Effect of Ni Content on Rate Capability of Mg[sub 0.9]Ti[sub 0.06]V[sub 0.04]Ni[sub x] Negative Electrodes for Nickel/Metal Hydride Batteries

Abstract

Mg 0.9 Ti 0.06 V 0.04 Ni x (x ϭ 1.0-1.3) alloys were prepared by mechanical alloying and their charge-discharge characteristics were investigated with structural characteristics. Maximum discharge capacity obtained at the 1st cycle was decreased with an increase in Ni content, while cycle performance was improved due to the suppression of surface oxidation. High-rate dischargeability ͑HRD͒ was also markedly improved with increasing Ni content. In particular, the Mg 0.9 Ti 0.06 V 0.04 Ni 1.3 negative electrode showed HRD of 65% even at 1 A g Ϫ1 . From the results of differential chronopotentiometry and differential thermal and thermogravimetric analyses, it was suggested that hydrogen in the amorphous phase of the alloy was destabilized, leading to the accelerated hydrogen diffusion. This is likely to be a factor for the marked improvement of HRD.Mg-based hydrogen storage alloys are attractive materials for energy conversion and storage, but they need high temperature for the rapid and reversible hydrogen absorption and desorption. In addition, from the viewpoint of utilization for a negative electrode material in nickel/metal hydride batteries, the oxidation on the alloy surface was also a serious problem, leading to low discharge capacity, poor cycle performance, etc.Amorphous MgNi alloy 1-4 prepared by mechanical alloying ͑MA͒ and nanocrystalline Mg 2 Ni alloy 5-7 prepared by ballmilling showed more than ten times higher discharge capacity than the crystalline Mg 2 Ni alloy, but they showed poor cycle performance. For improving such disadvantages, two methods have been adopted. One is bulk modification, that is partial substitution of constituent elements with foreign metals, 8-18 and another is surface modification. [19][20][21] We have found that the discharge capacity was increased by surface modification, i.e., ballmilling of MgNi with carbon materials such as graphite, Ketjen black, etc., 22-26 while cycle performance was improved by bulk modification, i.e., partial substitution of Mg in a MgNi alloy with Ti and/or V in the ballmilling. 24,25,27 It was concluded that the Mg 0.9 Ti 0.06 V 0.04 Ni alloy was the most appropriate composition from the viewpoint of both discharge capacity and cycle performance.Besides discharge capacity and cycle performance, rate capability is an important factor. However, to our knowledge there have been few investigations on the rate capability of the Mg-based alloy negative electrodes, 13,15,18,19,28 and all HRD values obtained in the investigations were low. In this study, Mg 0.9 Ti 0.06 V 0.04 Ni x (x ϭ 1.1-1.3) alloys increasing Ni content from a basic composition of Mg 0.9 Ti 0.06 V 0.04 Ni were prepared by ballmilling of raw constituent elements, and their rate capability, in particular high-rate dischargeability ͑HRD͒, is examined in relation to the other chargedischarge characteristics and structural characteristics. Because Ni is well known to be an effective catalyst for hydriding and dehydriding, the increase in Ni content is expected to improve the HRD value. Expe...

Electrochemical hydrogen storage properties of nonstoichiometric amorphous MgNi1+xMgNi1+x–carbon composites (x=0.05x=0.05–0.3)

“…They found that the electrode surface area and the materials oxidation rate constant play an essential role in capacity fading. Recently, Inoue et al [2] reported that the decay of the discharge capacity for Mg 0.9 Ti 0.06 V 0.04 Ni x alloys followed the mechanism proposed by Notten et al Here, we found that nonstoichiometric MgNi 1+x alloys also obey the mechanism of Notten. with various carbon powders.…”

“…This suggests that the alloys prepared by mechanical alloying have an amorphous structure. However, the center of the broad peak for alloys with higher Ni content slightly shifts to higher angle compared to lower Ni content alloys, implying that the amorphous phase partially includes nanocrystalline Ni [2]. Fig.…”

“…However, the discharge capacity of MgNi decreases rapidly with charge/discharge cycles because of the formation of Mg(OH) 2 during the charge/discharge in alkaline solution. Three methods have been adopted to overcome such a disadvantage: (1) bulk modification, that is partial substitution of constituent elements with foreign metals [1][2][3][4]; (2) nonstoichiometric amorphous Mg-based alloys, we have found that the nonstoichiometric amorphous Mg-based alloys exhibit higher discharge capacities and better cycle life than the stoichiometric one [5]; (3) surface modification [6][7][8].…”

Enhanced electrochemical properties of nonstoichiometric amorphous Mg2Ni1.3 electrodes

“…Mg 2 Ni 1.3 coated with graphite shows the highest plateau potential. These results suggest that the increase in the equilibrium hydrogen pressure and improvement in the electrochemical reactivity on the alloy surface leads to improved dischargeability [13]. It has been well known since the 1960s that amorphous metals and alloys can be made by rapid quenching from the vapor or the liquid phase.…”

“…In particular, a high RC value (78%) was obtained for the Mg 2 Ni 1.3 coated with graphite. It was reported that the hydrogen in the amorphous phase of the alloy was destabilized, leading to accelerated hydrogen diffusion [13]. This is likely to be a factor for the improvement in rate capability in amorphous Mg 2 Ni 1.3 electrodes.…”