Energy Fuels 2009, 23, 4678–4682 Published on Web 08/06/2009
: DOI:10.1021/ef900528d
Effect of Mechanical Milling on the Structure and Electrochemical Properties of Ti2Ni Alloy in an Alkaline Battery Xiangyu Zhao,† Liqun Ma,*,† Xinxin Qu,† Yi Ding,† and Xiaodong Shen*,†,‡ †
College of Materials Science and Engineering and ‡State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology, 5 Xinmofan Road, Nanjing 210009, People’s Republic of China Received May 25, 2009. Revised Manuscript Received July 15, 2009
Mechanical milling (MM) has been used to treat the crystalline Ti2Ni alloy prepared by solid-state sintering. Changes of the structure, morphology, and electrochemical properties were investigated. The particle size of the alloy decreases first and then increases after the milling. Moreover, MM contributes to a decrease in grain size and the formation and increase of the amorphous phase by increasing the milling time, resulting in improvements of the antipulverization ability and cycle life after the milling. The result of linear polarization indicates that the exchange current density, determined by the competition between the specific surface area and amorphous phase, increases first and then decreases. The amorphous phase is beneficial to hydrogen diffusion according to the result of potential-step measurement. The electrochemical properties of Ti2Ni alloy are significantly improved by the non-equilibrium processing technology.
to increase the cycle life of Ti2Ni alloy. Nevertheless, the substitution shows a slight benefit for improving the cycle life of Ti2Ni alloy. Recently, Hu et al.17 showed that (Ti1-xVx)2Ni (x = 0.05-0.3) alloys mainly consisting of the icosahedral quasicrystalline phase have large discharge capacities of approximately 250 mA h g-1, which is much higher than that of the Ti2Ni alloy reported by Luan et al.10 Moreover, the cycle life of Ti2Ni alloy could be significantly improved by the substitution of Ti by V and/or the change of the crystalline structure. It is known that the bulk alloy structure plays an important role in the electrochemical properties of hydrogenstorage alloys.18-21 To our best knowledge, there is no report regarding the effect of the bulk structure on the electrochemical performance of Ti2Ni alloy. Mechanical milling (MM), a process of milling powders, has often been used because of its advantages, such as simplicity, low cost, and facility to produce non-equilibrium structures.22,23 In this work, solidphase sintering and subsequent MM have been used to prepare Ti2Ni alloys. Changes of the structure, morphology, and electrochemical properties of the alloys with different bulk structures were studied.
