High temperature performances of yttrium-doped spherical nickel hydroxide

Electrochimica Acta 49 (2004) 3361–3366

High temperature performances of yttrium-doped spherical nickel hydroxide X. Mi a , X.P. Gao a,∗ , C.Y. Jiang b , M.M. Geng c , J. Yan a , C.R. Wan b a

Institute of New Energy Material Chemistry, Nankai University, Weijin Road 94, Tianjin 300071, China b Institute of Nuclear Energy Technology, Tsinghua University, Beijing 100084, China c Tianjin Peace Bay Power Sources Group Co. Ltd., Tianjin 300384, China Received 5 January 2004; received in revised form 18 March 2004; accepted 18 March 2004 Available online 6 May 2004

Abstract The regular and yttrium-doped spherical ␤-phase nickel hydroxides were synthesized by means of chemically co-precipitation. The yttrium-doping with long needle-like nanocrystallites observed by TEM promoted the formation of the spherical nickel hydroxide with the larger diameter of about 5 ␮m. The discharge capacity of the yttrium-doped spherical nickel hydroxide was measured to be slightly lower than that of the regular spherical nickel hydroxide at room temperature. At temperatures of above 50 ◦ C, however, the discharge capacity of the yttrium-doped nickel hydroxide is much higher than that of the regular spherical nickel hydroxide. The improvement of discharge capacity at elevated temperatures was contributed to the increase of the charge acceptance of yttrium-doped nickel hydroxide. The formation of an yttrium-rich surface layer on nickel hydroxide particles raised the oxygen evolution over-potential, leading to performance improvements of the nickel hydroxide electrode. The improvement of high temperature charge acceptance of yttrium-doped nickel hydroxide remarkably contributed to the high temperature charge–discharge efficiency of the nickel–metal hydride (Ni–MH) batteries with a commercial AAA size. © 2004 Elsevier Ltd. All rights reserved. Keywords: Nickel hydroxide; High temperature performance; Nickel metal hydride battery; Yttrium-doped nickel electrode

1. Introduction Metal hydrides as negative electrodes and ␣/␤-type Ni(OH)2 as positive electrodes in rechargeable alkaline nickel–metal hydride (Ni–MH) batteries have been studied intensively due to their high reversible energy storage capacity, excellent long-term cycling stability and good charge/discharge kinetics [1–5]. As power sources for electric and hybrid electric vehicles (EV and HEV) as well as electric tools, the Ni–MH batteries are required to work in a higher-temperature environment of over 40 ◦ C in which a number of cells are connected in series to provide a high system voltage. The higher temperature performance of Ni–MH batteries is directly related to the behavior of the nickel hydroxide electrode materials. Because of oxygen evolution readily on positive electrode at elevated tempera∗ Corresponding author. Tel.: +86-22-23500876; fax: +86-22-23502604. E-mail address: [email protected] (X.P. Gao).

0013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2004.03.005

tures (>50 ◦ C), the charge efficiency of positive electrodes significantly declines, leading to poor performances of Ni–MH batteries at high temperatures. In order to increase the performances, many studies on addition of cobalt oxide (CoO) [6–9], zinc oxide [10], cadmium oxide [11] and lanthanide oxides [12–14] on the positive electrodes were made. On the basis of these studies, the addition of cobalt oxide into nickel hydroxide electrode increased the oxygen evolution potential, improve material utilization and suppress ␥-NiOOH formation during charge–discharge cycling [6,7]. The utilization, charge–discharge reversibility and oxygen evaluation potential of the nickel hydroxide electrode have been further improved after surface modification with metallic cobalt or addition of cobalt hydroxide [15–17]. However, the amount of cobalt added or coated must be limited due to a cost and drop of discharge potential. The addition of heavy lanthanide oxides was found to be particularly attractive for improving charge acceptance of positive electrodes at elevated temperatures as reported in a literature [13].

