Garnet-type Li₇La₃Zr₂O₁₂ (LLZO) with its competitive ionic conductivity (~ 1 mS/cm), wide electrochemical stability (above 5.0 V vs. Li/Li+), and compatibility with Li-metal, is a promising electrolyte for solid-state Li-metal batteries. However, fabricating a full battery cell with a high energy density remains a challenge mainly due to the processing difficulties with the co-sintering of composite solid-solid cathode/electrolyte while maintaining thin electrolyte thicknesses of the tape around 20 µm.
In this project, we investigate the scalable powder-to-device fabrication for a cathode/electrolyte half- and full-cell with LiCoO₂ as the active material and LLZO as the ion conducting electrolyte phase, combined with a Li-metal anode. Gradient composite cathodes can dramatically improve the battery performance due to high cathode active material loadings and good ionic and electronic conductivity through the cathode. To manufacture a cell with low interfacial resistance, it is important to prevent secondary phase formation between LiCoO₂ and LLZO during conventional high temperature co-sintering. We find that minor addition of Al- in Ta-oxide doping of 0.05 and 0.4 stoichiometric amounts to LLZO stops Co interface-diffusion at high temperatures of 1050 °C. Additionally, synergistic effects of the co-doping and tailoring the characteristics of the LLZO lead to high ionic conductivities up to 0.85 mS/cm and high relative densities up to 97% in pellets, enabling the comparison of properties between pellets and tapes.  Furthermore, we demonstrate the fabrication of a co-sintered half-cells with a 100 µm gradient composite cathode and a 30 µm electrolyte via free-standing tapes.  To accomplish such a cell-design, we reveal the importance of controlling various processing parameters such as particle size distributions, mixing specifications, and sintering conditions. Our findings are important steps towards the ceramic manufacturing strategies for energy-dense, high-quality tape-based all-solid-state batteries