Cation-Acceptor DMI Boosts NaNO ₃ Solubility via Solvation Regulation for High-Performance Nonaqueous Sodium Rechargeable Batteries

Sodium nitrate (NaNO₃) serves as an effective film-forming additive, facilitating the formation of inorganic, nitride-rich solid electrolyte interphase (SEI) layers at the electrode interface. However, its limited solubility in conventional ether-based electrolytes constrains its practical application. To overcome this limitation, a high donor-number solvent, 1,3-dimethyl-2-imidazolidinone (DMI), was introduced to enhance the solubility of NaNO₃ in hard carbon (HC)-based sodium-ion batteries. Due to its strong donor number (DN), DMI acts as a cation acceptor, which can disrupt the ion-pairing limitation by attracting Na⁺ ions and weakening the interaction between Na⁺ and NO₃, thereby increases the solubility of NaNO₃ in the ether-based electrolyte to 0.05 M. Meanwhile, in order to overcome the problem of thicker film formation caused by the introduction of DMI, it was preferred that the addition amount of DMI in the electrolyte be 10%. The optimized electrolyte system generates a nitrogen-rich SEI film. As a result, the HC||Na half-cell achieved a specific charge capacity of 300 mAh·g^-1 at 0.5 C and a 91% capacity retention after 400 cycles. This work proposes a strategy to manipulate solvation structures by improving NaNO₃ solubility via high donor-number solvents, offering a straightforward approach to designing advanced electrolytes for sodium-ion batteries.