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LJournal of Alloys and Compounds 313 (2000) 4752www.elsevier.com/locate/jallcom

Gas atomization of metal hydrides for NiMH battery applications

*Matthew L. Anderson, Iver E. Anderson126 Metals Development, Ames Laboratory, Iowa State University, Ames, IA 50011, U SA

Received 23 May 2000; received in revised form 6 July 2000; accepted 17 July 2000

Abstract

The use of high pressure gas atomization (HPGA) to produce fine, spherical powders of metal hydride (MH) alloys has been shown to

improve the properties of the alloy, as well as reducing costly processing steps. Electrochemical cycling was used to compare three

electrodes constructed of LaNi Sn particulate, a simplified AB -type alloy for battery applications. Two electrodes consisted of gas4.75 0.2 5 5

atomized powders and the other used particulate produced from casting and crushing an ingot, the conventional method of MH particulate

production. Gas atomized powders showed improved cyclic stability over the cast and crushed particulate. A commercial cast and crushed

MmNi Co Mn Al alloy was also cycled and showed reduced degradation when compared against the electrode composed of3.55 0.75 0.4 0.3

LaNi Sn cast and crushed particulate, as was expected. 2000 Elsevier Science B.V. All rights reserved.4 .7 5 0 .2 5

Keywords: Metal hydride powders; Gas atomization; Electrochemical testing; Annealing effects; Gas phase hydrogen sorption

1. Introduction repeated gas-phase hydrogen absorption / desorption cy-

cling [811].

Nickelmetal hydride batteries are considered as a Therefore, it is of interest to investigate the use of

promising battery chemistry for use in electric and hybrid spherical, gas atomized powders as electrode materials in

vehicles. Their combination of high energy density, no NiMH batteries. Electrochemical cycling studies werememory problems, and a MH alloy composition consist- conducted for both helium and argon atomized

ing of abundant, low toxicity elements make nickelmetal LaNi Sn powders, as well as conventionally pro-4. 75 0. 25

hydride (NiMH) batteries a popular choice for applica- cessed (casting and crushing) LaNi Sn and4 .75 0 .25

tions requiring rechargeable batteries [1]. However, elec- MmNi Co Mn Al alloy particulate. This approach3 .5 5 0.75 0.4 0.3

trochemical degradation issues and processing costs are allowed electrodes composed of spherical, gas atomized

preventing a large surge in its application. powders to be directly compared to an electrode comprised

Particle fracturing within the MH electrode during of cast and crushed material of the same composition. The

cycling has been observed by several researchers and is performance of the MmNi Co Mn Al alloy was3 .55 0.75 0.4 0 .3

considered to reduce discharge capacity by oxidation of the also evaluated to compare alloy design effects for conven-

fracture surfaces [25]. The cracking can be attributed to tionally processed particulate.

tensile stresses developed at particle surfaces during

volumetric contraction of the particle during dehydriding[6]. Therefore, methods of reducing the volumetric expan- 2. Experimental procedure

sion, or the associated surface tensile stress, have been

considered. One common method is to substitute for the 2.1. Powder production

nickel in the LaNi structure with other transition ele-5

ments, or 3A and 4A elements [3,7]. It has also been The powders were produced by two methods: gas

shown that spherical, gas atomized LaNi Sn powders atomization and casting/ grinding. The gas atomization4 .75 0 .25

smaller than 20 mm in diameter do not fracture after process utilized the high pressure gas atomization (HPGA)

system at Ames Laboratory [12]. Lanthanum (99.0%

purity with respect to rare earths), nickel (99.99% purity),

and tin (99.99% purity) were prealloyed to create charge*Corresponding author.E-mail address:andersoni@ameslab.gov (I.E. Anderson). material with a composition of LaNi Sn for atomiza-

4. 75 0. 25

0925-8388/00/$ see front matter 2000 Elsevier Science B.V. All rights reserved.P II: S 0 9 2 5 -8 3 8 8 (0 0 )0 1 1 7 0 -1

