Preparation Of High-performance Mesoscopic Size And Above Silicon/Carbon Composite For Lithium-ion Battery Anodes

Since the emergence of lithium-ion batteries(LIBs),graphite has gradually become its negative electrode material due to its excellent stability.However,as the demand for energy storage markets(especially electric vehicles)grows,the shortcomings of its low theoretical capacity(372 mAh/g)are also increasing.Because of its high theoretical capacity(theoretically,the capacity of Li4.4Si alloy is 4200 mAh/g),silicon has become an attractive candidate material to replace commercial graphite anodes.However,large volume changes during charging and discharging cause the particles to be easily pulverized,separated from the current collector,and the battery capacity decays quickly;poor conductivity leads to poor battery rate performance.These shortcomings limit its wide commercial application.The combination of small-sized silicon and carbon to form a silicon/carbon composite anode material is currently the focus of research to alleviate the above-mentioned problems,but the cost is high,the process is complicated,and even pollutes the environment.In order to reduce costs and processes,this paper uses a cheap millimeter-scale industrial silicon-calcium alloy as the silicon source,and uses molten salt chemistry to synthesize silicon/carbon composite anode materials at a low cost.The following research results are obtained:(1)In the calcium chloride-sodium chloride molten salt at 600℃,the industrial siliconcalcium alloy was oxidized by calcium carbonate,and a high-performance flake silicon/carbon composite lithium-ion battery anode material was prepared by one-step co-deposition.Its first discharge specific capacity is 1805.6 mAh/g,the corresponding first coulombic efficiency is 80.5%.The first discharge specific capacity of the silicon/carbon negative electrode material after ball milling is 1544.7 mAh/g,the corresponding coulombic efficiency is 52.7%,and there is still 911.8 mAh/g after 200 cycles of charge and discharge.When tested at a current density of 2 A/g,the capacity after 200 cycles is 765.4 mAh/g,which is 95.1%of the capacity when the stable cycle starts on the 25th circle.In the rate performance test,at a current density of 2 A/g,the capacity is 82.9%of that at 0.2 A/g,showing an excellent capacity retention rate.Moreover,studies have found that the elemental silicon in the industrial silicon-calcium alloy has an effect on improving the discharge specific capacity of the composite material.The research on the influence of synthesis temperature and synthesis time on the product shows that the flake Si/C composite material prepared by reacting at 600℃ for 1 hour has better electrochemical performance.(2)In the same environment,flake silicon was prepared by using silicon dioxide silicon oxide calcium alloy.By comparing Si,Si/G,Si@C,and Si/G@C(1:1)composite materials,it was found that the electrochemical cycle of Si/G@C(1:1)is the most stable.After 200 cycles,the capacity can still remain 644.7 mAh/g,and with the increase of the silicon mass ratio,the capacity of the composite material also increases,but the stability of the long cycle relatively declines.The cycle and rate performance of the HF-Si obtained after removing oxides by HF etching on the prepared flake silicon is greatly improved.After 500 cycles,there is still a capacity of 889.5 mAh/g.Studies have shown that the first discharge specific capacity of flake silicon obtained by extending the synthesis time is also higher.