Synthesis And Properties Of Multielement Vanadium-based Anode Materials For Lithium-ion Batteries

Lithium-ion batteries?LIB?have been widely used in portable electronic devices,electronic vehicles,and other fields as the energy storage and conversion facility,due to its high energy and power densities.To date,graphite serves as the mature anode material for commercial LIBs.However,the limited theoretical specific capacity(372mA h g^-1)and the safety problem related to the formation of lithium dendrites hinder the comprehensive performance of the LIBs.In recent years,great efforts have been devoted to the exploration of the alternative anode materials for LIBs.Among them,the intercalation-type type materials are of particular interest owing to the controllable volume expansion,leading to the superior cycling performance and the high capacity retention.Layered vanadium-based oxides are a series of intercalation-type materials,which provide a two-dimensional ion diffusion channel for the reversible insertion/extraction of Li-ions.Besides,the multiple valences of vanadium are beneficial for the multi-electron electrochemical reactions with the result of high theoretical specific capacity.In this thesis,several multielement vanadium oxides have been designed and synthesized via various methods,and the electrochemical properties and lithium-ion storage mechanism have been comprehensively investigated.The main conclusions are obtained as following:First of all,orthorhombic LiCuVO₄ was prepared by solid-state method.In the voltage range of 3.0-0.01 V,LiCuVO₄ delivers a specific capacity of 447 mA h g^-1after 50 cycles at the current density of 200 m A g^-1,corresponding to the capacity retention of 91%.Moreover,LiCuVO₄ exhibits superior rate performance with the specific capacity of 259 mA h g^-1 after 50 cycles at 2 A g^-1.The lithium-ion storage mechanism of LiCuVO₄ was studied via ex-situ X-ray diffraction?XRD?and transmission electron microscope?TEM?.LiCuVO₄ decomposes into Li₃VO₄ and Cu metal nanoparticles after initial discharge process.Then,lithium-ions could reversibly insert into/extract from Li₃VO₄ in the subsequent electrochemical processes.In contrast,Cu metal nanoparticles are found to be coated on the surface of Li₃VO₄,beneficial to the improvement of electronic conductivity.Secondly,Cu₃V₂O₇?OH?₂·2H₂O/graphene oxide?GO?nano-composite was synthesized via co-precipitation method.Compared with pristine materials,the reversible specific capacity and rate performance of nano-composite electrode materials are significantly improved owing to the introduction of high-conductive GO.The specific capacity achieves as 870 mA h g^-1 at 100 mA g^-1,and remains as 158 m A h g^-1 at a high current density of 5 A g^-1.According to the electrochemical impedance spectroscopy?EIS?data,the excellent electrochemical performance of nano-composite electrode materials is derived from the synergistic effect of Cu₃V₂O₇?OH?₂·2H₂O nanoparticles and GO with high electronic conductivity.The introduction of GO could not only improve the electronic conductivity leading to the optimization of rate performance,but also provide more active sites for lithium-ion storage resulting in the increase of specific capacity.Then,we prepared Cu₃V₂O₈ nanoparticles with a diameter of 40-50 nm via co-precipitation reaction,and investigated the electrochemical properties within voltage range of 3.0-0.01 V.Cu₃V₂O₈ nanoparticles shows a high initial specific capacity of 1157 m A h g^-1 at 100 mA g^-1,and 462 mA h g^-1 after 10 cycles.Then,the capacity trends to increase upon cycling,which is ascribed to the reversible formation and de-formation of gel-like film on the electrode surface based on the EIS data.According to the ex-situ XRD and TEM data,Cu₃V₂O₈ decomposes into Cu and Li₃VO₄ nanoparticles with the dimeter of 5 nm in the initial discharge process.In the subsequent cycles,the lithium-ion storage mechanism is similar with LiCuVO₄ in Chapter 1:Cu metal nanoparticles do not participate the electrochemical reaction,while lithium ions insert into and extract from Li₃VO₄ based on intercalation reaction.Finally,the performance of orthorhombic?-Cu₂V₂O₇ was investigated as the anode materials of LIBs.?-Cu₂V₂O₇ shows a specific capacity of 395 m A h g^-1 at the current density of 100 mA g^-1,while exhibits a specific capacity of 190 m A h g^-1 under a high current density of 5 A g^-1.Besides,the long-range and short-range ordered structure,as well as the valence change in the initial lithiation process have been studied via ex-situ X-ray diffraction,X-ray absorption near edge structure,and Raman spectra.It can be conclude that copper ions are reduced to Cu metal after initially being discharged to 1.6 V,and Cu metal could not be re-oxidized in the charging process.Meanwhile,Li₃VO₄ generated during discharging stage transforms to a new intermediate phase in the lower voltage range,and V ions could be re-oxidized to V⁵⁺after being charged to 3.0 VIn this dissertation,we designed and synthesized several multielement vanadium oxides,and investigated their lithium-ion storage properties via various electrochemical tests.Then,the lithium storage performance,including the cycle stability and rate performance,was further improved by morphology control and surface tailoring to overcome the poor electronic conductivity.Finally,the corresponding lithium-ion storage mechanism was proposed according to the phase evolution,valence and morphology change at different discharge-charge depth.This dissertation provides a strong insight into the design of novel transition metal oxide for the application as the anode materials in LIBs.