Hydrothermal Modification and Characterization of Ni (OH) 2 with High

ABSTRACT: The influence of hydrothermal treatment on the crystallinity and discharge capability of Al-substituted R-Ni(OH)2 was discussed in this pape...
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Hydrothermal Modification and Characterization of Ni(OH)2 with High Discharge Capability H. B. Liu, L. Xiang,* and Y. Jin Department of Chemical Engineering, Tsinghua UniVersity, Beijing 100084, China

CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 1 283-286

ReceiVed March 30, 2005; ReVised Manuscript ReceiVed September 15, 2005

ABSTRACT: The influence of hydrothermal treatment on the crystallinity and discharge capability of Al-substituted R-Ni(OH)2 was discussed in this paper. R-Ni(OH)2 with 15% Al substitution prepared at room temperature was converted to a mixture of β-Ni(OH)2 and R-Ni(OH)2 after treatment in alkaline solution at 140 °C for 1.0 h, leading to increase of the tap density and the discharge capacity owing to improvement of the crystallinity, increase of the interlayer distance, and occurrence of more proton vacancies. 1. Introduction Ni(OH)2 is widely used as the positive electrode active material in rechargeable alkaline batteries, such as Cd/Ni, MH/ Ni, Zn/Ni, and Fe/Ni. It crystallizes in a layered hexagonal structure and exists in two polymorphic forms known as R-Ni(OH)2 and β-Ni(OH)2. β-Ni(OH)2 crystallizes with a hexagonal brucite structure with an interlayer distance of co ) 4.60 Å and a Ni-Ni atom distance of ao ) 3.12 Å. The structural changes involved in the electrochemical cycling were illustrated by the famous Bode diagram.1 In most cases, the active material of the positive electrodes belongs to the β structure type owing to its high stability in strong alkaline electrolytes, and the corresponding electrode reaction in the charge-discharge process is as follows:

β-NiOOH + H2O + e a β-Ni(OH)2 + OH-

(1)

This is a one-electron exchange reaction, and the theoretical capacity is 289 mA h per gram of Ni(OH)2. β-Ni(OH)2 has a good reversibility when charged to β-NiOOH, which has a similar layered structure with lattice parameters co ) 4.85 Å and ao ) 2.82 Å. But β-NiOOH may convert to γ-NiOOH, which has an expanded co parameter of 7 Å, when the electrode is overcharged. The large difference between the lattice constants causes mechanical strains, leading to swelling and irreversible damage of the nickel electrode. R-Ni(OH)2, normally having a disordered lamellar structure with a nearly identical co parameter (7.6 Å) to that of γ-NiOOH, can be transformed to the γ-NiOOH phase reversibly without mechanical deformation or constraints. The cycle life and the float charging life can be prolonged owing to diminishment of the swelling phenomenon. Moreover, the R/γ couple can also exhibit a higher theoretical capacity compared with that of the β(II)/β(III) couple owing to the higher electron transfer (1.41.5) involved in the R/γ couple since the oxidation level of nickel in the γ-phase is 3.3-3.7.2-4 But the R-Ni(OH)2 synthesized by conventional methods is labile in alkaline or aqueous media and is readily converted to β-Ni(OH)2 through a dissolution-recrystallization reaction.5-8 The low tap density or volumetric energy density of R-Ni(OH)2 is another main problem inhibiting its commercial application. Recent work has shown that the partial substitution of trivalent or divalent foreign cations such as Al3+,9-21 Co3+,22-27 Fe3+,17,28,29 Mn3+,17,30-32 * Corresponding author. E-mail: [email protected].

