The research on solid state electrolytes (SSE) is motivated by their excellent safety properties, their potential for battery miniaturization, and their possibility to enhance the energy density of the battery. However, their often low Li-ion conductivity compared to liquid electrolytes is a drawback. In this respect garnet-type Li₇La₃Zr₂O₁₂ (LLZO) attracted much attention, since it combines features such as a very good electrochemical stability over a wide potential range as well as against lithium metal and a reasonably high Li-ion conductivity of around 10^‑6 S·cm^‑1 in tetragonal and 10^‑4 S·cm^‑1 in cubic modification.^[1]

Regarding fabrication, chemical vapor deposition (CVD) is a suitable method to grow functional thin-films for Li-ion batteries, since it allows for a homogeneous growth over large areas with high deposition rates and a very high purity.^[2] Despite these advantages, reports on CVD grown thin-films for Li-ion battery application are rare. One reason may be that with conventional CVD precursor delivery systems it is often difficult to achieve the correct composition of complex, multicomponent materials such as typically used for Li-ion batteries. Recently, our group established a novel CVD precursor delivery system applying CO₂-laser flash evaporation of solid precursors (LA-CVD) and demonstrated its applicability for Li-ion battery research.^[3]

In this contribution we report on the latest progress in LLZO thin-film deposition using LA-CVD. The detailed characterization includes results on microstructure, phase composition and electrochemical performance. Thin-films that are dense, free of cracks and mainly consist of tetragonal LLZO show a Li-ion conductivity of about 10^‑6 S·cm^‑1, probed with impedance spectroscopy. This compares with lithium phosphorus oxynitride (LiPON), which is a glassy solid electrolyte typically used in current all-solid-state cells. To further improve the Li-ion conductivity of the LLZO thin-films two approaches are followed up: one is to increase the bulk conductivity by stabilization of the cubic LLZO phase via doping. The second is to reduce the grain boundary contribution by tuning the morphology of the thin-films.

References:

[1] R. Murugan, V. Thangadurai, W. Weppner, Angew. Chem. Int. Ed. 2007, 46, 7778-7781.

[2] M. L. Hitchman, K. F. Jensen, Chemical Vapor Deposition: Principles and Applications, Academic Press, London 1993.

[3] C. Loho, A. J. Darbandi, R. Djenadic, O. Clemens, H. Hahn, Chem. Vap. Deposition 2014, 20, 152-160.