Study On The Construction Of High Content Lithium Sulfur Battery Cathode Based On Core-shell MnO₂@S Composite Material

Energy storage and power batteries play an important role in the current energy usage pattern.At present,the energy density（280Wh/kg）of high-performance lithium-ion batteries widely used is approaching its limit（350Wh/kg）,so it is urgent to develop secondary batteries with higher energy density.Because sulfur has a high specific capacity（1675 m Ah/g）and it is cheap and easily available.The lithium-sulfur battery is composed of sulfur as active material for positive electrode and lithium metal anode.Its theoretical energy density is up to 2600 Wh/kg,which makes it one of the important candidates for the next generation of secondary batteries to replace lithium-ion batteries.However,lithium-sulfur batteries still need to solve problems such as poor conductivity,large volume variation,and the dissolution and shuttle of polysulfide before they can be used in practical applications.The core of this method is to construct a highly efficient sulfur cathode to promote the smooth development of the complex apparent electrochemical reaction process of lithium and sulfur.Meanwhile,in order to ensure the energy density of the lithium sulfur battery and the sulfur load of the active material in the cathode should be as high as possible.For the sake of practical application in the future,its preparation process should be convenient and controllable.Therefore,in this study,a relatively easy path for the preparation of sulfur composites was chosen.Sphallic nano-sulfur particles were prepared by simple hydrothermal synthesis method and then metal oxide MnO₂adsorption material was coated on the sulfur surface.The core-shell MnO₂@S composites with different structures and morphologies were obtained by controlling the preparation conditions.These MnO₂@S composites,conductive matrix and binder were used to prepare sulfur cathode,and lithium metal anode was assembled to obtain lithium sulfur battery.The performance of charge and discharge performance was studied,and the performance of the battery under the preparation path was analyzed and investigated.The main research results are as follows:（1）The elemental sulfur nanoparticles were prepared by simple hydrothermal synthesis method,which provided the shape and structure basis for the synthesis of composite materials.First of all,the main factors（Na₂S₂O₃concentration and PVP concentration）that affect the morphology and structure of sulfur particles were found by designing orthogonal experiments.Then,sulfur particles with basically the same morphology,structure and size were synthesized by changing the concentration of the two factors.The experimental results were analyzed by SEM images.Finally,spherical sulfur particles with a diameter of about 500nm are obtained.（2）Two kinds of core-shell MnO₂@S composites with different morphology and density of MnO₂were prepared by REDOX reaction using two different reaction materials,and then used as cathode materials for lithium-sulfur batteries,and their electrochemical properties were tested.The results show that:i)Compared with pure solid sulfur spherical positive electrode,the positive electrode of MnO₂@S-1 composite with cored shell structure shows better electrochemical performance.At 0.05C discharge rate,the specific capacity is more than 1300 m Ah/g,and at 0.1C discharge rate,the specific capacity is also about 900m Ah/g.The capacity retention rate is also higher after 100 cycles,and the cycle performance is significantly improved;ii)Compared with the nanoblock MnO₂@S-2 composite,the nanosheet matrix showed higher discharge voltage,discharge capacity and lower overpotential;iii)The comparison of two kinds of MnO₂coated MnO₂@S composites with different density shows that the thin MnO₂coated laminates exhibit better cyclic and scaling properties,and the capacity recovery rate can reach 75.1%after 100cycles.In general,through hydrothermal synthesis of sulfur nanoparticles,the preparation of MnO₂@S composite material,and then the construction of sulfur cathode with high sulfur content to obtained lithium-sulfur batteries.It shows excellent performance and indicates that the process path has great feasibility,and it is necessary to make further research and improvement.This study lays a preliminary foundation for the further optimization of this kind of sulfur composites,high sulfur positive electrode and positive electrode structure.