Optimization of Sodium Bis(oxalato)borate (NaBOB) in Triethyl Phosphate (TEP) by Electrolyte Additives

The electrolyte solution of NaBOB in TEP is a low-cost, fluorine-free and flame-retardant electrolyte with ionic conductivity of 5 mS cm⁻¹, recently discovered to show promises for sodium-ion batteries. Here, the abilities of this electrolyte to effectively form a solid electrolyte interphase (SEI) was augmented with five common electrolyte additives of fluoroethylene carbonate (FEC), vinylene carbonate (VC), prop-1-ene-1,3-sultone (PES), 1,3,2-dioxathiolane 2,2-dioxide (DTD) and tris(trimethylsilyl)phosphite (TTSPi). Full-cells with electrodes of Prussian white and hard carbon and industrial mass loadings of >10 mg cm⁻² and electrolyte volumes of <5 ml g⁻¹ were used. X-ray photoelectron spectroscopy (XPS) and pressure analysis were also deployed to investigate parasitic reactions. Cells using electrolyte additives of PES, PES+DTD and PES+TTSPi (3 wt%) showed significantly increased performance in terms of capacity retention and initial Coulombic efficiency as compared to additive-free NaBOB–TEP. The best cell retained 80% discharge capacity (89 mAh g⁻¹) after 450 cycles, which is also significantly better than reference cells using 1 M NaPF₆ in EC:DEC electrolyte. This study sheds light on opportunities to optimize the NaBOB–TEP electrolyte for full-cell sodium-ion batteries in order to move from low-mass-loading lab-scale electrodes to high mass loading electrodes aiming for commercialization of sodium-ion batteries.