Pseudo-Solid Aqueous Electrolyte with Widened Potential Window By Encapsulating Water-in-Salt in Silica Matrix
Authors: Sugiyama Takaya , Muramatsu Keisuke , Sugimoto Wataru , Dunn Bruce S. 
Published: December 2024

Pseudo-solidification of liquid electrolytes is an effective means to prevent electrolyte leakage, and various polymeric materials have been proposed. Recently, silica gel prepared by sol-gel methods has been proposed as an inorganic polymer than can confine liquid electrolytes and has shown to be a promising pseudo-solidification method providing rigidity and stability owing to the inorganic matrix while maintaining the ionic conductivity of liquid electrolytes. For example, silica gel containing an ionic liquid, known as an ionogel, enables solidification while maintaining the properties of the ionic liquid1,2. Furthermore, by utilizing the high processability of the sol state, it becomes possible to achieve uniform interfaces with substrates and form thin films. In this study, we focused on Water-in-Salt (WiS) electrolytes3 as the liquid electrolyte to encapsulate in the silica matrix. WiS electrolytes are highly concentrated aqueous electrolytes with high safety and a wide electrochemical window.
As the WiS, 21 m (m: mol kg–1) LiTFSI (lithium bis(trifluoromethane sulfonyl)imide) was chosen and confined within a silica matrix by sol-gel methods. The 21 m LiTFSI silica gel was prepared by mixing tetraethoxysilane into a 21 m LiTFSI solution by modification of the synthesis of an ionogel.2 During this process, the water content within the matrix is sensitive to the humidity of the environment, thus control of humidity during the synthesis is crucial.
21 m LiTFSI silica gel obtained under suitable humidity exhibited high transparency. The LiTFSI concentration measured by thermogravimetry was 20.5 m, close to the intended concentration of 21 m. The activation energy evaluated from electrochemical impedance spectroscopy using a two-electrode Swagelok cell (Pt | 21 m LiTFSI silica gel | Pt) showed a value similar to that of the 21 m LiTFSI solution, showing that the silica matrix does not affect the conduction mechanism of the confined 21 m LiTFSI solution.
Interestingly, in the 21 m LiTFSI silica gel, a change in the activity of water molecules was observed compared to the liquid system with the same concentration. From the Raman spectrum, the 21 m LiTFSI silica gel exhibited an S–N–S bending vibration peak at a higher wavenumber compared to the liquid system. It is known that with the increase in salt concentration and the decrease in free water content in LiTFSI aqueous solutions, the interaction between Li+ and TFSI– strengthens, resulting in a shift of the S–N–S bending vibration peak of TFSI– to a higher wavenumber.REF This result suggests that the activity of water molecules in the 21 m LiTFSI within the silica matrix is reduced compared to the liquid system. The potential window of the 21 m LiTFSI silica gel was evaluated by cyclic voltammetry. A glassy carbon electrode coated with 21 m LiTFSI silica gel as the working electrode was immersed in a 21 m LiTFSI solution (v = 2 mV s–1, E = –2.2 to 2.2 V vs. RHE). The potential window of the 21 m LiTFSI solution was 2.97 V, while for 21 m LiTFSI silica gel the potential window was widened to 3.33 V. In highly concentrated aqueous electrolytes, the activity of water molecules affects the decomposition potential, and it is known that the potential window expands as the concentration of salt increases, due to the decrease in the activity of water.3,4 Therefore, the expansion of the potential window in the case of 21 m LiTFSI silica gel can be attributed to the decrease in the activity of water.

References

(1) D. S. Ashby, R. H. DeBlock, C.-H. Lai, C. S. Choi and B. S. Dunn, Joule, 1, 344 (2017).
(2) G. Tan, F. Wu, C. Zhan, J. Wang, D. Mu, J. Lu and K. Amine, Nano Lett., 16, 1960 (2016).
(3) L. Suo, O. Borodin, T. Gao, M. Olguin, J. Ho, X. Fan, C. Luo, C. Wang and K. Xu, Science, 350, 938 (2015).
(4) Y. Yamada, K. Usui, K. Sodeyama, S. Ko, Y. Tateyama and A. Yamada, Nat. Energy, 1, 16129 (2016).

Figure 1