Mixtures Ionic Liquids-Ethylen Carbonate for Lithium Batteries and Supercapacitor

Lithium batteries and supercapacitors are two most important electrochemical energy storage (EES) technologies. The demand for EES is nowadays extremely large and it addresses very different applications from small portable devices, over electric vehicles, to large stationary application. Therefore, the requirements for EES are including not only of specific power and energy but also the cycle life and security. Many research works have developed new electrode materials and novel electrolytes; designed new system concepts in order to maximize the stored and converted energy [1,2].

The security in the EES is quietly prioritized, that required the replacement of volatile solvent or at least using the inflammable additives. The ionic liquids, also called “room temperature molten salts” have been considered a promising electrolyte for lithium batteries or supercapacitors [3]. Given their interesting properties, such as nonvolatility, non-flammability, high thermal stability, wide liquid range and wide electrochemical window, many ILs have been suggested based on tailoring of cation and anion structures [4,5,6].

In our work, ILs based on ammonium cations (quaternary ammonium, pyrrolidinium and piperidinium) and 1-ethylmethyl imidazolium cation combined with bis(trifluoromethanesulfonyl) imide anion – TFSI were investigated for EES. The fluorinated ILs using fluoro-alkyl chain length were also studied due to enlargement of the electrochemical window. In addition, the new synthesis pathway was reported via derivated tosylate compound [7].These synthesized ILs were stable in oxidation, at least 5,2 V vs Li⁺/Li. The fluorinated ammonium ILs increase the oxidation potential up to 500 mV compared to the non-fluorinated ILs [5,6]. However, the high viscosity of ILs still remains a big challenge for EES application. One solution to design safer electrolytes based on ionic liquids which exhibit simultaneously all the required properties (i.e. low viscosity, electrochemical stability, non-flammability), is the addition of molecular additive (organic solvent such as: EC, acetonitrile,…). In our previous report, the incorporation of 20 %vol. EC has no positive effect on the pure IL dissociation, but decreases the viscosity as well as improves the electrochemical window stability [8]. Thus, 20% vol. EC was chosen for cycling test of batteries and supercapacitors.

The battery cycling test was performed using ILs + 20 % vol. EC + 0.25M LiTFSI. The results seem to be promising for N₁₁₂₃TFSI + 20% vol. EC +0.25M LiTFSI compared to the commercial electrolyte 1M LiPF₆/EC-DMC. The charge-discharge profile of LiMn₂O₄ in N₁₁₂₃TFSI + 20% vol. EC and in EMITFSI + 20% vol. EC in Comparing to the LiPF₆/EC-DMC (1:1). The discharge capacity and coulombic efficiency in ILs is higher, 85 mAh/g and 80% (N₁₁₂₃TFSI) and 60 mAh/g and 78% (EMITFSI) vs 70 mAh/g and 70% for LiPF₆/EC-DMC after 30 cycles at C/10 rate. The results will be further discussed.

References

 

[1]. D. Cericola, Electrochimica Acta 72 (2010) A536.

[2]. D. R. MacFaclane, Acc. Chem. Res. 40 (2007) 1165 – 1173.

[3]. J. Devynck et al., J. Electrochem. Soc. 131 (1984) 2274.

[4]. M. Buzzero et al., Chem. Phys. Chem. 5 (2004) 1106.

[5]. Le M.L. Phung et al., J. Phys.Chem B. 114 (2010) 894.

[6]. Le M.L. Phung et al., Ionics (2012).

[7]. Tran N. Anh, J. Fluorine Chemistry 164 (2014) 38 – 43

[8]. Le M.L. Phung al., J. Phys.Chem C. 116 (2012) 7712.