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Available online at www.sciencedirect.com

Electrochimica Acta 53 (2007) 1979–1986

Effects of the coating characteristics of Y(OH)3 on the electrochemicalperformance of spherical Ni(OH)2 at elevated temperature

Jing Fan, Yifu Yang ∗, Yanbo Yang, Huixia ShaoCollege of Chemistry and Molecular Science, Wuhan University, Wuhan 430072, PR China

Received 24 July 2007; received in revised form 23 August 2007; accepted 25 August 2007Available online 1 September 2007

bstract

The effects of the coating characteristics of Y(OH)3 on the high-temperature performance of spherical Ni(OH)2 were studied. The coatingas performed by chemical surface precipitation under different conditions, and the characterization of Y(OH)3-coating layer was evaluated

ccordingly. The electrochemical properties of three as-prepared samples with different coating surface were characterized and compared atifferent temperature. The results reveal that the sample with dense and porous surface retains the excellent performance at elevated temperatures,

nd the necessary coating amount is only 0.55 at.%. The mechanism analysis suggests that the chemical nature and morphology of the coatingayer plays an important role in oxygen evolution reaction (OER). The coating surface with dense and porous morphology, larger relative surfaceontent and higher utilization ratio of yttrium is more effective on controlling OER and improving the high-temperature performance of Ni(OH)2

lectrode for nickel–metal hydride (Ni/MH) batteries.2007 Elsevier Ltd. All rights reserved.

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eywords: Nickel hydroxide; Yttrium hydroxide; High-temperature performan

. Introduction

Nickel–metal hydride (Ni/MH) batteries are widely usedowadays due to their excellent performance and low envi-onmental load. Although Ni/MH batteries are commerciallyvailable, further research is still required to improve the high-emperature performance of them considering their use inlectric vehicles (EV) and hybrid electric vehicles (HEV). Theigh-temperature performance of Ni/MH batteries is directlyelated to the behavior of the nickel hydroxide electrode mate-ials [1].

It is well known that the oxidation reaction of Ni(OH)2 occursn the earlier period of the charging process.

i(OH)2 + OH− → NiOOH + H2O + e−

owever in the subsequent period, oxygen evolution reaction

OER) on Ni(OH)2/NiOOH electrode gradually becomes theominant.

OH− → O2 + 2H2O + 4e−

∗ Corresponding author. Fax: +86 27 68754067.E-mail address: yang-y-f1@vip.sina.com (Y. Yang).

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013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2007.08.055

xygen evolution reaction; Nickel–metal hydride battery

ecause the overpotential of oxygen evolution oni(OH)2/NiOOH electrode drops rapidly at elevated tem-erature, the conflict between the charging of Ni(OH)2 andER becomes more serious [2,3]. Therefore, the charge

fficiency of the electrode declines significantly, leading tooor performances of Ni/MH batteries at higher temperature.

For that reason various methods were employed to increasexygen evolution overpotential and to improve the charge effi-iency of Ni(OH)2/NiOOH electrode [1,4–17]. These includedhe addition of zinc [4], cadmium [5], cobalt [6–10], calcium11], and lanthanide or yttrium elements [1,12–17] into theecipe of Ni(OH)2 electrodes. On the basis of these studies, rarearth hydroxides formed on the surface oxide layer of nickelydroxide active material were found to be particularly attrac-ive for improving the charge acceptability of Ni(OH)2 electrodet elevated temperature. However, previous researchers focusedheir major attentions to the optimum adding amounts of var-ous rare earth elements, but paid much less attentions to theffects of surface distribution of these elements on the high-

emperature electrochemical performance of Ni(OH)2 electrode.n general, OER occurs on the surface oxide layer of Ni(OH)2ctive material [14,18,19]. Therefore, it is necessary to obtainetailed information about the chemical nature and morphology

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f the surface oxide layer of Ni(OH)2 with different distribu-ion of adding elements, and to compare their effects on thelectrochemical properties of Ni(OH)2 electrode.

In this study, the spherical Ni(OH)2 were coated with Y(OH)3y chemical surface precipitation under different conditions. Theffects of Y(OH)3 coating with diverse surface distribution butimilar chemical compositions were compared by three differentoated samples. And then the function of Y(OH)3-coating layeras interpreted reasonably in detail.

