Investigation On The Structural Properties And Electrochemical Properties Of Molybdate Electrode Materials

In recent years,molybdate has become a research hotspot in the field of energy storage such as lithium ion battery due to its stable structure,excellent physicochemical properties.Mo and metal ion have synergistic effect in molybdate,which can further improve the electrochemical reactivity of the material.However,molybdate still has the problems of large volume variation and low initial Coulombic efficiency in the electrochemical reaction process,which can be effectively addressed by structural regulation or composition optimization of molybdate.In this thesis,iron molybdate and cobalt molybdate with high theoretical capacities were selected as the research objects.On the one hand,the electrochemical properties of the molybdate nanomaterials were improved by constructing hollow nanostructures and other modification strategies,and the electrochemical energy storage mechanism was studied.On the other hand,carbon coating and the introduction of oxygen defects were applied to improve their reaction kinetics and ionic electronic conductivity,so as to improve their electrochemical performance.The research contents of this thesis are as follows:1. Fe₂?MoO₄?₃ hollow microspheres?Fe₂?MoO₄?₃-HMS?were synthesized by solvent thermal method using the bubbles generated by HNO₃ as templates.The microspheres were composed of nanoparticles,so the carrier diffusion path could be reduced and the electrochemical reactivity could be improved.The hollow hierarchical structure can effectively relieve the structural stress caused by volume expansion,reduce the side reaction between the material and the electrolyte,improve the mechanical and chemical stability of the SEI,and thus improve the coulombic efficiency and cyclic stability of the material.As an anode material for lithium ion batteries,Fe₂?MoO₄?₃-HMS has a high discharge capacity?1205 m A h/g after 200 cycles at 0.5 A/g?,excellent rate performance?565 m A h/g at 10 A/g?,good low-temperature performance?281 m A h/g at 1 A/g at-20??,and high areal capacity?5.2 m A h/cm² with a high active material mass loading of2.5 mg/cm²;4.8 m A h/cm² with a high active material mass loading of 5 mg/cm²?.In addition,the full battery matched with Li Fe PO₄ exhibited a high reversible capacity of1115 m A h/g at 0.2 A/g.Finally,the electrochemical reaction mechanism of Fe₂?MoO₄?₃as anode material of lithium ion battery was investigated by in situ XRD.The Fe₂?MoO₄?₃electrode material provides a new idea for the development of new efficient and long-life lithium ion battery electrode material.2. Carbon coated Fe₂?MoO₄?₃ nanosheets?Fe₂?MoO₄?₃/C-NS?were synthesized by a simple solution method and a solid phase sintering method using urea as both the template and carbon source.The nanosheet structure can provide a shortened diffusion path for charge carrier and improve the reaction dynamics of the material.The carbon coating can not only improve the electronic conductivity,but also improve the structural stability of the material and reduce the fracture of the SEI on the material surface.The Fe nanoparticles generated in the electrochemical reaction process can be used as catalysts for the reversible formation and decomposition of the SEI,thus further improving the initial Coulombic efficiency?ICE?of the material.Therefore,Fe₂?MoO₄?₃/C-NS electrode material has excellent electrochemical properties,with a high discharge capacity of 1376m A h/g after 250 cycles at 0.5 A/g,A discharge capacity of 983 m A h/g at 5 A/g,and ICE over 87%.These advantages make Fe₂?MoO₄?₃/C-NS become a potential anode electrode material that can be applied to the next-generation lithium ion batteries.3. CoMoO₄ nanorods were synthesized by one-step hydrothermal method and calcination under N₂ to introduce oxygen vacancies with a proper concentration,which can be used not only as the electron charge carrier to improve the conductivity of the material,but also as the active sites to improve the capacity of the material.Electron paramagnetic resonance?EPR?proved the presence of the oxygen vacancies in CoMoO₄,and Raman and XPS characterizations further confirmed this result.As the anode material of lithium ion battery,the capacity of the oxygen vacancy-containing CoMoO₄ nanorods is maintained at 999 m A h/g after 500 cycles at 0.5 A/g;650 and 461 m A h/g after 1000cycles at 2 and 5 A/g,respectively,showing excellent electrochemical performance.We calculated the formation energy of two oxygen vacancies by density functional theory?DFT?,and found that the probability of O loss in Co-O-Mo was greater.The formation of oxygen vacancies caused slight distortion on the material surface,resulting in defects that could be used as active sites,and the adsorption of Li⁺around the oxygen vacancies and the charge density significantly increased.The above research opens up a new idea for optimizing the electrochemical properties of binary metal oxides.