Investigation of hydrogen absorption in Li7VN4 and Li7MnN4

Abstract

The hydrogen storage properties of Li₇VN₄ and Li₇MnN₄ were investigated both by experiment and by density functional theory calculations. Li₇VN₄ did not sorb hydrogen under our experimental conditions. Li₇MnN₄ was observed to sorb 7 hydrogen atoms through the formation of LiH, Mn₄N, and ammonia gas. An applied pressurized mixture of H₂/Ar and H₂/N₂ gases was helpful to mitigate the release of NH₃ but could not prevent its formation. The introduction of N₂ also caused weight gain of the sample by re-nitriding the absorbed products LiH and Mn₄N, which correlated with the presence of Li₂NH, LiNH₂, and Mn₂N detected by X-ray diffraction. While our observed results for Li₇VN₄ and Li₇MnN₄ differ in detail, they are in overall qualitative agreement with our theoretical work, which strongly suggests that both compounds are unlikely to form quaternary hydrides.