(Invited) Designing Electrodes with Low Tortuosity Pores By Freeze Tape Casting

Abstract

The tortuosity of pores in composite electrodes for conventional lithium-ion batteries (LIBs) or “beyond lithium-ion batteries” is an extremely important parameter for optimizing performance. The lower the tortuosity, the shorter the path lengths that ions have to travel, which should result in better rate capability and charge acceptance. By using a method known as freeze tape casting (FTC), we are able to design electrodes with nearly unidirectional pores approximately normal to the film thicknesses. In this technique, an aqueous or non-aqueous slurry containing materials of interest is tape cast over a bed set at a temperature below the freezing point of the solvent. As the solvent freezes, the ice crystals that form exclude the solid particles, which segregate into columns. A freeze-drying step removes the ice, leaving behind the porous structure. In the case of conventional composite battery electrodes meant for use in cells with liquid electrolytes, the active materials, binder, and conductive additives can be cast together, similar to what is done by conventional tape casting. For solid-state batteries, a ceramic ionically conducting scaffold is first formed by the FTC process and then sintered to strengthen the pore walls. The scaffold is then infiltrated with active material and other components to form the electrode. In both types of systems, the film thickness, porosity, pore size and shape, and column thickness can be adjusted over a wide range by varying experimental parameters such as temperature, loading, and solvent type. For this talk, we will present several examples from our recent work including solid-state batteries using Li7La3Zr2O12 (LLZO) electrolytes, LIB graphite anodes for fast-charge applications, and sulfur electrodes for lithium/sulfur batteries.