Preparation And Properties Of High Cycle Life Silicon Carbon Composite For Lithium-ion Batteries

The silicon-based material with the high theoretical specific capacity of 4200 mAh·g^-1 and a low delithiation potential of 0.37 V vs.Li/Li⁺,which is a very promising anode material for high performance lithium-ion batteries.However,the silicon-based material is accompanied by a large-volume change up to 400%during the intercalation/deintercalation of Li⁺,resulting in the crushing and pulverization of the silicon particles and the loss of electroactivity of the electrode material,which severely hinders the commercialization of silicon-based material as an anode for lithium-ion batteries.Carbon materials with excellent electrical conductivity,outstanding chemical/electrochemical stability and unique physical properties,and have a wide range of sources and low production costs.Therefore,carbon materials are often used as the preferred substrate for the preparation of silicon-based composite.The preparation of silicon-carbon composite not only improves the conductivity of silicon,but also effectively suppresses the volume expansion Si anode,prevent the agglomeration of silicon nanoparticles,promote the formation of a stable SEI?solid electrolyte interface?film,thereby increasing the electrode's coulombic efficiency,charge/discharge specific capacity,cycle performance and rate capability.Based on the analysis of silicon and carbon materials,the mechanical properties of different pyrolytic carbons,such as hardness,friction coefficient and wear resistance,are characterized.The electrochemical properties of silicon-carbon composites are measured to study the effect of different pyrolytic carbon mechanical properties on the electrochemical performance of silicon-carbon composites.Meanwhile,new-type of silicon-carbon composite structures are being explored to prepare silicon-carbon anode materials with high cycle life and large lithium storage capacity,which provides experimental basis and ideas for the preparation of high-performance silicon-based energy storage materials.The main research contents are as follows:?1?Different pyrolytic carbons were prepared with sucrose,glucose,and fatty alcohol polyoxyethylene ethers?AEO-7?as carbon sources,respectively.The mechanical properties of the pyrolytic carbon,such as hardness,friction coefficient,and wear resistance were measured,and different silicon-carbon composites were successfully prepared.The results showed that the pyrolytic carbon of aliphatic alcohol polyoxyethylene ethers with small friction coefficient,strong wear resistance and high hardness,which is beneficial to improve the cycle life of silicon-carbon composites.Moreover,the single carbon source coating cannot completely solve the problem of volume expansion of Si.The synergistic effect of various carbon sources and the preparation of porous silicon carbon composites are needed to suppress/accommodate volume effect of silicon.?2?A novel silicon-carbon composite with a 3D porous bird-nest structure denoted as Si@SiOx/CNTs@C was designed and prepared.The results showed that fatty alcohol polyoxyethylene ether as a surfactant effectively reduced the surface tension of Si NPs and improved the dispersion of Si NPs.Meanwhile,the pyrolytic carbon generated from fatty alcohol polyoxyethylene ether and the SiO_x layer formed by pre-oxidation treatment effectively inhibited the volume effect of Si NPs in lithiation/delithiation process and promoted the formation of a stable SEI film.In addition,the 3D conductive network formed by carbon nanotubes,graphite,and pyrolytic carbon increases the conductivity,and forms a rich pore structure,which is beneficial to the rapid penetration of the electrolyte and the rapid conduction of lithium ions.The specific capacity of the Si@SiO_x/CNTs@C composite can be maintained above 1740 mAh·g^-1 at a current density of 0.42 A·g^-1 after 700 cycles,and it has no attenuation compared with the initial capacity.The reversible specific capacity of the composite can reach450 mAh·g^-1 at a high current density of 4.2 A·g^-1,showing excellent cycle performance and rate capability.?3?Si@C microsphere composite with multiple buffer structures was prepared via hydrothermal treatment.By virtue of ferric citrate as the elaborately chosen coated carbon source to prepare mesoporous carbon by pyrolysis and leaching.The silicon nanoparticles with SiO_x layer are encapsulated in the homogeneous mesoporous carbon layer,the mesoporous carbon layer and SiO_x layer with toughness can primely suppress the volume expansion of silicon.Meanwhile,the plentiful mesopores in the carbon layer can effectively buffer and accommodate the volume change of silicon,and greatly improve the infiltration of the electrolyte to the anode.The support network formed by carbon nanotubes with high electrical conductivity and good mechanical strength which can greatly buffer the volume expansion of Si@C microspheres and improve the fatigue resistance of the composite.Therefore,with the multiple buffer structures described above,the Si@C electrodes exhibited a high initial charge/discharge capacity of 2956/4197 mAh·g^-1 at a current density of 0.42 A·g^-1,and the reversible capacity can be maintained above 1127 mAh·g^-1 after 800 cycles at a current density of 2.1 A·g^-1,showing superior rate capability and good cycle performance.