Redox Shuttles for Lithium-Ion Batteries at Concentrations up to 1 M Using an Electroactive Ionic Liquid Based on 2,5-di-tert-butyl-1,4-dimethoxybenzene

In this work, we designed two redox shuttles with high solubility (up to 1 M) in conventional carbonate-based lithium-ion battery (LIB) electrolytes. At this high concentration, redox shuttles ensure improved overcharge protection than lower concentrations. We developed electroactive imidazolium salts by modifying imidazolium with 2,5-di-tert-butyl-1,4-dimethoxybenzene. Two salts with the cation 1-(3-(2,5-di-tert-butyl-1,4-methoxyphenoxy) propyl)-3-methyl-1H-imidazol-3-ium (EMIm) were synthesized using either hexafluorophosphate (DDB-EMIm-PF6) or bis(trifluromethanesulfonyl) amide (DDB-EMIm-TFSI)) anions. The electrochemical properties of DDB-EMIm-PF6 and DDB-EMIm-TFSI dissolved in ethylene carbonate : diethyl carbonate (EC:DEC), in the presence of either LiPF6 or LiTFSI, were evaluated. Cyclic voltammetry showed a compatible potential (similar to 3.85 V vs. Li/Li+) for use in LIBs using LiFePO4 as cathodes. Electrolytes using 0.1 M of DDB-EMIm-PF6 or 0.3, 0.7 and 1 M of DDB-EMIm-TFSI were prepared and evaluated in Li/LiFePO4 (LFP) test cells to demonstrate overcharge protection. Electrochemical cycling at C/10 showed an overcharge protection for all concentrations of the redox ionic salts under 100% overcharge conditions. Among these salts, DDB-EMIm-TFSI, at a concentration of 0.7 M, was effective in shuttling excess current for over 200 cycles, representing over 6000 operating hours, while maintaining nominal values for the discharge capacity of LiFePO4. (C) 2015 The Electrochemical Society.