. Experimental

.1. Preparation and characterization of sphericali(OH)2 coated with Y(OH)3

The spherical Ni(OH)2 used in this investigation was com-ercial obtainable �-Ni(OH)2 product. The coating of Y(OH)3

n spherical Ni(OH)2 particles was performed in the followingay: a certain amount of Ni(OH)2 particles were added into a

ontainer with distilled water under agitation to form suspen-ion solution. Subsequently, 20 mmol L−1 YCl3 solution wasropped stepwise into the suspension solution under magnetictirring at 50 ◦C, during the process the pH value of the reactionolution was controlled between 7.5 and 8.5 using an ammoniaqueous solution. Finally, the product was washed with ultrapureater, filtered and dried at 65 ◦C. Similar procedures were car-

ied out for obtaining optimum coating content of yttrium, excepthat in these procedures the adding amount of YCl3 solution wereifferent.

For comparison reason, sample A was prepared by addingmmol L−1 YCl3 solution and maintaining the reaction solu-

ion pH between 6.5 and 7.5, sample B was prepared by adding0 mmol L−1 YCl3 solution and maintaining pH between 7.5nd 8.5, sample C was prepared by adding 40 mmol L−1 YCl3olution and maintaining pH between 8.5 and 9.5, while in thesexperiments other reaction conditions were kept the same.

Crystal phase structure of the samples was analyzed by pow-er X-ray diffraction (XRD-6000, Shimadzu, Japan) with Cu� radiation. Surface images were characterized with a scan-ing electron microscope (SEM, FEI Quanta 200, Holland). Theurface condition of Ni(OH)2 coated with Y(OH)3 was tested byn X-ray photoelectron spectrum (XPS, XSAM800, KRATOS).quantitative measurement of the ratio of yttrium to nickel was

btained by dissolving the samples in 1.2 mol L−1 HCl and bynalyzing the solutions with Inductively Coupled Plasma (ICP)pectrometry (Intrepid XSP Rasial, Thermo, USA).

.2. Electrode preparation and electrochemical property

To evaluate the electrochemical properties of the coated andncoated Ni(OH)2, the pasted nickel hydroxide electrodes wererepared as follows: 88 wt.% Ni(OH)2 sample, 7 wt.% CoO andwt.% Ni powder were thoroughly mixed with 2 wt.% PTFE

olution to form a slurry. Then the slurry was pasted into nickeloam to form a 2 cm × 2 cm or 0.8 cm × 0.8 cm electrode. Afterrying at 65 ◦C for 1 h, the pasted Ni(OH)2 electrode was pressedt a pressure of 30 MPa for 1 min.

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The electrochemical performance test of the pasted electrodesas carried out with a classical three-electrode cell and with an

lectrochemistry workstation (CHI660B, Chenhua Instrumenttd., Shanghai). The cell was made in such a way that the elec-

rolyte temperature can be controlled and adjusted. The pastedickel electrode (0.8 cm × 0.8 cm) was used as working elec-rode, the counter electrode was a Pt foil, and the referencelectrode was a home made Hg/HgO immersed in 6 mol L−1

OH electrolyte prepared with KOH (G.R) and ultrapure waterresistivity ≥18 M�, water purification system). The referencelectrode was connected to the cell by a luggin capillary. Steadytate polarization curves for OER were measured at a scanningate of 0.1 mV/s. The test temperatures of 25 ◦C and 60 ◦C wereelected for the comparison test purpose. A 6 mol L−1 KOHolution was applied as the electrolyte. The potentials shown inhese measurements were calibrated by correcting the potentialith respect to Hg/HgO for the ohmic drop between the top of

he luggin capillary and the working electrode.Charge/discharge studies were conducted with a Battery Test-

ng Equipment (Land Test Equipment, CT2001A) in a sealedest cell, including a pasted nickel electrode (2 cm × 2 cm) ashe cathode, a commercial hydrogen-storage alloy electrode ashe anode and a 6 mol L−1 KOH solution as the electrolyte, thenode and cathode were separated by a commercial availableeparator. Before test, the cell was charged and discharged at.2C rate for several cycles for activation. The cell capacity waseasured at 1C rate with about 10% overcharge and a cut-off