Cr3+,17 and Zn2+ 33,34 into the lattice of Ni(OH)2 could improve the stability of R-Ni(OH)2. R-Ni(OH)2 was stabilized by the electrostatic interactions between the cation intercalated into the layer and the anion inserted into the interlayer. Among these elements, Al3+ has attracted much attention because of its high stability. It was reported that the stability of R-Ni(OH)2 in strong alkaline medium could be much improved if more than 20% of Ni was substituted by Al. However, the available capacity and energy density usually decrease with the increase of Al substitution since Al does not take part in the electrochemical redox reaction. Another problem is that the volumetric energy density of R-Ni(OH)2 is usually lower than that of β-Ni(OH)2 due to the lower tap density of the Al substituted R-Ni(OH)2. Therefore, the development of a reliable technical route to synthesize R-Ni(OH)2 with a low Al substitution, a perfect stability, and a high volumetric energy density is a meaningful scientific challenge for exhibiting the advantages of R-Ni(OH)2 in rechargeable alkaline batteries. The electrochemical performance of Ni(OH)2 is connected closely with its synthesis conditions. The influence of the process parameters at normal temperature (lower than 100 °C), such as the concentration of reagent solution, the temperature, pH, the complex agent, and the doping elements, on the microstructure and the electrochemical characteristics of Ni(OH)2 has already been described in many papers. Recently, some researchers have tried to modify the Ni(OH)2 at hydrothermal conditions with the aim of improving the stability and crystallinity of R-Ni(OH)2. Sugimoto14 and Hu21 have developed a method to synthesis Ni(OH)2 with perfect crystallity, long cycle time, and high halfdischarge potential, in which they added the mixing solution containing nickel and aluminum salts into an alkaline solution at room temperature, then treated the slurry directly at 100150 °C for 16-72 h, but problems such as the low tap density (about 1.2 g/cm3) and the long hydrothermal reaction time still inhibited its further application. In this work, an advanced technology has been developed to synthesize Al-substituted Ni(OH)2 particles with high tap density and perfect discharge capability via a specific precipitationhydrothermal modification route. The Al-substituted R-Ni(OH)2 was formed at room temperature by adding alkaline solution slowly into the mixing solution containing NiSO4 and Al2(SO4)3. The solid product formed at room temperature was then treated at hydrothermal conditions in alkaline solution containing NaOH and NH4OH. The samples before and after hydrothermal modification are characterized by X-ray diffraction (XRD), Fourier transform infrared spectronetry (FT-IR), thermogravim-

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Liu et al.

etry (TG), and Raman spectroscopy; the relationship between the structure and the electro-chemical behavior is discussed. 2. Experiment 2.1. Sample Preparation. R-Ni(OH)2 was prepared by adding 100 mL of the alkaline solution composed of 3.0 M of NaOH and 3.0 M NH4OH to 150 mL of a mixed solution containing 0.8 M NiSO4 and 0.05 M Al2(SO4)3, while stirring at 30 ( 2 °C. The reaction was stopped at pH 7.5; the suspension was then filtered, washed with distilled water, and dried at 110 °C for 12 h. Five grams of the precipitate formed at room temperature was then redispersed ultrasonically in 50 mL of alkaline solution containing equal molar concentrations of NaOH and NH4OH (0.5-1.0 M). The suspension was then transferred to a Teflon-lined stainless steel autoclave with an inner volume of 70 mL. After sealing, the autoclave was heated (5 °C/min) to 140 °C and kept for 1.0 h. Then the autoclave was cooled to room temperature naturally; the product was filtered, washed with distilled water, and dried at 110 °C for 12 h. 2.2. Characterization. The structures and the functional groups of the samples were characterized by X-ray diffraction (XRD; model D/max, Rigaku, Japan) and Fourier transform infrared (FTIR) spectrometry (model Nicolet 560, Nico, American), respectively. The thermal behavior of the samples was identified by thermal gravimetric (TG) analysis (model 2050, Beijing Optical Products Co., Ltd., China). The defects of the samples were characterized by Raman spectrometry (model RM2000, Renishaw, United Kingdom).The Ni and Al contents were determined by the ethylenediaminetetraacetic acid (EDTA) titration method. 2.3. Preparation of the Ni(OH)2 Electrode. The active material paste containing 90% Ni(OH)2 and 10% Co(OH)2 was inserted into a nickel foam with porosity over 95%, using 0.5% carboxymethyl and 1% polytetrafluoroethylene as the binder agents. After drying at 110 °C for 2.0 h, the above electrode was sandwiched between two identical Ni foam electrodes and pressed at 50 MPa for 5 min. 2.4. Evaluation of Electrochemical Properties. The Ni(OH)2 electrode was immersed into the alkaline electrolyte containing 6.0 M NaOH and 0.3 M LiOH, using a Ni foam as the counter electrode and Hg/HgO (6.0 M NaOH) as the reference electrode. The cell was then charged at 30 mA/g for 15 h, rested for 1.0 h, and then discharged at 100 mA/g until the potential decreased to 0.1 V. The charge-discharge experiment was repeated until the discharge capacity became stable. The test at high discharge current was performed and repeated at the following conditions: charged at 30 mA/g for 15 h, rest for 0.5 h, discharged at 1500 mA/g to -0.4 V.

