Synthesis and Electrochemical Properties of Electrolytes Based on Phosphonium, Sulfonium and Imidazolium Ionic Liquids for Li-Ion Batteries

Abstract

This work describes synthesis and characterization of three ionic liquids (IL) with imidazolium, phosphonium or sulfonium cations. These ILs were used as precursors for the preparation of electrolytes by adding various types of lithium salts. Thermal and electrochemical properties of the electrolytes were studied to assess their feasibility for usage in lithium-ion batteries (LIB). Ethylmethylimidazolium phosphonic acid ethyl ester [EtMeIm]⁺[EtPITE]^– with added Li salts showed very low conductivities of about 0.02 mS cm^–1 and a rather narrow electrochemical stability window of 4.3 V. Of all the ILs under study, this imidazolium-based IL exhibited the worst thermal properties both neat and with additional Li⁺ salt. Therefore, using this IL as an electrolyte for LIB is unlikely. Methyl­tributylphosphonium bis(trifluoromethyl sulfonyl) imide [P₁₄₄₄]⁺[TFSI]^– with Li salts had already significantly higher conductivities of 0.4–0.6 mS cm^–1 and showed a very wide electrochemical stability window of 7 V. TGA curves of [P₁₄₄₄]⁺[TFSI]^– show excellent thermal stability up to 300 °C. DSC measurements showed semicrystalline behaviour of [P₁₄₄₄]⁺[TFSI]^–; this unfavourable fact was overcome by the addition of Li⁺ salts. Potential of this IL is very high for the use in LIB. At room temperature, trimethylsulfonium bis(trifluoromethyl sulfonyl) imide [S₁₁₁]⁺[TFSI]^– is a solid crystalline material, so it was mixed with [P₁₄₄₄]⁺[TFSI]^–. The resulting mixture [P₁₄₄₄]⁺[S₁₁₁]⁺[TFSI]^– achieved the highest conductivities of 0.6–1.0 mS cm^–1 in comparison to other prepared ILs. The electrochemical stability is also interesting, its ESW value being 5.5 V. Thermal properties are comparable to those of [P₁₄₄₄]⁺[TFSI]^–, but it does not show semicrystalline behaviour. This mixture is a promising electrolyte for LIB.

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