Synthesis and Electrochemical Studies of Ionic Liquid Electrolytes for Lithium-Air Batteries
Authors: Ono Yuji , Rahman Md Mijanur , Delgertsetseg Byambasuren , Ui Koichi , Takeguchi Tatsuya 
Published: December 2020

Lithium-air battery (LAB) is one candidate of energy sources beyond conventional lithium ion batteries for overcoming future energy need without worsening environmental issues. Presently, LABs have attracted interest due to its high energy density, while maintaining a long operational life. Although, significant progress has been made in developing electrochemical performance of these batteries, many technical challenges still remain unsolved. For example, the electrolytes used in rechargeable LABs have inferior conductivity, high viscosity, high volatility and flammability, decomposition on Li, and overpotential during charging [1]. To overcome these limitations, it is crucial to replace the conventional electrolyte with an ionic liquid (IL) because of its non-volatility, non-flammability, high ion density and chemical stability [2], [3]. However, there are a few issues that need to be solved when using ILs in LABs; they have high viscosity, low oxygen solubility, and low cyclability. Therefore, synthesis of an appropriate IL electrolyte is necessary due to improve the overall performance of LABs.

In order to synthesis of ILs, bis(trifluoromethanesulfonyl)amide (TFSA) anion was used for its anionic similarity with Li salts. Five different types of cations such as ammonium, phosphonium, piperidinium, sulfonium, and pirolidinium were used. Subsequently, Lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) was completely dissolved at a concentration of 1.0 mol dm−3 in these ILs, and then stirred for 8 h at 60 ºC. The viscosities of prepared IL electrolytes were measured by viscometer. Then, Ketjen black (KB1600) and polyvinylidene difluoride (weight ratio = 9:1, respectively) were mixed in N-methylpyrrolidone solution. After stirring at RT for 5 min, the slurry was coated on the carbon paper followed by drying in a vacuum oven at 80 ºC for 12 h to obtain the cathode. The electrode area was set to 2.0 cm2 in which carbon loading was adjusted to 0.5 mg cm−2. Approximately 50 µl of IL electrolyte drop was deposited on the surface of as-prepared cathode (2.0 cm2 area), and subsequently anode was placed onto the center. The coin cells were assembled inside an Ar filled glove box in which moisture content was less than 1.00 ppm. To observe micro structures on the electrode surface, FE-SEM was used. The electrochemical performances were evaluated in coin cells using mainly cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) tests.

Fig. 1 shows the result of viscosity of IL electrolytes. The viscosities of these IL electrolytes are different, and depend strongly on temperatures. The viscosity decreased with increasing temperature. It can be attributed to the reduced forces of cohesion among surfactant molecules. The viscosities of the IL electrolytes at specific temperature decrease in the order: piperidinium> pirolidinium> ammonium> phosophonium> sulfonium. The result of CV analysis has confirmed similar pattern of chemical reactions in which composition and subsequent decomposition of Li2O2 occurred. However, the current density obtained for the case of sulfonium IL electrolyte is higher than other electrolytes. GCD tests were performed to determine the cycle performance and capacity. The LAB which used low viscosity IL electrolyte, especially sulfonium IL electrolyte showed better cycling performance and capacity than that of high viscosity IL electrolytes. We investigated the temperature effects on sulfonium IL electrolyte to determine optimum performance characteristics. In this regard, the Li metal was immersed in the sulfonium IL electrolyte for 1 week at 25 ºC, 50 ºC, and 75 ºC. Subsequently, the SEM observation was performed to evaluate the morphologies of the Li metal surface. No significant change is observed in the Li metal surface after immersion when compared with the pristine lithium. This result indicates that the Li metal is highly stable in the sulfonium IL electrolyte. Then GCD measurements were performed in dry air at 25 ºC, 50 ºC, and 75 ºC. The discharge/charge capacities of LAB were found to increase with increase in temperature, and a lower overvoltage was obtained at high temperature. However, an initial discharge/charge cycle was retained up to 2nd cycles. To further examine the cycle performance, the measurement atmosphere has been changed to O2 instead of dry air. Fig. 2 shows the discharge/charge curves obtain at 75 ºC under O2 and dry air. As shown in figure, the initial cycle performance is retained up to 10th cycles, and a lower overvoltage is obtained in O2 as compared with dry air. These results indicate a notable improvement in cycle performance. In summary, the sulfonium IL electrolyte has exhibited very high performance in electrochemical tests, even worked at a high temperature under O2 atmosphere. Therefore, the proposed electrolyte design concept has great potential for not only LAB, but also next-generation batteries for various applications.

Figure 1