Scalable Processing Routes for Solid-State Energy Conversion and Storage Devices
Solid-state electrochemical devices all have at least one central component in common, an ion-conductive solid electrolyte (SE). The SE is the basis of the design of both all-solid-state batteries (ASSBs) and solid oxide fuel cells (SOFCs) and is a major determinate for their performance and the economic value in electrochemical storage and conversion applications. The optimization of the SE in ASSBs and SOFCs along with the entire electrochemical cell is mired with challenges concerning efficient transport of electronic, ionic, and fuel species (in the case of fuel cells) when designing cells to be made with scalable processes. Nonetheless, the transport properties of structures in SOFCs and ASSBs as they would be in a product design (thin coatings) are critical issues related to performance limitations and manufacturability. Chapter One briefly reviews solid-state electrochemistry, and Chapter Two provides a brief review specifically of SOFCs and ASSBs with emphasis on scalable processing of cells. Chapters Three and Four investigate scalable processes for SEs and composite cathodes for sulfide-based ASSBs and demonstrate how polymer affects dispersion stability and electrochemical properties. Chapter Three reveals how strict control of solid loading of a slurry prepared for tape casting SEs enables highly uniform, dense coatings suitable for ASSBs, resulting in ionic conductivity similar to that of the pristine powder but with 11 times less area-specific resistance due to a reduction in thickness of the SE. It is revealed that only 3 wt.% changes in the solid loading allow for significant changes in the dispersion stability based upon extended forms of Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory and result in coatings four times more uniform, based upon an internally developed evaluation tool. A dry, solvent-free coating process for composite cathodes was demonstrated in Chapter Four and outperformed coatings prepared with a wet, tape casting process by more than 30%. The key difference between preparing SE and cathode coatings is the requirement of cathode films to serve as a mixed ionic and electronic conductor. The dry method preserved both ionic and electronic transport properties that improved battery performance at higher current density. This could have significant cost benefits as well in large-scale manufacturing when solvents are removed from processing the cathode at a large scale. Chapter Five investigates the porosity in anode-supported SOFCs and the application of freeze casting to create aligned pore channels to improve fuel transport. The alignment of porosity is shown to reduce concentration polarization by 40% under lean operating conditions, which is an operating state when high fuel utilization is achieved. When the porosity is maximized using freeze casting, the permeability of the tubes is increased by nearly 29%, and higher power density and conversion efficiency by 40% were demonstrated with the highest level of porosity. Improved electrochemical performance, however, comes with a cost of decreasing the mechanical integrity of the tubes by nearly two times as demonstrated within our study. In summary, this dissertation focuses on scalable processing routes to address key performance-limiting factors in solid-state electrochemical energy devices such as sulfide-based ASSB and anode-supported tubular SOFCs. Just by changing processing parameters and methodologies, great effects are observed on the electrochemical performance of solid-state cells.
Solid-State Battery, Solid Oxide Fuel Cell
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