Effect Of Different Ion Substitution On Superionic Conductivety Of The Compound Li₃Sc₂（PO₄）₃

Li₃Sc₂（PO₄）₃ ceramics with NASICON structure may have good ionic conductivity due to its favorable three-dimension tunnel for ion conduction. As a pure solid electrolyte material, Li₃Sc₂（PO₄）₃ has good chemical stability, which can overcome many of the safety hazards such as flammability and explosion caused by the use of organic liquid electrolyte for the present lithium-ion batteries. However, pure Li₃Sc₂（PO₄）₃ ceramics shows poor ion conductivity, which greatly limits its application in the lithium-ion batteries. Therefore, how to improve ion conductivity of the Li₃Sc₂（PO₄）₃ ceramics was paid much attention in the area of all-solid lithium-ion batteries.In the paper, based on our research on Si O44- substitution for PO43- in the Li₃Sc₂（PO₄）₃, partial substitution of Al3+ for Sc3+ in the Li3.15Sc2（PO4）2.85（SiO4）0.15 was initially used to further improve ion-conduction performance of the compound. Our results proved that the ionic conductivity of the samples varied in parabolic curve with an increase in the Al3+ content, and the compound with x=0.45 presented the maximum conductivity of 1.04×10-3 S/m at room temperature, and the minimum activation energy for ion conduction of 42.45 k J/mol. Then, we used Li BO2 as a fluxing agent to modify performance of the Li3.15Sc2-x Alx（PO4）2.85（Si O4）0.15（x=0⁰.6） compounds. It proved that Li BO2 addition improved sinterability of the samples, made them denser, changed little on their ionic conductivity. 27 Al MAS-NMR results showed that most of the doped Al（OH）3 existed in the form of octahedron, which meant that the added Al3+ played a role on partial substitution of Sc3+ in the Li3.15Sc2（PO4）2.85（Si O4）0.15.By using V₂O₅ as a starting material, partial substitution of VO₄^3- for PO43- was used in the compound Li₃Sc₂（PO₄）₃ to prepare Li₃Sc₂（PO₄）₃-y（VO4）y（y=0².0） series. In the range of y≤1.2, the Li₃Sc₂（PO₄）₃-y（VO4）y formed a uniform compound. Further increase in the VO₄^3- content resulted in appearance of impurities such as VPO5, V（PO3）3 and Sc VO4 phases. The VO₄^3--substituted compounds showed good sintering properties and electrical conductivities. Their ionic conductivity increased with an increase in the VO₄^3- content, and the compound with x=1.5 presented the maximum conductivity of 8.41×10-3 S/m at room temperature, and the minimum activation energy for ion conduction of 39.65 k J/mol. This conductivity value was two orders of magnitude larger than that of the unsubstituted one, and 8⁹ times larger than that of the Zr4+- and Si O44--substituted samples. On the other hand, electronic conductivity measurement indicated that electronic conductivity of the samples with y≤1.5 was lower than 1% in the total conductivity and then the VO₄^3--subsittuted Li₃Sc₂（PO₄）₃ samples were pure lithium ion conductors.In conclusion, the VO₄^3- anion substitution was an powerful method of improving ion conductivity for the Li₃Sc₂（PO₄）₃ compound.