Flexible Electrolyte Sheets Using Sulfonated Cellulose Nanofiberfor Sheet-Type All-Solid-State Batteries
Authors: Hino Minato , Sonoda Yushiro , Azuma Shota , Tani Ryotaro , Ozawa Fumisato , Saito Morihiro 
Published: December 2024

With the rapid spread of electric vehicles that do not use fossil fuels for suppression of global warming, all-solid-state batteries (ASSBs) have been studied as a next-generation battery to improve the cruising range and safety. In general, ASSBs use solid-state electrolyte such as polymer or inorganic electrolytes instead of organic electrolyte solutions, which prevent from electrolyte leakage and its degradation especially at high temperatures. Therefore, it is possible to simplify for the cell design and to increase the energy density. However, for the practical use relatively high interface resistance between electrode and solid-state electrolyte is one of the critical problems. Recently, addition of cellulose nanofiber (CNF) into polyethylene oxide (PEO) electrolyte was reported to improve flexibility and mechanical strength. Also, this method can reduce the interface resistance and improve the cycle performance [1]. In this study, we prepared new composite electrolyte membranes by using sulfonated CNF (S-CNF, Yokokawa Bio Frontia Inc.), together with a low molecular weight PEO, i.e. tetraglyme (G4), and some amounts of ceramic powders to support the ionic conductivity and mechanical strength. The electrochemical properties were investigated by using SUS | SUS and Li | Li symmetric cells to elucidate the mechanism of ionic transport.
Mixtures of S-CNF, Lithium-Ion Conducting Glass-Ceramics (LiCGCTM, OHARA) or SiO2 powders, Lithium Bis(trifluoromethanesulfonyl) imide (LiTFSI) salt and G4 solvent were prepared in a predetermined ratio. The resulting solutions were coated onto PET film and dried in vacuum to form membranes, and then peeled off to obtain the S-CNF composite electrolyte membranes. The ion conductivity was measured by AC impedance method at a frequency range from 1 MHz to 100 mHz and applied voltage of ±50 mV by using a SUS | SUS symmetric cell. The interface resistances were evaluated by using a Li | Li symmetric cell to subject to a Li dissolution/ deposition test at 25°C. The tests was conducted in a constant current mode with a voltage range of -2.0 to 2.0 V, a current density of 0.50 mA cm-
2 and a capacity limit of 0.50 mAh cm-2.

Fig. 1 shows Nyquist plots of a LICGC pellet and the S-CNF composite membranes containing LiCGC or SiO2 powder. The composite membranes has no semicircular component different from the LICGC pellt. This indicates that the interface resistance between the electrolyte menbranes and Li metal is drastically reduced owing to the flexibility. Also, both case of S-CNF composite menbrane exhibited almost same plots. This indicates that the contribution for ionic conductivity by the LICGC solid-state electrolyte is smaller than that by the polymer electrolyte conponent of LiTFSI/G4. In fact, the ion conductivity calclated from the Nyquist plots were 55.0 and 51.7 μS cm-1 for the S-CNF composite menbranes containing LICGC and SiO2 powder, respectively. These values were five times higher than that of pure LICGC pellet (9.53 μS cm-1). On the other hand, Li |Li symmetric cell test exhibited a quite stable polarization curves for the both S-CNF menbranes remaining the overpotential below 5 mV for up to 200 cycles. This also indicates the good chemical stability against Li metal.
The mechanism of ionic conductivity, the role of S-CNF in the composite menbranes and the mechanical strength will be totally discussed in the meeting.
References
[1] Y. Li et al., Energy Environ. Mater., 4, 439 (2021).

Figure 1