Jordi Sastre-Pellicer1 Agnieszka Priebe2 Ayodhya Tiwari1 Yaroslav Romanyuk1 Stephan Buecheler1

1, Laboratory for Thin Films and Photovoltaics, Empa - Swiss Federal Laboratories for Materials Science and Technology, Dübendorf, Zürich, Switzerland
2, Laboratory for Mechanics of Materials and Nanostructure, Empa–Swiss Federal Laboratories for Materials Science and Technology, Thun, , Switzerland

The concept of all-solid-state Li-ion batteries has gained broad attention in recent years as it has the potential to overcome the energy density and safety limitations of the nowadays widely used Li-ion batteries based on liquid electrolytes. Thin film manufacturing opens up the possibility of further enhancing volumetric and gravimetric energy densities by reducing the amount of inactive materials, lower the costs of production, and enable new applications, like on-chip batteries.

Garnet-type Li7La3Zr2O12 (LLZO) electrolyte is a promising ionic superconductor for the development of all-solid-state thin-film batteries. While high ionic conductivities (above 1 mS/cm at RT)1 have been demonstrated for bulk material in pellet form, processing LLZO in the form of thin films still poses some challenges. Ionic conductivities reported so far lag behind by some orders of magnitude,2 and density and conformability are difficult to achieve at the limited sintering temperatures required.

In our work we investigated LLZO thin films deposited by co-sputtering from LLZO, Li2O and Al2O3 targets, and subsequently annealed under a controlled oxidizing atmosphere at a limited temperature of 700°C. Prepared samples were investigated using in-situ grazing-incidence X-ray diffractometry, to assess the crystalline phase and its stability. Scanning electron microscopy and focused-ion-beam time-of-flight secondary ion mass spectrometry were used to analyze the density and the distribution of the substitutional element in the film. The ionic conductivity was measured by in-plane impedance spectroscopy.

In this contribution we will present successful approaches to increase the ionic conductivity of LLZO thin films above 10-5 S/cm and achieve higher density, uniformity and surface stability, which are key attributes necessary for the implementation of a LLZO-based thin-film battery with a metallic Li anode. In particular, we will discuss how we tackled common challenges in the fabrication of thin-film ceramic Li-ion electrolytes: the stabilization of highly conductive phases at room temperature, the compensation of Li losses during annealing, the densification of the films at relatively low temperatures, and the mitigation of moisture-induced surface degradation.

1 E. Yi, W. Wang, J. Kieffer, and R. M. Laine, “Key Parameters Governing the Densification of Cubic-Li7La3Zr2O12 Li+ Conductors,” Journal of Power Sources, vol. 352, pp. 156–164, Jun. 2017.
2 M. Rawlence et al., “Effect of Gallium Substitution on Lithium-Ion Conductivity and Phase Evolution in Sputtered Li7–3xGaxLa3Zr2O12 Thin Films,” ACS Applied Materials & Interfaces, vol. 10, no. 16, pp. 13720–13728, Apr. 2018.