Preparation And Energy Storage Performance Of Silicon-based Composite Materials For Lithium Ion Battery Anode

Lithium-ion batteries have matured over the past few decades and have become one of the most important commercial energy storage devices.In commercial applications,increasing the capacity of the battery and prolonging its service life are the goals that people always pursue.Silicon-based materials have the highest theoretical capacity(?4200 mA h g-1)currently known,which is more than ten times that of graphite.In addition,the low discharge potential and weak polarization effect make silicon the most promising anode material for lithium ion batteries after graphite.However,the huge volume expansion of the silicon negative electrode during the charging and discharging process,the continuous formation of the solid electrolyte interface film and its own weak conductivity have limited its practical application.In order to solve the above-mentioned problems,in this paper,starting from two aspects of material and structure,through careful design and reasonable process,a series of silicon-based composite materials with optimized structure have been prepared.Compound Si with good conductor materials to try to improve the electronic conductivity of silicon materials;through reasonable design,optimize the spatial structure of the materials,provide buffer space for the dramatic volume changes of silicon during charging and discharging,and stabilize the electrode structure to achieve silicon High capacity and long life of negative electrode.(1)Based on the electrostatic self-assembly and hydrothermal process of Si nanoparticles and MXene nanosheets,a silicon-based composite material SiNP@MX 1/MX2 with a three-dimensional porous structure and double MXene coating was constructed.The electrostatic self-assembled MXene layer(MX 1)can effectively avoid agglomeration between Si particles and reduce the volume expansion tension of silicon;the three-dimensional MXene network skeleton(MX2)effectively improves the electronic conduction and ion transmission performance of the electrode material,greatly improving the diffusion power At the same time,the synergy of dual MXene(MX1 and MX2)provides sufficient buffer space for the volume expansion of Si.The material shows excellent cycling performance(the reversible capacity after 200 cycles at a current density of 500 mA g-1 is 1422 mA h g-1)and good rate performance(at 50,100,200,500,1000,The corresponding first discharge capacities at 5000 mA g-1 current density are 2865,1895,1686,1407,1177,852,and 574 mA h g-1,respectively,with high coulombic efficiency and long life.(2)Based on the microemulsion template method,flower spherical SiO2 precursors were prepared at different temperatures by hydrolysis of ethyl orthosilicate.Using glucose as the carbon source for carbon coating and then performing magnesium thermal reduction,a Si@C composite material was prepared.Studies have shown that the precursors prepared at 70? have the most stable morphology and structure,the most uniform particle size distribution,and a well-developed radial pore structure.The Si@C composite material after magnesia reduction largely retains the unique structure of the precursor,and the in-situ etched holes are additionally formed by removing the by-product MgO,which further enriches the pore structure.The test results show that the specific surface area of the Si@C-5 sample is 589 m2 g-1,the pore size distribution is 10-12 nm,and the optimized structure provides sufficient space for Si expansion.In addition,the effect of different carbon coating amounts on the electrochemical properties of Si@C composites was explored.Among them,Si@C-5 showed the most excellent cycle performance(reversible capacity of 808 mA h g-1 after 100 cycles at 500 mA g-1 current density)and excellent rate performance(at 50,100,200,500,1000,5000 mA g-1 current density reversible capacity is 1492,1273,954,783,597,536 and 425 mA h g-1),indicating that the increase in carbon content synergistically enhances electrode materials Ion transport and electron conduction,and help maintain the structural integrity of the silicon electrode.(3)Si-Cu alloys with different mass ratios were prepared by solid-state mechanical ball milling.Through the constant current charge and discharge test,the effect of copper and silicon-copper mass ratio on the initial coulombic efficiency(ICE)of the electrode material was studied.The results show that when Si/Cu is 15:1,10:1,5:1,and 1:1,the corresponding ICEs are 76.0%,89.4%,82.5%,and 73.2%,respectively,which are higher than 65.8%for pure silicon samples.Indicating that the introduction of Cu acts as a buffer protection layer,improves the mechanical strength of the material,reduces the resistance of the silicon material,and can effectively improve the ICE of the silicon electrode.The Si-Cu alloy was further combined with graphene,and the graphene network-wrapped Si-Cu@rGO composite material was prepared by hydrothermal driving and heat treatment.As a highly conductive material,graphene enhances the electrical contact of active materials,optimizes the transmission path of electrons and ions,and its continuous pleated structure provides free space for the volume expansion of silicon nanoparticles.The cycle test shows that the introduction of graphene significantly improves the capacity retention rate of the composite(at 500 mA g-1 current density,the specific capacity after 200 cycles is 954 mA h g-1,and the capacity retention rate is 50.9%)and structural stability.