1. Introduction Many strategies have been employed to find new hydrogenstorage alloys for the application in nickel/metal-hydride batteries in the last few decades.1-6 A2B-type alloys, such as Mg2Ni7 and the Zr-based laves phase,8 have been widely investigated. Moreover, the A2B-type Ti2Ni alloy has also attracted much attention because of its rich interstitial sites for hydrogen storage.9-13 However, severe capacity loss during cycling limited its application, which was attributed to the formation and accumulation of the irreversible Ti2NiH0.5 phase for hydrogen absorption/desorption.10 Elemental substitution of Ni by Al,14 Co,11 K-B,12,15 Nb, and Pd16 was used *To whom correspondence should be addressed. Telephone: þ86-2583587243/7234. Fax: þ86-25-83240205. E-mail:
[email protected] (L.M.);
[email protected] (X.S.). (1) Kuriyama, N.; Sakai, T.; Miyamura, H.; Tanaka, H.; Ishikawa, H.; Uehara, I. Vacuum 1996, 47, 889–892. (2) Hong, K. J. Alloys Compd. 2001, 321, 307–313. (3) Furukawa, N. J. Power Sources 1994, 51, 45–59. (4) Akiba, E. Curr. Opin. Solid State Mater. Sci. 1999, 4, 267–272. (5) Zhao, X. Y.; Ma, L. Q. Int. J. Hydrogen Energy 2009, 34, 4788– 4796. (6) Feng, F.; Geng, M.; Northwood, D. O. Int. J. Hydrogen Energy 2001, 26, 725–734. (7) Niu, H.; Northwood, D. O. Int. J. Hydrogen Energy 2002, 27, 69– 77. (8) Kim, D. M.; Jang, K. J.; Lee, J. Y. J. Alloys Compd. 1999, 293295, 583–592. (9) Luan, B.; Cui, N.; Liu, H. K.; Zhao, H.; Dou, S. X. J. Power Sources 1994, 52, 295–299. (10) Luan, B.; Cui, N.; Zhao, H.; Liu, H. K.; Dou, S. X. J. Power Sources 1995, 55, 101–106. (11) Luan, B.; Cui, N.; Liu, H. K.; Zhao, H. J.; Dou, S. X. J. Power Sources 1995, 55, 197–203. (12) Luan, B.; Cui, N.; Zhao, H. J.; Liu, H. K.; Dou, S. X. Int. J. Hydrogen Energy 1996, 21, 373–379. (13) Luan, B.; Kennedy, S. J.; Liu, H. K.; Dou, S. X. J. Alloys Compd. 1998, 267, 224–230. (14) Luan, B.; Cui, N.; Zhao, H.; Zhong, S.; Liu, H. K.; Dou, S. X. J. Alloys Compd. 1996, 233, 225–230. (15) Luan, B.; Liu, H. K.; Dou, S. X. J. Mater. Sci. 1997, 32, 2629– 2635. (16) Wang, C. S.; Lei, Y. Q.; Wang, Q. D. J. Power Sources 1998, 70, 222–227. r 2009 American Chemical Society
2. Experimental Section Powders of TiH2 (300 mesh, 99.9 atomic %) and Ni (350 mesh, 99.9 atomic %) were mixed together to form a nominal composition of (TiH2)2Ni and, subsequently, cold pressed into a pellet at a pressure of 30 MPa. The pellet was sintered at 850 °C for 8 h and then mechanically crushed into powders below 200 mesh. The powders were loaded into a stainless-steel container with (17) Hu, W.; Wang, J. L.; Wang, L. D.; Wu, Y. M.; Wang, L. M. Electrochim. Acta 2009, 54, 2770–2773. (18) Smardz, L.; Smarda, K.; Jurczyk, M.; Jakubowicz, J. J. Alloys Compd. 2000, 313, 192–200. (19) Drenchev, B.; Spassov, T. J. Alloys Compd. 2007, 441, 197–201. (20) Feng, Y.; Jiao, L. F.; Yuan, H. T.; Zhao, M. Int. J. Hydrogen Energy 2007, 32, 1701–1706. (21) Liu, B. Z.; Wu, Y. M.; Wang, L. M. Electrochim. Acta 2007, 52, 3550–3555. (22) Suryanarayana, C. Prog. Mater. Sci. 2001, 46, 1–184. (23) Koch, C. C. Nanostruct. Mater. 1997, 9, 13–22.
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pubs.acs.org/EF
Energy Fuels 2009, 23, 4678–4682
: DOI:10.1021/ef900528d
Zhao et al.
Figure 1. SEM morphology of Ti2Ni alloys as a function of the milling time: (a) nonmilled, (b) 20 min, (c) 1 h, (d) 5 h, and (e) 10 h.
