Multilayered Cobalt Oxide Platelets for Negative Electrode Material of a Lithium-Ion Battery

Journal of The Electrochemical Society, 155 �12� A903-A908 �2008� A903

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Multilayered Cobalt Oxide Platelets for Negative ElectrodeMaterial of a Lithium-Ion BatteryWenli Yao, Jun Yang,*,z Jiulin Wang, and Yanna Nuli

Department of Chemical Engineering, Shanghai Jiao Tong University, Shanghai, 200240 China

Layer-controllable CoO and Co3O4 platelets were prepared by calcination of hexagonal �-Co�OH�2, which was synthesized via asurfactant-free hydrothermal method. As negative electrode material for lithium-ion batteries, CoO and Co3O4 platelets demon-strated high reversible capacity �more than 800 mAh/g for CoO and 600 mAh/g for Co3O4� and excellent electrochemical cyclingstability. The multilayered CoO platelets showed larger capacity and much better cycling performance than the monolayer CoOplatelets and CoO nanoparticles. The effect of dimension and morphology of CoO particles on the electrode behavior wasdiscussed.© 2008 The Electrochemical Society. �DOI: 10.1149/1.2987945� All rights reserved.

Manuscript submitted May 15, 2008; revised manuscript received August 20, 2008. Published October 7, 2008.

0013-4651/2008/155�12�/A903/6/$23.00 © The Electrochemical Society

High lithium storage capacity, coulombic efficiency, and longcycling life are still the major challenges for designing electrodematerials for rechargeable lithium batteries.1,2 Although graphite-based anode materials are widely used in commercial lithium-ionbatteries due to the excellent charge and discharge cycling behavior,the theoretical Li-storage capacity of graphite is limited to372 mAh/g.3,4 Various materials have been intensively investigatedfor the anode application.5-9 Transition metal oxides, such as cobaltoxides �CoO, Co3O4�, are attractive due to the high Li-storage ca-pacities about three times larger than those of graphite.5,10-13 How-ever, the large irreversible capacity in the first cycle and poor capac-ity retention during charge and discharge cycling restrict theirpractical applications.12-15

As reported, the electrochemical properties of as-synthesizedCoO and Co3O4 strongly depended on their structures andmorphologies.5,8,10 For example, Co3O4 nanotube synthesized viathe anodic aluminum oxide template showed the capacity about500 mAh/g after 100 cycles at the current density of 50 mA/g.8 Louet al. reported a one-step self-supported topotactic transformationapproach for synthesis of needlelike Co3O4 nanotubes, which deliv-ered high capacities �1000 mAh/g for 30 cycles and 400 mAh/gafter 80 cycles.14 Although these synthesized Co3O4 nanotubesshowed higher capacities and better capacity retention than those ofcorresponding nanoparticles, their cycling performance should bestill further improved for the next-generation of lithium-ion batter-ies. Thus far, there has been no report on the synthesis and applica-tion of microcobalt oxides platelets for lithium-ion batteries.

In this study, we report the preparation of layer-controllable CoOand Co3O4 platelets through thermal decomposition of as-synthesized �-Co�OH�2 precursors. The as-synthesized �-Co�OH�2samples with different dimensions were synthesized by a “green”hydrothermal route without organic solvents, toxic reagents, andsurfactants. As negative electrode material for lithium-ion batteries,the resulting mesoporous CoO and Co3O4 samples exhibit largercapacity and much better cycle performance than CoO and Co3O4nanoparticles. The possible reason for the remarkable improvementhas been discussed.

