Journal ofMaterials Chemistry A

PAPER

Publ

ishe

d on

03

July

201

3. D

ownl

oade

d by

RM

IT U

ni o

n 23

/08/

2013

16:

27:1

3.

View Article OnlineView Journal | View Issue

aSchool of Materials Science and Enginee

50 Nanyang Avenue, Singapore 639798. E-

edu.sgbTUM-CREATE Center for Electromobility,

Nanyang Drive, Singapore 637459

† Electronic supplementary information (EFESEM images, the comparative cycling pethe tted data extracted from the EIS spe

Cite this: J. Mater. Chem. A, 2013, 1,10935

Received 18th April 2013Accepted 2nd July 2013

DOI: 10.1039/c3ta11549f

www.rsc.org/MaterialsA

This journal is ª The Royal Society of

The facile synthesis of hierarchical porous flower-likeNiCo2O4 with superior lithium storage properties†

Linlin Li,ab Yanling Cheah,a Yahwen Ko,a Peifen Teh,a Grace Wee,a Chuiling Wong,a

Shengjie Peng*a and Madhavi Srinivasan*ab

In this work, we demonstrate the facile fabrication of 3-dimensional (3D) hierarchical porous flower-like

NiCo2O4 and its application as an anode material in high-performance lithium ion batteries (LIBs). The

uniform flower-like NiCo2O4 is built from porous nanoplates with thicknesses of approximately 25 nm.

A detailed investigation reveals that PVP plays an important role, not only in controlling the formation

of the delicate hierarchical flower-like structure, but also in creating the uniform pores of each

nanoplate. Furthermore, a possible formation mechanism for this unique structure is proposed based on

the experimental results. As a virtue of its beneficial structural features, the as-prepared NiCo2O4

exhibits an enhanced lithium storage capacity and excellent cycling stability (�939 mA h g�1 at 100 mA

g�1 after 60 cycles). This remarkable electrochemical performance can be attributed to the hierarchical

structure and sufficient void space within the surface of the nanoplates, which effectively increases the

contact area between the active materials and the electrolyte, reducing the Li+ diffusion pathway and

buffering the volume change during cycling.

Introduction

Hierarchical porous structures with unique surface propertiesand superior architectural features have been an attractive andimportant family of functional materials in recent years.1–3 Inparticular, it is anticipated that 3D hierarchical structuresassembled from low dimensional building blocks shouldinherit the unique advantages of their pristine building blocksand obtain additional benets from their superior secondaryarchitecture. Furthermore, porous subunits should endowthese superior structures with intriguing properties such as anincreased surface area, facilitating the diffusion of foreignsubstances throughout the bulk material, relieving mechanicalstress and so on.4,5 Therefore, the combination of poroussubunits with a hierarchical architecture could provide bettercontrol for the tailoring of various properties and meet therequirements for application in a myriad of elds includingwater treatment,6 catalysis,7 magnetic devices,8 and, particu-larly, applications related to energy storage devices.9,10

The key to alleviating environmental and energy issues is theoptimization of energy storage devices. The precise design of

ring, Nanyang Technological University,

mail: sjpeng@ntu.edu.sg; madhavi@ntu.

Nanyang Technological University, 62

SI) available: TGA curves, FTIR spectra,rformance of the different samples andctra. See DOI: 10.1039/c3ta11549f

Chemistry 2013

materials with advanced functions provides one of the mostdesirable ways of developing high-performance energy devices.Recently, various nanostructured transition metal oxides (TMO)have been exploited as candidates for potential application inenergy-related devices.11–13 As an important member of the TMOfamily, spinel nickel cobaltite (NiCo2O4), an abundant multipleoxidation state ternary metal oxide, appears to be a promisingcandidate for various applications, including electrocatalyticwater splitting,14 photodetectors,15 and energy storagedevices.16–19 To date, NiCo2O4, especially its 1D nano-structures,20–26 has been extensively investigated as an electrodematerial for supercapacitors because of its notable electricalconductivity and electrochemical activity, compared to pureNiO and Co3O4.27,28 The attractive features of NiCo2O4 arebelieved to be of huge benet for anode materials in LIBs.However, there are only a few reports on the application ofNiCo2O4 as an anode material for LIBs.29,30 Therefore, researchgaps still exist in this eld. In addition, it is well known that theelectrochemical performance of LIBs highly depends upon theunique structural properties of the electrode materials (partic-ularly for the anode), including the surface texture, morphol-ogies, and particle sizes, etc. Therefore, the synthesis of NiCo2O4

with a rationally designed micro-/nanostructure is imperative ifit is to be used as an anode material in high-performance LIBs.Generally, electrode materials with hierarchical porous struc-tures exhibit many advantageous properties, which are favor-able for improving the electrochemical performance of LIBs,such as their ability to alleviate volume changes, shorten Li+ andelectron diffusion pathways, improve the electrode–electrolyte

