Preparation And Electrochemical Property Of High Energy Density Silicon Oxide/carbon Composite Anode Materials

Micrometer sized SiO_x is the preferred alternative for high energy density lithium-ion batteries anode materials due to its high mass specific capacity（2200-2500 m Ah/g）and low preparation cost.However,the huge volume change（～200%）,low Initial Coulombic Efficiency（ICE）and sluggish charge transfer rate greatly limit the practical application of SiO_x anode materials.The aim of this thesis is to address these issues in the way of controlling the composition and structure of SiO_x/C composite anode materials.Different structures of SiO_x/C composite were designed to enhance its charge transfer kinetics and cycling stability.The detailed research work is as follows:Firstly,in order to resolve the volume expansion and low conductivity of SiO_xanode materials,the composite structure（SiO_x@C@CNTs）was construced by combing a CVD-based carbon coating layer and a spray drying-based CNTs conducting network.It was found that the CVD-deposited carbon coating is tightly bonded to SiO_x,which helped to alleviate the mechanical strain of SiO_x and to form a stable Solid Electrolyte Interface（SEI）film.The CNTs conductive network greatly improved the conductivity of SiO_x and significantly reduced its charge transfer resistance.As a result,the composite showed remarkable cycling stability and the full cell assembled with Li Ni_0.8Co_0.1Mn_0.1O₂（NCM811）provided a high energy density of 401.8 Wh/kg.Secondly,in order to increase the binding force between the conducting network layer and SiO_x,plasma enhanced chemical vapor deposition（PECVD）was used to directly grow vertically structured carbon nanosheets（CNs）on the surface of disproportionated SiO_x（d-SiO_x）powders（d-SiO_x@CNs）.The fabricated CNs bind closely with d-SiO_x to alleviate the volume change of d-SiO_x.Furthermore,CNs ensure the sufficient contact between the active materials with the electrolyte interface,forming rich transport channels for charge exchange at the interface and significantly enhancing the electrode dynamic process.A full cell based on this composite anode assembled with commercially available NCM811 displays a high energy density of 424.2 Wh/kg.Finally,the effect of the heteroatom-doping modified of the carbon coating layer on the SiO_x/C composite anode materials was investigated.A nitrogen-doped carbon and graphene-co-modified SiO_x（SiO_x@GNC）composite anode materials was prepared by combing mechanical grinding method with heat treatment process.Graphene not only well relieved the internal stress generated by SiO_x during cycling,also improves the conductivity of SiO_x.The nitrogen-doped carbon coating formed by highly dispersed melamine provides more reactive sites,mean while it anchored graphene to the surface of SiO_x.As a result,the composite exhibits excellent electrical conductivity and electrochemical reversibility（reversible capacity of 582.1 m Ah/g after 1000 cycles at 2A/g,capacity retention rate of 81.7%）.