X. Mi et al. / Electrochimica Acta 49 (2004) 3361–3366

The lanthanide oxides usually translate to hydrous hydroxide in an alkaline solution, and their uniform distribution on the surface of nickel hydroxide particles plays a critical role in increasing the performances of nickel electrodes. The small amounts of additive (usually 3.5–5 wt.% because large amounts of additive will decrease the energy density per weight or volume) are not easily handled to mix uniformly with a mass of nickel hydroxide powder having a particle size of 1–10 ␮m in an agglomeration. In order to overcome these disadvantages, in this work, we chemically synthesized yttrium element (Y3+ )-doped spherical nickel hydroxide, which makes each nickel hydroxide particle have an yttrium-rich surface layer. The charge–discharge properties of the yttrium-doped spherical nickel hydroxide electrodes were then investigated in the temperature range of 20–70 ◦ C, and compared with regular nickel hydroxide electrodes (without yttrium element). To evaluate its application in Ni–MH batteries, AAA size Ni–MH batteries were assembled using the yttrium-doped spherical nickel hydroxide electrodes as positives and MmNi5 -type alloy electrodes as negatives. The battery performances were examined at elevated temperatures.

2. Experimental details 2.1. Powder preparation and characterization A sodium hydroxide aqueous solution (5 mol/l) and ammonia aqueous solution (13 mol/l) were gradually added into a mixed aqueous solution of nickel sulfate, cobalt sulfate, and zinc sulfate (molar ratio of Ni:Co:Zn is 100:2.5:5) to produce regular (Ni, Co, Zn)(OH)2 spherical powders. The concentration of NiSO4 aqueous solution was 1.5–2.0 mol/l. pH value (11.5–12) was controlled using an ammonia aqueous solution under a stirring speed of 1500–2000 rpm. The nickel hydroxide precipitation was carried out at the temperature of 50 ◦ C. The precipitate was then washed with distilled water, filtered and dried at 80 ◦ C. A yttrium-doped spherical nickel hydroxide powder was also synthesized by adding sodium hydroxide aqueous solution and ammonia aqueous solution into the mixed aqueous solution of nickel sulfate, cobalt sulfate, zinc sulfate and yttrium chloride (molar ratio of Ni:Co:Zn:Y is 100:2.5:5:1). The two kinds of nickel hydroxide powders obtained were then characterized by X-ray diffraction (XRD, Rigaku D/max-2500), transmission electron microscopy (TEM, FEI Tecnai 20) and X-ray photoelectron spectroscopy (XPS, PHI-5300 ESCA), respectively. 2.2. Electrode preparation and electrochemical property The regular and yttrium-doped nickel hydroxide powders were, respectively, mixed with cobalt oxide powder in a weight ratio of 10:1 together with a small amount (1 wt.%) of polytetrafluoroethylene (PTFE) aqueous solution as a binder and then the mixture was forced into a nickel foam to form

a 2 cm × 2 cm × 0.06 cm electrode. After drying at 60 ◦ C for 1 h, the foamed nickel hydroxide electrode was pressed at a pressure of 30 MPa for 1 min. In an experimental cell with a three-electrode system, the positive electrode was placed in a middle of two metal hydride electrodes. These electrodes were separated using two polyolefin separators and two stainless steel sheets were used to fix these electrodes. An Hg/HgO electrode was used as a reference electrode. A Luggin capillary tube, which connected the reference electrode to the working electrode, was placed close to the working electrode in order to minimize the ohmic drop across the electrolyte solution. The electrolyte used was a 6 M KOH aqueous solution. The experimental cells were set up in a temperature-controlling water bath and the cells were tested at temperatures in the range of 20–70 ◦ C. The charge/discharge tests of the experimental cells were conducted using a Land battery testing equipment (CT2001A). The positive electrodes were charged at 0.2 C for 7.5 h and discharged at 0.2 C to a cut-off potential of 0.1 V versus the Hg/HgO electrode. The AAA-size Ni–MH batteries (700 mAh) with MmNi5 -type alloys as negative electrodes and regular or yttrium-doped nickel hydroxides as positive electrodes were manufactured in Peace Bay Power Sources Group Co. Ltd. The capacity utilization of AAA-size Ni–MH batteries was measured under a charge–discharge current of 140 mA to a cut-off voltage of 1.0 V at different temperatures.

3. Results and discussion To evaluate the effect of the yttrium-doped content on the high temperature performances of nickel hydroxide electrodes, the discharge capacities of nickel hydroxide electrodes with different yttrium-doped amount at 60 ◦ C were measured at 0.2 C as shown in Fig. 1. It was found that the optimum yttrium-doped content (atomic concentration) was near 1 at.% to obtain higher discharge capacity at 60 ◦ C, 300 250

Discharge capacity (mAh/g)

3362

200 150 100 50 0

0.0

0.5

1.0

1.5

2.0

2.5

Yttrium content ( at %)

Fig. 1. The effect of the yttrium-doped content on the discharge capacity of nickel hydroxide electrodes at 60 ◦ C (0.2 C).

