Electrochimica Acta 54 (2009) 6161–6165

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Electrochimica Acta

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Preparation and electrochemical performance of micro-nanostructured nickel

Xiaoyan Han, Feng Zhang, Jiangfeng Xiang, Caixian Chang, Jutang Sun !

Department of Chemistry, Wuhan University, Wuhan 430072, PR China

a r t i c l e i n f o

Article history:Received 24 March 2009Received in revised form 11 May 2009Accepted 16 May 2009Available online 27 May 2009

Keywords:Rheological phase reaction methodMicro-nanostructured nickelAnodeLi ion batteries

a b s t r a c t

Micro-nanostructured nickel has been prepared as anode materials for Li ion batteries, via a rheologicalphase reaction method. Ni2C2O4·xH2O (x = 2 or 2.5) as precursors are obtained from the solid–liquidrheological mixture of (NH4)2C2O4·H2O and Ni(NO3)2. The nickel powders are prepared by thermaldecomposition of the precursors. The structural, morphological and electrochemical performance areinvestigated by means of thermogravimetry (TG), differential scanning calorimetry (DSC), powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscope (TEM) andtypical electrochemical tests. The micro-nanostructured nickel displays an initial discharge capacity of457 mAh g"1. It also has a remarkable cycling stability with an average capacity fade of 0.17% per cyclefrom 13th to 50th cycle in 0.01–3.00 V versus Li at a constant current density of 100 mA g"1.

© 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Nanosized materials with novel properties present a widepotential application in many fields [1–6]. Constructions of well-ordered and realizations of their potential applications haveresulted in intensive research for the past few years. The demandsof superfine metal powders have increased dramatically in theelectronics, biotechnology, powder metallurgy and energy sourcesfields. Micro-nanostructured nickel with unique morphology, sizeand structure may exhibit superior functionality or provide newpossibilities due to the composite structure, porosity, stability andthe inherent properties of nanosized materials.

So far as we know, there are many researches on nickel includingalloy used as anode materials [7–12]. However, a well-known factis that the pulverization, which brings the fast capacity fade duringcycling, is the key issue to restrict metal or alloy being widely usedas the anode materials for Li ion batteries. There have been manyattempts to resolve the cracking and crumbling problems of metalor alloy materials for Li ion batteries. Nanosized particles or com-posite materials have been considered. Nevertheless, just reducingthe particle size or synthesizing simple composite materials can-not effectively improve the cycleability or capacity of the anodematerials [10].

Various methods have been used to prepare superfine nickelpowders such as mechanical milling, chemical reduction, evap-oration, polyol process, spattering and atomize method [13–18].

! Corresponding author. Tel.: +86 27 87218494; fax: +86 27 68754067.E-mail address: jtsun@whu.edu.cn (J. Sun).

However, some of the above-mentioned methods need rigorousreaction conditions such as high temperature or high pressure,some need organic solvent, and some need the addition of extraprecipitator to the reaction system, which cause the safety prob-lem, complicated manufacture equipment, high production cost,poor quality, low production efficiency and so on. As a result, thedesign and synthesis of nanosized materials is the current subjectof intense research.

In this study, micro-nanostructured nickel was prepared viaa rheological phase reaction method. Rheological phase reactionis a simple, economical and effective route to prepare functionalmaterials [19–25], which does not need complicated processes. Thethermal decomposition of the precursors carried out at 300–400 #Cin a self-assembly device without protective gas.

2. Experimental

2.1. Material preparation

(NH4)2C2O4·H2O, Ni(NO3)2·6H2O and anhydrous ethanol wereall analytical-grade reagents. Solutions were prepared by use ofdeionized water.

(NH4)2C2O4·H2O (48.87 g) was added in 5.0 M nickel nitratesolution [Ni(NO3)2·6H2O (100 g), H2O (20 mL)] at 70–90 #C to forma rheological body with viscoelasticity. The ropy rheological bodywas fully dispersed with an appropriate amount of deionized wateradded, then filtered, washed, dehydrated with anhydrous ethanoland dried at 80–90 #C to yield NiC2O4·2H2O (Precursor A), the over-all yield was 99.76%. The nickel nitrate was added in supersaturatedammonium oxalate solution to give NiC2O4·2.5H2O (Precursor B),the overall yield was 97.98%.

0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved.doi:10.1016/j.electacta.2009.05.069

6162 X. Han et al. / Electrochimica Acta 54 (2009) 6161–6165

Fig. 1. TG–DTG–DSC curves of Precursor A (a) and B (b) in covered Al crucible.

An appropriate amount of nickel oxalate was cased in sealedvessel. The air inside expanded and let out slowly when the thermaldecomposition was carried out, which ensured the precursor wassaturated with self-atmosphere. The vessel was heated up to 335 !Cin air furnace with a heating rate of 5 !C min"1, kept at 335 !C for6 h to give a black sample. The nickel obtained from the PrecursorA and B was marked as NiA and NiB, respectively.