oltage of 1.0 V for discharge at 25 ◦C and 60 ◦C. In the sub-equent charge–discharge cycles, the cell was charged at 1C,C and 5C rate to 80% state of charge (SOC). After 30 minest the cell was discharged at 1C rate to 1.0 V. The test tem-erature ranged from 25 ◦C to 60 ◦C. For the same sample, theharge/discharge tests were repeated with at least three cells andhe same experiment process, and then the charge efficiency ofickel electrode was calculated through averaging these exper-mental results. In this paper, the term charge efficiency standsor the proportion of the output electric charge counted by 1Cate to 1.0 V to the input of a cell at a given temperature.

. Results and discussion

.1. Characterization of spherical Ni(OH)2 coated with(OH)3

Fig. 1(A) and (B) shows the X-ray diffraction patterns ofhe regular spherical Ni(OH)2 and 0.55 at.% Y(OH)3 coatedi(OH)2 (sample B). The results indicate that there is no dif-

erence in the diffraction patterns between uncoated and coatedi(OH)2, probably it is because the coating amount of Y(OH)3

s too small to be detected by XRD analysis. For comparisoneason, the X-ray diffraction pattern of the Y(OH)3 precipitaterepared under the same condition of the preparation process ofample B with the absence of Ni(OH)2 was also detected and the

ata is given in Fig. 1(C). It is found that the yttrium hydroxides amorphous.

The SEM images of the samples are illustrated in Fig. 2.t can be observed that the uncoated Ni(OH)2 particles (sam-

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ig. 1. XRD patterns of different samples. Spectrum (A), the regular spheri-al Ni(OH)2; spectrum (B), 0.55 at.% Y(OH)3 coated Ni(OH)2; spectrum (C),(OH)3.

le A) show a smooth and porous surface which is made up oftacks of needle-like and flake-like Ni(OH)2 tiny crystals, andhe morphology of coated Ni(OH)2 particles depend strongly onhe experimental conditions of the surface coating. The condi-ion of relatively low-solution pH value (6.5–7.5) and yttriumoncentration (5 mmol L−1) yielded a continuous coating withmooth and dense surface (sample A), the condition of relativelyigh-solution pH value (8.5–9.5) and yttrium concentration40 mmol L−1) resulted in a flake-like coating with rough andoose surface (sample C), whereas moderate solution pH value7.5–8.5) and yttrium concentration (20 mmol L−1) leaded to aarticle-like coating with coarse and dense surface (sample B).

The XPS patterns of Ni(OH)2 coated with 0.55 mol%(OH)3 (sample B) are given in Fig. 3. It can be seen that the

haracteristic peak of Ni2p appears at 856 eV (Binding energy),nd the characteristic peak of Y3d appears at 158.4 eV. The sur-ace and the overall molar ratios of element Y to Ni for samples, B and C were determined by XPS and ICP, respectively, and

he results are listed in Table 1. ICP analyses show that there

s no obvious difference in the overall additive amounts amonghe three as-prepared samples; while XPS results exhibit that the

olar ratio of element Y to Ni on the surface of samples A, Bnd C is 2.19, 7.77 and 3.72, respectively.

able 1elative element content for different samples

ample Molar ratio on the surfacea Molar ratio in totalb

Y:Ni Y:Ni (%)

2.19 0.5487.77 0.5553.72 0.531

a Molar ratio of Y to Ni as determined by XPS.b Molar ratio (%) of Y to Ni as determined by ICP.

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Therefore, it is no doubt that the coating element is enrichedn the surface layer of the particles. Moreover, depending onhe preparative conditions the surface morphology of Ni(OH)2articles and the element relative content on the particle surfaceere changed greatly.

