A Poly(ethylene carbonate)/Lithium Bis(fluorosulfonyl)imide/Titanium Oxide Composite Electrolyte Containing a Pyrrolidinium-based Ionic Liquid

Abstract:

Solid polymer electrolytes (SPEs), formed by the combinations of flexible polymers and metal salts, can be battery electrolyte alternatives to the conventional liquid-based ones because they are not flammable and leakage-free. In the present work, poly(ethylene carbonate) (PEC) has been selected as a polymer matrix. Notably, poly(alkylene carbonate)s, such as PEC, are synthesized by alternating copolymerizations between carbon dioxide (CO₂) and epoxides. Therefore, we initially focused on the CO₂/epoxide copolymers as polymer hosts for the SPEs, being motivated by a concept of CO₂ utilization as a raw carbon source [1,2]. We have already reported extraordinary ion-conductive behaviors of PEC, including high ionic conductivities at quite high Li salt concentration regions and extremely high Li transference number (t_Li+) values [3,4]. For instance, an electrolyte containing 80wt% of LiTFSI showed a 120 times higher ionic conductivity than that of a sample containing 20wt% of the LiTFSI salt [3]. In addition, the t_Li+ of a PEC-LiFSI (80wt%) electrolyte with TiO₂ particles was estimated to exceed 0.7, according to an electrochemical measurement, as well as to a pulse-field-gradient NMR technique [4].

In the present work, we propose a quaternary PEC-LiTFSI mixture-based composite electrolyte membrane containing N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr₁₄TFSI) ionic liquid (IL) as a plasticizing additive and a submicron-sized electrospun SiO₂ fiber (SiF) as an inorganic filler, in view of creating more feasible battery materials. We demonstrate that the resulting mechanically stable membrane reinforced by SiF shows a conductivity of the order of 10^-7 S cm^-1 at 40 ^oC combined with a favorable t_Li+ value of 0.36, which both are enhanced by the simultaneous presence of the IL and SiF. Furthermore, we demonstrate that the electrolyte works as a promising solid-state electrolyte in a galvanostatic cycling for a prototype battery having a LiFePO₄-based cathode at an elevated temperature (75 ^oC). Further advancements can be achieved by the use of lithium bis(fluorosulfonyl)imide (LiFSI) as a salt and N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)imide (Pyr₁₃FSI) as an IL. We have confirmed that a quaternary PEC-LiFSI-Pyr₁₃FSI-TiO₂ electrolyte leads an ionic conductivity of the order of 10^-5 S cm^-1 at 40 ^oC which is more than an order of magnitude greater than that of the LiTFSI-based analogous sample mentioned above. Further detailed properties of the PEC-LiFSI mixture-based composite electrolytes, which are currently under investigation in our laboratory, will be presented.

References:

[1]     Y. Tominaga, T. Shimomura, M Nakamura, Alternating Copolymers of Carbon Dioxide with Glycidyl Ethers for Novel Ion-Conductive Polymer Electrolytes, Polymer, 51, 4295 (2010).

[2]     M. Nakamura, Y. Tominaga, Utilization of Carbon Dioxide for Polymer Electrolytes II: Synthesis of Alternating Copolymers with Glycidyl Ethers as Novel Ion-Conductive Polymers, Electrochim. Acta, 57, 36 (2011).

[3]     Y. Tominaga, V. Nanthana, D. Tohyama, Ionic Conduction in Poly(ethylene carbonate)-Based Rubbery Electrolytes Including Lithium Salts, Polymer J., 44, 1155 (2012).

[4]     Y. Tominaga, K. Yamazaki, Fast Li-ion Conduction in Poly(ethylene carbonate)-Based Electrolytes and Composites Filled with TiO₂ Nanoparticles, Chem. Commun., 50, 648 (2014).

Acknowledgement:

This work was financially supported by a Grant-in-Aid for Scientific Research (B) of JSPS KAKENHI (25288095), Japan.