Effects of Ti on the cycle life of amorphous MgNi-based alloy prepared by ball milling

Abstract

Amorphous MgNi alloys have a critical problem of poor cyclic behavior in spite of their high capacity for the negative electrode of a Ni/MH rechargeable battery. In order to improve the cycle life of the alloys, the degradation mechanism of amorphous MgNi alloy is suppressed. It is found that the surface property of the MgNi alloy is a critical factor for improvement of cycle life because an increase of the thickness of the magnesium oxide layer on the alloy surface results in a decrease of the discharge capacity of the alloy. To prevent further oxidation of magnesium, both titanium surface coating and substitution methods are adopted. Ti substitution is found to be very effective for improving the cycle life while Ti surface coating proves to be hardly effective. Magnesium in the Mg 50Ni 50 alloy is replaced by titanium. The amount of titanium ( x in Mg 1− x Ti x Ni 1.0) is varied from 0.1 to 0.3. In the case of x=0.3 (Mg 0.7Ti 0.3Ni 1.0), the alloy shows the best cycle life. To explain this phenomenon, both electrochemical and phenomenological analyses are conducted. Electrochemical impedance spectroscopy (EIS) analysis shows that the charge transfer resistance ( R ct) between the Mg 0.7Ti 0.3Ni 1.0 alloy and the electrolyte does not increase during charge–discharge cycles. It is also found by auger electron spectroscopy (AES) that the thickness of the surface oxide layer on the Mg 0.7Ti 0.3Ni 1.0 alloy is thinner than that of the oxide layer on the MgNi alloy. At the same time, a nickel-enriched layer is found on the surface of the Mg 0.7Ti 0.3Ni 1.0 alloy. X-ray photoelectron spectroscopy (XPS) also shows that magnesium in the Mg 0.7Ti 0.3Ni 1.0 alloy exists in metallic state while titanium forms an oxide layer. These results indicate that the titanium oxide layer not only prevents further oxidation of magnesium, but also induces a nickel enriched layer on the alloy surface.