Design And Applications Of Carbon-Based Materials For High Volumetric Energy Density Lithium-Sulfur Battery

Traditional fossil fuels cause serious environmental pollution,and the development of clean energy is very important.With the rapid growth of new energy vehicles,drone and portable electronic products,traditional lithium-ion batteries have been unable to meet the requirements of long life.Lithium-sulfur batteries with multi-electron reactions have attracted much attention due to their high energy density.However,the polysulfide dissolved in the electrolyte to produce shuttle effect,self-discharge and loss of active material,which reduces the electrochemical stability and restricts the industrialization.In addition,the low tap density of cathode structure and the thick barrier layer have affected the volumetric energy density of lithium-sulfur batteries.For the above problems,this paper on the one hand has prepared the composite of expanded graphite with sulfur by ball-milling in the cathode.On the other hand,a hybrid dense coating of lithium titanate embedded in graphene and an ultra-thin and light barrier prepared by nanofiber have been designed.Finally,cycle stability and volumetric energy density of lithium-sulfur batteries have been improved.In this paper,firstly,we prepared expanded graphite at different scales,and sulfur was penetrated in expanded graphite by melt-diffusion.Using the ball-milling process could adjust the layer structure to reduce the contact of polysulfides to electrolyte and improve the tap density,thereby reducing the polysulfide shuttle effect and improving the cycle stability and volumetric energy density.It was found that EG75%expanded graphite/S cathode exhibited the best electrochemical performance due to the large specific surface area and high utilization of sulfur,and the performance of EG50%expanded graphite/S cathode got the most obvious improvement after ball milling.Secondly,in the modified separators,we got a fast lithium-ion transport in dense coating.The coating was prepared by mono-dispersed lithium titanate nanospheres uniformly embedded in graphene to effectively reduce polysulfide shuttle and enhance the volumetric energy density.Due to ingenious combination of the physical barrier from graphene and chemical adsorption of lithium titanate,coupled with lithium titanate to ensure the fast transport of lithium ions,cells using the dense coating of hybrid showed excellent electrochemical performance(697 mAh g^-1 after 500 cycles at 1 C with 85.7%capacity retention).Lastly,an ultra-thin and light barrier was prepared by nanofiber to minimize volume and enhance volumetric energy density,then studying the structural morphology and electrochemical performance of barrier with different contents.Due to the balance of physical block and ions transport by adjusting the coverage rate for holes on separator,combined with electrostatic repulsion for polysulfides,cells with nanofiber barrier exhibited the well electrochemical performance.