Preparation And Electrochemical Performance Of Nano-scale Silicon-based Anodes

Among multitudinous anode materials for lithium-ion batteries,silicon has been regarded as the most potential anode material for the next generation of lithium-ion batteries(LIBs)due to its high theoretical specific capacity(3579 m Ah g^-1)in application.Unfortunately,the huge volume changes during cycling causes the particles fragmentation and pulverization,which leads to the sharp degradation of cycle performance.Moreover,the high cost and extremely harsh synthesis conditions of silicon anode material seriously hinder its application in LIBs.The above problems can be effectively solved by constructing reasonable nanostructures and compounding silicon with high conductivity carbon materials,the electrochemical performance of silicon-based anode materials can be significantly improved.In this thesis,Si/C nanocomposites have been designed and fabricated via magnesium thermal reduction reaction and chemical vapor deposition(CVD)by using hydrolyzed Si O₂and low-cost attapulgite as raw materials.The lithium storage performances of the composites have been investigated.The main results are listed as follow:The spherical nanoporous silicon was prepared using spherical Si O₂which hydrolyzed by TEOS as precursor,and Na Cl as endothermic agent.By simply adjusting the particle size of magnesium powder,the heat accumulation in the reaction can be reduced,so as to maintain the integrity of the morphology after magnesium thermal reduction.When magnesium powder(s-Mg and m-Mg)with smaller particle size is used as reducing agent,too fast reaction speed caused the release of a large amount of heat,resulting in serious heat accumulation.Therefore,the structure of as-prepared m-si and s-si is greatly different from that of Si O₂.However,the prepared l-si can maintain its original shape and obtain highly developed porous nanostructures when the particle size of magnesium powder is adjusted to l-mg because of the thermal accumulation in the reaction is reduced.As a result,the l-si material has the initial discharge specific capacity of 2067 m Ah g^-1and the initial coulombic efficiency of 70%.After 100 cycles,the l-si material have the specific capacity of 1412 m Ah g^-1at 0.5 A g^-1,the specific capacity retention rate is 68%,which is higher than s-si and m-si.Silicon nanoparticles(MRR Si)with uniform morphology and controllable morphology was prepared by magnesium thermal reduction reaction with low cost attapulgite as raw material,furthermore,MRR Si@C composite aggregate can be manufactured by CVD process by depositing a layer of carbon material on the surface of silicon.The as prepared MRR Si have a specific capacity of 2362 m Ah g^-1at the current density of 0.2 A g^-1with the initial coulombic efficiency of 69%,and a capacity of 1148 m Ah g^-1is still kept after 100cycles at 0.5 A g^-1.In contrast,MRR Si@C composite reveals the discharge capacity of 2494m Ah g^-1and higher initial coulombic efficiency value of 76%.The cyclic properties show that the specific capacity of the MRR Si@C composite can reach 1292 m Ah g^-1after 100 cycling at the current density of 0.5 A g^-1.In addition,the composite can still exhibit a high specific capacity of 734 m Ah g^-1at a high current density of 5 A g^-1.The MRR Si@C composite shows excellent rate performance and favourable application prospect.