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

In order to make garnet based all-solid-state batteries (ASBs) attractive for industrial application, their rate capability has to be significantly improved. Recently, cubic Li_6.4Ga_0.2La₃Zr₂O₁₂ (LLZO:Ga) was shown to have by far the highest total ion conductivity of all oxide solid-state electrolytes, ranging 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 would be the ideal solid-state electrolyte for high performance ASBs. However, the fabrication of such ceramic composite cathodes at elevated temperatures requires careful material selection to avoid material incompatibility, leading to low electrochemical performance. We therefore systematically studied the co-sintering behavior of cubic LLZO:Ga in combination with common cathode active materials, namely, LiCoO₂ (LCO), LiNi_1/3Mn_1/3Co_1/3O₂ (NCM111), and LiNi_0.8Mn_0.1Co_0.1O₂ (NCM811) by X-ray diffraction, Raman spectroscopy, and scanning electron as well as transmission electron microscopy. Additionally, the experimental conditions were chosen to allow a direct comparison with our previous study on LLZO:Ta. Thus, for the first time, it was possible to elucidate the impact of different LLZO compositions on the 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 structure itself was found, which reduces its conductivity and thus limits the performance of the final cell. Therefore, the processing window for producing LLZO:Ga based composite cathodes is even narrower than for LLZO with other dopants, requiring careful tailoring and tight control over the processing conditions when manufacturing garnet based ASBs.