Synthesis,Regulation And Electrochemical Properties Of Vanadium Oxide-Based Materials

Rechargeable lithium-ion battery has been regarded as a revolutionary technology of renewable energy and extensively studied in recent decades.As an indispensable part of lithium-ion batteries,anode materials play an essential role in improving electrochemical performance of lithium-ion batteries.However,conventional lithium-ion batteries with graphite anode have been gradually unable to meet demands in different application scenarios(such as electric vehicles and large-scale power supplies).To meet the needs of contemporary society for energy density and power density,researchers have made tremendous endeavors to design and synthesize alternative anode materials with high specific capacity and long cycle life.Vanadium oxide-based materials are considered as one of the most promising systems to be studied owing to the wide range of oxidation states from+2 to+5 of vanadium ion,the large variety of crystal structures,high specific capacity and proper operate voltage.Unfortunately,low intrinsic electronic conductivity of vanadium oxide-based materials leading to poor rate performance and large volume change during cycling process results in rapid capacity decline.Cumbersome and dangerous synthetic methods also obstruct their development.Therefore,from the perspective of synthetic chemistry,the synthesis design and structural regulation of vanadium oxide-based materials optimize the electrochemical performance and further reveal the mechanism of electrochemical reaction kinetics,which are of great significance to promote the development of vanadium oxide-based materials in lithium-ion batteries.This thesis takes VO and Li₃VO₄ as the research objects.From a synthetic chemistry point of view,several simple and green methods have been developed to synthesize vanadium oxide-based materials with special micro-structure to alleviate volume change during cycling process.Meanwhile,ion doping is employed to optimize electronic structure and to further improve electronic conductivity and electrochemical performance,so as to deeply clarify effect of doping on structure,electrochemical performance and reaction kinetics.We hope to promote promising potential of vanadium oxide-based materials in lithium-ion battery.The main research results obtained are as follows:1.Construction of conversion-type VO nano-rings and porous carbon composite and mechanism of improved cycling performance.A simple topochemical self-reduction method is developed to synthesize VO nano-rings and porous carbon composite.This composite displays excellent specific capacity and cycling stability(like 336 m A h g^-1 after 400 cycles at 10 A g^-1 with capacity retention of 85%),as well as prominent rate capability(like 235 m A h g^-1 at 20 A g^-1).The systematic characterizations show that the composite gives a low charge transfer activation energy of 54 me V and a high lithium-ion diffusion D_Li+of 10^-11 cm^-2 s^-1,and reveals that the enhanced electronic conduction and the special structure of nano-ring play essential roles on electrochemical performance.A full cell was fabricated by using this composite as anode and commercial Li Fe PO₄ as cathode.It delivers a discharge capacity of 213m A h g^-1 after 100 cycles at 0.1 A g^-1,revealing a great potential for practical application.The innovative design and approach reported here may provide hints for synthesizing plenty of functional materials.2.Defective structure regulation of cationic Fe doped insertion-type Li₃VO₄ and mechanism of improved rate capability.Compared to conversion-type VO,insertion-type Li₃VO₄ deliver a stable structure during cycling,but it is a semiconductor with a wide bandgap and poor electrons.A series of Fe³⁺doped Li₃VO₄ samples were synthesized via freeze-drying and heating-treatment.Fe³⁺doping leads to a slight lattice expansion and the appearance of oxygen vacancies,which change electronic structure of Li₃VO₄.Electrochemical impedance spectroscopy further shows that Fe³⁺doping reduces energy barrier of electron transfer.In all Fe³⁺doped samples,Li₃V_0.99Fe_0.01O_4-?electrode gives the best performance,delivering the specific capacity of 484 m A h g^-1 at 100 m A g^-1 after 100 cycles,and 375 m A h g^-1 at 200 m A g^-1 after 200 loops,is superior to those of Li₃VO₄ based electrodes ever reported in the literatures.The outstanding electrochemical performance is ascribed to the appearance of oxygen vacancy and lattice expansion leading to an improved Li⁺and electrons diffusion process.Doping with 3d metals to regulate the defective structure of electrode materials and improve the electronic conductivity may be an effective way to enhance the electrochemical properties of electrode materials.3.Defective structure regulation of anion F doped Li₃VO₄ and its improved cycling stability.In the previous work,cationic Fe doping was employed to adjust electronic structure of Li₃VO₄ and improve rate performance,but its cycling stability needs to be enhanced.F^-doping is performed to regulate defective structure of insertion-type Li₃VO₄ and further improve cycling properties.A series of F^-doped Li₃VO₄ samples were synthesized by sol-gel method.F^-doping for Li₃VO₄ lattice was demonstrated to give a lattice expansion,the presence of oxygen vacancy and lower valence state of V⁴⁺,which resulted in a decrease of band gap and improved electronic conductivity.Electrochemical tests show that 5 at%F doped Li₃VO₄ gives excellent cycling stability as an anode for lithium-ion battery.It can deliver specific capacity of450 m A h g^-1 after 1100 cycles at 500 m A g^-1.Electrochemical kinetics analyses indicate that the reduced energy barrier for electron and ionic transfer,the complete structural rearrangement and the optimized solid electrolyte interface film during cycling process are favorable for achieving long cycle life.F doping chemistry may provide a new clue for designing high-performance electrode materials.4.Ultrafast solvothermal synthesis and electrochemical properties of carbon coated Li₃VO₄ microspheres.The rate performance and cycling performance of insertion-type Li₃VO₄ was improved by cationic and anion doping in above two charpters,but the particles size of Li₃VO₄ synthesized in two chapters was uneven.Uniform particle plays an important role in electrochemical perporties.Based on this,an ultrafast solvothermal method was designed to synthesize uniform Li₃VO₄microspheres.The average particles size of carbon coated Li₃VO₄ microspheres is about 1.8?m.The uniform microspheres give outstanding cycling stability and rate performance.A discharge capacity of 233 m A h g^-1can be obtained after 1000 cycles at a rate of 4 A g^-1 with a high-capacity retention of 94%.Even at a high rate of 10 A g^-1,it can also deliver a capacity of 152 m A h g^-1.Electrochemical tests indicate that the excellent electrochemical performance is attributed to the uniform microsphere morphology,which can effectively alleviate stress change during charge/discharge process.Carbon coating can improve electronic conductivity and accelerate electrons transfer.A full cell was assembled by employing carbon coated insertion-type Li₃VO₄microsphere and commercial Li Fe PO₄ as anode and cathode,respectively.Three red LEDs can be lightened by three full cells.Such ultrafast synthetic method provides hint for shortening synthetic process of oxide electrodes and shows great application potential in practical application.