An experimental study on the structure-property relationship of composite fluid electrodes for use in high energy density semi-solid flow cells

Abstract

A novel electrochemical energy storage device, the semi-solid flow cell (SSFC), has recently been demonstrated. The device features a complex fluid composite as its anode and cathode. Both electrodes incorporate particles of a lithium storage compound suspended in a carbon black electrolyte gel. This design of a mixed conductor gel host and electrochemically active filler allows for fluid electrodes to be pumped, from storage tanks, through reaction cells. The de-coupling of energy and power capacity in a high energy density device opens up new opportunities for low cost, high performance energy storage. This thesis explores the microstructure of these fluid composites and establishes links to macroscopic properties that determine the device's energy and power density, efficiency, and cycle life. The rapid agglomeration of colloidal carbon black aggregates leads to gelation by diffusion limited cluster aggregation. The low density, percolating network of carbon provides conduction paths for both ions and electrons. The gel's yield stress stably suspends density mismatched particles of lithium storage compounds, which can readily access the electrochemical reactants via the gel matrix. Application of shear reversibly destroys the gel network, allowing for flow. Flow-induced heterogeneities are also investigated and methods of maintaining macroscopic homogeneity are presented.