Effects of Metal Interlayers between Li7La3Zr2O12 Solid Electrolyte and Li Anode on Li Dissolution/Deposition Cycles
Authors: Haruta Masakazu , Okazaki Erika , Onodera Hitoshi , Yoshida Shuhei , Inaba Minoru 
Published: December 2024

Garnet-type oxide solid electrolyte, Li7La3Zr2O12 (LLZ), has relatively high ionic conductivity and high reduction stability for metallic Li. It is considered as a promising candidate for use in all-solid-state batteries. However, practical applications of LLZ are currently hindered by large interfacial resistances at LLZ electrolyte/Li electrode interfaces. One of the reasons for this issue is believed to be the poor wettability of metallic Li on the LLZ surface, resulting in a poor contact interface. To reduce the interfacial resistance, intermediate layers have been introduced.[1-3] In this study, we investigated the effects of intermediate layers such as Sn, In, Bi, etc., which alloy with Li, at the LLZ/Li interface on Li dissolution/deposition reactions.

Li6.7La3Zr1.7Ta0.3O12 (LLZ, φ10 mm, 1 mmt) pellets, produced by TOSHIMA Manufacturing Co., Ltd., were used as solid electrolytes. The LLZ surfaces were polished with sandpaper in an Ar-filled glovebox. Bi interlayers were deposited on both sides of the LLZ pellets by thermal evaporation with varying thickness. Subsequently, Li electrodes (10 μmt) were deposited to obtain Li symmetric cells (Li|LLZ|Li). Electrochemical impedance measurements and Li dissolution/deposition tests of the Li|LLZ|Li cells were performed at the temperature of 60°C. In the dissolution/deposition test, the current density was set to 0.2 and 0.5 mA/cm2, and the direction of current flow was switched every 0.5 hours. After the Li dissolution/deposition test, cross-sections of the LLZ pellet were observed by scanning electron microscope (SEM) and analyzed by energy dispersive X-ray spectroscopy (EDX).

Figure 1 shows Li dissolution/deposition cycle performance in Li|LLZ|Li cells both with and without a Bi interlayer. The Li|LLZ|Li cell without the interlayer exhibited stable cycle operation for 50 cycles at a current density of 0.2 mA/cm². However, when the current density was increased to 0.5 mA/cm², the overvoltage increased rapidly due to the formation of voids, which decreased the contact area at the LLZ/Li interface. On the other hand, when an 80 nm Bi interlayer was introduced, stable cycling was observed even at a current density of 0.5 mA/cm² with no increase in overvoltage during cycling. EDX analysis of the LLZ/Li interface after the dissolution/deposition tests revealed the presence of the Bi layer at the interface, as shown in Fig. 2, which remained even after repeated dissolution/deposition cycles. It is believed that the Bi layer remains at the LLZ/Li interface after dissolution/deposition cycles, thereby maintaining a good contact interface between LLZ and Li, and improving cycle life.

Acknowledgments

This work was supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI Grant Number 22H01967.

References

[1] K. Fu et al., Angew. Chem. Int. Ed. 56, 14942 (2017).; [2] H. Koshikawa et al., J. Electroanal. Chem. 835 143 (2019).; [3] W. Feng et al., Adv. Mater. 2306111 (2024).

Figure 1