Li-ion transport studies of NASICON-type LiZr2(PO4)3 solid electrolyte crystallizing in rhombohedral structure at room temperature

Abstract

Li-ion conducting solid electrolytes exhibiting NASICON structure have received promising research interest for their potential applications in safer all-solid-state Li-ion batteries. LiZr2(PO4)3 (LZP) belongs to the NASICON-type of materials exhibiting Li-ion conduction and furthermore offers lower interfacial resistance and good electrochemical stability against the Li-metal. LZP is known to crystallize in four polymorphs, namely monoclinic, orthorhombic, triclinic, and rhombohedral, and further, LZP crystallizing in rhombohedral structure is known to exhibit the highest ionic conductivity. The present work demonstrates the synthesis of LZP exhibiting rhombohedral structure at room temperature using the modified sol-gel method at a lower sintering temperature (900 °C). The effect of sintering temperature on the structural and transport behavior of LZP is studied by XRD, SEM, and impedance spectroscopy techniques. The complex impedance plots show the presence of two depressed semicircles associated with different capacitance values, indicating both bulk and grain boundary conductions. The bulk, grain boundary and total conductivity of LZP sintered at different temperatures are calculated. The bulk conductivity of LZP is found to increase with increase in the sintering temperature from 800 °C to 1100 °C, whereas LZP sintered at 900 °C exhibits the highest total conductivity value at room temperature, indicating the significant role of the grain boundary conductions in the total conductivity of LZP material. The transport results of LZP sintered at different temperatures are discussed with their unit cell volume (XRD), relative density (Archimedes method), and microstructure (SEM). Furthermore, the ionic transport number measurement by dc polarization shows that the major charge carriers are the ions and the electrochemical stability measurements by both cyclic voltammetry and linear sweep voltammetry show its stability with Li-metal is up to 5.1 V.