A New “Solution” Towards Wet Chemical Processing of Lithium Garnet Films

Lithium solid electrolytes are expected to improve next-generation energy storage technology on the basis of energy density, safety, and cost. However, there has been limited success in mass producing a solid separator with transport properties and comparable thickness to a traditional polypropylene separator used in lithium-ion batteries. This problem is especially difficult with oxide-based solid electrolytes, such as Li7-3xAlxLa3Zr2O12 (LLZO), where thin films between 200 nm and 5 µm in thickness using scalable solution-based techniques have heretofore not been realized. Several vacuum-based1,2 and sol-gel-derived3,4 methods have shown promise in making thin garnet-based solid electrolyte films with relatively high ionic conductivity (~10-6 - 10-5 S/cm at room temperature), yet lithium loss during post annealing can alter the phase as well as significantly affect the lithium ion conductivity. Only a recent report5 has overcome this issue by using an alternative ceramic processing strategy establishing lithium reservoirs directly in lithium garnet-based films (through Li3N multilayers) that allow for lithiated and fast-conducting cubic LLZO electrolytes at unusually low processing temperatures using pulsed laser deposition. Still, the exploration of scalable fabrication techniques is of special importance to develop thin films of garnet-based solid electrolytes with high ionic conductivity. Here, we present a new technique based on spray pyrolysis for growing LLZO thin films. We show that the crystallization and the phase transformation can be modulated to lower temperatures (<750 °C) by tuning the concentration of cations and the boiling point of the solvents used in the spray solution. Also, by altering the chemistry of the spray solution and the post-annealing conditions, we show that the surface roughness can be modulated while still maintaining dense and continuous membranes of LLZO. Our results highlight a new opportunity for manufacturing garnet-based solid electrolytes with tunable electrochemical surface areas. The insights from this work are expected to serve as fundamental guidelines for future optimization toward solution-based processing of thin film superionic garnet-based materials for next-generation lithium metal batteries. Acknowledgements This research was supported by Samsung Electronics and a portion of this research was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. References [1] R.J. Chen, M. Huang, W.Z. Huang, Y. Shen, Y.H. Lin, and C.W. Journal of Materials Chemistry A, 2014, 2(33), 13277-13282. [2] M. Bitzer, T. Van Gestel, and S. Uhlenbruck. Thin Solid Films, 2016, 615,128-134. [3] I. Garbayo, M. Struzik, W.J. Bowman, R. Pfenninger, E. Stilp, and J.L.M. Rupp. Advanced Energy Materials, 2018, 8(12), 1702265. [4] M. Rawlence, A.N. Filippin, A. Wäckerlin, T.Y. Lin, E. Cuervo-Reyes, A. Remhof, C. Battaglia, J.L.M. Rupp, and S. Buecheler. ACS Applied Materials & Interfaces, 2018, 10(16), 13720-13728. [5] R. Pfenninger, M. Struzik, I. Garbayo, E. Stilp and J.L.M. Rupp. Nature Energy, 2019, in press.