Preparation And Electrochemical Properties Of Novel Vanadium-Based Lithium Ion Battery Electrode Materials

As an electrochemical energy storage system,lithium-ion batteries?LIBs?are widely applied in portable,wearable electronic devices and electric vehicles?EVs?due to their small size,light weight,high energy density,long cycle life and good safety.At present,graphite is mainly used as an anode material for commercial lithium-ion batteries.However,the overall performance of lithium-ion batteries is severely limited owing to the relatively low theoretical specific capacity of graphite(372 mAh g^-1)and the discharge voltage platform?-0.2 V?which may generate lithium dendrites and trigger safety issue.Therefore,finding novel anode materials as a potential substitution of the commercial graphite anode has become a research focus.The novel vanadium-based anode material not only has low preparation cost and wide source,but also can provide high lithium storage during the multi-electron electrochemical reaction of vanadium ion rich valence state.In this dissertation,several vanadium-based electrode materials were synthesized by morphology control,ion doping,carbon coating and structural optimization.Besides,the electrochemical properties and lithium storage mechanism were analyzed in detail.The main contents and results are listed as follows:1.To improve the electrochemical performance of Li₃VO₄,a series of Li₃Nb_xV_1-x-x O₄?0?x?0.3?compounds are prepared via sol-gel method.The similar ionic radii of Nb⁵⁺and V⁵⁺in octahedral coordination make it possible to form single phased Li₃Nb_xV_1-xO₄?0?x?0.15?.The larger lattice constant caused by the substitution of V with Nb enhances the cross-sectional area along c-axis diffusion channel,which indirectly provides a higher diffusion coefficient for Li-ion migration.Benefitting from high electronic conductivity?around two orders of magnitude?and good lithium-ion diffusion coefficient,the Li₃Nb_xV_1-x-x O₄?x=0.02?exhibits the best electrochemical property as an anode of LIBs.In the potential window of 0.1–3.0 V,at the current density of 30 mA g^-1,the charge capacity of pure Li₃VO₄ is around 440 mAh g^-1,whereas the Li₃Nb_xV_1-xO₄?x=0.02?shows a higher capacity of 550 mAh g^-1.Additionally,the charge/discharge capacities for Nb-doped samples are almost twice to those of the undoped Li₃VO₄ when the current density increases to 1500 mA g^-1.2.The binary metal oxide?VOMoO₄?is successfully prepared via sol-gel method for the first time.By using citric acid as carbon source and chelating agent,the sample particles become smaller and the surface of the sample are coated by uniform thin carbon layer.Benefiting from structural optimization,improved electronic conductivity,and multi-electron reactions of V and Mo,VOMoO₄ exhibits excellent electrochemical performance as anode material for lithium ion battery.In the potential window of 0.1-3.0 V,at a current density of0.2 A g^-1,the VOMoO₄ electrode delivers a high specific capacity of over 830 mAh g^-1 after250 cycles.Moreover,VOMoO₄ can also exhibits good rate performance,and the VOMoO₄electrode has a high and stable specific capacity even at an extreme current density of 10 A g^-1.In addition,in-situ XRD tests further confirmed the conversion reaction mechanism of VOMoO₄ for lithium ion battery.3.The novel vanadium-based metal oxides NiV₂O₆ and NiV₂O₆/C were synthesized by sol-gel method,and they were studied for the first time as negative electrode materials for lithium ion batteries.X-ray diffraction and Rietveld refinement results show that NiV₂O₆/C is a single-phase structure with space group P-1,which is composed of VO₅ pentahedron and NiO₆ octahedron alternately linked to form a layer.The citric acid as the carbon source and chelating agent in the synthesis scheme optimizes the structure and particle size of the NiV₂O₆sample,which not only improves the electronic conductivity of the material itself,but also effectively improves the interface kinetics of the material.Besides,the cyclic voltammetry test,charge and discharge curve and EIS spectrum were used to simply analyze the electrochemical reaction process of NiV₂O₆/C electrode during charge and discharge process.As a negative electrode material for lithium ion batteries,NiV₂O₆/C exhibits better electrochemical performance.