Design And Modification Of Hollow Structured Anode Materials For Sodium Ion Batteries

In September 2020,China proposed at the United Nations General Assembly to"increase the country’s independent contribution,adopt stronger policies and measures,strive to peak CO₂ emissions by 2030,and strive to achieve carbon neutrality by 2060.In this context,the demand for efficient and clean energy is becoming more and more urgent.Energy storage technology has ushered in greater opportunities and challenges.Most of the energy storage systems with high energy density and long cycle life use lithium-ion battery technology.However,with the depletion of lithium resources and the severe shortage of lithium resources in China,academia and industry are looking for a new sustainable alternative product or technology to meet the growing global demand for energy storage.Due to the abundant sodium resources and similar physical and chemical properties to lithium,sodium ion batteries have received renewed attention.Sodium ion batteries operate on a similar principle to lithium-ion batteries,which are also essentially a concentrated differential battery.Although its energy density is not as high as that of lithium-ion batteries,its abundant reserves will reduce our dependence on lithium resources,which is one of the biggest advantages of sodium-ion batteries.Although fabricated with cheaper raw materials,the sodium ion battery system differs greatly in the industrial distribution from lithium-ion battery system from.Therefore,we need to improve the manufacturing process,and continuously improve the existing cathode and anode materials or develop new cathode and anode materials of sodium ion batteries to improve their energy density.The aim of this thesis is to study and develop high performance anode materials of sodium ion battery.Through the construction of special morphology and anion doping to play a synergistic role,the shortcomings of existing sodium ion battery anode materials are improved and their electrochemical performance is enhanced.At the same time,new sodium ion battery anode materials are developed to improve the safety performance of sodium ion full batteries.This thesis conducts an investigation of the relevant contents,as follows.（1）Sn O₂ is a traditional anode material for sodium ion batteries,which has attracted much attention due to its high abundance,high theoretical capacity,and non-toxicity.However,the large volume expansion leads to rapid capacity decay after cycling,and also result in the intrinsically low ionic and electronic conductivities responsible for its poor reaction kinetics and multiplicative performance.These limitations greatly hinder its practical application.Here we prepared hollow nanospheres of Sn O₂ via one-step hydrothermal preparation by hard template method and systematically investigated the effect of hollow structure on sodium storage performance.It is shown that the special morphology can effectively mitigate the volume change during charging/discharging,and thus ensure the structural integrity as well as a better kinetic behavior of sodium ions and electrons in the embedding/detachment process.（2）Although Sn O₂ performs well,its discharge potential is too high at 0.6-1 V（vs.Na⁺/Na）to achieve high energy density.The second part of this paper studies Na₂Ti₃O₇,which is considered as an ideal anode material with high energy density because of its relatively low discharge potential at 0.3 V（vs.Na⁺/Na）.However,the Na₂Ti₃O₇ nanostructure lacks a rational design,thus limiting its rate performance.A special morphology of hollow nanospheres is also constructed in this paper.Shortening the transport path of sodium ions slows down the accumulation and release of stress during cycling,thus suppressing the volume expansion of the material during cycling.The highest occupied state of Na₂Ti₃O₇ was found to be at the top of the valence band by density flooding theory calculations,and the calculated band gap of Na₂Ti₃O₇ was about 2.41 e V,which exhibited the electronic properties of an insulator.The system after doping by F element has additional electronic occupied states compared to undoped Na₂Ti₃O₇,and the Fermi energy level moves up from the top of the valence band of Na₂Ti₃O₇ through the conduction band,indicating the transformation of the material to a metal-like nature.The conductivity of Na₂Ti₃O₇ electrons and ions can be increased by F-element doping,thus improving the electrochemical performance of the cell.The F-element doping also derives oxygen vacancies as shown by XPS tests.It further promotes the transport of sodium ions,thus enhancing its electrochemical performance.（3）The advantages brought by the hollow nanostructure are obvious,not only shortening the diffusion path of sodium ions,but also suppressing their volume expansion problem during cycling.The electrochemical performance of the battery is greatly improved,paving a way for the large-scale application of sodium ion batteries.However,the safety performance of sodium ion batteries is also one of the primary issues,affected by the respective problems of positive and negative electrodes for a full battery.It is easy to make the battery disorderly because of a slight perturbation.Therefore,the third part of this paper starts from the hollow structured V₂O₅ sodium ion negative material,investigating its performance as a sodium ion positive electrode.In this way,a symmetric full cell is assembled by one material,thus further reducing the process level and price of sodium ion batteries.The electrochemical performance of the as-fabricated batteries is greatly improved through the hollow structure.Besides,its safety performance is improved by constructing a symmetric electrode full cell,which provides new ideas and methods for the research of large equipment energy storage power plants.