Application of Reference Electrode for All-Oxide Solid-State Battery to Reveal Electrode Reactions and Degradation Mechanisms
Authors: Fujihara Yui , Kutsuzawa Dai , Kobayashi Takeshi 
Published: December 2024

All-solid-state batteries are attracting attention as the next generation batteries for their wide operating temperature range, high energy density and safety. Among the several types of all-solid-state batteries, all-oxide solid-state batteries (AOSSBs) have the advantage of their particular safety due to their outstanding chemical and thermal stability without a risk of generating toxic gases. However, AOSSBs have difficulty in reassembling electrodes and reference electrodes being placed, which is essential for understanding electrode reactions during cycling and degradation mechanisms, because their fabrication process needs high-temperature sintering for sufficient inter-particle contact to have robust cells. In fact, few reports have developed the AOSSB cells with a reference electrode,[1] although there are more reports of developing reference electrodes in sulfide-type solid electrolytes, which do not need sintering.[2,3,4,5] Furthermore, even when the reference electrode is inserted into the AOSSB, it is difficult to fabricate the three-electrode cells which operates under conditions as close as possible to those of the actual full cells because the reference electrode placed in between positive and negative electrodes affects the sintered state of the solid electrolyte and many of the reference electrode configurations have influence on dimensions of AOSSB full cells.
Here, we have developed a new multi-layered cell system, which incorporates reference electrodes into AOSSBs and enables the operation under conditions close to those of the actual full cells. Moreover, the reference electrodes of this system are also used as temporal carrier ion sources to adjust the redox state of each electrode.
The scheme of this multi-layer cell system is shown in Fig. 1. Because the reference electrodes are located on the backside of the positive and negative electrodes, the full cell configuration in the center can work in the same way as an actual full cell. Moreover, by applying the external power source between the reference electrode and positive or negative electrode, the redox state of the positive or negative electrode can be adjusted (In this case, the reference electrode works as a counter electrode.).
In this study, we demonstrate this system in Na-ion battery system with Na3Ti2(PO4)3, a bipolar active material, as both positive and negative electrode active materials, Na3Zr2(SiO4)2(PO4)3 as a solid electrolyte, and Na3V2(PO4)3 as a reference electrode active material. Using these materials, the reference electrode system successfully works to show the same electrode potential of positive and negative electrodes as reported in the literature with the liquid-electrolyte cell [6]. Furthermore, we analyzed the degradation mechanism of this full cell when cycling at various temperatures. On the meeting site, the change in voltage curves upon cycling will also be presented and the dominant degradation mode in this cell is to be discussed.

Acknowledgments

This study is partially based on the results obtained from a project, JPNP14004, commissioned by the New Energy and Industrial Technology Development Organization (NEDO). We thank Zyun Siroma and Hikaru Sano (National Institute of Advanced Industrial Science and Technology (AIST)) for fruitful discussions.

References

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Figure 1