J. Mater. Chem. A, 2013, 1, 10935–10941 | 10935

Journal of Materials Chemistry A Paper

Publ

ishe

d on

03

July

201

3. D

ownl

oade

d by

RM

IT U

ni o

n 23

/08/

2013

16:

27:1

3.

View Article Online

interface, and enhance the structural stability.9,10 So, it can beenvisioned that a hierarchical porous NiCo2O4 material couldcombine the merits of not only hierarchical nanostructures, butalso porous morphologies.

Herein, we report the synthesis of a micro-sized hierarchicalporous ower-like NiCo2O4, which is constructed of 2D nano-plates, through a facile hydrothermal approach followed by asuitable calcination in air. Remarkably, a detailed investigationdemonstrates that PVP is crucial in controlling the morphologyof the as-prepared NiCo2O4 and is responsible for the formationof a uniform distribution of pores on the surfaces of thenanoplates. Meanwhile, a PVP-induced self-assembly mecha-nism may account for the formation of the ower-like NiCo2O4.Due to its unique architecture, the porous ower-like NiCo2O4

yields a high initial discharge capacity of 1508.7 mA h g�1 andhas excellent cycling stability (939 mA h g�1 at 100 mA g�1 aer60 cycles), indicating that the 3D hierarchical porous ower-likeNiCo2O4 could serve as a promising candidate for next-genera-tion LIBs. More importantly, our present work may shedsome light on the easy preparation of hierarchical porousmicro-/nanostructures and the development of materials withadvanced functions for energy storage.

Experimental sectionPreparation of hierarchical porous ower-like NiCo2O4

All chemical reagents were used as received. A typical prepara-tion process of hierarchical porous ower-like NiCo2O4 wasconducted as follows: 0.1 g of poly(vinylpyrrolidone) (PVP,Mw ¼360 000 g mol�1) was rst dissolved in a mixed solution con-taining 15 ml of ethanol and 15 ml of deionized water at roomtemperature, followed by the addition of 0.5 mmol ofNi(NO3)2$6H2O and 1 mmol of Co(NO3)2$6H2O, under stirring.Aer being stirred for a further 10 min, the mixture was trans-ferred to a 50 ml Teon-lined stainless steel autoclave. It wasthen heated to 180 �C andmaintained at this temperature for 20h. Aer cooling to room temperature, the precipitate wascollected, washed with deionized water and ethanol, and nallydried at 80 �C for 10 h. In order to obtain crystallized NiCo2O4,the dried product was calcinated at 450 �C, under air, for 1 h togive hierarchical porous ower-like NiCo2O4.

Characterization

The crystallite structures and morphologies of the sampleswere characterized using powder X-ray diffraction (Bruker D8Advance, Cu-Ka radiation, l ¼ 1.5418 A), a eld emissionscanning electron microscope (FESEM, JEOL JSM-7600F) withan energy dispersive X-ray (EDX) attachment, and a trans-mission electron microscope (TEM, JEOL 2100F). The surfacearea analysis was conducted using Brunauer–Emmett–Teller(BET) theory (Micromeritics, ASAP 2020). Fourier transforminfrared (FTIR) spectroscopy was performed using a PerkinElmer Spectrum GX instrument between 400 to 4000 cm�1.Thermogravimetric analysis (TGA, Q500) was carried outusing a temperature ramp of 10 �C min�1 under an airatmosphere.

