Hydrogen release and absorption in mixed anion lithium amide/lithium ternary nitride systems

Abstract

In this work, reactions of either LiBH$_4$, ZnCl$_2$ or Zn$_3$N$_2$ with LiNH$_2$ have been studied. The presence of CoO significantly affected the products and hydrogen release on heating mixtures of χLiBH$_4$-γLiNH$_2$. The ratios of the l4$_1$/amd and the P2$_1$/c polymorphs of Li$_3$BN$_2$ in the products have been changed under different conditions studied. On addition of CoO, the temperature of hydrogen release from the χLiBH$_4$-γLiNH$_2$ systems was greatly reduced, starting from 100°C and peaking around 250°C, much lower than 240°C and 330°C without catalyst. Ball-milling helped to improve the amounts of hydrogen desorbed from 3–4 wt% up to ≥10 wt%. In the reactions of ZnCl$_2$ + nLiNH$_2$ (where $\textit n$ = 2–6), main products were LiCl, Zn$_3$N$_2$, and LiZnN. NH$_3$ was the main gas released from these reactions and the addition of LiH changed NH$_3$ into H$_2$, which was released around 90°C, much lower than in the absence of LiH. A mixture of LiZnN and LiCl obtained from this reaction was partly rehydrogenated to form Li$_2$NH and Zn. The reaction of Zn$_3$N$_2$ and LiNH$_2$ was found to produce pure LiZnN without LiCl. Neither pure LiZnN nor Zn$_3$N$_2$ could be hydrogenated under the conditions tried, but a mixture resulting from the reaction could react with H$_2$ to form LiNH$_2$ and Zn. The cyclability of the Li–Zn–N system showed an ability to release and take up gases under different pressure conditions. Mg-doping in LiZnN was examined to improve reversibility of the Li–Zn–N system but was not successful