Development of mxene-sulphur cathodes for lithium sulphur batteries

Today, increasing energy demand on a global scale, the limitation of fossil fuel resources and their environmental impacts have made the need for clean, sustainable and efficient energy solutions imperative.The continuous reliance on traditional energy sources like coal, oil, and natural gas results in major environmental problems, including global warming, climate change, and pollution stemming from greenhouse gas emissionsIn this direction, solar, wind, hydro and geothermal energy systems come to the forefront with their clean and renewable nature to reduce the dependence on fossil fuels. However, the high dependence of these systems on natural conditions and fluctuations in production continuity make it difficult to maintain the supply-demand balance. This situation has made the need for energy storage technologies that can operate independently from generation sources and at the same time offer high energy density and long cycle life in order to ensure the flexibility, security and continuity of energy systems inevitable. Electrochemical energy storage systems have made significant progress in recent years to meet this requirement, and have been used as a key component in many applications ranging from portable devices to electric vehicles to grid-scale storage solutions. Among these systems, lithium-ion batteries have become the most widely used energy storage technology in the commercial market due to their high energy density, long cycle life, high efficiency and low maintenance requirements. Especially since the introduction of the first commercial Li-ion battery with LiCoO₂ cathode and graphite anode in 1991, this technology has developed rapidly and has become the standard in many fields, most notably in portable electronics. However, the sustainability and long-term adequacy of these technologies are increasingly being questioned as a result of multiple structural and economic constraints. In particular, electrode structures based on the intercalation mechanism have approached their theoretical capacity limits, suggesting that lithium-ion batteries have reached saturation level in terms of energy density. At the same time, the high dependence on minerals, among which are lithium, cobalt and nickel, poses significant constraints in terms of economic and environmental sustainability, particularly due to the toxic nature of cobalt and the increasing costs of these elements. In parallel with increasing energy demand and emerging technologies, the expectations for higher capacity, light weight and safety of next generation applications that include electric vehicles and portable electronics make the need for alternative battery systems that go beyond current lithium-ion technology inevitable. In this context, Li-S batteries are one of the most attractive alternatives for advanced energy storage applications. Sulphur, which is used as the active material in these systems, is a low-cost, environmentally friendly and abundant element in the earth's crust and has a high theoretical specific capacity and energy density. However, some fundamental problems in the practical application of Li-S batteries remain unresolved. First of all, both elemental sulphur and the discharge product Li₂S exhibit poor electrical and ionic conductivity. This insulating structure makes it difficult to use sulfur electrochemically effectively; it also causes passivation by precipitating on the cathode surface and slows down the reaction kinetics. For this reason, sulphur is often used in combination with conductive carbon, polymer or metal matrices; however, the addition of these structures can have a negative impact on the specific capacity as it reduces the radio of active material. Besides that, the high solubility of lithium polysulfide species, which are formed as intermediate products during the discharge process, in commonly used organic electrolyte environments causes serious problems for the stability of the system. The "shuttle effect" caused by the continuous movement of soluble polysulfides between the anode and cathode leads to loss of active sulphur, decrease in coulombic efficiency and rapid increase in capacity loss. Furthermore, the migration of these soluble species to the lithium anode surface where they precipitate as insoluble Li₂S and Li₂S₂ compounds forms a passive layer on the anode surface, limiting ion passage. Due to the non-conductivity of sulphur, reduction reactions can only take place on conductive surfaces. In this context, the presence of soluble polysulfides favors this process on the one hand, but on the other hand, it leads to uneven redistribution of sulphur and an increase in impedance in the cathode structure. In addition to all these processes, about 80% volumetric expansion occurs on sulphur particles during the conversion from elemental sulphur to Li₂S in Li-S batteries. This high volume change causes severe mechanical stresses in the electrode structure during charge-discharge cycles. As a result, the active sulphur particles separate from the conductive matrix and become electrochemically isolated, resulting in capacity losses. Designing appropriate cavity structures in the cathode architecture to maintain the structural integrity of the electrode and absorb volume variations is critical to mitigate these limitations. In order to overcome these limitations, various dopants have been integrated into sulphur cathodes to improve both electrical conductivity and structural stability. Structures such as carbon nanotubes, graphene, conducting polymers and metal oxides have been investigated in this direction. However, these materials have not fully met the expectations due to their limited interaction with sulphur, low adsorption capacity, unstable interaction with the electrolyte and inadequacy in cycle stability. MXene materials, which have 2D layered structures derived from transition metal carbides and nitrides in recent years, are among the most attractive additives in Li-S battery systems thanks to their unique structural and surface properties. Their high electrical conductivity, large specific surface area, hydrophilic character and the presence of functional groups such as -O, -OH and -F on the surface make MXenes multifunctional structures that can strongly interact with lithium polysulfide species through both physical adsorption and chemical bond formation. These interactions play a critical role in suppressing the "shuttle effect" that occurs during the charge-discharge cycle and causes loss of active substance. Furthermore, the layered structure of MXene maintains the mechanical stability of the cathode structure by buffering against the volumetric expansion that occur during the conversion of sulphur to discharge products; it also increases the kinetic efficiency of redox reactions by facilitating ion and electron transport pathways. Thanks to this comprehensive functionality, MXene structured materials have been widely researched in the literature as advanced cathode hosts and interlayer materials for Li-S batteries. In this thesis, Ti₃C₂Tₓ, a highly conductive MXene derivative, was successfully synthesized from Ti₃AlC₂ MAX phase by selective etching method using hydrofluoric acid (HF) and then composited with sulphur and used as cathode material. The synthesis of the composite structure was carried out by wet chemistry method and the morphological and crystal structure of the obtained Ti₃C₂Tₓ/S structure was analyzed in detail by SEM and XRD techniques. XRD analyses showed that the characteristic diffraction peaks of the Ti₃AlC₂ phase disappeared after HF acid treatment and were replaced by diffraction patterns specific to the Ti₃C₂Tₓ phase. SEM images revealed an accordion-like multilayer structure with sulphur particles homogeneously dispersed between the MXene layers. This structure enables not only better dispersion of the active substance, but also increased surface interaction and more efficient diffusion processes. Electrochemical evaluations were carried out in CR2032 type coin cells and confirmed by galvanostatic charge-discharge tests. According to the results obtained, the Ti₃C₂Tₓ/S composite cathode offered an initial discharge capacity of 1030 mAh/g at 0.1C current density, while the comparative pure sulphur cathode showed a capacity of 745 mAh/g. After 100 cycles, the composite structure retained 70% of its capacity, offering a significant advantage in terms of cycle stability. In conclusion, it has been demonstrated that the Ti₃C₂Tₓ/S composite structure developed in this thesis can effectively mitigate the main limitations faced by Li-S batteries. The composite structure offers high capacity performance by balancing the electrical insulativity of sulphur, reduces the loss of active material by limiting the shuttle effect, and increases the cycle life. In this context, the developed Ti₃C₂Tₓ/S composite cathode structure is considered as a potential cathode material for both academic research and advanced energy storage applications.