Li₇La₃Zr₂O₁₂ (LLZ) is an attractive candidate for oxide solid electrolyte for various Li-metal-based batteries such as all-solid-state-Li battery, Li-S battery, and Li-air battery. This is because LLZ has a high shear modulus and a wide electrochemical stability window and high Li⁺ conductivity (can be 10⁻³ S cm⁻¹ depending on dopant element) at room temperature. However, LLZ still cannot perfectly suppress short-circuiting during Li plating/stripping cycles. Although several approaches to suppress the short-circuiting phenomenon have been reported¹, why the short-circuiting occurs during Li plating/stripping cycles on LLZ has not essentially been understood. This study applies an in-situ scanning-electron microscope (SEM) observation technique² to the investigation on the Li plating/stripping reactions on Li_6.6La₃Zr_1.6Ta_0.4O₁₂ (LLZT).

A LLZT plate (Toshima Manufacturing Co. Ltd.) with a thickness of 0.5 mm was used as the electrolyte¹. A Cu current collector (CC) film was deposited on one side of a LLZT plate by pulsed laser deposition (PLD). A Li film with a thickness of 3 μm and a diameter of 9.0 mm was deposited on the other side of the LLZT plate as the counter electrode by vacuum evaporation. A fabricated Cu/LLZT/Li cell was sandwiched between Cu and brass plates. The Cu plate has a view port in the center. The cell was transferred into the SEM camber without exposure to the air. Li plating/stripping was conducted under galvanostatic conditions at room temperature during the SEM observation.

Figure 1shows the voltage transient during Li plating at 100 uA cm⁻² on Cu-coated LLZT. Li does not only nucleate at grain boundaries but also on grain cores. After 450 seconds, the voltage drops to 0 V indicating the occurrence of short-circuiting. This result indicates that only a small number of the nucleation sites exist on LLZT surfaces whereby the local current density is supposed to increase. We will also discuss the detail of short-circuiting mechanism of LLZT.

Acknowledgement

The authors gratefully acknowledge JSPS 17H04894 for the financial supports

References

[1] Motoyama et al., J. Electrochem. Soc.,162, A7067 (2015).

[2] Yonemoto et al., J. Power Sources, 343, 207 (2017).