Safety is a highest priority issue for large-scale applications of lithium-ion batteries. Commercial lithium-ion batteries use organic electrolytes, which are highly volatile and flammable to cause serious fires and explosions. An effective approach for minimizing such risks is to replace the flammable organic electrolyte for nonflammable (or fire-extinguishing) one. Organic solvents containing phosphorous or fluorine have been extensively studied as flame retardant additives. However, they cannot passivate carbonaceous negative electrodes; thus, using those additives generally degrades charge-discharge cycleability. As a result, there has been a trade-off between non-flammability and cell cycleability in designing organic electrolytes. Recently, superconcentrated electrolytes are widely studied as a new class of liquid electrolyte for battery applications, which enables salt anion-derived excellent passivation of negative electrodes.¹ Our group applied this strategy to LiN(SO₂F)₂ (LiFSA) / trimethyl phosphate (TMP) electrolytes to achieve both non-flammability (and even fire-extinguishing function) and outstanding cycling performance of carbonaceous negative electrodes.² However, such superconcentrated electrolytes have several disadvantages of high viscosity, low ionic conductivity, and high cost for practical application.¹ To circumvent these issues, Watanabe and coworkers proposed the dilution of superconcentrated electrolytes (specifically solvate ionic liquids) with low-polar solvent (“diluent”).³ They demonstrated that diluted solvate ionic liquids retains their original functions but have much lower viscosity and higher ionic conductivity. In this work, we studied a fire-extinguishing LiFSA/TMP superconcentrated electrolyte diluted with low-polar 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (HFE) which is also a flame retardant. We investigated the effect of dilution on passivation abilities toward graphite negative electrodes, and discuss the passivation mechanism from the viewpoint of local coordination structure.

We studied three electrolyte compositions: LiFSA/TMP (1:1.3 by molar ratio, “concentrated”), LiFSA/TMP:HFE (1:1.3:8 by molar ratio, “concentrated + diluted”), and LiFSA/TMP:HFE (1:8:1.3, “dilute”). The last one is named as “dilute” electrolyte, because it has an excess amount of high-polar solvent (TMP) as with conventional dilute electrolytes. Figure 1 shows charge-discharge curves of graphite in the three electrolytes. In “dilute” electrolyte, plateaus appeared at around 1.0 V on charging and then almost no discharge capacity was observed, which is due to the co-intercalation of TMP. On the other hand, “concentrated” electrolyte enables highly reversible Li⁺ intercalation reaction due to the formation of FSA anion-derived inorganic solid electrolyte interphase (SEI).² Notably, the high reversibility could be retained by diluting the concentrated electrolyte with low-polar HFE (“concentrated + diluted”). Upon electrode surface analysis with X-ray photoelectron spectroscopy (XPS), we found FSA anion-derived SEI film analogous to that formed in “concentrated” electrolyte, indicating that the unusual passivation ability could be retained even after dilution. Raman spectroscopy and Fourier transformed infrared spectroscopy (FT-IR) of the electrolyte solutions revealed that “concentrated + diluted” electrolyte had specific local coordination structure similar to “concentrated” electrolyte. These results suggest that passivation ability can be controlled by tuning the local coordination structure in electrolyte solutions.

References

1. Y. Yamada et al., J. Electrochem. Soc., 161, A2406 (2015)
2. J. Wang et al., Nature Energy, 3, 22 (2018)
3. K. Dokko et al., J. Electrochem. Soc., 160, A1304 (2013)