High Performance Lithium-sulfur Battery Cathode Material Is Constructed By Molten Salt Method

Secondary energy storage systems with low costs and environmentally friendly features are becoming more important as the energy density of traditional lithium-ion batteries can no longer meet people’s needs as demand for energy storage systems grows.Among the many conditions for energy storage systems,lithium-sulfur batteries（Li-S）are considered a promising next-generation energy storage system because of their high specific energy.However,the insulating properties of the active material S₈ and the discharge final product Li₂S,the irreversible loss of capacity due to the easy dissolution of intermediate products in the electrolyte,and the safety of the lithium cathode have been limiting commercialization.In this thesis,we accelerate the redox kinetics of the conversion process by modulating the structure and chemical components of the material,followed by physical characterization and electrochemical tests to illustrate the accelerated reaction mechanism.In order to further verify the reaction mechanism,we performed first-principle calculations on the adsorption model and diffusion model of the material,and the calculation results also verified the transformation mechanism.（1）Fe₃C encapsulated in nitrogen-doped carbon nanotubes was synthesized as an additive for lithium-sulfur battery applications using one-pot molten salt-assisted pyrolysis.Among them,graphene as a conductive network with strong chemisorption of N atoms improves the adsorption efficiency of Li PSs,while Fe₃C with good catalytic properties can accelerate the conversion of liquid Li PSs to solid Li₂S₂/Li₂S.The addition of the additive can significantly improve the cycling performance of the battery,and the initial discharge capacity of Fe₃C@NCNT/G/S at 0.5 C is 950 m Ah g^-1.The initial discharge capacity of Fe₃C@NCNT/G/S at 1 C was 674 m Ah g^-1（79%capacity retention）,and the cell decay rate per cycle was only 0.049%after 500 cycles.In addition,there is still a good capacity retention after 400 cycles at a load of 4 mg cm^-2 and a liquid-sulfur ratio of 7μL mg^-1.（2）Phosphorylation of Fe₃C@NCNT at various times can yield the Fe₃C-Fe₃P heterojunction structure.This structure can facilitate the balance of the intermediate product"adsorption-diffusion-transformation"of the active substance sulfur reaction process,which is beneficial to the cell performance;finally,the catalytic active center Fe₃C intermediate can enhance the redox kinetics and effectively improve the phase transition from liquid Li PSs to solid sulfur product Li₂S₂/Li₂S.The abundant active sites ensure the homogeneous arrangement of Li PSs,which significantly enhances the utilization of active material sulfur in lithium-sulfur batteries and improves the multiplicative performance of the batteries.Through the first principles calculation,it can be obtained that compared with Fe₃C,Fe₃P has a higher adsorption energy for polysulfides,indicating that it can be better adsorbed for polysulfide;the lithium ion diffusion barrier at the Fe₃P interface is higher,indicating that polysulfides can be more easily converted at the Fe₃C interface.Theoretical calculations and experimental results can be corroborated by each other.It can significantly improve the electrochemical performance of the battery,which can have a high initial specific capacity of 1370 and 1251m Ah g^-1 at 0.2 and 0.5 C,respectively,and an initial discharge capacity of 1095 m Ah g^-1 at 1C.After 300 cycles,it still has 706 m Ah g^-1,with a decay of only 0.11%per cycle.In addition,it has a decay rate of only 0.10%and 0.11%per turn at 2 and 5 C cycles.