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48 M.L. Anderson, I.E. Anderson / Journal of Alloys and Compounds 313 (2000) 4752

tion [10,11]. The alloy was remelted in the atomization ram with a 1.4 cm (0.55 in.) diameter, at a pressure of 207

system to a superheat temperature of 16508C (30008F) and MPa (30 k.s.i.). Excess powder was blown from the

then atomized using a close coupled discrete jet gas surface and a second 1.534 cm (0.5931.57 in.) piece of

atomization nozzle [811]. Two separate atomization foam was wrapped around the pressed section for protec-

experiments were conducted, one using argon (99.998% tion and to further bind the powder within the foam. A

purity) at a pressure of 7.58 MPa (1100 p.s.i.), the other chromel wire was attached to the side of the foam shroud

helium (99.995% purity) at a pressure of 5.52 MPa (800 and light pressure was applied to secure the entire assem-

p.s.i.) as the atomization gas [11]. The force exerted by the bly.supersonic gas on the molten metal stream caused it to Electrochemical cycling studies were performed using

break up into small, spherical droplets, which rapidly an open cell construction with the metal hydride electrode

solidify during freefall within an environmentally inert as the working electrode and a large Ni(OH) electrode2

spray chamber.. used as the counter electrode. A Hg /HgO electrode with a

The spherical, gas atomized metal hydride powders were luggin capillary was used as a reference electrode. A

size classified to obtain the ,25 mm diameter particles for Teflon electrode holder was employed to keep the working

the study. The size classified particles were heat treated by and counter electrodes parallel to one another and 6 cm

loading each batch into a sealed fused silica tube with an apart within an electrolyte bath consisting of a 6 M KOH

inert atmosphere present and held at 9008C for a desig- solution. Twenty cycles were performed on each of the

nated time. Two separate dwell times were investigated for samples with a charge of 75 mA/ g (C / 4 rate assuming a

this study: 15 min and 4 h. A heat treatment is required to theoretical capacity of 300 mAh/ g) for 4 h and discharged

homogenize the microstructure due to tin segregation at the same current to20.5 V vs. the Hg/HgO electrode.

during solidification [810,13].

A cast and crushed LaNi Sn powder sample was4. 75 0. 25

generated by DC arc melt chill casting the same con- 3. Results

stituents into a button. The button was heat treated at

9508C for 72 h. Mortar and pestle grinding was used to Directly comparing the properties of gas atomized

obtain a coarse powder sample of,20 mesh (850 mm powder to cast and crushed powders of the same com-

sieve openings). Then, one gas-phase hydrogen absorption/ position is important to determine the viability of using gas

desorption cycle was used to further pulverize the powder, atomization to produce hydrogen storage materials used in

and a ,500 mesh (25 mm sieve opening) sample was NiMH batteries. Powders produced by these two methods

obtained. Also, a ,35 mesh (,500 mm) sample of cast are shown in Fig. 1. There are several advantages spheri-

MmNi Co Mn Al alloy was obtained from San- cal, gas atomized powders possess over the irregularly3.5 5 0.75 0.4 0.3

toku America, Inc. and was gas-phase hydrogen cycled one shaped powders resulting from the casting and crushing

time to obtain a ,500 mesh (,25 mm) powder sample. process. First, the microstructural segregation upon solidi-fication is uniform within each atomized particle and is

2.2. Hydrogen gas cycling much finer, reducing the heat-treatment time required to

homogenize the composition [9,13]. Also, the grinding and

Gas phase hydrogen cycling was performed to investi- hydrogen attriting steps needed to produce powder from a

gate the fracture stability of the powders. Small powder cast ingot can be completely eliminated, since atomization

samples of |0.10 g were loaded into a Seiverts-type is a direct powder production process. The reduction or

apparatus [7] and hydrogen absorptiondesorption cycled a elimination of the annealing and pulverization steps saves

total of five times. The powder sample was then analyzed time and energy, thereby reducing cost.

using secondary electron imaging on a Amray 845FE A large volumetric expansion occurs, in the range of

scanning electron microscope. Three separate samples 1525vol.% [2], upon hydriding of AB materials. Stress5

were examined: argon atomized and annealed at 9008C for can develop from this expansion which can easily exceed

15 min, argon atomized and annealed at 9008C for 4 h, and the fracture stress of the brittle material, causing cracks to

helium atomized and annealed at 9008C for 15 min. The form and propagate. Particle size plays an important role in

effects of atomization gas and annealing time were tested determining the stress state at the powder surface. General-

on the three heat-treated samples. ly, larger powders will encounter greater stresses [10].

