Preparation And Application Of Carbon-based Functional Separator For Lithium-sulfur Battery

Batteries are the power source of electronic information systems.Lithium-ion batteries,as an efficient energy storage device,have been widely used in mobile electronic devices,large energy storage systems,new energy vehicles and other fields.However,with the rapid development of electric vehicles and electronic equipment,people put forward higher requirements on the energy density and cost of batteries.In many new secondary battery systems,the theoretical specific capacity of Lithium-sulfur battery is 1675 m Ah g^-1,the theoretical energy density theory is 2600 Wh g^-1,is the traditional Lithium-ion battery 3-5times,and elemental sulfur has rich resources,easy mining,environmentally friendly characteristics,more economic and environmental protection.However,there are some problems such as low actual energy density,fast capacity decay,short cycle life and poor safety performance,which limit the further development of Lithium-sulfur batteries.To solve the above problems,researchers have carried out a large number of pioneering studies,such as the structure of sulfur host materials,electrolyte additives,separator modification optimization,etc.This paper mainly studies the modification direction of Lithium-sulfur battery separator.The traditional separator is modified by carbon matrix composite material,and the functional separator is endowed with the ability to adsorb lithium polysulfide,improve the conversion efficiency and protect the lithium negative electrode.The main research work is as follows:（1）Molybdenum carbide/Molybdenum nitride heterogeneous structure and 2D nitrogen-doped carbon composite（Mo₂C-Mo₂N/C）supported by one-step nitride structure were synthesized for the positive side modification of Lithium-sulfur battery separator.Using g-C₃N₄as nitrogen source and template agent,glucose as carbon source and ammonium molybdate as molybdenum source,Mo₂C-Mo₂N heterostructure was obtained by simultaneous carbonization and nitriding at high temperature through ball milling and high temperature.Meanwhile,glucose was carbonized with g-C₃N₄as template to obtain two-dimensional sheet structure.In this structure,on the one hand,N-C plays a physical adsorption effect on lithium polysulfide,and can also reduce the interfacial resistance and improve the utilization rate of sulfur.On the other hand,Mo₂C-Mo₂N can accelerate the conversion of lithium polysulfide by chemical adsorption,which can effectively inhibit the shuttle effect under the synergistic action of Mo₂C-Mo₂N.The battery with Mo₂C-Mo₂N/C functionalized separator had an initial discharge capacity of up to 1407 m Ah g^-1at 0.1 C,and maintained 690 m Ah g^-1capacity after 500 cycles at 0.5 C current density,and the capacity attenuation rate per cycle was only 0.07%.（2）In order to explore the feasibility of metal fluoride and fluorine doped carbon as modified materials for Lithium-sulfur battery separator,magnesium fluoride and fluorine doped carbon composite（Mg F₂/C）was synthesized in one step by mechanical grinding method.Using polytetrafluoroethylene as carbon source and fluorine source,carbon composite magnesium fluoride material was prepared by high energy ball milling with metal magnesium powder.It is proved by adsorption experiment and XPS test that as lithium polysulfide adsorption material,fluorine doped carbon has strong adsorption,acetylene black（AB）has weak physical adsorption,Mg F₂has almost no adsorption on lithium polysulfide.The battery with Mg F₂/C-AB functionalized separator has a high initial discharge capacity of1280 m Ah g^-1at 0.1 C,and a specific discharge capacity of 1097 m Ah g^-1at 0.5 C.After 350cycles,the capacity is still 630 m Ah g^-1,and the capacity decay rate per cycle is 0.12%.（3）In addition to the problems on the positive side,the unstable solid electrolyte interface（SEI）and lithium dendrite problems on the negative side of lithium metal need to be solved.Therefore,the obtained Mg F₂/C was used to modify the negative side of the separator to explore the influence of lithium metal deposition behavior.The results show that Mg F₂/C coating has the characteristics of lithium affinity,uniform current density and low surface diffusion barrier.The lithium deposition behavior of lithium copper battery using Mg F₂/C separator was analyzed,and the"reverse deposition"model was established.In the process of lithium deposition,the Li₂CO₃-rich SEI is first produced on the surface of Mg F₂/C coating,and a dendrite smooth lithium layer grows towards the negative electrode side,which significantly increases the deposition capacity of lithium.In the lithium copper battery with Mg F₂/C modified separator,the fixed capacity of 0.5 m Ah cm^-2was deposited and then fully stripped,achieving a 97%Coulomb efficiency stable cycle of 180 cycles,while the unmodified PP separator battery failed after 40 cycles.Then in the long cycle performance test,the battery was stably cycled for 800 h,extending the cycle life of the lithium copper battery by 10 times.