Study On Composite Materials Of Lithium-sulfur Based On Porous Carbon Support Materials

Lithium-Sulfur batteries are considered to be the most promising next-generation lithium secondary batteries because they have high discharge specific capacity of 1675mAh/g and a theoretical energy density of 2600 Wh/kg.Compared to traditional lithium-ion batteries,lithium-sulfur batteries have 3-5 times energy density.In addition,elemental sulfur has the advantages of low cost,easy availability and environmental friendliness.However,the current energy density of lithium-sulfur batteries is much lower than its theoretical value,and the commercial application of lithium-sulfur batteries are extremely difficult.There are many problems such as low utilization of active materials and low conductivity of sulfur.Thus,in order to realize the commercial application of lithium-sulfur batteries,various materials have been explored for the design and construction of sulfur-based composite materials.Based on the existence of sulfur cathodes in lithium-sulfur batteries,this paper has carried out a series of work to construct new types of sulfur-based composites.In this paper,a simple spray drying technique was successfully used to prepare different composites,which was treated as host for lithium-sulfur batteries.The results showed that the performance of lithium-sulfur batteries was improved to some extent.The main research results of this paper are as follows:(1)Based on the principle of secondary use of waste,outdate milk was selected as a raw material for the construction of a new type of carbon material carrier.Combined with the template method and spray drying technology,waste milk is converted into hollow porous nitrogen-doped carbon material(HNMCM).The carbon material prepared by this method has both hollow and porous structure,which is favorable for the loading of sulfur.The rich nitrogen of milk can achieve nitrogen doping in the material,greatly improving the conductivity of the electrode.The material was used as a host for lithium-sulfur batteries.The initial discharge capacity was 781 mAh/g at a current density of 0.5 C,and the reversible specific capacity was 634 m Ah/g after 100 cycles.In addition,raw materials are cheap,easy to use and waste resources are used twice.This avoids the waste of resources and allows for large-scale production.(2)A pomegranate-like TiN@graphene material(PTG)was constructed by spray drying.This structure successfully distributed hollow titanium nitride spheres inthree-dimensional graphene.The titanium nitride inside the material and the graphene coated on the surface of the titanium nitride,which can achieve the dual effect of polysulfide in physical adsorption and chemisorption.Electrochemical test results show that the PTG carrier can significantly increase the discharge specific capacity of the lithium-sulfur battery,and the cycle performance of the battery is stable.At 0.5 C rate,the capacity was maintained at 810 mAh/g after 200 cycles,which was significantly higher than that of graphene(PG)materials with similar structures.The significant improvement in the performance of the battery is mainly ascribed to the excellent conductivity of the graphene and the good binding of the polar titanium nitride to the polysulfide.(3)The vanadium nitride quantum dot is attached to the three-dimensional graphene skeleton by a spray drying method and template method.A series of characterizations of the electrode material was conducted to discuss the main factors affecting the electrochemical performance of the battery.The experimental results show that the polar vanadium nitride has a positive effect on inhibiting the shuttle effect of the lithium-sulfur battery and can play a role in capturing the active material.The capacity up to 1498 m Ah/g after the activation process was completed at 0.2 C current density,and the capacity was stabilized at 960 m Ah/g after 200 cycles.