Nickel-positive electrodes obtained by electrochemical impregnation of aluminum-substituted Î±-nickel hydroxide are found to deliver a reversible discharge capacity of ca. 450 mAh/g. This is much higher than the capacity of Î²-nickel hydroxide electrodes 200 mAh/g: this work; 225 mAh/g: Dixit et al., J. Power Sources, 63, 167 (1996) prepared under identical conditions and pasted electrodes comprising cobalt-doped nickel hydroxide 345 mAh/g: Faure et al., J. Power Sources, 36, 497 (1991). These observations suggest that the theoretical target-capacity for high-performance nickel-positive electrodes must be revised from the currently accepted value of 289 mAh/g (1e exchange) to 491 mAh/g 1.7e exchange: Corrigan and Knight, J. Electrochem. Soc., 136, 613 (1989). Â© 1999 The Electrochemical Society. S1099-0062(98)08-044-4. All rights reserved

Dixit, M.

Ganesh Kumar, V.

Jayashree, R.S.

Munichandraiah, N.

Shukla, A.K.

Vishnu Kamath, P.

Nickel-positive electrodes obtained by electrochemical impregnation of aluminum-substituted alpha-nickel hydroxide are found to deliver a reversible discharge capacity of ca. 450 mAh/g. This is much higher than the capacity of beta-nickel hydroxide electrodes [200 mAh/g: this work; 225 mAh/g: Dixit et al., J. Power Sources, 63, 167 (1996)] prepared under identical conditions and pasted electrodes comprising cobalt-doped nickel hydroxide [345 mAh/g: Faure et al., J. Power Sources, 36, 497 (1991)]. These observations suggest that the theoretical target-capacity for high-performance nickel-positive electrodes must be revised from the currently accepted value of 289 mAh/g (le exchange) to 491 mAh/g [1.7e exchange: Corrigan and Knight, J. Electrochem. Soc., 136, 613 (1989)]. (C) 1999 The Electrochemical Society. S1099-0062(98)08-044-4. All rights reserved

Dixit, Mridula

Jayashree, RS

Kamath, P Vishnu

Shukla, AK

Kumar, V Ganesh

Munichandraiah, N

Name not available

Electrochemically impregnated aluminum-stabilized alpha-nickel hydroxide electrodes

