Sodium Bis(oxalato)borate in Trimethyl Phosphate: A Fire-Extinguishing, Fluorine-Free, and Low-Cost Electrolyte for Full-Cell Sodium-Ion Batteries

Sodium-ion batteries based on all-naturally abundant elements, in which no cobalt, nickel, copper, and fluorine is used, can lead to a major breakthrough in making batteries more sustainable. Safety aspects—in particular, flammability of electrolytes—in the state-of-the-art battery technology is another important concern, especially for applications in which large numbers of cells are employed. Nonflammable battery electrolytes studied so far are based on highly fluorinated compounds or high salt concentrations, which suffer from high cost and toxicity. We here propose an electrolyte based on a single solvent and low-cost and fluorine-free salt at a lower range of "standard" concentrations. Our results show—for the first time—that sodium bis(oxalato)borate (NaBOB) is soluble in the nonflammable solvent trimethyl phosphate (TMP). This finding enables a nonflammable electrolyte with high ionic conductivity and promising electrochemical performance in full-cell sodium-ion batteries. An electrolyte of 0.5 M NaBOB in TMP provides an ionic conductivity of 5 mS cm–1 at room temperature, which is comparable to the commonly used electrolytes based on sodium hexafluorophosphate (NaPF6) and organic carbonate solvents. The proposed electrolyte shows a Coulombic efficiency of above 80% in the first cycle, which increased to about 97% from the second cycle in sodium-ion battery full-cells consisting of a hard carbon anode and a Prussian white cathode. This work opens up opportunities to design safe electrolytes which can further be optimized with electrolyte additives such as vinylene carbonate for industrial applications.