Preparation And Electrochemical Performance Research Of High Performance Of Nano Silicon

In the 21st century,in response to energy and environmental crises,especially the increasingly severe pressure of energy saving and emission reduction,the transformation of automotive energy power systems has become a common choice of the international automotive industry.The exit of fuel vehicles from the market is becoming the general trend.So the development of new energy vehicles is of vital importance.Currently,lithium-ion batteries(LIBs)are widely used in various portable mobile electronic devices.It has gradually become the main power source for electric vehicles and hybrid vehicles.However,the energy density of power batteries for new energy vehicles can no longer meet our needs.Therefore,the further development of new energy vehicles requires the development of power batteries with higher energy density.The theoretical specific capacity of silicon is 4200mAhg-1,which is the anode material with the highest theoretical specific capacity found so far.In addition,silicon has a series of characteristics such as a wide range of sources,cheap,high energy density,suitable working voltage and high rate safety.Therefore,silicon materials are considered as the negative electrode of the next generation of LIBs with great potential.However,the insertion of lithium ions into silicon can form a lithium-silicon compound during charging progress which will cause serious volume expansion(about 300%).It will bring about pulverization of the negative electrode material and make the material lose electrical contact with the current collector.Which eventually causes irreversible degradation of battery capacity.Studies have found that reducing the particle size of silicon materials to nanometers can effectively alleviate the volume expansion during charging which can significantly improving the cycle stability of LIBs.The existing nano-Si preparation methods can be divided into physical methods and chemical methods.The physical methods are high-energy ball milling,physical vapor deposition,laser processing of bulk silicon and so on.Chemical methods are chemical vapor deposition,chemical reduction,electrochemical etching and so on.Nano-Si prepared by the physical methods has the characteristics of more impurities,wide particle size distribution,serious agglomeration and complicated operation.So they are not suitable for mass production.Nano-Si prepared by chemical methods is relatively widely used.Chemical vapor deposition(CVD)can prepare silicon nanospheres,nanotubes,nanowires and so on.All of which exhibit excellent electrochemical performance.But silane is expensive,pyrophoric and highly toxic which is not an ideal precursor material.Moreover,CVD requires expensive equipment which is only suitable for small batch preparation in the laboratory and cannot be produced on a large scale.The reduction method is difficult to prepare high-purity nano-Si while the unreacted silicon oxide will reduce the electrochemical performance.Existing methods for preparing nano-Si often require high temperature conditions or expensive reagents and the preparation process is cumbersome.There were potential safety hazards and pollution if the post-processing were improper These reasons have caused the current high industrial production cost of nano-Si which severely limits the application of materials.This thesis is based on the previous study on the preparation of nano-Ge from LiZnGe.Now we propose a method for efficiently preparing nano-Si.The ternary intermetallic compound Li2ZnSi is synthesized from Li,Zn,and Si as the precursor through the solid-phase synthesis method.The precursor can be spontaneously decomposed in the air,but it takes a relatively long time.In order to accelerate the decomposition process,we make it react with water,acid and buffer solution.The results show that they can all decompose the precursor,but the reaction is too violent and the product is not nano-Si.We tried a low-speed ball milling reaction at room temperature and found that it only takes 36 hours to complete the precursor reaction.The products are lithium oxide,zinc and amorphous silicon.The amorphous silicon could be crystallized by a 30-minutes annealing treatment.After purification,pure crystalline nano-Si can be obtained that the size is between 15-45 nm.The nano-Si is used as the negative electrode material of the LIB s that are charged and discharged.It is found that the first cycle discharge specific capacity is 3248.8mAh·g-1 and the first cycle coulomb efficiency is 82.78%.After 100 cycles,the specific capacity is 1780 mAh·g-1 which the performance is better than commercial Alfa nan-Si.In this paper,the decomposition mechanism of the precursor is discussed.Li2ZnSi has a layered structure.Zn atoms and Si atoms constitutes an anion framework and Li atoms are filled between layers to maintain the stability of the structure.Lithium is more active.When reacts with oxygen and other substances,it would detach Li atoms from this structure which cause the anion framework to collapse.There is no binary phase between Zn and Si,so the atoms of Zn and Si are assembled separately.They could form Si and Zn nanomaterials in a low temperature.Compared with the traditional method,this method has the characteristics of simple operation,low energy consumption,short cycle,safety and no pollution.So it is suitable for batch preparation of nano-Si materials.