Efficient Construction Of The Cathodes In Lithium-sulfur Batteries And Research On Their Properties And Mechanisms

With the continuous development of social economy,people put forward the higher requirements for energy storage equipments with high energy density and long endurance capacity.At present,the energy density of the latest lithium-ion batteries has reached the threshold,and it is difficult to have a large room for improvement.Therefore,it is urgent to develop the new energy storage systems with higher power densities.Among all kinds of energy storage systems,Li-S battery stand out among all kinds of energy storage systems because of its high theoretical specific capacity(1675m Ah g^-1)and energy density(2600 Wh kg^-1),which are several times of the latest theoretical specific capacity and energy density of lithium-ion battery.It becomes one of the most promising preferred alternative systems for the next generation high energy density energy storage technologies.In addition,Li-S battery,which uses elemental sulfur as cathode materials,also has the advantages of abundant sulfur resources,low cost and environmentally friendly.Therefore,Li-S battery is one of the most promising alternatives to traditional lithium-ion battery in the field of energy storage.However,Li-S battery is faced with some problems and challenges to be solved,such as the low conductivity of elemental sulfur and its volume expansion in the cycling,shuttling effect of polysulfides(LiPSs)and it's dissolved into the electrolyte,the discharge end product of Li₂S deposition onto the anode and its volume change,the sluggish LiPSs transformation kinetics and lithium dendrite growth.These problems and challenges simultaneously result in the low sulfur utilization of active substances and poor Coulombic efficiency of Li-S batteries,which leads to the capacity attenuation of the battery and seriously restricts the commercial applications of Li-S batteries.In order to effectively deal with and solve the challenges and problems faced by Li-S batteries,the majority of researchers have proposed a number of strategies to synchronously improve the comprehensive performance of Li-S batteries.In this paper,starting from the design of key materials for Li-S battery,a series of S cathode and Li anode host materials were efficiently constructed by using the experimental method combined with the theoretical calculations,which synergetically solved the above problems and challenges restricting the commercialization progress of Li-S battery,and provided a guide for the design and synthesis of key materials for Li-S battery.The main points of this paper are summarized as follows:(1)A self-assembly strategy was adopted to construct N-doped carbon coated WS₂and Co₉S₈(WS₂@NC and Co₉S₈@NC)hierarchical structures to explore the regulation mechanism of d band of metal cations on the interfacial redox and conversion kinetics of LiPSs.This self-assembly strategy has a high degree of versatility.By adjusting the amount of NH₃×H₂O,different tissue structures(such as flake-like,carambola-like,flower-like and sandwich structure)can be obtained.Among all kinds of tissue structures,the flower spherical structure shows the best electrochemical performance.At the same time,compared with Co₉S₈@NC,the WS₂@NC shows more excellent electrochemical performance.The mechanism study shows that the better electrochemical performance of WS₂@NC is due to the obvious shift tendency of the d band of W atom in WS₂towards the Fermi level and more charges compensation from the d band into the adsorbed LiPSs molecules,which promotes the interfacial electron transfer kinetics.(2)MS₂(M=Co and Ni)polyhedral nanoparticles were constructed in situ by template carbonization combined with hydrothermal method and decorated in situ on N,S co-doped hierarchical porous carbon(GC)derived from natural ginkgo-nut to obtain3D electrocatalytic cathode material(GC-MS₂).Compared with Ni S₂,Co S₂can be used as a more effective electrocatalyst to enhance the redox kinetics of active sulfur and promote the conversion kinetics of LIPSs.Adsorption experiments and DFT theoretical calculations revealed that Co S₂has a stronger adsorption capacity for LIPSs and a lower diffusion energy barrier for Li⁺.(3)Co₉S₈nanoparticles protected by N-doped thin carbon layer chainmail and uniformly embedded in CNT-supported N-doped carbon nanofiber composite(CNT@NC/Co₉S₈)was constructed as a kind of chainmail-like catalyst by adopting a simple electrospinning route combined with gas phase sulfurization strategy.This composite material has good flexibility.It can be used as a self-supporting cathode electrode without using collector,and its sulfur loading can reach up to 50 mg cm^-2.The mechanism analysis showed that the chainmail-like catalyst promoted the charge redistribution on the surface of the carbon protective layer under the induction of the electron contact between Co₉S₈and the carbon layer,thus enhancing the anchoring effect of LiPSs,and promoting the redox and conversion kinetics of LiPSs.(4)Fe₂N-VN heterostructure nanoparticles uniformly embedded in a hierarchical porous N-doped carbon nanofiber scaffold(Fe₂N-VNIPNCF)dominated by the ordered macroporous structures was constructed as a flexible self-supporting sulfur host material for Li-S battery cathode materials under high sulfur loadings.This rationally designed heterostructure combines the advantages of the strong adsorption capacity of VN towards LiPSs and the better catalytic conversion capacity of Fe₂N towards LiPSs.By the synergistic effect between VN and Fe₂N,which synergistically enhanced the anchoring effect of LiPSs and the redox and conversion kinetics.First of all,the hierarchical porous structure dominated by the macroporous structures provides sufficient space for the loading of sulfur,which can achieve a higher loading of sulfur,and at the same time alleviate the volume expansion of active sulfur in the charge-discharge process.On the other hand,uniformly inlaid Fe₂N-VN nanoparticles heterostructures provid the sufficient chemical binding sites and catalytic conversion sites for the adsorption and diffusion of LiPSs,which synergically enhanced the chemical anchoring and redox and conversion kinetics of LiPSs.(5)Vertically grown Ni Fe-LDH nanoarrays on carbon cloth by one-pot hydrothermal method,and then transformed them into Ni₄N-Fe₂N heterostructures was constructed through the subsequent carbon coating and nitridation processes(CC@Ni₄N-Fe₂N@C).These heterostructures have good sulfiphilic and lithiophilic properties and can be directly used as host materials both for S and Li.Based on this,the doping of In element was introduced,which was doped into Ni₄N-Fe₂N heterostructure.When used as the host material for S cathode,the performance of the half cells shows that the 6%In doping shows the best electrochemical performance.Research on symmetric cells show that 6%In has the lowest overvoltage and the best cycle stability among the different doping amounts.When used as host materials both for Li and S,the performance of the assembled full cells is significantly better than that of the half cells,which further confirms that In doping can more effectively inhibit the growth of lithium dendrites,thus can obtain the better electrochemical performance.