Electrolyte Design with Dual -C≡N Groups Containing Additives to Enable High-Voltage Na3V2(PO4)2F3-Based Sodium-Ion Batteries.

Abstract

Na3V2(PO4)2F3 is recognized as a promising cathode for high energy density sodium-ion batteries due to its high average potential of ∼3.95 V (vs Na/Na+). A high-voltage-resistant electrolyte is of high importance due to the long duration of 4.2 V (vs Na/Na+) when improving cyclability. Herein, a targeted electrolyte containing additives with two -C≡N groups like succinonitrile has been designed. In this design, one -C≡N group is accessible to the solvation sheath and enables the other -C≡N in dinitrile being exposed and subsequently squeezed into the electric double layer. Then, the squeezed -C≡N group is prone to a preferential adsorption on the electrode surface prior to the exposed -CH2/-CH3 in Na+-solvent and oxidized to construct a stable and electrically insulating interface enriched CN-/NCO-/Na3N. The Na3V2(PO4)2F3-based sodium-ion batteries within a high-voltage of 2-4.3 V (vs Na/Na+) can accordingly achieve an excellent cycling stability (e.g., 95.07% reversible capacity at 1 C for 1,5-dicyanopentane and 98.4% at 2 C and 93.0% reversible capacity at 5 C for succinonitrile after 1000 cycles). This work proposes a new way to design high-voltage electrolytes for high energy density sodium-ion batteries.