Non-crystallizing solvent mixtures and lithium electrolytes for low temperatures

Abstract

A new class of solvent mixture and new liquid electrolytes are reported. New non-crystallizing materials may improve the performance of lithium-ion batteries at extremely low temperatures. Solvents were prepared by mixing EC (ethylene carbonate) with PEG250 (poly(ethylene glycol) dimethyl ether with an average molar mass of 250gmol−1). Some of these mixtures show only a glass transition, which takes place at temperatures below −70°C. It is possible to prepare such non-crystallizing electrolytes with both the novel imidazolide lithium salt LiTDI (lithium 4,5-dicyano-2-(trifluoromethyl)imidazolide) and the benchmark commercial salt LiPF6 (lithium hexafluorophosphate). Most state-of-the-art electrolytes crystallize at temperatures above −40°C. Thus, below this temperature a rapid decrease in conductivity is observed. New solutions show very good thermal and electrochemical properties (for instance: high conductivity, wide range of electrochemical stability window). This type of electrolyte may also reach high conductivity values of around 0.014mScm−1 at −60°C. This new class of electrolyte may be the answer to the numerous present issues with applications below 0°C, and also gives the possibility of battery storage at low temperature without cell damage.