Preparation Of Nano-silicon-based Anode Materials And Lithium Storage Performance

With the rapid development of portable electronic devices and electric vehicles and other industries,the demand for high energy density batteries is becoming more and more urgent.The development of high performance anode materials has become the fastest breakthrough to improve energy density as the energy density of cathode materials is getting slower and slower at this stage.However,the poor intrinsic conductivity of silicon anode materials and the severe volume expansion（up to 300%）of the embedded lithium make silicon anode materials suffer from low Coulomb efficiency and poor cycling stability,while the high cost of silicon precursors and the complicated preparation process also limit the large-scale development of silicon anode materials.In response to the above problems,this paper starting from preparation of silicon materials,carries out structural design,element doping and silicon-carbon composite modification studies respectively,and explores the crystal structure and material morphology of the modified silicon materials.And the changes in pore structure,etc.,and related electrochemical performance tests were carried out at the same time.The main research conclusions obtained are as follows:（1）The cobalt-containing silicon oxide precursor（Co-Si O₂）was prepared experimentally using the simple and efficient St（?）ber method,followed by the reduction of the precursor using the magnesium thermal reduction method,and the porous silicon material was obtained by removing the Co template after acid treatment.The prepared silicon material showed a porous pomegranate-like structure when the molar ratio of silicon to cobalt was controlled as 10:1,and the reversible discharge specific capacity was 1182.2 m Ah/g at a multiplicity of 4 A/g.At a current density of2 A/g,the first coulomb efficiency was as high as 84.7%,and the charge specific capacity was 616.2 m Ah/g after 300 cycles,showing excellent multiplicity performance and cycling stability.The main reason for the excellent performance is that the removal of the Co template optimizes the pore structure of the silicon material,forming a pomegranate-like porous silicon with inter-cross-linked shells and abundant internal pores,which reserves space for volume expansion and shortens the Li⁺transport path,while ensuring the stability of the electrode structure.（2）Although the construction of a porous structure can reduce the volume expansion problem,the low conductivity problem of silicon materials has not been improved.In this section,phosphoric acid is used as the phosphorus source,simple and efficient pre-hydrolysis and thermal reduction reactions are used to realize the phosphorus doping strategy for silicon materials,and the influence of phosphorus doping concentration on the microstructure and lithium storage performance of silicon materials is explored.The results show that the doping of phosphorus improves the intrinsic conductivity of silicon materials and increases the electron transmission rate.The best doping ratio of phosphorus is obtained through research.When the molar ratio of phosphorus to silicon is 1:8(i.e.P_0.125Si),silicon materials have the best microstructure and electrochemical properties.Compared with pure Si,P_0.063Si and P_0.188Si,P_0.125Si has coral-like structure,which is beneficial to reduce the stress during charge and discharge and ensure the mechanical stability of electrode structure.The P_0.125Si electrode has a specific charge capacity of 2460.4 m Ah/g at a current density of 0.2 A/g.The cycle performance of P_0.125Si electrode was tested at 2 A/g.After 100cycles,the charge specific capacity was as high as 1564 m Ah/g,and at a high rate of16 A/g,the reversible charge specific capacity was 911 m Ah/g,and after 500 cycles,the capacity was 757 m Ah/g,showing good high rate and long cycle performance.（3）Using low-cost natural sepiolite and sodium alginate as the silicon source and carbon source,the structure design and conductivity improvement of the silicon material were carried out at the same time.After simple magnesium thermal reduction and carbonization,a nano-silicon fiber composite carbon composite material（Sep-Si@SA-450）.TEM analysis shows that Sep-Si@SA-450 has a fibrous structure.This fiber structure can alleviate the volume effect of the electrode during the cycle.The combination of carbon materials improves the conductivity of the electrode,reduces the electrode polarization,and maintains the stability of the electrode structure during the cycle.Sep-Si@SA-450 has excellent rate performance and cycle stability.At a current density of 8 A/g,the Sep-Si@SA-450 electrode has a discharge specific capacity of 891.3 m Ah/g,at 2 A/g The discharge specific capacity after 200cycles at a current density of 1354.2 m Ah/g is 1354.2 m Ah/g,and the capacity retention rate is as high as 78.2%.