10936 | J. Mater. Chem. A, 2013, 1, 10935–10941

Electrochemical measurements

The working electrode was prepared using the active material,polyvinylidene uoride (PVDF) and a conductive agent (Super-P-Li carbon) in a weight ratio of approximately 6 : 2 : 2. Theelectrochemical properties were measured using a CR 2016 coincell assembly, with metallic lithium as the counter electrode,Celgard 2400 as the separator and 1 M LiPF6 in ethylenecarbonate–diethylene carbonate (EC : DEC ¼ 1 : 1) as the elec-trolyte. The cells were assembled in an Ar-lled glove box. Thegalvanostatic discharge–charge cycling (0.005–3 V) and CyclicVoltammetry (CV) (0.005–3.0 V, 0.1 mV s�1) were measuredusing a Neware battery tester at different current densities and aSolartron 1470E, respectively. Electrochemical impedancespectroscopy (EIS) was conducted in the frequency range100 kHz to 0.1 Hz, applying a 10 mV bias voltage. The data wereanalyzed using Zplot and Zview soware (version 2.2).

Results and discussion

Hierarchical porous ower-like NiCo2O4 can be obtained bycalcining a Ni–Co-based intermediate (prepared via a hydro-thermal method at 180 �C for 20 h) at 450 �C for 1 h. The phasepurity and crystallinity of the NiCo2O4 were characterized usingXRD, as shown in Fig. 1a. All of the diffraction peaks can beunambiguously indexed and assigned to cubic NiCo2O4 with aspinel structure (JCPDS card no. 73-1702, a ¼ 8.114 A) and noother phases or impurities can be detected. Elemental compo-sition analysis of the NiCo2O4 obtained from EDX corroboratesthe XRD study (Fig. S1, ESI†). Moreover, according to the TGAresults (Fig. S2, ESI†), there is a sharp weight loss between 100and 400 �C, conrming that the calcination temperature of450 �C, in air, is high enough to ensure the formation ofNiCo2O4 as a single phase and that thorough elimination of PVPpolymer occurs, which is consistent with previous reports.26–28

Furthermore, the transformation of the Ni–Co-based interme-diate to NiCo2O4 is also supported by the results of the FTIRspectroscopy (Fig. S3, ESI†), with the disappearance of bandsassigned to PVP and the appearance of bands belonging toNiCo2O4.31–33 The morphology of the Ni–Co-based intermediateand as-prepared NiCo2O4 were then analyzed using FESEM. Theintermediate product is formed of many uniform and discretemicrospheres with sizes around 1.5 mm (Fig. S4a, ESI†). Thesemicrospheres possess a 3D hierarchical ower-like nano-structure, which is constructed from 2D nanoplates withsmooth surfaces (Fig. S4b and c, ESI†). Remarkably, the 3Dhierarchical ower-like structure is maintained even aerthermal treatment and there is no discernible structuralcollapse or breakage in the sample, revealing excellent struc-tural stability, as shown in Fig. 1b and c. In addition, it can beseen that the neighboring nanoplates are loosely inter-connected and that obvious open spaces exist between them.More importantly, a large number of small pores can be clearlyobserved on the surface of the nanoplates, whichmay have beencreated by the release of gas during the crystal transformation at450 �C (Fig. 1d). The transmission electron microscope (TEM)image shown in Fig. 1e shows conspicuous hierarchical porous

This journal is ª The Royal Society of Chemistry 2013

Fig. 2 N2 adsorption/desorption isotherm of hierarchical porous flower-likeNiCo2O4. The inset shows the pore size distribution.

Paper Journal of Materials Chemistry A

Publ

ishe

d on

03

July

201

3. D

ownl

oade

d by

RM

IT U

ni o

n 23

/08/

2013

16:

27:1

3.

View Article Online

structures that look like owers and are composed of nano-plates. As shown in Fig. 1f, the typical thickness of a nanoplateis about 25 nm and in each nanoplate there are a lot of nano-pores with sizes ranging 5–20 nm. The lattice fringe has aninterplanar spacing of 0.469 nm (Fig. 1g), corresponding to the(111) crystal plane of the cubic NiCo2O4 phase, which furtherindicates the formation of NiCo2O4 under these conditions. Inaddition, the SAED pattern (see the inset in Fig. 1g) indicatesthat the material has polycrystalline features, with diffractionrings that can be assigned to the (311), (111), (220), and (440)planes of the cubic NiCo2O4 phase, which is in agreement withthe XRD analysis.

