Study On Design And Catalytic Performance Of Cobalt And Nickel Based Compound Cathode Materials For Lithium-Sulfur Batteries

Since,primary energy sources such as coal,oil and natural gas are non-renewable and their use causes environmental pollution,the proportion of fossil fuels in the future energy consumption structure is predicted to gradually decrease and the proportion of electrical energy is gradually increasing,driving the development of new battery systems.Due to the low cost of sulfur as the cathode active material,low environmental pollution and high theoretical energy density and theoretical specific capacity,lithium-sulfur batteries have attracted the attention of many researchers,and in the past decade or so,lithium-sulfur batteries have received a big bang-like rapid development and are regarded as energy storage batteries that can be widely used in the future.In addition to the significant advantages,there are problems such as poor electrical conductivity of cathode active substance sulfur and lithium sulfide,volume expansion due to different active substance densities during charging and discharging,shuttle effect of liquid polysulfide,and lithium anode dendrites.In order to solve the above defects,this thesis regulates the conversion behavior of lithium polysulfide by designing a catalytic sulfur cathode carrier and functional intercalation layer.The main studies include the following:1.The development of lithium-sulfur batteries is very promising,but they also face the problems of hindered polysulfide conversion and lithium dendrite growth.In contrast to previous reports using one or two catalysts to overcome these problems,we present here a catalytically active Ni₁₂P₅-Ni₂P interface,which is partially converted in situ from Ni₁₂P₅using a gas-phase phosphorylation method.The unique electronic structure of the crystal interface confers effective electrocatalysis of Ni₁₂P₅-Ni₂P for S species during reduction and oxidation and enhances the reaction kinetics.the Ni₁₂P5-Ni₂P interface also lowers the barrier to lithium formation and promotes uniform lithium deposition.the Ni₁₂P₅-Ni₂P@CNT/S cathode exhibits a high capacity of over 1500 m Ah g^-1 and excellent multiplicative performance up to.In addition,the full battery has a capacity of 1345 m Ah g^-1 at 0.2C and still has a capacity of 931 m Ah g^-1 after 500 cycles.This study provides an all-in-one substrate with an active Ni₁₂P₅-Ni₂P crystal interface,which is beneficial for the practical application of lithium-sulfur batteries.2.A two-dimensional conducting MOF（Co₃（HITP）₂）with abundant periodic Co-N₄centers was grown in situ on carbon fiber paper and used as a functional interlayer for lithium-sulfur batteries to promote conversion kinetics,thereby effectively suppressing the shuttle effect of polysulfides.Theoretical calculations show that the Co-N₄ center in Co₃（HITP）₂ has stronger electron density and more negative electrostatic potential distribution than the single-atom catalyst in the carbon matrix,thus lowering the Gibbs free energy and decomposition energy barriers and promoting the electrochemical performance.The optimized cell provides a high capacity of over 400 m Ah g^-1 at 4C current and a satisfactory capacity decay rate of 0.028%at 1C for 1000 cycles.The designed lithium-sulfur cell assembled with Co₃（HITP）₂ intercalation has excellent performance at high temperature conditions（45°C）:762 m Ah g^-1 reversible capacity and 79.6%capacity retention after 500cycles,which means it has great potential for practical applications.