Surface and interface properties of glass-ceramic garnet-type solid electrolytes for the application in solid-state batteries
Next-generation secondary batteries with enhanced energy and power density gain in importance, especially due to the growing demand for electric vehicles. Solid-state batteries (SSBs) are considered as one of the most promising alternatives to state-of-the-art lithium ion batteries. Conventional liquid electrolytes, that are volatile and ... flammable, are replaced by solid electrolytes (SEs). In addition to improved cell safety due to the high thermal stability of SEs, they promise to enable the application of the lithium metal anode (LMA). The LMA provides a very high theoretical capacity, offering significantly improved gravimetric and volumetric energy density. SEs comprise polymers, oxides, sulfides, halides, phosphates or composites thereof. In the class of oxides, the garnet-type Li7La3Zr2O12 (LLZO) has attracted significant interest. In addition to its high room-temperature ionic conductivity, it is one of the few candidate materials that is stable in contact with lithium metal. However, several obstacles still hinder the practical application of LLZO, such as its instability in ambient air. Most importantly, despite the high mechanical stability of LLZO, lithium dendrite propagation in interconnected pores, grain boundaries and even single crystals, which leads to internal short circuits, is a severe issue. The focus of this thesis is on the application of the novel class of glass-ceramic LLZO and its surface and interface properties. It is produced at the SCHOTT AG in a specific and industrially scalable melting route. Firstly, the surface properties of the powder and the related degradation mechanism in ambient air were studied by kinetic analysis. Both hydration and carbonation were found to follow the core shrinking model with the hydration being an essential intermediate step. A linear dependence on the specific surface area of the powders was observed. Secondly, the effect of the amorphous phase intrinsically contained in glass-ceramic LLZO on the sintering and the interface to the LMA was investigated. Besides its potential to reduce sintering temperature and time due to liquid phase sintering, it was found to segregate into the grain boundaries and pores, mechanically and electrochemically preventing the growth and propagation of lithium metal dendrites. Thirdly, another strategy to overcome the obstacles of LLZO is the integration of LLZO particles into a polymer solid electrolyte (PSE). However, the effect of the garnet/PSE interface on the lithium ion transport in such hybrid electrolytes remains unclear. Hence, the influence of the phase compatibility between LLZO and a polyethylene oxide (PEO)-based PSE on the interface resistance was investigated, finding no positive correlation. The interface resistance is too high to achieve ion transport through LLZO in the HSE, resulting in decreased ionic conductivity compared with the pure PSE. Overall, the results of this thesis provide profound insights into the application of glass-ceramic LLZO. Besides practical knowledge on its surface properties for the processing of LLZO, this work revealed the outstanding properties of the amorphous phase at the interface with potential LMAs, making it a promising material for future SSBs.