The porosity of the NiCo2O4 is further investigated using BETanalysis. Fig. 2 depicts the N2 adsorption/desorption isothermand the corresponding Barrett–Joyner–Halenda (BJH) pore-size-distribution plot (inset of Fig. 2) of the sample. The isothermcan be described as a type IV isotherm with H3 hysteresis loopsin the relative pressure range of 0.5–0.95, suggesting the pres-ence of a mesoporous structure. The BET surface area of thesample is around 80.1 m2 g�1. Moreover, the pore size distri-bution (Fig. 2, inset) indicates that the majority of the pores areless than 25 nm, which corroborates the TEM observations. Inaddition, the N2 adsorption/desorption isotherm of the as-prepared intermediate (Fig. S5, ESI†) shows no apparenthysteresis loop, indicating the absence of a porous structure,which results in a BET surface area of about 15.7 m2 g�1. Thisfurther demonstrates that the calcination process leads to theformation of a porous structure and an increase of the specic

Fig. 1 (a) XRD pattern; (b, c and d) FESEM images at different magnifications;(e and f) TEM images; and (g) HRTEM image of the hierarchical porous flower-likeNiCo2O4 after calcination at 450 �C for 1 h in air. The inset in (g) shows the cor-responding SAED pattern.

This journal is ª The Royal Society of Chemistry 2013

surface area of the hierarchical ower-like NiCo2O4. It is knownthat a relatively large surface area and porous features arecritical, offering more active sites within the pores for fastelectrochemical reactions and facilitating Li+ and electrontransfer at the electrolyte/electrode interface, which results ingreatly enhanced lithium storage properties.

In order to investigate the formation process of the 3D hier-archical ower-like nanostructure, a time series analysis wasperformed. The morphological evolution of the precursor wasanalyzed using SEM (Fig. 3 and S6, ESI†). When the duration ofthe reaction was as short as 1 h, the sample was composed of�400 nm nanospheres with smooth surfaces (Fig. 3a). Aer the

Fig. 3 FESEM images of the Ni–Co-based intermediate obtained at 180 �C andafter different reaction times: (a) 1 h, (b) 3 h, (c) 8 h, and (d) 15 h.

J. Mater. Chem. A, 2013, 1, 10935–10941 | 10937

Scheme 1 The formation mechanism of hierarchical flower-like NiCo2O4 with aporous structure.

Fig. 5 EIS spectra of hierarchical porous flower-like NiCo2O4 after differentcycles.

Journal of Materials Chemistry A Paper

Publ

ishe

d on

03

July

201

3. D

ownl

oade

d by

RM

IT U

ni o

n 23

/08/

2013

16:

27:1

3.

View Article Online

reaction time was increased to 3 h, the sphere-like structureshad grown signicantly larger to �700 nm in size, with clearlyobservable cracks on their surfaces (Fig. 3b). When the reactiontime was increased to 8 h, several nanoplates could be identiedgrowing on the surfaces of the nanospheres. In addition, itseemed as if these nanoplates had been inserted into thesurfaces (Fig. 3c). Aer a reaction time equal to 15 h, the newlyformed nanoplates appeared to have grown larger in a lateraldirection and the diameter of the inner core had greatlydecreased, resulting in the formation of a �1.5 mm ower-likestructure (Fig. 3d). Eventually, aer the duration of the reactionhad been increased up to 20 h, the inner core disappeared andthe sample demonstrated an entirely 3D hierarchical ower-likenanostructure (Fig. S4c, ESI†).

It is believed that PVP can be used to control the morphologyof products, due to a coordination effect.33 In the present

Fig. 4 Electrochemical characterization of the hierarchical porous flower-like NiCo2for the 1st, 2nd, 10th, and 60th cycles; (c) specific capacity at different current densities3.0 V vs. Li/Li+; (d) the rate performance in the voltage range 0.005–3.0 V vs. Li/Li+.

10938 | J. Mater. Chem. A, 2013, 1, 10935–10941

reaction system, PVP is crucial for obtaining uniform ower-likeNiCo2O4, because of its ability to coordinate to metal ionsthrough its N and/or C]O functional groups. We speculate thatthe role PVP plays in controlling the morphology is as follows:on the one hand, PVP can help to direct the anisotropic growthof the NiCo2O4 and further assemble it into the 3D hierarchicalower-like architecture, as well as uniformly disperse theproducts.34–36 On the other hand, the release of gas from thedecomposition of PVP produces a large number of nano-sizedpores in each nanoplate of the ower-like NiCo2O4 structureduring the calcination process in air. Moreover, an investigationof the effect that different amounts of PVP have on themorphology of the NiCo2O4 hierarchical micro-owers partlyconrms our hypothesis (Fig. S7, ESI†). When no PVP is used

O4 electrode. (a) The first four consecutive CV curves; (b) discharge–charge curvesand the coulombic efficiency versus the cycle number in the voltage range 0.005–

This journal is ª The Royal Society of Chemistry 2013

Scheme 2 Schematic illustration showing the diffusion of electrons, Li+ and theelectrolyte. (a) The electrolyte can easily diffuse into the inner region of thehierarchical porous flower-like NiCo2O4 structure from the open spaces betweenneighboring nanoplates. (b) The hierarchical porous structure ensures that eachnanoplate is in contact with electrolyte, and that electrons and Li+ can diffuse withlittle resistance.

