Preparation Of Anode Materials Deriving From Silicon/Carbon Nanospheres For Lithium Ion Batteries

As a portable and efficient energy storage device,lithium-ion batteries are now used in various fields.However,the theoretical specific capacity of graphite anode materials that most commonly used in commercial lithium-ion batteries is lower,making it increasingly difficult to meet the pursuit of high energy density.Among the many anode materials,silicon materials are regarded as the new anode materials with the greatest development potential due to their extremely high theoretical specific capacity,non-toxic and harmless,environmental protection and suitable working voltage.However,in practical applications,silicon anode often has two key shortcomings of poor conductivity and huge volume expansion,resulting in high initial capacity but extremely fast electrochemical attenuation or even battery failure.Therefore,improving the two major disadvantages of silicon-based anode materials is crucial for the development of high-performance lithium-ion batteries.For silicon/carbon anode materials,this paper synthesized silicon nanodots impregnated with single-shell hollow carbon structure by using the structural advantages of silicon nanocrystals,carbon composite and core-shell structure.Based on the silicon/carbon nanospheres,further structural design was carried out and a variety of silicon/carbon anode materials with different nano or micron structures were prepared.This kind of material is not only beneficial to enhance the conductivity,but also can effectively alleviate the volume expansion effect during the electrochemical cycle.The morphology and phase composition of the composites were characterized,and the electrochemical properties and reaction kinetics of the electrode materials were investigated.Specific studies are as follows:（1）In chapter 3,the structure of silicon nanodots prepared by magnesium thermal reduction technology and distributed uniformly inside and on the carbon shell of single shell hollow carbon nanospheres was proposed（Si NDs@SSHC）.Compared with pure silicon,the obtained Si NDs@SSHC nanospheres exhibit better capacity and cycling performance when used as anode materials.After 500 cycles at 0.3A g^-1,the specific capacity of Si NDs@SSHC nanospheres is stable at 519 m Ah g^-1,while that of pure silicon is only 95 m Ah g^-1.It is mainly due to the ultra-small range of silicon nanodots can reduce the effect of lithium-stress and shorten the transport path,the formation of carbon shell stable SEI film and the pore adaptation of the core shell structure to volume changes.（2）On the basis of the above single-shell silicon/carbon nanosphere structure,a layer of hollow carbon structure with uniformly distributed silicon nanodots is added to form a structure in which silicon nanodots are distributed inside and on the double shell hollow carbon（Si NDs@DSHC）.The results show that the double shell structure has better lithium storage performance than the single shell structure.At 0.3 A g^-1,the specific capacity of Si NDs@DSHC can be stabilized at 1350 m Ah g^-1 after 500 cycles.Due to the double shell structure can not only continue the advantages of single shell hollow carbon,the most important is that the double shell hollow carbon has high mesoporous properties,which can be used as chemical reactor to restrict silicon nanodots and improve the thermodynamic stability.In addition,the outer carbon layer provides an additional layer of protection,further increasing the structural stability of the material,while the inner carbon shell acts as a bridge for electron/ion transfer.（3）The integrated paper electrode（SHCM/NCF）without binder is prepared by electrospinning technique,in which nitrogen doped carbon fiber crosslinked silicon/carbon self-assembled microspheres.The structure also continues the advantages of the single-shell silicon/carbon nanospheres.More importantly,the single-shell nanospheres are connected and aggregated by the nitrogen/carbon conductive network and maintain electrical contact,which is conducive to increasing the conductivity of the paper.At 1 A g^-1,the reversible specific capacity of SHCM/NCF paper after 800 cycles is 1442 m Ah g^-1 in lithium-half battery,and 450 m Ah g^-1 at 0.5 A g^-1 after 200 cycles in lithium-full battery.This integrated paper synthesis method is simple and saves the cost of polymer binders and conductive additives,making it show great application potential.（4）Considering the advantages of building blocks agglomeration structures,different dimensional silicon/carbon aggregation structures derived from the same 0D silicon/carbon nanospheres building blocks are discussed from the perspective of dimensional design,including 1D SHC nanospheres interconnected with nitrogen doping carbon necklace fiber,2D SHC nanospheres directional arranged plane,and 3D SHC nanospheres self-aggregated microsphere.After 1200 cycles at 0.5 A g^-1,the specific capacities of 0D nanospheres,1D fiber paper,2D plane and 3D microspheres can be maintained at 600,2000,1002 and 1810m Ah g^-1,respectively.These three different dimensions of composite materials also have the advantages of single-shell nanospheres in terms of electrical properties.In addition,these show unique advantages in chemical,physical and electronic properties,including 1D high aspect ratio,2D rapid electron/ion diffusion kinetics and 3D efficient conductive networks,thereby effectively improving the corresponding electrochemical properties and structural stability.