Studies on rechargeable NiMH batteries

International Journal of Hydrogen Energy 31 (2006) 525 – 530 www.elsevier.com/locate/ijhydene

Studies on rechargeable NiMH batteries T.-K. Yinga,1 , X.-P. Gaob , W.-K. Huc,∗ , F. Wud , D. Noréusc a College of Biology and Chemistry, Zhejiang Normal University, Zhejiang, Jinhua 321004, China b Institute of New Energy Material Chemistry, Nankai University, Tianjin 300071, China c Department of Structural Chemistry, Arrhenius Laboratory, Stockholm University, S-106 91 Stockholm, Sweden d School of Chemical Engineering and Materials Science, Beijing Institute of Technology, Beijing, China

Available online 9 June 2005

Abstract Nickel-metal hydride (NiMH) batteries offer some advantages in the aspects of power, cycle life and environment. However, they are encountering challenge from other rechargeable batteries such as Li-ion batteries. In this paper, the possibility of performance improvements and cost reduction of NiMH batteries were analyzed. Some approaches to improve energy densities and reduce the battery cost were discussed. An attempt to increase the energy density of NiMH batteries was made through improvements in the specific capacity of rare earth-based AB5 -type hydride alloys in the negative electrodes and capacity enhancement of nickel hydroxide electrode. A relatively lower cost small cylindrical NiMH cell was constructed and its properties were evaluated. 䉷 2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: NiMH battery; Hydrogen storage alloys; Hydride electrode; Nickel hydroxide; Nickel electrode

1. Introduction As petroleum oil reserves in the world declines and gasoline price arises, electric and hybrid vehicles (EV and HEV) have attracted more and more people. Rechargeable NiMH batteries as power sources of the EV and HEV have many advantages such as higher power densities and environmental friendliness [1–3]. Based on the current hydride technology, however, the cost of NiMH battery is too high (about 250 US$/kWh), limiting the widespread applications as power sources for EV and HEV. In addition, the NiMH cells are encountering competition from Li-ion cells or Li-polymer cells in the applications of small electronic devices such as mobile phones, tap-top computers, small power tools and

so on. Therefore, how to reduce the cost of the rechargeable NiMH batteries and improve battery performances is significantly important. In this paper, we reviewed some literature and discussed the possibility of performance improvements and cost reduction of NiMH batteries. Some approaches to enhance the cell performance and reduce the battery cost were discussed. The cost share among the battery components was analyzed. Finally, an attempt to construct a lower cost and higher performance cylindrical NiMH cell was made and its performance was evaluated.

2. Brief review of literature 2.1. Improvement of NiMH cell energy density

∗ Corresponding author.

E-mail addresses: [email protected], [email protected] (W.-K. Hu). 1 Also for correspondence.

There are two ways to increase the energy density of a NiMH battery: (a) to reduce the total weight of the NiMH battery; (b) to increase the battery capacity output. The

0360-3199/$30.00 䉷 2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2005.04.018

T.-K. Ying et al. / International Journal of Hydrogen Energy 31 (2006) 525 – 530

60

Weight percentage

50 MH electrode

40 30 20 10

Nickel electrode

Cell can

Insulator Header Separator & wrapper

0 Cell components Fig. 1. Weight percentage distribution of battery components.

weight distribution of NiMH cell components is shown in Fig. 1. As seen, the MH electrode has the highest percentage in weight, which indicates that the energy density of NiMH cells will increase if the specific capacity of the MH alloy is improved. During the past 15 years, a great effort has been made to develop and optimize AB5 -type hydrogen storage alloys as negative materials in the NiMH cells. Many researchers have intensively investigated the effect of rare earth composition, metallurgical processing and alloy microstructure on the hydrogen storage capacity and cycling stability [4–11]. At present, the specific capacity of AB5 type hydride alloys can reach > 320 mAh/g, 20% more than the earlier capacity of 250–270 mAh/g about 15-years ago. For the small cylindrical NiMH cells, the energy density has increased to more than 80 Wh/kg from the earliest value of about 45 Wh/kg. However, further improvement in the specific capacity of AB5 -type alloys gets more and more difficult because this value is close to the theoretical capacity of 372 mAh/g of prototype LaNi5 alloys. Zr–Ti–Ni-based AB2 -type multiphase alloys and mischmetal-Mg–Ni based AB3 -type hydride alloys probably will be the candidate materials for the future NiMH cells since they have the higher specific capacity of more than 380 mAh/g [12,13]. The positive electrode occupies about 30% of total weight, as shown in Fig. 1. Equally, the energy density of NiMH cells will significantly increase if a lightweight nickel electrode is available or a higher energy density’s nickel electrode is developed [14]. In the earlier NiMH cells, sintered nickel electrodes with an energy density of 400–500 mAh/ml were widely employed. This is one of reasons leading to low energy densities of NiMH cells in the early stage. In order to replace the conventional sintered nickel electrodes, a pasted nickel electrode has been developed [15–17]. The pasted nickel electrode has the energy density of 580–620 mAh/ml or higher, depending on the utilization and loading amount of the active material per unit volume. To improve the utilization and loading amount, high packed density of beta-phase spherical nickel hydroxide has been developed [16,18]. Re-