3. Results and Discussion Figure 1 shows the XRD patterns of the samples before and after hydrothermal treatment. For the sample without hydrothermal treatment (sample a), the large asymmetric peak at 35.16° reflected the typical turbostratic feature of R-Ni(OH)2.35 The interlayer distances (d values) were 7.07 Å according to the (003) peak at 12.52° and 3.76 Å according to the (006) peak at 23.68°, indicating periodic disorder in the (003) planes since the d values were not increased in multiple proportion.30 For sample b, the R-Ni(OH)2 peaks became sharper and symmetric after hydrothermal treatment, indicating the improvement of the crystallinity and the conversion of the turbostratic structure to the regular plate structure.22,27 The (003) peak shifted from 12.52° to 11.34°, corresponding to an increase of the interlayer distance from 7.07 to 7.80 Å. The increase of the interlayer distance was favorable for the intercalation of H2O into the Ni1-xAlxO2 layers and the acceleration of the proton diffusion since H2O molecules among the layers usually acted as a proton diffusion passage along the layers.14,20-21 The occurrence of peaks at 2θ ) 19.2°, 38.7°, and 52.0° showed the formation of the β-Ni(OH)2 phase with the lattice parameters of a ) 3.08 Å and c ) 4.62 Å. The IR spectra of the samples before and after hydrothermal treatment are given in Figure 2. The broad band in the region

Figure 1. XRD patterns of the samples before (a) and after (b) hydrothermal treatment: ([) R-Ni(OH)2; (b) β-Ni(OH)2.

Figure 2. IR spectra of the samples before (a) and after (b) hydrothermal treatment.

of 3000-3800 cm-1 was ascribed to the stretch vibration of O-H. In the case of sample a, the peak at 1630 cm-1 was ascribed to the bend vibration of H2O; the peaks at 1120 cm-1 (υ3 vibration), 1038 cm-1 (υ3 vibration) and 603 cm-1 (υ4 vibration) were attributed to the interaction between SO42- in C3V symmetry and the metallic cations; the low interlayer distance (7.07 Å) induced from XRD data indicated that SO42ions may be adsorbed on the surface of Ni1-xAlxO2 layers since the interlayer distance should be in the range of 8.8-9.4 Å if SO42- were inserted into the interlayers.26 In the case of sample b, the sharp peak at 3650 cm-1 represented the free OH group in the β-Ni(OH)2 phase.4 The appearance of the peak at 1370 cm-1 (corresponding to the υ3 vibration of CO32- in D3h symmetry26) and the shrinkage of the peak at 1120 cm-1 indicated that some of the SO42- ions may be replaced by CO32ions after hydrothermal treatment, and the CO32- ions were hydrogen bonded between the layers.26 The intercalation of CO32- into the Ni1-xAlxO2 layers may lead to an increase of the interlayer distance and enhancement of the stability of R-Ni(OH)2. Figure 3 displays the thermal behavior of the samples before and after hydrothermal treatment, where W0 and WT represent the weight of sample before heating and at temperature T, respectively. The TG curve for the sample a can be divided

Ni(OH)2 with High Discharge Capability

Crystal Growth & Design, Vol. 6, No. 1, 2006 285

Figure 4. Raman spectra of the samples before (a) and after (b) hydrothermal treatment.

