Structure Design And Carbon-coating Of Nanoporous Si/graphite Composite Anode And Their Li Storage Performance

Lithium-ion batteries（LIBs）are gaining increasing attentions and great commercial success in the fields of mobile electronic devices,electric vehicles and large-scale energy storage systems.However,the traditional graphite anode approaches its theoretical capacity,which cannot meet the current demand for higher specific energy in batteries.Therefore,it is urgent to develop anode with higher theoretical energy density.Alloy anode represented by Si can realize reversible storage of lithium by alloying with lithium,and its theoretical capacity can reach 3579 m Ah g^-1,which is about ten times that of graphite.At the same time,it is considered the most promising anode material because of its low cost and low working voltage（～0.4 V vs.Li/Li⁺）.Unfortunately,its poor conductivity and huge volume evolution（>300%）during（de）lithiation seriously restrict their practical applications.These problems result in large electrode polarization,repeated particle breakage,continuous generation of solid electrolyte interface（SEI）layer,which eventually lead to the battery failure.How to improve the cyclic stability of the Si-base anode through electrode structure design and surface interface control is an important problem.In order to solve the above problems,this paper proposes the nanoporous design of silicon-base anode,in-situ composite with graphite materials,and material design ideas for carbon coating on the surface of composite materials.It also systematically studies the reversible lithium storage performance of composite materials.The specific research is as follows:（1）Carbon coated nanoporous Si（NP-Si@C）has been prepared from commercial low-cost Al-Si alloy,pitch,etc.,using chemical dealloying,liquid phase surface asphalt,and pitch pyrolytic carbon coating.Pitch pyrolytic carbon coated porous Si/graphite（NP-Si@C/Gr）composites were prepared by the composite of the above materials with graphite.The optimum conditions for pitch pyrolysis were determined by adjusting the annealing temperature,and the effects on the electrochemical properties of the composites were studied systematically.The effect of the coating structure of pitch pyrolytic carbon layer on the electrochemical properties of the composite was investigated by comparing the electrochemical properties of porous Si and graphite composites.As a result,NP-Si@C/Gr shows a high specific capacity of 764.2 m Ah g^-1after 100 cycles;it delivers a discharge capacity of 1165.1 m Ah g^-1 at 100 m A g^-1 and can still recover to 1138.4 m Ah g^-1 when it returns to the initial state after experiencing cycles at 2000 m A g^-1,with a retention rate of up to 97.7%.Furthermore,the NP-Si@C/Gr//LFP full battery was assembled to test its potential application in LIBs(a high discharge capacity of 98.7 m Ah g^-1 after 80 cycles at 0.5 C).The results show that the presence of pitch pyrolytic carbon layer is beneficial to improve the interfacial compatibility of Si and graphite,which promotes the synergistic effect between different materials,and significantly improves the electrochemical performance of the composites.（2）In view of the difficult problems such as particle agglomeration in the composite process of nano Si and graphite materials,a preparation method of Si/graphite/carbon composite anode materials which is easy to be expanded on a large scale was developed based on the above research.The construction of porous structure in micron silicon particles and the close bonding of porous silicon and graphite in composite materials were realized by wet-chemical method.SEM characterization of the recycled electrode plates showed that the porous silicon particles still maintained a complete spherical morphology after up to 50 cycles,indicating that the porous structure in silicon can effectively buffer volume changes,so as to effectively avoid particle breakage.The close combination of silicon and graphite gives full play to the synergistic effect between the two,which is conducive to the graphite as a conductive and buffering substrate.The pitch pyrolytic carbon was further used to cover the material,and the carbon coating amount of the composite was explored and optimized.When the mass ratio of porous silicon/graphite composite to asphalt precursor was 5:1,the electrochemical performance of the composite was the best.The NP-Si/Gr@C anode prepared under the optimal ratio conditions showed a high specific capacity of567.9 m Ah g^-1 after 300 cycles.The results show that the porous structure,highly dispersed graphite,and multiple buffer structures of the carbon layer outside the composite significantly improve the cyclic stability of the composite.Finally,the full cell(134.75 m Ah g^-1 high discharge capacity after 100 cycles at 0.5 C)was coupled with LFP cathode to verify the practical potential of the composite.