STRUCTURAL AND ELECTROCHEMICAL ANALYSIS OF SOLID STATE ELECTROLYTE FOR RECHARGEABLE LITHIUM-ION BATTERIES

Abstract

The global transition toward carbon-neutral energy systems, transportation, and resilient grid-storage infrastructures has intensified the demand for rechargeable energy-storage technologies that simultaneously deliver high energy density, safety, long cycle life, and robust operational stability. Lithium-ion batteries (LIBs), despite of their widespread acceptance and decades of technological refinements, remain constrained by certain limitations associated with their liquid-electrolyte-based architecture. Organic carbonate electrolytes composed of LiPF6, LiClO4, or other similar salts dissolved in ethylene carbonate, dimethyl carbonate, and related solvents also offer high ionic mobility. But due to volatility, flammability, and electrochemical instability toward both the highly reducing lithium metal and the oxidative surfaces of high-voltage cathodes, they resist the use of a liquid electrolyte system. These limitations of liquid electrolytes have motivated a paradigm shift toward solid-state electrolytes (SSEs) as a pathway to develop fundamentally safer, more stable, high-performance next-generation rechargeable batteries. SSEs restrict the flammable liquid phase and introduce solid ion conducting media. They are capable of withstanding high voltages, suppressing dendrites, and operating across a broad temperature range. The inherent high thermal stability, wide electrochemical window, and compatibility with lithium metal anodes create opportunities for transformative gains in energy density and long-term reliability. Ceramic electrolytes offer highly ordered three-dimensional ion-migration networks with conductivities competing with those of liquids. The structure of LISICON-type, vii NASICON-type, perovskite-type, sulfide-based, and garnet-type materials exhibits diverse frameworks for Li-ion conduction, each defined by distinct lattice polarizability, defect chemistries, and conduction topologies. Among the growing class of ceramic SSEs, garnet-type Li7La3Zr2O12 (LLZO) has emerged as a compelling SSE material for next-generation solid-state lithium-ion batteries. LLZO offers a unique combination of high ionic conductivity (~10-4 Scm-1) in its cubic phase, exceptional stability against lithium metal, and a wide electrochemical window extending beyond 5 V. In contrast, the tetragonal polymorph exhibits ordered Li-site occupation, higher activation energy, and substantially reduced conductivity. Despite its intrinsic advantages, LLZO still has scientific challenges, such as high-temperature sintering, which exacerbates lithium volatilization, leading to impurity phases like La2Zr2O7 or other oxides that impede ionic transport. The tendency of LLZO to react with atmospheric CO2 and H2O forms resistive Li2CO3 surface layers that degrade interfacial conductivity. Hence, to obtain high density microstructures, there is an essential requirement of careful optimization of sintering schedules, dopant chemistry, and powder-processing routes. Similar to the garnet-type SSEs, phosphate-based solid electrolytes such as LiTa2PO8 (LTPO) represent a structurally robust family of materials with excellent thermal stability, versatile cation substitution, and favourable electrochemical behaviour. The LTPO and Zr doping in LTPO, abbreviated as LTZPO, introduces controlled lattice strain, modifies grain-boundary behaviour, and affects the sintering behaviour, resulting in improved densification and transport pathways. These characteristics also render LTZPO an effective sintering additive for garnet electrolytes, enabling microstructure viii refinement and grain-boundary engineering in composite systems. Therefore, the convergence of these scientific insights motivates a comprehensive investigation of solid-state electrolytes: phosphate-based LiTa2PO8, Zr-doped LiTa2PO8 (Li1+xTa2 xZrxPO8), and oxide-based Ce-doped Li7La3Zr2O12 (Li7La3Zr2-xCexO12), and finally the LLZCO–LTZPO composite electrolytes. Hence, each system offers a distinct window into the interplay between composition, structure, microstructure, and ionic transport. This research work also aims to deepen the scientific understanding of how structural heterogeneity, grain-boundary effects, dopant incorporation, and composite design govern ionic transport in solid-state electrolytes. By establishing clear correlations between structural, microstructural, and electrochemical performances. This research contributes to the rational design of next-generation SSEs capable of enabling safe, high energy density, and durable all-solid-state rechargeable Li-ion batteries. The findings of the present research have been organized into seven chapters within the thesis, each outlined briefly as follows: Chapter 1 introduces the motivation behind developing solid-state electrolytes for next generation rechargeable lithium-ion batteries. It outlines the limitations of conventional liquid electrolytes and explains how solid-state electrolytes offer better safety, wider electrochemical