Experimental

All reaction reagents, purchased from Sinopharm Chemical Re-agent Co. �China�, are analytic grade reagents and directly usedwithout further treatment. In a typical reaction, 0.56–4.47 g ofCo�NO3�2·6H2O were dissolved into 100 mL of aqueous solution ina three-necked round bottom. This fresh solution was stirred for0.5 h under an argon flow. Then 20 mL of aqueous solution contain-ing 0.5–2.0 g of ammonia �25 wt % NH3·H2O� was added into thesuspending solution and aged for 10 min to form the Co�OH�2 gel at

* Electrochemical Society Active Member.z E-mail: [email protected]

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room temperature. The Co�OH�2 gel precursor was then transferredinto a Teflon-lined autoclave with 75–80% filling ratio in argon andhydrothermally treated at 120°C for 4 h without agitation. After theautoclave was cooled down to room temperature, the �-Co�OH�2product can be obtained by filtering and drying under vacuum at70°C. The corresponding CoO and Co3O4 platelets were finally ob-tained by calcining the �-Co�OH�2 precursor respectively at 550°Cin Ar flow for 2 h and at 500°C in air for 2 h. The precursor forCoO or Co3O4 nanoparticles was prepared as follows: 4.47 gCo�NO3�2·6H2O were dissolved into 100 mL of isopropyl alcohol-water �1:1, v/v� solution in a three-necked round bottom. Then20 mL of aqueous solution containing 2.4 g of NH4HCO3 wasadded into the above solution and aged for 2 h to form the precursorat room temperature under Ar. Finally, the precursor was obtainedby filtering and drying under vacuum at 70°C. The CoO and Co3O4nanoparticles were obtained by calcining the dried precursor, respec-tively, at 550°C in Ar flow and at 500°C in air for 2 h.

The resulting samples were analyzed by X-ray diffraction �XRD�on a Rigaku diffractometer D/MAX-2200/PC at a scanning rate of5°/min with a 2� ranging from 20–80°, using Cu K� radiation�1.5406 Å�. Morphology of the powders was observed by scanningelectron microscopy �SEM�. The prepared sample was mixed withethanol under sonication to form dispersion, and a small amount ofthe dispersion was dropped on an aluminum sheet. After the solventevaporation, SEM micrographs were recorded with a JEOL field-emission microscope �JSM-7401F�. Specific surface area was deter-mined by gas adsorption on ASAP 2010 M + C �Micromeritics Inc.USA�.

Electrodes were fabricated by coating a slurry containing80 wt % active material, 10 wt % acetylene black, and 10 wt %polyvinylidene fluoride binder dissolved in N-methyl-2-pyrrolidinone, on a copper foil and drying under vacuum at 120°Cover 3 h. A typical electrode disk contained active material of1.0–1.5 mg cm−2. Electrochemical performance of the compositematerials was examined via CR2016 coin cells with lithium metalcounter electrode, Celgard 2700 membrane separator, and electro-lyte with 1 M LiPF6 dissolved in the mixture of ethylene carbonateand dimethyl carbonate �DMC� �1:1, v/v�. The test cells were as-sembled in an argon-filled glove box containing �1 ppm each ofoxygen and moisture. Cyclic voltammogram measurements wereperformed using a CHI660A electrochemical workstation at a scan-ning rate of 0.2 mV/s. Galvanostatic charge �delithiation� and dis-charge �lithiation� was cycled between 3 and 0.01 V vs Li+/Li onLAND CT2001A cycler at a current density of 100 mA/g at 25°C.

Results and Discussion

The phase purity and crystallinity of CoO and Co3O4 plateletswere characterized using XRD. Figure 1a shows the XRD pattern ofCoO samples. All the diffraction peaks at �111�, �200�, and �220� canbe indexed as the cubic symmetry of CoO phase �space groupFm3hm, JCPDS card no. 48-1719�. The clear diffraction peaks in

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Fig. 1b represent a typical character of the crystalline face-centered-cubic Co3O4 phase �space group Fm3hm, JCPDS card no. 78-1970�.No peaks related to �-Co�OH�2 phase are observed from CoO orCo3O4 products, indicating the complete decomposition of hydrox-ides under the corresponding experimental conditions.

