(Keynote) The Role of Interlayer Chemistry in Li-Metal Growth through a Garnet-Type Solid Electrolyte

Although garnet-type LLZO has been considered one of the most promising solid electrolyte for solid-state batteries (SSB), the instability of electrode/LLZO interface has been obstacle for the practical application of LLZO into the batteries.[1] Thus, a fundamental understanding of the lithium deposition behavior in the interface would aid in elucidating the underlying mechanisms of the short circuit failure due to unstable interface and designing the interface in SSBs. Herein, we successfully used an in operando microscopy technique to probe Li deposition through the LLZO electrolyte in an anode-free solid-state battery setup. More importantly, we carefully examined the interface with artificial interlayers, which revealed that Li plating is strongly dominated by the kinetics of alloying and precipitation through the metal interlayer. In addition, we confirmed that the interlayer also affects the sequential stripping process, influencing the electrochemical performance of the cell. Supported by these intriguing observations, we propose the dynamic roles of the interlayer during battery operation: as a buffer layer and a seed layer. The in-house cell and microscope system shown in Figure 1a were used for the in operando observations of Li deposition on LLZO. Li foil was attached to the bottom of the LLZO pellet and the top surface was pre-coated with the selected interlayer metal. We investigated electrochemical Li deposition behavior in the absence of an interlayer metal. Li metal begins to appear in island shapes under an applied galvanostatic current (0.1 mA cm-2) and continues to grow over time as shown in Figure 1b. It is worth noting that all of the small Li-metal islands first form at pre-existing defects on the pristine LLZO surface during initial lithiation and Li grows preferably at these islands during the subsequent plating. When the metal interlayer was introduced, the deposition behavior was significantly altered. With 30-nm-thick Au layer, we observed that Au interlayer changed color under electrochemical bias, which is indicative of the formation of a Li-Au alloy. These color changes are followed by the formation of island-type small precipitations. From this observation, we propose that Au interlayer plays the unexpected role of a “buffer layer”, which dynamically functions as a medium for Li redistribution by propagating alloying reactions. Given the proposed new role of the interlayer, we expected the Li-metal deposition behavior to critically depend on the thermodynamic and kinetic properties of the interlayer metal and its alloying nature with Li. Si and Ag were chosen in this study, considering the availability of various alloys and the appreciable Li diffusivities in their alloys. When Si layer is applied (Figure 2a), the color changed during the early stages as gold, but subsequent behavior was noticeably different from that of the Au; Li metal preferentially precipitates at only a few sites, and whisker-shaped Li metal rapidly grows at these sites. On the other hand, in Ag interlayer (Figure 2b), the alloying reaction occurs first, followed by the uniform formation of numerous small nuclei. Interestingly, the two reactions occur nearly simultaneously, implying that both the alloying reaction and precipitation in the alloy are so fast that the sequential processes are unable to be distinguished. Regarding on this, the metal interlayers can be regarded as seed matrix for lithium precipitation. Further, Ag interlayer is more reversible upon Li stripping than Au or Si. The better efficacy of the lithium deposition/stripping process with Ag interlayer was further validated by comparative electrochemical testing of two electrode cells constructed with ‘100-nm-thick metal layer│LLZO │Li’ configuration. The results show that the coulombic efficiencies of cells using the various interlayers are closely related to the Li deposition and stripping behavior. The cell with Ag layer, which induces the most-uniform and reversible Li deposition among the tested metals, exhibits the highest efficiency of 71%, while that with Si layer shows 28%. Thus, it indicates that the interlayer can significantly affect the electrochemical performance of anode-free SSBs that employ solid electrolytes by regulating lithiation and de-lithiation behavior at the interface. Considering the ease of interlayer deposition on the LLZO surface and its wide applicability, we expect that our findings will provide useful guidelines for securing optimal interfaces for SSBs. REFERENCES [1] Aguesse, F.; Manalastas, W.; Buannic, L.; Lopez del Amo, J. M.; Singh, G.; Llordés, A.; Kilner, J., ACS Applied Materials & Interfaces 9, 3808-3816. (2017) Figure 1