Dual-ion batteries (DIBs) based on anion (de)intercalation into low-cost graphitic carbon cathodes hold great promise in gridscale energy storage. Different from the electrolyte in rockingchair batteries, which only serves as a charge transporter, both cations and anions in the electrolyte for DIBs participate in battery reactions. Hence, the impact of the electrolyte formulation on cycle life, energy density, as well as cost has become a subject of vital importance. This review discussed the challenges and recent progress of electrolytes for DIBs, with a particular focus on the exploration of electrolytes with high oxidation stability, high salt concentration, high ionic conductivity, and low cost. Moreover, the influence of varied ion concentrations at different state-of-charge levels on the electrolyte properties such as ionic conductivity and electrochemical stability is analyzed. Finally, perspectives on the current limitations and future research directions of electrolytes for DIBs are provided.
Aqueous sodium‐ion batteries are promising candidates for grid‐scale energy storage due to their high safety and low cost. However, the constant dissolution of vanadium‐based electrodes and protective solid‐electrolyte interphase (SEI) in aqueous electrolytes severely limits the cycle life. Herein, using adiponitrile as a functional co‐solvent, we designed a super‐concentrated aqueous sodium electrolyte (Na+/H2O=1, molar ratio) to confine almost all water molecules within the primary solvation shell of Na+ with decreased activity. Such a unique solvation structure not only expands the electrochemical stability window of the electrolyte to 2.75 V, but also greatly alleviates the dissolution of vanadium‐based electrodes and NaF‐rich SEI. The assembled Na3V2(PO4)3/NaTi2(PO4)3 battery delivers an average Coulombic efficiency of 99.6 % with 71 % capacity retention after 1000 cycles at 5 C. In addition, the freezing point of the electrolyte could be reduced −79 °C while retaining appreciable low‐temperature conductivity due to the disrupted hydrogen bond among water molecules.
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