UCTEA Chamber of Metallurgical & Materials Engineers Proceedings Book

770 IMMC 2016 | 18th International Metallurgy & Materials Congress

Development of Rare Earth-Free Negative Electrode Materials for Ni/MH Batteries

Ezgi Onur Şahin, Cavit Eyövge, Tayfur Öztürk

Middle East Technical University - Türkiye

Abstract In this study, activation behavior of AB2 type (Ti0.36Zr0.64)(V0.15Ni0.58Mn0.20Cr0.07)2 alloy, is investigated. Effects of particle size, ball milling and surface modification via NAFION coating were considered. Galvanostatic cycling in open cells showed that bare alloy initially had almost no capacity, but reached a value of 220 mAh/g after 14 cycles. Experiments showed that coarse particles activate faster yielding an improved capacity 245 mAh/g. The surface modification via NAFION coating yielded improved performance with regard to high rate dischargeability. 1. Introduction

Several alternatives are available as negative electrode material in NiMH batteries. Among them, AB2 type Laves phase alloys are promising negative electrode materials [2]. It is possible to attain high electrochemical discharge capacity and good cyclic performance in these electrodes, even higher compared to rare earth alloys if proper treatments were applied [3]. Nevertheless, activation requirement is the key issue that limits the application of these alloys in practice.

There are several methods proposed for activation of AB2 alloys. Liu et al. [4] and Matsuoka et al. [5] used ball milling for activating the alloy which creates fresh new surfaces. McCormack et al. [6] have claimed that the application of voltage pulses in cycling helps breaking the oxide layers in the particles, and also reveals fresh surfaces as in the case of milling. An alternative approach that may be adopted for activation would be to aim for surface modification. Thus Sun et al. [7] achieved to obtain activated C15 Laves phase alloy by milling it with Ni powder. It was claimed that Ni rich particle formed on the surface of the particles could be activated easily due to the catalytic effect of Ni.

This study focuses on the activation behavior of an AB2 alloy, namely (Ti0.36Zr0.64)(V0.15Ni0.58Mn0.20Cr0.07)2 which has a Laves phase structure. A number of methods; particle size refinement, ball milling, and surface modification via NAFION coating were used

so as to activate the alloy. The aim of the study is to determine suitable methods and parameters for activating the alloy faster and attaining a high capacity especially at high discharge rates.

2. Experimental Procedure

The AB2 alloy used in this study had a composition of (Ti0.36Zr0.64)(V0.15Ni0.58Mn0.20Cr0.07)2. Rietveld refined X-ray spectrum of this alloy is given in Fig. 1. It is seen that the alloy has a C14 Laves phase crystal structure. Calculated lattice parameters are a=0.4958 and c=0.8091 nanometers. A second phase is also present in this alloy as there are additional peaks. The second phase present measured by EDS had a composition of (Ti0.50Zr0.50)Ni [8]. The phase is however quite small not more than 5%.

Electrochemical charge-discharge capacity of the alloys was measured with a three electrode cell exposed to open atmosphere. For negative electrode, the active powder was mixed with copper powder (1:3) and pressed under a pressure of 400 MPa into pellets 10 mm in diameter. A nickel mesh was used to wrap the pellet and the electrode was connected to the galvanostat with a nickel wire spot welded to it. The counter electrode was a nickel mesh spot welded to a Ni wire. Nickel mesh had a larger surface area than that of the negative electrode.

Fig.1 Rietveld refined X-ray diffraction pattern of AB2 alloy. Note that the alloy has a C14 Laves Phase crystal structure. Also note presence of additional peaks arising from the second phase.

