Unravelling LaNi5 intrinsic degradation: New insights from DFT study on thermodynamics and hydrogen diffusion

The use of LaNi5 intermetallic for hydrogen storage is drastically limited due to its degradation during cycling. Despite extensive efforts in the literature to address this issue, a fundamental challenge remains - no one has been able to pinpoint the exact origin of this degradation. In this work, we elucidate the fundamental mechanisms behind this degradation using DFT calculations, a feat that couldn't be achieved through experimental methods alone. Particularly, our findings underscore the stability of the (2b) site in comparison to (6c1) and (6c2) sites. Hydrogen migration in the (x,y)-plane from (6c1)-site to (2b)-site is confirmed to be fast within the crystal structure, owing to a low barrier of 0.24 eV, which is much lower than that of MgH2. On the other hand, hydrogen diffusion from the (2b)-site to the (6c1)-site exhibits the highest energy barrier of 0.45 eV. These results emphasize the stability of the (2b)-site and provide a fundamental explanation for the degradation of LaNi5. Indeed, the occupation of the (2b)-site serves as the primary driving force behind the formation of stable hydrides that cannot be desorbed under ambient conditions.