Development of argyrodite-type Li₆PS₅Cl and Li7P3S11 solid electrolytes for fully solid state lithium sulfide batteries

The growing demand for energy storage technologies, particularly for electric vehicle applications and renewable energy integration, has spurred the development of safer, more efficient systems with higher energy densities. Among these, solid-state lithium-sulfur (SSLS) batteries have garnered increasing interest due to their potential to surpass traditional lithium-ion batteries in terms of energy density and safety. Lithium-sulfur (Li-S) batteries, renowned for their high theoretical energy density of up to 2600 Wh/kg, are considered a promising candidate for next-generation energy storage solutions. However, the commercialization of these batteries has been impeded by challenges such as limited cyclic stability, shuttle effects of polysulfides, and degradation of the electrode-electrolyte interface, which collectively contribute to performance losses over prolonged cycling. This thesis delves into the properties and electrochemical behavior of argyrodite solid electrolytes, specifically Li₆PS₅Cl and Li₇P₃S₁₁, in lithium-sulfur batteries. The study emphasizes their performance when combined with two types of anode materials: lithium metal (Li-metal) and lithium-silicon (Li-Si) composite anodes, aiming to evaluate their stability, ionic conductivity, and cyclic performance. The synthesis of Li₆PS₅Cl and Li₇P₃S₁₁ electrolytes was conducted via a solid-state reaction method, a process chosen for its simplicity and scalability. The resulting materials were extensively characterized using X-ray diffraction (XRD) to confirm crystallographic properties and phase purity, as well as scanning electron microscopy (SEM) to investigate morphological features critical for ensuring uniform ionic transport pathways. These characterizations provided valuable insights into the microstructural properties of the electrolytes, highlighting their dense and homogeneous structure, which is crucial for minimizing ionic resistance and maximizing conductivity. Electrochemical impedance spectroscopy (EIS) was employed under varying conditions to assess the ionic conductivity of these solid electrolytes, revealing that both materials exhibit high ionic conductivities suitable for solid-state applications, particularly at elevated temperatures. For instance, the room-temperature ionic conductivity of Li₆PS₅Cl was determined to be approximately 1.4 × 10⁻³ S/cm, while Li₇P₃S₁₁ exhibited a slightly higher conductivity of 2.1 × 10⁻³ S/cm, confirming their viability as solid electrolytes in high-performance batteries. Electrochemical cells were assembled using the synthesized electrolytes paired with Li-metal and Li-Si anodes to examine their interface stability and overall battery performance. The electrochemical behavior of these cells was systematically studied through linear sweep voltammetry (LSV), cyclic voltammetry (CV), EIS, and charge/discharge cycling tests, providing insights into their reaction mechanisms and stability over repeated cycles. It was observed that both Li₆PS₅Cl and Li₇P₃S₁₁ exhibited enhanced ionic conductivity and improved performance when paired withlithium-silicon alloy anodes, attributed to the alloy's higher compatibility and reduced interfacial resistance. Furthermore, the powder form of the Li-Si alloy facilitated easier assembly and contact within the electrochemical cells, suggesting its practical advantages as a solid-state anode material. On the other hand, the combination of Li₆PS₅Cl with a Li-metal anode demonstrated superior initial stability and efficiency, though capacity degradation was evident with prolonged cycling. The degradation mechanisms observed in Li-metal-based cells were attributed to the formation of lithium dendrites at the anode-electrolyte interface. These dendrites, which grow progressively during repeated cycling, penetrate the solid electrolyte, leading to short circuits and capacity loss. To address this challenge, surface coatings and interface modifications have been proposed as potential solutions to enhance the stability of Li-metal anodes. In contrast, the Li-Si anode exhibited better compatibility with Li₆PS₅Cl, despite some limitations related to volume expansion during lithiation and delithiation. The electrochemical results demonstrated that, while the Li-metal anode achieved an initial capacity of 582 mAh/g with Li₆PS₅Cl, this capacity declined to 430 mAh/g after 20 cycles, accompanied by a drop in efficiency to 73.9%. Conversely, the Li-Si anode achieved an initial capacity of 940 mAh/g with the same electrolyte, maintaining 58.51% efficiency after 100 cycles. These findings highlight the trade-offs between energy density, stability, and cycle life, underscoring the need for material innovations to optimize these parameters. Further analysis was conducted to understand the role of Li₇P₃S₁₁ as a solid electrolyte, which exhibited distinct advantages over Li₆PS₅Cl in terms of stability and ionic transport properties. When paired with a Li-Si anode, Li₇P₃S₁₁ achieved an impressive initial capacity of 1450 mAh/g, albeit with a significant drop in efficiency to 52.41% after 100 cycles. This performance was attributed to the high ionic conductivity of Li₇P₃S₁₁ and its relatively stable interface with the anode. However, challenges such as the formation of an intermediate reaction layer at the interface and the partial decomposition of the solid electrolyte during cycling were identified as critical factors limiting long-term performance. The study also explored strategies to mitigate these issues, including the use of protective coatings, incorporation of buffer layers, and optimization of electrolyte composition to enhance interfacial stability. In addition to electrochemical performance, the thermal stability and mechanical properties of Li₆PS₅Cl and Li₇P₃S₁₁ were investigated to assess their suitability for practical applications. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) revealed that both materials exhibit excellent thermal stability up to 300°C, making them ideal candidates for high-temperature operating environments. Moreover, their mechanical properties, characterized through nanoindentation tests, demonstrated sufficient hardness and elasticity to withstand the mechanical stresses associated with battery assembly and operation. These findings underscore the robustness of argyrodite electrolytes and their potential to overcome the limitations of traditional liquid electrolytes in lithium-sulfur batteries. The study further emphasizes the importance of optimizing the sulfur cathode to complement the performance of solid electrolytes. Sulfur, being highly insulating, requires conductive additives and binders to enhance its electrochemical activity. The development of composite cathodes incorporating sulfur, carbon, and polymeric binders was explored, revealing that the choice of cathode composition significantly influences the overall performance of SSLS batteries. Advanced fabrication techniques, such as vacuum infiltration and freeze-drying, were employed to achieveuniform sulfur distribution within the cathode matrix, resulting in improved energy density and rate capability. In conclusion, this thesis provides a comprehensive investigation into the behavior of Li₆PS₅Cl and Li₇P₃S₁₁ solid electrolytes in lithium-sulfur batteries, highlighting their potential to revolutionize energy storage technologies. The findings demonstrate that the combination of Li-Si anodes with argyrodite electrolytes offers a promising pathway for achieving high-performance solid-state batteries. However, significant challenges remain, particularly in terms of capacity retention, interfacial stability, and scalability. Addressing these challenges will require a multidisciplinary approach, integrating advances in materials science, electrochemistry, and engineering. This work lays the foundation for future research aimed at developing more efficient and durable solid-state lithium-sulfur batteries, paving the way for their commercial adoption in electric vehicles, renewable energy systems, and other critical applications.