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.