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Abstract
Water-in-salt electrolytes have successfully expanded the electrochemical stability window of aqueous electrolytes to up to 3 V. While particularly the reductive stability of water-in-salt electrolytes can be further improved by simply increasing the salt concentration, this approach has its limitations as it leads to very viscous and hence poorly conducting solutions. An alternative strategy is the partial substitution of water by ionic liquids, which boosts the lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) solubility while maintaining adequate transport properties at very high salt concentrations.
Here, we study such ternary electrolytes based on LiTFSI, water, and imidazolium-type ionic liquids in terms of their thermal, electrochemical, and transport properties. We find that the LiTFSI solubility increases from 21 mol kg−1 in water to up to 60 mol kg−1 in the presence of these ionic liquids. Deconvolution of the different contributions to the LiTFSI solubility shows that particularly the ionic liquid 1-ethyl-3-methylimidazolium trifluoromethanesulfonate acts as solubility enhancer.
The increased reductive stability of these ternary electrolytes enables stable cycling of both TiO2 and Li4Ti5O12 anodes. We further show that the low water content of these electrolytes also allows stable cycling of LiNi0.8Mn0.1Co0.1O2 (NMC811) cathodes. For instance, a TiO2/NMC811 full cell based on such a hybrid electrolyte shows an energy density of 121 Wh kg−1 on the active material level, a Coulombic efficiency of >99.7% at C/2, and retains 80% of its initial capacity after 290 cycles at this rate. Owing to the 300 mV lower redox potential of Li4Ti5O12 compared to TiO2, Li4Ti5O12/NMC811 full cells reach energy densities of 141 and 150 Wh kg−1 at a rate of 1C and C/2, respectively. These cells still show Coulombic efficiency of 99.4% and 99.2%, respectively, and maintain 80% of their initial capacity after 230 and 154 cycles, respectively. Considering the large number of potential lithium salt–ionic liquid combinations, we anticipate further improvements in electrolyte properties and subsequently cell performance.
Here, we study such ternary electrolytes based on LiTFSI, water, and imidazolium-type ionic liquids in terms of their thermal, electrochemical, and transport properties. We find that the LiTFSI solubility increases from 21 mol kg−1 in water to up to 60 mol kg−1 in the presence of these ionic liquids. Deconvolution of the different contributions to the LiTFSI solubility shows that particularly the ionic liquid 1-ethyl-3-methylimidazolium trifluoromethanesulfonate acts as solubility enhancer.
The increased reductive stability of these ternary electrolytes enables stable cycling of both TiO2 and Li4Ti5O12 anodes. We further show that the low water content of these electrolytes also allows stable cycling of LiNi0.8Mn0.1Co0.1O2 (NMC811) cathodes. For instance, a TiO2/NMC811 full cell based on such a hybrid electrolyte shows an energy density of 121 Wh kg−1 on the active material level, a Coulombic efficiency of >99.7% at C/2, and retains 80% of its initial capacity after 290 cycles at this rate. Owing to the 300 mV lower redox potential of Li4Ti5O12 compared to TiO2, Li4Ti5O12/NMC811 full cells reach energy densities of 141 and 150 Wh kg−1 at a rate of 1C and C/2, respectively. These cells still show Coulombic efficiency of 99.4% and 99.2%, respectively, and maintain 80% of their initial capacity after 230 and 154 cycles, respectively. Considering the large number of potential lithium salt–ionic liquid combinations, we anticipate further improvements in electrolyte properties and subsequently cell performance.
