Coating Modification Of Polypropylene Separators And Their Application In Lithium-Sulfur Batteries

Lithium-sulfur battery has the advantages of ultra-high theoretical energy density(2,600Wh kg^-1)and low cost,and it is a kind of next generation lithium secondary battery with high energy density which has great application potential.However,there are some problems in lithium-sulfur batteries,such as low conductivity of positive sulfur,dissolution and diffusion of polysulfide as the intermediate product of charge and discharge,and disordered growth of anode lithium dendrites.Especially under the practical conditions of high sulfur load,these problems are more prominent,leading to the low reversible capacity and poor cycle performance of lithium-sulfur batteries.Based on the coating modification of commercial polypropylene PP separator,this paper studies the influence of the structural characteristics of multi-functional coating,such as porous structure,interface functional group and electrocatalytic activity,on the performance of lithium-sulfur batteries,and finally improves the performance of lithium-sulfur batteries with high sulfur load.The main research contents of this paper is as follows:（1）Study on commercial carbon black coated polypropylene separators and their electrochemical performance.Through a simple coating process,commercial carbon black Super-P（SP）was coated on the side of polypropylene（PP）separator,and the effects of surface loading and interface functional group characteristics of SP coating on the performance of lithium-sulfur batteries with high sulfur loading were studied.The results show that the conductive porous SP coating can suppress the shuttle effect of lithium polysulfide and improve the performance of lithium-sulfur batteries with high sulfur loading.When the surface load of SP coating is 1 mg cm^-2,the lithium sulfur battery with SP-1@PP as the separator(cathode sulfur surface load>4 mg cm^-2)has the highest reversible capacity,and the first cycle discharge capacity is 819.5 m Ah g^-1at 0.1 C.It still has a reversible capacity of 660.1 m Ah g^-1 after 100 cycles.After further acid oxidation of SP,the interface of OSP coating contains abundant negatively charged groups,which can further inhibit the shuttle of polysulfide through electrostatic repulsion and improve the cycling stability of lithium-sulfur batteries.（2）Preparation and electrochemical performance of MoS₂/MoO₂/rGO composite separator coatings.Study on MoS₂-MoO₂/rGO composite coated polypropylene separator and its electrochemical performance.Through a simple hydrothermal method,MoS₂-MoO₂/rGO composites were prepared by in-situ growth of MoS₂-MoO₂heterostructures on graphene surface,and were used to coat and modify commercial polypropylene（PP）separator.The effect of MoS₂-MoO₂/rGO@PP separator on sulfur cathode,lithium metal anode and battery performance of lithium sulfur battery was studied.The results show that,compared with single MoS₂/rGO or MoO₂/rGO,heterogeneous MoS₂-MoO₂/rGO has stronger adsorption and catalytic conversion ability for polysulfide.Therefore,the MoS₂-MoO₂/rGO@PP separator can more effectively improve the redox reaction rate of sulfur and inhibit the diffusion shuttle of lithium polysulfide from the cathode side to the anode side.In addition,MoS₂-MoO₂/rGO@PP membrane can also make the lithium ion diffusion flow uniformly distributed,thus reduce the interface of the anode of lithium metal deposition overpotential,promote the uniform deposition of lithium metal,inhibit the growth of the disorder of lithium dendrite.Therefore,the lithium-sulfur battery with MoS₂-MoO₂/rGO@PP as the separator has more excellent electrochemical performance:the discharge capacity of the first cycle is 1123.6 m Ah g^-1 at 0.5 C current density,and the discharge capacity after 100 cycles is still 886.7m Ah g^-1.At 1C current density,the specific discharge capacity of the first cycle is 1036.8 m Ah g^-1,and the capacity retention rate is 75.2%after 200 cycles.Even under the condition of high sulfur loading of 8.9 mg cm^-2,the lithium-sulfur battery with MoS₂-MoO₂/rGO@PP separator still has 530 m Ah g^-1discharge capacity and 4.7 m Ah cm^-2 surface capacity,and the capacity retention rate is 71.7%after 100 cycles.