Preparation And Modification Of Silicon-Based Anode Composites For Lithium-Ion Batteries

As portable and efficient energy storage devices,lithium-ion batteries are widely used due to long cycle life and high energy conversion efficiency.The performance of anode materials directly affects the performance of lithium-ion batteries.Currently,the commonly used anode material is graphite.However,graphite has a low theoretical discharge specific capacity（only 372 m Ah/g）and poor compatibility with the electrolyte,making it difficult to use in lithium-ion batteries with high energy density.Si and Si O₂,with high specific capacities of 3590 and 1965 m Ah/g at room temperature respectively,can be used in lithium-ion batteries with high capacity and high energy density,showing promising application prospects.However,volumes of Si and Si O₂rapidly expand during the insertion and removal of lithium ions,resulting in the fragmentation of material particles,continuous exposure of new surfaces to the electrolyte,repeated breakage and regeneration of the surface solid electrolyte layer（SEI）,and weakening of electrical contact between particles and between particles and current collectors,ultimately leading to the failure of the lithium-ion battery.To solve these problems caused by volume expansion and low conductivity of Si and Si O₂,the effects of metal doping,particle size,porous carbon coating,particle size control and novel MXene carrier on Si and Si O₂ performance were investigated in this work.The specific research contents are as follows:（1）The industrial wasted micro-silicon powder,ammonium sulfate and critic acid were used as silicon source as template and carbon source.Ball milling was taken to reduce the particle size of silicon and mix iron powder into silicon.Afterwards,the carbon coated porous silicon-based composites were prepared by heat treatment.The effects of the amount of citric acid and ammonium sulfate,the pre-carbonization time and temperature of citric acid on morphology and electrochemical properties of materials were studied.Results showed that the obtained material had excellent electrochemical properties when the weight ratio of ball-milled silicon powder,ammonium sulfate,and citric acid was 1:1:7,and the time and temperature for pre-carbonization of citric acid were 90 min and 300℃,respectively.The initial discharge specific capacity was 1376.11 m Ah/g,and initial coulombic efficiency was 70.81%at the current density of 0.2 A/g.After 150 cycles,the discharge specific capacity remained at 791.78 m Ah/g.At the current density of 12 A/g,the specific discharge capacity was 196.21 m Ah/g.When the current density returned to 0.1 A/g,it still had the specific discharge capacity of 903.88 m Ah/g.These results demonstrated that large specific surface area could allow material to better soak in the electrolyte,and the iron doping and void space in the material could reduce the strain accumulation inside the material during lithiation.（2）Silicon carbon composites were prepared by heat treatment according to the obtained optimal pre-carbonization time and temperature.Ammonium sulfate and critic acid were used as template and carbon source,respectively.Silicon with different particle sizes（30 nm,100 nm,500 nm and 1μm）were used to study the effects of particle size of silicon on the electrochemical performance.Results showed that the material prepared by 30 nm silicon with initial discharge specific capacity of 2006.32m Ah/g exhibited the best cycle performance.After 200 cycles,the discharge specific capacity decayed to 384.85 m Ah/g.The material prepared by 100 nm silicon showed a discharge specific capacity of 58.74 m Ah/g at 12 A/g.And when the current density returned to 0.1 A/g,it still exhibited a specific capacity of 135.32 m Ah/g,indicating the best rate performance.During the charging and discharging process,larger grain sizes of silicon would form more lithium rich Li-Si alloys and larger stress in these particles,leading to material breakage and the separation of active materials,and resulting in poor cycling performance.Although the smaller size of silicon often possessed better cycling performance,the narrow grain boundaries of silicon with smaller grain sizes limited the diffusion of lithium ions at high rates,resulting in limited rate performance.（3）Layered MXene/Si O₂ materials were prepared by growing Si O₂ on the surface of MXene using the St（?）ber method.Si O₂ contents were changed by adjusting the hydrolysis time of tetraethyl orthosilicate（TEOS）.Results showed that the sample with hydrolysis time of 2 hours had a suitable thickness of Si O₂on the surface of the sheet,and Si O₂ particles did not agglomerate.And it exhibited better stability and electrochemical performance.When tested at the current density of 0.2 A/g,the initial discharge specific capacity was 227.27 m Ah/g,and the initial coulomb efficiency was88.75%.After 140 cycles,the discharge specific capacity was 181.82 m Ah/g.It had a discharge specific capacity of 77.95 m Ah/g at 3 A/g.When the current density returned to 0.1 A/g,the discharge specific capacity could reach 197.21 m Ah/g,demonstrating that excellent electronic conductivity of MXene could greatly improve the rate performance of the material.