Fabrication And Electrochemical Performance Of Si-based Composite Andoe Materials For Lithium-ion Batteries

In recent years,lithium-ion batteries have been widely used in portable electronic devices such as mobile phones and laptops,as well as new energy vehicles.At present,graphite materials are major commercialized anode in LIBs.However,the theoretical specific capacity of graphite anode materials is lower,which is far from the development requirements of power energy batteries.Currently,silicon anode is an emerging material with fast charge and highly safe characteristics,its intercalation-deintercalation lithium potential(0.02?0.6 V vs.Li/Li+),theoretical discharge specific capacity of 4200 mAh g-1,environmental friendliness,abundant raw material sources,and low cost.It has been attracted much attention and extensively researched by academicians and scientists.It is considered to be an ideal anode material for high-performance and high-safety lithium-ion power batteries instead of graphite materials.However,its further application and development were largely restricted by the following two major factors:its poor rate performance caused by the low conductivity and depressed ion diffusion kinetics;and poor cycle performance caused by huge volume expansion(320%)and unstable SEI film.To solve the above problems,this paper designs and synthesizes nanoporous silicon and its composites with different morphologies from different angles.The structure and lithium storage properties of the materials are studied,and the microstructural properties are analyzed.The main tasks as follows:(1)The nano silicon/reduced graphene oxide(Si/rGO)composites have been prepared by self-assembly method,and porous Si/rGO composites(Si@void@rGO)that etched by HF acid have been prepared to provide space for silicon volume expansion.The effects of different ratios between silicon and carbon on the morphology,structure and electrochemical properties of Si/rGO composites have been investigated.An initial discharge specific capacity of 2252.4 mAh g-1 and a charge specific capacity of 2020.9 mAh g-1 of Si/rGO-1 at a current density of 100 mA g-1,corresponding to the initial Coulombic efficiency(CE)of 89.7%.After 50 cycles,the reversible specific capacity is maintained at 1155.3 mAh g-1.The reversible specific capacity of Si@void@rGO achieves 1900 mAh g-1 after 50 cycles at a current density of 100 mA g-1,even delivers 1142.1 mAh g-1 at a high current density of 1000 mA g-1.The synergistic effect between the wrinkle structure of rGO and free space structure alleviates the volume change for silicon materials,showing excellent cycle life and rate performance.(2)SiO2 anchored on graphene oxide(Si02@GO)has been obtained by hydrolysis of tetraethyl orthosilicate(TEOS)under alkaline conditions.The porous silicon anchored on reduced graphene oxide(P-Si@rGO),the graphenelike porous silicon anchored on reduced graphene oxide(GP-Si@rGO),and graphene-like porous silicon nanosheets(GP-Si)have been obtained by direct magnesiothermic reduction(MR),NaCl-assisted MR,and high-temperature heat treatment combined with NaCl-assisted MR,respectively.The first charge specific capacity of P-Si@rGO-3 is 715.7 mAh g-1,which retained at 296 mAh g-1 after 50 cycles at 100 mA g-1.The first specific discharge capacity of GPSi@rGO is 1417.8 mAh g-1 at a current density of 100 mA g-1,the reversible capacity reaches 549.1 mAh g-1 after 50 cycles,corresponding to a capacity retention rate of 71%.The higher electrochemical performance of GP-Si@rGO than P-Si@rGO-3 is attributed to the fact that the molten salt assists the MR process to produce uniform porous silicon.It is beneficial to alleviate the volume effect of silicon and improve cycle stability.The reversible specific capacity of GP-Si still maintains at 1276.9 mAh g-1 at 200 mA g-1 after 200 cycles,its electrical properties are much higher than those of samples that come from a direct MR or NaCl-assisted MR.This may be due to the graphene oxide is reduced in advance by heat treatment to increases the conductivity and porosity of the material and facilitate the Li+ion diffusion and transmission.The pore structure of GP-Si can relieve the volume change of silicon to improve mechanical stress and cycle stability.(3)The spherical ordered mesoporous silica has been prepared by the onestep soft template method,and the mesoporous silicon future has been obtained via direct MR.The effect of the MR mechanism on the spherical mesoporous silica porous channel and electrochemical performance is investigated in this chapter.The results show that the morphology and ordered pore channel of spherical ordered mesoporous silica are destroyed.The specific surface area is reduced,and the size of the pore is increased after MR.To improve the properties of the material,the spherical mesoporous silicon@carbon(SP-Si550@C)composite material was obtained by in-situ polymerization of phenolic resin carbon to coat spherical mesoporous silicon.It can be observed that as the current density increases simultaneously,the first CE also increases due to the disordered pore structure that can perform charge and discharge processes in multiple directions,and the irreversible capacity loss is small.The initial discharge specific capacity and first CE of the sample is 2929.6 mAh g-1 and 79.97%,respectively,and the reversible specific capacity is still maintained at 1325 mAh g-1 after 50 cycles at a current density of 100 mA g-1.(4)A controllable self-assembled mesoporous silicon nanocrystals framework(MRHDE-Si)was adjusted directly by employing NaCl as a liquid interface via a(MR)method.The mass ratio of Mg powder to NaCl has a significant effect on the pore structure,crystallinity,and electrochemical properties of MRHDE-Si.The reversible specific capacity and discharge specific capacity of MRHDE-Si are 1290.9 mAh g-1 and 1309.6 mAh g-1 after 50 cycles at 200 mA g-1.The reversible capacity is as high as 567.5 mAh g-1 at 2 A g-1.Its smallest particle size(23 nm),appropriate and regular closed pore structure,pore volume(0.69 cm-3 g-1),and the most appropriate pore size(12.64 nm)can provide more lithium ions to be inserted into the vacancy,shorten the diffusion path of Li+,enhance the charge transfer capacity,and serve as a buffer area to effectively release the stress caused by Si volume expansion.This selfassembly strategy provides a simple and easy to operate method for the design and preparation of silicon materials with controlled porous structures,which has great potential in the field of new energy.