Effect of Ionic Structure on Transport Properties of Weakly-Coordinating Polyanions Dissolved in Non-Aqueous Solvents
Authors: Sakashita Yusuke , Yang Xiaoxiao , Dokko Kaoru , Watanabe Masayoshi , Ueno Kazuhide 
Published: December 2020

In recent years, secondary battery for electric vehicles have a problem of limited charge-discharge rate capability. In order to achieve high charging / discharging rate capability, it is necessary to improve the transference number of cations serving as carriers for ion transport as well as improving the ion conductivity. Moreover, it was suggested that electrolytes with high lithium transferece number can suppress dendrite formation. Therefore, it is neccesary to improve cation transference number to achieve high-energy-density lithium metal batteries. Poly(ionic liquid)s (PILs) have recently attracted much attention as a new class of polyelectrolyte materials to improve the cation transference number[1],[2]. However, polyelectrolytes in a solution, including PILs, suffer from insufficient ionic conductivity because of low dissociation degree caused by the counter-ion condensation on the polymer backbone. According to Manning’s theory, the counter-ion condensation can be suppressed by extending the distance between charges on the polymer chains or dissolving in a solvent with high dielectric constant[3]. Whereas Manning’s counter-ion condensation is often argued for aqueous solution of polyelectrolytes, there are few studies on non-aqueous systems. Furthermore, the charge is also described as a simple electric charge, and the effect of ionic species on actual dissociation state is not considered in the theory. In this study, we synthesized polyanion-type polymers with different counter cations and investigated the effects of fixed anion and cation species on the solubility and ion transport properties in non-aqueous system. In a preliminary study, we found that the ionic conductivity of PILs solutions increases as monomer composition ratio of neutral monomer (butyl acrylate) increases even in the non-aqueous systems. In this study, we synthesized the polyanions having weakly coordinating amide-type anions and different cations and investigated the effect of cation species on counter ion condensation and ion transport properties. The cation transference number of the PIL solutions was also determined.

A random copolymer was synthesized by polymerizing sodium 4-styrenesulfonyl-(trifluoromethylsulfonyl)amide (Na-TfNS) having a weakly coordinating imide side chain and nonionic butyl acrylate at a composition ratio of 1:1. Then, cation exchange was performed for the obtained Na-based polyanion copolymer, and three types of polyanions were obtained, including Li salt and [C2mim] salt, which have the same length of polymer chain. The ion transport properties were compared by measuring the ionic conductivities and viscosity of these polyanions having different cation species in a non-aqueous solvent. The effects of cation structure on ion transport and dissociation properties were confirmed by Raman spectroscopy and estimation of ioniciy, respectively. In addition, another copolymer in which Na-TfNS was replaced by Na styrenesulfonate (Na-StSO3) was synthesized in a similar manner and the effect of anion structure on ion transport and dissociation properties was investigated. The cation transference number was measured by potentiostatic polarization method combined with electrochemical impedance spectroscopy.

Effects of the cations, Na, Li, and [C2mim] cation, on the ionic conductivity was studied at a constant concertation of 0.1 mol kg⁻¹ in propylene carbonate (PC) and dimethyl sulfoxide (DMSO). It was confirmed that the ionic conductivities were higher in DMSO systems than in PC systems for each cation. The polyanion with [C2mim]cation showed the highest conductivity in each non-aqueous solvent. Compared to smaller cations such as Li and Nacation, charge-delocalized [C2mim] cation is considered to be more dissociated and is easier to escape from condensation layers. It is confirmed that weakly coordinating TfNS-anion based polyanion show a higher dissociation degree compared to StSO3-anion based polyanion. We will further discuss the ion transport properties of PILs in non-aqueous solvents in detail.

References

[1] Chen, H et al., Macromolecules
42, 4809–4816 (2009). [2] Mecerreyes, D. Prog. Polym. Sci.
36, 1629–1648 (2011). [3] Manning, G. S. Ber. Bunsenges. Phys. Chem.
100, 909–922 (1996).

Figure 1