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Fig.2 SEM Micrograph of AB2 alloy powder taken from a sample mounted on epoxy polished and etched to reveal the phase distributions. The reference electrode used is a zinc rod (99.995 %) of 4 mm in diameter, although the use of Zinc electrode has its problems, it was not critical for the purpose of this study. The measurements were carried out in 6 M KOH electrolyte with a battery tester Land CT 2001A. Charging was done at 50 mA/g. For comparison two discharge rate regimes were experimented: one was 50 mA/g and the other was significantly higher 320 mA/g. Following charging for a duration of 8 h, the alloys were discharged down to a cut-off voltage of -0.75 V vs. Zn.

3. Results and Discussion

Electrochemical performance of the powders in the as–received condition is given in Fig. 3. The initial capacity was found to be very low, but increased with cycling and reached a value of 220 mAh/g at 14th cycle.

PCT curve of the current alloy yields a reversible hydrogen storage capacity of 1.18 wt. % hydrogen at a hydrogen pressure of 1 atm. This corresponds to an electrochemical discharge capacity of 316 mAh/g. It was surprising that this capacity was not reached in the as-received powders even after 20th cycle. In order to activate the alloy to obtain maximum attainable capacity, three activation methods were used; particle size refinement, ball milling and surface modification via NAFION coating

Particle Size Refinement: As received alloy powders were sieved to and separate groups were collected. Collected powders divided into three batches having average particle size, d(0.5), of 82.5, 62.7 and 37.3 microns.

Discharge capacity of each batch were measured with respect to the cycle number, and plotted as seen in the Fig. 3. There was no capacity measured in the first few cycles as in the case of as-received powder. However,

coarse powders, having average particle size larger than 37.3 microns, reached the maximum discharge capacity of 245 mAh/g only after 7th cycle. Although there is a decrease in the discharge capacity with further cycling, this was thought to be due to particle drop-out and was not considered to be the real behavior.

Fine powders on the other hand, behaved quite similar to the as-received powders. The maximum capacity reached was less. This was thought to be due to the presence of isolated powders, i.e. not in contact with the electrode. Following an initial 20 cycle, the electrode was tested with a high discharge rate. However, there was a drastic reduction in discharge capacity for all samples.

Ball Milling: As-received powders were ball milled to investigate the effects of particle size reduction on activation. 10 mm stainless steel balls were used in milling with a ball-to-powder ratio of 20:1 in a 250 cc vial. Milling was carried out using RETSCH PM 400 planetary ball mill with 250 rpm rotation speed. Powders were milled for durations of 1h, 3h and 5h. SEM micrographs of milled powders are given in Fig. 4. Fragmentation of the coarse powder particles were clearly seen after 1h of milling. With further milling particles agglomerate yielding larger particle sizes. Discharge capacities of the milled powders are shown plotted in Fig.5 against cycle number. It is clearly seen that milling had a positive effect on activation and that all milled powders seem to have reached nearly their respective capacity after 6th cycle. The maximum capacity attained in the milled powders varied with the milling time. For 1h, the discharge capacity was 330 mAh/g, which is slightly above the expected capacity of 316 mAh/g, while 3h and 5h yielded capacities of 260 mAh/g and 210 mAh/g respectively.

Fig. 3 Effect of particle size on discharge capacity. First 20 cycles refer to moderate discharge rate 50 mA/g, and the last 20 cycles refer to fast discharge rate, 320 mA/g.

UCTEA Chamber of Metallurgical & Materials Engineers Proceedings Book

772 IMMC 2016 | 18th International Metallurgy & Materials Congress

Fig. 4. SEM images of pristine powder, the powder milled for 1h and 5h.

Fig. 5. Discharge capacity vs. cycle number for powders milled for 1h, 3h and 5h. First 20 cycles refer to moderate discharge rate 50 mA/g, and the last 20 cycles refer to fast discharge rate, 320 mA/g.

