Reverse Monte Carlo analysis of NaI-LiI solid electrolyte based on the neutron total scattering data

In the present study, the origin of Li+ ion conduction in an Li+/Na+ mixed solid electrolyte, 9NaI∙LiI, was investigated from the perspective of atomic configuration. The co-existence of Li+ and Na+ ions in solid electrolytes is known to result in a deterioration in conductivity, known as the mixed alkali effect. While dominant Li+ conduction has been reported in Li+-doped NaI where 10 mol% of Li+ ions are substituted for the host Na+ ions in NaI, the structural origin of the Li+ conduction in NaI is not well understood. From the results of reverse Monte Carlo (RMC) modeling based on the neutron total scattering data, it was revealed that the position of the Li+ is significantly off-centered from the ideal LiI6 octahedra despite the symmetrical distribution of the Na+ ions in the cation sites. The peculiar distribution of Li+ was explained by the large mismatch between the size of the Li+ ion and the lattice volume of NaI. These off-centered Li+ ions were shown to result in void spaces in the LiI6 octahedra. It is likely that the free volume in the LiI6 octahedra provides the hopping sites for the neighboring Li+ ions. Since the ionic radius of Na+ ions is larger than that of Li+ ions, the spaces in the LiI6 and NaI6 octahedra were not sufficient for Na+ hopping. One of the factors which contributed to the different ion conduction mechanism for Li+ and Na+ in the Li+/Na+ mixed solid electrolytes was shown to be the different local structures around Li+ and the host Na+ in Na compounds. [Display omitted]