Research And Development Of Sulfur Cathode For High-Performance Lithium-Sulfur Battery

With the increasingly prominent energy and environment problems, as well as the fast development of electric and electronic equipments e.g., the Electric Vehicles, the conventional Li-ion battery has been unable to fulfill the market demand. The persuit of advanced battery system with high energy density has become the hot spot in energy storage research. Lithium-sulfur (Li-S) battery with merits such as high energy density, low cost and environmental friendliness, is perceived as one of the most promising next-generation energy storage systems. However due to the poor conductivity of the active material, large electrode volume change during cycling, the solubility of the reaction intermediates and the accompanied "shuttle effect", Li-S battery exhibits low utilization of active material and poor cycling life, which inhibit its practical application.To address these problems, this dissertation mainly focuses on the structural improvement for sulfur cathode via the design and optimation of conductive additive and functional binder. The macroporous carbon (MPC) with 3D conductive network was synthesized and applied as conductive matrix for sulfur electrode. Compared to carbon materials with different dimensions, the 3D MPC with abundant pore structure and high specific surface area leads to uniform sulfur distribution, easy access to electron and ion, and good physical adsorption to polysulfides (PS), effectively inhibiting the dissolution and shuttling of PS. The influences of template amount, pyrolysis temperature, sulfur content in MPCS composite and sulfur loading in electrode are also studied to achieve improved electrochemical performance.The binder, as another critical part for sulfur electrode, is also studied to further improve the performance of Li-S battery.1) To address the PS shuttling, a PNS composite binder is applied in sulfur electrode. The synergistic effect of PVP and Nafion component constrains the PS inside the cathode and suppresses the dissolution and migration of PS. The addition of SiO2 further enhances the adorption to PS and also improves the Li+ conduction so as to obtain higer capacity and cycling stability.2) On the basis, mPBI was synthesized as a multi-functional binder for sulfur electrode. The excellent mechanical properties of mPBI ensure an highly stable electrode structure to hold the volume change during cycling, while the abundant functional groups in mPBI provide strong chemical adsorption to PS to inhibit the PS shuttling. Meanwhile the mPBI modifed separator was also prepared to build a PS block between cathode and anode, to hinder the penetration of PS through the separator and further suppress the PS shuttle effect. The obtained Li-S battery exhibites high coulombic efficiency above 95% even in electrolyte without LiN03, high areal capacity with raised sulfur loading, and good capability of working at high temperature.3) In order to avoid the usage of toxic organic solvent in electrode fabrication, an aqueous bio-derived Gum Arabic (GA) binder is applied for sulfur cathode. The physical and chemical characterizations reveal the uniform and stable structure of GA based electrode, which buffers the volume change during the cycling; the chemical interactions between GA and sulfur species endow the binder with great effect in inhibition for PS shuttling. The obtained Li-S battery exhibites long lifesapn with a high capacity of 841mAh g-1 after almost half a year’s cycling, and excellent rate capability of 460 mAh g-1 at even large current density of 10 C. This work offers a novel and effective idea for the developemnt of high electrochemical performance Li-S batteries, as well as a green and economical strategy for large-scale fabrication of sulfur electrode.Based on the above results, MPC conductive substrate and GA binder are combined to fabricate high-performance Li-S coin cell, which delivers a high capacity of 780 mAh g-1 at 1C rate even after an ultra-long 1000 cycles’s test. Soft package Li-S cell was also prepared to investigate the practicality of the aforementioned design. The obtained soft package cell is capable of 300 cycles’ deep charge-discharges, with areal capacity higher than that of the commercialized Li-ion battery, revealing a great application prospect.