Investigation of Redox Potential of Li2so4 Using All-Solid-State Cells
Authors: Fujita Yushi , Motohashi Kota , Sakuda Atsushi , Hayashi Akitoshi 
Published: December 2024

The demand for power sources for electric vehicles and portable devices has increased rapidly. All-solid-state lithium-ion batteries are promising attention due to their safety and energy density. Especially, the development of solid electrolytes is necessary to realize all-solid-state batteries. Solid electrolytes with high ionic conductivity like sulfides, oxides, and chlorides are used for long-distance conduction in the entire all-solid-state batteries. In addition, those with low ionic conductivity like lithium salts are also used as coating materials and sintering aids. Our group reported active materials and electrolytes using Li2SO4 [1, 2].
One of the features required for solid electrolytes is wide electrochemical windows. Although various sulfide solid electrolytes have been studied due to their high ionic conductivity and ductility, they have narrow electrochemical windows [3], leading to oxidative and reductive decomposition during cell cycling. Thus, it is important to consider the electrochemical windows of solid electrolytes.
Conventionally, the electrochemical windows of solid electrolytes have been investigated using electrochemical techniques like cycling voltammetry and linear sweep voltammetry (LSV). The working electrode is composed of the objective solid electrolytes and conductive carbon. However, for some materials with low ionic conductivity, it is challenging to ensure ionic conduction at long distances in the composite working electrode. Therefore, few reports have shown that the redox potential of a low ionic-conductive material including Li2SO4 is experimentally determined by electrochemical tests, although the electrochemical stability of lithium ion conductors was investigated using first-principles calculation methods [3, 4].
In this study, we used LSV measurements using all-solid-state cells to investigate the redox potential of Li2SO4. The composite working electrode comprised Li2SO4, vapor-grown carbon fiber (VGCF), and oxide solid electrolyte. Li2SO4 and VGCF were ball-milled and the obtained Li2SO4-VGCF and oxide electrolyte were mixed in an agate mortar and pestle to prepare composite working electrodes. As oxide electrolytes, 90Li3BO3·10Li2SO4 glass-ceramic (LBSO) [2] was used in cells for an anodic scan, and 67Li2O·33LiI glass (LOI) [5] was used in cells for a cathodic scan because they have better oxidation or reduction tolerance than sulfide electrolytes. In addition, it was found that the decomposition of sulfide electrolytes affected the LSV curves when sulfide electrolytes were used in a composite working electrode or as a separator layer that directly contacted with the composite working electrode. Therefore, the cell configuration was Li-In/Li3PS4/LBSO (or LOI)/Li2SO4-VGCF-LBSO (or LOI). To confirm the decomposition of Li2SO4 after the LSV measurements, X-ray photoelectron spectroscopy (XPS) measurements were performed on the cells, and the redox mechanism of Li2SO4 was discussed.
In the anodic scan, a current peak derived from the oxidation of Li2SO4 was observed at approximately 4.3 V vs. Li-In (4.9 V vs. Li+/Li), which was close to the standard electrode potential of oxidation from SO4
2– to S2O8
2– [6]. In addition, in the cathodic scan, a current peak derived from the reduction of Li2SO4 was observed at approximately 1.5 V vs. Li-In (2.1 V vs. Li+/Li), which almost corresponds to the potential of reduction from SO4
2– to SO3
2– [6]. Furthermore, the XPS analyses confirmed that Li2SO4 was finally reduced to Li2S and Li2O.
In summary, the LSV measurements were performed with the all-solid-state cell using oxide electrolytes instead of sulfide electrolytes, which eliminated the overlap of a current caused by the decomposition of sulfide electrolytes. The oxidative and reductive decomposition voltages of Li2SO4 were close to each standard electrode potential. We also found a way to experimentally determine the electrochemical potential of any materials by modifying the cell design. This study will help spread the importance of potential windows and the realization of all-solid-state lithium-ion batteries.

Reference
s
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[1] K. Nagao et al., Sci. Adv. 6 (2020) eaax7236.
[2] M. Tatsumisago et al., JCS-Japan, 125 (2017) 433.
[3] Y. Zhu et al., ACS Appl. Mater. Interfaces, 7 (2015) 23685.z
[4] W. D. Richards, et al., Chem. Mater., 28 (2016) 266.
[5] Y. Fujita et al., Electrochemistry, 89 (2021) 334.
[6] A. J. Bard et al., Standard Potentials in Aqueous Solution, Routledge (2017).