2.2. Material characterization

The powder X-ray diffraction (XRD) patterns of the precursorsand the products were collected by a Shimadzu model XRD-6000diffractometer with a Ni filter and Cu-K!1 radiation (! = 1.54056 Å)in the range of 10–60! (2") and 10–120! (2") with a scanningrate of 4! min"1, respectively. The refined crystal lattice parame-ters were calculated by the JADE5.EXE procedure (Rietveld analysisof the powder XRD patterns using on a step-scan mode with a stepof 2! in the 2" range of 20–130!). Chemical analysis was carriedout by inductively coupled plasma atomic emission spectrometry(ICP-AES, model IRIS, TJA). The particle size and morphology wereobserved using scanning electron microscope (SEM, Hitachi 400)and transmission electron microscope (TEM, JEM-2010 FEF). Thethermal analysis experiments were carried out in covered Al cru-cible with a Netzsch STA 449C thermal analyzer from 50 to 600 !Cat a heating rate of 5 !C min"1. The sample mass was kept about6.0–8.0 mg.

2.3. Electrochemical measurements

The electrochemical experiments were examined on a Newarecell test system at room temperature. The charge–discharge testswere carried out with the coin-type cell (size: 2016), which con-sisted of a working electrode and a lithium foil counter electrodeseparated by a Celgard-2300 microporous membrane. The workingelectrode was prepared by mixing the nickel powders, acetyleneblack and polytetrafluoroethylene (PTFE) binder, in a weight ratioof 80:15:5, compressing the mixture onto a stainless steel currentcollector. A 1 M LiPF6 EC/DMC (1:1, volume ratio) was used as theelectrolyte. The cells were assembled in an argon-filled glove box(Mikrouna, Super 1220/750, China). The cells were discharged andcharged between 0.01 and 3.0 V versus Li at a constant currentdensity of 100 mA g"1.

3. Results and discussion

3.1. Thermal decomposition reaction

Fig. 1 presents the TG–DTG–DSC curves of Precursor A(6.223 mg) and B (7.620 mg). The specific temperature and masslosses are labeled on DSC–TG curves. Analysis shows that themolar compositions of Precursor A and B are NiC2O4·2H2O andNiC2O4·2.5H2O, respectively. The DSC peaks closely correspondto the weight changes observed on the TG curves. The thermal

X. Han et al. / Electrochimica Acta 54 (2009) 6161–6165 6163

Fig. 2. XRD patterns of oxalate nickel precursors (a) and nickel products (b).

decomposition generally proceeds in two steps: dehydration anddecomposition of the anhydrous oxalate. The mass loss before200 !C, characterized by a small endothermic peak on DSC curves, isascribed to the dehydration of adsorptive water. XRD analysis con-firms that the final products of thermal decomposition in Al crucibleare nickel powders, which is different from the common reports onthermal decomposition of nickel oxalate in air [26,27]. The thermaldecomposition reaction formula of the precursors can be expressedas follows (x = 2 or 2.5):

NiC2O4·xH2O " NiC2O4 + xH2O (1)

NiC2O4 " Ni + 2CO2 (2)

But thermal decomposition behaviors of the two precursors aredifferent: the decomposition rate of Precursor B is twice than thatof Precursor A (shown in DTG curves).

3.2. Powder X-ray diffraction analysis

Fig. 2a shows the XRD patterns of Precursor A and B preparedvia rheological phase reaction method. The partial refined crys-tal lattice parameters calculated by the JADE5.EXE procedure arepresented in Table 1. All characteristic diffraction peaks of two pre-cursors are well indexed to the monoclinic phase of nickel oxalate,with the space group P2/m (No. 10). The broken line in Fig. 2a

Table 1Lattice parameters of Precursor A and B.

Parameter Precursor A Precursor B

S. G. P2/m (No. 10) P2/m (No. 10)a (Å) 7.8167 (16) 9.8874 (40)b (Å) 2.6607 (6) 4.2409 (19)c (Å) 5.9066 (11) 6.5671 (47)V (Å3) 122.84 266.13

Table 2Lattice parameters of NiA and NiB.

Parameter NiA NiB

S. G. Fm3̄m(225) Fm3̄m(225)a (Å) 3.5232 (1) 3.5245 (1)V (Å3) 43.73 43.78Z 4 4Dcalc (g cm#3) 8.9150 8.9058

indicates that the two precursors are different from each other.Diffraction relative intensity and peak position of two XRD patternsare different from those reported in the literature (JCPDS File CardNo. 25-0581).

XRD patterns and lattice parameters of the products are givenin Fig. 2b and Table 2. In order to get an accurate lattice param-eters silicon element (internal standard) is added to compensatefor sample displacement error (Fig. S4 and Fig. S5, Supplemen-tary Information). Results show that lattice parameters and theposition of main peaks of face centered cubic nickel are well consis-tent with the standard file (JCPDS File Card No. 04-0850), provingthe sole existence of nickel particles. No sign of the formation ofoxides species could be observed in Fig. 2b. All diffraction peaksare very sharp, which indicates that the high crystallinities of theproducts.