However, other factors can contribute to whether a powder

2.3. Electrochemical cycling particle will fracture: compositional homogeneity, micro-

structural feature size, and solidification morphology of the

Electrodes were fabricated by blending 75 wt.% metal particle. Fig. 2 shows images of LaNi Sn powders4. 75 0. 25

hydride particulate with 25 wt.% nickel powder (INCO after five gas-phase hydrogen absorption and desorption

SPP, Wycoff, NJ), and applied to a 1.531.5 cm (0.593 cycles with atomization gas and annealing time as the

0.59 in.) section of nickel foam (INCO SPP). The powder variables. From these images, one can determine that an

was pressed into the voids of the foam using a cylindrical increased annealing time and an atomization gas (He) with

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M.L. Anderson, I.E. Anderson / Journal of Alloys and Compounds 313 (2000) 4752 49

Fig. 1. Secondary electron images of: (a) cast, annealed, and crushed; and

(b) as gas atomized and annealed LaNi Sn powders. Note the4 .7 5 0 .2 5difference in the size scales for each image.

an enhanced thermal conductivity improved the fracture

stability of the powder. The extra time for annealing

allowed for improved compositional homogenization to be

achieved, and relief of any residual stresses resulting from

the rapid solidification. Using a gas with a higher thermal

conductivity for atomization allows for greater solidifica-

tion undercooling to be achieved, promoting the develop-

ment of finer microstructural features.

Since the change in atomization gas showed the most

improvement in fracture stability, its effect on electro-Fig. 2. Secondary electron images of gas atomized powders after fivechemical performance was also investigated. Fig. 3 showshydrogen absorption and desorption cycles. (a) Ar-atomized, 9008C for 15

the discharge capacity plots of the argon and heliummin (LaNi); (b) Ar-atomized, 9008C for 4 h (LaNi); and (c) He-atomized,

atomized powders (both annealed for 15 min at 9008C). 9008C for 15 min (MmNi).After 20 cycles, there is a noticeable improvement in

cyclic stability for the electrode constructed of helium

atomized powder. Polished cross-sections of these elec- materials are known to degrade by reactions with the

trodes after cycling are shown in Fig. 4a and b. A electrolyte [3,4,14]. As the amount of fracturing increases,

difference in the fracture behavior exists between these more surfaces are exposed to the 6 M KOH electrolyte. A

two electrodes; significantly less fracturing occurred with reaction between the electrolyte and the hydrogen absorp-

helium atomized powders. This is important because these tion material can reduce the capacity by consuming

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50 M.L. Anderson, I.E. Anderson / Journal of Alloys and Compounds 313 (2000) 4752

Fig. 3. Cyclic discharge capacity curves for four separate electrodes: Ar-atomized LaNi Sn , He-atomized LaNi Sn , cast and crushed4.75 0.25 4.75 0.25

LaNi Sn , and cast and crushed MmNi Co Mn A1 . No loss of powder during cycling was observed for these electrodes.4.75 0.25 3.55 0.75 0.4 0.3

hydrogen absorbing material to form non-hydrogen absorb- LaNi Sn alloy in Fig. 4c. Thus, less material was4. 75 0. 25

ing compounds. susceptible to corrosion. Secondly, some researchers have

Also, shown in Fig. 3 are discharge capacity plots of shown that cerium substituted alloys show improved

two samples of cast and crushed particulate composed of corrosion resistance [14,15]. Since cerium is a major

LaNi Sn and MmNi Co Mn Al . An im- component of misch-metal (Mm) [1], better surface passi-4.7 5 0.2 5 3 .55 0.75 0.4 0 .3

provement in electrochemical stability is obvious when the vation can be achieved, suggesting a thinner, more stable,

two electrodes comprised of gas atomized powders are corrosion product layer.

compared to the cast and crushed LaNi Sn par-4. 75 0. 25ticulate electrode. Also, an improvement in stability is

evident when the cast and crushed 4. Discussion

MmNi Co Mn Al alloy particulate is contrasted3.5 5 0.75 0.4 0.3

with the cast and crushed LaNi Sn (see Fig. 3). The In 1997, Bowman et al. [16] published an article4. 75 0. 25

fracture and surface corrosion behavior can explain the concluding that the gas atomized alloys had comparable

differences observed for each of the curves in Fig. 3, when hydrogen storage properties to cast and crushed particulate.

