Fabrication And Electrocemical Performances Of Multifunctional Metal-based Sulfur Host Materials For Li-S Batteries

It is well-known that the lithium ion battery(LIB)has drawn extensive attention from various aspects and also has been wildly used in mobile electronics and power batteries because of the excellent long cycling life and safety,etc.While,the energy density and cycling life of the commercial LIBs material cannot afford the forward request of people,at present.Thus,it is urgently needs to exploit the next generation rechargeable battery which has an excellent long cycling life and higher energy density.Li-S batteries(LSBs)are regarded as one of the most prospective candidates for the novel energy-storage devices,due to the high energy density(2500?Wh kg^-1 or 2800 W·h L^-1).Furthermore,the sulfur cathode has a lot of advantages such as rich reserves,low price and environmentally friendness.Though LSBs have so many advantages,there are also three distinct drawbacks which greatly hinder their development and successful commercialization:1)the insulativity nature of sulfur cathode(5×10^-30 S cm^-1);2)the‘shuttle effect'created by the soluble intermediate lithium polysulfides(LPS,Li₂S_n,4?n?8)during the reaction process;3)the large volumetric change(up to 80%)during the reaction.Due to these serious issues,LSBs always have a low utilization of S,bad cycling life,fast capacity attenuation and even some safety problems.The flaw in the nature of sulfur is the primary cause which leads to so many problems in LSBs,so it is a useful way to modify the sulfur cathode to solve those drawbacks.At present,researchers pay more attention to the sulfur-carbon composite materials,but due to the non-polarity of carbon,it cannot effectively mitigate the influence of the"shuttle effect".So it is vital importance to develop other new sulfur host materials.In this thesis,a series of multifunctional metal based sulfur host materials have been synthesized and the influence on the electrochemical performance of LSBs also be deeply investigate.Firstly,the hollow nanosphere was synthesized of Mo-polymer with the ammonium molybdate tetrahydrate and dopamine hydrochloride as the reactants.After carbonization,the Mo₂C/C hollow nanospheres were obtained.To further improve the electrochemical performance of S cathode,Mo₂C/C nanospheres were further covered with N-doped carbon layers(Mo₂C/C@C).Through the in-situ XRD test,it is proved that Mo₂C/C@C nanospheres have the dual-absorption effect(physical and chemical absorptions)to restrain the‘shuttle effect'.Thus,the electrochemical performance was greatly improved.Secondly,it is found that the Mo₂C with metallicity can greatly improve the conductivity of the cathode.To further enhance the electrochemical performance of the sulfur cathode,a“green soy bean-like”B,N co-doped graphitic nanotubes loaded with Co nanoparticles(Co@BNTs)were designed and synthesized as the efficient polysulfide immobilizers for sulfur cathode.It is found that the dual doped elements and Co nanoparticles play a key role in improve the electrochemical performance.Though in-situ XRD test and theoretical calculation,it is proved that Co nanoparticles not only effectively restrain the‘shuttle effect',but also promote the transform of polysulfide.The long-term cycling stability of Co@BNTs-S is promising as retaining a high capacity of 1008 m Ah g^-1 after 400 cycles,with a low decay rate of 0.038%per cycle,shows a great electrochemical performance.Thirdly,a new element of P was introduced to obtain a CoP-based host material.A novel sulfur cathode of integrating sulfur,flexible carbon cloth,and metal-organic framework(MOF)-derived N-doped carbon nanoarrays with embedded CoP(CC@CoP/C)has been designed.The CC@CoP/C shows a higher specific capacity and longer cycle life.To further investigate the mechanism,the in-situ XRD test and theoretical calculation were carried out.The results indicate that though CoP has lower adsorption energy than metal Co shows a better electrochemical performance because CoP could more effect reduce the energy barrier of polysulfide conversion.As compared with the adsorption energy,the reduction of the energy barrier of conversion palys a decisive role in improving the electrochemical performance.Furthermore,this host material is covered on the carbon cloth which has a good flexibility,this sulfur cathode could realize the full flexible Li-S battery.Finally,in the last work we found that the CoP nanocrystalline could catalyze the conversion of the polysulfide to improve the electrochemical performance of sulfur cathode.In order to investigate that if the single atom catalyst could effectively reduce the shuttle effect and improve the conversion of the polysulfide,the single atom catalyst was introduced to the Li-S battery.So,the hollow carbon spheres which loaded with the atomic Co were designed and synthesized.Further study suggests that atomic Co could effectively improve the absorption of polysulfide and promote the transform of the polysulfide.