Optimization of the La substitution by Mg in the La2Ni7 hydride-forming system for use as negative electrode in Ni-MH battery

• Coexistence of two polymorphic structures in the La2−xMgxNi7 system.
• Mg-free compound exhibits lower reversible electrochemical capacity.
• Optimum Mg composition obtained around 25 at.% Mg sitting on the La site.

The crystallographic, thermodynamic and electrochemical properties of the hydrogen-sorbing system La2−xMgxNi7 has been examined as a function of x. X-ray diffraction shows the coexistence of two polymorphic structures, hexagonal (P63/mmc) and rhombohedral (R-3m), for the same compositions as already observed for other systems La–Mg–Ni. Thermodynamic properties regarding hydrogen uptake have been determined by means of solid–gas reaction. Capacity and equilibrium pressure have been measured and electrochemical properties in alkaline medium have been determined as a function of the magnesium content. Mg-free compound exhibits much lower reversible electrochemical capacity than the Mg-containing one and an optimum Mg composition has been obtained around 25 at.% Mg sitting on the lanthanum site.

Figure optionsDownload as PowerPoint slide