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

Abstract

The electrolyte solution of NaBOB in TEP is a low-cost, fluorine-free and flame-retardant electrolyte with ionic conductivity of 5 mS/cm, 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/cm2 and electrolyte volumes of <5 mL/g 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) 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.

Tris(trimethylsilyl) Phosphite and Lithium Difluoro(oxalato)borate as Electrolyte Additives for LiNi0.5Mn1.5O4‐Graphite Lithium‐Ion Batteries

“…The 31 P NMR spectra confirm the TMSPi peak at 115.1 ppm, in addition to the presence of tris(trimethylsilyl) phosphate (TMSPa) and bis-(trimethylsilyl) phosphate (BMSPa) as possible side products both in the pristine sample and in the EC-DEC solvents at around À 24.06 and À 13.02 ppm, respectively. [33] Further, the 19 F NMR spectra show a LiDFOB peak in the EC-DEC at around À 154.9 to À 155.0 ppm. Thereby, the NMR experiments indicate that there is no reaction between TMSPi and LiDFOB with the EC-DEC solvent mixture.…”

“…Moreover, the reactivity of the additives with EC-DEC solvents was also investigated in the absence of LiPF 6 salt (Figure 3c-e) to isolate the effect. In general, the 1 H, 19 F, and 31 P NMR spectra of the main electrolyte components EC, DEC, and [PF 6 ] À do not show significant change in their chemical shifts in the presence of additives (Figure S1), which could be due to their high concentration that leads to negligible change in their environment. However, the 1 H NMR spectra (Figure 3c) show the trivial shielded TMSPi molecules due to interactions with the solvents, but there is no reaction observed between the additives and EC-DEC solvents, which is also corroborated by the 19 F and 31 P NMR spectra (Figures 3d and 3e).…”

“…In general, the 1 H, 19 F, and 31 P NMR spectra of the main electrolyte components EC, DEC, and [PF 6 ] À do not show significant change in their chemical shifts in the presence of additives (Figure S1), which could be due to their high concentration that leads to negligible change in their environment. However, the 1 H NMR spectra (Figure 3c) show the trivial shielded TMSPi molecules due to interactions with the solvents, but there is no reaction observed between the additives and EC-DEC solvents, which is also corroborated by the 19 F and 31 P NMR spectra (Figures 3d and 3e). The 31 P NMR spectra confirm the TMSPi peak at 115.1 ppm, in addition to the presence of tris(trimethylsilyl) phosphate (TMSPa) and bis-(trimethylsilyl) phosphate (BMSPa) as possible side products both in the pristine sample and in the EC-DEC solvents at around À 24.06 and À 13.02 ppm, respectively.…”

“…Among the various high‐voltage additives available, tris(trimethylsilyl) phosphite (TMSPi) has been introduced as an efficient bifunctional additive that could scavenge the HF generated during cycling and build a robust CEI upon its oxidative decomposition [18,19] . Song et al.…”

“…[16,17] Among the various high-voltage additives available, tris(trimethylsilyl) phosphite (TMSPi) has been introduced as an efficient bifunctional additive that could scavenge the HF generated during cycling and build a robust CEI upon its oxidative decomposition. [18,19] Song et al found that TMSPi prevented the dissolution of Mn 2 + and Ni 2 + from LiNi 0.5 Mn 1.5 O 4 electrodes by scavenging HF molecules at a high charge potential of 5 V. [20] The stability of additives and their ability to build suitable electrolyte interphase layers are generally determined through oxidation and reduction potential values. [21,22] The oxidation potential (OP) and reduction potential (RP) of TMSPi were previously calculated using density functional theory (DFT) by Yim et al [23] and the OP was reported as 4.29 V vs. Li/Li + , which is lower than that of ethylene carbonate (EC, 6.92 V vs. Li/Li + ).…”