As can be seen from Fig. 5, there was some positive effect on rate dischargeability Especially after 1h milling the capacity at high rate discharge rate was not too far off that of moderate charging. NAFION coating: For NAFION coating a pellet was prepared in the usual way. Thus AB2 powder were pressed with copper powder. A drop of NAFION solution (5% in isopropyl alcohol) was applied and the pellet was dried. This was repeated 4 times 2 for each surface. Coated pellets were dried in the furnace at 80°C for 12 hours. The discharge capacity versus cycle number data were plotted in Fig. 6. It can be seen that the activation behavior was not improved with NAFION coating. Capacity reached after 20 cycles, though the profile was different, were similar to the pristine alloy. The capacity with high discharge rate in AB2 modified by nafion coating was exceptionally good and reaches 349 mAh/g. In addition, the capacity does not seem to have saturated at the end of the 40th cycle.

Fig. 6. Discharge capacity vs. cycle number for powders coated with NAFION. Note that the high rate discharge behavior is better than that of the pristine powder. 4. Conclusions The effects of particle size, ball milling and surface modification by NAFION coating on activation behavior and high rate performance of the (Ti0.36Zr0.64)(V0.15Ni0.58Mn0.20Cr0.07)2 Laves phase AB2 alloy were examined. From the current study the following remarks can be made: i) Large particle size provides faster activation. This is due to fragmentation of large particles exposing fresh new surfaces. The capacities with fine particle were less. But they show better discharge capacity at higher discharge rates ii) The milled powders also provide better activation yielding a capacity of (349 mAh/g) after 1 h of milling. However prolonged milling decreases the saturation capacity. This decrease was attributed to the presence of isolated fine particles not in contact with the electrode. iii) NAFION coating did not lead to any improvement in the activation behavior. However, the alloy performance was improved significantly at high discharge rate.

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This work is in progress and improved performance reported with nafion coating is being re-examined if it is real or due to some artifacts. 5. Acknowledgements We gratefully acknowledge the financial support of TUB TAK (Project No 112M193). We also acknowledge BASF (Kwo Young) for the powder used in this study. A more comprehensive treatment of activation of AB2 is reported by Tan et al. given below. 6. References [1] Tan S., Shen Y., Onur ahin E., Noreus D., Öztürk

T. Activation behavior of an AB2 type metal hydride alloy for NiMH batteries. Int. J. Hydrogen Energy 2016. http://dx.doi.org/10.1016/j.ijhydene.2016.03.196

[2] Cuevas F. Joubert J.-M., Latroche M., Percheron-Guegan A. Intermetallic compounds as negative electrodes of Ni/MH batteries. Appl. Phys. A 2001; 72:225–238.

[3]Young K, Chao B, Bendersky LA, Wang K. Ti12.5Zr21V10Cr8.5MnxCo1.5Ni46.5 x AB2-type metal hydride alloys for electrochemical storage application: Part 2. Hydrogen storage and electrochemical properties J Power Sources 2012; 218:487-94.

[4] Liu B-H, Jung J-H, Lee H-H, Lee K-Y, Lee J-Y. Improved electrochemical performanc of AB2-type metal hydride electrodes activated by the hot-charging process. J Alloy Compd 1996; 245:132-41.

[5] Matsuoka M, Tamura K. Effects of mechanical grinding on initial activation and rate capability of Zr-Ti based Laves phase alloy electrode. J Appl Electrochem 2007; 37:759-64.

[6] McCormack M, Badding ME, Vyas B, Zahurak SM, Murphy DW. The role of microcracking in ZrCrNi hydride electrodes. J Electrochem Soc 1996;143(2): L31-3.

[7] Sun D, Latroche M, Percheron-Guegan A. Activation behaviour of mechanically Ni-coated Zr-based Laves phase hydride electrode. J Alloy Compd 1997; 257:302-05.

[8] Song X, Chen Y, Sequeira C, Lei Y, Wang Q, Zhang Z. Microstructural evolution of body-centered cubic structure related Ti–Zr–Ni phases in non-stoichiometric Zr-based Zr–Ti–Mn–V–Ni hydride electrode alloys. J Mater Res, 2003;18:37-44.