The Study Of Metal Compound/Nitrogen Doped Carbon Composite Materials For Lithium-sulfur Batteries

Lithium sulfur batteries are one of the present research hotspots being a very promising systemfor the next generation of energy storage because of their high theoretical energy density（2600 Wh kg-1）.The dissolution of polysulfide and resulted"shuttle effect are the key problems that influences the electrochemical performance of lithium sulfur batteries seriously such as short capacity life,low discharing capacity and coulombic efficiency.Aim to inhibit the serious"shuttle effect",we constructure multifunctional carbon composite materials（iron carbide/nitrogen doped porous carbon nanosheets,flexible nano Nd₂O₃/nitrogen doped carbon fibers membrane,and flexible VN/nitrogen doped carbon fibers membrane）,based on the synergistic effect between nonpolar carbon materials and polar transition metal compouds.The as-prepared sulfur cathodes based on multifunctional carbon composite materials can obtain an improved electrochemical performance due to the dual confined effect of physics and chemical adsorption/bonding for polysulfides.Based on the above research ideas,the main research contents of this paper are divided into the following aspects:1.With a cheap biological waste as raw material,carbon iron/nitrogen doped porous carbon （Fe₃C/N-C）composite material is obtained by introducing Fe source.Nitrogen doped porous carbon sheets have the grapheme-like appearance.As a comparison,the blocky nitrogen doped porous carbon is prepared without Fe source,and the carbon material has a high BET surface and pore volume.As a host material for sulfur,the influence of the Fe₃C/N-C composite materials on the electrical properties of sulfur cathode was studied.The sulfur positive electrode based on Fe₃C/N-C composite materials shows good cycle performance and rate performance.This is due to the special pore structure of carbon materials which provide good reaction sites for the conversion of sulfide,thus the good physical confined effect on polysulfides,inhibit polysulfide secession from the positive skeleton.Moreover,the electrical conductivity of the carbon materials has been increased because of higher graphitization degree.At the same time,the strong adsorption of polysulfide by doped nitrogen and Fe₃C enhances the ability of anchoring polysulfides and further inhibits the travel effect of polysulfide.2.We use electrostatic spinning technology which is suitable for batch production,followed by subsequent heat treatment to prepare a series of Nd₂O₃/nitrogen doped carbon fiber（NCF）composite membrane（Nd-X/NCF）with neodymium nitrate as the source of metal,polyvinylpyrrolidone（PVP）as carbon source.With the flexible Nd-X/NCF composite as the functional intermediate layer between the sulfur cathode and the sepatater and the lithium-sulfur battery showed good electrical properties.Under the charge and discharge conditions of 2C,the initial discharge capacity is 911.2 mAh g^-1 and the capacity after200 cycles is still 548 mAh g^-1.On the one hand,nitrogen doped carbon fibers have strong interation with polysulphides,on the other hand the polar neodymium oxide nanoparticles has strong cooperation with polysulfides,therefore the interlayer can improve the cycle stability and ratio performance of the lithium-sulfur battery through the synergistic effect.3.Transition metal nitride has higher electrical conductivity nitride than transition metal oxide.So we use ammonium vanadate as metal source,polyvinylpyrrolidone（PVP）and polyacrylonitrile（PAN）as carbon source,and adopt electro-spinning method to obtain metal nitrides/carbon nanofibers composite materials（VN-X/NCF）.As a functional interlayer,it can be used to improve the solution of polysulfide in Li-S batteries.Lithium-sulphur batteries containing VN-2/NCF has the best performance.The initial discharge capacity is 933 mAh g^-1 at 1C,and a reversibale capacity of 549.6 mAh g^-1 could be maintained after 150 cycles.The initial discharge capacity of the battery without the interlayer is 836 mAh g^-1,and the capacity of the 150^th cycleis is only 86.1 mAh g^-1.Similarly with nano Nd₂O₃/nitrogen doped carbon fiber composites,the lithium-sulphur battery containing the VN-X/NCF interlayer has improved electrochemical properties due to the synergistic effect of VN and nitrogen doped carbon on anchoring polysulfides.