placement of Ni by small amounts of Zn and Co was investigated to suppress formation of
-phase NiOOH during repeating cycling [19,20]. Surface modification of nickel hydroxide particles by cobalt hydroxide and metal cobalt films was suggested in order to improve conductivity between nickel hydroxide particles or at interface connecting the particles and the current collector [15,21–23], which is considered to be the critical factor to improve utilization of the active material. It was claimed that the NiMH battery composed of such a pasted nickel electrode in which the surface of beta-phase nickel hydroxide was modified by cobalt hydroxide had a higher capacity and long cycle life. It is clearly seen from Fig. 1 that other components have litter opportunity to make contributions to energy densities of NiMH cells by reducing their weight. Thus, increase in the specific capacities of MH alloy and utilization of nickel hydroxide in positive electrodes is key issues for improvements in the energy density of NiMH cells.

2.2. Analysis of NiMH battery cost The battery cost consists of two parts, the battery manufacturing cost and battery material cost. The battery manufacturing cost depends on production volumes and the manufacturing technology. It is well known that increasing production volume and improving manufacturing technology would reduce the battery cost. It was reported that a dry powder roller pressing technology had a lower manufacturing cost for the NiMH cells [24]. The battery material cost is mainly made up of following parts: the negative electrode, the positive electrode, separator, cell hardware and other. The cost share of these components is shown in Fig. 2. The negative electrodes occupy the highest cost share, some 45% in a total material cost. Therefore, R & D of a cheap MH electrode is significant for cost reduction of NiMH batteries. The most effective way to reduce the cost of MH electrodes is to reduce the cobalt content in the AB5 -type 60 50 Cost share %

526

MH electrode

40 30

Nickel electrode

20 Ni foam

10

hardware

other

0 Cell components Fig. 2. Cost share distribution of battery components.

T.-K. Ying et al. / International Journal of Hydrogen Energy 31 (2006) 525 – 530

3. Experimental 3.1. Electrode preparation Two metal hydride alloys having particles sizes of 36–63
m were used as the negative materials. The first one is a commercial composition AB5 -type hydrogen storage alloy (LmNi3.55 Co0.75 Mn0.4Al0.3 ), in which Lm is La-rich rare earth metal. The other one is a low-cobalt rare earthbased AB5 -type alloy, which has the chemical composition of Lm0.96 Ti0.04 Ni4.0 Co0.2 Mn0.2Al0.2 Cr0.2 Cu0.2 Si0.01 . The La-rich mischmetal composition is 78% La, 15.04% Ce, 5.51% Nd and 1.45% Pr. The negative electrodes were prepared by a plastic-bound process using a poly(tetrafluoroethylene) (PTFE) solution. A slurry containing 95 wt% MH alloy powder, 2 wt% carbon powder and 3 wt% PTFE solution was made and then pasted into nickel net substrates. After drying for 24 h at room temperature, the MH electrode was cold-pressed to a thickness of 0.35–0.40 mm. For the positive electrodes, -phase spherical nickel hydroxide powder having a tap density of 2.1 g/ml was used as active materials. The surface modification of nickel hydroxide powder is necessary and was done using the similar method of literature [54]. The positive electrodes were prepared as follows: nickel hydroxide powder, nickel powder (Inco 210) and cobalt oxide powder were mixed at a weight ration of 85:10:5. A binder of 1.0 wt% carboxymethyl cellulose (CMC) was added into the mixture to obtain slurry. Nickel foam with a thickness of 1.6 mm was used as the nickel electrode substrate. The nickel substrate

1.00 0.2C rate 0.5C rate 1.0C rate 2.0C rate

-Potential / V

0.95 0.90 0.85 0.80 0.75 0.70

0

50

100

150

200

250

300

350

Discharge capacity / mAh/g Fig. 3. Discharge curves of commercial AB5 -type alloy electrodes. Charge at 0.2 C for 6 h and 30 min rest at room temperature.

