Optimizing Ti substitution for the enhanced densification, ionic conductivity, and microstructure of garnet-type Li$_7$La$_3$Zr$_2$O$_{12}$ solid electrolytes

Abstract

Garnet-type lithium lanthanum zirconium oxide Li$_7$La$_3$Zr$_2$O$_{12}$ (LLZO) is a favorable solid electrolyte for all-solid-state Li-ion batteries due to its wide electrochemical stability, compatible ionic conductivity, and good safety. However, further improvement in ionic conductivity is required for its practical applications. In this work, titanium (Ti) is doped into LLZO to enhance its Li-ion transport properties and structural stability. The series Li$_7$La$_3$Zr$_{2-x}$Ti$_x$O$_{12}$ has been successfully synthesized using conventional solid-state reaction method. The content of Ti has been varied from 0 to 0.20 atoms per formula unit (a.p.f.u). The conducting cubic phase has been confirmed by the X-ray diffraction technique (XRD). Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) have been used for structural analysis, and elemental distribution. Density measurements have been carried out for all the samples. Electrochemical impedance spectroscopy revealed that the high ionic conductivity of $8.08\times 10^{-5}$ Scm$^{-1}$ is offered by the Li$_7$La$_3$Zr$_{1.9}$Ti$_{0.1}$O$_{12}$ sample, which has the lowest activation energy of 0.37 eV. The DC polarization analysis verified that the main contribution to conductivity in the 0.10 Ti sample comes from ions. A one order of magnitude increase in room temperature ionic conductivity is observed for the 0.10 Ti sample, making it a strong candidate for solid electrolyte applications.