Sulfonylimide-Based Cation Exchange Membranes As Electrolyte for Lithium-Ion Batteries. Synthesis and Characterization

The use of cation-exchange membranes as electrolytes in lithium-ion batteries is considered to be one of their highly promising applications. Polystyrene-based membranes containing sulfonylimide functional groups are the promising materials for these application due to the low crystallinity of polymer, high dissociation degree of functional groups, as well as thermal and electrochemical stability of obtained electrolytes. There are two different approaches for the synthesis of sulfonylimide based polymers: polymerization of functional monomer [1], and functionalization of the polymer [2]. The aim of this work was to synthesize membranes based on polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene (SEBS) containing functional sulfo R–SO₃Li and sulfonylimide groups [R–SO₂NSO₂–X]Li, where X= –CF₃, –CCl₃, –CH₃, –C₆H₅, –p-NO₂C₆H₄ and –p-CF₃C₆H₄, and investigation of Li⁺ conductivity of polyelectrolytes solvated by mixtures of ethylene carbonate/dimethylacetamide (EC-DMA) and ethylene carbonate/propylene carbonate (EC-PC). Membranes were synthesized according to the Scheme.

To prepare plasticized polymer electrolytes, the synthesized dry SSEBS and SSEBS-X membranes in the Li⁺ form were transferred to a dry argon box (humidity level <10 ppm) and placed in the solutions containing equal volumes of EC-DMA or EC-PC for 24 h. Solvation degrees (n) of the membranes with EC-DMA and EC-PC mixtures were calculated as a ratio of the number of solvent molecules to the total number of membrane functional groups.

The acid-base titration was used to determine full ion-exchange capacity (IEC) of the SSEBS membrane containing functional sulfo groups, as well as the residual amount of R–SO₃ ^- functional groups in the SSEBS-NH₂ membrane after the amination.

The structure of SEBS, SSEBS and SSEBS-X were proven by ATR IR spectroscopy, using Nicolet iS5 IR in the frequency range 4000-500cm^-1, and CHNS analysis, using EuroVector ЕА3000 equipment.

Ionic conductivity of the obtained polymer electrolytes in the Li⁺ form plasticized by EC-PC and EC-DMA mixtures was investigated by impedance spectroscopy in the temperature range 0...+50 °С under the argon atmosphere, using the alternating current bridge Elins Z-1500J (frequency range from 2 MHz to 10 kHz) on symmetrical cells carbon/membrane/carbon. Resistance was determined from the intercept with the axis of active resistances in Nyquist plot.

Optimal concentrations and equivalent excesses required for complete transformation of

R–SO₂NH₂ groups into sulfonylimide functional groups [R–SO₂NSO₂–X]M by the Hinsberg reaction were found using phenylsulfonyl chloride. In particular, 1.5М is the minimal concentration of sulfonyl chloride in the mixture, and minimal excesses of sulfonyl chloride and triethylamine with respect to the amount of R–SO₂NH₂ groups are 10 and 3, respectively.

IEC of the obtained membranes SSEBS-X are 1.54-1.68 mmol∙g^-1. Solvation of the membranes with the EC-PC solvent mixture, which is the most common solvent in lithium-ion batteries, is not efficient for this purpose due to low solvation capability of organic carbonates and a high viscosity of the mixture. The solvation number of the membranes with this mixture does not exceed 1.7 solvent molecules per functional group, and Li⁺ conductivity is equal to or below 2∙10^-3 mS∙cm^-1 at 25°С. The use of amide-containing solvents leads to an increase of ionic conductivity of the sulfonylimide-based membranes due to specific interactions that determine their high solvation capability. In particular, when the EC-DMA mixture is used the solvation degree and ionic conductivity of the SSEBS-CF₃ membranes at 25°С increase to 14.1 and 1 mS∙cm^-1, respectively.

The lowest solvation degree (1.9 solvent molecules per sulfonic group) and conductivity (0.1 mS∙cm^-1 at 25°С) was found for the SSEBS membrane and the largest values were found for the SSEBS-CF₃ polymer that contains trifluoromethanesulfonylimide functional groups (14.1 solvent molecules per 1 sulfonic group and 1.1 mS∙cm^-1 at 25°С)

According to acidity of [R–SO₂NHSO₂X] groups negative charge delocalization degree decreases along the following sequence X=CF₃>CCl₃>p-CF₃Ph>Ph, whereas the sulfonic group should be placed at the end of the sequence having the lowest acidity. Such consistency of measured ionic conductivity and calculated acidity.

This work was financially supported by the Russian Science Foundation (project No 17-79-30054)

Figure 1