| All-solid-state batteries are promising candidates in the field of new energy,smart grid,power station of electrical energy storage and other large-scale applications because of their high thermal stability,high resistance to shocks and no leakage.Sodium ion batteries were regarded as alternatives to lithium ion batteries due to the ubiquitous sodium source and low cost.Accordingly,all-solid-state sodium ion batteries have attracted much attention.Solid-state electrolyte(SSE)is the key unit of allsolid-state batteries and the performance of SSE plays an important role in the use of all-solid-state batteries.In this thesis,a new-type Na-ion SSE of Na0.33La0.55 Zr O3 with perovskite structure is designed by a simple chemical substitution of Li with Na and Ti with more stable Zr on the basis of Li-ion SSE of perovskite-type(ABO3)Li0.33La0.55 Ti O3.Strategy of heteroatom doping at A-site or B-site and codoping is performed to further improve the ion conductivity of synthesized Na0.33La0.55 Zr O3.The effects of composition,crystal structure and morphology on the ion conductivity of synthesized samples are investigated in detail.The specific research results are as follows:(1)Na3xLa2/3-xZrO3(x = 0.11)was prepared by a conventional solid-state method,the effect of sintering temperature and excess Na2CO3 on its ionic conductivity was characterized by XRD,SEM,and EIS.At a sintering temperature of 900 ?,the phase of perovskite structure has been formed and completely transferred to perovskite structure when the sintering temperature increased to 1200 ?.There is no effect on the crystal structure but is beneficial to the improvement of ionic conductivity with excess Na2CO3 raw material is added.At the sintering temperature of 1300 ?,the ionic conductivity of sample with the adding of excess Na2CO3 can reach 6.89 × 10-7 S cm-1,which is higher than the pure samples(6.02 × 10-7 S cm-1).(2)On the basis of synthesized Na0.33La0.55 Zr O3,element Sr is doped at A-site in the Na0.33La0.55 Zr O3 aimed to increase the ionic conductivity.The effect of different Sr-doping amount and sintered temperature on ionic conductivity,crystal structure and microscopic morphology was determined by EIS,XRD,and SEM.The enhancement of crystal constant of the sample with the increase of Sr doping amount is investigated by the refinement of XRD results.A high ionic conductivity of 1.025 × 10-5 S cm-1 is achieved with a composition of Na1/3La1/3Sr1/3Zr O3 when the sample was sintered at 1300 ?.(3)On the basis of synthesized Na0.33La0.55 Zr O3,element Nb is doped at B-site in the Na0.33La0.55 Zr O3 aimed to increase the ionic conductivity.The highest ionic conductivity 3.26 × 10-6 S cm-1 was achieved when 1/4 mol % Nb doping into Na0.33La0.55 Zr O3,and the crystal structure keep the perovskite structure with orthorhombic system.The crystal structure transformed to the Na Nb O3 with the increase of Nb-substituted amount,it may because of the higher amount of Nb in the sample.The enhancement of Nb doping amount also damage the decrease of ionic conductivity.(4)Strategy of Ca,Nb co-doping at A and B-site of Na0.33La0.55 Zr O3 was adopted to further increase the ionic conductivity.On the basis of previous chapter,introduced low constant amount of Ca in Asite of 1/4 mol Nb doped Na0.33La0.55 Zr O3 sample to increase the ionic conductivity.XRD result shows the crystal structure still belong to the perovskite after Ca doped.Ca doping will increase the density of pellets,decrease the number and volume of pores in the pellets.When the amount of Ca doping is 1/16 mol,the density of pellets was highest,and it will decrease when the doping amount change to 1/8mol.The introduced Ca distributed on the bulk and grain boundary of pellet.At sintering temperature of 1300 ?,the sample which 1/16 mol Ca doped shows the highest ionic conductivity,6.71 × 10-6 S cm-1. |
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