Effects Of Multi-component On The Phase Structure And Electrochemical Properties Of Zr-based Laves Hydrogen Storage Alloy

Zr-based AB₂ type Laves phase hydrogen storage alloy, which is regarded as the promising electrode material, has been extensively investigated, because of its much higher discharge capacity and longer life. In this paper, the effects of composition and preparation methods on the phase structures and electrochemical properties of alloys have been reviewed. On the basis of above discussions and the previous results in our lab, multi-component alloying were chosen to improve the discharge capacity, activation and high rate dischargeablity of Zr-based Laves alloys.The effects of substituting Mn and V for Ni on the phase structures and the electrochemical properties of the alloy ZrCr_0.7Ni_1.3 have been investigated. In ZrCr_0.7-xV_xNi_1.3 (x=0.1-0.6) alloys, withV content increasing, C14 Laves phase content increases and C15 Laves phase content decreases, and second phase Zr₇Ni₁₀ changes to Zr₉Ni₁₁, and the cell volumes of the alloys expanse, and discharge capacity of the alloys decreases sharply. In ZrCr_0.7-xV_xMn_yNi_1.3(x+y=0.2-0.4) alloys, withV and Mn increasing simultaneously, the phase constitutions of the alloys have not been changed evidently, but discharge capacity of the alloys increases. The alloy ZrCr_0.4Mn_0.2V_0.1Ni_1.3 exhibits the best electrochemical performance (318 mAh/g).The phase structure and the electrochemical properties of the non-stoichiometric alloy Zr(Cr_0.4Mn_0.2V_0.1Ni_1.3)_x(x=1.8-2.4) have been studied. When x <2, the content of C14 Laves phase in the alloys increases and Zr₇Ni₁₀ turns to be Zr₉Ni₁₁ with x decreasing. When x>2, with x increasing, C14 Laves phase and Zr₇Ni₁₀ in the alloys decrease gradually. The alloy consists only of C15 Laves phase when x=2.4. Hyper-stoichiometric alloys exhibit better activation properties, but the discharge capacity of the non-stoichiometric alloys is lower than that of stoichiometric alloy, and the high rate dischargeability of the non-stoichiometric alloys can not be improved.The effects of different operation temperature on the electrochemical properties of the alloy ZrCr_0.4V_0.1Mn_0.2Ni_1.3 have been investigated. At 70℃, the alloy reaches the maximum capacity after 3 cycles, and its high rate dischargeability preserves 52% at current 300 mA/g. The activation and rate capability of the alloy can be improved at high temperature, but the self-discharge property of the alloy is deteriorated. The alloy shows the excellent cycle stability at different temperature. It is mainly ascribe to the formation of oxide film maintaining Cr, which prevents oxidation and dissolution of the other elements in the alloys further.The effects of substituting Ti for Zr on the phase structure and the electrochemical properties ofdifferent alloys ZrCro.4Vo.1Mno.2Ni 13 and ZrMno.6Vo.2Coo.1Ni12 have been analyzed. The results show the same change trend for phase structure of both alloys with Ti content increasing. The content of C14 phase increases and that of C15 phase decreases, and second phase decreases and vanishes. The cell volume of the alloys contracts and the dendrite structure turns to be finer. However, the effects of substituting Ti for on the electrochemical properties of both alloys are different. When x=0.2, the alloy Zrj.xTixMno.6Vo.2Coo.1Ni12 (x=0.1-0.5) exhibits the maximum discharge capacity 358 mAh/g, and their activation and rate capability can be improved with increasing Ti content, on the other hand, the alloy Zr1.xTixCro.4Vo.1Mno.2Ni 1.3 (x=0.1-0.3) reveals the maximum discharge capacity 336 mAh/g when x=0.1, and the activation and rate capability decrease with increasing Ti content.To preserve the cell volume and phase structure of the alloy ZrCro.4V0.1Mno.2Ni 13, Ni is substituted by Mn and V respectively when Zr is substituted by Ti. The discharge capacity of the alloys can be improved, and the high rate capacity of the alloys containing Mn can be elevated with Mn content increasing.Effects of substituting La for Zr on the phase structure and electrochemical properties of the different alloys ZrCro.4Mno.2Vo.1Ni13 and ZrMno.6Vo.2Coo.1Niu are investigated. The phase structure of both alloys has not still changed at low content La. With increasing La content, however, the phase structure of the alloys Zr|.xLaxCro.4Mno.2Vo.iNii.3(x=0.03-0.1) can be changed from C15 to C14, but that of the alloys Zri.xLaxCr0.4Mn0.2V0.iNii.3 (x=0.03-0.1) has not still changed, and the second phase of both alloys decreases. Some diffluent phase appear on the surface of both alloys, because there some holes on the surface of alloys after corrasion. With increasing La content, the dendrite structure of the as-cast alloys turns to be blurred and the diffluent phase enlarges. The activation and the discharge capacity of the alloys increase after substituting La. With increasing La content further, the discharge capacity and the cycle stability of the alloys are destroyed. It is reasonable that the content of La in the alloys is at the range of 0.03-0.05.When substituted Ti and La for Zr in the alloy ZrMno.6Vo.2Coo.1Ni1.2 simultaneously, the content of C14 of alloys increases and the content of second phase decreases. The alloy Zro.75Tio2Lao.05Mno.6Vo.2Coo.1Ni12 reaches the maximum discharge capacity 372 mAh/g after 4 cycles. Its discharge capacity still preserves 70% after 300 cycles.In order to improve the discharge capacity and the rate dischargeability of the alloys, modifications of alloy Zro.5Tio.5Mno 6VojCoo.iNiu by ball milling with Zr7Nii0 have been carried out.It is found that the phase structure of the composite alloy is mainly affected by ball milling time, but not the content of Zr7Nii0. With ball milling time increasing, the alloys turn to be amorphous gradually. The discharge capacity and the rate dischargeability of the alloy can be improved after ball milling with 7% Zr7Niio for 1 h. However, the discharge capacity of the alloy decreases sharply after ball milling for 3h. The part of alloy recrystallizes after heat treatment for lh at 1073K, and its discharge capacity increases, but its activation decreases.The diffusion of the H atom in the alloys is also discussed. The diffusion coefficients of the different alloys have been calculated. They are in the range of 10"9-10"10, which is very low compared with LaNi5. The results are in accordance with above electrochemical results.