X. Mi et al. / Electrochimica Acta 49 (2004) 3361–3366 001

100

101

3363

yttrium-doped Ni(OH)2 110 111

102

Intensity

(b)

regular Ni(OH) 2

(a) 10

20

30

40

50

60

70

2θ Fig. 2. XRD patterns of the regular (a) and yttrium-doped nickel hydroxide (b).

corresponding to 1.5 wt.%, which was smaller than that mechanically addition content of lanthanide oxides (at least 3.5 wt.%) into the nickel hydroxide electrode [13]. The structure and electrochemical properties of the yttrium-doped nickel hydroxide electrode with the content of 1 at.% were investigated in detail hereafter. Fig. 2 shows X-ray diffraction patterns of the yttriumdoped nickel hydroxide and regular nickel hydroxide powders. It was found from XRD patterns that the peaks of the yttrium-doped nickel hydroxide powder, close to that of the regular nickel hydroxide powders, can be mainly indexed to ␤-phase nickel hydroxide with a brucite-type structure and a hexagonal unit, indicating that the yttrium was entirely incorporated in the lattice of nickel hydroxide. The lattice parameters as shown in Table 1 are calculated from the above XRD patterns. It was indicated that the lattice volume of the yttrium-doped nickel hydroxide powder was slightly larger than that of the regular nickel hydroxide powders due to the difference of Ni2+ (0.69 Å) and Y3+ (0.89 Å) ionic radii. The TEM images of the regular nickel hydroxide and yttrium-doped nickel hydroxide powders are illustrated in Fig. 3. It was observed that the regular nickel hydroxide and yttrium-doped nickel hydroxide were shown to have the uniform spherical shapes with the diameters of 2 and 5 ␮m, respectively. After ultrasonic vibration, the thin broken pieces of regular nickel hydroxide and yttrium-doped nickel hydroxide powders were clearly found to consist of the needle-like nanocrystallites inside particles as shown in Fig. 4, which were also observed in pure Table 1 Lattice parameters of the regular and the yttrium-doped spherical Ni(OH)2 Lattice constants

Yttrium-doped Ni(OH)2 Regular

a (Å)

c (Å)

3.1288 3.1255

4.6246 4.6287

Unit cell volume (Å3 )

39.21 39.16

Fig. 3. TEM images of the regular spherical (a) and yttrium-doped nickel hydroxide (b).

␤-phase nickel hydroxide obtained by chemical precipitation [18]. The needle-like nanocrystallites were believed to be beneficial to the agglomeration and formation of nickel hydroxide with a spherical shape. The yttrium-doping with long needle-like nanocrystallites promoted the formation of the spherical nickel hydroxide with larger diameter. As showed in Fig. 5, the discharge capacity of the regular spherical nickel hydroxide electrode reaches a maximum capacity after four cycles at 0.2 C. In the meanwhile, the discharge capacity of the yttrium-doped spherical nickel hydroxide electrode reaches a maximum capacity after eight cycles. The regular spherical nickel hydroxide electrode appeared easier activation than the yttrium-doped spherical nickel hydroxide electrode. The easy activation of the regular spherical nickel hydroxide electrode could be related to the smaller particle size in diameter as observed above. It was also identified that the faster activation of the yttrium-doped spherical nickel hydroxide electrode was achieved at high

3364

X. Mi et al. / Electrochimica Acta 49 (2004) 3361–3366

Discharge capacity (mAh/g)

300 250 200 150 Regular Ni (OH) 2

100

Yttrium-doped Ni(OH)2

50 0

20

30

40

50

60

o

Temperature ( C) Fig. 6. The dependence of temperature on discharge capacity of the regular and the yttrium-doped nickel hydroxide electrodes at 0.2 C.

