Advanced sintering of garnet-based ceramic composite cathodes for all-solid-state lithium batteries

Oxide-ceramic based All-Solid-State Lithium Batteries (ASSLBs) can provide high intrinsic safety, extended operational temperature range, and high energy density. As the first two are intrinsic to the materials system, one prerequisite to obtain high energy densities with such ASSLBs is the manufacturing of thick, mixed electronic and ionic conductive composite cathodes, analogous to those of conventional liquid electrolyte-based lithium batteries. The preparation of composite cathodes using oxide-ceramic electrolytes is challenging since high temperature sintering steps are necessary during electrode and cell manufacturing to achieve proper mechanical stability, contact between the individual phases, and good ionic and electronic conductivity. Since oxide-based electrolyte materials like Li${}_7$La${}_3$Zr${}_2$O${}_{12}$ (LLZ) require sintering temperatures above 1050 °C, they often exceed the thermal stability windows of the cathode active materials like spinels (e.g. Li${}_2$NiMn${}_3$O${}_8$) or layered materials (e.g. Li[Ni${}_{1-x}$${}_{-y}$Co${}_x$Mn${}_y$]O${}_2$ and LiCoO${}_2$ (LCO)). One well established method to achieve consolidation at lower temperatures is high-pressure assisted Field-Assisted Sintering Technique/Spark Plasma Sintering (FAST/SPS). The first part of this dissertation will focus on the material selection and show that LCO is compatible with LLZ. Through the application of high-pressure assisted FAST/SPS, the sintering temperature was reduced to 675 °C – 750 °C and the dwell time to 10 minutes, reaching a relative density of 95 % for LCO/LLZ pellets. The high mechanical pressure was found to be a crucial factor for achievement of high density and phase purity. However, low sintering temperature leads to a significant influence of surface impurities in the initial powders and influences the crystallinity and impedance of the sintered LCO/LLZ interface. Therefore, a special heat treatment for the cleaning of the starting powders and post-sintering annealing was conducted. The annealed composite cathode can provide a high areal capacity in fully inorganic and polymer-ceramic ASSLBs. The all-solid-state composite cathode with rigid interfaces is mechanically stable during electrochemical cycling, which is attributed to the high relative density of the cathodes achieved via FAST/SPS processing. Although, no mechanical degradation is observed, the composite cathodes show a fading electrochemical performance due to increasing LCO/LLZ:Ta interface impedance while cycling. Detailed interface characterization and thermodynamic calculations imply an electrochemical driven Al-Co exchange that can explain the fading electrochemical performance. A subsequent thermal annealing step helps to nearly fully recover the electrochemical performance of the composite cathode.