Garnet-Based Composite Cathodes for All-Solid-State Lithium Batteries

Abstract

Garnet-based all-solid-state lithium batteries (ASSLB) 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 composite cathodes. Preferably, cathode active material (CAM) and electrolyte should form an intertwined 3D-network with an intimate extended contact between two phases. The interface between the CAM and the electrolyte should feature high total surface area with a low impedance to enable an efficient charge transfer and transport and minimize the resistance losses in a battery.Composite solid cathode microstructures can be formed via a co-sintering of solid electrolyte and CAM powders. However, many CAMs such as Li[Ni1-x-yCoxMny]O2 (NMC), Li[Ni1-x-yCoxAly]O2 (NCA) or Li2NiMn3O8 (LMO) cannot withstand sintering temperatures required, for oxide class solid electrolytes as Li7La3Zr2O12 (LLZ) or Li1+xAlxTi2-x(PO4)3, to obtain sintered composite cathodes. Furthermore, the thermodynamic stability of CAMs significantly decreases in the presence of solid electrolytes. Still, several groups have demonstrated that thermodynamic limitations can be significantly lessened via a kinetic control of the process such as reduction of reaction times and a careful control of powder morphology during ceramic sintering process.Following this work, we demonstrate that Field Assisted Sintering Technique / Spark Plasma Sintering (FAST/SPS) enables fabrication of different functional solid-state battery components within minutes such as dense LLZ separator layers with a high ionic conductivity, thick LiCoCO2 (LCO)/LLZ composite cathodes without any interfacial reactions, and integrated cathode half-cells consisting of a composite cathode and a LLZ separator layer at temperatures as low as 675 °C. . The analysis of these composite cathodes revealed well sintered pure phases and a homogenous distribution of cubic LLZ and cathode active material. Electrochemical tests showed promising electronic and ionic conductivity (0.4 mS cm-1) values and an increased capacity (4 mA cm-2) compared to cathodes with pure active material.The structural analysis after the electrochemical characterization shows the mechanical integrity of the LLZ/LCO interface, which means that a mechanically stable rigid LLZ/LCO interface can be obtained.Such a mechanically stable LLZ/LCO cathode allows to analyze possible electrochemistry induced degradation, which is suggested by theoretical work but lacks experimental validation. A detailed analysis of the LLZ/LCO interface by SEM, TEM, Raman, and XRD provide insights into the processes occurring during electrochemical cycling. The findings can pave the way to increase the cycling stability of ceramic all-solid-state batteries via interface optimization Figure 1