0.95

0.2C 1.0C 2.0C

0.90

-Potential / V

hydrogen storage alloy. This is because the cobalt takes up about 10 wt% and 40–50% share of the total cost in the commercial MmNi3.6 Co0.7 Mn0.4Al0.3 alloy [25]. Therefore, intensive attempts in development of lowcobalt and cobalt-free AB5 -type alloys are ongoing at many laboratories around the world [26–49]. In addition, it was reported [50–53] that a low-cost plastic bonded electrode technology enabled the material cost to reduce about US$ 50/kWh. If the technology were combined with a bipolar battery configuration, another US$ 50/kWh reduction of material cost would be expected due to eliminating much of the hardware needed for conventional cell designs. It was said that a nearterm goal of US$ 150–200/kWh for EV or HEV NiMH batteries could be achieved through a plastic-bonded electrode technology combined with a bipolar battery configuration. In this work, we attempted to use low-cobalt AB5 -type alloys as negative materials and beta-phase spherical nickel hydroxide modified by coating cobalt hydroxide on surface as active materials of positive electrodes to construct small cylindrical NiMH cells. The electrochemical performances of the NiMH cells were evaluated.

527

0.85 0.80 0.75 0.70

0

50

100

150

200

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300

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Discharge capacity / mAh/g Fig. 4. Discharge curves of low-cobalt AB5 -type alloy electrodes, which has the composition of Lm0.96 Ti0.04 Ni4.0 Co0.2 Mn0.2Al0.2 Cr0.2 Cu0.2 Si0.01 . Charge at 0.2 C for 6 h and 30 min rest at room temperature.

was then filled with the slurry, dried at room temperature for 24 h and cold-pressed to obtain a pasted nickel electrode. 3.2. Cell assembly and testing AA and AAA size cylindrical NiMH cells were assembled by spirally winding the nickel hydroxide positive electrodes and the MH negative electrodes along with a 0.12 mm thick polyolefin non-woven cloth. The electrolyte was a 6 M KOH solution containing 1 M of LiOH. Before charge and discharge, the cells were placed in a thermostatic bath at 40 ◦ C for 24 h. Then, the cells were charged and discharged at room temperature (25 ◦ C) with a rate of 0.1 C for 14 h and the cut-off voltage was chosen to be 0.9 V for 2–4 cycles. After activation, a 0.2 C rate charging for 6 h and discharging at the same rate were applied for cycling.

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4. Results and discussion

300

Fig. 3 shows the electrochemical discharge characteristics of the MH electrode, in which the commercial AB5 alloy with the chemical composition of LmNi3.55 Co0.75 Mn0.4Al0.3 was used. The MH electrode exhibited the specific capacity of 330 mAh/g and a flat discharge plateau was observed between 0.2 C and 2.0 C rates. According to this value and the volume of MH electrodes, the volumetric energy density of the MH electrode could reach 1450–1500 mAh/ml. Another MH electrode, where the low-cobalt rare earth-based AB5 -type alloy was employed, is shown in Fig. 4. This MH electrode had a specific capacity of some 310 mAh/g and the volumetric energy density of 1340–1400 mAh/ml. The both electrodes had similar electrochemical discharge curves. However, the latter electrode had a relatively lower capacity and energy density than the former one. But, the cost of the latter MH electrode was reduced by about 30% because the cobalt content declined considerably from original 10 to 2.8 wt% in the low-cobalt AB5 -type alloy. 4.2. Positive electrode performance Fig. 5 gives the discharge curves of two pasted nickel hydroxide electrodes. Clearly, the average discharge potential and the discharge capacity increased after the nickel hydroxide particles were coated through a layer of cobalt oxide hydroxide film. This indicates that the electrode polarization was reduced and the active material utilization was improved after surface treatments, especially at a higher discharge rate. Fig. 6 gives the dependence of discharge capacity on the discharge current density. As seen, the rate capability of the nickel electrode was much superior to the electrode with no surface treatments. The performance im-

0.6 (a) 5 wt% coating (b) no coating

Potential / V

0.5 0.4 0.3

(a)

0.2 (b)

0.1 0

50

100

150

200

250

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Capacity / mAh/g Fig. 5. Discharge curves of two -phase nickel hydroxide electrodes. (a) Surface modification by coating 5 wt% cobalt hydroxide; (b) without surface modification.

