Electrode characteristics of C15-type laves phasealloys

Journal of the Less.Common Metals, 172-174 (1991) 1211-1218

1211

Electrode characteristics of C15-type Laves phase alloys Y. Moriwaki, T. Gamo, H. Seri and T. Iwaki Living Systems Research Centre, Matsushita Electric Industrial Co. Ltd., 3-15 Yagumonakamachi, Moriguchi, Osaka 570 (Japan)

Abstract Extensive studies on hydrogen storage alloys such as LaNi 5, MmNi5, Ti(Zr)-Ni etc. have been actively carried out for the negative electrodes of Ni-H 2 batteries. The present authors, on the contrary, have focused on C14-type and C15-type Laves phase alloys of the AB2 type based on TiCr2, TiMn 2 and ZrMn2, and have succeeded in optimizing them for electrode materials which can supply a high energy density by inspecting the relation between the composition of the alloys, the P - C - T characteristics and the electrode performance. The Mn:Cr ratio, A:B ratio and addition of vanadium to the C15-type Laves phase ZrMno.6Cro.2Nil.2 were examined to investigate the alloy phases, hydride characteristics ( P - C - T ) and electrode performance. As a result, electrodes composed of such alloys as ZrCro.sNil.2 and ZrMno.sCro,2Vo.lNi~.2 were found to provide a large discharge capacity in a cell test with an abundant electrolytic solution.

1. Introduction A n i c k e l - h y d r o g e n b a t t e r y having a hydrogen storage alloy for the negative electrode has a higher discharge capacity t h a n the conventional n i c k e l - c a d m i u m battery. The ABs-type alloys based on LaNi5 and MmNi~ [1-3] and T i ( Z r ) - N i system alloys based on Ti2Ni and TiNi [4, 5] are well known as hydrogen storage alloys. Among the two-element systems of T i - N i and Z r - N i , AB2-type Laves phase alloys do not exist. With the T i - N i and Z r - N i system alloys, in the region where the q u a n t i t y of nickel exceeds those of TiNi and ZrNi, which are not Laves phases, cell tests showed t h a t both systems had decreasing hydrogen storage capacity and electrical discharge capacity as the q u a n t i t y of nickel increased. Also, neither of these systems could achieve sufficient performance in discharge capacity with any q u a n t i t y of nickel [6]. We decided to add other elements so t h a t the alloy phase would be adjusted to a new type and the performance would be improved. From this viewpoint, we have focused on C14-type and C15-type, AB2-type Laves phase alloys [7], such as TiCr2, TiMn2 and ZrMn2, for use in hydrogen utilization technologies [8-10] and for a negative electrode for a high energy n i c k e l hydrogen battery [6, 11, 12].

0022-5088/91/$3.50

9 Elsevier Sequoia/Printed in The Netherlands

1212

Using the C15-type Laves phase ZrMno.6Cro.2Ni,.2 alloys and adjusting their alloy composition, by changing the Mn:Cr and A:B ratios, and trying a new type of alloy by adding vanadium to these alloys, we tried to make electrodes of high capacity. We attempted this by examining their P - C - T characteristics and their performances as electrodes.

2. E x p e r i m e n t a l details First the ZrMno.6Cro.2Ni,.2 alloy among the C15-type Laves phase alloys was selected to check its P - C - T characteristic and electrode performance. Next, ZrMno.s_xCrxNi,.2 (x = 0 . 2 - 0 . 8 ) , obtained by changing the ratio of manganese and chromium in ZrMno.~Cro.2Ni,.2, and ZryMno.4Cro.4Ni,.2 (y = 0.8-1.2), obtained by changing the ratio of the A site (zirconium) and the B site (manganese, chromium, nickel) in ZrMno.6Cr0.2Ni,.,~, were used to study improvements in the electrode performance. Also, ZrMno.6_xCro.2VxNi,.2 system alloys were examined to inspect the effect of adding vanadium to the B site to increase the crystal lattice constants of the C15-type Laves phase alloys. Each alloy was produced by compounding the raw materials and melting them four times in an argon arc furnace, with the alloys being turned over each time. Furthermore, portions of each alloy were heated for 12 h at 1080 ~ ~ in a vacuum to examine heat treatment effects. The alloys were powdered, to below 37 ~m (under 400 mesh), their crystal structure was inspected by X-ray, their hydrogen storage performance was measured by determination of their P - C - T characteristics and their electrode characteristics were also evaluated. The electrodes were produced by applying pressure to the alloy powder after mixing it with nickel powder. That is, 1.0 g of alloy powder, 3.0 g of nickel powder (INCO 270) and 0.12 g of ultrafine polyethylene powder were mixed and then pressed into a pellet of 25.4 mm diameter and 2.5 mm thickness. The characteristics of these negative electrodes were examined by a vented-type sintered n i c k e l - h y d r o g e n cell controlled under a negative electrode capacity.

