The Application Of Pollen-based Carbon Material In The Cathode Of The Lithium Sulfur Battery

With the over-exploitation and utilization of fossil energy,the problem of environmental pollution has become increasingly severe.Renewable energy has become a research hotspot for scientists in the era of today.Because of the unstable and discontinuous characteristics of solar and wind power,it is more stable and efficient to use batteries for energy storage obviously.Lithium-sulfur batteries have a high energy density and theoretical specific capacity among all these battery systems.At the same time,as the cathode material,monomeric sulfur is easily accessible,cheap and naturally abundant.For this reason,lithium-sulfur batteries have attracted much attention from researchers.However,the elemental sulfur is poor conductivity and easy to occur volume expansion.The intermediate of the products in cathode usually occurs shuttle effect causing the performance of batteries unsatisfying and achieving large-scale popularization and promotion difficult which has also sparked the thinking of scientific researchers.The host material in cathode electrode is a binding framework of elemental sulfur,and meanwhile,carbon based host materials have good compatibility with sulfur.Combining these two materials can reduce the volume expansion of sulfur during discharge and improve the conductivity of the cathode electrode.At the same time,the recombination of carbon based material with the substance which has good inhibition of shuttle effects can improve battery performance better.In this thesis,we selected rapeseed pollens from a wide range of sources as carbon-based material to design and meanwhile choosed two substances（Mo₂C,BP）that significantly inhibit shuttle effects to synthesize.Combined with rapeseed pollen as the host material for the cathode,the material was obtained through sulfur loading.After assembling the batteries,it is characterized and tested for electrochemical performance.The specific research contents are as follows:（1）Three dimensional Mo₂C/N-C host material in cathode electrode is synthesized in one step using ammonium molybdate tetrahydrate and pretreated rapeseed pollen as raw material through high-temperature carbonization reaction.In this material,molybdenum carbide adsorbs polysulfide lithium through chemical adsorption effectively,limiting the shuttle effect;The unique three-dimensional porous structure accelerates the transfer rate of ions and electrons,and manwhile,it is conducive to the uniform loading of sulfur and accelerates reaction rates;This composite material contains mesopores and macropores.The presence of mesopores is conducive to the diffusion and transport of Li⁺,while the presence of macropores is able to provide channels for the electrolyte,which is beneficial for improving battery performance.The test results show that:at 0.2 C,the initial capacity of Mo₂C/N-C/S cathode is 1255.4 m Ah g^-1.After 200 cycles,and the capacity is 1105.4 m Ah g^-1,with a capacity decay of 0.05%per cycle;At 1 C,the initial discharge specific capacity is881.85 m Ah g^-1.After 300 cycles,the discharge specific capacity is 736.88 m Ah g^-1,and the average Coulombic efficiency is 99.4%.（2）Black phosphorus nanosheets prepared by the liquid phase stripping method is combined with carbonized rapeseed pollen to form black phosphorus nanosheet and carbonized pollen composite material（BP/N-C）.The results show that the carbonized pollen and black phosphorus nanosheets bonded through a stable C-P bond,indicating that black phosphorus is synthesized on the surface of carbonized pollen successfully.The black phosphorus on the surface can absorb polysulfide lithium（Li PSs）and convert them into Li₂S quickly,which inhibits the shuttle effect effectively.At the same time,the presence of mesopores carbon enhances the transmission rate of Li⁺.The test results show that:At 0.2C,the initial capacity of BP/N-C/S cathode is 1009.6 m Ah g^-1,and after 150 cycles,the capacity is 879 m Ah g^-1;At a current density of 1.0 C,BP/N-C/S cathode exhibits an initial discharge capacity of 755.66 m Ah g^-1.After 300 cycles,the remaining specific capacity is626.42 m Ah g^-1,and the Coulombic efficiency remain at a high level（99.6%）.