Development of solid inorganic lithium (Li) ion conducting materials for the use as solid electrolytes is indispensable for the realization of next-generation all-solid-state Li batteries with high safety and reliability. Among various oxide-based solid electrolyte materials, a garnet-type oxide with the formula of Li₇La₃Zr₂O₁₂ (LLZO) has attracted much attention because of its high Li ion conductivity at room temperature, excellent thermal performance, and high stability against Li metal.^1,2) However, the formation of a solid-solid interface between LLZO and the Li metal anode is challenging. Poor interfacial connection causes non-uniform Li plating and intergranular penetration of Li dendrite in polycrystalline LLZO when the cell is cycled particularly at high current densities, resulting in internal short-circuit failure.^3–5)

There is no doubt that the establishment of prevention technology for short-circuit failures is a top priority issue for the development of all-solid-state Li metal batteries.⁵⁾ On the other hand, from the viewpoint of effective use of material resources, the possibility of reusing LLZO extracted from a solid-state battery after a short-circuit failure occurred is worth considering. In this work, we investigated the reusability of a Ta-doped Li_6.55La₃Zr_1.55Ta_0.45O₁₂ (Ta-LLZO) solid electrolyte shorted by Li dendrite growth during electrochemical Li plating/stripping testing for a Li/Ta-LLZO/Li symmetric cell. Ta-LLZO was taken out of a tested cell after the degradation by Li dendrite growth occurred, and then annealed at 700 ºC in air. The annealing temperature was set to suppress possible excess Li loss from Ta-LLZO during post-annealing.⁶⁾

In the first Li plating/stripping test, the cell was shorted at 0.85 mA cm^-2 and the dark gray area with possible Li dendrite growth was confirmed on the surface of Ta-LLZO. This dark gray area turned white but slightly different from the original color of Ta-LLZO by post-annealing. The ionic conductivity of as-synthesized and post-annealed Ta-LLZO was measured and compared. Post-annealed Ta-LLZO retained high room temperature ionic conductivity of 0.82 mS cm^-1, which is slightly lower than the conductivity of as-synthesized one (= 0.90 mS cm^-1). We also prepared a symmetric cell with the post-annealed Ta-LLZO and Li metal electrodes and the second Li plating/stripping test was carried out. Symmetric cell with post-annealed Ta-LLZO showed stable voltage response. This indicates the possibility of reusing the degraded garnet-type solid electrolyte by Li dendrite growth for an another solid-state Li battery.

References

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