Ionic transport in pure and doped LiZr2(PO4)3 polymorphs

Abstract

Safety issue associated with the high flammability and volatility of organic electrolytes used in commercial rechargeable lithium-ion batteries has led to significant attention to ceramic-based solid electrolytes. A detailed study of ion conduction mechanisms in these ceramics is one of the key features for the development of solid electrolytes for application in rechargeable lithium batteries. In this regard, the NaSICON-type framework has been extensively investigated in a bid to design new compounds with improved room temperature lithium ionic conductivity. LiZr2(PO4)3 is an important compound in view of the reported high reduction potential against lithium metal which makes it an attractive candidate for use as an electrolyte in high energy density batteries. However, reported ionic conductivity in LiZr2(PO4)3 is still lower than what is required for a potential electrolyte in practical battery applications. This thesis, therefore, deals with investigations on the lithium ion conduction in monoclinic and rhombohedral phases of LiZr2(PO4)3. Influence of (SiO4)4- substitution for (PO4)3- on the structure and physical properties of LiZr2(PO4)3 ceramics is studied using some experimental techniques.Chapter 1 introduces the field of solid-state lithium ion conductors or solid electrolytes (for lithium-ion batteries) as well as their need in the present global scenario. Chapter 2 describes the working principle of a typical lithium-ion battery, classification of different solid electrolytes and fundamentals of ion transport in these solids. The literature on fast ion conducting electrolytes is reviewed. Finally, the objective of the present study is stated.Chapter 3 describes the various techniques used in this project, which include, both synthesis and characterization techniques. Various experimental techniques such as density measurement, X-ray diffraction, Field Effect Scanning Electron Microscopy and room temperature and high-temperature dielectric measurements are employed to characterize structural, microstructural, and electrical properties of ceramic solid electrolytes.Results of the present work are presented and discussed in Chapter 4. X-ray diffraction data revealed that the LiZr2(PO4)3 crystallized in two polymorphs depending on the calcination temperature and confirmed that P21/n and R3¯c are the appropriate space groups for the sample synthesized at 900 ⁰C and 1300 ⁰C, respectively. Further, density value has been increased with (a) higher sintering temperature and (b) increase in concentration of Si in the general formula Li1+3xZr2(P1-xSixO4)3 for (x = 0, 0.05 and 0.1). FE-SEM result of LiZr2(PO4)3 ceramics shows good sintered samples with few micro-cracks due to anisotropic growth of the rhombohedral unit cell. Finally, results of the temperature dependent dielectric measurements were analyzed, and it was found that conductivity of high-temperature phase (Rhombohedral) was two orders higher than the low-temperature phase (Monoclinic). It also shows that activation energy (EA) value also changes with (a) change in calcination/sintering temperature, (b) concentration of the (SiO4)4- dopant in the LiZr2(PO4)3.The conclusions of the present work are presented in Chapter 5.