Although the Ni-MH battery has many advantages that are superior to the conventional Ni-Cd battery and Pb-acid battery, many researchers are still striving to develop the new negative electrode materials to improve the performances of Ni-MH battery. Recently, it is reported that Ti-Zr-V-Mn-Ni based alloys are one of the Laves phase alloys that have a very high discharge capacity and are easy to be activated. Their capacities, however, decrease rapidly within tens of charge-discharge cycles, making them unsuitable as the negative electrode material in the Ni-MH battery. The fast degradation of Ti-Zr-V-Mn-Ni based alloys is caused by the deterioration of surface properties followed by the growth of Ti-oxide on the surface. Therefore, the degradation behavior of Ti-Zr-V-Mn-Ni alloys should be prevented in order to utilize them as MH electrode. Considering the bulk and surface properties, the two prevention methods can be suggested.: one is the alloy design and the other is surface modification of Ti-Zr-V-Mn-Ni alloys.
In order to improve the bulk properties(corrosion resistnace and surface oxide composition, etc) of the alloy, the Mn is partially substituted by Cr. The cycle life of the can be improved by Cr substitution for Mn However, the discharge capacity decreases with the Cr substitution owing to the formation of second phase having small hydrogen storage capacity.
The effect of Cr substitution on the cycle life of Ti-Zr-V-Mn-Ni alloy is concluded as followed : first, the ductility of the alloy increased with Cr substitution. The pulverization rate of the alloy is greatly diminished due to the increased ductility of the alloy. This decreased pulverization rate of the alloy might be one of the factors improving the cycle life. Second, chromium in the matrix plays an important role on restraining the dissolution of vanadium.
However, the Cr in 2nd phase can't restrain it because V-Cr 2nd phase is more easily dissolved into KOH solution. Third, chromium prevents the oxidation of the constituent element. Chromium forms the dense $Cr_2O_3$ layer on the alloy surface and this oxide layer prevent the further oxidation of alloy inner part,
However, the Cr substitution is not good in view of high discharge capacity. In order to avoid this disadvantage of Cr substitution, Ti is partially substituted by Zr in the Cr-substituted Ti-based alloy. After Zr substitution, the cycle life of the alloy is improved. After the optimization of the alloy composition, the $Ti_{0.6}Zr_{0.4}Mn_{0.2}Cr_{0.3}V_{0.5}Ni_{0.9}$ alloy has a good cycle life and high discharge capacity(371mAh/g),
Second, the surface of $Ti_{0.8}Zr_{0.2}V_{0.5}Mn_{0.5}Ni_{0.8}$ alloy was modified by ball-milling with Ni powder. After surface modification with Ni, the cycle life of alloy is improved, but its discharge capacity is decreased. Decrease of discharge capacity is attributed to the hydrogen storage capacity loss due to amorphization of the alloy. In order to avoid this disadvantage of surface modification, the Ni powder is pre-ball milled and the shape of Ni powder is changed from spherical to flake. The flake Ni powder can modify the alloy surface within short ball-milling time because the highly-strain value promotes the diffusion of element. The optimal ball-milling condition is as followed : Ni amount(10wt.%), time (25min), ball-to-powder ratio(5:1). After ball-milled with flake Ni powder, the $Ti_{0.8}Zr_{0.2}V_{0.5}Mn_{0.5}Ni_{0.8}$ alloy shows a good cycle life($C_{180}$/$C_{max}$ = 92%) and very high discharge capacity (429mAh/g).
The roles of surface modification on the cycle life of the $Ti_{0.8}Zr_{0.2}V_{0.5}Mn_{0.5}Ni_{0.8}$ alloy can be explained as surface alloying effect and amorphization effect. Surface alloying provides the alloy with the active and protective surface layer. This layer is Ni-enriched and very immune in a KOH solution. The amorphization can improves the cycle life of alloy because the thin, compact oxide layer is formed on the amorphous alloy surface, As the amorphization goes on, the corrosion current is decreased and corrosion potential is moved toward positive value.
