Research On Highly Flexible Lithium-Sulfur Batteries Based On Key Materials And Devices

Traditional Li-ion batteries are facing problems due to the intrinsic limitation of their low energy density.As one type of promising Li battery,lithium–sulfur(Li–S)batteries have an advantage of high theoretical energy density(2600 W h/kg).Nevertheless,the highly insulating nature of elemental sulfur,the huge volume expansion of sulfur upon lithiation/delithiation,the notorious polysulfide shuttle and low sulfur loading are the main obstacles to widespread practical utilization of Li–S batteries.Thus,improving the sulfur loading of the cathode in Li-S batteries to enhance the energy density of the whole devices has been drawing researchers'sight during recent years.In this paper,we focus on surfur loading and energy density enhancement of the lithium-sulfur batteries through the improvement of both positive and negative electrodes of lithium-sulfur batteries and the preparation process.Firstly,we design and construct a pyrrole modified graphene aerogel foam(Py-GF)by a simple hydrothermal and freeze drying method as the sulfur host;secondly,we replaced Li metal anode with GP@Li and fabricated GP@Li/Py-NGF@S full cell;thirdly,we warp sulfur cathode with g-C3N4 film through spray method.Compared with traditional process,this method is highly scalable and flexible.The main contents and research results are as follows:(1)We design and construct a pyrrole modified graphene aerogel foam(Py-GF)by a simple hydrothermal and freeze drying method as the sulfur host,where pyrrole provides strong chemical bonding for polysulfide anchoring and graphene aerogel foam serves as a matrix to enhance the conductivity as well as increase the sulfur loading of the cathode simultaneously.The Py-GF@S cathode,with a high sulfur loading of about6.2 mg/cm~2,displays an improved initial specific capacity(1220 mA h/g at 0.2C and985.8 mA h/g at 0.5C)and cycle stability(capacity retention of 81%after 100 cycles at0.5C).We anticipate that the work described here will be helpful to develop Li–S batteries that meet the requirements of practical applications.(2)On the basis of the first experiment,ascorbic acid was added to the positive electrode to further increase the reducibility of graphene to improve the specific surface area and porosity and to improve the sulfur content of the positive electrode.In the negative electrode,we use graphene self-assembly technology to prepare graphene paper.We assemble Li-S full cell based on cathode and anode.The full cell,with a high sulfur loading of about 5.4 mg/cm~2,displays an improved initial specific capacity(990mA h/g at 0.5C),high energy density(540W h/kg based on the mass of full cell)and cycle stability(capacity retention of 87.6%after 400 cycles at 0.5C).(3)We prepared graphite carbon nitride sheet,and the use of spray technology in the positive pole piece surface prepared carbon nitride film.Through the visualization experiment and the electrochemical performance test,it is proved that the carbon nitride thin film can effectively inhibit the shuttle effect during the charge-discharge process and the self-discharge phenomenon during standing still.This is mainly due to the strong adsorption of polysulfides by carbon nitride.Compared with the traditional scraping and other technologies,spray technology can facilitate the simple and large-scale membrane modification,low cost and broad prospects for development.