Fig. 4. TEM images of pieces of the regular (a) and yttrium-doped nickel hydroxide (b). 300

temperature. No obvious change in discharge capacity was observed after 20 cycles. The dependence of temperature on discharge capacities of the yttrium-doped spherical nickel hydroxide and regular spherical nickel hydroxide after activation at 0.2 C is illustrated in Fig. 6. It indicated that the regular nickel hydroxide had a slightly higher discharge capacity than the yttrium-doped nickel hydroxide at 20 ◦ C. However, the discharge capacity of the yttrium-doped nickel hydroxide is higher than that of the regular nickel hydroxide over 50 ◦ C. The ratio of the discharge capacity of yttrium-doped nickel hydroxide at 60 and 20 ◦ C can reach 82.3%. In contrast, the ratio of the discharge capacity of the regular nickel hydroxide at 60 and 20 ◦ C is only 41%. The discharge curves of the yttrium-doped nickel hydroxide and regular nickel hydroxide at 0.2 C and at 20 and 60 ◦ C after activation are indicated in Figs. 7 and 8, respectively. It could be seen that the charge and discharge curves of the yttrium-doped nickel hydroxide and regular nickel hydroxide at 20 ◦ C were almost identical. However, the charge potential of the yttrium-doped spherical nickel hydroxide electrode was

0.6

200

0.5

Regular Ni(OH)2

Yttrium-doped Ni(OH) 2

150

Potential (V)

Discharge capacity (mAh/g)

250

Regular Ni(OH)2 100

Yttrium-doped Ni(OH)2

0.4 0.3 0.2

50

0.1

0 1

2

3

4

5

6

7

8

9

10

Cycle number (n) Fig. 5. The initial activation of the regular and yttrium-doped nickel hydroxide electrodes at 20 ◦ C (0.2 C).

0

100

200

300

400

500

Capacity (mAh/g) Fig. 7. The 0.2 C charge and discharge curves of the regular and the yttrium-doped nickel hydroxide electrodes after activation at 20 ◦ C.

X. Mi et al. / Electrochimica Acta 49 (2004) 3361–3366 0.6

Potential (V)

0.5

Regular Ni(OH)2

0.4

Yttrium-doped Ni(OH)2 0.3

0.2

0.1 0

100

200

300

400

Capacity (mAh/g) Fig. 8. The 0.2 C charge and discharge curves of the regular and the yttrium-doped nickel hydroxide electrodes at 60 ◦ C.

lower and discharge potential was higher at 60 ◦ C, compared with the regular spherical nickel hydroxide electrode. The results are in agreement with the literature [13] by mechanical addition of lanthanide oxides into the nickel hydroxide electrode. The lanthanide oxides were distributed around nickel hydroxide particles as measured by EPMA mapping [13]. In addition, the oxygen over-voltage increased due to the formation of the active oxygen evolution site on the surface of sintered nickel positive electrodes, which was almost coated with yttrium hydroxide by chemically immersing [19]. Therefore, the enriched yttrium on the surface of nickel hydroxides is beneficial to control the oxygen evolution of nickel hydroxide electrodes at high temperature. It was identified from XPS result in our experiment that the surface layer composition of Ni, Co, Zn and Y was different from the original preparation composition. The molar ratio of surface layer Ni, Co, Zn and Y was measured to be 100:6.1:0.8:10.0. Comparably, the yttrium element

3365

tended to segregate to the surface layer of nickel hydroxide particles prepared chemically, which was 10 times higher than that of the original preparation composition. The formation of yttrium-rich surface layer possesses the function similar to coated surface layer with yttrium hydroxide in the spherical nickel hydroxide electrodes [20] or distributed interface with lanthanide oxides around nickel hydroxide particles [13]. Therefore, the charge and discharge performances of the nickel hydroxide positive electrode at higher temperatures were improved by doping chemically the yttrium into the nickel hydroxide during the spherical powder preparation although the yttrium-doped content was only 1 at.%. Two types of commercial AAA-size Ni–MH batteries were manufactured using the fresh yttrium-doped nickel hydroxide and the regular nickel hydroxide as positive electrodes, respectively. The normal capacity of AAA-sized Ni–MH batteries was 700 mAh. As shown in Fig. 9, the discharge capacity of the battery using the yttrium-doped nickel hydroxide as electrode materials is almost identical to that of the battery using the regular nickel hydroxide at 20 ◦ C and lower temperatures after activation. However, the discharge capacity of the battery using the yttrium-doped nickel hydroxide is much higher than that of the battery using the regular nickel hydroxide at temperatures of over 40 ◦ C. The reason was mainly attributed to the improvement of the charge and discharge performances of the yttrium-doped nickel hydroxide electrode at higher temperatures. In addition, it was observed that the higher-temperature performances of the sealed Ni–MH batteries using the yttrium-doped nickel hydroxide electrodes were more efficient compared with the yttrium-doped nickel hydroxide electrode in the half-cell test. This result was probably related to the inner temperature increase in the sealed Ni–MH batteries due to recombination of oxygen and hydrogen during overcharging and reinforce of the

Discharge Capacity (%)

100

80

60

Regular Ni(OH)2 Yttrium-doped Ni(OH)2

40

20

0 0

10

20

30

40

50

60

70

80

o

Temperature ( C) Fig. 9. Capacity utilization of AAA-size Ni–MH batteries using the regular and the yttrium-doped nickel hydroxide as active electrode materials at different temperatures (0.2 C).

