HYDROGEN ABSORBING ALLOY FOR ALKALINE STORAGE BATTERY AND METHOD FOR PREPARING THE SAME

A hydrogen storage alloy, for use in alkaline storage batteries, having a CaCu5-type crystal structure and represented by the compositional formula MmNixCoyMnzM1-z (wherein M represents at least one element selected from the group consisting of aluminum (Al) and copper (Cu); x is a nickel (Ni) stoichiometry and satisfies 3.0 ≤ x ≤ 5.2; y is a cobalt (Co) stoichiometry and satisfies 0 ≤ y ≤ 1.2; z is a manganese (Mn) stoichiometry and satisfies 0.1 ≤ z ≤ 0.9; and the sum of x, y and z satisfies 4.4 ≤ x+y+z ≤ 5.4). The hydrogen storage alloy includes a bulk region having a CaCu5-type crystal structure and a substantially uniform composition and a surface region surrounding said bulk region and having a graded composition. When the sum in percentage of numbers of cobalt (Co) atoms and copper (Cu) atoms present in the surface region is given by a and that in the bulk region by b, the relationship a/b ≥ 1.3 is satisfied.