Synthesis and Morphological Control of Nickel Hydroxide for Lithium

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Synthesis and Morphological Control of Nickel Hydroxide for Lithium-Nickel Composite Oxide Cathode Materials by an Eddy Circulating Precipitation Method Jutang Sun, Jinguo Cheng, Chiwei Wang, Xiaoling Ma, Ming Li, and Liangjie Yuan* College of Chemistry and Molecular Science, Wuhan UniVersity, Wuhan 430072, People’s Republic of China

Spherical nickel hydroxide with high density was synthesized via an eddy-circulating precipitation method, in which a Ni(NO3)2 solution was reacted with a NaOH solution at 65-70 °C, and NH4NO3-NH3‚H2O was used as a buffer reagent. The resulting nickel hydroxide particles, with a tap density of 2.20 g/cm3 and the average particle size of 30.6 µm, are aggregated by the granular crystallites. The morphology of the crystallite can be controlled by changing the anions. The LiNi0.8Co0.2O2 that has been prepared using this nickel hydroxide shows excellent electrochemical performance, which delivers an initial discharge capacity of 190.90 mAh/g at a current rate of 25 mA/g between 3.0 V and 4.35 V and exhibits good cycle stability. Introduction

Scheme 1. Flow Chart for the Synthesis of Nickel Hydroxide

Nickel hydroxide is not only widely used for the cathode material of rechargeable alkaline batteries (Ni/Cd, Ni/MH, Ni/ H2, and Ni/Zn batteries),1-5 but also is applied in the manufacture of lithium-nickel composite oxide cathode material for lithium ion batteries.6-9 Many methods for the synthesis of nickel hydroxide have been reported, such as solid-state reaction,10 homogeneous precipitation,11,12 electrochemical method,13 ion-exchange resin method,14 and chemical precipitation method,15-18 etc. Recently, Subbaiah et al.19 and Konishi et al.20 prepared nickel hydroxide via a complexation-precipitation route and hydrolytic stripping in organic solvent, respectively. The chemical precipitation method is extensively used in the industry, in which nickel hydroxide is generally prepared from NiSO4 solution, because the SO42- ions obviously facilitate precipitation and decrease the absorbed impurities on the surface of nickel hydroxide particles.15,16 Mrha and co-workers determined that the presence of some SO42- anions in nickel hydroxide benefits the electrochemical performance of alkaline Ni/Cd batteries;21 however, the nickel hydroxide prepared from Ni(NO3)2 shows lower activity. Lithium-nickel composite oxides are considered to be promising cathode materials for lithium ion batteries to replace commercial LiCoO2, because of their low cost and high specific energy.22-26 Nickel hydroxide, especially spherical nickel hydroxide, is the primary raw material used to synthesize lithium-nickel composite oxides by diverse routes, such as solid-phase reaction,27,28 low-temperature synthesis,29 microwaveassisted synthesis,30 and the sol-gel method.28 The effect of SO42- remnants in cathode materials for lithium ion batteries is different from the case in Ni(OH)2-based alkaline batteries. We detected that the electrochemical activities of the prepared lithium-nickel composite oxides using Ni(OH)2 from NiSO4 were lower than that from Ni(NO3)2. In this paper, the objective of the work is the preparation of spherical nickel hydroxide with high density that is suitable for the production of lithium-nickel composite oxide cathode materials for lithium ion batteries, and a novel eddy-circulating precipitation method is provided to synthesize Ni(OH)2. The particle morphological control and properties of nickel hydroxide are investigated. The influence of Ni(OH)2 from different raw materials on electro* To whom correspondence should be addressed: Tel.: +86-2787218264. Fax: +86-27-68754067. E-mail: [email protected].

chemical performances of the lithium-nickel composite oxides are discussed. Experimental Section Materials. Ni(NO3)2‚6H2O, NiSO4‚6H2O, NaOH, NH4NO3, and NH3‚H2O (25-28 wt %) were all analytical-grade reagents. Solutions were prepared using deionized water. Ni(NO3)2 and NaOH Solutions. On a per mole of Ni(NO3)2 solution basis, 0.3 mol of NH4NO3 and 0.7 mol of NH3‚H2O was added to form a buffer solution, and the Ni2+ ion concentration was controlled to be within the range of 1.8-2.0 mol/L. There was no precipitation of Ni(OH)2 in the solution, and the pH was 7. The molar concentration of NaOH solution was 3.6-4.0 mol/L. Apparatus and Procedure. Scheme 1 shows the flowchart for the synthesis of nickel hydroxide by the eddy-circulating precipitation method. The reaction installation for the eddy-circulating precipitation method is shown in Figure 1. It consisted of a cylindrical stainless steel reaction vessel with a sleeve, a windstick stirrer, two automatic liquid feeders for Ni(NO3)2 and NaOH solutions, and a container to collect effluent. The inner diameter of the reaction vessel was 13 cm, and the height was 18 cm. The stirrer with the diameter of 6 cm was placed 2 cm above the bottom of the vessel.