3366

X. Mi et al. / Electrochimica Acta 49 (2004) 3361–3366

charge acceptance utilization of the yttrium-doped nickel hydroxide electrode.

4. Conclusion The yttrium-doped nickel hydroxides having a ␤-phase structure were synthesized by chemically co-precipitation. The optimum yttrium-doped content was shown to be around 1 at.% to obtain higher discharge capacity at 60 ◦ C. It was demonstrated that the charge–discharge capacity utilization of the positive electrode at higher temperatures was improved by doping chemically the yttrium into the nickel hydroxide due to the formation of an yttrium-rich surface layer. In addition, the higher-temperature performances of the sealed Ni–MH batteries using the yttrium-doped nickel hydroxide as positive electrodes were much better than those using the regular nickel hydroxide electrodes. Acknowledgements This work was supported by the 973 Program (2002CB 211800) and NSFC (50134020) of China. References [1] C. Faure, C. Delmas, M. Fouassier, J. Power Sources 35 (1991) 279.

[2] M.S. Wu, C.M. Huang, Y.Y. Wang, C.C. Wan, Electrochim. Acta 44 (1999) 4007. [3] M. Dixit, R.S. Jayashree, P.V. Kamath, A.K. Shukla, V.G. Kumar, N. Munichandraiah, Eletrochem. Solid State Lett. 2 (1999) 170. [4] W.K. Hu, D. Noreus, Chem. Mater. 15 (2003) 974. [5] C.Y. Wang, S. Zhong, D.H. Bradhurst, H.K. Liu, S.X. Dou, J. Alloys Compd. 330–332 (2002) 802. [6] R.D. Armstrong, G.W.D. Griggs, E.A. Charles, J. Appl. Electrochem. 18 (1988) 215. [7] A.K. Sood, J. Appl. Electrochem. 16 (1986) 274. [8] K. Watanabe, M. Koseki, N. Kumagai, J. Power Sources 58 (1996) 23. [9] V. Pralong, A. Delahaye-Vidal, B. Beaudoin, J. B Leriche, J.M. Tarascon, J. Electrochem. Soc. 147 (2000) 1306. [10] C. Tessier, C. Faure, L.G. Demourgues, C. Denage, G. Nabias, C. Delmas, J. Electrochem. Soc. 149 (2002) A1136. [11] M. Oshitani, Y. Sasaki, K. Takashima, J. Power Sources 21 (1984) 219. [12] Y. Morioka, S. Narukawa, T. Itou, J. Power Sources 100 (2001) 107. [13] M. Oshitani, M. Watada, K. Shodai, M. Kodama, J. Electrochem. Soc. 148 (2001) A67. [14] K. Ohta, K. Kayashi, H. Matsuda, Y. Toyoguchi, M. Ikoma, Proc. Electrochem. Soc. 94–27 (1994) 296. [15] V. Pralong, A. Delahaye-Vidal, B. Beaudoin, J.B. Leriche, J.M. Tarascon, J. Electrochem. Soc. 147 (2000) 1306. [16] X.Y. Wang, J. Yan, H.T. Yuan, Z. Zhou, D.Y. Song, Y.S. Zhang, L.G. Zhu, J. Power Sources 72 (1998) 221. [17] F. Lichtenberg, K. Kleinsorgen, J. Power Sources 62 (1996) 207. [18] R.S. Jayashree, P.V. Kamath, G.N. Subbanna, J. Electrochem. Soc. 147 (2000) 2029. [19] K. Shinyama, Y. Magari, A. Funahashi, T. Nohma, I. Yonezu, Electrochemistry 71 (8) (2003) 686. [20] X. Mi, C.Y. Jiang, J. Yan, X.P. Gao, C.R. Wan, Chin. J. Inorg. Chem. 20 (2) (2004) 164.