Calculation Study On The Energy Storage Mechanism Of Transition Metal Oxide Aqueous Zinc-ion Battery Cathode

With the increasing demand for energy and the yearning for a green earth,the utilization of clean and renewable energy has become an important strategic goal of national development.Zinc-ion batteries（ZIBs）are expected to replace lithium-ion batteries as a new generation of ideal energy storage devices due to their low cost,good safety,abundant resources and high ionic conductivity,among which transition metal oxide as its cathode material has attracted wide attention of researchers.First-principles calculations based on density functional theory are not only helpful to understand the basic electrochemical properties of electrode materials,but also helpful to further explore the energy storage mechanism and explain the experimental phenomena,thus providing a new approach for the selection and design of electrode materials.In this thesis,MnO₂and VO₂,the common transition metal-based zinc ion battery electrode materials,were used as the objects.With the aid of theoretical calculation,we study the stable surface of two oxides,the energy changes of H₂O molecules in the process of adsorption and dissociation,and the free energy changes of H⁺diffusing to the near surface or inside the electrode during the energy storage process.The main research contents and calculation results are as follows:（1）Using the first principles,the crystal structure and electronic properties ofβ-Mn O₂and VO₂（M）were calculated,and the surface structures with different crystal orientations were constructed.We found thatβ-Mn O₂is metallic,while VO₂（M）is semiconducting,and Mn O₂（110）and VO₂（011）are the most stable surface structures.DOS calculation results show that on the surface of Mn O₂（110）,the electrons near the Fermi level are mainly provided by O atoms,among which the bridge O atoms on the surface are the most active.On the surface of VO₂（011）,the overall DOS peak moves to low energy,and the band gap decreases from 0.81 e V to 0.36 e V.Different from the surface of Mn O₂,the electrons near the Fermi level are mainly provided by V atoms,among which the internal V atoms contribute the most.（2）The adsorption configuration and adsorption energy of H₂O molecules on Mn O₂（110）and VO₂（011）surfaces and the influence of the concentration of H₂O molecules on the adsorption energy were studied.For molecular adsorption,the adsorption of H₂O molecules at Mn₅and V₅sites is the strongest,and the adsorption of H₂O molecules on Mn O₂surface is stronger than that on VO₂surface at the same site,because the hybridization between H₂O molecules and Mn₅-d orbital is greater than that of V₅-d atom.For dissociative adsorption,the adsorption energy of H₂O molecule on Mn O₂surface is also greater than that on VO₂surface.For different adsorption concentrations,the H₂O molecules attract each other in the[100]direction and repel each other in the[010]direction.（3）The dissociation barrier of H₂O molecules on the oxide surface and the adsorption and diffusion barrier of protons on the oxide surface are discussed.At all adsorption concentrations,the dissociation barrier of H₂O molecules on Mn O₂surface is negative,and the dissociation barrier of H₂O molecules on VO₂surface is positive,indicating that on Mn O₂（110）surface,H₂O molecules tend to be adsorbed by molecularly,while on VO₂（011）surface,they tend to be adsorbed by a dissociation manner.Studies on the adsorption and diffusion of protons on oxide surface show that protons are more likely to be adsorbed on O_brsite.When protons are embedded into the electrode,they first diffuse from the O_brsite to the O_ssite,and then from the O_ssite to the surface layer.After entering the surface layer,the protons will first diffuse to the entire surface layer through the[100]tunnel direction,and then diffuse to the lower layer.The kinetic barrier of proton diffusion from the oxide surface to the interior is much larger than the dissociation barrier of H₂O molecules,indicating that the dissociation rate of H₂O molecule on the oxide surface is double restricted by the high diffusion barrier of proton at O_brand O_ssites,and the diffusion barrier of Mn O₂surface is higher than that of VO₂,the restriction is more obvious.