Research On Modified Sulfur Cathode For Lithium Sulfur Battery

With the increasing demand for mobile energy storage,the current lithium-ion battery energy storage system is difficult to meet its needs.Therefore,high energy density storage equipment has become a research hotspot in recent years.Among them,lithium sulfur batteries have many advantages,such as high specific capacity,high specific energy,abundant reserves,low price and environment friendliness.Despite the above advantages,sulfur as the positive electrode of lithium-sulfur batteries still faces the following challenges in the actual energy storage process:a)the dissolution of lithium polysulfide,the former charge product of sulfur,resulting in loss of active substances and shuttle effect;b)volume expansion caused by low density of sulfur charge product;c)the poor electronic conductivity of sulfur.In order to solve the problems above,the major work of the thesis includes:a)Firstly,through the method of addition vulcanization,elemental sulfur was successfully fixed to the polymer skeleton based on the double-bond enriched polymer polyisoprene(PIP),by covalent bonding.Finally,PIPS copolymer composites(90%sulfur)were successfully synthesized.This method can ensure the covalent immobilization of elemental sulfur to ease the problem of shuttle effect.Moreover,in the design of composite electrodes,PIP was chosen to replace PVDF as the binder of the electrodes,which effectively improved the volume expansion of sulfur cathode.In the study of PIPS composite cathode,the nano-polycrystalline structure of the copolymer was explored experimentally,and the thermal stability of the cathode material itself was also improved.Based on PIPS composite cathode material and PIP binder,the stable cycling performance of high areal capacity was achieved,about 7.0 mAh/cm2,which is five to ten times as much as reported in the literature.Through the charge-discharge curve of PIPS electrode and introducing shuttle factor,it was concluded that the modified PIPS composite cathode material encapsulated the dissolution of polysulfide.Finally,through SEM and EDS analysis,it shows that the polysulfides are better dispersed around the PIPS electrode after cycles of lithiation/delithiation,which is consistent with the results of electrochemical characterization.It also shows that the PIPS cathode improves the utilization of sulfur.b)In this paper,a new multicomponent composite sulfur cathode material MPS(MnO2/Ppy/S)was synthesized by one-step method.On the one hand,the composite cathode hindered the shuttle effect of polysulfides.On the other hand,the conductivity of the composite cathode material was improved due to the doping of conductive polymer polypyrrole.The morphology and structure characteristics of MPS as cathode material were demonstrated by SEM.And the effect of pyrrole addition on the morphology of MPS was investigated.The successful coating of MnO2 and Ppy was proved theoretically by TEM、EDS、XPS.In addition,compared with S/C cathode,the modified MPS composite cathode shows a better electrochemical performance.The discharge capacity of the first cycle reached 1079 mAh/g.In the following 100 cycles,the capacity decay is 70.7%,and the average Coulomb efficiency is 97.38%in 200 cycles.In addition to good cycling performance,the high-loading MPS material still exhibits good electrochemical performance.When the sulfur load is 4.9 mg/cm2,the surface density of the electrode exceeds the commercial standard.The first cycle capacity is 1002 mAh/g.In the subsequent cycle,the specific discharge capacity of the electrode remains stable at 650 mAh/g.It can be seen that with the addition of MnO2 and Ppy,the electrodes can maintain a good electrochemical performance under high sulfur loading,leading to a promising prospects.