In Situ Transmission Electron Microscopy Study On Ion Transport Of Copper Oxide Nanoelectrodes

Being a low cost metal oxide,copper oxide（CuO）nanomaterials also have the merit of possessing a very high electrochemical capacity（674 m Ah/g）.Therefore,they are considered as promising candidates for the electrode materials of ion batteries.Applying in situ transmission electron microscopy techniques,this work focuses on exploring the microscopic growth mechanism of CuO nanowires（NWs）and revealing the charging-discharging behavior of CuO NWs based composites/heteroustructures as the electrode in ion batteries.The feasibility of these composites/heteroustructures used as electrode materials has been proved.The detailed research contents and achievements are summarized as follows:1.The structural characterization of CuO NWs and the exploration of their microscopic growth mechanism.According to crystallographic theory and experimental results,it is indicated that the growth direction of CuO NW is<110>.Furthermore,aided by in situ transmission electron microscopy,the atomic-scale dynamic oxidation process of cuprous oxide（Cu₂O）nanoparticles has been investigated.The oxidation proceeds via two steps:Cu₂O is firstly oxidized into an unknown intermediate CuO_x phase,which is then oxidized into the final product CuO.Different mechanisms have been found for the above steps.In the former step,the oxidation is controlled by the evolution of dislocations;for the latter step,the oxidation happens layer by layer.Based on crystallographic analysis and simulation of high-resolution transmission electron microscopy image and diffraction patterns,CuO_x with an orthogonal structure has been constructed from the Cu₂O unit cell to explain the observed intermediated phase.Simulation results match with experimental data,proving the applicability of the constructed structure.2.The electrochemical behaviors of CuO NWs based composites.The charging-discharging cycles of CuO NWs based composites,i.e.CuO/C NWs,CuO/Au NWs,are directly visualized and in situ investigated in TEM.Partial reversibility has been found for the structural phase transitions of CuO during electrochemical reactions.For sodium ion batteries using anode materials based on above composites,this partial reversibility,when compared to full reversibility,would cause obvious capacity decay during charging-discharging cycles,thus deteriorating their service performance.Two impacts of the introduced surface coating layer（C and Au）on CuO NWs have been found.On one hand,the conductive surface would provide extra conductivity to the NWs,thus accelerating the electrochemical reactions;on the other hand,the coating layer confines the NWs geometrically,resulting in a reduced axial（<110>direction）expansion of CuO NWs.These two effects would improve the reliability of CuO NWs based composites used as the electrode materials in ion batteries.The intercalation-deintercalation of Na⁺in the surface coated Au layer has been proved fully reversible during the charging-discharging cycles of CuO/Au NWs.This finding directly demonstrates that Au nanocrystalline can effectively act as storage materials for Na.3.The mechanical behaviors of CuO NWs and CuO/C NWs.The lithiation reaction of CuO/C NWs induces the generation of cracks on the C coating layer.These cracks propagate forward along the coating layer in the subsequent reaction.In the sodiation process,the integrity of the C coating layer of CuO/C NWs cannot be retained for a long time,leading to the formation of cracks on the C coating layer.However,these cracks do not propagate forward in the subsequent reaction.Statistically speaking,reducing the lithiation speed of CuO/C NWs can alleviate the generation of cracks and restrain their propagation.The mechanical properties of the nanowires after electrochemical reactions have been investigated.When lithiated or sodiated,pure CuO NWs become plastic and have good ductility,while CuO/C NWs are less plastic and prone to brittle fracture.This finding provides some guidances toward the design of dedicated ion batteries used in specific environments.