Properties of Mechanically Milled Nanocrystalline and Amorphous Mg–Y–Ni Electrode Alloys for Ni–MH Batteries

Abstract

Nanocrystalline and amorphous Mg₂Ni-type Mg_20−x Y _x Ni₁₀ (x = 0, 1, 2, 3 and 4) electrode alloys were fabricated using mechanical milling. The effects of the Y content and milling time on the microstructures and electrochemical performances of the alloys were investigated in detail. X-ray diffraction and transmission electron microscopy analyses revealed that the substitution of Y for Mg yields an obvious change in the phase composition and micro morphology of the alloys. When the Y content x ≤ 1, the substitution of Y for Mg does not change the major phase Mg₂Ni, but with a further increase in the Y content, the major phase of the alloys transforms into the YMgNi₄ + YMg₃ phase. A nanocrystalline and amorphous structure can be obtained by mechanical milling, and the amorphisation degree of the alloy visibly increases with increased milling time. Electrochemical measurements indicate that the discharge capacity of the alloys first increases and then decreases with increasing Y content and milling time. The substitution of Y for Mg dramatically ameliorates the cycle stability of the as-milled alloys, and the mechanical milling more or less impairs the cycle stability of the alloys. Furthermore, the high rate discharge ability, electrochemical impedance spectrum, Tafel polarisation curves and potential step measurements indicate that the electrochemical kinetic properties of the as-milled alloys first increase and then decrease with increasing Y content and milling time.