Optimization of the manufacturing process of Copper(II)-hexacyanoferrate as a cathode material for Zinc-ion-batteries
Zinc-ion batteries are a promising technology for modern stationary energy storage devices. The Prussian Blue analogue material copper(II)-hexacyanoferrate (CuHCF) has been shown to have favorable ion storage properties as well as electric conductivity and a straight forward production routine. The main goal of this thesis is the investigation of an upscaled production process of CuHCF and the effects of process time and synthesis temperature on the quality of the product. CuHCF is synthesized via co-precipitation and the material is cleaned, dried and milled into a powder. The CuHCF powders were investigated using Raman spectroscopy, scanning electron microscopy and thermogravimetric analysis coupled with mass spectrometry. Then the powders were used as cathode active material in zinc-ion cells. For this the CuHCF powders were mixed with a binder (polyvinyliden diflueride PVdF) and a conductivity enhancing additive (C65 carbon powder) to form a slurry, which has been coated on a carbon cloth substrate. These cathodes were assembled against zinc foil as anode and an aqueous zincsulfate solution as an electrolyte in different cell designs. The reaction behaviour and specific capacities as well as cyclability of the cells in dependance to the variable CuHCF synthesis conditions have been investigated. Analysis of the powders have shown impurities like lattice water and residues of the educt materials, here potassium, in the material. Microscope images also have shown non-crushed agglomerates inside the powder within micrometer range, with primary particle sizes in the nanometer range. In general, the electrochemical performance of the cathode materials was similar to lab scale experiments like shown by lab cell testing. The best results were generated by the CuHCF sample synthesized at the highest temperature (80 C) with process times that allowed for optimal mixing conditions (up to 2 h). A second task of the work was to develop a compact cell design on the basis of pouchbag setups. Several challenges appear for stable operation, mainly driven by the system-related hydrogen gas evolution upon cycling. Due to the water as solvent of the electrolyte, electrolysis is a constant problem inside the pouchbag cells causing concentration changes inside the electrolyte and bloating of the pouchbag through formation of gas leading to contact losses. Finally the scale-up of the CuHCF synthesis and implementation of the reactor and postprocessing devices have been successfully proven. The produced CuHCF material and specific capacity of the CuHCF cathode is in line with literature values. While the cyclability is limited in the pouchbag setup due to gas evolution, over 100 cycles are achievable with high efficiency in a lab cell test setup with excess electrolyte. Future investigations are needed with regard to electrolyte and anode materials as well as cell design to further increase cyclabilityand reduce capacity losses in zinc-ion batteries.
Bremerhaven, Hochschule, Master Thesis, 2021