Preparation Of Rice Husk-derived Si@C Based Anode Material,and Characterization Of Its Structure And Electrochemical Performance

Nowadays,Li-ion batteries cannot meet the requirements of long-range electric vehicles,due to its lower energy density.This has become one of the greatest barriers limiting the development and application of electric vehicles.It has revealed that energy density of Li-ion batteries is mainly associated with electrode materials.At present,capacity of commercial graphite anode material is very close to its theoretical capacity(372 m Ah g^-1).It is found that theoretical capacity of Si material is much higher than that of graphite anode,approaching as high as 4200 m Ah g^-1(forming Li_4.4Si alloy).However,Si anode materials suffer an enormous volumetric change as high as 300%during the long-term charging and discharging process,resulting in severe pulverization of Si particle and electrode fracture,and thus serious capacity fading.Recent studies demonstrate that the combination of nano-silicon with various carbon species can effectively avoids the drawbacks of simple Si anode material.This thesis concerns preparation of Si@C based anode materials by a novel and environmentally benign route of ball milling and molten salt coactivated aluminothermic reduction at low temperature,using rice husk（RH）as the starting material.The relations between their compositions,structures,morphology and electrochemical properties were investigated by the technologies of X-ray diffraction,field emission scanning electron microscopy,transmission electron microscopy,X-ray photoelectron spectroscopy,cyclic voltammetry,galvanostatic charge/discharge and electrochemical impedance spectroscopy,in order to reveal the feasible preparation approaches and relative mechanisms for achieving high-performance Si@C based anode materials derived from waste RH.The research contents and main results are as follows:Firstly,preparation of nano-Si derived from rice husk（RH）and its combination with different-size carbon materials were studied.Amorphous silica was obtained by calcining RRHH in air,subsequently it was reduced to nano-Si bbyy aa mechanical milling and molten salt coactivated aluminothermic redduuccttiioonn..Si@C anode materials weree prepared by combining tthhee nano-Si powder wiitthh different-size mixed carbon（reed rod-derived fiber,glucose and conductive carbon black）.The relations between their compositions,structures and electrochemical performance were investigated.Results show that carbon fiber and spherical carbon black constituted a porous three-dimensional conductive network,and the amorphous carbon coated nano-Si particles,whose size is～50 nm,were attached to the carbon conductive network or filled in the pores of the network.At a current density of 0.05 A g^-1,the Si@C anode materials shows a maximum capacity of 1990 m Ah g^-1,and its capacity decreased to1118 m Ah g^-1at 2 A g^-1.After cycling at 0.5 A g^-1 for 200 cycles,it still delivered a high reversible capacity of 823 m Ah g^-1.Compared with the nano-Si anode,the Si@C anode material shows greatly improved performance in terms of rate capability and cycling stability.But its specific capacity and 1^st coulombic efficiency were apparently compromised.Secondly,preparation methods and mechanisms were investigated to obtain high-performance Si@C based anode materials,only using rice husk as silicon and carbon sources.Rice husk（RH）was carbonized at different temperatures（550-1000°C）,then the carbonized RH was reduced to Si@C based anode materials by a mechanical milling and molten salt coactivated aluminothermic reduction.Effects of carbonization temperature were studied on the compositions,structures and electrochemical properties of the Si@C based anode materials.Results show that the carbonized rice husk samples obtained at lower temperatures（≤850°C）are mainly composed of amorphous carbon and amorphous silica,while the one obtained at a higher temperature（1000°C）was revealed to contain a small amount of cristobalite-type silica.The amorphous silica in the low-temperature carbonized RH can be readily reduced to crystalline nano-Si,while the cristobalite-type silica in the high-temperature carbonized RH is difficult to be reduced.The formed nano-Si with a particle size of～5 nm was uniformly embedded in the amorphous carbon and silica matrix.Among them,the Si@C based anode material derived from the850°C-carbonized RH exhibits superior electrochemical performance.The anode material shows a maximum capacity of 2205 m Ah g^-1 at a current density of 0.05 A g^-1,and its capacity was decreased to 740 m Ah g^-1at 2 A g^-1.After cycling at 0.5 A g^-1for 200 cycles,it still delivered a high reversible capacity of 794 m Ah g^-1.In brief,waste RH cann be efficiently transformed to hhiigghh--ppeerrffoorrmmaannccee Si@C based anode mmaatteerriiaallss by the novel and environmental benign method based on the mechanical mmiilllliinngg aanndd mmoolltteenn ssaalltt ccooaaccttiivvaatteedd aalluummiinnootthheerrmmiicc rreedduuccttiioonn..Finally,preparation methods and possible mechanisms were explored for obtaining higher-capacity Si@C based anode material by adding other silicon source into rice husk.Using tetraethyl orthosilicate（TEOS）as silicon source,a sol-gel method was used to add other silicon source into rice husk,resulting in higher-silicon-content Si@C based anode materials.Effects of different silicon-adding methods were studied on the compositions,structures and electrochemical performance of the Si@C based anode materials.Results demonstrate that the Si@C based anode material（TEOS-RH）prepared by a‘rice husk-silicon increasing-carbonization-reduction’route shows higher carbon content,higher graphitization degree and poor uniformity of silicon distribution in carbon matrix,while the anode material（TEOS-CRH）prepared by a‘rice husk-carbonization-silicon increasing-reduction’route exhibits lower carbon content and lower graphitization degree,but higher homogeneity of silicon distribution in carbon matrix.Discharge capacities of TEOS-RH and TEOS-CRH were determined to be 3020 m Ah g^-1 and 2277 m Ah g^-1 at a current density of 0.05 A g^-1,respectively,and their specific capacities are 942 m Ah g^-1 and 828 m Ah g^-1 at 2.0A g^-1.After 200 cycles at 0.5 A g^-1,the specific capacities of TEOS-RH and TEOS-CRH are 981 m Ah g^-1 and 875 m Ah g^-1,respectively.The results show that both of the silicon-increasing techniques can significantly enhance specific capacity of the Si@C based anode material without its rate capability and cycling performance obviously degraded.Among them,the‘rice husk-silicon increasing-carbonization-reduction’route is more significant to increase specific capacity.