ePrints@Bangalore University

Journal of Power Sources 63 ( 1996) 167-171 Abstract An electrochemically impregnated sintered-nickel porous electrode with a capacity of 225* IO mAh per g of active material has been developed. This capacity is comparable with any state-of-the-art nickel hydroxide electrode reported in the literature. such as the stabilized a-nickel hydroxides fhal contain aluminium, iron and other trivalent cations. A technical update on various types of nickel positive ekctmdes is given. 1. Introduction and technical update Nickel hydroxide is the positive-electrode material for a number of nickel-based alkaline secondary cells such as Nil Cd, Ni/Fe, Ni/Zn and NL’MH cells. Since nickel hydroxide is an electronic semiconductor. the positive electrodes have to include a conductive support. The latteraddsconsiderably to the weight of the electmde and, therefore. the specific capacity is lowered. Consequently, continuous efforts are being made to fabricate high-performance nickel hydroxide electrodes for a range of storage batteries. In general, these efforts encompass three crhical aspects: (i) synthesis and stabilization of novel nickel hydroxide phases [ I-41 that are capable of delivering nmre than I e- charge per nickel atom during reversible cycling and, hence, will result in an increased specific capacity, i.e. Ah per g of active material (SCAM) ; (ii) development of ultra-light conductive sup- ports [5-S] with a high surface area and a high porosity that are capable of accepting high loads of active material and will therefore lead to a high specific capacity in terms of the total electrode weight (SCEW), inclusive of the weight of the support: (iii) development of suitable methods to load tbeactivematerialontodifferentsupports [9-121 withaview to achieving maximum material utilization at high loading levels. good mechanical and morphological characteristics under prolonged cycling, and high cycle life. Ofthemany novel nickel hydroxidephases [ 13-151,those mcdelled on the layered double hydroxides that contain 20 0378.7753/%/$15.03 0 19% Elsevirr Science S.A. All righrr reserved PlfSO378-7753196)02460-3 a 25 mol% of trivalent ions such as Co [II. Fe 121 OT Al 13.41 have been the most successful. Although thin films of nickel hydroxide azc known toexchange 1.67 e- perNi atom [ 161 (corresponding to a SCAM of 482 mAh g-l), nickel hydroxide doped with cobalt [I] is reported to deliver the highest known SCAM in bulk electrodes, viz.. 345 to 375 mAb g- ’ that corresponds to 1.2 to 1.3 c- exchange per metal (Ni. Co) atom. Further improvements in the SCAM of these materials. which are now known to be isostmctural with a-nickel hydroxide [ 171, will be the subject ofa forthcoming publication by the authors. The sintered-nickel porous plaques [Ill] that arc cowen- tionally used as conductive supports are heavy and constitute nearly 60% of the weight of the positive electrode. These are being replaced by lighter supports. such as sintered-nickel fibrous plaques [ 191 that are known to reduce the electrode weight by about half. Using these lightweight supports, Osh- itani et al. [7] have achieved a record SCEW of 170 mAh g-‘ataSCAMof250mAhg-‘.Zhuetal. [20] reportan SCEW of I41 mAb g-’ at 85-904s utilization of active material (SCAM: 245-260 mAh 8-l) by electmchemical impregnation of sintered-nickel plaques that contain both nickel fibres and particles (FibrexlP supports) [21]. The fibrous plaques have porosity of up to 90% (as against the 80% porosity of porous plaques), but they have a much smaller surface areas0 that the utilization ofmaterialislower in fibrous than in conventional porous plaques. This situation is somewhat improved in FibrexlPsuppnrts. The use ofalter- native supports drastically reduces the material utilization to 38-B% (SCAM: I IO-170 mAh g-‘) [fi]. As a result, the sintered-nickel porous plaques are still the best supports as they have a much higher surface area compared with the lighter plaques and, therefore, provide better electrical con- tact and material utilization at higher loads. In fact, Oshitani et al. [22] have used sintered-nickel porous plaqoesto realist 100% material utilization (SCAM: 289 mAh g-‘) in a dip- loaded electsode. Tbere are many methods by which the active material can be bonded to the conductive support [ 231. Many loading techniques, especially dip loading, expose the support to aggressivecorrosion by thenickel nitratesolution. Asaresult, the possibility of over-estimating the specific capacity is very real 1241. Of dl these techniques, electrochemical impreg- nation (ECI) is considered to be the best. Cathodic