Construction Of Transition Metal Oxide (sulfide)/carbon Composite Structure And Its Performance As Negative Electrode For Lithium-ion Batterie

Rechargeable batteries have always been regarded as one of the reliable ways of energy storage and transportation.Lithium ion batteries（LIBs）are highly expected for their high energy density,environmental friendliness,safety and stability.It can complement unstable energy sources,such as tidal energy and solar energy.After decades of efforts of researchers,human beings have rich theoretical basis for the charging and discharging motor system of lithium-ion battery,and have realized its application in electronic equipment,aerospace and military.Despite the disadvantages of high cost and high price of LIBs,graphite,which has been commercialized,can not meet the requirements of higher capacity density.Due to the advantages of LIBs,researchers are still looking for more suitable anode materials to meet the requirements of higher energy density.Both transition metal oxides and transition metal disulfides have much higher theoretical capacity than graphite.As a typical transition metal oxide,Fe₃O₄ has a theoretical capacity of 924 m Ah g^-1.It has the advantages of rich reserves and low price on the earth.Due to the disadvantages of poor conductivity of transition metal oxides and particle crushing caused by volume change caused by conversion reaction in charge discharge process,they are usually combined with amorphous carbon to increase conductivity and limit volume change at the same time;MoS₂ is a common anode material of LIBs in transition metal disulfide（TMD）.It has a layered structure like graphene.The reaction mechanism is intercalation reaction,with small volume change,and the theoretical capacity is slightly lower than that of transition metal oxide.Monolayer MoS₂ is thermodynamically unstable and easy to stack under the action of van der Waals force,losing the advantage of layered structure.At the same time,the thermodynamically stable 2H phase MoS₂ is a semiconductor with large internal resistance.It usually uses the synergy with other materials（usually carbon materials）to increase the layer spacing,reduce the resistance of lithium ion insertion and prevent the stacking under the action of van der Waals force.Carbon materials have high conductivity and good mechanical properties,and are widely used in the preparation of and cathode materials.This paper mainly describes the use of carbon materials to improve the performance of LIBs,which is mainly divided into the following two parts.（1）in the first part of the work,the author first used the one-step solvothermal method to prepare Fe₃O₄ nanoparticles with a diameter of 80 nm at atmospheric pressure.At the same time,gelatin was used as the medium,and Fe₃O₄ nanoparticles were firmly anchored to the conductive network formed by single wall carbon nanotubes（CNTs）by spray drying method,and then annealed.Fe₃O₄@C/CNTs composites.Electrochemical test data show that:the specific capacity of Fe₃O₄@C/CNTs still be maintained at 348 m Ah g^-1 after 700times charged and discharged cycles at a current density of 5 A g^-1,while the specific capacity of Fe₃O₄/CNTs and Fe₃O₄@C decays to 155 and 70 m Ah g^-1.The use of gelatin and CNTs greatly improves the conductivity of the composites and limits the volume expansion of Fe₃O₄ nanoparticles.It can be shown from the data of scanning electron microscope and transmission electron microscope that Fe₃O₄ nanoparticles are evenly distributed in the conductive network of CNTs.（2）In the second part,using Na₂MoO₄·2H₂O as molybdenum source,thiourea as sulfur source and glucose solution as solvent,MoS₂ nanosheets were grown in situ on carbon fiber cloth（CC）by solvothermal method to form self-supporting MoS₂@CC,where carbon fiber cloth is obtained by electrospinning polyacrylonitrile（PAN）solution and then carbonization.MoS₂ nanosheets grown on CC can not only improve the conductivity of MoS₂ nanosheets and prevent the stacking of nanosheets,but also avoid using adhesives such as PVDF（polyvinylidene difluoride）to reduce the specific capacity.The effects of different proportions of Na₂MoO₄·2H₂O and thiourea on the morphology and electrochemical properties of MoS₂ composites were studied.The results show that MoS₂ grown on CC becomes more and more uniform and dense and the specific capacity also increases gradually,with the increasing proportion of thiourea.It displays the best electrochemical performance,the specific capacities of 810 m Ah g^-1after 200 cycles at a current density of 0.5 A g^-1,while the mass ratio of Na₂MoO₄·2H₂O to thiourea is 1:3.In the same at a current density of 0.5 A g^-1,the The specific capacities are 450 m Ah g^-1（1:0.05）,617 m Ah g^-1（1:1）,608 m Ah g^-1（1:1.5）,621 m Ah g^-1（1:2）,and 703m Ah g^-1（1:4）,respectively.