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

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).