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Figure 1. Schematic diagram of the device for the synthesis of Ni(OH)2 via the eddy-circulating precipitation method.

First, a proper amount of deionized water was added to the reaction vessel, heated to 65-70 °C, and churned up to create an eddy circulation. The Ni(NO3)2 and NaOH solutions were respectively fed, in an alternating fashion, into the reaction vessel from the top via the feeders. The feeding rates of Ni(NO3)2 and NaOH solutions were both controlled at a rate of 20-30 times per minute, with a volume of 1-2 mL per time. The reaction temperature was 65-70 °C, and the pH was 11-12. The stirring speed was 300-400 rpm. After the reaction ended, the nickel hydroxide was sedimentated for 12 h in the reaction vessel. The precipitate then was washed with the deionized water, to remove the Na+ and NO3- ions, and dried at 130 °C. Grass-green Ni(OH)2 (a) product with excellent dispersivity and fluidity was obtained, and the yield reached 98%. In addition, Ni(OH)2 (b) product was synthesized from NiSO4 via the same method and technology, under the same conditions. Sample Analysis. X-ray diffraction (XRD) patterns were collected using a Shimadzu model XRD-6000 diffractometer with Cu KR1 radiation (λ ) 1.54056 Å) in the range of 10°80° (2θ) at a scanning rate of 4°/min. Specific surface areas were measured (Micromeritics, model ASAP-2020) according to the Brunauer-Emmett-Teller (BET) multipoint method, via nitrogen physisorption at 77.35 K. The tap density was obtained by placing nickel hydroxide powders in a 25-mL graduated cylinder and tapping it until a constant volume was obtained. The particle size distribution was obtained using a laser granulometer (Malvern, model Mastersizer-2000), and water was used as the dispersing agent. The particle morphology was observed using scanning electron microscopy (SEM) (Hitachi, model SEM X-650). Infrared (IR) spectra were collected using a Nicolet model Avatar-36 FT-IR spectrometer in KBr pellets. The chemical analysis of NO3- and SO42- was performed using ion chromatography (IC) (Dionex model ICS-2500, coupled with column AS11-HC). Preparation of Cathode Material for Lithium Ion Battery. LiNi0.8Co0.2O2 cathode material was prepared using LiOH‚H2O, Co2O3, and Ni(OH)2 as the starting materials by the rheological phase reaction method31,32 and the once sintering method at 750 °C for 10 h in an oxygen atmosphere. The electrochemical cell consisted of a LiNi0.8Co0.2O2 working electrode and a lithium foil counter-electrode. The electrolyte was a 1 mol/L solution of LiPF6 dissolved in a 1:1 mixture (by volume) of ethylene carbonate (EC) and diethyl carbonate (DEC). To obtain a discharge-charge profile of the materials, the cells were subjected to 15 cycles in the voltage range of 4.35-3.00 V vs Li+/Li at a current rate of 25 mA/g. Results and Discussion Synthesis of Nickel Hydroxide. The preparation of Ni(NO3)2 solution is an important step to synthesize spherical nickel

Figure 2. XRD pattern of the nickel hydroxide from Ni(NO3)2.

Figure 3. Granularity distribution of nickel hydroxide prepared using Ni(NO3)2 (curve a) and NiSO4 (curve b).

