Electrochemical Properties of Novel Poly(ethylene carbonate)-Based Electrolytes Including Ionic Liquids for Lithium Rechargeable Batteries

Polymer electrolytes are promising candidates as electrolytes for batteries because of their appropriate properties such as safety (non-flammability, non-leakage), lightweight and flexibility. In this study, poly(ethylene carbonate) (PEC) has been selected as a polymer matrix.  It is noticeable that PEC can be synthesized by alternating copolymerization of CO₂ with epoxide, which is the good way to utilize CO₂ as a row material, can be easily processed and it is biodegradable and highly transparent.  PEC is completely amorphous phase and it has the carbonate group (-O-(C=O)-O-) as a polar group on the main chain.  We have already reported extraordinary ion-conductive behaviors of PEC such as high conductivity at quite high Li salts concentration region and higher Li transference number (t₊) compared to common polyether-based electrolytes [1]. For instance, the PEC-based electrolyte containing 80 wt% of LiTFSI showed ionic conductivity of 5 × 10^-6 S cm^-1, which was more than 120 times higher than that of the sample containing 20 wt% of the salt [1].  The Li transference number (t₊) of the TiO₂composite (5 wt%) of PEC-based electrolyte including 80 wt% of LiFSI was estimated to be 0.8 according to the electrochemical measurement [2].  These characteristics might come from strong dipole moment of the carbonate group (-O-(C=O)-O-) that allows to dissociate large amount of Li salts and relatively weaker ion-dipole interaction between this group and Li cation.

Room temperature ionic liquids (RTILs) that generally consist of large organic cations and anions have appropriate properties for the application of electrolytes such as negligible vapor pressure, non-flammability, and high thermal, electrochemical stability.  Recently, to incorporate RTILs into polymer electrolytes as plasticizers is considered as a promising approach to improve not only ionic conductivity, but also the electrochemical stability and compatibility with electrodes.

Here, we introduce the novel poly(ethylene carbonate)-based electrolytes consisting of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and RTILs such as N-alkyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr_1ATFSI) (alkyl = e.g. butyl).  The electrochemical properties such as ionic conductivity, and thermal properties of the electrolytes have been investigated to evaluate the applicability of the (x) PEC_n:LITFSI + (1-x) Pyr_1ATFSI to Li rechargeable batteries.

References:

[1] Y. Tominaga, et. al., Ionic conduction in poly(ethylene carbonate)-based rubbery electrolytes including lithium salts, Polymer J., 44, 1155 (2012)

[2] Y. Tominaga, et. al., Extraordinary ion-conductive behavior of poly(ethylene carbonate)-based electrolytes and composite, Polymer Preprints, Japan, 62, 4038 (2013)

Acknowledgment:

One of the authors (KK) acknowledges financial supports from Program for Leading Graduate Schools of Tokyo University of Agriculture and Technology that is organized by Japan Society for the Promotion of Science (JSPS) to perform overseas collaborative research.