Capacity / mAh/g

4.1. Negative electrode performance

250

(a)

200 (b) 150 (a) 5 wt% coating (b) No coating

100 50 0

0

200 400 600 800 1000 1200 1400 1600 Current density/ mA/g

Fig. 6. Dependence of capacity on discharge current densities of the two nickel electrodes. (a) Surface modification by coating 5 wt% cobalt hydroxide; (b) without surface modification.

provements were ascribed to the reason that the polarization resistance decreased after coating a layer of conductive cobalt oxide hydroxide film on the nickel hydroxide particles, which results in improvements in charge efficiency and reaction kinetic of nickel electrodes. The pasted nickel electrode using the surface modified nickel hydroxide particles had a volumetric energy density of 600–630 mAh/ml and a weight energy density of 190 mAh/g including the electrode substrate, respectively. 4.3. Characteristic of NiMH cells Fig. 7 shows the discharge curves of cylindrical AA and AAA size NiMH cells using the nickel hydroxide modified by a layer of cobalt oxide hydroxide film on surface for the positive electrode and a commercial AB5 -type MH alloy for the negative electrode. The AA and AAA size cells have a capacity of some 1650 and 600 mAh at a 0.2 C rate, respectively. The specific energy density of AA size cells is about 75 Wh/kg, which is much higher than 50–60 Wh/kg of the earlier AA size cells having a discharge capacity of 1200 mAh. Thus, the battery energy density dramatically increases through improvements in the specific capacity of negative and positive electrodes. Fig. 8 shows the performance of a relatively low cost NiMH cell where the lowcobalt AB5 -type alloy was used as negative materials. The relationship between the cell capacity and the cycle number for this NiMH cell is shown in Fig. 9. For comparison, the cycling stability of the NiMH cell using the commercial AB5 -type alloy containing about 10 wt% Co is also shown in the Fig. 9. The both NiMH cells show satisfactorily cycling stability after 200 cycles with 100% depth of discharge (DOD). Thus, the low-cobalt alloy is considered to be an idea candidate to reduce the cost of NiMH batteries. The future EV or HEV NiMH battery cost probably can be close to the requirements of the price target, US$ 150/kWh, if the

T.-K. Ying et al. / International Journal of Hydrogen Energy 31 (2006) 525 – 530 1.6

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800 700

Capacity / mAh

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Fig. 9. Cycling stability of two kinds of AAA size NiMH cells. (a) Commercial AB5 -type alloy electrodes; (b) low-cobalt AB5 -type alloy electrodes.

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5. Conclusion 1.0

0.2C rate 1.0C rate 2.0C rate

0.8 0.6

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(b)

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Fig. 7. Discharge performance of NiMH cells using commercial AB5 -type alloy electrodes. (a) AA size; (b) AAA size.

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Voltage / V

0

Energy densities and cost of NiMH batteries in applications of future EV or HEV as power sources are the critical factors. This paper briefly reviewed the progress of NiMH cells in the aspects of performance improvements and cost reduction. With the purpose of the two points, we have made an attempt to construct small cylindrical NiMH cells, in which low-cobalt AB5 hydrogen storage alloys were used as negative electrodes and -phase spherical nickel hydroxide after surface modification was used as positive electrodes. Our results showed that the energy density could increase to 75 Wh/kg, compared with 50–60 Wh/kg of the earlier cells with the same sizes and weight. Battery cost could be reduced considerably if the negative electrode is composed of a durable low-cobalt AB5 type alloy and combined with a low-cost battery manufacturing technology.

1.2

Acknowledgements

1.0

This work was supported partly by the 973 program (2002CB211800) of China.

0.2C rate 1.0C rate 2.0C rate

0.8 0.6

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300 400 500 Cell capacity / mAh

References 600

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Fig. 8. Discharge performance of AAA size NiMH cells using low-cobalt AB5 -type alloy electrodes.

anode of NiMH cells is made up of the low-cobalt AB5 type alloy and combined with a low-cost battery manufacturing technology.

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