correlated with microstructural analysis of the cycled However, it was reported that the gas atomized powders

electrodes. Fig. 4c shows the cast and crushed had reduced capacities and greater degradation than the

LaNi Sn electrode after 20 electrochemical cycles. conventionally processed materials during electrochemical4. 75 0. 25

The ending particle size is smaller (45 mm), on average, cycling [16]. Nearly the opposite can be concluded from

than the final particle size in the electrodes composed of the results described in this article. Initially, the electrode

gas atomized powders (|7 mm), as shown in Fig. 4a and b. composed of cast and crushed LaNi Sn particulate4 .75 0 .25

Thus, more new surfaces have been exposed to the had higher discharge capacities, but as cycling continued,

electrolyte, thereby, reducing the capacity of the electrode the electrodes composed of gas atomized powders de-

by the formation of corrosion/ oxidation products, as graded less rapidly than the cast and crushed sample, as

previously described. shown in Fig. 3. One difference which could account for

The use of the cast and crushed the discrepancy between the present results and previous

MmNi Co Mn Al alloy also shows dramatic im- reports is annealing. The gas atomized powders examined3.5 5 0.75 0.4 0.3

provement in electrochemical stability over the similarly in the 1997 publication were not annealed prior to cycling,

processed LaNi Sn alloy. Two factors can contribute whereas a short annealing process was performed for the4 .75 0 .25

to the improvement. First, as shown in Fig. 4d, there was powders investigated in this article. During solidification,

less fracturing of the electrode particles after twenty cycles regions of increased tin concentration reside at the cell

for the MmNi Co Mn Al alloy than for the boundaries of the gas a tomized powder particles [83 .55 0.75 0.4 0 .3

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M.L. Anderson, I.E. Anderson / Journal of Alloys and Compounds 313 (2000) 4752 51

Fig. 4. Backscattered electron images of polished cross-sections for (a) an Ar-atomized LaNi Sn electrode after 20 electrochemical cycles; (b) a4 .7 5 0 .2 5

He-atomized LaNi Sn electrode after 20 electrochemical cycles; (c) a cast and crushed LaNi Sn electrode after 20 electrochemical cycles; and4.75 0.25 4.75 0.25

(d) a cast and crushed MmNi Co Mn A1 electrode after 20 electrochemical cycles.3.55 0.75 0.4 0.3

10,13]. Therefore, if an annealing step was not conducted, state. Two recent articles have investigated the use of

the stress state during hydrogen absorption and desorption similar alloys processed by gas atomization [17,18]. Both

would not be uniform. Cracks would form more readily suggest that annealing improved discharge capacity, how-

along the cell boundaries because the tin-rich regions ever, not to the levels of similar cast and crushed material.

would absorb hydrogen more quickly than the tin-poor These results should not discourage further research into

regions due to a lower plateau pressure. The influence of the use of gas atomization as a powder processing method

the annealing step was shown in Fig. 2; the powders for metal hydride materials. Better understanding is needed

exposed to a longer annealing step showed better fracture of the effects processing has on hydrogen storage prop-

stability. As the mechanical stability of the powders erties.

improves the fresh surface available for corrosion and Although the benefits of gas atomization of AB metal5

oxidation is reduced, thus permitting the capacity of the hydride materials have been mainly described for applica-

electrode to be maintained for more electrochemical tions in nickelmetal hydride batteries, many of the same

cycles. benefits may also apply to alloys used for hydrogen gas

The performance of the cast and crushed storage. The direct formation of hydrogen storage alloy

MmNi Co Mn Al alloy demonstrated the benefits powder and the reduced annealing times could benefit the3.5 5 0.75 0.4 0.3

that a highly substituted AB alloy can provide as an manufacturability of the material without degrading the5

electrode material. The constant output from the 14th cycle hydrogen absorption and desorption properties. Thus, the

and beyond (Fig. 3) suggests improved corrosion resist- atomization approach may enable the technology needed to

ance once the particles have reached a mechanically stable store and transport hydrogen in a safe and efficient manner.

