| Considering the current demand of electric vehicles and other industries for secondary batteries with high specific energy and high specific power,and the theoretical specific capacity(more than 4000mA h g-1)of silicon as the anode of lithium-ion battery is very high,so how to apply silicon materials to lithium batteries has become a research hotspot.However,in the lithium-ion battery,with the charging and discharging process,the volume expansion of single silicon is very large(more than 300%),which makes it impossible to form a stable SEI film on its surface.With the continuous renewal of SEI film,lithium ion is no longer involved in the charging and discharging cycle of the battery,resulting in the overall capacity of the battery.As the anode of lithium-ion battery,carbon material has the advantages of rich micropores,high strength and strong conductivity,and the precursor used to prepare carbon material has a wide range of sources,which is easy to be modified according to the demand.Therefore,in this study,nano silicon and carbon material are compounded,and the performance of the material is explored through the structure design and preparation method optimization.The main content and conclusion of this paper as follows:(1)Synthesis and electrochemical properties of silicon carbon composite with different thickness of carbon core-shell structure.The low molecular weight resorcinol formaldehyde resin(RF)produced by the reaction of formaldehyde and resorcinol was coated on the surface of nano silicon(SiNPs)and carbonized to obtain the conformal coated silicon carbon composite.The effect of surfactant cetyltrimethylamonium bromide(CTAB)on the morphology and cyclic properties of the material in the RF coating process and the effect of different RF coating amount on the conformal properties were explored The influence of the electrochemical properties of the coating materials.The results show that CTAB can improve the dispersion of SiNPs in aqueous solution,make RF polymerize on SiNPs more easily,and finally achieve uniform conformal coating.The composite material with core-shell structure of silicon carbon can alleviate the volume expansion of SiNPs,stabilize the SEI film on the surface of the material,strengthen the conductivity of the negative electrode,and achieve a reversible specific capacity of 1124 mA h g-1 after cycling for 100 cycles under the current density of 200 mA g-1.(2)Carbon coating carbon/silicon composites were prepared with RF of different proportions through the spray drying.The negative performance of the composites was analyzed,and the influence of technological parameters on the properties of the composites was also explored.The spherical carbon silicon polymer was redispersed in aqueous solution,then formaldehyde and resorcinol were added to produce low molecular weight RF,then spray dried,collected and carbonized.The relationship between the amount of RF addition and the morphology of the materials and the properties of the negative electrode was investigated.The results show that the more phenolic resin is added,the closer the shape of the sample is to the sphere,and the larger the specific surface area is.However,when the carbon content reaches 66.1 wt.%,the mass transfer resistance of lithium ion in the negative electrode increases obviously,resulting in the decrease of the specific capacity.When the carbon content is 51.5 wt.%,the cycle performance of the material is relatively stable.After 500 charge discharge cycles,the reversible specific capacity is 764.3 mA h g-1,and the capacity is only 13.2%decreased.(3)Glucose and FeSO4 were added to the uncarbonated materials in the second chapter,for catalytic carbonization,and the influence of different proportion of catalyst on the negative electrode performance of the materials was investigated.The experimental results showed that the method improved the proportion of graphitized carbon in the materials,enhanced the charge conductivity of the materials,and the first coulomb efficiency.When the carbon content of the sample is 51.8 wt.%,the specific capacity of the material after 500 cycles is 786.3 m h g-1,and the capacity is only decreased by 11.8%. |
PDF Full Text Request