3.3. Particle size and morphological characterization

Particle sizes and morphology changes throughout the ther-mal decomposition are shown in Figs. 3 and 4 by SEM and TEMobservations. The particles size and morphology of the PrecursorA and B are obviously different from each other. The PrecursorB consists of monodispersed quadrate crystals with the parti-cle sizes of 0.6–1.0 !m (Fig. 3b1). The Precursor A consists ofmany irregular block crystals with the particle sizes of 0.1–0.2 !m(Fig. 3a1). After thermal decomposition, Precursor A changes intoclose-grained spherical heteromorphy with the average particlesizes about 0.1 !m (Fig. 3a2 and Fig. 4a3). The product NiB withthe average particle sizes about 0.5 !m keeps the quadrate mor-phology of the Precursor B and the single quadrate crystal stillconsists of many nanospheres with the average particle sizes about20 nm (Fig. 3b2 and Fig. 4b3). The results from SEM and TEM imagesagree well with the observations from the XRD and electrochemicalmeasurements.

3.4. Electrochemical characterization

Fig. 5 shows the typical discharge and charge curves of NiA andNiB electrode at a constant current density of 100 mA g#1 between0.01 and 3.0 V. The three obvious plateaus in the first dischargecurve suggest that there are three Ni atoms and three Li atoms ina unit cell. More detailed results of the formation and compositionof the LiNi alloy are given in Table S1, Table S3 and Fig. S2, Supple-mentary Information. The reaction mechanism of intercalation ofLi ions can be expressed as follows:

1.60–0.84 V 1/3Li + Ni " Li1/3Ni (3)

0.84–0.62 V 1/3Li + Li1/3Ni " Li2/3Ni (4)

0.62–0.01 V 1/3Li + Li2/3Ni " LiNi (5)

The initial discharge capacity of NiA electrode is 376 mAh g#1

and the coulombic efficiency is 55.3%. However, the initial dischargecapacity of NiB electrode is 457 mAh g#1 and the coulombic effi-ciency is 62.4%. The low initial coulombic efficiency is attributedto the formation of the alloy framework and the solid electrolyteinterface (SEI) film. As shown in Fig. 5, the first discharge curve is

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Fig. 3. SEM images of Precursor A (a1) and NiA (a2), Precursor B (b1) and NiB (b2).

characterized by four plateaus. The small plateau at about 1.5 V cor-responded to the irreversible capacity loss, which disappears in thefollowing cycles. The NiB electrode delivers higher initial dischargeand charge capacity of 457 and 285 mAh g!1, respectively. The rea-son for the higher initial capacity of NiB electrode may be attributedto the particle size and the microstructure. The deintercalation pro-cess of Li ions between the layers of the anode is a diffusion process[25]. Therefore, the larger or close-grained particles, which would

lead to a long path for Li ions, are not advantageous to deintercala-tion.

Fig. 6 shows the cycling performances of NiA and NiB elec-trode. The NiA electrode delivers an initial reversible capacityof 208 mAh g!1 and maintains 87 mAh g!1 in the 50th cycle. Incomparison, the NiB electrode delivers an initial reversible capac-ity of 285 mAh g!1. In the 50th cycle, the charge capacity stillretains 143 mAh g!1, which is much better than NiA electrode. After

Fig. 4. TEM images of NiA (a3) and NiB (b3).

X. Han et al. / Electrochimica Acta 54 (2009) 6161–6165 6165

Fig. 5. Discharge–charge curves for the typical cycles of NiA (a) and NiB (b) over 0.01–3.0 V.

Fig. 6. Cycling performance of NiA and NiB over 0.01–3.0 V.

13 cycles there is little capacity fade of the NiB electrode, theaverage capacity fade of 0.17% per cycle. It indicates that the micro-nanostructure is stable enough for intercalation–deintercalationof Li ions and the NiB electrode has suppressed the pulverization[10]. The rate of capacity fade of the NiA electrode is faster thanthat of NiB electrode, which may be related to the particle sizes.The NiB particles with unique structure and appropriate particlesize show the good electrochemical performance. Moreover, theNiB particles keep the quadrate morphology; this microstructureavoids the agglomeration of nanosized particles and reduces thecontact between the nanosized particles and electrolyte. This loosestructure can buffer the volume changes of the nickel electrode andavoids the collapse of the composite structure.

4. Conclusions

In conclusion, two different nickel oxalate precursors have beenprepared using a rheological phase reaction route. Two differ-ent nickels (NiA and NiB) have been successfully got by thermaldecomposition of the precursors, which confirmed the effects ofthe precursors on the final products from the structure, particlesize and morphology. Electrochemical tests show that NiB with themicro-nanostructure shows better cycling ability and electrochem-ical performance.

Acknowledgements

This work was supported by The National Natural Science Foun-dation of China (No. 20771087).

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.electacta.2009.05.069.

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