Synthesis and characterization of garnet-type solid state electrolytes for lithium-ion batteries

  • Synthese und Charakterisierung von granatartigen Festkörperelektrolyten für Lithium-Ionen Batterien

Paulus, Anja; Eichel, Rüdiger-Albert (Thesis advisor); Simon, Ulrich (Thesis advisor)

Aachen (2018, 2019)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2018

Abstract

Due to the growing interest in renewable energies and the reduction of the use of fossil fuels, the interest in reliable energy storage is growing. One contemplated storage system besides fuel cells and others are batteries. Conventional lithium-ion batteries use toxic, flammable electrolytes containing lithium salts and organic solvents that pose safety hazards. One approach to solve these problems are solid electrolytes. This thesis is concerned with the production of these electrolytes and characterization by means of X-ray diffractometry, thermogravimetry, dilatometry and scanning electron microscopy. The material classes of garnet-type or garnet-related compounds offer a high electrochemical stability in addition to a high lithium-ion conductivity among the potential solid electrolyte materials. Based on the classical solid state synthesis, phase pure sintered ceramics for aluminum substituted garnet-type and iron substituted garnet-related compounds were obtained. This was also achieved using a nitrate citrate stabilized sol gel method, which was applied to Li7-3xFexLa3Zr2O12 for the first time. For the differently synthesized materials it was also possible to show the sintering behavior by means of in situ dilatometer measurements for the first time. It turned out that the densification is completed for the solid state syntheses after a dwell of 2 h and for the sol gel syntheses after a dwell of 20 min, at an optimized temperature of 1225 °C, regardless of the substituent. Prolonged sintering times or increasing sintering temperature to 1250 °C lead to secondary phase formation accompanied by an increase in crystallite sizes, which was shown by the solid state synthesis of Li6.4Fe0.2La3Zr2O12. Li7-3xFexLa3Zr2O12 ceramics produced by the solid state synthesis and sintered for 2 h at 1225 °C have crystallite sizes in the order of magnitude of 10 μm. In comparison pellets of the sol gel synthesis have an order of magnitude larger crystallites, sintered under the same conditions. This shows that with the choice of synthesis, as well as the sintering conditions the crystallite size of sintered ceramics of these materials is controllable. Relative densities obtained after sintering were about 10 percentage points higher in the sol gel synthesis than in the solid state synthesis. In addition, iron improves the densification of the material during sintering, which could be shown by changing the iron content in Li7-3xFexLa3Zr2O12 for the solid state synthesis. Therefore, the density of Li6.4Fe0.2La3Zr2O12 is about 10 percentage points higher than that of unsubstituted or aluminum substituted Li7La3Zr2O12 and Li6.4Al0.2La3Zr2O12. Whereas only partially cubic stabilized powders calcined at 950 °C for 12 h were obtained for the solid state synthesis due to diffusion problems, the use of the sol gel synthesis enabled the production of phase pure powders for both substituents. For the first time, the morphology of calcined powders of Li7-3xAlxLa3Zr2O12 can be controlled via the pH. This is accompanied by a pH-dependent pyrolysis behavior of the dried gels. In the presence of iron in the sol gel synthesis of Li7-3xFexLa3Zr2O12, the pH shows a markedly reduced effect on pyrolysis. Therefore, no apparent change in the microstructure of calcined powders was observable. Nonetheless, the pH shows an influence on the sintering behavior and the microstructure of the sintered pellets, which could be demonstrated by thermal etching. In addition, a reaction with water was found for Li7-3xFexLa3Zr2O12, which, although known for other lithium-ion conducting garnet compounds, was previously unknown for the iron substituted garnet-related compounds.

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