hydroxide. In the Ni(NO3)2 solution, adding a suitable amount of NH4NO3 before adding ammonia can avoid the formation of nickel hydroxide precipitate in the raw solution. If NH4NO3 is not added into the Ni(NO3)2 solution beforehand, but, instead, ammonia is added directly, the nickel hydroxide precipitate will be easily formed. After the precipitate is formed, the pipeline system would be blocked. Herein, NH4NO3 and NH3‚H2O formed a buffer system, which could promote the formation of a nickel-ammonia complex and restrain the NH3‚H2O ionization to NH4+ and OH- ions. In the course of reaction with NaOH solutions, the nickel-ammonia complex has the role of controlling the release of Ni2+ ions. It could decrease the precipitate reaction speed, which is beneficial to the growth of Ni(OH)2 particles. The eddy circulation and feeding mode, operated in an alternating fashion, are the key techniques for performing spheroidization and obtaining high density, which can effectively decrease nucleation speed and the chance of nucleation of nickel hydroxide. In this eddy-circulating system, OH- and Ni2+ ions are alternatingly adsorbed and deposited on the surface of the formed Ni(OH)2 particles that are ceaselessly rotating. The eddy circulation is occurring continuously, and big spherical particles are obtained. This method has many advantages, such as small reactor volume, high yield, and controlled particle size and morphology. In addition, the products can be prepared in batch mode as well as continuously. Characterization of Products. The XRD patterns of Ni(OH)2 (a) are shown in Figure 2. All the diffraction peaks are indexed to the hexagonal phase of β-nickel hydroxide, with lattice constants of a ) 3.126 Å and c ) 4.605 Å, which is consistent with those reported in the literature (JCPDS File Card No. 140117). The specific surface area is 9.28 m2/g, and the tap density is 2.20 g/cm3, which is close to the industry standard of 2.12.2 g/cm3 for spherical nickel hydroxide.33 Figure 3 shows the granularity distribution of Ni(OH)2 powders. The particle size

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Figure 4. Scanning electron microscopy (SEM) images of nickel hydroxide prepared using (a) Ni(NO3)2 and (b) NiSO4.

Figure 5. Infrared (IR) spectra of nickel hydroxide prepared using Ni(NO3)2 (spectrum a) and NiSO4 (spectrum b).

distribution of Ni(OH)2 (a) is narrow, with an average size of 30.6 µm (d10 ) 14.4, d50 ) 28.6, and d90 ) 50.4 µm). The average size of Ni(OH)2 (b) is 31.5 µm (d10 ) 12.4, d50 ) 26.0, and d90 ) 51.8 µm), and the tap density is 2.05 g/cm3. Figure 4 shows the SEM images of nickel hydroxide powders. The Ni(OH)2 (a) are close-grained and compact spherical-like particles, which consist of huge amounts of small granules and are different from Ni(OH)2 (b). The latter is the noncompact aggregation of needlelike crystallites. It is revealed that the anions have the role of controlling the granule shape and crystallite morphology of nickel hydroxide in precipitation process.34 The spherical particles and the narrow granularity distribution result in high density. In addition, the particle size and the density also can be controlled by changing the reaction conditions, such as concentration, feeding rate, and stirring speed. Figure 5 shows the IR spectra of nickel hydroxide. The strong bands at 3630 cm-1 correspond to the presence of OH-. The bands around 1100-1 and 1040 cm-1 are characteristic absorption bands of SO42- anions, and the sharp band at 1380 cm-1 corresponds to the presence of NO3-. Effect on Electrochemical Properties of Lithium-Nickel Composite Oxide. The LiNi0.8Co0.2O2-A and LiNi0.8Co0.2O2-B cathode materials were prepared using Ni(OH)2 (a) and (b), respectively. Figure 6 shows the initial charge/discharge and cycle stability curves of these materials. The discharge plateau was mainly located at 4.2-3.5 V, and the charge plateau was in the range of 3.6-4.35 V. The LiNi0.8Co0.2O2-A has an initial charge capacity of 215.5 mAh/g, followed by a discharge capacity of 190.9 mAh/g. The Coulombic efficiency is 88.6%. After 15 cycles, the discharge capacity still kept at 180.2 mAh/g with an average capacity fade of 0.37% per cycle. For LiNi0.8Co0.2O2-B, the first charge capacity is 212.4 mAh/g, followed by a discharge capacity of 173.5 mAh/g. The Coulombic efficiency is 81.7%. After 15

Figure 6. Initial charge/discharge curves and cycle performance of LiNi0.8Co0.2O2-A and LiNi0.8Co0.2O2-B at a current rate of 25 mA/g and in the voltage range of 3.0-4.35 V. Table 1. Chemical Analysis of NO3- and SO42- in the Samples by Ion Chromatography (IC) sample

NO3- content (wt %)

SO42- content (wt %)

Ni(OH)2 (a) LiNi0.8Co0.2O2-A Ni(OH)2 (b) LiNi0.8Co0.2O2-B

0.09392 0 0 0

0 0 1.597 1.459

cycles, the discharge capacity is 159.8 mAh/g. It is obvious that the former has better electrochemical performance. However, the latter shows more irreversible capacity loss and capacity fade, because of the fact that a trace quantity of residual sulfate from the raw material remains. The Ni(OH)2 synthesized by the chemical precipitation method contains some anions such as NO3- or SO42- unavoidably.1,17 Those impurities obviously affect the electrochemical performances of the prepared LiNi0.8Co0.2O2 materials. Table 1 shows the chemical analysis of the NO3- and SO42- ion (in terms of weight percent) in the samples by ion chromatography (IC). It is obvious that the trace of NO3- could be eliminated completely in the sintering process for LiNi0.8Co0.2O2 cathode materials, but the SO42- is difficult to remove. Therefore, the

