Investigation of hydriding properties and structure of MlNi5-xSnx system

The hydriding performance, electrochemical properties and structure of MlNi5−xSnx (x=0–0.5) hydrogen storage alloys containing La-rich mischmetal were investigated by the use of pressure–composition isotherms, electrochemical measurements, and XRD, XPS, SEM and atomic parameters to improve properties and reduce cost. The substitution of Ni by Sn leads to an increase of the unit cell volume and charge–discharge cycle life and decrease of the plateau pressure, hysteresis and the hydrogen storage capacity. The standard enthalpy of hydride formation decreases with increasing tin substitution. The main factor which influences the standard enthalpy of the hydriding reaction is the number of the outer orbit electrons and not the atomic size factor. With a small amount of tin substitution, the cycle life increases by 52% (0.5C) and 42% (1.0C), but maximum discharge capacity decreases only by 3.0 and 3.5%, respectively. The cycle life is obviously improved by the low volume expansion and the formed SnO2 surface layers upon electrochemical cycling.