A microcontact impedance study on NASICON-type Li1xAlxTi2−x(PO4)3 (0 ≤ x ≤ 0.5) single crystals

We successfully demonstrated the applicability of microcontact impedance spectroscopy (MC IS) on Li conducting solid electrolytes and measured the Li bulk conductivity (sb) of LiTi2(PO4)3 (LTP) and Li1+xAlxTi2 x(PO4)3 (LATP) single crystals independent of microstructural effects (e.g., grain boundaries, pores, and density). The crystals had a size of about 100 mm in each direction and crystallized with NASICON-type structure (R 3c). Finite element calculations were performed to validate the impedance data analysis. A strong increase in sb in the order of three magnitudes (3.16 10 6 to 1.73 10 3 S cm ) was found after incorporating 0.1 mol Al per formula unit into LTP. Moreover, since the crystal structural changes are almost linear in the LATP system up to x 1⁄4 0.5, the increase of sb is most probably related to additional Li sites at the M3 (36f) position. The additional Li + leads to a displacement of Li occupying the M1 (6b) sites towards the nearest-neighboring M3 position, and therefore opens the fast-conducting pathway within the NASICON structure. A significant change in sb was also observed as the Al content further increased (x 1⁄4 0.1 to 0.5). The highest sb value of 5.63 10 3 S cm 1 was obtained for samples with x 1⁄4 0.4.