Partial nitridation of Li4SiO4 and ionic conductivity of Li4.1SiO3.9N0.1

The Li content and anion lattice of Li4SiO4 were modified to improve ionic conductivity. Li2CO3 and Si3N4 were mixed in a ratio of Li/Si = 4.5 and heated in NH3 at 820 °C, which resulted in the formation of the oxynitride, Li4.1SiO3.9N0.1. Powder X-ray diffraction analyses revealed Li4.1SiO3.9N0.1 and Li4SiO4 to be isostructural with a subtle variation in the lattice constants. Diffuse-reflectance absorption spectroscopy, however, showed a significant decrease in the band gap, from 5.6 eV in Li4SiO4 to 4.8 eV in Li4.1SiO3.9N0.1. X-ray photoelectron spectra of the Li 1s and Si 2p levels revealed enhanced lattice covalency in Li4.1SiO3.9N0.1 compared to the oxide phase. The ionic conductivity of Li4SiO4 and Li4.1SiO3.9N0.1 were measured by ac impedance spectroscopy over the temperature range 100-400 °C. Non-linear fitting analysis of the equivalent circuit revealed that the ionic conductivity of Li4.1SiO3.9N0.1 was approximately one order of magnitude higher than that of Li4SiO4.