polari- zation of the plaques creates a hydrogen gas blanket that protects the plaques from corrosion. EC1 is a one-step process that takes place by nitrate reduction [ 25 1. EC1 is the only technique that is effective in filling the cavities of the support without blacking the pore outlets. In all other loading tech- niques, pore cIosttre due to surface deposition is inevitable. A survey of the literature reveals that the sintered-nickel porous plaques loaded by the EC1 route are still the preferred nickel hydroxide positive electrodes and are being employed extensively in commercial batteries. Since the technical details of EC’1 techniques are protected, a vast majority of researchers in this area are deprived of valuable data. Ho [25] and Ho and Jome [2S] have studied the EC1 route in detail, both experimentally and theoretically, to optimize the various experimental parameters, but they do not report any capacity measurements. Law and Sapjeta [26] haIre opti- mized the EC1 process for Fibrex/P supports hut, again, do not report either the SCAM or SCEW vahtes for the &X&&S. Two factors affect the performance of nickc. Hydroxide as an electrode material. First, the conductivity varies with the state-of-charge and the discharged phase has paor conductiv- ity; during the discharge cycle, the material close to the conductive support discharges first and forms an insulating layer that prevents discharge of the active material away from the support. Second, the charged and discharged phases have different unit cell volumes, especially on overcharge; the electrode swells due to y-NiO(OH) formation and this causes mechanical strain, shedding of the active material and morphological changes that adversely affect electrode performance. Additives are invariably added to nickel hydroxide to improve its conductivity and prevent swelling. These addi- tives icclude Zn(OH)*, Co(OH), and Co0 at levels in the S-10 mol% range. In this composition range. such additives do not affect the phase or the structural chemistry of nickel hydroxide. Inclusion of additives further complicates the EC1 procedure. Zimmerman [ 271 has studied the effect of differ- entECIpammetersonthecobaltcontentoftheactivematerial deposited from a mixed metal nitrate bath. Pickett and Malay [28] have used a 4C-SO vol.% ethanol- water mixture as the solvent for a mixed metal nitrate bath. Later. Earl and Noble [ 291 studied the ageing of the bath used for the EC1 procedure. Neither of these (or other) stud- ies have investigated the role of ethanol on the stmctural and spectral properties of the active material or its role in enhanc- ing electrode performattce. Electrodes made up of @nickel hydroxide exhibit stabi- lized capacities (SCAM) of only I88 mAh g-’ [30] in the absence of any additives. Except for Oshitani et al. 1221 who report lM)% material utilization (SCAM 289 mAh g-l), all other studies report a lower material utilization in the 70- 85% range (SCAM 202-245 mAh g- ‘) in the presence of a variety of additives. Of late. nickel foam substrates pasted with nickel hydroxide have shown promise. Watanabe et al.[3l I report a SCAM of ZOO* 10 mAh g-‘, while Hoi et al. [32] report 210-250 mAh g-l. The values correspond to 70and72-86%mmaterialutilization,respectively.Thespecific capacity values realised by diffcrcnt authors are summarized in Table I. It is noteworthy that in view of the experimental results of Delmas and co-workers [ I .2], Corrigan and Knight [ 161 and themodel ofIsmailet al. [ 331. the potential theoreticalcapac- ity of nickel hydroxide has to be revaluated on the basis of a 1.67 e- change per Ni atom during reversible cycling, which amotmts to aSCAM of4gOmAh g-‘. Thus. a new target has been set for high-performance electrodes. Nevertheless,since this value has not been widely accepted, the reported per- centage utilization has been converted into SCAM values. in this study, by using the more widely accepted theoretical capacity of 289 mAh g-’ for the nickel hydroxide electrode. Although sintered-nickel porous plaqttesloadedby the EC1 procedure are widely used. an examination ofTable I reveals that there are no adequate data in the literature on the SCAM values of such electrodes. This communication reports SCAM values of nickel hydroxide electrodes that are obtained by EC1 of sintered-nickel porous plaques. 