The recent interest in solid state batteries has been motivated by the potential for higher energy density, longer cycle life, and improved safety compared to liquid electrolyte based Li-ion batteries. Deposition of thin solid electrolytes on 3D structures could enable a wide range of thin film and bulk battery architectures that offer energy and power density improvements compared with planar geometries. Traditional deposition methods are unable to form dense, uniform, pinhole-free films on complex non-planar geometries, however Atomic Layer Deposition (ALD) excels at conformally coating even ultrahigh aspect ratio substrates with uniform thickness and composition, even multi-component films.¹

To date, the development of ionically conductive solid state electrolytes by ALD has failed to produce films with ionic conductivities comparable to that of sputtered LiPON (~2*10^-6 S/cm at 298K), the current state-of-the-art in thin-film batteries.^2–4 Despite the limited ionic conductivities, several reports have demonstrated the potential of ALD films for both interfacial engineering of bulk batteries, and for thin film batteries.^5,6 Films with improved conductivity would enable faster charging rates, more robust electrolyte films, and serve as a better platform for 3D battery development.

Here, we demonstrate a novel ALD process for ternary lithium borate thin films with ionic conductivities above 10^-6 S/cm at 298K. This represents almost a 2x improvement over the previous best reported value in an ALD film.² The stability and structure of the deposited films are characterized and compared with those calculated with Density Functional Theory and Molecular Dynamics. The film remains an ionic conductor when in contact with metallic Li, and displays stable cycling when paired with a thin-film cathode and Li metal anode. The composition, conductivity, and stability are studied as a function of deposition temperature showing tradeoffs between process conditions and performance, and demonstrating the precise control afforded by the ALD process.⁷

References

(1) Kazyak, E.; Chen, K.-H.; Wood, K. N.; Davis, A. L.; Thompson, T.; Bielinski, A. R.; Sanchez, A. J.; Wang, X.; Wang, C.; Sakamoto, J.; Dasgupta, N. P. Atomic Layer Deposition of the Solid Electrolyte Garnet Li 7 La 3 Zr 2 O 12. Chem. Mater. 2017, 29 (8), 3785–3792.

(2) Kozen, A. C.; Pearse, A. J.; Lin, C.-F.; Noked, M.; Rubloff, G. W. Atomic Layer Deposition of the Solid Electrolyte LiPON. Chem. Mater. 2015, 27 (15), 5324–5331.

(3) Cao, Y.; Meng, X.; Elam, J. W. Atomic Layer Deposition of LixAlyS Solid-State Electrolytes for Stabilizing Lithium-Metal Anodes. ChemElectroChem 2016, 3 (6), 858–863.

(4) Xie, J.; Sendek, A. D.; Cubuk, E. D.; Zhang, X.; Lu, Z.; Gong, Y.; Wu, T.; Shi, F.; Liu, W.; Reed, E. J.; Cui, Y. Atomic Layer Deposition of Stable LiAlF4Lithium Ion Conductive Interfacial Layer for Stable Cathode Cycling. ACS Nano 2017, 11 (7), 7019–7027.

(5) Pearse, A. J.; Schmitt, T. E.; Fuller, E. J.; El-Gabaly, F.; Lin, C. F.; Gerasopoulos, K.; Kozen, A. C.; Talin, A. A.; Rubloff, G.; Gregorczyk, K. E. Nanoscale Solid State Batteries Enabled by Thermal Atomic Layer Deposition of a Lithium Polyphosphazene Solid State Electrolyte. Chem. Mater. 2017, 29 (8), 3740–3753.

(6) Kazyak, E.; Wood, K. N.; Dasgupta, N. P. Improved Cycle Life and Stability of Lithium Metal Anodes through Ultrathin Atomic Layer Deposition Surface Treatments. Chem. Mater. 2015, 27 (18), 6457–6462.

(7) Kazyak, E.; Yu, S.; Chen, K.H.; Davis, A.L.; Sanchez; Lasso, J.; Thompson, T.; Sakamoto, J.; Siegel, D. J.; Dasgupta, N. P., Atomic Layer Deposition of Ultrathin Lithium Borate Solid Electrolytes, Submitted