Ester based electrolyte with lithium bis(trifluoromethane sulfonyl) imide salt for electrochemical storage devices: Physicochemical and electrochemical characterization

Esters co-solvent has been used in ternary system electrolytes for electrochemical storage using LiTFSI as salt. Three linear esters have been studied: methyl propanoate (MP), ethyl propanoate (EP) and ethyl butanoate (EB). The ionic conductivity, viscosity, and electrochemical performances of ethylene carbonate (EC)/ester/dimethyl carbonate (DMC) mixtures have been determined and compared to the reference ternary mixture: EC/propylene carbonate (PC)/DMC in proportion (1/1/3) by volume. These electrolytes have been tested in half cells of the type: Li | electrolyte | activated carbon (AC) or graphite (Gr). These active materials are designed to be used in hybrid Li-ion capacitors. Electrochemical results show that MP presents the best performances at the AC electrode in terms of capacitance, especially at low temperatures (−10 °C). However, MP based electrolytes give the higher irreversible capacity when used with the Gr electrode, and are the most corrosive toward the aluminum current collector. This problem has been resolved by adding a small amount of LiPF6 (1% in mol) to the electrolyte solution as a passivating additive. Thus, MP based electrolytes exhibit the best performances in terms of capacity at the Gr electrode and capacitance at the AC electrode.