EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Cubic Li7La3Zr2O12 (LLZO) garnets have attracted lots of attention in previous years as they show promising properties for Solid-state Li batteries (SSLB) as solid electrolyte including a high electrochemical stability of up to 6 V and a high ionic conductivity in the range of 1 mS cm-1.1 Despite of these favorable prerequisites, its current use in such devices is limited as high temperature treatment is required to form a good contact between LLZO and, e.g., LiCoO2 leading to a high interface resistance by the formation of interdiffusion layers.2
In this study, we present that Co does not only lead to Co diffusion into the LLZO lattice but further induces the formation of resistive interlayers, such as La2CoO4. Since Co is a transition metal, its incorporation could change, e.g., the band gap and, thus, the electrochemical stability window or ionic conduction.
Herein, we simulated the properties of the LLZO|LiCoO2 interface by incorporating Co from LiCoO2 powder into transparent Czochralski-grown Li6.4Ga0.2La3Zr2O12 single crystals over the gas phase at high temperatures. We noticed a color change of the crystals from yellow, orange to dark blue depending on the heating history. To investigate the role of incorporated Co in LLZO garnets, we applied a wide spectrum of techniques, such as UV-VIS, TOF-SIMS, SC-XRD, 57Emission Mößbauer spectroscopy or impedance spectroscopy.
For example, we found that the optical band gap decreases to 1.5 eV by the incorporation of 0.92 Co per formula unit. By conducting impedance analysis a significant change in the Li ion transport was revealed resulting in an increase of the activation energy Ea of 0.54 eV and a decreased room temperature ionic conductivity σ = 7.60 x 10-7 S cm-1 for dark blue LLZO compared to Ea = 0.30 eV and σ = 1.20 x 10-4 S cm-1 for pristine LLZO, respectively.