High Lithium-Ion Conducting Nasicon-Type Li1+x-YAlxNbyTi2-x-Y(PO4)3 Solid Electrolyte

Abstract

Recently, the demand of a high energy density battery for electric vehicles (EVs) has been significantly increased, because the driving range of EVs with conventional lithium-ion batteries is too short compared with vehicles with internal conversion engines. Lithium-air batteries are attractive candidate for the high specific energy density batteries, because of their high theoretical energy density as 3560 Wh kg-1 for the non-aqueous and 1910 Wh kg-1 for the aqueous system. Non-aqueous systems have some severe problems that still need to be addressed such as decomposition of the electrolyte by the reaction product, lithium corrosion by water, and high polarization for reduction of the reaction product of Li2O2. While, some of these problems are not appreciable to the aqueous system. The aqueous system cannot operate without a water stable lithium metal electrode, because lithium reacts violently with water. The idea of the water stable lithium electrode was proposed by Visco et al. in 2004 [1]. The lithium metal was separated with an aqueous electrolyte by the water stable NASICON-type lithium conducting solid electrolyte of Li1+XAlxTi2-x(PO4)3. In the last half century, many types of high lithium-ion conducting solid electrolytes have been reported, the conductivities of which are in a range of 10-4-10-2 S/cm at room temperature. Of them, Li1+xAxTi2-x(PO4)4 and Li7La3Zr2O12 were reported to be stable in the LiCl saturated aqueous solution. The stability in the aqueous solutions is essential for the lithium protect solid electrolyte in aqueous lithium-air batteries. The Ta doped Li7La3Zr2O12 exhibited a high lithium ion conductivity of 1x10-3 S cm-1 at 25 °C and is stable in contact with lithium metal, but it is difficult to prepare a thin film, because of lithium evaporation during the high temperature sintering. The thin film of Li1+xAxTi2-x(PO4)4 was prepared by a tape casting method and the ion conductivity of the Li1.4Al0.4Ti1.6(PO4)3-3wt% TiO2 film was 7.7 x10-4 S cm-1at 25 °C [2]. . Since the report by Aono et al. in 1989 [3], the NASICON-type lithium-ion conducting solid electrolytes have been extensively examined. Zhang et al. reported that the electrical conductivity of Li1+xAlxTi2-x(PO4)3 was enhanced by a partial substitution of Ge, Co, and Fe for Ti. The highest ion conductivity of 1.3×10-3 S cm-1 at 25 °C was observed in Li1.4Al0.4Ge0.2Ti1.4(PO4)3. In this study, we have examined dependence of the substitution of Nb5+ for Ti4+ in Li1+xAlxTi2-x(PO4)3 on the electrical conductivity and mechanical properties. Ionic radius of Nb5+(0.064 nm) is slightly higher than that of Ti4+ (0.061 nm) and higher than that of Ge4+ (0.053 nm). Nb2O5 is considerably cheaper than GeO2. Li1+x-yAlxNbyTi2-x-y(PO4)3 was synthesized in a range of x=0-0.60 and y=0-0.4 using a conventional solid-state reaction method. The lithium-ion conductivity of 7.5×10-4 S cm-1 and three-point bending strength of 67 N mm-2 for Li1.3Al0.5Nb0.2Ti1.3(PO4)3 and the lithium-ion conductivity of 3x10-4 S cm-1 and the three-point bending strength of 120 N mm-2 for Li1.35Al0.55Nb0.2Ti1.25(PO4)3at room temperature were observed. The water-stable high lithium-ion conducting solid electrolytes with excellent mechanical properties have potential applications for lithium-air batteries.