Freeze Tape Casting Derived Li7La3Zr2O12 Electrolyte Architecture with Oriented Ion Conduction Channels

Abstract

The high interfacial impedance of garnet electrolyte based solid state batteries has been addressed recently by constructing 3D interfaces in which a random network of solid electrolyte extends to the anode/cathode layer to serve as anolyte/catholyte. Thicker active material layers can be achieved with minimal performance loss compared to deposition derived approaches in such configurations as well. The cell architecture can be further improved by forming directional solid electrolyte channels with low pore tortuosity approaching unity compared to randomly oriented pores, thus imparting higher ion mobility along the transport direction during charge/discharge. Freeze casting, magnetic alignment, and laser ablation are some of the methods that have been used in forming aligned ion conduction channels in lithium ion battery applications and have shown performance improvement over the randomly oriented counterpart. In order to build 3D interfaces with vertically aligned solid electrolyte channels at practical sample thicknesses with potential for scaling, we introduce freeze tape-casting, a combination of tape-casting and freeze casting, to continuously produce green tapes of Li7La3Zr2O12 with vertically aligned pore channels. LLZO was selected as it is one of the most promising solid electrolytes and is stable with Li metal anodes. On sintering, LLZO scaffolds in which LLZO sheets are vertically aligned are obtained. In parallel, processing optimization has been made on producing sintered thin films of LLZO by tape-casting and sintering. We show that porous/dense bi-layers and porous/dense/porous tri-layers of LLZO are produced readily by stacking and sintering the tape-cast and freeze tape-cast LLZO green tapes. Pores are filled with active material and current collector components to complete the cell, and the porous layers of various thicknesses can be mixed and matched as desired per loading requirements. The thicknesses of the porous and dense layers could be reduced down to 100 and 30 µm, respectively. This novel solid electrolyte framework shows great potential to advance the field of solid state battery research. Furthermore, the approach can readily expand to other Li or Na conducting oxide electrolytes to take advantage of the unique architecture.