Ionic liquids remain in a liquid state over a wide temperature range, permitting their use in both high- and low-temperature conditions. These liquids are also thermally, chemically stable and exhibit low vapor pressure; they can therefore be used under extreme conditions such as a vacuum. In addition, ionic liquids are non-flammable and conductive.

Al-air battery. Metal-air batteries use a catalytic air cathode in combination with an electrolyte and metal anode such as lithium, aluminum, magnesium or zinc. With very high theoretical energy densities, metal air technology is considered a promising technology candidate for “beyond Li-ion” next-generation batteries enabling future long-range battery-electric vehicles—assuming the development obstacles can be overcome.

Aluminum-air batteries offer a theoretical specific energy of 8.1 kWh/kg (with respect to aluminum)—second only to the Li-air battery (13.0 kWh/kg). However, aluminum-air technology suffers from parasitic hydrogen evolution caused by anode corrosion during discharge; this has been a long-standing barrier to the commercialization of aluminum–air batteries. Not only does it cause additional consumption of the anode material, but it also increases ohmic loss in the cell.

Dr. Ryohei Mori, in charge of the Al-air project at Fuji Pigment, earlier attempted to suppress anode corrosion and byproduct accumulation by modifying the aluminum-air battery structure by placing ceramic and carbonaceous materials between aqueous NaCl electrolyte and electrodes as an internal layer.

From cyclic voltammetry experiments, Dr. Mori found that some of the ceramic oxide materials underwent an oxidation–reduction reaction, indicating the occurrence of a faradaic electrochemical reaction. He then tried a TiO₂ film as an internal layer. While this approach successfully prepared an aluminum–air battery with secondary battery behavior (Dr. Mori called it semi-rechargeable), cell impedance increased as the charge/discharge reactions proceeded. In a paper published earlier this year, Dr. Mori said that results of quantum calculations and x-ray photoelectron spectroscopy suggested the possibility of developing an aluminum rechargeable battery using TiO2 as an internal layer.

Now, Fuji Pigment Co., Ltd. is attempting to create a fully rechargeable aluminum–air battery by replacing the aqueous electrolyte with an ionic liquid. The company is proposing that by using an ionic liquid such as 1-ethyl-3-methylimidazolium chloride or 1-butyl-3-methylimidazolium chloride as the electrolyte, it is possible to create a rechargeable aluminum–air battery.

Electrolytes for other batteries. Lithium-ion batteries (LIBs) using an ionic liquid as electrolyte are observed to have better electrochemical properties as compared with those using the conventional carbonate-based electrolyte. Ionic liquid electrolytes are also stable and non-flammable, countering the flammability of typical LIBs due to their carbonate-based electrolytes.

Other potential applications for ionic liquids are:

• Dissolution of cellulose and cellulose nanofibers. Ionic liquids are able to dissolve insoluble materials such as cellulose.

• Formation of carbon nanotube dispersions and gels. Mixing carbon nanotubes with an ionic liquid allows the formation of a good dispersion with the resin, which is needed to create a gel that maintains the high conductivity and other interesting properties.

• Use as an antistatic material. Because of their non-volatility and high heat resistance, ionic liquids can be mixed with resin in a high-temperature process. In particular, optimizing the ionic liquid's structure and compatibility with the resin results in a material that has excellent antistatic property, high transparency, and high conductivity.

• Solvent for organic synthesis. Because of the urgent need for "green chemical processes", replacing the strongly volatile organic solvents used in syntheses with more environment-friendly solvents is necessary. Since ionic liquids exhibit almost no vapor pressure and non-flammable and safety, they have potential use in such processes. Furthermore, Ionic liquids can be easily reused and can be recyclable many times because they can be separated from other liquids by heating, distilling, and other processes.

• Ionic liquids can also be used as CO₂-absorbing materials, wetting agents or lubricants that can be used under vacuum conditions, and additives for colored products, among various other applications.

Resources

• Mori, R. (2016) “Semi-rechargeable Aluminum–Air Battery with a TiO₂ Internal Layer with Plain Salt Water as an Electrolyte” Journal of Elec Materials 45: 3375 doi: doi:10.1007/s11664-016-4467-8

• Ryohei Mori (2015) “Addition of Ceramic Barriers to Aluminum–Air Batteries to Suppress By-product Formation on Electrodes Batteries and Energy Storage” J. Electrochem. Soc. 162(3): A288-A294; doi: 10.1149/2.0241503jes