Oxide Ionic Conduction And Its Mechanisms Of Compounds With Layered Structure Characteristic

The development of new energy has always been a research hotspot for sustainable development in modern society.Solid oxide fuel cell（SOFC）is the focus of new energy development.Its solid electrolyte can reduce the working temperature,but the solid oxide electrolyte materials have low ionic conductivity(σ>10^-2 S/cm,500-700℃),low oxygen transfer number（t～1）,poor chemical stability and other bottlenecks.In addition to oxide anionic conductor materials,alkali metal cationic conductors have been widely studied for it’s high efficiency and high conductivity.With the further study of ionic conductors,some studies had shown that in some materials with（similar to）layered structures,the oxygen ions can migrate along the intralayer（2D）or interlayer（3D）paths,which enhances the oxygen ion migration efficiency within the material structure.However,most of the layered oxygen ionic conductor materials only show two-dimensional ion migration path,which reduces the ionic conductivity of the materials.The shortage and high cost of lithium resources in alkali metal cation conductors with significant layered structure limit the long-term application of lithium ion batteries.It is an ideal research scheme to replace lithium ion conductor materials with Na⁺/K⁺ion conductor materials（Na⁺and K⁺are abundant and cheap）.Therefore,in this paper,some materials with（similar to）layered structures were found,such as In₂Ge₂O₇,MBO₃（M=In,Sc,Lu）,Na₂Zn₂TeO₆,Rb Na Mg P₂O₇.Structural defects were introduced through different valence ion doping,and various characterization methods were used to explore the phase crystal structure,material stability and ionic conductive mechanism of the materials.The main research contents are as follows:1.For the germanium oxide ionic conductor In₂Ge₂O₇ material,the low price Zn²⁺ions were doped to partially replace the high price In³⁺to produce oxygen vacancy defects.The In_2-xZn_xGe₂O_7-0.5x material was synthesized and the ionic conductivity,thermal stability and conductive mechanism were investigated.The results showed that the solid solution limit of In_1-xZn_xGe₂O_7-0.5x was x=0.2,and the cell parameters of the material gradually decreased with the increase of doping amount;The introduction of Zn²⁺improves the conductivity of the intrinsic material.The conductivity of the undoped sample is 6.53×10^-5 S/cm（1000℃）.When the doping amount x=0.2,the conductivity is increased to 1.62×10^-2 S/cm（1000℃）.The material has mixed electron oxygen ion conductivity.The migration number of oxygen ions increases with the temperature when it is lower than 750℃,and it is mainly electronic conductivity when it is higher than 750℃.The oxygen vacancy occupies the bridged oxygen O1 position of the Ge₂O₇ dimer unit and is stabilized by sharing oxygen ions with adjacent Ge₂O₇dimers.2.For MBO₃（M=In,Sc,Lu）materials,use low price Zn²⁺ions to replace high price M³⁺ions to create oxygen vacancies to obtain M_1-xZn_xBO_3-0.5x（M=In,Sc,Lu）ceramics.The conductivity of the materials was investigated.The results showed that the highest conductivity of MBO₃（M=In,Sc,Lu）reached 10^-5 S/cm,and the replacement of Zn²⁺ions increased the conductivity of the intrinsic materials by about2-3 orders of magnitude:the total conductivity of In_0.9Zn_0.1BO_2.95 at 1000℃reached4.4×10^-3 S/cm,conductivity of Sc_0.7Zn_0.3BO_2.85 is 1.1×10^-2 S/cm,the conductivity of Lu_0.9Zn_0.1BO_2.95 is 3.6×10^-3 S/cm.Changing the coordination environment of anionic polyhedron can improve the conductivity of materials.3.Based on the stacking faults and displacements of the P2-type structure and the unoccupied Na sites in the Na₂Zn₂TeO₆ materials,the materials were doped with Na sites,and excess alkali metal ions were added to synthesize Na_xZn₂TeO₆ materials,and their Na⁺ion occupancy and electrical conductivity were studied.By analyzing the occupation of Na⁺ions by NPD data,the actual Na content of the materials with x=1.8and x=2.2 is relatively large,and the rich Na tends to preferentially enter the Na1 site.Regarding the electrical properties of the material,the intrinsic sample has the highest conductivity,2.2×10^-3 S/cm at 100°C.While the conductivity of the x=2.1(1.3×10^-3 S/cm),x=2.2(6.18×10^-4 S/cm),and x=2.3(9.49×10^-5 S/cm)components decreasing in turn.This shows that with the increase of Na content,the interlayer distance of the structure decreases,the ion channel decreases,and the migration difficulty of Na increases.Excessive addition of Na element content is not conducive to the improvement of the ionic conductivity of the material.Alkali metals are incorporated into the intrinsic materials,and the Na₂A_xZn₂TeO₆（A=K,Rb,Cs）components cannot be synthesized to obtain pure phase samples.4.For Rb Na Mg P₂O₇,a nonlinear optical material with a non-centrosymmetric（quasi）layered structure,we use Ca²⁺ions to replace Na⁺to create cation vacancies.Doping does not significantly reduce the reversible phase transition temperature of the material.The conductivity of the intrinsic sample at 550℃is 4.82×10^-4 S/cm,Rb Na_0.96Ca_0.02Mg P₂O₇ conductivity is 2.1 at 550℃×10^-3 S/cm,the conductivity has not been significantly improved after doping.At the same time,the anisotropic borate materials with non-centrosymmetric structure were investigated,and the solid-state synthesis conditions and electrical properties of Na_3-3xLa_2+x（BO₃）₃,K_3-xTa_3-xW/Mo_xB₂O₁₂ and Gd_1+xCa_4-1.5xO（BO₃）₃ were preliminarily explored.The experimental results show that the synthetic conditions of borate are complex,and it is difficult to obtain pure intrinsic samples by solid phase method.