The �-Co�OH�2 precursor, synthesized from 0.128 M Co�NO3�2solution, can be converted into CoO or Co3O4 under different cal-cination conditions. Figure 2 exhibits SEM images of �-Co�OH�2,CoO, and Co3O4 platelets. A typical SEM image of the obtained�-Co�OH�2 sample shown in Fig. 2a indicates a multilayered hex-

Figure 1. XRD pattern of as-synthesized cobalt oxide platelets: �a� CoO and�b� Co3O4.


Figure 2. SEM images of different samples: �a� �-Co�OH�2 precursor, �b�CoO, and �c� Co O .

Figure 3. N2 adsorption/desorption iso-therm and BJH pore size distributions ofcobalt oxides: �a� CoO and �b� Co3O4.



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agonal platelet structure with average discal size of 15 �m diam and6 �m thick. The corresponding SEM images of the obtained CoOand Co3O4 samples �Fig. 2b and c� indicate that the original featuresare fully preserved. The representative N2 absorption/desorption iso-therms shown in Fig. 3 and the corresponding Barret–Joyner–Halenda �BJH� pore size distribution curves �inset� reveal the poros-ity of CoO and Co3O4 platelets after thermal treatments for 2 h.Both samples exhibit bimodal pore distribution with pore sizes ofabout 2–4 nm range and average mesopore sizes of �30 nm. Incontrast, the porous character of the �-Co�OH�2 precursor is notobvious. Brunauer–Emmett–Teller �BET� surface areas of the corre-sponding CoO and Co3O4 platelets after calcinations process are9.88 and 9.32 m2/g, respectively, much larger than 3.16 m2/g forthe initial �-Co�OH�2 samples.

Figure 4 shows cyclic voltammograms of electrodes made fromCoO and Co3O4 platelets at a scan rate of 0.2 mV/s. In the firstcycle, a large cathodic current peak appears at 0.53 V for CoO andat 0.82 V for Co3O4 respectively, representing electrochemicallithiation of these oxides. The cathodic peak at 0.5–0.80 V is alsopartly related to the formation of a polymer/gel-like film, whichoccurs below 0.8 V.16,17 Anodic peaks at 2.08 and 2.06 V corre-spond to the oxidation �delithiation� of CoO and Co3O4. As Poizot etal. have proposed,5,6 the electrochemical reaction mechanism of Li

Figure 4. Cyclic voltammograms of cobalt oxide platelets: �a� CoO and �b�Co3O4, scan rate: 0.2 mV/s.


with transition-metal oxides, such as cobalt oxides, mostly involvesa displacement redox reaction �widely called “conversion reaction”in the lithium battery community� as follows

CoO + 2Li �delithiation

lithiation

Co + Li2O �1�

Figure 5. The electrochemical behaviors of the cobalt oxide electrodes at acurrent rate of 100 mA/g: charge and discharge profiles of CoO platelets �a�and Co3O4 platelets �b�, the capacity-cycle number curves of platelike CoOand Co3O4 �c�. Solid and hollow symbols in �c�, respectively, representlithium insertion and extraction.




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Co3O4 + 8Li �delithiation

lithiation

3Co + 4Li2O �2�

Compared to the first cycle, the main cathodic peak is shifted to1.26 V for CoO and 1.04 V for Co3O4 in the second cycle. Thesignificant difference in the lithiation voltage may be related to thechanges in the CoO and Co3O4 phase structure and the interfacialproperty. The decrease in the following peak intensity and integralareas results from the incomplete conversion reaction and partlyirreversible lithium loss for the formation of the polymer/gel-likefilm.18,19 The peak feature of the third cycle is closer to that of thesecond one, indicating that the electrochemical reversibility isgradually set up after the initial cycle.

