(Invited) Towards Scalable Manufacturing of Solid State Batteries

The use of ceramic electrolytes like LLZO (variants of Li7La3Zr2O12) hold great promise for improving safety and increasing practical energy densities of batteries with lithium metal anodes. However, the thin film configurations that are currently employed in devices based on ceramic components are not readily scalable, limiting the applications for which these batteries can be used. Moreover, practical energy densities fall far short of the theoretical values because of the low areal capacities of the cathodes. We are currently exploring an alternate approach to the fabrication of solid state batteries based on a low-cost, eco-friendly, and continuous processing method known as freeze tape casting (FTC). In this method, an aqueous slurry of LLZO (or any other ceramic electrolyte of interest) is tape cast over a cold surface. If the conditions are selected correctly, columnar ice crystals form in such a way as to exclude particles. The ice is then removed by freeze-drying, resulting in a porous green body, which then can be sintered to strengthen the pore walls. By changing formulations, temperatures, and other experimental parameters, the thicknesses, porosities, and pore sizes of the scaffolds can be varied. The resultant porous scaffolds can then be infiltrated with active material and other components to make composite cathodes. The low tortuosity of the pores (approaching t=1) in a typical FTC scaffold is beneficial for the infiltration process and also shortens lithium diffusion pathways. A goal is to make electrodes similar in thicknesses and loadings to what is used for cathodes in conventional lithium ion batteries (~100 mm, >75 vol.% porosity), to maximize energy density. Freestanding scaffolds this thin are somewhat fragile. Supporting the porous layer on a thin, dense electrolyte layer results in a more robust configuration. A lithium foil can then be attached to this bilayer to form the cell. A porous/dense/porous trilayer has additional manufacturing advantages. In this sort of configuration, lithium metal or another anode material is infiltrated into one of the porous layers, and a cathode material (and other components) into the other. For this talk, we will discuss the challenges of fabricating solid state cells for large scale applications like electric vehicles, and show examples of FTC components and cells made in our labs.