Freeze Casting - from Battery Separators to Ceramic Scaffolds

Citation

Wu, Chun-Wei Vince (2023) Freeze Casting - from Battery Separators to Ceramic Scaffolds. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/ghvz-q712. https://resolver.caltech.edu/CaltechTHESIS:06092023-230729474

Abstract

Freeze casting is a versatile pore-forming technique which allows tunability of pore structures including pore size, size distribution, morphology, and alignment in various material systems. It is that versatility that makes freeze casting a prospective candidate for fabrication of porous components used in a wide range of fields, ranging from biomaterials to supercapacitors. This work explores freeze casting as the processing route to fabricate battery separators and ceramic scaffolds.

The first part of this study assesses the feasibility of tape/freeze casting, a combination of tape casting and freeze casting, in fabricating battery separators for sodium-ion batteries. Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) is chosen as the separator material due to its chemical inertness in battery environments, and dioxane is selected as the solvent for PVDF-HFP owing to its dendritic crystal structure and the absence of demixing upon freezing the solution. PVDF-HFP membranes fabricated by a bi-directional tape/freeze casting with dioxane exhibit through-thickness, directionally aligned pore structures. Although PVDF-HFP is shown to surpass reference separators in electrolyte affinity and electrochemical performance, composite strategies are designed to provide enhanced mechanical and electrochemical properties.

Firstly, the effects of alumina, a reinforcing agent introduced via ball milling with dioxane to form suspensions prior to tape/freeze casting, are examined. Composite PVDF- HFP/Al2O3 membranes show similar microstructures to their polymer counterpart, with enhanced resistance to thermal shrinkage, elastic modulus, electrolyte uptake, and ionic conductivity. Moreover, coin cells made with composite membranes deliver better rate performance and cycling stability than those with polymer membranes and filter paper reference materials.

Secondly, an alternative route to incorporate inorganic reinforcing elements into PVDF- HFP membranes is found through a co-solvent process. Silica particles from a sol-gel reaction of tetraethoxysilane (TEOS) are introduced into PVDF-HFP membranes via a co- solvent method in conjunction with dimethyl sulfoxide (DMSO). The tape/freeze-cast PVDF-HFP membranes fabricated with DMSO alone exhibit directionally aligned pores, while a hierarchical pore morphology with circular pores on the aligned pore walls is observed in composite membranes fabricated with TEOS, and hence, silica additions. Composite PVDF-HFP/SiO₂ membranes outperform their unreinforced polymer counterpart in terms of elastic modulus, thermal stability, electrolyte affinity, and ionic conductivity, along with capacity retention and cycling performance when assembled into coin cells.

The final portion of this study evaluates the capability of freeze casting for highly permeable ceramic scaffolds using a polymethylsiloxane preceramic polymer with tert- butyl alcohol (TBA), a solvent creating prismatic pores without side arms that affords high permeability. A double-sided freeze-casting configuration results in more controlled freezing of the polymer solutions in comparison with the conventional single-sided counterpart, and hence a more aligned pore structure is obtained. Further improvement in pore alignment accompanied by an eight-fold increase in water permeability is realized by templating the substrate in freeze-casting molds.

Thesis Files