3. R e s u l t s a n d d i s c u s s i o n 3.1. Mn:Cr ratio and A:B ratio The X-ray diffraction patterns of ZrMno.s_x CrxNi,.2 (x = 0.2-0.8) show that their structures are mainly C15 type in any composition, and the homogeneity and the crystal lattice constants of the alloy (a = 7.054-6.061 .~) did not change very much when the amount of chromium was increased. However, both the hysteresis and the flatness of the plateau pressure decreased as the amount of chromium increased (Fig. 1).

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Fig. 1. P - C - T curves of ZrMno.a_xCrxNi~.2 (x = 0.2-0.8) systems. F r o m the X-ray d i f f r a c t i o n p a t t e r n s of ZryMno.4Cro.4Nit. 2 (y = 0.8-1.2) s h o w n in Fig. 2, t h e C15 p h a s e c l e a r l y a p p e a r s as the q u a n t i t y of z i r c o n i u m in the s t o i c h i o m e t r i c c o m p o s i t i o n of the a l l o y is d e c r e a s e d . I n this case, as the a m o u n t of z i r c o n i u m decreases, t h e c r y s t a l l a t t i c e c o n s t a n t s of the a l l o y d e c r e a s e ( y = 0.8, a = 7.010 ~ ; y = 1.0, a = 7.055/~), t h e h y d r o g e n e q u i l i b r i u m p r e s s u r e in P - C - T measurements increases and the

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1214

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measured hydrogen storage capacity decreases (Fig. 3). On the contrary, the homogeneity of the alloy decreases and the crystal lattice constants increase when the amount of zirconium increases excessively. As a result, the hydrogen storage capacity at the first P - C - T cycle increased, but the performance of the absorption and desorption cycle deteriorated and the quantity of hydrogen desorbed decreased. The discharge capacity increases as the amount of chromium is increased in the case of ZrMno.8_x CrxNil.2 (x = 0.20.8), as shown in Fig. 4. In the case of ZryMno.4Cro.4Nil. 2 (y =0.8-1.2), the discharge capacity was the largest and the rising performance of the initial charge-discharge cycle was good when y = 1.0. This means that the alloy composition should not differ from the stoichiometric composition. The relation between the discharge capacity calculated from the P - C T data and the actual discharge capacity determined by cell tests of

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Fig. 5. Comparison of calculated capacity (from P - C - T data) and discharge capacity (measured) of ZrMno.s_x Crx Nil.2 (x = 0.2 - 0.8) systems. Fig. 6. Comparison of calculated capacity and (from P - C - T data) and discharge capacity (measured) of ZryMno.4Cro.4Nil.~ (y = 0.8-1.2) systems.

ZrMn0.s_x Crx Nil.2 (x = 0.2-0.8) is shown in Fig. 5 and the same relation for ZryMn0.4Cr0.4Ni].2 (y = 0.8-1.2) is shown in Fig. 6. The standard of the capacity calculated from P - C - T data was set supposedly for a region where the hydrogen desorption is less than 0.5 M P a at 20 ~ The utilization ratio for the discharge cycle was calculated from the actual discharge capacity divided by the P - C - T calculated capacity. The P - C - T capacity does not change very much with the composition in the case of ZrMno.s_xCr~Nil. 2 (x =0.2-0.8) but the actual discharge capacity exhibited an improvement in utilization ratio as the amount of chromium increased. The discharge capacity changes largely according to the heat treatment of the alloy and the radius of the alloy particles. Also, differences in the surface conditions of the alloys, such as oxide phase and segregation phase, are expected to related to the discharge capacity. Therefore, controlling the surface of the alloy is important for improving the utilization ratio as well as improving the P - C - T capacity. 3.2. Z r M n C r V N i systems with vanadium substitution In the case of ZrMno.G_xCro.2VxNil.2, based on ZrMno.6Cro.2Nil.2, the X-ray diffraction analysis and capacity calculated from P - C - T data were examined (Table 1). The homogeneity of the effective Laves phase of the alloys decreased with added vanadium and the crystal lattice constants increased (Table 1). Some P - C - T curves for these alloys, compared with that of the ZrMnCrNi without vanadium, shown in Fig. 7 for 70 ~ An improvement in hydrogen storage capacity resulting from the addition of vanadium can be observed. Also, a large increase in the discharge capacity was observed in the cell test (Fig. 8).

