The effects of titanium on the phase structure and charge/discharge performance of Ml(NiCoMnTi)(5) alloy

Substitutional effect of small amount of Ti in hydrogen storage electrode alloy Ml(NiCoMnTi)5 was systematically studied. The results show that Ti in the alloy is almost entirely separated out as a second phase of TiNi3 at boundary of the as-cast dendrite. After annealed, TiNi3 phase disappeared, but SEM and EDS analyses indicate that the partially substituted element by Ti is mishmetal Ml, not Ni in B side as expected, i.e. Ti is an A side substituent element. The optimal substitution amount is 5% (atomic fraction), with which the corresponding alloy, both as-cast and annealed, shows a high discharge capacity over 310 mA.h/g. If more Ti is added, a greatly decreased discharge capacity will be received with only a little increase of better cycle stability.