The Electrochemical Performance Of Ionic Liquid Electrolyte For Lithium Ion Batteries

Lithium-ion batteries attract the researcher's intresets on the field of the energy for their non-pollution, high voltage, high energy density and long cycle life. The electrolyte as an important component of lithium-ion batteries, not only plays a very important role in current conduction and ion transport between the positive and negative, but also largely effectes on the reversible capacity,safety,cycle life of the batteries. Lithium-ion batteries commonly use flammable organic electrolyte, which has a significant negative impact on the battery's safety. Especially in these days, with the development of reseach on Electronic Vehicles (EVs), lithium battery as a part of EVs, higher safety requirements is needed on the electrolyte for lithium-ion batteries. Ionic liquid (IL) as a new electrolyte solvent, for its less volatile, non-flammable, high conductivity, electrochemical stability window characteristics causes widespread concerns. In this article, piperidine based ionic liquid-N-methl-N-propylpiperidinium bis (trifluoromethanesulfonyl) imide (PP13TFSI) was selected as the research object, the physical and electrochemical properties of ionic liquid were investagited, and the performance of positive and negative electrodes in this kind electrolyte was also tested Piperidine based IL and imidazolium based IL mixture were successfully prepared. The phiscal and chemical properties of IL were measued. The performance of the batteries using ionic liquids mixed electrolyte was also tested. The results show that mixed electrolyte have good thermal stability, wide electrochemical window, and high ion conductivity whose values are close to the conventional organic electrolyte. Because reduction potential of imidazole-based ionic liquids is close to 1V vs.Li/Li ⁺, the graphite materials can not get the higher coulombic efficiency and revisable capacity. SEM, XPS and other test results show that ionic liquid can not form stable SEI on surface of carbon anode. However, when we used the ionic liquid mixed electrolyte instead the imidazole-based ionic liquids, the LiFePO₄,Li₄Ti₅O₁₂,Carbon materials have gained a good reversible capacity and cycle stable in this system.Ionic liquid and conventional organic electrolyte (1 mol L^-1LiPF₆/EC-DEC) mixture was prepared. The effects of 2wt% LiBOB as film additives on the the performance of mixed electrolyte were also investigated.The test results show that the LiBOB effectively decreased the voscosity and increased the ion conductivity of mixed electrolyte, the thermal stability and safety of mixed electrolyte are not ruduction, LiBOB additive clearly improves the performance of IL on carbon anode beacaued it decompoed and formed SEI at 1.75V. SEI formed by LiBOB can provent the IL decompositon and co-interalation of PP13⁺on carbon anode, thus improve the cycle performance and coulombic efficency of cell using mixed electrolyte. We also investigated 5%VC additive on the performance of IL-organic electrolyte mixture .VC can effectively formes SEI on carbon electrode and improve the cycle performance of Li/C cell. But the decomposition of VC in 1^st cycle greatly decreases the coulombic efficiency of the cell.Because the ionic liquid and organic electrolyte mixed electrolyte shows the well properties on ion conductivity, electrochemisty window and safety, the effects of VC-LiBOB bianry additive on the performance of electrolyte were also investagited, the studies shows that the binary additives obviously amended the SEI on carbon anode, the performance of the Li/C cells using binary additives containing mixed electrolyte are improved at room temperature and 55℃, Li/LiFePO₄ cells using binary additives also show reasonable reversible capacity and coulombic efficency.To improve the performance of the solid-state electrolyte (PEO), without the loss of its security, we prepared the ionic liquid-polymer composite electrolyte. Compared with the traditional organic additives and inorganic nano-additives, the addition of ionic liquid effectively improves the conductivity of PEO without loss the capacity and safety of battery. The ionic liquid effectively reduces the activation energy of composite electrolyte, cathode materials LiFePO₄, and anode material Li₄Ti₅O₁₂ show good cycling performance and the reversible capacity in this kind electrolyte. We also further studies effectes of the the functional ionic liquid electrolyte on the properties of the polymer electrolyte. The electrochemical window of PEO is well improved; LiFePO₄ and Li₄Ti₅O₁₂ materials get higher reversible capacity and coulombic efficiecncy in this kind electrolyte.