.2. Effects of surface coating of Y(OH)3 on thelectrochemical performance of spherical Ni(OH)2

To evaluate the effect of the coating amount of yttriumn the charge acceptability of Ni(OH)2 electrodes, theharge/discharge performance of the electrodes with differentoating amount of yttrium at 25 ◦C and 60 ◦C were measuredt 1C rate. The results are shown in Fig. 4. It is found thathe optimum amount of yttrium addition is near 0.55 at.% tobtain higher charge acceptability at 60 ◦C, which is smallerhan that of yttrium doping (1 at.%) [1], Ca3(PO)4 coating2 at.%) [11], Yb(OH)3 coating (2 at.%) [12] and Lu(OH)3 coat-ng (0.8 at.%) [13]. The electrochemical properties of Ni(OH)2lectrode coated with 0.55 at.% yttrium were investigated inetail hereafter.

The charge/discharge curves of regular Ni(OH)2 and.55 at.% Y(OH)3 coated Ni(OH)2 (sample B) at 1C rate andwo different temperatures (25 ◦C and 60 ◦C) are shown inigs. 5 and 6, respectively. It can be seen that the charge andischarge curves of uncoated Ni(OH)2 and sample B are almostdentical at 25 ◦C. However, the discharge capacity of sample

is much higher than that of uncoated Ni(OH)2 at 60 ◦C, theatio of the discharge capacity of sample B at 60 ◦C over thatt 25 ◦C can reach 88.1%. In contrast, the ratio for uncoatedi(OH)2 is only 45.4%. Additionally, for sample B, a rise in

harging potential at the end of charge indicates that the charg-ng reaction of Ni(II) to Ni(III) and OER proceed in series way.ncoated Ni(OH)2, by contrast, exhibits no such rise of the

harging potential at the end of charge, which indicates that theain and side reactions occur in parallel way.Fig. 7 shows the stationary polarization curves for OER on

wo kinds of electrodes (regular Ni(OH)2 and sample B). Obvi-usly, the electrocatalytic activity of Ni(OH)2 for OER decreasesfter Y(OH)3 coating especially at elevated temperature. Forxample, at potential of 0.6 V versus Hg/HgO electrode, thexygen evolution current on uncoated Ni(OH)2 electrode is43.1 mA at 60 ◦C, however it is only about 96.1 mA on Ni(OH)2lectrode coated with 0.55 at.% Y(OH)3.

A relation of the logarithm of the anodic current with thepplied potential (log i–E) at 25 ◦C based on the data shownn Fig. 7 is plotted in Fig. 8. The results manifest that theverpotential of oxygen evolution increased in the presencef Y(OH)3 coating. The Tafel slope is about 66.6 mV/dec inhe presence of Y(OH)3 coating, but it is only 57.4 mV/dec inbsence of Y(OH)3 coating. The value of Tafel slope indicateshe increase rate of overpotential as the current density increases.-coated Ni(OH)2 electrode with high-Tafel slope means that

he increases of oxygen evolution overpotential is higher on ithan that before coating.

In conclusion, Y(OH)3 coating is an effective way to improvehe charge acceptability of Ni(OH)2 at elevated temperature, and

1982 J. Fan et al. / Electrochimica Acta 53 (2007) 1979–1986

Fig. 2. SEM images of different samples. (A) and (B) for regular spherical Ni(OH)2; (C) and (D) for sample A; (E) and (F) for sample B; (G) and (H) for sample C.

J. Fan et al. / Electrochimica Acta 53 (2007) 1979–1986 1983

Fig. 3. XPS patterns of spherical Ni(OH)2 coated with 0.55 at.% Y(OH)3.

Fig. 4. Discharge capacity of spherical Ni(OH)2 coated with various ratios ofY(OH)3 at 25 ◦C and 60 ◦C at 1C rate.

Fig. 5. Charge/discharge curves of regular Ni(OH)2 and 0.55 at.% Y(OH)3

coated Ni(OH)2 at 1C rate and 25 ◦C.

Fig. 6. Charge/discharge curves of regular Ni(OH)2 and 0.55 at.% Y(OH)3

coated Ni(OH)2 at 1C rate and 60 ◦C.

Fig. 7. Polarization curves of OER on the electrode containing regular Ni(OH)2

and 0.55 at.% Y(OH)3 coated Ni(OH)2 at a scanning rate of 0.1 mV/s at 25 ◦Cand 60 ◦C.

