Ionic Transport Properties in Sulfone-Based Solid Polymer Electrolytes
Authors: Doi Chihiro , Saika Yuichiro , Kokubo Hisashi , Dokko Kaoru , Watanabe Masayoshi , Ueno Kazuhide 
Published: December 2020

Solid polymer electrolytes (SPEs) consist of polymers and metal salts, which are safer than liquid electrolytes owing to non-leakage of electrolytes. They have many practical advantages compared with inorganic solid electrolytes; for instance, flexibility and processability. However, SPEs have several challenges: low ionic conductivity and Li+ transference number. Typical SPEs such as a polyethylene oxide (PEO)-based electrolyte show low ionic conductivity of ~10−5 S cm−1 and low Li+ transference number of ~ 0.2 at room temperature 1, 2. Therefore, it is essential to design novel SPEs with high ionic conductivity and Li+ transference number for practical battery applications.

Recently, our group has focused on highly concentrated liquid electrolytes and found that sulfolane (SL)-based molten Li salt solvates show high Li+ transference number (0.7 ~ 0.8) via Li+ hopping/exchange conduction mechanism 3. In the SL-based electrolytes, oxygen atoms of the sulfone groups coordinate to two different neighbouring lithium cations to form a unique chain-like Li+ coordination structure, which enables predominant Li+ hopping/exchange conduction between the coordination sites. This was considered to accelerate Li+ transport and result in the high Li+ transference number despite relatively high viscosity of the electrolytes.

Here, we expect that high Li+ transference number can be achieved in SPEs containing the sulfone moieties similar to the SL-based molten Li salt solvates. In this study, we attempted to incorporate the sulfone groups into the polymer side chains to achieve high Li+ transference number. Thermal, ionic transport and electrochemical properties of the prepared sulfone-based SPEs were investigated. We synthesised poly(3-acryloyl sulfolane) (PASL) containing the sulfone groups into the side chains of polyacrylates as polymer matrix of SPEs and prepared two types of SPEs: Li[TFSA] (lithium bis(trifluoromethanesulfonyl)amide)/PASL = 1:2 and Li[TFSA]/PASL/SL = 1:1:1. Li[TFSA]/PASL was prepared by mixing Li[TFSA] and PASL at a molar ratio of 1:2 using DMF (N,N-dimethylformamide) as co-solvent followed by vacuum drying for five days. Li[TFSA]/PASL/SL was prepared by mixing equimolar amounts of Li[TFSA], PASL and SL.

In addition to ionic conductivity, we have also evaluated Li+ transference number in these electrolytes by the potentiostatic polarization method using a Li metal symmetric cell. 4, 5 As shown in Figure (a), ionic conductivity of Li[TFSA]/PASL/SL is higher than that of Li[TFSA]/PASL. The plasticization of the SPE in the presence of SL molecules resulted in the higher ionic mobility for Li[TFSA]/PASL/SL. The ionic conductivity of Li[TFSA]/PASL at the glass transition temperature (T
g : 45 °C) was 3.9×10−8 S cm−1 and that of Li[TFSA]/PASL/SL(T
g : −45 °C) was 1.7×10−14 S cm−1. The ionic conductivity at T
g of Li[TFSA]/PASL is much higher than that of PEO-based polymer electrolytes of about 10−14 S cm−1 as well as that of Li[TFSA]/PASL/SL. Ionic conduction behavior in SPEs can be discussed in terms of coupling/decoupling of Li+ conduction with/from polymer segmental motions 1. In the coupling system, ionic motions are coupled with the segmental motions of polymer chains and therefore ionic conductivity becomes very low at T
g (~10−14 S cm−1). On the other hand, in the decoupling system, the ionic motions is decoupled from the polymeric motions, and ionic conductivity at T
g is higher than that in the coupling system. Whilst Li+ conduction is dominated by the polymeric segmental motions in PEO-based electrolytes, Li[TFSA]/PASL can be classified as the decoupling system owing to its much higher ionic conductivity at T
g of ~10−8 S cm−1. Li+ transference number of Li[TFSA]/PASL/SL was found to 0.75 at 80 °C, which is higher than that of PEO-based polymer electrolytes (t
Li = 0.2 at 70 °C). This suggests that the unique Li+ hopping/exchange mechanism contributes to ionic conduction in the solfone-containing SPE as well as a SL-based molten Li salt solvates.

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Figure 1