Preparation And Electrochemical Properties Of Nano-silicon-based Composite Materials

High-performance lithium-ion batteries(LIB)are fundamentally important devices for electric vehicles,portable electronics and renewable energy storage.As for anode materials,although the graphite anode has already been commercialized,its low theoretical specific capacity(375 m Ah g^-1)cannot sufficiently meet the requirement of the next-generation LIBs emphasizing higher energy densities.Due to the high theoretical specific capacity(4200 m Ah g-¹),abundant resource and environmental friendliness,silicon is expected to be new generation anode of lithium-ion batteries.However,poor conductivity and serious volume expansion(?300%)during charge and discharge cycles easily result in pulverization of the particles,aiding the formation of an unstable solid electrolyte interface(SEI)film which results in a rapid decrease in charge-discharge capacity.These shortcomings seriously hamper the commercialization of silicon anode materials.In order to improve the conductivity of silicon-based anode materials and alleviate the volume expansion effect during charge and discharge,Following works were conducted:1)A mesoporous-Si embedded by Sn nano-particles(denoted as Sn@MP-Si)has been successfully synthesized by magnesiothermic reduction,impregnation and hydrogenation reduction.Compared with pure MP-Si,Sn@MP-Si shows more excellent electrochemical performance.After 100 cycles,Sn@MP-Si anode delivers a reversible capacity of 1128.6m Ah g^-1.Even at a current density of 1000 m A g^-1,Sn@MP-Si can maintain the reversible capacity of 589.7 m Ah g^-1after 200 cycles,while showing excellent rate performance.The high-resolution transmission electron microscope shows that Sn particles around 50 to 120nm in diameter are tightly anchored to the surface of MP-Si.Meanwhile,some smaller Sn particles are embedded in the MP-Si pores which effectively support the pore structure and build a bridge network for electrical conductivity.Therefore,Sn@MP-Si has a lower charge transfer resistance and a higher lithium ion diffusion rate,improving the electrode kinetics performance.2)We prepared a Si-C anode material with the yolk-shell structure(denoted as Si@Void@C/rGO),which was synthesized by nano-Si oxidation,surface carbonization and Si O_xetching.The thickness of the oxide layer can be controlled by temperature.The void size in the range of 2-7 nm can be obtained.At the same time,reduced graphene oxide(rGO)wraps the material so that Si@Void@C particles are fixed on the rGO sheet without agglomeration.It was found that Si@Void@C/rGO with a void size of 5 nm possesses the best electrochemical performance among these samples.The discharge capacity is 1294m Ah g^-1at a current density of 500 m A g^-1after 100 cycles.It also offers excellent rate capability and kinetics performance.These enhancements can be attributed to appropriately sized(5 nm)voids which sufficiently accommodate the volume expansion of silicon and stabilize the carbon shell.At the same time,these voids effectively inhibit the growth of the solid electrolyte interface(SEI)film by depressing the decomposition of the electrolyte on the surface of Si in Si@Void@C/rGO.Furthermore,interfaces between Si@Void@C particles and rGO sheets construct bridges for electron conduction,which improves the kinetic performance.3)We synthesized highly packed Si-C,Sn-C core-shell nano-composites(Si@C/Sn@C/rGO).High-resolution transmission electron microscopy shows that Sn particles with a diameter of 100 nm are covered with carbon layer and surrounded by Si@C particles around 40 nm in diameter.The expansion of Si and Sn particles is limited by carbon,and reduced graphene oxide sheets provide flexible support for volume changes.At the same time,Sn and reduced graphene oxide with excellent conductivity act as a bridge for electrical transportation among Si particles.The prepared Si@C/Sn@C/rGO electrode owns high initial coulombic efficiency(78%)and rate capability,and greatly improves the capacity retention rate(close to 1900 m Ah g^-1after 60 cycles).Even at a current density of1000 m A g^-1,a high discharge capacity around 1000 m Ah g^-1can be maintained after 300cycles.