Synthesis And Electrochemical Performance Evaluation Of Si/CNFs Protected By Extra Carbon Coating

In recent years, the development and utilization of renewable energy has attracted widely interest due to the serious energy and environment problems,which in turn drive the intense study on energy storage and provide a good opportunity for the traditional textile field with the application of fiber in energy storage, leading to the emergence of energy storage fibers. The energy storage fibers should consist of three essential factors:(1) fibers, electrospinning is a simple way to prepare nanofibers;(2) energy storage materials, silicon has the highest theoretical capacity;(3) the electrical conductivity, carbon is a good conductor for electron and ion. Considering this, electrospun Si/CNFs is a good breakthrough point for the research of energy storage fibers. However, there remains a critical shortcoming regards to the poor cyclic stability that arises when Si nanoparticles and their aggregates are exposed on the fiber surface. Without protection, these exposed Si particles would come into direct contact with the electrolyte and easily fracture during cycling. This, in turn, would lead to the repeated fracture and regeneration of the solidelectrolyte interphase(SEI) film, resulting in excessive decomposition of the organic electrolyte, low columbic efficiency, and capacity loss. Thus, this paper was aimed to provide additional protection to the exposed Si nanoparticles and to study the electrochemical performance of the carbon coated electrospun Si/CNFs based on the anode of lithium-ion batteries.Sucrose is first used as the carbon precursor to be coated on the fiber surface through impregnation and converted to carbon after pyrolysis. The carbon coating produced by this process significantly improves the electrochemical performance of the resulting anode material with regards to its electrical conductivity, cyclic stability, and rate performance. It has been shown, for example, that a prepared composite anode can deliver a high discharge capacity of 1215.2 mAh/g at a current of 0.6 A/g after 50 cycles. To optimize the coating material, graphene was then applied to protect the Si/CNFs by electrostatic self-assembly method and hydrothermal dehydration. The Si/CNFs@rGO exhibites superior electrochemical performance as an anode, with a high specific capacity of 1055.1 mAh/g up to 130 cycles at 0.1 A/g, with slight capacity loss. After the introduction of carbon layer on the fiber membrane surface, there still exit some exfoliation of Si nanoparticles due to the incomplete protection to the Si nanoparticles on the surface of inner fibers. To defend individual Si particles within the electrode, Si/C composite particles with yolk-shell structure were inserted into the CNFs. The structure has SiNPs as the“yolk” and amorphous carbon as the “shell” and there is void space between the SiNPs yolk and carbon shell, which allows for the SiNPs to expand upon lithiation without breaking the carbon. Upon this, the SiNPs will not fall off from the fibers even if undergo fragmentation. The novel anode material display excellent electrochemical, with a high specific capacity of 1076.5 mAh/g up to 100 cycles at 0.2 A/g, with slight capacity loss. The improved electrochemical performance can be attributed to the protection of the introduced carbon layer, and these methods is suitable for the preparation of other kinds of anode materials with large volume expansion.