stability, and compatibility with lithium metal along with the detailed literature review on solid state electrolytes developed so far. Chapter 2 covers the synthesis approach and characterization techniques to prepare SSEs. All solid-state electrolytes are prepared through the conventional solid-state ix reaction route. Thermal behavior has been examined using Thermogravimetric Analysis and Differential Scanning Calorimetry (TGA/DSC), while structural analysis and phase identification have been performed using X-ray diffraction (XRD) patterns. The microstructural feature study and dielectric properties investigation are carried out using Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), and an LCR meter, respectively. The electrochemical performance for symmetric and full cell testing has been performed using a potentiostat and galvanostatic battery cycling system. Chapter 3 investigates the synthesis, structural evolution, and ionic transport behaviour of the LiTa2PO8 (LTPO) solid-state electrolyte. LTPO has been synthesized via a solid state reaction route. The thermal analysis (TGA/DSC) up to a temperature of 1100 °C guided the selection of a multi-step sintering profile. XRD measurement confirmed monoclinic LTPO as the primary phase with traces of trigonal LiTaO3 phase, while microstructural characterization revealed highly dense and heterogeneous grain distribution, particularly under step-sintering conditions. Dielectric studies indicate Maxwell-Wagner relaxation with clear frequency-dependent dispersion, and impedance spectroscopy identified distinct grain and grain-boundary contributions to ion transport. The electrochemical performance of LTPO has been investigated using a buffer layer at the electrolyte/electrode interface in a quasi-solid-state cell. Chapter 4 focuses on enhancing the ionic conductivity of LTPO through zirconium substitution doping at the Ta site to prepare Zr-doped LTZPO solid electrolytes x synthesized via the solid-state reaction route. X-ray diffraction confirmed the formation of monoclinic phase LTZPO with the appearance of a secondary LiTa3O8 phase upon Zr incorporation. Pre-sintering particle size reduction indicates a significant role in modifying the structural characteristics, while finer particles induced compressive stresses in the pellet. An electrochemical study has been performed using symmetric Li|LTZPO|Li cells across, showing remarkable stability, maintaining continuous cycling for up to 1500 hours. Chapter 5 includes the structural and impedance analysis of Ce-doped Li7La3Zr2O12 (LLZCO) solid electrolyte synthesized through the solid-state reaction method. X-ray diffraction confirmed the stabilization of the cubic garnet phase upon Ce substitution, accompanied by a minor Ce0.8La0.2O1.9 impurity phase. The detailed impedance de convolution has been carried out to separate bulk, grain-boundary, and electrode interface contributions, allowing a clearer interpretation of relaxation processes influencing lithium-ion conduction. Electrochemical analysis the electrolytes has been measured in both symmetric and full solid-state cell configurations. Chapter 6 investigates the electrochemical performance of LLZCO-LTZPO composite solid-state electrolytes designed to improve densification, grain-boundary characteristics, and overall ionic transport of the composite electrolyte system. The composite electrolytes were synthesized using a conventional sintering approach, where LTZPO is incorporated as a sintering additive into LLZCO. XRD analysis confirmed the successful composite formation for samples containing variable weight percentage (0 xi 25%) LTZPO in LLZCO. Complementary SEM and DRT analyses revealed the distribution of LTZPO predominantly along grain boundaries. The electrochemical behaviour of these composite electrolytes has been measured in both symmetric and full solid-state cell configurations. Chapter 7 summarizes the major outcomes of the research work conducted in the thesis research,. The chapter highlights the successful synthesis and characterization of LTPO, LTZPO, LLZCO, and LLZCO-LTZPO, composite electrolytes, along with insights into their structural evolution, densification mechanisms, ionic transport pathways, and electrochemical performance. This chapter also outlines the future directions for improving ion kinetics in solid-state batteries, such as microstructure refinement, interface surface engineering, advanced sintering strategies, in-situ diagnostic studies, scalable fabrication routes and the further incorporation of these ceramic solid electrolytes in polymer membranes for polymer-ceramic hybrid solid state electrolyte development and long-term cycling assessments for pre-commercial readiness along with the broader societal relevance of solid-state electrolytes, particularly their role in enabling safer, non-flammable batteries, extending storage lifetimes for electric vehicles, and renewable-energy systems, reducing environmental footprint, strengthening energy security, and supporting technological leadership in next-generation energy-storage technologies.