Research On Preparation And Electrochemical Performance Of Silicon-based Composite Anode Materials For Lithium Ion Batteries

In recent years,with the development of electronic devices,electric vehicles,hybrid electric vehicles,and large-scale energy storage equipment,it has become a task of top priority to develop low-cost lithium ion batteries(LIBs)with high energy density and long cycling life.Anode material is an important component of LIBs.At present,the commercial graphite anode material with the low theoretical specific capacity cannot meet the requirements of high-capacity LIBs.Si and SiO2,with theoretical specific capacities as high as 4200 mA h g-1 and 1965 mA h g-1 respectively,are regarded as promising candidate anode materials for next-generation LIBs.However,both Si and SiO2 anode materials suffer from fracture and pulverization due to huge volume change during charge and discharge processes,which eventually leads to rapid capacity degradation.Additionally,the low electrical conductivity of Si and SiO2 also induces a poor rate capacity.In this work,aiming at preparation high-capacity and long-life anode materials for LIBs,Si and SiO2 anode materials were studied.Spherical micro-sized porous AlSi with carbon layer(AlSi@C)and SiO2 coated by amorphous TiO2(SiO2@a-TiO2)composite anode materials were prepared.The specific details are as follows:(1)The low-cost commercial AlSi alloy microspheres are used as the silicon source to synthesize porous AlSi by a facile acid etching process.The porous AlSi precursors with various A1 contents were synthesized by delicately controlling acid etching period.Through the polymerization and carbonization process,the AlSi@C composite was prepared by wrapping carbon coating layer on the surface of the porous AlSi precursor.The properties,morphologies,and electrochemical performances of AlSi@C composites with different A1 contents were tested.It is found that a small amount of A1 in AlSi@C is beneficial to maintain the spherical morphology and enhance the electrical conductivity of the anode material.Profiting from the porous structure used for relieving large volume change and dense carbon coating layer used for protecting internal silicon,micro-sized porous AlSi-4@C composite with Al,Si,and C contents of 4.9 wt.%,45.0 wt.%and 50.1 wt.%respectively exhibits the best cycling performance,which exhibits a high initial coulombic efficiency of 84.2%,delivers an initial discharge specific capacity of 1650 mA h g-1 at a current density of 0.50 A g’1,and still retains 824 mA h g-1 after 300 cycles.(2)The SiO2@a-TiO2 composite was synthesized by conformally depositing amorphous TiO2 shells on commercial SiO2 nanoparticles via a facile sol-gel coating approach using the hydrolysis of titanium isopropylate in an alkaline alcohol system.The SiO2@a-TiO2 composite with different TiO2 contents were synthesized by delicately controlling the amount of titanium isopropylate.The properties,morphologies,and electrochemical performances of SiO2@a-TiO2 composites with different TiO2 content were tested.There are three effects after coating amorphous Ti02 layer on commercial SiO2 nanoparticles.First of all,the conductivity of composite could be improved due to the formation of the lithiated Ti02 during the discharge process.Furthermore,the amorphous TiO2 layer coating on the surface of commercial SiO2 nanoparticles could inhibit the side reactions between SiO2 and electrolyte by preventing SiO2 from contacting with electrolyte.Finally,elastic amorphous TiO2 shell could be coated on commercial SiO2 nanoparticles without breaking during repeated volume expansion.The SiO2@a-TiO2-3 composite with TiO2 content of 12.18 wt.%possesses the best performance.At the current density of 0.1 A g-1,the SiO2@a-TiO2-3 composite still retains the reversible specific capacity of 647 mA h g-1 with the high capacity retention rate(relative to the second cycle)of 87.3%after 300 cycles.