Flexible Membranes Based On Silicon Or Vanadium Pentoxide As Binder-free Anodes For Sodium-ion Batteries

Lithium-ion batteries are widely used in electronic products and large-scale grid energy storage due to their long life and high specific capacity.However,it is an increasingly intensified contradiction between the high cost of limited lithium resources and the growing market demand.The sodium ion battery has received extensive attention from researchers due to its resource advantages and similar physical and chemical properties compared with lithium.In recent years,flexible and wearable devices have gradually gone into people’s lives and played an increasingly vital role.The operation of such devices requires light-weight,flexible and high-performance drives that match their needs.The key is the development of flexible electrodes.In this dessertation,commercial silicon nanocrystals and vanadium pentoxide are selected as research objects based on the problems mentioned above.It is expected to prepare flexible and self-supporting anodes with superior energy storage performance through surface modification and composite design.The main contents include two parts as follows:（1）A self-supporting composite composed of carbon-coated silicon nanocrystals and a 3D graded carbon network of interconnected multi-walled carbon nanotubes was successfully prepared based on the shielding effect of electrostatic repulsion,vacuum filtration and thermal treatment.And it was directly used as anodes for reversible sodium storage.This structure has rich interface active sites that can be used for capacitively-controlled Na storage.Furthermore,the highly conductive 3D network composed of carbon nanotubes and coated carbon provides a high-speed way to facilitate charge transportation and an effective buffer to accommodate volume changes in silicon nanocrystals during Na⁺insertion/extraction.As a binder-free anode in SIBs,the self-supporting electrode delivers outstanding electrochemical performance such as stable cyclability with a capacity of 80%at a current density of 0.2A g^-1 after 1000 cycles and high-rate capability with the capacity of 105 mAh g^-1 at current density of 2A g^-1.The results show that the self-supporting Si-based anode materials prepared by this simple process have great application potential in high-performance SIB.（2）A flexible and self-supporting film composed of vanadium pentoxide nanobelts embedded by calcium ions and multi-walled carbon nanotubes were successfully prepared by two-step hydrothermal method,vacuum filtration and freeze-drying technique.The raw materials include vanadium chloride,commercial vanadium pentoxide and multi-walled carbon nanotubes.The layer space of the vanadium pentoxide nanobelts embedded by calcium ions is further expanded,which offers an influential channel for rapid insertion/extraction of sodium ions,improves migration dynamics of sodium ion,and holds structural stability during insertion/extraction of sodium ion.Furthermore,three-dimensional cross-linked network not only facilitates rapid conduction of electrons,but also buffers the volume expansion of active materials during sodium ion insertion/extraction,and improves the stability of the electrode structure.The flexible and self-supporting film shows superior cyclic stability and high rate performance when it is directly used as anodes for sodium-ion batteries.The self-supporting composite fabricated by the simple procedures has great possibility for application in high-performance SIBs and wearable devices.