Optimization of the lithium-ion cell electrolyte composition through the use of the LiTDI salt

Abstract

Optimization of electrolyte composition for future traction batteries is presented in this paper. Extensive systematic studies of electrochemical performance, material utility and subsequently cost are reported in order to obtain optimal combination. Lithium salt of TDI anion (4,5-dicyano-2-trifluoromethanoimidazole) is used due to its superior thermal stability (compared to other commercially available lithium salts), stability in case of moisture presence, electrochemical stability and possible material savings when used in electrolyte. Critical selection of solvent mixtures is made with respect to the price of components.Conductivity dependence of salt concentration is shown. For all studied systems the increased conductivity region is observed in the salt low and broad concentration range of 0.3 to 0.8molkg−1. The structural and rheological explanation of the mentioned feature is performed. Lithium cation transference numbers are measured for highly conductive samples and used as the secondary parameter in the optimization procedure. The highest values are recorded for the very low salt content: 0.31molkg−1 LiTDI in EC:DMC (1:2 weight ratio) (σ=5.09mS cm−1, tLi+=0.622) and 0.4molkg−1 LiTDI in EC:DMC:DME (8:16:1 weight ratio) (σ=6.17mS cm−1, tLi+=0.648) giving the opportunity to substantial material savings in batteries.