| Lithium-sulfur(Li-S)battery with high theoretical specific capacity(1675 mAh g1)and energy density(2600 Wh kg-1)is one of next-generation energy storage system.However,Li-S batteries are still difficulties in fast charging and long-cycle stability with high practical energy density.In this dissertation,as the two major issues for the practical application of Li-S batteries,we mainly focus on the interfacial reaction of cathode side.The cathode-electrolyte interface,including the interlayer,was regulated to obtain fast electron/ion conductive skeleton and the cathode-interlayer interface was regulated to maintain long-cycle stability with high sulfur loading/lean electrolyte.A method to measure the efficiency of interlayer materials/other inactive components and a strategy for high sulfur loading/lean electrolyte Li-S batteries were proposed,which can provide scientific basis for high power density,high energy density and long-cycle stability of Li-S batteries.Main results of this dissertation are as follows:(1)A vertically-aligned carbon nanotube(VACNT)was grown on the surface of aluminum foil by chemical vapor deposition,and vertically-aligned carbon nanotube/sulfur(VACNT/S)were prepared by melt impregnation method.The VACNT/S electrode can realize fast charging Li-S batteries,obtain long-cycle stability under high-rate,the electrochemical performances present nearly no decrease with the increase of electrode thickness.Compared with conventional coating electrodes,the VACNT/S electrode has following advantages:First,the VACNT array has ultra-high electronic conductivity,which can reduce the electron transfer resistance greatly and improves the utilization efficiency of sulfur.Second,the highly ordered structure of VACNT provides short and directional pathways for high-speed ion transport,which are crucial for the kinetics of electrode reactions and the thickness-independent electrochemical performances.Finally,the excellent mechanical properties of CNTs also provide enough space to accommodate the volume expansion of sulfur and maintain the integrity of the electrode,thereby achieving long cycle life.Therefore,the VACNT/S electrode can have stable cycling at 5 C,with 894 mA h g-1 in the first cycle,and a stable discharge capacity of 486 mA h g-1 after 400 cycles.The capacity decay rate is only 0.1%per cycle.(2)Boron nitride nanosheet/single walled carbon nanotube(BN/SWCNT)as an ultrathin and highly efficient interlayer(UHEI)was prepared for high sulfur loading/lean electrolyte Li-S batteries.UHEI possesses bifunctional Lewis acid/basis sites,high Li+conductivity,and highly efficient polysulfide conversion capability.Besides,UHEI facilitates the realization of high charge-discharge specific capacity and long-cycle stability with high sulfur loading/lean electrolyte.UHEI has the following advantages:First,UHEI is very light,thin and highly efficient,which improves the practical specific capacity of the battery.Second,UHEI inhibit the diffusion of polysulfides,accelerate the conversion of lithium polysulfides to Li2S/Li2S2,and improve the electrochemical performances of Li-S batteries with high sulfur loading/lean electrolyte.Third,UHEI facilitates the transport of Li+across the interlayer,significantly promote the redox kinetics of polysulfides conversion and enhance the efficiency of interlayer materials.Therefore,the Li-S pouch cells with UHEI@PP provide 6.6 mA h cm-2 after 100 cycles at 0.2 C rate,which is about 1.5 times of the cathode of normal lithium-ion battery.(3)Dual reaction mechanism interlayer(DRMI)was prepared based on a new strategy called "dual reaction mechanisms interface regulation strategy".The DRMI was composed of high conductive Ketjen Black(KB),SWCNT and 1,3,4-Thiadiazole2,5-dithiol(C2H2N2S3,DMTD),which can regulate both the "solid-liquid-solid" and"solid-solid" reaction process.The DRMI has the following advantages:First,the carbon-nitrogen double bond in the DMTD molecule can adsorb the long-chain lithium polysulfides generated by the "solid-liquid-solid" conversion process,while the thiol groups can in situ polymerize polysulfides and convert them into sulfur-containing polymer,which can further participate in the subsequent reactions and reduce the concentration of polysulfide ions in the electrolyte.Second,the conductive network formed by KB and SWCNT acts as a conduction and a support for the further transformation of the adsorbed lithium polysulfides and the sulfur-containing polymer.Next,DMTD molecules are polymerized during discharge and depolymerized during charge,and the DMTD molecules are reversible during the charge-discharge cycle.Finally,compared with interlayers by vacuum filtration,the DRMI was prepared by coating method,which is more conducive to large-scale production.Therefore,DRMI@PP can significantly improve the electrochemical performance with lean electrolyte(E/S ratio of 8 μL mg-1)and the charge/discharge specific capacity and cycling stability with high sulfur loading(3.5 mg cm-2). |
