February 2019

Driven by the interest in high-performing and safe solutions for energy storage, continuous progress was made in the field of all-solid-state lithium batteries over the past years. Although modern thiophosphate electrolytes provide ion conductivities comparable to the widely used liquid materials, their low stability against lithium is still a major bottleneck for application. However, less conductive oxide electrolytes, e.g. Li7La3Zr2O12 (LLZO), show superior stability, especially in contact with metallic lithium, which is a targeted anode material in all-solid-state cells.While the symbiotic combination of materials to composites is used for a variety of purposes, composite electrolytes represent a comparably unexplored field in research. By combining different compounds, the individual benefits may lead to new possibilities for cell design. As ionic transport is naturally the key performance parameter for electrolytes, the interfacial resistance was investigated in a first step. While simply pressing solids only leads to insufficient contact (upper figure), pulsed laser deposition was employed to ensure an intimate junction between the materials. In this work, the influence of surface treatment of a LLZO pellet on the resulting interfacial resistance towards a thiophosphate electrolyte was investigated. LLZO is known to form a passivating layer on its surface, which suppresses charge transfer. By impedance analysis, the contribution of the interface can be resolved. Polishing of the surface leads to an interfacial resistance as low as 7 Ω∙cm2 at room temperature, while heating the pellet increases the interfacial barrier. Besides a more detailed analysis on the developing interface, a bilayer electrolyte will be implemented into an all-solid-state cell as a next step. (Picture submitted by Georg Dewald.)