1216 TABLE 1 The X-ray d i f f r a c t i o n analysis ZrMno. 6 _ x Cro.2V x Ni~.2 s y s t e m s

Alloy composition

T= 70"C

g

capacity

calculated

from

X-ray diffraction analysis

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Capacity calculated from P C - T data 0.353 A h g-1 0.405 A h g - 1 0.395 A h g - l

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1217 These results on AB2 Laves phase alloys with vanadium addition show t h a t vanadium is effective in increasing the electrical discharge capacity and also providing some increase in the hydrogen absorption and desorption capacity. However, the homogeneity of the alloy decreases with addition of vanadium as it expands the crystal lattice. Therefore, vanadium has to be added in an appropriate quantity in order to maintain the plateau property. 4. C o n c l u s i o n We have tried to optimize the alloy compositions of AB2 type Laves phase alloys, which can realize high energy densities, by investigating the effects of the Mn:Cr ratio and the A:B ratio on the performance of the ZrMno.6Cr0.2Nil. 2 alloys and we have tried to increase the high capacity by adding vanadium. The following results have been obtained from these attempts. (1) In the ZrMno.~_xCrxNi,.2 (x =0.2-0.8) system alloys, manganese contributes to flattening the plateau pressure and chromium contributes to decreasing the hysteresis characteristics. (2) It is important to keep the ratio A:B of the stoichiometric composition close to 1:2 in order to acquire a large quantity and high stability of hydrogen absorption in the case of ZryMn0.4Cr0.4Nil. 2 (y = 0.8-1.2). (3) Substitution of v a n a d i u m for manganese is effective in increasing the electrical discharge capacity and to some extent, the q u a n t i t y of hydrogen absorbed and desorbed. Since there is a limit to the substitution of vanadium for manganese, correct quantities of manganese and chromium are also essential for m a i n t a i n i n g plateau characteristics. (4) The discharge capacity of the electrodes does not always correspond to the P - C - T capacity. Although it is important to improve the real P-C-T capacity, the surface condition of the alloy is the most important factor for improving the utilization efficiency of the electrode reaction in the future. As a result of our investigation, electrodes composed of alloys such as ZrCr0.sNi~. 2 and ZrMno.aCro.2Vo.3Ni,. 2 exhibited an improved discharge capacity of 0.33 A h g - ' and 0.36 A h g - ' . References 1 J. J. Willems and K. H. J. Buschow, J. Less-Common Met., 129 (1987) 13. Philips J. Res., 39 (Suppl. 1) (1984). 2 H. Ogawa, M. |koma, H. Kawano and I. Matsumoto, Power Sources, 12 (1989) 393. 3 T. Sakai, H. Miyamura, N. Kuriyama, A. Kato, K. Oguro and H. Ishikawa, J. Electrochem. Soc., 137 (1990) 795. 4 M. A. Fetcenko, S. Venkatesan, K. C. Hong and B. Reichman, Power Sources, 12 (1989) 411. 5 S. Wakao, H. Sawa, H. Nakano, S. Chubachi and M. Abe, J. Less-Common Met., 131 (1987) 311.

1218 Moriwaki, T. Gamo, A. S h i n t a n i and T. Iwaki, Denki Kagaku, 57 (1989) 488. Nakawichi, Nonstoichiometric Metal Compounds, Nippon Kinzoku Gakkai, 1975, p. 277. Gamo, Y. Moriwaki, N. Y a n a g i h a r a and T. Iwaki, J. Less.Common Met., 89 (1983) 495. Gamo, Y. Moriwaki, N. Yanagihara, T. Yamashita and T. Iwaki, Int. J. Hydrogen Energy, 10 (1985) 39. 10 Y. Moriwaki, T. Gamo, I. Takeshita and T. Iwaki, Nippon Kagaku Kaishi, 8 (1988) 1282. 11 Y. Moriwaki, T. Gamo, A. S h i n t a n i and T. Iwaki, Proc. 28th Battery Syrup. in Japan, Tokyo, November 1987, p. 111. 12 Y. Moriwaki, T. Gamo, A. S h i n t a n i and T. Iwaki, Proc. 29th Battery Symp. in Japan, October 1988, p. 117. 6 7 8 9

Y. T. T. T.