Controlled Synthesis And Electrochemical Performance Of Vanadates As Anode Materials For Lithium Ion Batteries

Lithium ion batteries（LIBs）possessing high energy density and power density are keeping rapid development in field of electronic products.If LIBs could meet the requirements of higher-energy density,higher-power density and shorter charging time,they will be further developed as power batteries for vehicular transports.However,it is difficult for current LIBs to meet the demands for advanced plug-in electric vehicles and hybrid electric vehicles.One strategy to improve the energy and power densities is to search for the electrode materials which could provide higher theoretical capacity.Another strategy is to find the ideal anode materials which show the potential as close to that of Li metal as possible.In addition,improving the conductivity of the electrode materials is an effective method to short the charging time of LIBs.Nanomaterials with small size and high specific surface area could effectively shorten Li⁺and e^-diffusion pathways,which could improve the conductivity of electrode material.Moreover,nanomaterials also could provide abundant active sites for Li storage and more space for volume expansion during charge/discharge process to enhance the structural stability of the electrode material.In this thesis,we prepared series of vanadate nanomaterials with different microstructures including MoV₂O₈,Zn₃V₂O_8,Cu₅V₂O_10,Co₃V₂O_8,Zn₂V₂O₇,and so on.We also tested lithium storage performance of these materials and discussed their corresponding lithium storage mechanism.These research works could be summarized as following:（1）A solvothermal method was successfully developed for the preparation of MoV₂O₈ precursor.MoV₂O₈ nanorods and nanobelts could be obtained by adjusting the calcining temperature.The MoV₂O₈ nanorods show enhanced lithium storage performance than the MoV₂O₈ nanobelts.The MoV₂O₈ nanorod electrode could deliver a specific capacity of over 1325 mAh g^-11 after 50 cycles at a current density of0.2 A g^-1.When the electrode is cycled at the current density as high as 10.0 A g^-1,it still maintains a high specific capacity of around 570 mAh g^-1.Besides,we also studied lithium storage mechanism of the MoV₂O₈ nanorods.The result demonstrates that the intercalation-deintercalation and partial redox processes are responsible for the lithium storage mechanism of MoV₂O₈ nanorods.（2）Zn₃V₂O₈ nanocages were prepared by calcining a pre-prepared hollow nanosphere precursor.The formation mechanism of the hollow nanospheres precursor is attributed to the Ostwald ripening.Because of the unique architecture and the synergistic effect of different metal ions,the Zn₃V₂O₈ nanocages present an excellent lithium storage performance as the anode materials for LIBs.At a current density of0.1 A g^-1,the electrode could deliver a capacity of 1400 mA h g^-1 after 80 cycles.When the current density is changed to 0.5 A g^-1,the capacity of 1347 mA h g^-1 could be kept after 200 cycles.Furthermore,we also assembled the full cell based on the Zn₃V₂O₈ anode and LiFePO₄ cathode.The Zn₃V₂O₈//LiFePO₄ full cell demonstrates good cycle performance at the voltage window of 0.9-3.9 V.By ex-situ XRD,XPS and TEM measurements,one mechanism has been proposed for addressing the electrochemical behavior of the Zn₃V₂O₈ nanocage electrode,which involves the synergistic effects including the conversion,intercalation and alloying.（3）Interconnected Cu₃V₂O₇（OH）₂·2H₂O nanosheet precursor was successfully prepared by reasonably adjusting reaction parameteres.Porous Cu₅V₂O₁₀0 nanosheets could be attained by calcining the Cu₃V₂O₇（OH）₂·2H₂O nanosheet precursor.The interconnected porous Cu₅V₂O₁₀0 nanosheets show excellent lithium storage performance in term of specific capacity,cycling stability and rate performance.At a current density of 0.5 A g^-1,the electrode could deliver a discharge capacity of ca.1000 mA h g^-1 after 100 cycles.When the current density is increased to 2.0 A g^-1,the discharge capacity could be kept at ca.900 mA h g^-1 after 280 cycles.The Cu₅V₂O₁₀/LiFePO₄ full cell shows good cycle performance.The excellent performance is attributed to the interconnected porous structure of Cu₅V₂O₁₀nanosheets.By ex-situ XRD,XPS and HRTEM measurements,the reaction mechanism of synergistic effects including the conversion and intercalation has been verified to explain the excellent electrochemical behavior of the interconnected porous Cu₅V₂O₁₀ nanosheets.（4）Porous Co₃V₂O₈ nanosheet and nanoflower arrays directly grown on Cu substrate with robust adhesion were prepared by a facile and scalable low temperature hydrothermal deposition process followed by a simple calcination.NH₄F plays a vital role for the formation of porous Co₃V₂O₈ nanosheet arrays.The porous nanosheet arrays as a binder-free electrode for LIBs show excellent Li storage performance.The electrode reveals a reversible capacity of 1150 mA h g^-1 after 100 cycles at a current density of 0.5 A g^-1.When the electrode is cycled at a higher current density of 3 A g^-1,it could maintain a high specific capacity of 552 mA h g^-1 after 600 cycles.（5）A universal method was developed to synthesize Zn₂V₂O₇,Co₃V₂O₈ and CoMoO₄ nanosphere precursors.Hollow nanocages were realized by calcining the resultant precursors.We investigated the Li storage performances of Zn₂V₂O₇,Co₃V₂O₈ and CoMoO₄,respectively.For example,the Zn₂V₂O₇ porous hollow nanospheres at a current density of 0.2 A g^-1 exhibit higher specific capacity of 1123mAh g^-1 after 70 cycles.At a current density of 1.0 A g^-1,the Zn₂V₂O₇ porous hollow nanocages could provide the specific capacity of 780 mAh g^-1 after 600 cycles.When Co₃V₂O₈ porous hollow nanocages were used as the anode material,it could keep a high specific capacity of 1149 mAh g^-1 at 0.2 A g^-1 after 70 cycles and also show a specific capacity of 1240 mAh g^-11 at the current density of 1.0 A g^-1 after 600 cycles.CoMoO₄ porous hollow nanocages also showed good Li storage performance.All of the above materials as the anode materials for LIBs exhibit excellent lithium storage performance.