The electrochemical performance of as-synthesized CoO andCo3O4 platelets was investigated and compared. Figures 5a and bshow the discharge �lithiation� and charge �delithiation� profiles forCoO and Co3O4 platelets. The voltage trends are well indicative oftypical characteristics of CoO and Co3O4 electrodes,5,12 namely, along voltage plateau at about 0.8 V for CoO �Fig. 5a� and 1.07 V forCo3O4 �Fig. 5b�, followed by a sloping curve down to the cutoffvoltage of 0.01 V during the first discharge step. A small plateau at1.7 V for the first Li insertion into Co3O4 in Fig. 5b is probablyrelated to the formation of the intermediate LixCo3O4 phase.20,21 Thefirst specific lithiation capacity is 1107 mAh/g for CoO and1134.4 mAh/g for Co3O4 platelets, respectively, higher than theirtheoretical values �715 and 890 mAh/g�. The extra capacity mayresult from the formation/dissolution of the polymer/gel-likefilm.10,18,19 Indeed, such polymers that have been intensively studiedturn out to form from a multistep degradation mechanism includingelectrochemical and chemical processes.22 Moreover, the excess re-versible capacity in the sloped regime has also been explained in


terms of a heterogeneous interfacial storage mechanism,23-26 whereexcess lithium is accommodated at the Co/Li2O interface which isformed in the course of the heterogeneous solid-state reaction. Thecoulombic efficiencies for CoO and Co3O4 platelets rapidly risefrom 74 and 75.4% in the first cycle to 99 and 90% in the secondone, respectively, and then an efficiency of mostly �98% is main-tained in the following cycles for both the samples. Two possibilitiescan explain why the charge capacity is lower than the dischargecapacity. First, side reactions such as electrolyte decompositioncould consume a small amount of lithium for the formation of theinterfacial film. Second, the formation nanodispersed metallic Coduring the discharge process is not completely transformed back intoCoO or Co3O4 upon the following charge sweep. Even after100 cycles, the reversible capacity of the CoO sample is still kept at800.8 mAh/g �Fig. 5c�. The capacity fade is rapid during initialcycling for Co3O4, but its following cycles remain stable and thecapacity keeps at ca. 600 mAh/g for the 100th cycle. The multilay-ered CoO platelets show larger capacity and better charge and dis-charge performance than the multilayer Co3O4 platelets.

The CoO platelets with different dimension in Fig. 6 are used aselectrode material for electrochemical lithium insertion and extrac-tion. To study the dimension effect of reaction phase, CoO nanopar-ticles in average size of 50 nm are compared �see Fig. 6d�. In Fig. 6,platelike CoO-1, CoO-2, and CoO-3 respectively, represent the CoOsamples synthesized from starting solutions containing 0.032, 0.064,and 0.128 M of Co�NO3�2. The average discal size and thickness ofCoO-1, CoO-2, and CoO-3 platelets are summarized in Table I. Inspite of the same calcinating temperature and time for the materialpreparation, the cycling performance is considerably different forthe three samples. As shown in Fig. 7, the capacity turns to be stable

Figure 6. SEM images of �a� CoO-1platelets, �b� CoO-2 platelets �c� CoO-3platelets, and �d� CoO nanoparticles.



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after the first cycle for the platelike CoO-3 electrode but declinesnoticeably within 10 cycles for platelike CoO-2. Particularly, rapidcapacity drop occurs within 20 cycles for CoO-1 platelets and CoOnanoparticles. With the increase of the dimension for CoO platelets,the electrode behavior becomes better. The multilayered CoO plate-lets with average discal size of 15 �m diam and 6 �m thick pre-sents larger capacity and much better charge and discharge perfor-mance than the monolayer CoO platelets and CoO nanoparticles.Similarly, the great influence of the particle size of 3d-metal oxideson the electrochemical performances was also proved by the experi-ments carried out on copper oxides.27 As for the multilayered Co3O4platelets, the electrode can remain stable with the capacity of ca.600 mAh/g for 100 cycles �see Fig. 5c�. By contrast, nano-Co3O4electrode shows very high capacity of �1000 mAh/g for initialcycles, but the capacity decreases rapidly to 300 mAh/g after

Table I. Average discal size and BET surface area of differentcobalt oxide materials.