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52 M.L. Anderson, I.E. Anderson / Journal of Alloys and Compounds 313 (2000) 4752

[2] T. Sakai, M. Matsuoka, C. Iwakura, in: K.A. Gschneidner, L. Eyring5. Conclusions(Eds.), Handbook on the Physics and Chemistry of Rare Earths, Vol.

21, Elsevier Science, Switzerland, 1995, p. 133.The process simplification that gas atomization can

[3] J.J.G. Willems, Philips J. Res. 39 (suppl. 1) (1984) 1.provide to the manufacturing of metal hydride powders has [4] A.H. Boonstra, G.J.M. Lippits, T.N.M. Bernards, J. Less-Commonbeen thoroughly described [810,12,13]. However, the Met. 155 (1989) 119.

[5] P.H.L. Notten, R.E.F. Einerhand, J.L.C. Daams, J. Alloys Comp.ability to improve electrochemical performance has been231 (1995) 604.questionable, until now. This articles shows that gas

[6] S.B. Biner, J. Mater. Sci. 33 ( 1998) 3935.

atomization processing dramatically improved the electro- [7] G. Sandrock, in: Y. Yurum (Ed.), Hydrogen Energy System, Kluwerchemical cycling stability of the LaNi Sn metal Academic Publishers, Netherlands, 1995, p. 135.4 .75 0 .25hydride alloy when directly compared to an electrode [8] I.E. Anderson, V.K. Pecharsky, J. Ting, C. Witham, R.C. Bowman

Jr., Materials for Electrochemical Energy Storage and Conversion IIcomposed of the same alloy that was conventionally Batteries, Capacitors, and Fuel Cells, MRS Symposium, Boston,processed. The cycling stability was improved when theMA, December 15, 1997, p. 37.

alloy was helium atomized instead of argon atomized, a[9] I.E. Anderson, J. Ting, V.K. Pecharsky, R.C. Bowman Jr., Advances

result of the finer solidification microstructure developed in Powder Metallurgy and Particulate Materials, 1997, p. 5.31.with helium atomization. Future investigations intend to [10] J. Ting, Gas atomization processing of tin and silicon modified

LaNi for nickelmetal hydride battery application, Dissertation,focus on the electrochemical performance of gas atomized 5Iowa State University, 1998.MmNi Co Mn Al , or a similar composition, to

3.5 5 0.75 0.4 0.3 [11] M.L. Anderson, Surface passivation and electrochemical behavior ofsee if comparable improvements in electrode stability can

gas atomized LaNi Sn for battery applications, Thesis, Iowa4 .7 5 0 .2 5

be achieved. State University, 2000.[12] I.E. Anderson, M.G. Osborn, T.W. Ellis, JOM 48 (3) (1996) 38.

[13] M.L. Anderson, J. Ting, I.E. Anderson, Powder Materials: Current

Research and Industrial Practices, Minerals, Metals and MaterialsAcknowledgementsSociety, AIME, November 1999, p. 49.

[14] G.D. Adzic, J.R. Johnson, S. Mukerjee, J. McBreen, J.J. Reilly,Funds for this study were provided by the Materials Electrochemical Surface Science of Hydrogen Adsorption and

Science Division of DOE/ BES under contract W-7405- Absorption, Electrochemical Society, May 1997, p. 288.[15] T. Sakai, H. Miyamura, N. Kuriyama, A. Kato andH, K. OguroEng-82 at Ames Laboratory, Iowa State University. The

kawa, H. Ishikawa, J. Electrochem. Soc. 137 (3) (1990) 795.authors would like to thank Bob Terpstra for the atomiza-[16] R.C. Bowman Jr., C. Witham, B. Fultz, B.V. Ratnakumar, T.W. Ellis,

tion processing, Fran Laabs for his SEM expertise, andI.E. Anderson, J. Alloys Comp. 253254 (1997) 613.

Viktor Balema for his help with the electrochemical [17] H.S. Lim, G.R. Zelter, D.U. Allison, R.E. Haun, J. Power Sourcestesting. 66 (1997) 101.

[18] H. Yuexiang, Z. Hong, J. Alloys Comp. 305 ( 2000) 76.

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