Targeted shape formation of glasses inside a stirred media mill and their potential application as novel anode materials in next generation lithium-ion batteries

Wet comminution in stirred media mills is a well-established, scalable and versatile top-down processing unit operation in process engineering as well as particle technology. Mills are designed to effectively stress particles and reduce their respective particle size. Here, mostly crystalline materials and their respective material-specific size reduction behavior has been studied. Though, recent publications have shown that vitreous materials, namely SiO2 glass, can be plastically deformed to yield flake-like particles using the right comminution solvent. However, no comprehensive experimental study has been published for different glasses, studying the influence of different process parameters on the deformation behavior during stressing in a stirred media mill. Within the framework of this experimental work, the comminution and deformation behavior of three commercially available glasses, namely SiO2, soda lime and borosilicate glass, was investigated as a function of different process parameters in a stirred media mill. Process parameters were varied by altering the respective stress energies, due to different stirring speeds and grinding bead diameters, and stress numbers, due to varying processing times. Significant particle size reduction was observed in the first hour of stressing followed by prolonged deformation stage, resulting in the formation of flake-like particles with high aspect ratios. Deformation behavior is significantly influenced by the glass compositions, here the addition of glass network formers such as B2O3 or glass network modifiers such as CaO or Na2O result in a softer glass prone to increased plasticity rather than brittle fracture in the submicron particle size range. The altered mechanical behavior can even be observed on the macroscopic scale in terms of reduced Young’s modulus as well as Poisson ratio. Naturally, a stressing limit in terms of resulting particle size can be observed for all chosen process parameters and glass systems. Furthermore, the complementary micromechanical characterization of sodalime and borosilicate glass single particles in the size range of 0.4 – 4.0 μm was conducted using an electron microscopy supported in situ micromanipulation device. The broad particle size range was chosen to assess possible size-dependent deformation phenomena, commonly seen for SiO2 glass particles. The evaluation of experimental deformation data with well-established contact and deformation models, gave access to mechanical properties such as Young’s modulus and revealed increased plasticity with decreasing particle size. For both glass systems, no significant fragmentation behavior was observed for a certain particle size range. After initial plastic deformation, both glasses show the formation of surface cracks in the meridian plane to relieve compressional stress and release elastically stored energy. However, further plastic deformation did not result in catastrophic failure in terms of fatal fragmentation into several fragments, yet further surface cracks formed accompanied by subsequent plastic deformation, yielding flake-like particles with high aspect ratios over a broad particle size range. Both glasses showed increased plasticity compared to SiO2 glass, which can be directly linked to the chemical composition and therefore altered glass structure, since both studied glass systems can be characterized as multicomponent glasses. In the second part of this thesis, the knowledge gained was used to produce glasses with tailored chemical compositions via a simple melt quenching technique. Here, two common glass network formers, namely B2O3 and SiO2 were used to incorporate electrochemically active components, such as Co3O4 and GeO2, into the glass matrix. The as-produced glasses were subsequently processed and shaped into flake-like particles using the previously explored wet comminution process. It was found that the shown brittle-to-ductile transition held true for all produced glasses, confirming the versatility of the technique without limitations in regard to the chemical composition. Electrochemical characterization as novel anode materials in lithium-ion-batteries showed that all compositions showed electrochemical activity towards Li-ions. In particular, the influence of particle size (and therefore shape) on lithiation behavior and cycling stability was investigated. Electrochemical characterization revealed a strong influence of the chemical composition as well as particle size on the electrochemical activity, lithiation behavior and cycling stability. In total, decreasing the particle size and therefore increasing the mass-specific surface area had a positive effect on the cycling stability of the investigated glasses. Especially glasses containing less than 25 mol.-% GeO2, either B2O3- or SiO2 based, showed promising specific capacities with good capacity retention after 100 galvanostatic discharge-charge cycles. Lastly, based on previously elaborated electrochemical results, a targeted heat treatment was performed on a chosen SiO2-GeO2 glass composition to study the influence of partial crystallization on the electrochemical cycling behavior. The study displays that a suitable annealing can induce the crystallization of Ge-rich GexSiyO6 (2 ≤ x ≤ 3; 0.1 ≤ y ≤ 1) crystallites embedded in a SiO2-GeO2 glass matrix that can function as electrochemically active sites for the reaction with Li-ions. The proposed glass-ceramic material could convince with improved electrochemical activity during initial discharge, good lithiation kinetics and cycling stability at various charging currents.