Atomic-scale Electron Microscopic Characterization Of Structural Modification Of Nickle-based Sodium-ion Layered Oxide Materials

In the increasingly severe situation of the energy crisis,the design and development of new energy devices and low-cost large-scale energy storage systems play a vital role.Sodium-ion batteries（SIBs）have become the most promising new-generation energy storage devices after lithium-ion batteries（LIBs）due to their abundant resources and low cost.As cathode materials largely determine the electrochemical performance of SIBs,the structural characterization and spectroscopic study of the cathode materials at the atomic scale are essential to obtain the rela-tionship between the atomic structure and the macroscopic properties and provide important guiding significance for improving their electrochemical performance.Among them,nickel-based layered transition metal oxides have higher discharge platforms and are simple to syn-thesize,which are highly expected.Therefore,this thesis focuses on the nickel-based layered transition metal oxide Na Ni_0.5Co_0.2Mn_0.3O₂ material and uses aberration-corrected electron mi-croscopy characterization technology to study its atomic-scale structure and electronic structure information.This study provides guidances for the structure optimization of the material and obtain a cathode material with an excellent specific capacity,rate performance,and cycle sta-bility.The main research contents and results are as follows:（1）Atomic-scale ex-situ/in-situ characterization analysis of the structural degradation mechanism of Na Ni_0.5Co_0.2Mn_0.3O₂ material.（1）The surface and internal structure degradation of cycled Na Ni_0.5Co_0.2Mn_0.3O₂ material were studied.Combining atomic imaging and spectroscopic analysis,it can be seen that the surface degraded layer structure is transformed from a layered structure to a rock-salt structure,and accompanied by a decrease in the valence state of the transition metal and a large number of oxygen vacancies,indicating that the crystal surface directly in contact with the electrolyte is preferentially degraded.The internal degradation is manifested as lattice distortion caused by the Jahn-Teller effect,and multiple stress concentration points are found using geometric phase analysis techniques,indicating that the strong Jahn-Teller effect of Ni³⁺has an adverse effect on structural stability.（2）The thermal decomposition degradation of Na Ni_0.5Co_0.2Mn_0.3O₂ material was studied by in-situ heating technology.It was found that as the temperature increased,the structure of Na Ni_0.5Co_0.2Mn_0.3O₂ gradually degraded from a layered structure to a spinel structure,and the decrease of the valence state of the transition metal（mainly Ni）and the generation of oxygen vacancies were accompanied by thermal degradation.During the thermal decomposition pro-cess,the grain boundary with huge strain due to the presence of lattice mismatch will preferen-tially generate microcracks,while the internal oxygen release process will exacerbate the crack-ing of the microcracks,eventually leading to the dissociation and destruction of the crystal structure.Based on the evolution of the atomic structure and changes in electronic structure,the thermal decomposition degradation mechanism of Na Ni_0.5Co_0.2Mn_0.3O₂ material was sum-marized.（3）The atomic image and nano-beam diffraction technologies were used to study the twin grain boundary structure in Na Ni_0.5Co_0.2Mn_0.3O₂ material.The differential phase contrast im-aging technology was used to obtain the structure information of all atom-columns in the twin grain boundary.The twin grain boundary is a special P-type configuration,so the Na transpor-tation barrier at the twin boundary is lower,and it will preferentially generate a Na-depleted layer.Combined with geometric phase analysis,it is found that there is a phenomenon of stress oscillation in the lattice near the twin grain boundary.It is found in cycled Na Ni_0.5Co_0.2Mn_0.3O₂that the twin grain boundary is the location where cracks are preferentially generated and prop-agated.Based on these results,the crack growth mechanism introduced by the twin grain bound-ary was proposed.（2）Based on the degradation mechanism of the Na Ni_0.5Co_0.2Mn_0.3O₂ material,a scheme of Fe doping to optimize the structural stability was proposed,and the dual mechanism of Fe dop-ing was studied by combining imaging and spectroscopic technologies.（1）The electron energy loss spectroscopy technology was used to investigate the valence states of transition metals in different Fe-doped materials,and it was proved that the doping of Fe would lead to the reduction of Ni valence states.Thus,reducing the strong Jahn-Teller effect of Ni³⁺can eliminate the distortion structure caused by O′3 phase.（2）Combining atomic image and stress analysis,severe lattice distortion and multiple strain concentration points were found in cycled Na Ni_0.5Co_0.2Mn_0.3O₂ material.However,the strain distribution in Na[Ni_0.5Co_0.2Mn_0.3]_0.9Fe_0.1O₂ material was uniform and the crystal structure was intact.A structural explanation was given to improve the electrochemical performance with proper Fe doping.（3）The electron energy loss spectroscopy was used to study the redox pairs in different Fe-doped materials.It was proved that the charge capacity of Na[Ni_0.5Co_0.2Mn_0.3]_0.9Fe_0.1O₂ mainly comes from the oxidation process of Ni,and the crystal structure stays intact.The charge ca-pacity of Na[Ni_0.5Co_0.2Mn_0.3]_0.6Fe_0.4O₂ mainly comes from the oxidation process of Fe,and the phenomenon of transition metal migration to the Na layer occurred.（4）Using energy dispersive X-ray spectroscopy technology,atomic-scale element distribu-tion maps were obtained in cycled Na[Ni_0.5Co_0.2Mn_0.3]_0.6Fe_0.4O₂ material,which proved that Fe and Ni migrate preferentially to the Na layer.Excessive Fe doping will hinder the process of Na-ion re-insertion,leading to severe capacity degradation.In summary,this thesis carried out the ex-situ/in-situ characterization studies of atomic structure,electronic structure,strain analysis and element distribution in Na Ni_0.5Co_0.2Mn_0.3O₂and Fe-doped optimized Na[Ni_0.5Co_0.2Mn_0.3]_1-xFe_xO₂ materials using powerful aberration-cor-rected electron microscopy technology.And we successfully applied differential phase contrast imaging technology to the fine characterization of twin grain boundary structure.Combined with these results,material degradation mechanisms were proposed,which give guidance to the design of cathode materials.