In recent years, increasing efforts have been focused upon the developments of all solid-state batteries. As for the solid electrolyte, while sulfide-based systems represent excellent lithium ion conductivity, oxide-based systems still have an advantage in a stability in air. Therefore, it is greatly recommended to improve the conductivity of lithium ion conductive oxides. We have reported that lithium ion conductivity is enhanced by introducing LLTO (La_2/3-xLi_3xTiO₃) particles in LATP (Li_1.3Al_0.3Ti_1.7(PO₄)₃) matrix [1] as shown in Fig. 1. The d.c. electrolysis using stainless-steel / electrolyte / metallic lithium asymmetry cell confirmed the transport number of lithium ion as unity. While the sample was obtained by co-sintering the precursor of LATP and LLTO particles, X-ray diffraction suggested that LLTO was decomposed into LaPO₄. Then, we thought that dispersion of LaPO₄ attribute to the enhancement of lithium ion conductivity. Nevertheless, when LaPO₄ particle was employed as a starting member for co-sintering, conductivity enhancement was not observed, which is presumably due to the particle size growth of LaPO₄. The precise size of LaPO₄ formed from LLTO is uncertain from the back-scattered electron images of SEM. In addition, titanium component decomposed from LLTO might attribute to the conductivity enhancement for LATP.

In the present study, LaPO₄ particle dispersed in LATP-LLTP composite is confirmed by TEM observation. Thereafter we prepare the LATP-La₂O₃ composite to clarify that the conductivity enhancement of LATP-LLTO composite is not caused by the titanium introduction into LATP but simply the insulator-dispersion effect in lithium ion conductors.

Experimental

Sample preparation has been carried out according to the previous study [1]. Li_0.35La_0.55TiO₃ (LLTO) was prepared by the conventional solid-state reaction method sintered at 1300^oC, thereafter crushed and milled finely by using planetary ball mill. LATP precursor was synthesized by calcining Li₂CO₃, γ-Al₂O₃, TiO₂ and (NH₄)H₂PO₄ starting materials at 700^oC for 2 hours. The obtained LATP precursor was then mixed with LLTO particles or La₂O₃ powders (d < 100 nm) and milled for 1 hour with as small amount of ethanol. After drying, the composites are pressed into pellets and sintered at 1000^oC.

Results and Discussions

Fig. 2 shows the TEM images of the dispersed particles. From the EDX analyses, the composition of the particle is confirmed as LaPO₄. In addition, typical particle size is found to be 200 nm. Any void was not observed at the interface between LaPO₄ and LATP matrix.

As for the LATP-La₂O₃ composites, the introduced La₂O₃ appeared to be reacted into LaPO₄, since X-ray diffraction indicated the absence of La₂O₃ and formation of LaPO₄ in the composites. Fig. 3 shows the compositional dependence of conductivity for LATP-La₂O₃ composite measured at room temperature. Although the maximum conductivity is smaller than that of LATP-LLTO, conductivity is increased by introducing La₂O₃ with the maximum at 8wt% of La₂O₃ addition. Accordingly, the conductivity enhancement observed in these system is surely due to the insulator-dispersion effect as observed in LiI-Al₂O₃ system [2].

References

[1] H. Onishi, S. Takai, T. Yabutsuka, T. Yao, Electrochemistry, 84, 967-970 (2016).

[2] C.C. Liang, J. Electrochem. Soc., 120, 1289-1292 (1973).