Dispersion Effect of LaPO4 Particles in Lithium Ion Conductor LATP
Authors: Takai Shigeomi , Uematsu Masayoshi , Chen Wen-Jauh , Yabutsuka Takeshi , Yao Takeshi 
Published: February 2020

Introduction

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 (La2/3-xLi3xTiO3) particles in LATP (Li1.3Al0.3Ti1.7(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]. Li0.35La0.55TiO3 (LLTO) was prepared by the conventional solid-state reaction method sintered at 1300oC, 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 700oC 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 1000oC.

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).

Figure 1