Synthesis And Modification Of Nasicon-typed Na₃Zr₂Si₂PO₁₂ Solid State Electrolytes

There are many problems, such as leakage （pollution）, corrosion and explosion athigh temperature （safety）, especially the safety issues, for rechargeable batterieswhich are based on liquid organic electrolytes. Therefore the necessity of developingall-solid-state batteries arises owning to its safety, long-cycle life, miniaturization andversatile geometry. In addition, solid state electrolytes also act as the role of separatorin batteries, which simplifies the battery structure and decreases the dependence onthe protection ambient, such as Ar-filled glove box.One of the most promising electrolytes, NASICON-typed Na₃Zr₂Si₂PO₁₂hasdemonstrated excellent ionic conductivity, approaching0.20S/cm at598K, which iscomparable with liquid electrolytes or β"-Al₂O₃electrolyte. Na₃Zr₂Si₂PO₁₂displaysthe3D isotropic Na ion conductivity and inert characteristic with Na and H2O in alarge electrochemical window. However, Na₃Zr₂Si₂PO₁₂obtained by conventionalprocessing using ball milling techniques always contains a second phase of zirconiadue to the volatilization of sodium and phosphorus at high temperature （usually above1200℃）. Recently we found that phase-pure Na₃Zr₂Si₂PO₁₂can be obtained by addingexcessive sodium and phosphorus sources in the precursors. In this thesisNa₃Zr₂Si₂PO₁₂was synthesized by solid state reaction and sol-gel approachrespectively. Further attempts are focused on improving the ionic conductivity ofNASICON-typed Na₃Zr₂Si₂PO₁₂by changing the doping elements and/or adjustingthe doping contents. This thesis can be subdivided into the following parts,1. Synthesis and sintering conditions were explored in order to prepare phase-pureNa₃Zr₂Si₂PO₁₂samples through solid state reaction and sol-gel approach, respectively.It is revealed that the conducting properties of Na₃Zr₂Si₂PO₁₂ceramics are hardlyaffected by the presintering conditions, however controlling the resinteringtemperature is a key factor to avoid the formation of impurities and thus obtain highionic conductivity. The ceramics sintered at1100℃are phase pure Na₃Zr₂Si₂PO₁₂and possess ionic conductivity as high as1.71×10-4S/cm, which is slightly higherthan samples sintered at1200℃. Further analysis reveals that the evaporation ofphosphorus at high temperature would cause the formation of ZrO2impurity in thesamples, leading to a lower ionic conductivity. When compared with solid state method, samples with enhanced ionic conductivity as high as5.4×10-4S/cm at roomtemperature can be prepared at a rather lower temperature by sol-gel synthesis.2. Ca-doped Na₃Zr₂Si₂PO₁₂samples, viz. Na_3-2xCa_xZr₂Si₂PO₁₂（x=0.1,0.2）, wereprepared through sol-gel method. Nevertheless, electrochemical impendencespectroscopy （EIS） shows that the ionic conductivity was not enhanced obviously bythe Ca substitution for Na sites. Further analysis indicates that although morevacancies are introduced by Ca-doping on Na sites on the basis of charge neutrality，the channel for Na ion conducting is blocked by the heavy Ca ions, as a result theionic conductivity seems to be lower than expected.3. Y-doped Na₃Zr₂Si₂PO₁₂samples, viz. Na_3+xZr_2-xY_xSi₂PO₁₂（x=0.1,0.2） andLa-doped Na_3+xZr_2-xLa_xSi₂PO₁₂（x=0.05,0.1,0.15,0.2） samples were prepared bysol-gel method. EIS analysis reveals that La-doped samples are, as expected,exhibiting higher ionic conductivity while Y-doped samples are not, which might beattributed to the electrostatic interaction caused by the introduction of Y3+. The ionicconductivity of the system firstly increases and then decreases as the doping contentof La increases. When the doping content is0.15, the ionic conductivity reaches thevalue of7.47×10^-4S/cm. However the ionic conductivity is slightly lower than theundoped samples if the doping content reaches0.20. This is because the ionic radiusof La3+is much bigger than that of Zr4+, too much introduction of La3+will destroythe structure of NASICON skeleton and lead to the precipitation of ZrO2which isunfavorable for the conducting of Na+ion. This important improvement on the ionicconductivity of La-doped Na₃Zr₂Si₂PO₁₂samples lays a solid foundation for furtherinvestigation of all-solid-state sodium-ion batteries.