Graphene Oxide-based Ordered Composite Coatings And Their Applications In Lithium-sulfur Batteries

The significant demand for carbon peaks and carbon neutrality and the increasingly exhausted fossil energy force people to develop renewable clean energy and develop a green and sustainable economy.Lithium-sulfur（Li-S）batteries have attracted the most attention due to their high theoretical specific capacity(1675 m Ah g^-1)and theoretical energy density(2600 Wh kg^-1).The elemental sulfur as cathode materials is abundant,non-toxic and environmentally friendly in nature.However,Li-S batteries still have some problems that limit their practical use.Among them,the most serious is the shuttle effect of polysulfides（PSs）,which causes irreversible loss of active substances and constantly corrodes lithium metal,reducing the battery’s cycle life.The current commercial polyolefin separator cannot effectively block the shuttle of PSs,so it is challenging to realize Li-S batteries’high energy density.To solve the defects of polyolefin separator,the separator for the Li-S battery was studied in this paper.The composite coating with the ordered structure was employed to modify the commercial separator.The issues of thermal stability,electrolyte wettability,and PSs barrier of the separator were solved by optimizing the coating’s composition and structural design.The main research results are shown as follows:（1）Based on the covalent/non-covalent interaction between graphene oxide（GO）and di（aminobenzo）[18]crown-6（DB18C6-2NH₂,CE）,graphene oxide-crown ether complex（GO-CE）was prepared.The GO-CE nanocomposite separator with compact and ordered structure and controllable thickness was prepared by a simple coating approach.The distribution of CE in the plane and between the GO layers effectively prevented the self-stacking of GO nanosheets,promoted the rapid infiltration and preservation of the electrolyte,and improved the diffusion rate of lithium ions.The dense and ordered GO coating can physically block PSs.Simultaneously,the positively charged amino-functional groups on DB18C6-2NH₂ molecules can generate electrostatic adsorption with negatively charged PSs(S_x^2-),thus realizing the dual restriction for PSs.The Li-S battery assembled with GO-CE₁₅composite separator has optimized rate performance and cycle stability.The discharge-specific capacity retention rate is 41%after 1000 stable cycles at 1 C,and the corresponding capacity decay rate is as low as 0.059%per cycle.（2）The ordered MXene/GO-CE composite separators with a physical barrier and electrochemical conversion functions were prepared using highly oriented GO liquid crystal to induce the ordered distribution of highly conductive MXene.The separator has good thermal stability and thermal diffusion ability,which improves the battery’s cycle stability under uneven heating.MXene/GO interpenetrates with the overlapped structure to expand the individual layer spacing and effectively prevent nanomaterials’self-stacking.The composite coating with excellent electrical conductivity accelerates the charge transfer kinetics and reduces the batterys’impedance,thus improving their utilization of the active material and the performances.The resulting electrochemical performances show that the Li-S battery with MXene/GO₂₀-CE₃₀ composite separator has excellent anti-self-discharge performance.After standing for 72 h,the discharge specific capacity decay rate is as low as 14 %.