Preparation Of MXene-based Gel-state Multifunctional Sulfur-fixing Carrier And Research On The Performance Of Lithium-sulfur Batterie

Lithium-sulfur battery is an energy storage system which takes advantage of electrochemical reaction between sulfur and metallic lithium to convert chemical energy and electric energy.Because of its high theoretical energy density(2,600 Wh kg^-1),it has become one of the most promising battery systems of the next generation.However,the complex multi-electron conversion reaction mechanism of lithium-sulfur battery will lead to the solid-liquid-solid phase transition of elemental sulfur during charging and discharging,and the intermediate of polysulfide（LiPSs）will be dissolved in the electrolyte and diffused at the anode and cathode under the drive of concentration gradient and electric field.This phenomenon will lead to the decrease of coulombic efficiency and short lifespan of battery.At the same time,the insulating properties of active material sulfur make it difficult to be directly used as the electrode host,and the volume expansion and contraction during the discharge-charge process will lead to the collapse of the structure of sulfur cathode.The above problems have seriously hinder the commercialization process of lithium-sulfur batteries.Thus,the key to promote the practical process of lithium-sulfur battery is to suppress shuttle effect and improve conductivity of sulfur,develop materials suitable for lithium-sulfur battery.Therefore,in this paper,based on the suppression of shuttle effect,polar MXene（Ti₃C₂）is used as the matrix material to construct a gel-like multifunctional sulfur-fixing carrier,which can suppress the shuttle of LiPSs,accelerate the kinetics of redox reaction,improve the stability of electrode structure,and realize high cycle specific capacity and long cycle life of lithium-sulfur battery under multi-scale and multifunctional coupling.The main research contents of this paper are as follows:Firstly,we took advantage of the similarity of two-dimensional lamellar structure and unique material characteristics of GO and MXene to construct a three-dimensional multifunctional GO/MXene gel-like sulfur-fixing material（GM）by gel method.And the material was applied as sulfur cathode.In this strategy,two-dimensional（2D）GO sheets and highly conductive MXene nanosheets are integrated into a 3D porous gel structure,in which a 3D conductive network and a large polar surface of MXene can simultaneously accelerate ion/electron transport,enhance chemical anchoring of sulfur and promote redox reaction between LiPSs.Therefore,GM based cell realizes the high utilization rate of sulfur components and long cycle life of the battery through the multifunctional cooperation of physical structure and chemical adsorption.The prepared GM/S electrode battery provides a high initial capacity of 1255.62 m Ah g^-1 at 0.1C,and the capacity remains 615.7 m Ah g^-1 after nine months of cycling.Secondly,aiming at the limitation of disordered structure in accelerating ion migration and LiPSs transformation,sodium alginate（SA）/MXene gel sulfur-fixing material（SM）with vertical array structure was constructed by sol-gel and ice template method.In this strategy,the pore control of SM materials is realized by using the ability of SA to control the growth of ice crystals,and the rich pore structure can effectively provide physical confinement to LiPSs.The highly active polar surface of MXene（Ti₃C₂）on the hole wall can effectively anchor LiPSs and inhibit the shuttle effect.At the same time,the vertical array constructed by the ice template method provides a linear transport path for ions,which effectively promotes ion diffusion and electron transfer,thus accelerating redox kinetics.Therefore,the prepared SM/S electrode battery has high reversible specific capacity(the initial specific capacity is 1359.72 m Ah g^-1 at 0.1C),excellent rate performance(484.67 m Ah g^-1 at 3C)and long cycle stability（the cycle attenuation rate per cycle is only 0.0586%at 0.5C）.Finally,in order to solve the problems of slow conversion rate of LiPSs and poor cycle stability of the battery,a carbonized polyacrylic acid（PAA）/MXene gel sulfur-fixing material（CPM）with Ti₃C₂@Ti O₂ heterojunction was constructed by sol-gel method and in-situ conversion process.The three-dimensional structure constructed by two-dimensional lamellar MXene（Ti₃C₂）not only improves the overall conductivity of sulfur-loaded cathode as a conductive skeleton,but also provides more chemical adsorption sites to limit the dissolution and shuttle of LiPSs.In addition,the in-situ formation of Ti₃C₂@Ti O₂ heterostructure on the surface of the lamellar is beneficial to electron transfer and can effectively promote the conversion reaction of LiPSs.Therefore,combining the association mechanism of adsorption-capture-transformation,CPM gel material greatly inhibits the shuttle of LiPSs.When the composite material is used as a sulfur cathode frame,it shows a high specific capacity of 1261.41 m Ah g^-1 at a rate of0.1C.In addition,at the rate of 1C,the specific capacity of 66.5%is maintained after500 cycles.