Preparation Of Transition Metal-heteroatomic Co-doping Biochar And Its Application In High Performance Lithium Sulfur Batteries

As the most promising battery among many secondary batteries,lithium sulfur battery has attracted much attention and research because of its high energy density,abundant source of active material sulfur and low price.However,lithium sulfur battery still has the following problems:（1）sulfur and its discharge products have poor conductivity and poor charging-discharge performance;（2）the polysulfide easily dissolves in the electrolyte and causes the"shuttle effect",resulting in poor cycling performance.In recent years,through a variety of modification strategies,the electrochemical performance of lithium sulfur batteries has been significantly improved,but most of the reported sulfur load and energy density are low,the cycling stability is poor,and the preparation process is complex and the preparation cost is high.The reason is that when pure carbon material is used as the conductive skeleton of sulfur,its conductivity and polarity are low,and the polysulfide is mainly bound by physical adsorption to inhibit the occurrence of shuttle effect.Moreover,the existence form of pure carbon is mostly sp3 hybridization,with low graphitization degree and relatively poor conductivity.Therefore,looking for a simple and mild method to functional resorcinarene modification of carbon materials,to further improve the conductivity and chemical adsorption ability of polysulfide,and can effectively promote the sulfide in the REDOX reaction in the process of charging and discharging,to improve the lithium-sulfur battery cycle life especially under the condition of high sulfur surface density of discharge capacity and cycle stability,is particularly critical.This article introduce the impurity atoms or transition metal nonpolar of carbon materials,the polarity of heteroatomic/transition metal and sulfur can be formed between the strong chemical bonds,and polysulphide showed strong chemical adsorption,but also can promote more sulfur catalytic conversion,the battery of the electrochemical properties and therefore got improved significantly.Specific research contents are as follows:1.Application of biocarbon-based self-supporting cathode in high-volume energy-density lithium sulfur batteries with poor electrolyteUsing tobacco stem as carbon source,potassium permanganate as metal dopant,phosphoric acid as activator and reaction medium,pre-carbonization was performed at280℃in an oxygen-containing atmosphere.As the pre-carbonization temperature is higher than the boiling point of phosphoric acid（261℃）,it will form a boiling state,generate a strong convection effect,and aggravate the contact between the tobacco stem and the interfacial oxygen.Then combined with the high-temperature calcination treatment,the biological carbon material with rich oxygen-containing functional groups and strong hydrophilicity is prepared.The low binder self-supporting positive electrode was prepared by coating-drying-self stripping.As the catalytic activity center,Mn O₂nanoparticles can effectively combine the advantages of structure limitation and chemosorption,inhibit the"shuttle effect"generated in charging and discharging,and significantly improve the electrochemical performance.The capacity retention value of up to 526.2 mAh/g is up to1500 cycles at 2C high current density,and the capacity attenuation rate of each cycle is only0.025%.When the sulfur surface density is up to 6.3 mg/cm²,the discharge area capacity of7.94 mAh/cm²and discharge volume capacity of up to 881.7 mAh/cm³are available at 0.1C.The discharge area capacity of Mn-NOPC@S electrode at 0.1C is 4.65 mAh/cm²and the discharge volume capacity is also up to 801.5 mAh/cm³under the condition that the electrolyte dosage is 5 u L/mg and the sulfur surface density is up to 3.8 mg/cm².These results show that the high energy and low cost self-supporting anode Mn-NOPC@S provides the possibility for the practical application of lithium sulfur battery.2.Study on the application of porous biological carbon with nitrogen and iron co-doped in high performance lithium sulfur batteryWith sesame seed shell as raw material,ethylenediamine tetraacetic acid disodium iron as carbon source and the source of iron,phosphoric acid as activator,was prepared by a N-iron doped with Fe-NOPC porous carbon materials,interaction between N and the coexistence of Fe and significantly adjust the electron density of the carbon surface,promoted the more interaction between sulfur and carbon matrix,can effective adsorption polysulphide,inhibition of shuttle effect,can improve the speed of ion migration at the same time,to achieve higher sulfur capacity Fe-NOPC@S electrode can reach a high discharge capacity of 1349 mAh/g at a low current density of 0.1C.After up to 350 cycles at 1C high current density,the Fe-NOPC@S electrode still retained a capacity of up to 692 mAh/g,with a capacity attenuation rate of only 0.083%per cycle.Fe-NOPC@S electrode under different loads of sulfur electrode REDOX potential closer,voltage polarization is small,the low/high sulfur capacity when all has the very good lithium ion diffusion rate,under high loads can fully polysulphide transformation,that under the condition of high sulphur load remains high ability of ions and electrons will speed up implementation lithium-sulfur battery application.3.Facile synthesis of porous carbon for high-performance Li-S batteries obtained from tobacco stick via phytic acid activationUsing tobacco stem as carbon source,phytic acid as the doping agent,activator and P carbon source,the preparation of a kind of high output,has the rich functional groups,P doped NPOC-2,porous carbon materials doped impurity atoms in situ as catalytic active center,can strengthen the adsorption of polysulfide,reduce the dissolution of sulfide,inhibition of lithium battery charge and discharge of sulfur"shuttle effect"caused by the electrochemical performance significantly increased,exhibited lower polarization rate and high coulomb efficiency.At 0.1C,it has a very high discharge capacity of 1211 mAh/g.After300 cycles of 1C with high current density,it has a capacity retention value of up to 885mAh/g.The capacity attenuation rate of each cycle is only 0.029%.When the sulfur surface density is as high as 5.3 mg/cm²,the discharge capacity at 0.1C is 1110 mAh/g.These results show that NPOC-2@S electricity has a very good practical application effect.