In Situ Phase Transition Studies Induced By Ion Migration In Fast Ion Conductors

Fast ion conductor materials have received wide attention in the energy field because of their excellent physicochemical properties.The framework structure and ionic liquid type of fast ion conductor materials are the hotspots and frontiers of energy materials.In this paper,based on the in situ transmission electron microscopy method,we investigate the atomic structures of the domain boundaries and the growth mechanisms of materials.The structural evolution of the sodium tungsten bronze framework induced by the migration of sodium ions driven by the electric field is studied.And the phase transition dominated by the migration of Cu⁺in Cu₂Se is investigated.The details of the study and the results are shown as follows:Preparation and structural characterization of sodium tungsten bronze framework structural materials:Single crystals（800℃）and layered disordered superstructures（600℃）Na_0.36WO_3.14 were prepared by the thermal oxidation method.The rotation angles between domains are 60°,120°,and 180°.The growth mechanisms of the rotating domains are interpreted based on the group theory.The results provide a reference for regulating the structures and properties of sodium tungsten bronze materials and deepen the understanding of the growth mechanisms of disordered layered structure materials.Structural evolution of sodium tungsten bronze framework induced by Na⁺migration controlled by electric field:By applying a positive bias to the sample in the transmission electron microscope,Na⁺aggregation induces the phase transition from the pseudo-hexagonal Na_0.36WO_3.14 to the tetragonal Na_0.48WO₃ with higher sodium content;while under negative bias,Na⁺vacancies aggregation induces the phase transition from Na_0.36WO_3.14 to hexagonal Na_xWO₃ with lower sodium content.Combined with the first principles calculation,the overall twisting of the framework structure induced by Na⁺aggregation and W⁶⁺migration triggers the sodium-rich phase transition.The W-O suspension bonds introduced by Na⁺vacancies lead to the destabilization of the framework structure,and the migrations of W⁶⁺to the vacancies induced by the external field result in sodium-deficient phase transition.During the cyclic sodiation and desodiation of Na_0.36WO_3.14 nanosheets as electrode materials for ion batteries,the samples transform into amorphous after the high throughput of Na⁺insertion.And the samples remain amorphous after desodiation.Slight expansion and contraction of the samples occur during the reaction.The results provide an important reference for understanding the structural evolution of tungsten bronze framework type materials.Theα/βphase transition in Cu₂Se modulated by Cu⁺migration governed via temperature:The complex structures of oriented,reversed phase domains and the phase boundaries structures in Cu₂Se are investigated at the atomic scale.In the in situ heating experiments,it is found thatα-Cu₂Se near the oriented domain boundary preferentially transforms intoβ-Cu₂Se,while the anti-phase domain boundary would hinder the phase boundary migration leading to the elongation of the phase boundary,and the results provide an important reference for understanding the excellent thermoelectric properties of Cu₂Se near the phase transition temperature.The relationship between the phase transition temperature of the Cu₂Se nanobridge with the size and exposed surface of the nanobridge is investigated,and the corresponding mechanism is deduced by thermodynamic theory in conjunction with the first principles calculation.The modulation of the orientation domains in Cu₂Se is achieved by controlling the exposed surface of the nanobridges.The results optimize the theory related to the size effect on the solid-state phase transition temperature and provide an important reference for regulating the domain structure.