Electrical Conductivity of NASICON-Type Lithium-Ion Conductor Sintered from the Precursor Prepared by Sol-Gel Method

Water stable NASICON-type lithium ion conducting solid electrolytes Li1+xTi2-xAlxP3O12 (LTAP) have been used for the water stable lithium metal electrode. The water stable lithium metal electrode has a potential application as the anode in a high energy density lithium air electrode. For this application of the solid electrolyte, the electrical conductivity of it should be as high as possible. The conductivity of Li1+xTi2-xAlxP3O12 at room temperature is in a range of 10 to 5x10 Scm, which depends on the composition of LTAP and the preparation method. The bulk conductivity of Li1+xTi2-xAlxP3O12 was as high as 10 Scm at room temperature, but the grain boundary conductivity was low. We have observed that the grain boundary resistance of the sintered Li1+x+yTi2xAlxP3-ySiyO12 was suppressed by immersion in acetic acid solution. The grain boundary resistance suppression depended on the sintering temperature of the Li1+x+yTi2xAlxP3-ySiyO12 powder. In this study, the effect of the immersion in LiCl saturated aqueous solution on the electrical conductivity has been examined. The precursor of Li1.4Ti1.6Al0.4P3O12 (LTAP) was prepared by a sol-gel method with citric acid. The precursor was heated at 750 o C for 4 h and then the pressed sample sintered at 750~1,000 C for 6 h. The sintered samples were immersed in a LiCl saturated aqueous solution and a LiCH3COOH saturated HCH3COOH-H2O solution at 50 C for several days. The electrical conductivity of the sintered samples was measured using a Solatron 1260 frequency response analyzer in the frequency range 0.1 Hz to 1 MHz. The sintered samples were characterized by XRD, SEM and ESCA. Figure 1 shows the XRD patterns of the LTAP sintered at 950 C for 6 h along with that of LTAP immersed in the LiCl saturated aqueous solution and the LiCH3COOH(sat)-CH3COOH(90wt%)-H2O(10wt%) solution at 50 C for one week. That of as sintered LTAP is characterized as the NASICON-type structure and no extra peak was observed. The LTAP immersed in the LiCl saturated aqueous solution and the LiCH3COOH(sat)CH3COOH(90wt%)-H2O(10wt%) solution shows no change of the XRD pattern. The electrical conductivity measurements showed that the highest conductivity of 6.1x10 Scm was observed in the sample sintered at 950 C. The conductivity value is comparable to that reported previously. The electrical conductivity of LTAP was enhanced by immersing in the acetic acid solution and LiCl saturated aqueous solutions. Fig. 2 shows typical impedance profiles of the pristine LATP and the LATP immersed in the LiCH3COOH(sat)CH3COOH(90wt%)-H2O(10wt%) solution and LiCl saturated aqueous solution. These profiles show a semicircle due to the grain boundary resistance. The grain boundary resistance is diminished in the LATP immersed in the acetic acid solution and LiCl saturated solution. The grain boundary resistance of the pristine LTAP of 241 Ωcm decreases to 24.3 Ωcm for LTAP immersed in the acetic acid solution and to 13.9 Ωcm for LTAP immersed in LiCl saturated aqueous solution. The total conductivity of 6.1x10 Scm at 25 C was enhanced to be 1.36x10 Scm for LTAP immersed in acetic acid solution and 2.0x10 Scm for LTAP immersed in LiCl saturated aqueous solution. The SEM images and Raman spectra of these samples showed no significant change of the surface morphology by immersion into these solutions. The enhancement of the electrical conductivity may be due to the dissolution of a resistive interface layer at the grain boundary.