| As one of the most common energy storage technologies today,lithium batteries have been widely used in cameras,cell phones,computers and other electronic products.With the frequent use of Li-ion batteries in our daily life,the problems of their safety and energy storage capacity are pressing to be solved.All-solid-state batteries have become a hot research topic today because of their good safety and stability and high energy density.Various solid-state electrolyte materials have been developed,among which perovskite-type solid-state electrolyte Li0.33La0.557Ti O3（LLTO）has attracted much attention because of its high energy density,wide operating temperature range and considerable ionic conductivity.However,compared with liquid electrolytes,the ionic conductivity of LLTO needs to be further improved,which is the key to the market application of LLTO.In this paper,perovskite（ABO3）LLTOs are investigated in terms of A-site doping,B-site doping,A-and B-site double doping,and improvement of the preparation process to investigate their effects on the crystal structure,micromorphology,and electrochemical properties of LLTO electrolytes,and the following conclusions are drawn:（1）LLTO solid electrolyte specimens with pure phase LLTO and LLTO with different Bi2O3doping contents were successfully prepared by solid phase sintering method to investigate the effects of Bi doping on the physical phase,microscopic morphology and electrochemical properties of LLTO.It was found that all specimens were LLTO with tetragonal phase crystal structure and no heterogeneous phase was generated.The ionic conductivity of the pure phase LLTO was 1.34×10–5S·cm–1and the activation energy was 0.45 e V.The grain size of LLTO increased with the increase of Bi2O3doping content,and a large number of O ion vacancies appeared in the LLTO crystal.The increase of grain size can effectively reduce the grain boundary resistance,while the increase of O-ion vacancy concentration can provide more pore channels for Li ion migration（O4）,both of which can enhance the Li ion conductivity of LLTO solid-state electrolyte.The ionic conductivity of LLTO reaches a maximum value of 2.71×10–5S·cm–1when Bi2O3doping is 1wt%,which is twice the ionic conductivity of pure phase LLTO.And the doping of Bi2O3can also effectively reduce the activation energy of LLTO,and the activation energy of Li0.33La0.557Ti O3+2wt%Bi2O3is as low as 0.28 e V,which is nearly 40%lower compared with that of pure LLTO.In addition,the doping of Bi can greatly improve the critical current density of LLTO solid state electrolyte,compared with the critical current density of0.06m A of pure LLTO,the critical current density of Li0.33La0.557Ti O3+1wt%Bi2O3can reach 0.16 m A and still operate normally after 500 hours of cycling,showing excellent cycling stability.（2）The microstructure and electrical properties were investigated by XRD,SEM,Raman spectroscopy and electrochemical impedance spectroscopy using large radius Zr doping to replace Ti at the B-position in LLTO with the amount of Zr doping as a variable.With the increase of Zr doping content,the microstructure of LLTO solid electrolyte became more dense and the ionic conductivity also increased1.61×10–5S·cm–1at room temperature,and the ionic conductivity can reach 3.32×10–5S·cm–1at 40°C,which is 65%higher than that of the undoped LLTO under the same conditions.（3）Based on the A-site single doping,B-site doping was further introduced to study the interaction relationship between A and B-site co-doping and its effects on the microstructure and electrochemical properties of LLTO.The experimental results show that A and B-site co-doping is difficult to further enhance the ionic conductivity of LLTO by B-site doping on the basis of A-site doping.It is also interesting to note that the particle size of Zr and Bi elements doped alone increases more than that of double doping,which does not produce the expected synergistic effect in terms of ionic conductivity enhancement li0.33La0.557Ti0.97Zr0.03O3+1wt%Bi2O3ionic conduct-ivity is the largest in all double doped electrolyte samples,which can reach 2.16×10–5S·cm–1is 61%higher than that of the undoped LLTO.（4）Comprehensive analysis of the constitutive relationship and lithium ion transport mechanism of LLTO electrolyte,proposed to use the rapid cooling characteristics of the air-cooling process to introduce amorphous regions at the grain boundaries to reduce the grain boundary resistance,and prepared a high conductivity LLTO solid state electrolyte,the pure phase LLTO ionic conductivity prepared by the air-cooling process can be as high as 3.64×10–5S·cm–1,which is more than 2.7 times the conductivity of the LLTO prepared with the furnace cooling.However,the conductivity of LLTO doped with Bi and Zr elements was not improved but decreased by the air-cooling process.It is speculated that the reason is that the rapid cooling makes Bi and Zr not well doped into the crystal structure of LLTO to expand the lithium ion channels and reduce the grain boundary resistance,but hinders the migration of lithium ions resulting in the decrease of ionic conductivity. |
