Effective Synthesis of Cationic Cellulose with a High Degree of Substitution and Its Characteristics for Battery Applications

Cellulose is attracting attention for the development of environmentally friendly, carbon-neutral, sustainable materials. Cellulose derivatives with cationic groups have the potential for applications in various fields, e.g., electrolytes. However, the current situation is marked by a low degree of cationic group incorporation and a need for more efficient synthesis methods. In this study, cationic cellulose was synthesized using an epoxy derivative, 2,3-epoxypropyltrimethylammonium chloride (EPTMAC), in an aqueous pyrrolidinium hydroxide solution. Since an aqueous pyrrolidinium hydroxide solution is a strong alkaline solution, the solution not only exhibits a high cellulose solubility at room temperature but also facilitates the reaction between cellulose and the epoxy derivative. We investigated the influence of reaction time, temperature, cellulose concentration, cationic reagent concentration, and the selection of a precipitation solvent for purification on the degree of substitution (DS) value of cationic cellulose. The structure of the obtained cationic cellulose was examined using 1H NMR, 1H–13C HSQC, 1H–1H TOCSY measurements, and Fourier-transform infrared spectroscopy (FT-IR). As a result of increasing cellulose and EPTMAC concentrations, the DS value increased, reaching a maximum value of 1.9. Solubility tests indicated that the cationic cellulose with chloride counter-anions exhibited notable solubility even in ethanol when the DS values were over 1.2. Cationic cellulose with bis(trifluoromethylsulfonyl)amide (TFSA) anion synthesized with a view to battery applications was insoluble in water and exhibited a film-forming property. Thus, the solubility of cationic cellulose could be controlled by varying the anionic species. Cationic cellulose with TFSA anion showed a thermal decomposition temperature above 300 °C. The ionic conductivity of gel electrolytes composed of cationic cellulose and an ionic liquid was evaluated.