1984 J. Fan et al. / Electrochimica Acta 53 (2007) 1979–1986

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According to the results presented above, the high-temperature performances are different for samples A, B and

ig. 8. Tafel plots of OER on the electrode containing regular Ni(OH)2 and.55 at.% Y(OH)3 coated Ni(OH)2 at a scanning rate of 0.1 mV/s at 25 ◦C.

he necessary coating amount is only 0.55 at.%. This result isainly attributed to the increase of oxygen evolution overpo-

ential.

.3. Effects of surface distribution of Y(OH)3 on thelectrochemical performance of spherical Ni(OH)2 at highemperature

In general, altering the surface condition of sphericali(OH)2 will bring on the corresponding change of the per-

ormance of Ni/MH cells, such as specific discharge capacity,harge acceptance, etc. [10–13]. However, previous researchersocused their major attentions to the adding amounts of vari-us additives, but paid much less attentions to the distributionf these elements in surface oxide layer of spherical Ni(OH)2.o evaluate the effects of surface distribution of Y(OH)3 on

he high-temperature electrochemical performance of sphericali(OH)2, three different samples A, B and C were preparedy varying the formation condition of the coating layer in ouraboratory. The specific discharge capacities of the three as-repared electrodes are compared in Table 2. At 25 ◦C, theres no obvious difference in the discharge capacities betweenamples B and C, whereas sample A presents a bit lower dis-harge capacity. At 60 ◦C, the discharge capacity of sample

still retains 241.34 mAh g−1, corresponding to 88.1% of itsapacity at 25 ◦C, while samples A and C only show a retentionf 77.6% and 80.6% of their capacities at 25 ◦C, respectively.

In practical application, the charge efficiency of the cell is aritical standard to estimate the high-temperature performancef Ni/MH cells. In our studies, three types of sealed test cellsere manufactured with samples A, B and C as the positive

able 2ischarge capacity of samples A, B and C at 25 ◦C and 60 ◦C at 1C rate

emperature (◦C) Capacity (mAh g−1)

Sample A Sample B Sample C

5 261.55 273.86 275.460 203.04 241.34 222.15

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ig. 9. Average charge efficiency of the test cells using samples A, B and C asctive materials of positive electrode at 1C rate and at different temperatures.

lectrode active material, respectively, and at least three cellsere tested for each sample. The relation of the average charge

fficiency of the test cells for each samples with working temper-ture are shown in Fig. 9. Although the average charge efficiencyf all samples decreases as the temperature is elevated in general,he efficiency of the cell containing sample B decreases much

ore slowly than that of the other cells. It still retains 80.2%t 60 ◦C, while the efficiencies for samples A and C are only5.5% and 74.4%, respectively.

Fig. 10 shows the variation of the average charge efficiencyf the test cells under various charge rates. It is evident thathe charge efficiency increases with increasing charging rate.he explanation for this result is that higher charge current canause greater polarization of Ni(OH)2 electrode and elevate theverpotential of OER, so less electric charge is consumed onER. From Fig. 10, it is also found that the cell containing sam-le B shows higher charge efficiency than the cells containing

ig. 10. Average charge efficiency of the test cells using samples A, B andas active materials of positive electrode under different charge rate and 1C

ischarge rate at 60 ◦C.

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, although their overall coating amounts of Y(OH)3 are nearlyhe same. The sequence of their high-temperature capacity andharge efficiency performances from the best to the worst isample B > sample C > sample A. The interesting issue is whyhe effects of surface coating of Y(OH)3 are diverse for differentamples. This may be explained as follows.

It is widely accepted that the performance of Ni/MH cellseteriorates at high temperature due to the low overpotential ofxygen evolution on the positive electrodes [5,10–14]. Generallypeaking, OER occurs on the surface of Ni(OH)2 active material14,18,19]. Thus the chemical nature and morphology of theurface oxide layer plays an important role in OER. Oshitani etl. [14] assumed that heavy lanthanide oxide might exist in ahin film as hydrous hydroxides on the surface oxide layer ofhe nickel active material. The thin film may act as a barrieror electron transfer in the process of oxygen evolution [14,20]ecause of its poor conductivity. However, the chemical naturend morphology of this film is unknown.

