Research On The Performance Of Molybdenum Based Composite Materials For Lithium Sulfur Batteries

As one of the most promising technologies,lithium sulfur（Li-S）battery has attracted extensive attention because of its extremely high theoretical specific capacity(1675 m Ah g^-1)and energy density(2675 wh kg^-1).At the same time,sulfur is mainly obtained from petroleum refining,which is rich in reserves,cost-effective,non-toxic and environment-friendly.Despite these advantages,multi-step electrochemical reaction brings some challenges to the large-scale application of Li-S battery.The electronic/ionic insulation properties of S and sulfide（Li₂S₂/Li₂S）lead to slow reaction kinetics and low utilization of active materials.The volume expansion caused by the different density of sulfur and Li PSs also reduces the cycle performance during charge and discharge.In addition,the flow of soluble Li PSs at the positive and negative electrodes,commonly known as the"shuttle effect",will lead to low coulomb efficiency,continuous electrode degradation and rapid capacity attenuation.In the past several decades,with the application of advanced nanotechnology,the aim of cathode design is to inhibit the shuttle effect in Li-S battery and the greatly improve the electrochemical performance of sulfur cathode in cycle life and rapid charging performance.In this work,based on MoS₂,MoSe₂ and different substrate materials,different types of MoS₂ and MoSe₂ based composites were designed and prepared.The effects of the structure and components of the composites on the electrochemical properties of Li-S battery were studied.The main contents and research results are as follows:（1）C-MoS₂/rGO composites were prepared by a simple one-step hydrothermal strategy with both large surface area and high porosity,which were used as advanced electrode material in Li-S batteries.Double modified defect-rich MoS₂ nanosheets are successfully prepared by introducing reduced graphene oxide（rGO）and amorphous carbon.The conductibility of the cathodes can be improved through the combination of amorphous carbon and rGO,which could also limit the dissolution of polysulfides.After annealing at different temperatures,it is found that the C-MoS₂/rGO-6-S composite annealed at 600℃yields a noticeably enhanced performance of Li-S batteries,with a high specific capacity of 572 m Ah g^-1 at 0.2 C after 550 cycles,and551 m Ah g^-1 even at 2C,much better than that of MoS₂-S nanosheets(249 m Ah g^-1 and149 m Ah g^-1)and C-MoS₂/rGO-S composites(334 m Ah g^-1 and 382 m Ah g^-1).This electrode design protocol and annealing process may pave the way for the construction of other high-performance metal disulfide electrodes for electrochemical energy storage.（2）The MoS₂ grown on the 3D structured Carbon Cloth（CC@MoS₂）is firstly used to fabricate the Li-S battery and the sulfur loading can be freely tuned by adjusting thermal annealing time.A two-step melt-diffusion strategy is presented for fabrication of cathodes,which involves in melting and diffusion of sulfur instead of dissolution of sulfur in the toxic organic solvents.Compared with the non-polar carbon cloth,the CC@MoS₂ composites exhibit better adsorption capacity for polysulfides due to more edge active sites,which could effectively facilitate polysulfide redox kinetics.The SEM images of the CC@MoS₂ cathode after 300 cycles show that MoS₂ can still maintain the nanosheet morphology.After 300 cycles at 0.5 C,the CC@MoS₂cathodes loaded with 2 mg sulfur exhibit a better reversible capacity of 698 m A h g^-1 compared with CC@MoS₂ loaded with 1 mg sulfur(604 m A h g^-1)and CC@MoS₂ loaded with 4 mg sulfur(420 m A h g^-1).This work proposes an environmentally friendly method to fabricate the Li-S battery cathode material and the sulfur loading can be freely adjusted.（3）2H/1T MoS₂ loaded on annealed melamine foam（CF@2H/1T MoS₂）is prepared as a multifunctional interlayer to inhibit the shuttle effect,improve redox kinetics,and reduce the charge-discharge polarization of Li-S batteries.The CF@2H/1T MoS₂ becomes fragmented structures after assembling the cell,which not only benefits to adsorb and catalyze Li PSs but also to significantly buffer the volume expansion due to a large number of gaps between fragmented structures.Meanwhile,the batteries based on CF@2H/1T MoS₂ interlayer delivers high areal capacity of 5.1m Ah cm^-2 under high sulfur mass loading of 7.6 mg cm^-2 at 0.2 C.Importantly,the experiments of in situ Raman spectra demonstrate that the CF@2H/1T MoS₂ can obviously inhibit the shuttle effect by effectively adsorbing and catalyzing Li PSs.This novel design idea and low-cost melamine foam raw material open up a new way for the application of high-energy density Li-S batteries.（4）MoSe₂ is loaded on the annealed melamine foam（NCF@MoSe₂）as the interlayer of Li-S battery.In addition,MoSe₂/Mo P heterojunction was obtained by phosphating MoSe₂ powder material after high temperature annealing,which was melted with elemental sulfur and coated on aluminum foil.Therefore,it is reported for the first time that the battery is composed of interlayer and heterojunction material coated on aluminum foil.The NCF@MoSe₂ interlayer can effectively adsorb and catalyze Li PSs,more importantly,MoSe₂/Mo P heterojunction materials can also effectively adsorb and catalyze Li PSs.In this way,the assembled battery has a dual strategy to inhibit the shuttle effect.Thanks to these advantages,the batteries based on NCF@MoSe₂ interlayer and MoSe₂/Mo P heterojunction show good cycle performance and high area capacity even under high sulfur load.This design is also conducive to develop more advanced Li-S battery system.