fe_1/3Mn_1/3O₂ (NFM). While the NFM with the IRE exhibits a high specific capacity of ≈158 mAh g⁻¹ at 4.2 V_Na (130 mAh g⁻¹ at 4.0 V_Na), the aggressive cathode surface can still be effectively stabilized by the formation of favorable thin and inorganic‐rich cathode−electrolyte interfaces. Remarkably, under a high cut‐off voltage of 4.2 V_Na, an industrial ampere‐hour‐level NFM||hard carbon pouch cell with the IRE electrolyte shows an excellent long‐term cycling stability with 82.8% capacity retention after 800 cycles, largely outperforming the localized high‐concentration electrolyte (82.9% after 200 cycles).