Preparation Of Silicon/Carbon Composite Anode Materials And Performance Improvement For Lithium Ion Battery

Lithium-ion batteries have been commercialized and investigated for several decades,but essential issues including the low theoretical capacity of electrode materials?for example,³72 mAh/g of graphite in the form of LiC₆?restricts the development of lithium-ion batteries.Silicon is one of the most prospective anode materials for next-generation lithium-ion batteries due to extremely high theoretical specific capacity,low discharge voltage platform of⁰.5 V vs.Li⁺/Li and abundant resources in the earth's crust.However,the industrialization of silicon-based anodes is hindered by the low intrinsic electronic conductivity,poor structural stability and unstable solid electrolyte interface?SEI?film caused by huge volume change?>300%?,leading to a severe cracking or pulverization of electrodes and a consequently rapid capacity decay during the repeated charge-discharge cycling.In order to solve the problem of poor conductivity,huge volumetric expansion and low initial coulombic efficiency of silicon-based materials,the cubic H-Si@C NB composites were synthesized using the cubic-shaped Co₃O₄ nanoparticles as the sacrificial templates.The average size of H-Si@C NB particles is³60 nm,and the thicknesses of silicon layer and outer carbon shell are¹4 nm and¹6 nm,respectively.The first discharge specific capacity of H-Si@C-6 is 1305.1 mAh/g and the first charge specific capacity is 658.2 mAh/g,corresponding to an initial coulombic efficiency of50.4%.The reversible specific capacity after 100 cycles is 496.4 mAh/g and the corresponding capacity retention is⁷5.3%.The reversible specific capacity is 417.5mAh/g at 5 A/g and 674.5 mAh/g when current density is reduced to 0.1 A/g.Considering that silicon-based materials used in full lithium-ion batteries need to be low-cost,easy to synthesize,possessing high reversible capacity and stable cycle performance,low-cost micro-silicon/graphite/carbon?Si/G/C?composite anode materials were also prepared in a scalable operation process.The first reversible specific capacity of the Si/G/C composite anode materials is 872.6 m Ah/g,and the corresponding initial Coulomb efficiency is 80.5%.The subsequent cycling performance of this material is excellent.In the second cycle,the coulombic efficiency exceeds 96.4%and the capacity retention is 90.2%after 50 charge-discharge cycles.The potential for commercialization of silicon/graphite/carbon composite anode materials has been confirmed,but the initial coulombic efficiency and cycling performance shoule be improved further.The effects of different binders and electrolyte systems on the electrochemical performance of the anode have been investigated.FEC and VC both have a positive effect on the electrochemical performance of the Si/G/C composite materials.10 wt%FEC and 5 wt%VC as additives for electrolyte show the optimum performance,respectively.Li ODFB can significantly improve the cycling stability of Si/G/C anode and button-type batteries as a kind of lithium salt additive.The cycling performance of Si/G/C anode with electrolyte consisting of 0.6 mol/L LiODFB and 0.4 mol/L LiPF₆ is better than that with the commercial electrolyte.1wt%Cs₂CO₃can improve the initial coulombic efficiency of the Si/G/C composite materials but not improve the cycling performance of the anode.1²%Li BOB is beneficial for the formation of SEI film,but the thick SEI film reduces the electrode reaction activity.Among the five kinds of binders?CMC,CMS,PVA,GA,and agar?,CMC exhibited the best adhesion effect for Si/G/C anodes with the thickness of 75?m and 200?m.Using 15-Si/G/C materials?Si/G/C composite materials mixed with graphite in a mass ratio of 15:85?as anode active materials,a pouch cell with a theoretical capacity of 600 mAh was assembled.The cell shows an initial discharge capacity of 392.9 mAh at 0.1 C and coulombic efficiency of 64.8%,and coulombic efficiency increases to 97.9%at the second cycle.The discharge capacity of the pouch cell is maintained at 324 mAh after 170 cycles and corresponding capacity retention rate is 82.7%,indicating that the Si/G/C composite material has a great potential for commercialization.