stainless-steel balls (10 and 6 mm in diameter) under an argon atmosphere. The ball/powder ratio was 20:1. The milling was performed in a vibration mill with a rotation speed of 1200 rpm, and the milling time was 20 min, 1 h, 5 h, or 10 h. X-ray diffraction (XRD) with Cu KR radiation was carried out in a thermo ARL X’ TRA diffractometer system to determine the structure of the milled powders. The morphology observations of the samples were analyzed by a JSM-5610LV scanning electron microscope (SEM). The metal-hydride electrodes were prepared by pressing 0.15 g of alloy powders and 0.45 g of nickel powders into a pellet with 15 mm in diameter under a pressure of 15 MPa. The charge and discharge testing was conducted in a half cell consisting of a metalhydride electrode, a Ni(OH)2/NiOOH counter electrode, and a Hg/HgO reference electrode in a 6 M KOH solution under BT-2000 testing equipment (Arbin, College Station, TX) at 298 K. The electrodes were charged at 60 mA/g for 5 h, allowed to rest for 10 min, and then discharged at 60 mA/g to the cutoff potential of -0.6 V versus the Hg/HgO reference electrode. Linear polarization curves of the metal-hydride electrode were measured at a scan rate of 1 mV/s by scanning the electrode potential from -10 to 10 mV (versus open potential). For potential-step measurement, the electrodes in fully charged state were discharged with a potential step between open circuit potential and -0.6 V for 7200 s. All of the electrochemical tests were carried out at 298 K in the 6 M KOH solution on a CHI 660B electrochemical workstation.
Figure 2. XRD patterns of Ti2Ni alloys as a function of the milling time.
shape after 1 h of milling. Part of the particles agglomerates together, indicating the end of the fracture-dominant stage and that the particles have good ductility. In the second milling stage (above 1 h), the particles are severely deformed and cold-welded together, leading to an increase of particle size to 20-30 μm. The enhancement of the ductility of the alloy particles may be attributed to the change of the bulk alloy structure. When the milling time increases to 10 h, small particles are flattened and coated on the large particles, resulting in a continuous increase of the particle size. This corresponds to the cold-welding-dominant stage. Therefore, particle size and morphology of the powders are determined by the competition between fracture and cold welding. Figure 2 shows the XRD patterns of Ti2Ni alloys as a function of the milling time. The sample nonmilled exhibits the diffraction peaks corresponding to the Ti2Ni phase (PDF card 72-0442) with a cubic structure. The lattice parameter a and the unit cell volume are 11.3143 A˚ and 1448.38 A˚3, respectively. MM contributes to great broadening of the diffraction peaks. A wide diffraction bump at ∼41° appears
3. Results and Discussion 3.1. Mechanical Milling. Figure 1 shows the SEM morphology of Ti2Ni alloys as a function of the milling time. It can be seen that the morphology changes greatly after MM. It is well-known that cold welding and fracture are the two essential processes involved in the milling process.24,25 Two stages during the milling process can be observed, as shown in Figure 1. In the first milling stage (below 1 h), the particle size of the alloy decreases by increasing the milling time. MM contributes to a drastic decrease in particle size after 20 min of milling. This may be explained by the fracture-dominant process. The alloy shows a uniform distribution of particle size, 10-18 μm in average particle size, and a quasi-spherical (24) Zhou, J. B.; Rao, K. P. J. Alloys Compd. 2004, 384, 125–130. (25) Zhao, X. Y.; Ding, Y.; Ma, L. Q.; Shen, X. D.; Xu, S. Y. Int. J. Hydrogen Energy 2008, 33, 6351–6356.
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Energy Fuels 2009, 23, 4678–4682
: DOI:10.1021/ef900528d
Zhao et al.
