Preparation And Lithium Storage Performance Of Tin Oxide-based Self-supporting Anode

In recent years,with the proposal of the“two-carbon”goal,electric vehicles,energy storage stations and other new energy equipment are developing rapidly,and the demand for high-performance lithium-ion batteries is increasing day by day.However,the traditional graphite anode has approached the limitation of its theoretical specific capacity,so it is urgent to develop new anode materials.SnO₂ is considered as one of the most promising alternative anode materials due to its advantages of high theoretical lithium storage capacity(1494 m Ah g^-1),low reaction potential（0.25 V vs.Li/Li⁺）,and non-toxicity.However,SnO₂ anode also has some shortcomings,such as large volume change（300%）,poor reversibility and low intrinsic conductivity during charging and discharging,resulting in fast capacity decay and poor rate capability.In addition,with the development of flexible electrical devices,higher requirements for electrodes have been generated.The preparation process of self-supporting electrode is simple,without adding auxiliary materials such as binder and conductive agent,and the self-supporting electrodes are green,safe,and conducive to improving energy density of batteries.Therefore,the self-supporting electrode with certain flexibility shows its advantage and is one of the development trends of electrode materials in the future.Based on the above background,in this thesis,cellulose filter paper with abundant natural yield,green,renewable and low cost was used as the source of self-supporting carbon skeleton,and C/SnO₂ self-supporting material was prepared by using layer-by-layer self-assembly technique.Furthermore,in view of the structural stability and electrical conductivity of SnO₂ anode in lithium/delithium process,different kinds of guest materials,such as Ti O₂,metallic Sn and reduced graphene oxide（r GO）,were selected for composite,and the performance and modification mechanism of lithium storage were studied.The main work and conclusions are as follows:（1）SnO₂ and Ti O₂ were layer-by-layer self-assembled successively on the surface of filter paper fiber,and the C/SnO₂/Ti O₂ self-supporting anode materials with different Ti O₂ contents were prepared by adjusting the number of assembly layers.The macromorphology of the material is black flake,which has certain flexibility and strong compressive ability.The three-dimensional fibrous network porous structure of filter paper is perfectly retained in the microstructure of the material.And each fiber forms a core-shell structure with carbon nanofibers as the core,SnO₂ nanolayer and Ti O₂ nanolayer as the shell.As the number of Ti O₂assembly layers increase,surface roughness increases and particle agglomeration appears.The lithium storage performance test show that the best performance can be achieved with Ti O₂ assembled by 10 layers.In the current density of 0.78 m A cm^-2(0.1 A g^-1),the initial discharge specific capacity of C/SnO₂/Ti O₂-10 self-supporting anode is 4.6 m Ah cm^-2,and the initial coulombic efficiency is 62.9%.After 5 cycles,the specific capacity starts to rise and remains stable after 70 cycles.The reversible specific capacity is 3.1 m Ah cm^-2,and the capacity retention rate is as high as 67%,which is significantly improved compared with that of the C/SnO₂ self-supporting anode.By adjusting Ti O₂ content and coating morphology through layer-by-layer self-assembly technique,the“zero-strain”property of Ti O₂ can effectively stabilize the lithium/delithium microstructure of SnO₂,thus significantly improving its electrochemical reversibility and capacity retention rate.（2）Some SnO₂ layer-by-layer self-assembled on the surface of filter paper was reduced to metallic Sn in one step through heat treatment,using urea as the reducing agent.By adjusting the amount of urea,the C/SnO₂/Sn self-supporting anode materials with different proportions of SnO₂/Sn were prepared.The micromorphology of the materials presents a three-dimensional fibrous network porous structure.And each fiber forms a core-shell structure with carbon nanofibers as the core and SnO₂/Sn nanoparticle composite layer as the shell.The lithium storage performance test show that the best performance can be achieved with 0.2 g urea dosage.In the current density of 0.69 m A cm^-2(0.1 A g^-1),the initial discharge specific capacity of the C/SnO₂/Sn-0.2 self-supporting anode is 7.8 m Ah cm^-2,the first coulombic efficiency is 59.1%.The cycling curve remains basically stable after the 70th cycle.The reversible specific capacity is maintained at 3.5 m Ah cm^-2 after100 cycles.It is worth noting that its specific capacity is still 2.3 m Ah cm^-2 at a high current density of 13.8 m A cm^-2.Suitable SnO₂/Sn ratio can produce good synergistic effect,and Sn particles composite in SnO₂ nanolayer can provide good electrical conductivity while maintaining high specific capacity.Hence the rate performance is significantly improved.（3）SnO₂ and r GO prepared by the Hummer method were layer-by-layer self-assembled successively on the surface of filter paper.By adjusting the assembly layers of r GO,the C/SnO₂/r GO self-supporting anode materials with different r GO contents were prepared.r GO nanosheets were successfully loaded on the three-dimensional fiber network surface and pore structure of the C/SnO₂.The lithium storage performance test show that the best performance can be achieved with r GO assembled by 2 layers.In the current density of 0.55 m A cm^-2(0.1 A g^-1),the initial discharge specific capacity of C/SnO₂/r GO-2 self-supporting anode is 7.4 m Ah cm^-2,the first coulombic efficiency is 61.3%.The cycling curves remains basically stable after the 50th cycle.After 100 cycles,the reversible specific capacity remains at 3.8m Ah cm^-2,and 2.1 m Ah cm^-2 still retains at a higher current density of 2.75 m A cm^-2.With a proper amount of r GO nanosheet load,an overlapping conductive network is formed on the surface of the self-supporting material,which shortens the transfer pathway of lithium ions and electrons,and improves the dynamic performance of the electrode.In addition,the r GO with mechanical stability can effectively prevent the fall off of SnO₂ and the damage of material microstructure,thus improving the cyclic stability of the self-supporting electrode.