Preparation of Solid Electrolytes Containing Highly Concentrated Electrolytes and Effects of Solidifying on Physicochemical Properties
Authors: Mogi Rintaro , Furui Reita , Seki Shiro 
Published: December 2024

Currently, safety improvements for the electrolyte of conventional lithium-ion batteries is desired owing to their apprehending accidents of firing by contained flammable organic solvents. Safety ensuring is a very important issue for the expansion of larger sizes and applications. Recently, highly concentrated electrolytes containing an extremely high concentration of electrolyte salts are drawing attention as a new electrolyte design. The highly concentrated electrolytes reported unique characteristics such as greatly improved thermal stability and expanded electrochemical window(1). However, the solvation structure of highly concentrated electrolytes remains unclear. In this study, we proposed gel polymer electrolytes (GPEs) composed of highly concentrated liquid electrolytes using chemically cross-linked polyether-based polymers. We investigated the physicochemical properties and battery characteristics of GPEs. The aim of this study is to develop an electrolyte design that improved the safety and performance of rechargeable batteries.

In this study, all GPEs were prepared in an Ar-filled glovebox. LiFSA+xEC (0.8≦x≦4) liquid electrolytes (LE) were prepared by mixing EC and LiFSA, and GPEs were also prepared by LiFSA+xEC (0.8≦x≦4), polyether-based polymers (P(EO/PO = 0.8/0.2)) and DMPA as a photoinitiator, and irradiated UV for 5 min. Prepared GPEs samples were evaluated for thermal properties, ionic conductivity, and solvation structure by thermogravimetry (TG), The self-diffusion coefficients (D) and Raman spectrometric method, respectively.

To analyze the solvation structure of Li+ in the highly concentrated electrolytes, Raman spectra were measured in the wavenumber range of 680-860 cm−1. Figure 1 shows the obtained Raman spectra of prepared GPEs. The observed peak was attributed to the symmetric stretching mode of FSA- anion and confirmed shift to high wavenumber with LiFSA concentration. The peak shift of FSA- was suggested for the formation of aggregate structures by change of dissociation properties to contact ion pair (bound FSA-) from separated solvent ion pair (free FSA-).

Figure 2 shows the TG curves of each GPEs. TG data was obtained the heating scan rate of 10 K min-1 from 303 K to 753 K. The thermal stability of GPEs improved with LiFSA concentrations. GPEs of high Li salt concentration were suggested the formation of aggregate structure, and the strong interaction between Li+- ethylene carbonate, formed a quite strong solvation structure. In general, the aggregate structure is caused by strong interaction between lithium cation and ethylene carbonate. Also, LiFSA+xEC electrolyte solution exhibited decreasing tendency of free EC and improvement of thermal stability(2). From the above results, a change of solvation structure in GPEs was suggested to improve thermal stability. Although LiFSA+xEC electrolytes showed multiple steps of thermal weight loss processes, prepared GPEs exhibited that in one step. Therefore, proposed component materials of GPEs achieved sufficient homogeneously without phase separation and lower thermodynamic activity of the solvent.

The ionic conductivity(σ) of prepared GPEs was evaluated by alternating current(AC) impedance measurements using hermetically sealed cells. AC impedance measurements were conducted by cooling from 333 K to 263 K at 10 K intervals. The temperature dependence of the σ for the electrolyte using Arrhenius-type plots is shown in Figure 3. σ decreased with LiFSA concentration. σ also increased with temperature following with VFT equation. Also, LiFSA+xEC electrolyte solution exhibited tendency of the viscosity increased with LiFSA concentration(2). From the above results, decreased of σ with increasing concentration was due to increase viscosity.

The self-diffusion coefficients (D) were measured by PFG-NMR with a superconducting magnet and equipped with a pulsed-field gradient multiprobe. Figure 4 shows the self-diffusion coefficients (D
GPE and D
LE) of ions and EC were measured between 333 K and 303 K to evaluate the diffusion behaviors in the electrolytes of x = 4.0.

The order of D is D
EC > D
FSA > D
Li for the diluted electrolyte of x = 4.0 and increasing D
GPE with temperature. D
GPE decreased with solidification compared to D
LE. Figure 5 also shows the D
GPE of ions and EC were measured between 333 K and 303 K to evaluate the diffusion behaviors in the electrolytes of x = 0.8. D
Li increased compared to the electrolytes of x=4 relatively. Also, the order of D is D
Li > D
EC > D
FSA in highly concentrated electrolytes at 333 K.

The calculated t
Li of the electrolyte at 333 K is shown in Figure 6. The t
Li increases with the increased LiFSA concentration. The highest t
Li was obtained when x = 0.80. The significant increase in the t
Li at high LiFSA concentrations reflects faster diffusion of Li+ compared with anions.

[1] Y. Yamada, A. Yamada, J. Electrochem. Soc., 162(14), A2406-A2423 (2015).

[2] R. Furui, et.al, J. Phys. Chem. C, 127, 10748-10756 (2023).

Figure 1