2. Experimental Sintered-nickel porous plaques (porosity 80%) were rinsed in double-distilled water and degreased in 6 M KOH at 30 mA cm-‘. The plaques were washed and dried to con- stant weight at 90 “C. The degreased plaques were then immersed in nickel nitrate solution (0.2 M mixed with abso- lute alcohol in a 70:30 volume ratio) and allowed to stand for 2 to 3 min. Nickel hydroxide was then deposited galva- nostatically into the plaque under constant stirring by cathod- ically polarizing the plaque wtthrespecttoacylindricalnickel mesh anode. The pH of the bath solution was between S and 6 and the deposition current density was varied between 8 and 30 mA per cm2 geometric area of the plaque. The time of deposition was 10 to 30 min. Subsequent to deposition, the plaque was washed in double-disttlled water and dried to constant weight at 90°C toestimate the weight gain. At times, 2 3 4 5 6 7 8 9 Ill II I* 13 14 15 16 Ni-hbrex Propylene fibre Ni-Rbre Plexiglass Ni-foam Ni-porous plaque Ni-foam Ni-grid Ni- fom Ni-gnwc Ni-gWLC NI-Abrr Ni-fibre Ni-porous plaque Ni-porous pique Ni-foam SCAM tnAbg-‘1 m-168’ 17om 210 220 232 240 221-241 237-241 2.50 193 2.50-255 222-289 = >289’ 240-300 L 345’ SCEW mAb8-‘) 115 141 120 Rd. [81 161 [2oJ 1301 t3tJ l24bl [21 [4J 1321 t3bJ I3aJ Dal 171 [ I21 I211 IZW III the weight gain was marginal. and under such circumstances the plaque was resoaked and deposition was repeated. During the second deposition, a rapid weight gain was observed. In the event of surf~e deposition the deposit was brushed off with a stiff polypropylene brush and the plaque was re- weighed. Plaques loaded with 0.5 to 0.75 g of nickel bydmx- ide were used for capacity studies. Control experiments of nickel hydroxide deposited from prrre aqueous baths were carried out for comparison. The deposition setup was similar to that employed previously 191. The final deposition con- ditions were: bath temperature 70 “C PH 5.8-6 bath medium 30% ethanol:70% 0.2 M impregnation current Ni(NO,), tqueotts solution nature of counter electrode ~o%~%ctrcde, Ni mesh plaque drying temperature 9OT The impregnated plaques were then soaked overnight in 6 M KOH for wetting and galvanostatically charged to IZO- 2tY.Y% overcharge of the expected theoretical absolute capac- ity of the plaque. The potential of the plaque was measured against a Hg/HgO(MMO) reference electrode and two sin- tered-nickel plates (geometric area four times the plaque) were used as the counter electrodes. Galvanostaticdiscbarge was carried out at C/4 to Crates to a cutoff potential of 0.25 V. A Spectrum Engineers (India) d.c. power supply coupled with a MECO (India) milliammeter was employed for the charge/discharge studies. The potential of the working elec- trode was measured using a high-impedance. millivoltmeter. The tneasucements were restricted only to SCAM values. Each electrode was cycled through I5 to 20 cycles to deter- mine the stabilized capacity. At least two electrodes cotre- spending to each deposition condition were investigated for establishing reproducibility of the electrode behaviotu. All the experiments were conducted at 30 “C. The active material brushed off from overdeposited plaques was characterized by powder X-ray diffmction (IEOL JDXSP diffractometer) and infrared spectroscopy (Nicolet Impact 400D FT-IR spectrometer, KBr pellets, 4 cm-’ resolution). 3. Results The powder XRD pattern for nickel hydmxide deposited from an ethanolic bath is shown in Fig. 1 and is compared with the sample obtained From an aqueous bath. il is at once evident that the sample obtained from an aqueous bath has characteristicscloser to~hatof/%Ni(OH)~, whilethesample I I Da-~~:a=3.12~andc=22.11 A. ‘/3-phase.a=3.,2 Aandc=4.66 A. obtained from an ethanolic bath is much more disordered with poor intensities and shows some characteristics remi- niscent of a-nickel hydroxide. The prominent d-spacings together with their assignments are given in Table 2. Similar observations can also be made from the IR spectra (Fig. 2). The latter show signature vibrations that correspond to both the a- and &phases in the sample obtained from an ethanolic bath, while the sample obtained from aqueous bath is similar to &nickel hydroxide. The variation in specificcapacity as a function ofcycle life for the electrodes prepared under different conditions is pre- sented in Fig. 3. It is at once evident that plaques impregnated from pore aqtteous baths yield lower capacity ( 195 mAh g-- ’ SCAM) than plaques impregnated from ethanolic baths which show - 1.5% improvements to give a SCAM of 225 mAh g- ‘. A typical dischargecurve for an electrode impreg- nated from an ethanolic bath is shown in Fig. 4. In general, plaques loaded at low current densities (8 mA cm-‘) per- form better than those loaded at high current densities (up to 30 mA cm-‘). The addition of 10 owl% Co to the bath does not improve the electrode performance. All