Application Of Porous Biochar Loaded With Fe₃C Particles In Lithium Sulfur Battery

Lithium sulfur battery has the characteristics of high theoretical specific capacity(1675 m Ah g^-1)and theoretical energy density(2600 Wh kg^-1),low cost and good safety,which is expected to become the next generation of energy storage devices.However,the conductivity of elemental sulfur in lithium-sulfur battery cathode material is only 5×10^-30 S cm^-1 at room temperature,and the active material sulfur will produce serious volume deformation and strong shuttle effect during charging and discharging.The bio-porous carbon material has good electrical conductivity and can effectively adsorb polysulfides,which can better compensate for the defects of the sulfur cathode.Based on this,this article regulates the microstructure of the bio-carbon material,and makes the cathode material have the dual effects of physical confinement and chemical adsorption.The structure,electrochemical performance and mechanism of the lithium-sulfur battery cathode material are studied.The specific research content is as follows:（1）In order to inhibit the shuttle effect of polysulfides,potassium ferrate was used to activate the iron leaves,and the Fe₃C doping was realized at the same time of activation.The Fe₃C particles-loaded massive porous bio-carbon composite Fe₃C/PB was prepared in one step.Subsequently,the melting method was used to compound with sulfur,and its structure and performance were characterized.The results show that Fe₃C plays a key role in Li PSs chemisorption and other aspects.At the same time,Fe₃C/PB has abundant micropores and mesopores and a large specific surface area.Under the dual effects of physical limitation and chemical adsorption,the Fe₃C/PB electrode has an initial discharge capacity of 837.6 m Ah g^-1 at a rate of 1 C.After 250cycles,the capacity can still maintain 555.3 m Ah g^-1,and the single-turn capacity attenuation rate is 1.15%.（2）Compared with bulk biochar,the flake has a higher surface atom exposure rate and a faster charge transfer speed.Therefore,a low-concentration potassium ferrate solution was used as an activator to activate hibiscus petals,and a sheet-like porous biocarbon material（Fe₃C/GB）loaded with Fe₃C particles was synthesized in one step.Fe₃C/GB is combined with sulfur and used as a positive electrode in lithium-sulfur batteries.The test results show that the material can effectively shorten the ion diffusion length,improve the ability of charge transfer electrons,and accelerate the progress of electrochemical reactions.When the sulfur loading reaches80.0 wt%,the pole piece achieves a high sulfur loading of 3 mg cm^-2.Cycling 200times at 0.5 C,the reversible capacity of the battery is still 561.8 m Ah g^-1,and the single-turn capacity decay rate is only 0.0086%.（3）In view of the chemical activation method,it is difficult to control the pore size distribution of materials,especially the pore size distribution of macropores.Through the improvement of the process（high shear,freeze-drying combined with chemical activation method）,the selection of raw materials（good paste-forming material Tremella）,a hierarchical porous structure with adjustable three-dimensional macropores-mesopores-micropores is prepared Carbon material（HPBC-8）.And compare the effects of different shear rates on the distribution of macropores.As a sulfur carrier,HPBC-8 has excellent electrical conductivity,and the uniformly distributed large pores on the surface make the carbon wall thinner and shorten the Li⁺diffusion path.The special hierarchical porous structure effectively improves the cycle performance and rate performance of the battery.Therefore,the reversible capacity of 510.0 m Ah g^-1 can be maintained after 400 charge discharge cycles at 1 C.The prepared cathode can still reach a reversible discharge capacity of 616.3 m Ah g^-1at a rate of 5 C.