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

Abstract

? The Electrochemical Society ? The Author(s) 2015.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-PF 6) or bis(trifluromethanesulfonyl)amide (DDB-EMIm-TFSI)) anions. The electrochemical properties of DDB-EMIm-PF 6 and DDB-EMIm-TFSI dissolved in ethylene carbonate: diethyl carbonate (EC:DEC), in the presence of either LiPF 6 or LiTFSI, were evaluated. Cyclic voltammetry showed a compatible potential (?3.85 V vs. Li/Li +) for use in LIBs using LiFePO4 as cathodes. Electrolytes using 0.1 M of DDB-EMIm-PF 6 or 0.3, 0.7 and 1 M of DDB-EMIm-TFSI were prepared and evaluated in Li/LiFePO 4 (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, DDBEMIm-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 LiFePO 4.