the electrodes prepared by EC1 are found to exhihit their stabilized capaci- ties within 5 to 8 formation cycles. Fig. 3. Cycle-life data of nickel hydroxide elecvodes obtained from: (a) aqueous bath: (b) ethanolic bath with 10 mol% Co. snd fc) ethanolic bath. 0 40 60 120 160 Time (min) Typud discharge (atier IS cycles) data for a nickel hydroxide 4. Discussion Nickel hydroxide exists in two polymorphic forms, namely 01 and fl 1341, of which the former is known to exhibit a higher capacity. The EC1 procedure is known to yield the a- form, but the a-form is unstable and rapidly ages to the p form by a dissolution/reprecipitation mechanism 1351. The transformation is complete when impregnation is performed in a pore aqueous bath during work-up of the plaque. The p form, obtained by the ageing of the a-form has been char- acterized as & by Faure et al. [ 361 where ‘bc’ stands for ‘badly crystallized’. The pattern obtained in Fig. 1 cone- sponds to the & form. The presence of ethanol appears to retard the rr-p transformation kinetics. Organic additives in the precipitation bath have been shown to stabilize the (Y- form [ 131. While theexactmechanism isnotclear,it islikely that the additives adsorb on the freshly formed a-particles and retard their dissolution/reprecipiiation kinetics. In the present instance, the product is a mixture of a- and &forms and is poorly ordered. Poorly-ordered phases exhibit higher electrochemical activity by virtue of a large number of defects. The requirement of a high bath temperature and the ethanol in the bath have tbe secondary effect of increased wetting of the plaque which enables intimate penetration of the electro- lyte into the pores. Both these effects promote electrodeper- formance. The electrodes impregnated at room temperature from pure aqueous baths perform very poorly. Lower depo- sition currents also improve plaque performance by ensuring more uniform filling of the pores. The inclusion of cobalt does not improve the electrode performance. It is noteworthy to observe that the EC1 nickel hydroxide electrode reported here performs much better than the nickel hydroxide pasted electrode [ 301 and as good as the foam electrodes reported by Watanabe et al. [ 31 I. The perform- ance iscomparable with the performanceofpastedelectmdes made out of ‘improved’ stabilized-a double hydroxide mate- rials [ 2,3]. Only the electrodes of Oshitani et al. [221 and Faure et al. [ I] are superior to those reported hzre. PVK thanks the Department of Science and Technology, Government of India (GOI) for financial support. MD thanks the Council of Scientific and Industrial Research, GO1 for the award of a Senior Research Fellowship. I31 (a) D.H. Buss,J. *nuer,W. DiemhackmdD.Gkmser.J. Ckcnz.Soc. Chem. Cmnlnnu”.. (1985, 81; (b, 0. Gkmser. J. Baxr. D. BUSS. H.- 1. Harms and H. Low. Proc. 16rk Irrr. Power Sources Symp.. Eounwnourh. UK. 1998. Ch. I,. [4] P.V. Kwnath. M. Dixit. L. Indira. A.K. Shukla. V.G. Kumar and N. Munehandraiah. 1. Elecrrochem. Sot.. Ml C 1994) 2956. [6] W. Rekrc. I. Leczykirwicz and E. Frwkowiak. J. Power S~mrurce.~. 50(1994) 21. ] IO] D.F. Picket, and J.T. M&y. 1. Elecrrockem. SW. 125,1978) 1026. [I I I M.J. Avena, M.“. “azquez. R.E. Carbonio, C.P. de Pauli and V.A. Macagno. J. Appl. El*crwckem.. 24 t 1994) 256. [I21 W:H.Zbu.1:1. Ke. H.-M. YumdD:J. Zhang.l. PnwerSm,,ces. 56 (1995) 75. [t3, P.“. Karrmtb. 1. Irmtiil. M.F. Abmed. G.N. Subbvlna and J. Oopol*irsbnan. I Ma,e,. C,wm.. 3 t 1993) ,285. [ 14, J J. Bracannier. C. Delms., C. Fouasrier. M. FiBluz, B. Braudouin and P. Hagenmulter. Rrv. Ckbn. Min.. 2, , ,984) 496. 12.51 KC. fiu.,. Elrctruehern Sue. 134 (1987) 52C;K.C. “omdJ.Jame. I E,ec,mcbam. Six.. 137, ,990, 14% ,33,19%5, 1394. [26] H.“. Law and,. Sr,&r,ee.,. E,cc,rockcm. Sec.. 135, ,988) 2418: 136 ! (1989) ,603. ]27] A.H. Zimmemun,J. PowerSources. I2 C 19%) 233. [28] D.F. Pi&n and J.T. M&y. I Elrclnxkem. Sm.. 125 ( 1978) 1026. [29] M.W. Earl andT.F. Nobk. 1. PowerSourcer. I.2 (1984) 277. 1301 A. Del&ye-Vidal and M. Figlaw_ 1. A&. E~a?rnckem.. 17 C 1987) 589. I31 1 K. Wxzuxbe. T. Kibwkr and N. Kumagai. 1. Appf. Elecrroden~ 25 (1995) 219. (321 L. Hui. D. Yunehang, Y. Joinliang md W. Zeyuo. d Power Sources, 57(1995) 137. [33] 1. Isnwil. M.F. Abmed and P.V. Knmmh,l PowerSources. 36 t 19%) ,507. ,341 P. DIiva 1. Leonardi, J.F. Law”,. C. Del-, JJ. Bmconrdcr. 

Electrochemically Impregnated Aluminum-Stabilized Î±-Nickel Hydroxide Electrodes

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Electrochemically Impregnated Aluminum-Stabilized Î±-Nickel Hydroxide Electrodes

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