Figure 3. TG curves of the samples before (a) and after (b) hydrothermal treatment.

into three steps. The weight loss (18.0%) in the range of 40295 °C may be connected with the loss of the water adsorbed on the layers or intercalated in the interlayers. The weight loss (14.7%) in the range of 295-675 °C may be connected with the conversion of R-Ni(OH)2 to NiO, Al2O3, and NiSO4. The weight loss (7.1%) in the range of 675-1000 °C may be connected with the conversion of NiSO4 to NiO and SO3. The TG curve for sample b can be divided into three steps. The weight loss (10%) in the range of 40-268 °C was attributed to the loss of the water. The weight loss in 268-336 °C (7.5%) and in 336-400 °C (6.2%) may be connected with the decomposition of the mixing phases of R-Ni(OH)2 and β-Ni(OH)2. Chemical analysis indicated that 44.0 wt % of Ni and 3.5 wt % of Al were contained in sample a, corresponding to 48.1 wt % of Ni and 4.3 wt % of Al in sample b. The increased amount for Ni and Al was mainly due to the decrease of water content (from 18% to 10%, as shown in Figure 3) after hydrothermal treatment. The tap densities for samples a and b were 1.1 and 1.7 g/cm3, respectively. The higher tap density of sample b may be attributed to the formation of β-Ni(OH)2 and the improvement of R-Ni(OH)2 crystallinity. Sample a can be expressed as Ni0.85Al0.15(SO4)0.11(OH)1.93‚1.18H2O based on the chemical/ TG analysis and the general formula of aluminum-substituted R-Ni(OH)2, (Ni1-xAlx(SO4)x/2(OH)2‚mH2O). The Raman spectra of the samples before (a) and after (b) hydrothermal treatment are shown in Figure 4. The peak at 470 cm-1 corresponded to the stretch vibration of Ni-O and the peak at 550 cm-1 represented the stretch vibration of the deprotonated Al-O in Ni(OH)2.35-38 The enlargement of the peak at 550 cm-1 and the shrinkage of the peak at 460 cm-1 in sample b indicated that hydrothermal treatment was favorable for the formation of proton defects (Figure 4). The discharge curves of the samples before (a) and after (b) hydrothermal treatment under different discharge currents are shown in Figure 5. In the case of low discharge current (100 mA/g), the hydrothermal treatment led to the increase of the discharge capacity from 246 to 290 mA h/g, corresponding to the increase of the electron-transfer number per Ni atom from

Figure 5. Discharge curves of the samples before (a) and after (b) hydrothermal treatment at discharge current (mA/g) of (1) 100 and (2) 1500.

1.2 to 1.3. In the case of high discharge current (1500 mA/g), the discharge capacity of sample b (260 mA h/g) was bigger than that of sample a (172 mA h/g), indicating a higher specific power for sample b, which was favorable for the manufacture of advanced electrodes. It is known that the electrochemical performance of the nickel electrode depends on the proton mobility and the electrical conductivity. The hydrothermal treatment of the R-Ni(OH)2 led to the improvement of the crystallinity, which reduces the polarization, the increase of the interlayer distance, which is favorable for the diffusion of the proton, and the increase of the proton vacancies, which can improve the proton mobility and electrical conductivity, thus producing materials with high discharge capability. 4. Conclusion A 15% Al-substituted R-Ni(OH)2 was synthesized at room temperature via a coprecipitation route, using NiSO4 and Al2(SO4)3 as the reactants and NaOH and NH4OH as the coprecipitation agents. The R-Ni(OH)2 was converted to a mixture composed of R-Ni(OH)2 and β-Ni(OH)2, leading to increase of the tap density and the discharge capacity owing to improvement of crystallinity, increase of interlayer distance, and the occurrence of proton vacancies. Acknowledgment. This project was supported by National Natural Science Foundation of China (Grant No. 50174032) and the Open Fundation of Tsinghua University.

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