Preparation And Electrochemical Performance Of Bacterial Cellulose/MOFs-Derived Carbon-Based Composites For Lithium-Sulfur Batteries

Lithium-ion batteries have become predominant in the electronic and power batteries markets for more than 30 years,but they can no longer to meet the growing demand for higher energy storage technology.Lithium-sulfur batteries are becoming substitutes for the traditional lithium-ion batteries because of their high theoretical specific capacity(1675 mAh g-1),low cost and environmental protection.However,the commercialization of lithium-sulfur batteries is hindered due to some critical issues,such as poor electronic conductivity of sulfur and discharge products(Li2S2/Li2S),the volume variation during charge and discharge process,and the shuttle effect of polysulfides,all of which greatly affect the electrochemical performance of Li-S batteries.In order to solve the problems of Li-S batteries,metal-organic frameworks(MOFs)and their derivatives have been widely used in the researches of Li-S batteries,which significantly enhances the electrochemical performance of Li-S batteries.But MOFs-derived nanoparticles are independent and unconnected,which severely hinders the transfer of electrons between them.In addition,these particles are prone to stacking and agglomeration,resulting in reduced exposed active sites and specific surface area,which also weakens the adsorption to LiPSs.Herein,the active sites of MOFs materials are fully utilized from the structural design of materials,the polar co-based sulfide heterostructural particles anchored on 3D hyperfine carbon nano fiber(CNF)networks with the hierarchical structure from ZIF-67 and hyfine networks structure bacterial cellulose(BC)as raw materials are manufactured by the in-situ growth technology,hydrothermal method and high temperature heat treatment,which is used to synthesize the functional modified separator and self-supporting flexible electrode of Li-S batteries.The surface of BC is rich in hydroxyl functional groups,and the CNF derived from BC by high-temperature carbonization has excellent electrical conductivities,not only alleviating the stacking and agglomeration problems of ZIF-67 particles effectively,but also accelerate the transfer of ions and electrons between nanoparticles.The heterostructure of cobalt-based sulfide derived from ZIF-67 display a strong chemisorption for polysulfides and the CNF has excellent conductivity,which effectively alleviate the shuttle effect of polysulfides and improve the electrochemical performance of Li-S batteries.The main research contents are as follows:(1)ZIF-67 particles are grown on the surface of BC by an in-situ growth method,then BC/ZIF-67 is calcined at 700℃ and surfurized by thioacetamide.A composite material(CNF/Co-Co9S8-NC)derived from polar Co-Co9S8-nitrogendoped carbon(NC)particles anchored on a three-dimensional hierarchical network CNF is obtained,and used to modify the commercial polypropylene separator of Li-S battery.The effect of material structure and composition on electrochemical properties is investigated by some material characterization techniques and electrochemical measurements.The results show that ZIF-67 grow uniformly on BC surface,which effectively alleviates the agglomeration problem of ZIF-67 particles.The result of first-principles theoretical calculation shows that the heterostructure of Co and Co9S8 has strong adsorption and binding energy for Li2S6,which can effectively adsorb polysulfides.The initial discharge capacity of Li-S battery with CNF/Co-Co9S8-NC modified separator reached 1582.8 mAh g-1 at 0.1 C current density.The discharge capacity is 891 mAh g-1 after 100 cycles at 0.2 C with a capacity attenuation rate is 18.1%.(2)On the basis of the above experiments,in order to completely transform the Co component in the compound into cobalt sulfide,which has stronger adsorption and catalytic effect on polysulfides,the CNF/CoxSy-NC composites with heterostructure from BC/ZIF-67 as raw material are manufactured by the onestep vulcanization and carbonization with sulfur powders under the atmosphere of argon,and used as functional modification material for the separator of Li-S battery.The effects of heat treatment time on the material structure,composition and electrochemical properties is studied by material characterization techniques and electrochemical measurements.The results show that the high-temperature heat treatment time deeply affects the material composition and structural stability.When the heating time is 2 h,the CNF/CoS-Co9S8-NC composites with heterostructure and outstanding structural stability can be obtained.The Li-S battery with CNF/CoS-Co9S8-NC-2h separator can maintain a lower capacity attenuation rate of 17.9%after 100 cycles at 0.2 C,thus the capacity maintenance rate is higher than Chapter 3.This is due to the polar adsorption of CoS,the high electron conductivity and electrocatalytic activity of Co9S8,which effectively limit the shuttle effect of polysulfides and promote the rapid conversion of LiPSs to Li2S2/Li2S.(3)Based on the CNF/CoS-Co9S8-NC-2h composites prepared in the above experiments,a simple and efficient double-layer flexible self-supported electrode material(BC/CNT-S/CNF-CoxSy-NC)from carbon nanotubes encapsulate sulfur(CNT-S),BC and CNF/CoxSy-NC as raw materials is manufactured.The results show that the designed BC/CNT-S/CNF-CoxSy-NC self-supported electrode has a high sulfur loading(4.5 mg cm-2),which shows excellent structural stability and excellent electrochemical performance.The BC/CNT-S/CNF-CoxSy-NC-2 selfsupported flexible electrode delivers a high initial capacity of 1243.5mAh g-1 at 0.1 C when the thickness of CNF/CoxSy-NC layer is 30 μm.