251: Effect of pressure on ionic conductivity and microstructure of Ta-doped and high-entropy lithium garnet

Abstract: Lithium garnet material Li7La3Zr2O12 (LLZO) is favored for its high Li+ ionic conductivity (10-3-10-4 S.cm-1), wide electrochemical window (0V up to 6V), and outstanding mechanical properties. Nevertheless, the solid-solid interface often suffers from poor physical contact and void formation during lithium stripping, which increases interfacial impedance and current concentration. To address this issue, external stack pressure is commonly applied. Sufficient stack pressure leads to plastic deformation and creep of lithium metal, so that lithium metal will plastically flow and continuously replenish the interface. By compressing the cell, the poor contact gaps and the formation of interfacial voids are suppressed, which increases the effective ionic cross-sectional area and lowers the interfacial resistance. However, most of the prior studies have focused on the role of stack pressure in modifying the interface. Whether and how stack pressure directly influences the solid electrolyte itself is overlooked. In this study, we investigate how stack pressure affects the microstructure and electrochemical properties of Ta-doped LLZO with the chemical formula of Li6.75La3Zr1.75Ta0.25O12 (LLZTO), and high-entropy lithium garnet Li7La3Zr0.5Nb0.5Ta0.5Hf0.5O12 (LLZNTH). Phase stability was examined by X-ray diffraction (XRD), and microstructural features were characterized by scanning electron microscopy (SEM). We use electrochemical impedance spectroscopy (EIS) under pressures of 0.2 MPa and 5MPa. We found that external pressure decreases the ionic conductivity of LLZO, indicating that pressure may affect the electrolyte's intrinsic conductivity and crystal structure. Effects of pressure on the bulk and grain boundary conductivity are also studied.