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optimal nickel hydroxide suited to the lithium-nickel composite oxide cathode materials for lithium ion batteries is the one that is prepared using nitrate, but not sulfate. Conclusions The spherical nickel hydroxide was synthesized by means of an eddy circulating precipitation method from Ni(NO3)2. The spherical particles are agglomerated by large amounts of granular crystallites. The morphology and particle size can be controlled by changing the nickel salts and reaction conditions. The advantages of this technique are as follows: small reactor volume, high yield, controlled particle size and morphology, and flexible preparation methods for the products (they can be prepared in batch mode as well as continuously, etc.). The prepared Ni(OH)2 with high density and narrow particle size distribution is very suitable to prepare lithium-nickel composite oxide cathode materials for lithium ion batteries. The LiNi0.8Co0.2O2 prepared with this Ni(OH)2 from Ni(NO3)2 exhibits good cycle stability, the initial discharge capacity is 10% higher than that prepared with the Ni(OH)2 from NiSO4. Acknowledgment This work was supported by the National Natural Science Foundation of China (No. 20471044). Literature Cited (1) Casas-Cabanas, M.; Hernandez, J. C.; Gil, V.; Soria, M. L.; Palacı´n, M. R. Rationalization of the industrial nickel hydroxide synthetic process in view of optimizing its electrochemical performances. Ind. Eng. Chem. Res. 2004, 43, 4957. (2) Dell, R. M. Batteriessfifty years of materials development. Solid State Ionics 2000, 134, 139. (3) McBreen, J. Nickel hydroxides. Handb. Battery Mater. 1999, 135. (4) Ko¨hler, U.; Antonius, C.; Ba¨uerlein, P. Advances in alkaline batteries. J. Power Sources 2004, 127, 45. (5) Gille, G.; Albrecht, S.; Meese-Marktscheffel, J.; Olbrich, A.; Schrumpf, F. Cathode materials for rechargeable batteriesspreparation, structure-property relationships and performance. Solid State Ionics 2002, 148, 269. (6) Park, B.; Kim, Y.; Cho, J. Cathodes based on LiCoO2 and LiNiO2. Lithium Batteries 2004, 410. (7) Reisner, D. E.; Wang, M.; Ye, H.; Xiao, T. D.; Strutt, P. R.; Salkind, A. J.Nanostructured cathode materials for alkaline and lithium rechargeable batteries. Prog. Batteries Battery Mater. 1998, 171. (8) Iizaka, H.; Kubota, T.; Suzuki, S. Nickel hydroxide and its manufacture by crystallization for lithium nickelate and secondary battery. Jpn. Kokai Tokkyo Koho JP 2003261334, 2003. (9) Sato, S.; Araki, Y.; Nakayama, M.; Tamura, S. Manufacture of nickel hydroxide particles for Li-Ni complex oxides as positive electrode materials. Jpn. Kokai Tokkyo Koho JP 11060246, 1999. (10) Liu, X.; Yu, L. Synthesis of nanosized nickel hydroxide by solidstate reaction at room temperature. Mater. Lett. 2004, 58, 1327. (11) Soler-Illia, G. J.; Jobba´gy, M.; Regazzoni, A. E.; Blesa, M. A. Synthesis of nickel hydroxide by homogeneous alkalinization. Precipitation mechanism. Chem. Mater. 1999, 11, 3140. (12) Kato, A.; Kajiwara, A. Synthesis of spherical particles of Ni(OH)2 by a homogeneous precipitation method. AdV. Sci. Technol. (Faenza, Italy) 2003, 30, 515. (13) Jayashree, R. S.; Kamath, P. V. Nickel hydroxide electrodeposition from nickel nitrate solutions: mechanistic studies. J. Power Sources 2001, 93, 273. (14) Zhang, X.; Hu, Z.; Zhao, C. Production of ultrafine particles of Ni(OH)2 and NiO by method of basic ion-exchange resin. Gongneng Cailiao 2000, 31, 109.

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ReceiVed for reView September 18, 2005 ReVised manuscript receiVed January 16, 2006 Accepted January 31, 2006 IE051048U