Enhanced ionic conductivity in LAGP/LATP composite electrolyte**Project supported by the National Key Research and Development Program of China (Grant No. 2016YFB0100100), the National Natural Science Foundation of China (Grant Nos. 52315206 and 51502334), Fund from Beijing Municipal Science & Technology Commission, China (Grant No. D171100005517001).

Abstract

Nasicon materials (sodium superionic conductors) such as Li1.5Al0.5Ge1.5(PO4)3 (LAGP) and Li1.4Al0.4Ti1.6(PO4)3 (LATP) have been considered as important solid electrolytes due to their high ionic conductivity and chemical stability. Compared to LAGP, LATP has higher bulk conductivity around 10−3 S/cm at room temperature; however, the apparent grain boundary conductivity is almost two orders of magnitude lower than the bulk, while LAGP has similar bulk and grain boundary conductivity around the order of 10−4 S/cm. To make full use of the advantages of the two electrolytes, pure phase Li1.5Al0.5Ge1.5(PO4)3 and Li1.4Al0.4Ti1.6(PO4)3 were synthesized through solid state reaction, a series of composite electrolytes consisting of LAGP and LATP with different weight ratios were designed. XRD and variable temperature AC impedance spectra were carried out to clarify the crystal structure and the ion transport properties of the composite electrolytes. The results indicate that the composite electrolyte with the LATP/LAGP weight ratio of 80:20 achieved the highest bulk conductivity which shall be due to the formation of solid solution phase Li1.42Al0.42Ge0.3Ti1.28(PO4)3, while the highest grain boundary conductivity appeared at the LATP/LAGP weight ratio of 20:80 which may be due to the excellent interfacial phase between Li1+xAlxGeyTi2−x−y(PO4)3/LATP. All the composite electrolytes demonstrated higher total conductivity than the pure LAGP and LATP, which highlights the importance of heterogeneous interface on regulating the ion transport properties.