Preparation And Performance Of Silicon Monoxide-based Composite Electrodes For Lithium-ion Batteries

SiOx has high theoretical specific capacity(about 2600 mAh g-1),low lithium insertion potential(about 0.5 V)and low cost.It is a very promising anode material for lithium-ion batteries.However,SiOx has a large volume change and low intrinsic electrical conductivity during charging and discharging,which severely hinders the application.This thesis takes SiOx as the research object,improves the electrochemical lithium storage performance of SiOx electrodes from the aspects of compounding with high conductivity materials and optimizing the binder system,and prepares a series of SiOx-based electrodes with excellent performance.The main research results are as follows:(1)SiOx/rGO composites were prepared by a strategy of liquid phase ultrasound-freeze drying-high temperature reduction.In the composite material,rGO nanosheets are evenly coated on the surface of SiOx particles,which effectively buffers the volume variation of SiOx during charging and discharging,and improves the cycle performance of the material.In addition,rGO has also constructed an efficient conductive network,which promotes the diffusion of ions and the transmission of electrons,and improves the rate performance of the material.When the mass ratio of SiOx to rGO is 4:6,the SiOx/rGO composite exhibits the most excellent cycle performance and rate performance.The reversible specific capacity reaches 763.6 mAh g-1 at a current density of 0.1 C,after 50 weeks of cycling it remains 740.6 mAh g-1,the capacity retention rate is 97.0%.When the current density increase to 5 C,delivers a specific capacity of 295.3 mAh g-1.(2)For the problems of low conductivity of traditional polymer binders and poor bonding effect to materials with large volume expansion rates,a new type of high-performance flexible SiOx film electrode is prepared by vacuum filtration method using two-dimensional rGO as conductive binder.In this electrode,rGO firmly binds the SiOx particles in its three-dimensional conductive framework,which can well alleviate the volume expansion of the SiOx particles during the cycle,and significantly improve the conductivity of the electrode.In addition,this electrode avoids the use of traditional current collectors,which can improve the overall energy density of the battery.Based on the above advantages,the SiOx electrode with rGO as the conductive binder exhibits excellent lithium storage performance.It still has a reversible specific capacity of 1024.4 mAh g-1 after being cycled for 50 weeks at a current density of 0.1 C.The retention rate is 97.0%.It can still show a specific capacity of 295.0 mAh g-1 at a current density of 5 C.When the mass loading of the electrode is increased to 8 mg cm-2,its reversible specific capacity can reach 595.6 mAh g-1 after 50 cycles at a current density of 0.1 C,and the capacity retention rate is 94.1%.The above results indicate that graphene has potential application prospects as a conductive binder to prepare SiOx electrodes.(3)Using tannic acid as the binder and borax as the cross-linking agent,based on the dehydration condensation reaction between borax and tannic acid,a tannic acid-borax cross-linking binder system with a three-dimensional crosslinked structure was constructed.Compared with the traditional single binder,the tannic acid-borax system can form hydrogen bonds with the surface of SiOx particles,and its rich branched structure provides more bonding sites and enhances the structural stability of the electrode.In addition,based on the threedimensional cross-linked structure,the tannic acid-borax system also has higher strength,which is beneficial to alleviate the volume expansion of SiOx particles during charging and discharging and improve the cycle performance of the electrode.When the mass ratio of tannic acid to borax is 9:1,the SiOx electrode with the tannic acid-borax system as the binder shows good cycle and rate performance.It has a reversible specific capacity of 703.3 mAh g-1 after being cycled at a current density of 0.1 C for 50 weeks,and can maintain a reversible specific capacity of 442.0 mAh g-1 even at a high current density of 5 C.