Enhanced lithium ion conduction and the size effect on interfacial phase in Li2ZnI4–mesoporous alumina composite electrolyte

The enhancement of lithium ion conductivity was observed for the composite type electrolyte based on Li2ZnI4 and sol–gel synthesized mesoporous-Al2O3 with controlled pore size (2–10 nm). Compositional and pore size dependence of the electrical conductivity was investigated. More than fifty times enhancement of the electrical conductivity was observed for the composite with mesopore size greater than 5.0 nm. 6Li MAS-NMR measurement was performed for the composite electrolytes based on LiI and Li2ZnI4. Both LiI and Li2ZnI4 gave an identical 6Li NMR peak position at − 4.9 ppm. Two additional lithium ion environments, at − 0.8 ppm and − 2.8 ppm, respectively, were observed for the composite. Comparison with a 6Li cross polarization MAS-NMR measurement clarified the origin of − 0.8 ppm peak as a surface lithium ion adjacent to OH− groups. The high conductivity of the composite was suggested to originate from new interfacial phase that have 6Li NMR peak position at − 2.8 ppm.