Preparation And Performance Of Electrolyte Containing Ionic Liquid For Lithium Ion Batteries

Lithium ion batteries have been one of the main power supplies for electronic vehicle (EV) and hybrid electric vehicle (HEV) due to the advantages of high work potential, huge specific energy, long cycle life and low contamination. However, the conventional organic electrolyte of lithium ion batteries will cause safety issues because it is noxious, volatile and able to involve in the thermal decomposition reaction inside the batteries, which makes the safe and nontoxic electrolyte an urgent need. Ionic liquid arouses much attention from investigators and is considered to replace the conventional organic electrolyte and solve the safety issues by virtue of nonvolatility, non-flammability, high ionic conductivity and extended electrochemical steady window. In this dissertation, electrolytes containing ionic liquid were investigated in terms of physical-chemical properties electrochemical characteristics and the compatibility with electrode.Nine sorts of binary ionic liquid electrolyte were prepared by mixing buthy-3-methylimidazofium tetrafluoroborate, 1-buthy-3-ethylimidazolium bis (trifluoro methanesulfonimide), 1-methyl-3-ethylimidazolium bis(trifluoromethanesulfonimide) with lithium perchlorate, lithium hexafluorophosphate, lithium bis (trifluoro methanesulfonimide) respectively and the compatibilities with LiCoO₂, LiFePO₄, Li₄Ti₅O₁₂] and graphite (MAGD) were also evaluated. The results showed that EMITFSI+0.8mol·L-1LiTFSI ionic liquid electrolyte exhibited optimal electro-chemical properties with the ionic conductivity of 5.6×10-3, electrochemical stability window of 4.70V, lithium ion transference number of 0.79 and the satisfied compatibilities with LiFePO₄ and Li₄Ti₅O₁₂].Involvement of vinylene carbonate (VC) film-formation additive improved the compatibility of EMITFSI+0.8mol·L-1LiTFSI ionic liquid electrolyte with LiCoO₂. The LiCoO₂/ionic liquid electrolyte interface was characterized by SEM, EIS, XPS and FT-IR techniques to analyze the action mechanism of VC in enhancing the cycle performance of"Li/LiCoO₂"half cell. The results illustrated that VC can ameliorate the charge transfer at the LiCoO₂/ionic liquid electrolyte interface, promote adsorption and redox reaction of TFSI- anion on surface of electrode. The decomposition products of TFSI- anion were LiF, Li2CO3 and Li2O, which composed the main composition of passivation film on surface of LiCoO₂ electrode.Ionic liquid gel polymer electrolyte (ILGPE) was prepared by solution casting method with P(VdF-HFP), EMIPF6, LiPF6 as bases and small molecular ethylene carbonate (EC)-propylene carbonate(PC) as additives. The conduction properties and the compatibility with LiFePO₄, Li₄Ti₅O₁₂] were studied. The results showed that the conduction properties of ILGPE abided by Arrhenius equation, indicating that the conductivity of ILGPE mainly depended on migration of free ion and the movement of polymer chain segment was not primary. LiFePO₄ and Li₄Ti₅O₁₂] exhibited excellent cycle performance and rate capability in P(VdF-HFP)-EMIPF6-LiPF6 ILGPE.EMITFSI is one of the ionic liquid owning the lowest viscosity, which is considered the potential candidate of the organic electrolyte. EMITFSI-LiTFSI binary ionic liquid electrolyte exhibited satisfied application perspective in respect of thermal and electrochemical properties and compatibilities with electrode material. Combining the virtues of ionic liquid and polymer electrolyte, P(VdF-HFP)-EMITFSI-LiTFSI ternary ILGPE was prepared was expected to make great breakthrough in eliminating the safety issues of lithium ion batteries. The results illustrated that P(VdF-HFP)-EMITFSI-LiTFSI ternary ILGPE eliminated the flammability of electrolyte radically and no decomposition was occurred at the temperature range of -150¹50℃. Excellent compatibilities with LiFePO₄, Li₄Ti₅O₁₂] were also displayed by obtained ternary ILGPE. The investigations of electrode/ILGPE interface showed that EMITFSI could enhance the formation of passivation film at the LiFePO₄/electrolyte interface and impelled abundant Li+ to extract from Li₄Ti₅O₁₂], which explained why the initial discharge specific capacity of "Li/Li₄Ti₅O₁₂]" half cell was greater than the theoretic specific capacity.