Tuned Imidazolium Ionic Liquids for Li-Ion Batteries: Compatibility with Li4TiO12/LiFePO4 and Cgr/LiFePO4 at Elevated Temperature
Although there have been a lot of research studies concerning ionic liquids that use imidazolium-based cations (for example, synthesis solvent and catalysis), the problem of a stability limit up to the redox potential of lithium metal has been pointed out, and there have been very few to no reports on rechargeable batteries that use lithium metal or graphite as electrodes. In this research study, we examined the battery characteristics of Cgr/LFP electrodes using an imidazolium based ionic liquid, with an electron donating substituent at the second position of the imidazolium cation. We aimed to improve the reduction stability of imidazolium-based ILs and to realize a battery system that has both safety and high performance by the molecular design of the room temperature ILs.
In this work, we assembled and tested low-cost full cell configuration based on Cgr/LiFePO4 and containing ionic liquids (C1CnIm-NTf2 and C1C1CnIm-NTf2 / n=4, 6) based on bis(trifluoromethanesulfonyl)imide (NTf2-) anion as the electrolyte media at 60°C. The cycle performance, life, electrochemical stability of this cell is reported and compared with LTO/LFP technology, along with some post-test and electrochemical diagnostics of the components after cycling. According to the results, the effects of extending the alkyl chain were confirmed: the best electrochemical stability and charge-discharge performance is observed with modified ionic liquids and long alkyl chain (figure 1). It is worth to mention that lithium ion Cgr/LFP cells employing IL-electrolyte were realized and found to deliver a full discharge capacity. Even more important, we could accomplish the cycling at 60°C, which can’t be safely achieved with conventional organic solvent based electrolytes.
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