Sample

Averagediameter

��m�

Averagethickness

��m�

BETsurface area

�m2/g�

CoO nanoparticles 0.035 — 20.13CoO-1 2 0.1 15.09CoO-2 8 2 11.36CoO-3 15 6 9.88Co3O4 15 6 9.32

Figure 7. Capacity-cycle number curves of platelike CoO-1, CoO-2, CoO-3,and CoO nanoparticles. Solid and hollow symbols, respectively, representlithium insertion and extraction.


50 cycles �not shown here�.28 A similar result related to nanosizedCoO and Co3O4 electrodes was also observed by Wang et al.12

It has been revealed that the electrochemical lithiation of CoOleads to the formation of Li2O and nanodispersed metallic Co, alongwith the formation of the interfacial polymer/gel-like film.18,19 Morefinely divided active phase will result in more formation of polymer/gel-like film. The formation of this special film eventually consumessome charges and electrolyte solvent for the polymerization, andsimultaneously the outer layer of the active phase may take part inside reactions due to highly reactive nanodispersed Co, leading tothe irreversibility. It has been reported that a fast capacity fadingwould be favored by small particles or agglomerates due to poorelectronic conductivity of the metal-electrolyte interphase and thelost contact between nanograins.27,29 Another disadvantage of nano-sized active particles for electrode application is the particle aggre-gation driven by electrochemical insertion/extraction reactions.5,30

For these reasons, the nanodivided CoO granularity is unfavorablefor the electrochemical stability. However, in view of the volumeeffect of CoO during lithiation/delithiation and the limited reactionkinetics, a simple enhancement of the reactive phase dimension can-not improve the electrode performance. The platelike CoO in a suit-able size not only ensures a short Li+ diffusion depth, but also sup-plies a balance for the incompatible two sides mentioned above. Inaddition, the mesoporous CoO platelets �Fig. 8a� can accommodatepart of the volume expansion on lithium insertion by means of thefree space inside and thus ensure the structural integrity of CoOelectrode over many discharge–recharge cycles. These may explainthe effect of the dimension and morphology of CoO particles on theelectrode behavior. As shown in Fig. 8b, the original dimension andmorphology of the CoO platelet can be maintained even after50 cycles. Note that, for these SEM experiments, the delithiatedCoO platelet electrodes were taken out from cells, washed withDMC, and then dried in an argon dry box. A similar effect of thedimension and morphology on the electrochemical performance hasalso been observed for Co3O4 samples.

Conclusions

Layer-controllable CoO and Co3O4 platelets can be prepared bythermal treatment of highly crystalline �-Co�OH�2 templates, whichwas synthesized by a surfactant-free hydrothermal method. Theelectrochemical reversibility of the CoO and Co3O4 reactants towardlithiation and delithiation is strongly dependent on the dimensionand morphology. The multilayered CoO sample presents larger ca-pacity and much better charge and discharge performance than themonolayer CoO platelets and CoO nanoparticles. In addition, theCoO platelet is superior to the corresponding Co3O4 in the electro-chemical reversibility. The mesoporous and multilayered CoOsample with average discal size of 15 �m diam and 6 �m thickdelivers a stable specific capacity of 800 mAh/g for a life extending

Figure 8. SEM images of �a� a singleCoO platelet and its enlarged one �inset�,and �b� CoO platelets after 50 cycles.




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over hundreds of cycles at a current rate of 100 mAh/g. It is prom-ising for the use as negative electrode material in high-energy re-chargeable lithium-ion batteries.

Acknowledgment

This work was supported by National 973 Program �grant no.2007CB209700�.

Shanghai Jiao Tong University assisted in meeting the publication costsof this article.

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Multilayered Cobalt Oxide Platelets for Negative Electrode Material of a Lithium-Ion Battery

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