Screening And Design Of Cathode Materials For Lithium-Sulfur Batteries

Lithium-sulfur batteries have the theoretical specific capacity of 1675 m Ah g^-1 and theoretical energy density of 2600 Wh kg^-1,which is much higher than the current commercial lithium-ion batteries.Besides,sulfur,as the active material of cathode,is abundant in the earth and has the advantages of low price and environmentally friendly,so it is considered as the most attractive candidate for next-generation energy storage.However,lithium-sulfur batteries still have serious problems need to be solved:the charge/discharge products?S₈/Li₂S?have poor electronic conductivity and the dissolution of polysulfides in liquid electrolytes,leading to the"shuttle effect".Moreover,sulfur and Li₂S have a large difference in density,which causes the electrode to have serious volume expansion,poor cycling performance,low utilization of active materials,and safety problem and so on.Based on above problems,the main solution is to screen host materials with strong adsorption strength,good conductivity,and large specific surface area,thereby improving cycling performance and inhibiting the dissolution of polysulfide molecules.In this project,the electrochemical activity of transition metal organic framework?TM-BHT,TM=transition metal,BHT=benzenehexathiol?in lithium sulfur batteries is first studied.To speed up the screening of cathode materials,surface electron affinity and differential atom electronegativity are defined as descriptors.The related research work is as follows:?1?Metal organic frameworks are used as cathode materials for lithium-sulfur batteries.Using 3d transition metal atoms to substitute metal sites in metal organic framework as initial structures?TM-BHT?.Then,based on DFT calculation methods such as structural relaxation,DOS,Bader charge,and AIMD,Co/Ni/Cu-BHT are predicted to be ideal cathode materials for lithium-sulfur batteries.For their moderate adsorption strength,good electronic conductivity,and structural stability.In addition,it is shown that the energy barriers of the decomposition of S₈/Li₂S_n?n=2,4,6,8?on Co/Ni/Cu-BHT are0.91 e V,0.8 e V,and 0.87 e V,respectively,which are lower than 1.03 e V of graphene.Which is attributed to the stronger chemical interaction between Co/Ni/Cu-BHT and lithium polysulfides,especially short-chain polysulfides.In addition,the p-d-?electron conjugated framework and abundant electrochemically active sites not only can keep cathode conductive during the charge/discharge process,but also can be synergized discharged with the sulfur.?2?Establishing descriptors to speed up the screening of cathode materials.First,the correlation between the adsorption strength and charge exchange is calculated based on DFT.Then,we define the surface electron affinity energy to describe the strength of chemical interaction between the surface of substrate and polysulfide molecules.The surface electron affinity energy range?-2.74 e V?-7.65 e V?is determined by using desorption energy of Li₂S₈ and cohesion energy of S₈,respectively.To verify the rationality of the above descriptors.The Experimental results of Ti S₂,Ti O,Ti N,and CNT systems prove that Ti O with surface electron affinity within the above range has the excellent electrochemical cycle stability.Furthermore,based on the linear correlation between the electron affinity and the differential atomic electronegativity,we define screening ranges of differential atom electronegativity for host materials MX/MX₂ are 0.87-0.94 and 1.12-1.2,respectively.The prediction results show that divalent metal oxide,tetravalent transition metal sulfide,selenide and carbide can be used as the cathode material of lithium sulfur batteries with excellent electrochemical performances.