Research On Utilization Improving Strategies Of Active Substances Of Cathode Materials In Lithium Sulfur Battery

As the most widely used energy source of mobile electronic devices,lithium-ion batteries have been gradually unable to meet the needs of people for high specific energy batteries.In many new battery systems,lithium sulfur battery has been widely concerned because of its high energy density.The traditional mechanism of intercalation and detachment of lithium-ion battery makes lithium ion only enter into the fixed position of positive and negative materials,which greatly limits its energy density.In lithium sulfur battery,the conversion mechanism of positive sulfur and the stripping deposition mechanism of metal lithium anode make the theoretical specific capacity of positive and negative electrode materials up to 1675 and 3860 m A h g^-1,respectively.Based on this,the theoretical gravimetric and volume energy density of Li-S battery are 2600 W h kg^-1 and 2800 W h L^-1,respectively,which are much higher than the 420 W h kg^-1 and 1400 W h L^-1 of traditional Li-ion battery.In addition,due to the abundant reserves and low price of sulfur in nature,lithium sulfur battery is expected to become a cheap and easily available energy storage device.However,the commercialization of lithium sulfur battery still faces some urgent problems,such as low conductivity of active materials and their discharge products,shuttle effect caused by soluble lithium polysulfide formed during battery charging and discharging.According to the principle of multi-step reaction of Li-S battery,in the discharge process,the solid elemental sulfur first becomes soluble lithium polysulfide,which leads to the shuttle effect.However,if lithium polysulfide is insoluble in the electrolyte,because sulfur and its discharge product Li₂S are both electronic and ionic insulators,the internal sulfur will not participate in the electrochemical reaction,resulting in low utilization of active materials.Therefore,there is a great contradiction between the shuttle effect and the sulfur utilization.How to find a proper balance between the two issues has become the main problem in the design of positive materials.Therefore,more and more researchers began to pay attention to the catalytic ability of the host materials,which can improve the kinetic dynamics of the conversion reaction of lithium polysulfide,accelerate the consumption of lithium polysulfide,improve the utilization of active substances and inhibit the shuttle effect at the mean time.The current research idea is to design composite host materials with good conductivity and strong adsorption capacity,on the one hand,to provide conductive network for sulfur and improve the utilization rate of active substances;on the other hand,to limit the free movement of lithium polysulfides in the electrolyte through the adsorption capacity of the host,so as to inhibit the shuttle effect.However,the adsorption capacity of the host material is limited.How to consume the lithium polysulfide adsorbed rapidly and make the polar material continuously adsorb lithium polysulfide becomes the breakthough point of the host materials design.In a battery,the electrochemical reaction rate mainly depends on the electron conduction rate,the ion transfer rate and the reaction rate of the reaction itself.In this paper,the entry point is to improve the kinetic rate of lithium polysulfide conversion reaction in the process of charge and discharge of Li-S battery.In order to improve the electrochemical performance of Li-S battery,the cathode host materials and catalysts are designed from the aspects of improving the transmission rate of electrons in the carrier,shortening the transmission distance of electrons and ions in the active material,designing a catalyst to improve the kinetic rate of reaction,and improving the transmission rate of electrons in the active material.1.Nitrogen doped porous carbon and CNTs composites with 3D structure were constructed by spray drying methode using organic sodium salt as precursor.CNTs worked as conductive network in the composites.Organic sodium salt can be used as carbon source,nitrogen source and etching agent at the same time.The composite has strong adsorption capacity and good conductivity.It can improve the performance of Li-S battery from two aspects of adsorption and accelerating electronic conduction.After500 charge and discharge cycles,the cathode material can still maintain a reversible capacity of 785 m A h g^-1.At the current density of 0.3C,the average capacity decay rate is 0.08%per cycle.At 2 C current density,a high discharge specific capacity iof 880 m A h g^-1 is achieved,which showes excellent rate performance.In addition,using S/NPC/CNTs as cathode material,a single-layered Li-S pouch cell with a nominal capacity of 200 m A h can supply power continuously with a current of 38.6 m A.The single-layered pouch cell can simulate the working environment of the electrode material in the actual battery,which further proves that the cathode material prepared by this method has a good application prospect.2.The cathode material was prepared by one step spray drying in a rapid and large-scale way using ammonium sulfide solution as the reaction medium.In the cathode materials,CNTs are used as the mechanical support framework and conductive network,and sulfur is uniformly coated on the surface of CNTs with nano-scaled thickness,which shortens the conduction distance of electrons and lithium ions in the active material,and significantly improves the sulfur utilization and the cycling stability.In addition,theoretical calculation shows that the introduction of a little titanium nitride nanoparticles in the material preparation process can significantly improve the adsorption capacity of the cathode material for lithium polysulfide and promote the cycle stability of the battery.The S/T/CNTs–70 samples prepared by spray drying showed a high specific capacity of 1465 m A h g^-1 at the current density of 0.1C.The discharge specific capacity at 0.3C is 1200 m A h g^-1.After 200 cycles,and the specific discharge capacity remains at 842 m A h g^-1.In addition,a Li-S pouch cell with a nominal capacity of 2 A h was assembled by using S/T/CNTs–70 samples as cathode materials.The pouch cell can make 31 parallel light-emitting diodes work normally,indicating that the one-step spray drying method described in this paper has a good practical prospect.3.Under the guidance of theoretical calculation based on density functional theory(DFT),cobalt hexadecachlorophthalocyanine(CoPcCl)was selected as the electrolyte additive which function as soluble single atom catalyst for the first time in Li-S battery.When the catalyst molecules are dissolved in the electrolyte,the kinetic rate of lithium polysulfide conversion is increased,the energy barrier of Li₂S decomposition is reduced,and the nucleation of Li₂S during the discharge is promoted,so as to improve the utilization of sulfur.Compared with the host material with complex structure,the modification strategy of electrolyte additive is very easy to realize.As the catalyst dissolves in the electrolyte,the catalyst can contact with the active substance in the whole electrochemical reaction process,which significantly improves the sulfur utilization.4.The carbon-based host material with conducting network was prepared,and the phthalocyanine single atom catalyst was introduced on the surface of the carbon spheres.The performance of Li-S battery can be improved from three aspects:improving the intrinsic conductivity of active materials,building the external conductive network,adsorption and catalysis of the host materials.A hollow carbon sphere where CNTs was uniformly dispersed in was prepared by spray drying method.The electronic conductivity of the host was better,and the cathode material prepared had higher utilization of active materials.After CoPcCl catalyst was loaded on the surface of carbon host,sulfur utilization and the cycling stability of sulfur based cathode material have been greatly improved because of its better catalytic ability for the conversion of lithium polysulfide and its strong adsorption ability.In a battery,part of CoPcCl catalyst is dissolved in the electrolyte,which ensures the continuous contact between catalyst and active substance in the whole electrochemical reaction process.At the same time,the electrochemical properties of the Se S₂based cathode material which prepared by loading Se S₂ into the host materials show that the conductivity of the active material itself has a great influence on the improvement of the utilization rate and rate performance of the active material.