The results of SEM and XPS shown above clearly proves thathe distribution of Y(OH)3 on the spherical Ni(OH)2 surfaceayer is different for these samples. For sample A, a continuousnd dense film embeds into the surface oxide layer of the spheri-al Ni(OH)2. It can be seen that Y(OH)3 precipitates are partiallyormed inside the pores between tiny crystals of Ni(OH)2, thusample A shows the lowest relative content of yttrium on theurface. For sample B, a dense but porous film is formed on theurface of tiny crystals of Ni(OH)2. It is also found that the films composed of a great deal of irregular grains. So sample B hascoarse and porous surface. For sample C, a discontinuous and

oose film is formed on the surface of tiny crystals of Ni(OH)2.he film is uneven and made up of stacks of irregular grains andakes, and meanwhile some parts of the surface of the sphericali(OH)2 are still uncoated. So the molar ratio of element Y toi on the surface of sample C is less than that of sample B.To compare the inhibiting effects of these coating films on

ER, the steady state polarization curves for OER on the elec-rodes containing samples A, B and C were measured at 0.1 mV/snd 60 ◦C, respectively, the results are shown in Fig. 11. Inrinciple, for two electrodes with the same potential value, if

ta 53 (2007) 1979–1986 1985

ne of them has a lower current for OER then it means thathis electrode has better inhibiting effect for OER, or in otherords, if the current for OER is kept the same for these two

lectrodes, then the one with higher overpotential means it hasetter inhibiting effect for OER. From Fig. 11 one can see that at.6 V versus Hg/HgO electrode, the current of oxygen evolutionn the electrodes containing samples A, B and C are 159.6 mA,6.1 mA and 127.5 mA, respectively. Furthermore, to evaluatehe inhibiting effects of the surface coating of Y(OH)3 on OER,he potential required to produce anodic current of 80 mA is con-idered as the oxygen evolution potential (EOE) [21]. The valuef EOE for samples A, B and C is 572 mV, 589 mV and 578 mV,espectively. The above results clearly indicate that the inhibit-ng effects of the surface coating of Y(OH)3 on OER displays theollowing order: sample B > sample C > sample A. The resultsgree well with those of charge/discharge experiments. On theasis of these facts, it is deduced that the enriched yttrium onhe surface of Ni(OH)2 crystals is beneficial to the controlling ofER, while Y(OH)3 precipitated inside the pores between tiny

rystals is less effective. Accordingly, the better performance ofample B can be accounted for larger relative surface contentnd higher utilization ratio of yttrium.

As described vide anta, it is obvious that the improvementf the high-temperature performance of Ni(OH)2 electrode isot only related to the effects of additives themselves, but alsoepends on the distribution of these adding elements in surfacexide layer of Ni(OH)2. The coating surface with dense andorous morphology, larger relative surface content and highertilization ratio of yttrium is more effective for the controlling ofER and the improvement of the high-temperature performancef Ni(OH)2 electrode.

. Conclusions

In the present work, the synthesis, characteristics, and high-emperature electrochemical performance of Y(OH)3-coatedpherical Ni(OH)2 with different coating surface are system-tically studied. Following conclusions can be made based onhe results:

. Spherical Ni(OH)2 was successfully modified with Y(OH)3by chemical surface precipitation. Three distinct sampleswith diverse distribution of Y(OH)3 on the spherical Ni(OH)2surface layer were obtained under different experimental con-ditions.

. The coating of Y(OH)3 on spherical Ni(OH)2 is an effec-tive way to increase the overpotential of oxygen evolutionand to improve the charge acceptability of Ni(OH)2 at ele-vated temperature, and the necessary coating amount is only0.55 at.%.

. The chemical nature and morphology of the coating layerplays an important role in OER. The coated surface withdense and porous morphology, larger relative surface con-

tent and higher utilization ratio of yttrium is more effectiveon the suppression of OER and the improvement of thehigh-temperature performance of Ni(OH)2 electrode. Appli-cation of the coating technology to Ni(OH)2 active material

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of sealed Ni/MH cells yielded a charge efficiency of about80.2% at 60 ◦C and a capacity retention of 88.1% of its valueat 25 ◦C.

cknowledgement

The financial support by the 863 national research and devel-pment project foundation of China (Grant No. 2005AA501440)s gratefully acknowledged.

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