Table 1. Structural and Electrochemical Data of Ti2Ni Alloys as a Function of the Milling Time samples
grain size (nm)
strain (%)
Cmax (mAh/g)
S50 (%)
I0 (mA/g)
D (10-11 cm2/s)
nonmilled 20 min 1h 5h 10 h
146.5 23.4 21.1 12.4 N/A
0.03 0.29 0.31 0.35 N/A
278.0 212.3 172.3 170.0 124.7
33.0 72.4 73.4 79.5 80.8
44.6 49.2 97.9 78.3 68.1
0.62 0.63 1.11 2.52 4.43
after 20 min of milling. Moreover, part of the diffraction peaks disappears after the milling. Therefore, the milling process reduces the grain size, increases the internal strain, and produces a significant amount of amorphous phase. The grain size and internal strain listed in Table 1 were calculated from the line broadening according to the Hall-Williamson method.7,26 The grain size decreases and the internal strain increases with an increasing milling time. It is known that MM contributes to the formation of defects, such as dislocations, stacking faults, grain boundaries, and subgrain boundaries. The bulk structure can change to a new structure, including the nanocrystalline and/or amorphous state. Table 1 shows that significant changes for the grain size and internal strain occur at the short milling time, while slight changes occur at the long milling time, indicating that the energy introduced by MM is mainly operated on the refinement of the grain size at the initial stage of milling and the formation of amorphization at subsequent stages of milling. Moreover, by comparison to the analysis of SEM, it is noted that the decrease in the grain size occurs during the fracturedominant stage, while a significant amorphous phase is formed during the cold-welding-dominant stage. Similar behavior has already been observed in LaNi5-based alloy.27 The decrease in the grain size may be ascribed to the generation of dislocations and their subsequent interactions during the milling process.28,29 Furthermore, severe deformation occurs after 5 h of milling, leading to a drastic increase in the density of the dislocations28 and, subsequently, a lattice disturbance. When the milling time is 10 h, the alloy is predominantly amorphous. 3.2. Electrochemical Properties. Figure 3 shows the discharge capacities of Ti2Ni electrodes as a function of the cycle number at a current density of 60 mA/g. The nonmilled Ti2Ni alloy has a high initial or maximum discharge capacity of 278.0 mA h g-1, which is much higher than ∼160 mA h g-1 reported by refs 10-13. The Ti2Ni alloy shows rich sites for hydrogen storage. However, its discharge capacity depresses drastically by increasing charge and discharge cycles, only 91.8 mA h g-1 after 50 cycles. This decay is primarily attributed to the formation and accumulation of the irreversible Ti2NiH0.5 phase during the cycles, demonstrated by Luan’s group.10-13 We found that severe pulverization of the nonmilled Ti2Ni alloy occurs during cycling, as shown in Figure 4, resulting in a significant increase of specific surface area, and thus accelerates that deterioration. It is noted that MM is beneficial to a dramatic improvement against that capacity loss. The cycling capacity retention rate, expressed as S50 =C50/Cmax 100% (where Cmax is the maximum discharge capacity and C50 is the discharge capacity at the 50th cycle) of the alloys, is also listed in Table 1. The S50 of the alloy increased from 33.0 to 72.4% after short milling. Therefore, the alloy containing
Figure 3. Discharge capacities of Ti2Ni electrodes as a function of the cycle number at a discharge current density of 60 mA/g.
metastable phases can effectively restrain the capacity loss and/or the formation of the irreversible phase. Nevertheless, MM leads to a decrease of maximum discharge capacity as the amorphous phase increases. This may be ascribed to rich paths for hydrogen transport in the bulk amorphous alloy caused by MM and, thus, few sites for hydrogen storage. However, the amorphous Ti2Ni alloy has the highest S50 of 80.8%. The capacity loss regarding the irreversible phase for hydrogen absorption/desorption is evidently restricted by the structural modification for the crystalline Ti2Ni alloy. Moreover, the pulverization disappears for the alloys after 1 h of milling, indicating that the milled alloy has good strength and toughness. This is consistent with the coldwelding-dominant process mentioned above. The charge-transfer reaction at the electrode/electrolyte interface and the hydrogen diffusion within the bulk electrode are the two dominant factors that determine the electrochemical kinetic properties. They can be expressed as values of the exchange current density (I0) and hydrogen diffusion coefficient (D), respectively. Therefore, the linear polarization technique and potential-step measurement are performed to obtain I0 and D, respectively. Figure 5 shows the linear polarization curves of Ti2Ni alloys as a function of the milling time. The polarization resistance Rp can be determined from the ratio of η to I, and the corresponding exchange current density I0 is obtained from the linearized Butler-Volmer equation for low overpotential regions (
