Ionic Liquids For Lithium Batteries

Ionic liquids are salts that are composed entirely of ions and appear liquid at or near room temperature. There are many kinds of anions and cations in ionic liquid, the common cations are imidazolium, quaternary ammonium onium, pyrdinium, pyrrolidium, pyrazolium. The main anions are AlCl_4^-, Br^-, Cl^-, I^-, BF_4^-, PF_6^-, CF₃SO_3^-, TFSI- and others. Compared with traditional solvents, ionic liquids have the advantages of low vapor pressure, non-volatile, high thermal stability, no flammability, strong solubility and wide chemical window. Based on the above advantages, ionic liquids are suitable for lithium batteries as electrolytes, which are expected to improve the safety performance of lithium batteries.

Applications:

• Solvents for electrolyte salts: Ionic liquid, instead of the traditional carbonate solvent, can improve the safety performance of lithium batteries because of its non-volatile and non-combustible characteristics. At present, ionic liquids used as electrolyte salts for lithium batteries can be classified into imidazole ionic liquids, quaternary ammonium salts, pyrroles, piperidines and others, according to the types of cations. Imidazole ionic liquids are the first ionic liquids to be studied. They have the advantages of low viscosity and high conductivity, but have the disadvantages of narrow electrochemical stability window and poor stability to lithium metals. Compared with imidazole ionic liquids, quaternary ammonium salt ionic liquids have good chemical stability, and the electrochemical stability window is usually greater than 5V. Quaternary ammonium salt ionic liquids can withstand the electrochemical deposition and dissolution of lithium without oxygenolysis, and oxidative decomposition is not easy to occur on the cathode material surface. However, quaternary ammonium salt ionic liquids have the disadvantages of large cationic size, high viscosity and low ionic conductivity. The physical and chemical properties of pyrrole and piperidine ionic liquids are similar to those of quaternary ammonium salt ionic liquids, and they all have good chemical stability. Furthermore, the ring structure of pyrrole and piperidine is close to the plane and the motion resistance is smaller, which has a relatively low viscosity and higher ionic conductivity.
• Mixed with traditional electrolytes: When the solvent is used as a single electrolyte salt in lithium-ion batteries, it has the disadvantages of low ionic conductivity, poor charge-discharge cycle performance and ratio performance. Researchers often combine ionic liquids with conventional electrolytes with low viscosity to improve the electrochemical performance of batteries.

Figure1. An example of lithium batteries electrolyte by mixing ionic liquid with traditional electrolyte.

• Ionic liquid-based polymer electrolyte: When ionic liquid electrolyte is applied to lithium battery, the battery is prone to leakage because the electrolyte is liquid. The combination of ionic liquid and polymer electrolyte can avoid leakage and further improve the safety of battery. At present, there are two kinds of preparation methods of ionic liquid-based polymer electrolyte. One is polymer membrane adsorption ionic liquid electrolyte. The other is when oligomers or monomers are mixed with ionic liquid electrolytes to initiate polymerization.

Figure 2. An example of ionic liquid-based polymer electrolyte.

References

• Zhang R, Chen Y, Montazami R. Ionic Liquid-Doped Gel Polymer Electrolyte for Flexible Lithium-Ion Polymer Batteries[J]. Materials, 2015, 8(5):2735-2748.
• Eshetu G G, D Mecerreyes, Forsyth M, et al. Polymeric ionic liquids for lithium-based rechargeable batteries[J]. Molecular Systems Design & Engineering, 2019, 4(2).