Thermodynamic destabilization of Li–N–H system by Si addition

Li–N–H and Li–Si–N–H composites have been synthesized by ball milling starting from lithium amide, lithium hydride and silicon powders. Hydrogen sorption behaviour was investigated for the Si-containing composites and a Li–N–H reference sample. The as-milled Si-containing sample showed the presence of the constituent powders but after few absorption–desorption cycles an important amount of Li2SiN2 appeared in both de-hydrogenated and re-hydrogenated composites. This phase was present in higher amount in the de-hydrogenated sample than in the re-hydrogenated one. The presence of this phase was also confirmed by XPS measurements. Li–N–H and Li–Si–N–H composites in de-hydrogenated state also contained lithium imide. X-ray diffraction data indicated a reversible hydrogen generating reaction between lithium amide, lithium hydride and silicon to produce Li2SiN2, lithium imide and hydrogen. The plateau pressure of Li–N–Si–H system is twice the one for Li–N–H at the same temperature (about 265 °C), as shown by pressure–composition isotherms in desorption mode. For the first time it has been observed a thermodynamic destabilization of Li–N–H by mixing it with Si.