Recently, all-solid-state lithium batteries have attracted global attentions as next-generation batteries because of their higher safety due to the use of nonflammable inorganic solid electrolytes instead of flammable organic liquid electrolytes. Bulk-type all-solid-state batteries employ composite electrodes consisting of electrode active materials and solid electrolytes. There are a lot of solid-solid interfaces in composite electrode layers resulting in inhomogeneous reaction distributions. In order to achieve higher battery performance, it is important to investigate electrochemical reaction mechanisms at the solid-solid interfaces and evaluate reaction distributions in the electrode layers.

A graphite negative electrode is used in commercial lithium-ion batteries. However, there are few papers regarding all-solid-state batteries using graphite composite electrodes.[1] It is worth studying reaction mechanisms of graphite composite electrodes in all-solid-state lithium batteries for practical use in the near future. Colors of graphite particles change from black via dark blue and red to gold during a lithiation process.[2] Observation of color changes enables us to evaluate reaction distributions in a graphite electrode layer. In our previous paper, we compared reaction distributions of composite negative electrodes consisted of graphite and sulfide solid electrolytes with weight ratios of x : 100-x (x = 50, 60 and 70) by ex-situ optical microscopy.[3] The cell using the x = 50 electrode showed the highest reversible capacity of more than 250 mAh g^-1 and homogeneous reaction distributions. In this study, to monitor forming of reaction distributions during charge-discharge cycles, operando optical microscopy was conducted for a graphite electrode layer in an all-solid-state cell.

Composite electrodes were prepared by mixing graphite and 75Li₂S·25P₂S₅ (mol%) glass electrolyte particles with weight ratios of 50 : 50. 75Li₂S·25P₂S₅ glass and lithium-indium alloy were used as a solid electrolyte separator and a counter electrode, respectively. The cell (Li-In/75Li₂S·25P₂S₅ glass/Graphite) was cut to obtain flat cross-sectional observation areas. Operando optical microscopic observation was conducted for the cross-section of the graphite electrode layer at room temperature under a current density of 0.068 mA cm^-2. The cell was mounted under a low confined pressure of ca. 70 kPa in an Ar-filled vessel during optical microscopy.

Optical micrographs for the graphite electrode layer showed that lithiation and delithiation proceeded preferentially for the graphite particles near the solid electrolyte layer. The color changes in the graphite particles were quantitatively evaluated to compare SOC values. During the initial lithiation process, almost all the graphite particles in the electrode layer changed their colors from black to gold. However, after the 3rd lithiation process, only the graphite particles near the electrolyte separator layer showed color changes. The SOC value for the graphite near the separator layer side was more than twice as high as that near the current collector side. This suggested that inhomogeneous reaction distributions were formed in the graphite electrode layer, which resulted in degradation of cycle performances. In addition, the thickness changes of the graphite electrode layer during charge-discharge tests were also examined.

Acknowledgement:

Optical microscopic observation was supported by Lasertec Corp..

References:

[1] K. Takada, T. Inada, A. Kajiyama, H. Sasaki, S. Kondo, M. Watanabe, and R. Kanno, Solid State Ionics, 158 (2003) 269–274.

[2] S. J. Harris, A. Timmons, D. R. Baker, and C. Monroe, Chem. Phys. Lett., 485 (2010) 265–274.

[3] M. Otoyama, A. Sakuda, A. Hayashi, and M. Tatsumisago, Solid State Ionics, 323 (2018) 123-129.