Synthesis Of Nanocomposites Of High Capacity Lithium Storage Materials And Carbon In A Closed System And Their Performance

Lithium-ion batteries have become the most widely used energy storage devices because of light weight,small size,high safety,high working voltage,high energy density,high power,and long service life.Since the development of lithium-ion batteries,higher energy density has been pursued to meet application for electric vehicles.However,the commercial graphite anode has a low theoretical specific capacity?372 mAh/g?,which greatly limits the further improvement of energy density.Therefore,it is particularly important to develop anode materials with higher specific capacity.MoS₂,SiO_x?0<x<2?,and SnO_x materials possess higher theoretical specific capacity,which may play an important role in the next-generation high-energy-density batteries,thus it is of great significance for their research.However,there exist some common problems when MoS₂,SiO_x,and SnO_x are used as lithium-ion battery anode materials,mainly including large volume expansion during cycling and poor conductivity.At present,the most effective method to solve these problems is to make uniformly dispersed composites with carbon at nanoscale?<10 nm?.However,it is extremely difficult to synthesize such dispersed structures under conventional conditions due to large differences in the formation temperature of carbon and the above-mentioned compounds,which have been rarely reported.However,heating appropriate precursor to make them decompose in a closed system can result in vapor phase pressure.Under the pressure,the carbon and compound phases may be formed simultaneously,thus obtaining uniformly dispersed nanocomposites.Based on this point,MoS₂/C,SiO_x/C,and Sn/SnO₂/C nanocomposites with homogeneously dispersed structure are synthesized by heating decomposable precursors in the thesis.Furthermore,the effects of synthesis conditions on structure and performances of these composites are systematically studied.Besides,the formation mechanism of nanocomposites obtained by different precursors is discussed.Furthermore,the inherent law of the kind of precursors with the morphology and structure of materials is also discussed.The main research contents and results are as follows:MoS₂/C nanocomposites are prepared by using the solution of ammonium tetrathiomolybdate dissolved in dimethylformamide as precursor.In the preparation process,MoS₂ nanosheets are formed first,and then the carbon-containing gaseous substances produced by the decomposition of dimethylformamide adhere to the nanosheets to nucleate and grow.During this process,N and O atoms in the carbon-containing gaseous substances are combined with Mo atoms to form Mo-N-C and Mo-O-C bonds,respectively,thus forming the unique structure that MoS₂nanosheets are evenly dispersed in the N,O co-doped carbon matrix.The effects of mass ratio of ammonium tetrathiomolybdate to dimethylformamide and reaction temperature on the formation,structure,composition,and lithium storage performance of the composites are systematically studied.It is found that with the increase of mass ratio and the decrease of reaction temperature,the mass fraction of carbon in composites decreases.At the same time,with the increase of temperature,the amount of Mo-N-C and Mo-O-C bonds decreases.Higher carbon content in composites would result in the decrease of capacity.Both lower carbon content and lower amount of Mo-N-C and Mo-O-C bonds would decrease the cycling stability.The results show that MoS₂/C nanocomposites have the best lithium storage performance when the mass ratio is 1:2and the reaction temperature is 600 ^oC.The reversible capacity of MoS₂/C reaches702.3 mAh/g after 2700 cycles at a current density of 1.34 A/g,and 234.7 mAh/g at a current density of 13.4 A/g.SiOC powder materials are prepared by using liquid siloxane as precursor.In the preparation process,siloxane is decomposed into gaseous substances,and then SiOC nucleates and grows in the gas phase.As the pressure value of all directions in the closed system is uniform,which is favorable for the formation of spherical morphology,thus SiOC spheres are obtained.SiOC spheres can be converted into SiO_x/C spheres via subsequent heat treatment.The SiO_x/C composites consist of subnanoscopically and uniformly dispersed SiO_x and C phases.The effects of heat treatment technology,SiOC synthesis conditions,the nature of liquid siloxane,and coating carbon on SiO_x/C materials on on the formation,structure,composition,and lithium storage performance of the composites are systematically studied.It is found that SiOC could not be converted into SiO_x/C material when the heat treatment temperature is lower than 1000^oC.SiC would appear in SiO_x/C material when the temperature increases to 1300 ^oC.When the heat treatment time is lower than 3 h,SiOC could not be fully converted into SiO_x/C material,that is,the optimum heat treatment condition is that 1000 ^oC for 3 h.With the increase of synthesis temperature,mass loading of precursor,and heating rate of SiOC,the x value in SiO_x/C decreases.Except that the sample obtained at 0.2 ^oC/min is irregular bulk,other samples are well-developed spheres.By using siloxane with different atomic ratios of silicon to oxygen,the value of x in SiO_x/C can be controlled in the range from 0.08 to 1.28,and when x is less than 0.59,SiC appears in the samples.The lithium storage performance of SiO_x/C is related to x value,spherical morphology,and carbon coating.With decreasing the value of x from 1.28 to 0.59,the capacity of SiO_x/C increases,which would decrease when the x value continues to decrease.Well-dispersed spherical morphology possesses higher capacity and cycle stability.Furthermore,the capacity and cycle stability after carbon coating have been greatly improved.The capacity of the obtained C@SiO_0.59/C can reach 3.22 mAh/cm² after 100cycles at a current density of 0.1 A/g.Sn/SnO₂/C nanocomposites are prepared by using dimethyltinoxide as precursor.In the preparation process,SnO is formed first,then transformed into Sn and SnO₂.During this process,the carbon-containing gaseous substances produced by the decomposition of precursors adhere to the Sn and SnO₂ to nucleate and grow,thus obtaining Sn/SnO₂/C consisting of uniformly dispersed Sn,SnO₂,and C at nanoscale.The effects of the nature of precursors and reaction temperature on the formation,structure,composition,and lithium storage performance of Sn/SnO₂/C nanocomposites are systematically studied.It is found that only when the precursors have no long carbon chain at both ends of Sn-O/Sn=O in the molecular structure,uniformly dispersed structure can be obtained.The reaction temperature has a great influence on the structure,composition,and performances of the samples.The carbon content of the samples obtained at 400 ^o C is the lowest,thus decreasing the cycling stability of the composites.The oxygen content of the samples obtained at 800 ^oC is the lowest,which also results in poor cycling stability.The Sn/SnO₂/C samples obtained at 600 ^oC possess appropriate carbon and oxygen content,thus obtaining the best lithium storage performance.The reversible capacity can reach 477.0 mAh/g after 500 cycles at a current density of 1 A/g.The significance of this study is to propose that preparation of uniformly dispersed MoS₂/C,SiO_x/C,and Sn/SnO₂/C nanocomposites in a closed system.The lithium storage performance of the obtained composites is superior to that of commercial graphite anodes,which can further enhance energy density of lithium-ion batteries and promote development of lithium-ion battery anode materials.Besides,the intrinsic rules of the kind of precursor with the morphology and structure of material obtained in the closed system are established,which can promote the development of this method in material synthesis.