Optimization Design Of Micro-Structures In TiO₂ For Electrochemical Sodium Storage

Limited lithium sources and high cost hinder the further development and application of lithium-ion batteries?LIBs?.As such,this creates a need to develop new high performance battery systems beyond LIBs.Sodium-ion batteries?SIBs?have attracted extensive attentions for application in energy storage owing to abundant sodium resources,low cost and similar physicochemical properties between sodium and lithium.Studies have reported that the key of the development of SIBs is to research the electrode materials with high sodium storage performance.Thus,it is imperative to study and develop the applicable electrode materials for SIBs.Due to its high safety,low price,simple preparation methods,proper sodium storage voltage platform,stable structure in the sodium storage processes,and excellent theoretical electrochemical performances,TiO₂has become a potential applicable anode materials in SIBs.However,TiO₂ showed the limited electrochemical performances in SIBs,mainly due to its poor electrical and ionic conductivities.Therefore,in this paper,we adopted the TiO₂ anodes as the research object to improve their electrochemical performances in SIBs by the optimization design of the phase structure,micro-morphology,chemical environment and electronic structure of TiO₂in sequence.In other words,our study is designed to prepare the applicable TiO₂ anode in SIBs with high capacity,good rate performance,and long life cycling through revealing the relation among the micro-structure of the Ti O₂ anode,electrochemical performances,and sodium-ion storage optimization mechanism by the ex-situ spectroscopy and microscopy technologies.This paper may promote the development of SIBs by developing the applicable TiO₂ anode with excellent performances.The idea and method of this paper may provide guidance on design and synthesis of high performance electrodes for SIBs.The details of the paper are as follows:1.TiO₂ has a variety of phase structures.Therefore,in order to improve the electrochemical performances of TiO₂ in SIBs,we should study and optimize the phase structures of TiO₂ at first.We screened three phase structures of rutile,anatase and TiO₂?B?phases as research objects,which are the more electrochemically active hosts among the all phase structures in TiO₂,and are easy to be prepared with low cost.Then,we employed them as the anode materials for SIBs,and evaluated their electrochemical performances.After the systematic studies,we found that the anatase is the most electrochemically active host structure among the three prepared phase structures.2.The effective method to improve the ionic conductivity,sodium-ion storage and diffusion performances of TiO₂ anode material is to construct the nanoscaled TiO₂ with appropriate micro-morphologies.In this work,anatase TiO₂ which was selected in the previous chapter,was used as the research object,and its electrochemical performances were imrpoved through the optimization design of the micro-morphologies.Based on the relevant literatures as guidance,and easy preparation and low cost as the purpose,we selected and prepared the TiO₂ nanotubes,TiO₂ nanorods,TiO₂ nanofibers,self-assembled TiO₂ nanospheres,and MOFs based cake like TiO₂ as anode materials for SIBs,respectively.Through the electrochemical characterizations and analyses,we found that the morphology of nanotubes is the most appropriate structure among the all prepared morphologies.After the subsequent EIS spectroscopy technology,we revealed the relation among the micro-morphology of the TiO₂,electrochemical kinetic performances,and electrochemical performances.3.The main challenge limited the electrochemical performances of TiO₂ is its low electrical conductivity,which leads to the poor reversibility,low reversible specific capacity,and short cycle life.Therefore,in this chapter,we adopted the effective method of compositing with carbons to optimize the chemical environment of TiO₂ to improve their electrical conductivity and sodium storage performances.We adopted the optimized anatase Ti O₂ nanotubes as the research object,and graphene sponge with strong chemical stability,good electrical conductivity,abundant electrochemical active sites for ion storage and diffusion,and three-dimensional hierarchical pore structures as compositing carbons.Then,we prepared the anatase TiO₂ nanotubes/graphene sponge anode materials for SIBs,and evaluated their electrochemical performances.After the optimization of the chemical environment through adjusting and controlling the proportion of TiO₂ nanotubes and graphene sponge in the composite,the TiO₂ based anode showed much improved sodium storage performances.The resultant sample showed a high reversible capacity of 273 mA h g^-1 after 100 cycles at a current density of 50 mA g^-1,good rate performances,and long cycle life of 3000 cycles.4.Though the introduction of graphene sponge in TiO₂ greatly improved their electrochemical performances in SIBs,the carbon materials in the electrodes will produce the sodium dendrites during the electrochemical processes,which may puncture the separator,and cause the safety hazards of short circuit,combustion and explosion of SIBs.In this chapter,we adopted the method of ion doping to optimize the micro-structures of TiO₂,and thus improved its electrochemical properties in SIBs.Based on the work of the above-mentioned two chapters,we adopted the optimized anatase TiO₂ nanotubes as the research object.Through the consideration of the difficulty degree of ion doping,lattice mismatch after doping,doping contents,and structural stability of lattice after doping,we carried out three different doping methods including cation doping with same charge state(Sn⁴⁺),cation doping with different charge state doping(Ni²⁺)and anionic/cationic ions co-doping(Ni²⁺and N),to optimize the micro-structures and sodium-ion storage and diffusion performances of TiO₂.The optimized TiO₂ based anode showed the much improved sodium storage performances,and the resultant sample showed a high reversible capacity of 303 mA h g^-1 after 500 cycles at a current density of 50 mA g^-1 with capacity retention of 96%,good rate performances,and long cycle life of 8000 cycles.Then,we elucidated the universal law and special behavior of sodium-ion storage and diffusion in the TiO₂ electrode materials after the different types of ion doping or undoping in TiO₂through the ex-situ spectroscopy and microscopy technologies.We also revealed the effects of different types of ion doping and ion doping contents on the TiO₂ micro-structures?phase structures,micro-morphologies and electronic structures?,and the relation among the micro-structures,electrochemical performances,and sodium-ion storage optimization mechanism of the TiO₂ anode.