Study On The Synthesis And Properties Of Doped NASICON-type Lithium Fast Ion Conductors

Performances of all-solid-state Li-ion batteries depend on the conductivity of solid electrolyte.Because of high Li+ionic conductivity, wide electrochemical window and stable in air,NASICON-type lithium fast ion conductors are very promising solid electrolyte materials for all-solid-state Li-ion batteries.Different series of lithium fast ion conductor materials were prepared by sol-gel and solid-state methods using LiTi₂（PO₄）₃as matrix, and the influence of doped elements on the conductivitywas explored. Lithium ion diffusion dynamics was studied on lithium fast ion conductorLi_1.3Al_0.1Zn_0.1Ti_1.8P₃O₁₂. The main research contents are as follows:Lithium fast ion conductors Li_1+xAl_xTi_2-xP₃O₁₂（x=0、0.1、0.2、0.3、0.4�'�0.5） weresynthesized by sol-gel method using Ti（OC₄H₉）₄, LiNO₃·H₂O, Al（NO₃）₃·9H₂O and NH₄H₂PO₄. X-ray diffraction showed that all synthetic products had NASICON structure with space group R3C.The morphology analysis of Li_1+xAl_xTi_2-xP₃O₁₂（x=0.2） showed that it was lamellar structure. Theinfluences of different aluminium ion doping levels and test temperatures on the conductivity ofsolid electrolyte were studied by EIS complex impedance and galvanostatic step techniques, theresults show that the product with x=0.2presents highest ionic conductivity1.5310^-4S·cm^-1at523K.Four compositions of Nasicon-type solid electrolyte Li_1+x+2yAl_xZnyTi_2-x-ySixP₃xO₁₂weresynthesized by conventional solid state route. X-ray diffraction and Rietveld whole patternrefinement were applied to investigate the phase, structure and the unit cell parameters of this solidelectrolyte. EIS complex impedance method was carried out to obtain the Li+conductivity. Theresults show that Li_1+x+2yAl_xZnyTi_2-x-ySixP₃xO₁₂with x=0.1y=0.5presents highest ionicconductivity7.210-5S·cm^-1at ambient temperature and reaches2.010-3S·cm^-1at473K. Theactivation energies of Li+migration are calculated to be about20kJ·mol-1. The influence of Zn2+element on the structure and Li+mobility was preliminary explained.Nasicon-type solid electrolyte Li_1.3Al_0.1Zn_0.1Ti_1.8P₃O₁₂was prepared by citric acid-assistedacrylamide polymerisation gel method. X-ray diffraction pattern showed that the introduction ofZn2+in the parent matrix Li_1+xAl_xTi_2-xP₃O₁₂made it easier to get high-purity rhombohedralstructure （space group R3C） Li_1.3Al_0.1Zn_0.1Ti_1.8P₃O₁₂without the evidence of impurity secondary phase. The Li+kinetics were investigated by complex impedance in bulk pellet and ionicconductivity in battery-type composite cathode, respectively. Grain-interior resistance measured bygalvanostatic intermittent titration technique, potential step chronoamperometry, and ACimpedance spectroscopy at20°C varies in the range1.2-1.95×10^-4S·cm^-1, which is in goodagreement with that obtained by complex impedance method1.5×10^-4S·cm^-1.