Fully inorganic LLZO-based composite cathodes: the impact of Ga substitution on compatibility with cathode active materials during co-sintering

In order to make garnet-based all-solid-state batteries (ASSBs) attractive for industrial applications, their rate capability has to be significantly improved. Recently, cubic Li6.4Ga0.2La3Zr2O12 (LLZO:Ga) was found to have the highest total ionic conductivity of any oxide solid-state electrolyte by far, reaching up to 2 × 10-3 S/cm at room temperature. Since the rate performance of composite cathodes is directly linked to their ionic conductivity, LLZO:Ga is an ideal solid-state electrolyte for high-performance ASSBs. However, careful material selection is required for the fabrication of such ceramic composite cathodes at elevated temperatures in order to avoid incompatibility issues that could lead to low electrochemical performance. We therefore systematically studied the co-sintering behavior of cubic LLZO:Ga in combination with common cathode active materials, including LiCoO2 (LCO), LiNi1/3Mn1/3Co1/3O2 (NCM111), and LiNi0.8Mn0.1Co0.1O2 (NCM811). The analyses were performed using X-ray diffraction, Raman spectroscopy, scanning electron microscopy, and transmission electron microscopy. The experimental conditions were chosen to enable a direct comparison with our previous study on Li6.45La3Zr1.6Ta0.4Al0.05O12 (LLZO:Ta). For the first time, we were thus able to elucidate the impact of different LLZO compositions on material compatibility. While most of the observed secondary phases were similar to those found for LLZO:Ta-based composites, a more severe degradation of the cubic LLZO:Ga structure itself was observed, reducing its conductivity and thus limiting the performance of the final cell. Consequently, the processing window for producing LLZO:Ga-based composite cathodes is even narrower than for LLZO with other dopants, thus requiring careful tailoring and tight control over the processing conditions when manufacturing garnet-based ASSBs.