Development of ICP-based methods for laterally resolved stoichiometry determination of Li7-3xAlxLa3Zr2O12 garnets

The performance and safety of currently used Li-ion batteries is limited by the utilized instable organic electrolytes. Therefore, replacing these liquid electrolytes by inorganic solid ion conductors is of major interest. Due to their high Li-ion conductivity and chemical and electrochemical stability, Li7La3Zr2O12 (LLZO) garnets are among the most promising candidates for future all solid state Li-ion batteries. Since the highly conductive cubic modification of LLZO can be stabilized by the introduction of certain supervalent cations such as Al3+, especially doped LLZO variants like Li7-3xAlxLa3Zr2O12 have attracted attention. Despite extensive research in recent years, the reproducible synthesis of highly conductive LLZO remains challenging. Processes involved in the synthesis as well as the relationship between chemical composition and conduction behavior of the resulting LLZO garnets are still not completely understood. To investigate those, an analytical method capable of detecting and quantifying all metals of the material including Li is needed. The coupling of ICP-OES or ICP-MS to laser ablation (LA) not only fulfills this requirement, but also enables spatially resolved direct solid analysis without time-consuming sample preparation. In this work, the development of an ICP-based method for the laterally resolved stoichiometry determination of Li7-3xAlxLa3Zr2O12 garnets is described. Since reliable quantification of LA measurements is only possible using matrix-matched calibration standards, the preparation of suitable LLZO standards as well as the determination of their chemical composition was required beforehand. Homogeneous LLZO standards were successfully prepared by pressing ground LLZO powders with varying Al content into pellets. The stoichiometry of the utilized powders was determined using borax fusion for sample digestion in combination with ICP-OES analysis. Using these pressed LLZO pellets as standards for signal quantification, LA-ICP-MS as well as LA-ICP-OES experiments were performed. Thereby, two different calibration strategies were applied: A conventional univariate external calibration approach in combination with the use of La as internal standard and an internal-standard independent calibration strategy based on the normalization of the sum of all metal oxides to 100 wt%. Results show that LA-ICP-MS is not suitable for the quantitative analysis of LLZO due to a lack of the required precision of the Li-measurement, which is probably caused by mass discrimination due to space-charge effects. In contrast to that, a LA-ICP-OES method capable of quantitative and spatially resolved analysis of Li7-3xAlxLa3Zr2O12 could be found. Although both calibration strategies provide similar results, the 100 wt% normalization approach is superior to the conventional external calibration due to the capability of complete stoichiometry determination. To investigate possible changes of the chemical composition during the LLZO-synthesis, starting materials, intermediates and product of a “Li6.4Al0. 2La3Zr2O12” synthesis were analyzed using borax fusion and ICP-OES measurement. The results of the experiments indicate the loss/incorporation of CO2 and/or moisture during the different stages of the synthesis process