Effect of a pyrrolidinium zwitterion on charge/discharge cycle properties of Li/LiCoO2 and graphite/Li cells containing an ionic liquid electrolyte

Ionic liquids (ILs) containing zwitterions have been studied as electrolytes for lithium-ion batteries (LIBs). The effects of addition of a pyrrolidinium zwitterion in an IL electrolyte on the thermal and electrochemical stability and charge/discharge properties of Li/LiCoO2 and graphite/Li cells were investigated. The thermal decomposition temperature of the IL electrolyte composed of N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)amide ([P13][FSA])/lithium bis(trifluoromethylsulfonyl)amide (LiTFSA) with 3-(1-butylpyrrolidinium)propane-1-sulfonate (Bpyps) as the zwitterionic additive, the thermal decomposition temperature was about 300 °C. The electrochemical window of [P13][FSA]/LiTFSA/Bpyps was 0-+5.4 V vs. Li/Li+, which was almost identical to that of [P13][FSA]/LiTFSA. Li|electrolyte|LiCoO2 cells containing the IL/Bpyps electrolyte system exhibited high capacities in the cut-off voltage range of 3.0-4.6 V, even after 50 cycles. The increase in the interfacial resistance between the electrolyte and cathode with cycling was suppressed. In the cyclic voltammograms of cells employing a graphite electrode, the intercalation/deintercalation of lithium ions were observed in the range of 0 and + 0.4 V vs. Li/Li+. Further, graphite|electrolyte|Li cells containing [P13][FSA]/LiTFSA/Bpyps exhibited stable charge/discharge cycle behaviour over 50 cycles.