Paper Journal of Materials Chemistry A

Publ

ishe

d on

03

July

201

3. D

ownl

oade

d by

RM

IT U

ni o

n 23

/08/

2013

16:

27:1

3.

View Article Online

the primary nanoplates suffer from random aggregation,resulting in the formation of ill-dened nanostructures andtheir assemblies (Fig. S7a, ESI†). When the amount of PVP usedis 0.05 g, ower-like structures appear (Fig. S7c, ESI†). However,the size of the products is not uniform, which can be attributedto insufficient PVP. Moreover, increasing the amount of PVP to0.2 g leads to the generation of large and uneven hierarchicalower-like superstructures (Fig. S7e, ESI†). Apparently, exces-sive PVP in the system can not only provide many high-energysites for further growth, due to the many free PVP moleculesabsorbed on the surface of the nanocrystals,35 but also increasesthe viscosity of the solution, resulting in an uneven distributionof the products. This indicates that only a specic amount ofPVP benets the formation of the delicate hierarchical ower-like structures. Furthermore, it should be noted that the size ofthe pores on the surface of the nanoplates aer the calcinationprocess, is signicantly inuenced by the amount of PVP(Fig. S7b, d, and f, ESI†). An increase in the amount of PVP isaccompanied by an increase in the number of pores appearingin the nanoplates, further demonstrating that the formation ofpores is due to the decomposition of PVP.

Based on all of the observations described above, a forma-tion mechanism for the hierarchical ower-like structure can beproposed and is depicted in Scheme 1. In the initial stage, PVPmolecules are rst coordinated to the metal precursors, formingmetal–organic coordination particles, which can be identiedfrom the FTIR analysis (Fig. S3, ESI†). In the following growthstage, these primary particles quickly aggregate into nano-spheres, which serve as the cores of the ower-like structures(step 1 in Scheme 1). Meanwhile, there are many free polymermolecules that are absorbed onto the surface of the nano-spheres, whichmay be useful for avoiding the over growth of thenanospheres, resulting in uniformly discrete products.34,35

Moreover, a chemical equilibrium between the solid/liquidinterface is established at this stage. As the hydrothermalreaction time increases, some of the interior nanocrystallitesthat are still in a metastable state, are easily dissolved anddiffuse outwards. Consequently, supersaturation occurs in the

This journal is ª The Royal Society of Chemistry 2013

surrounding solution, leading to a non-equilibrium state. Inorder to bring the solution back into a state of equilibrium,many of the nanocrystallites in the solution transfer onto thenanospheres, nucleating spontaneously on their surfaces.37

More importantly, due to the highly intrinsic plate-like growthhabit of cobalt oxides and nickel oxides,37,38 the recrystallizationprocess results in the formation of nanoplate subunits (step 2 inScheme 1). Furthermore, during this process, the solid spher-ical cores are exhausted because of mass diffusion and Ostwaldripening, although plenty of the nanoplates continue to growlarger by combining with any remaining particles in the solu-tion. The inner core is then completely consumed through thisdissolution–recrystallization process, while the ower-likestructure is formed (step 3 in Scheme 1). It should be noted thatmany other factors, such as hydrophobic interactions, hydrogenbonds, crystal-face attractions, van der Waals forces, and elec-trostatic and dipolar elds, may be signicant in controlling theshape of the Ni–Co-based intermediate.37 Finally, the poroushierarchical structure is effectively obtained aer a calcinationprocess, which can be attributed to the release of gases, mainlyderived from the decomposition of the organic species, PVP(step 4 in Scheme 1).30

Considering that advanced materials with special hierar-chical porous structures are useful for application in LIBs, weinvestigated the electrochemical properties of the as-preparedhierarchical porous ower-like NiCo2O4 as an anode for LIBs.Fig. 4a shows the rst four consecutive cyclic voltammograms(CVs) of the electrode prepared from hierarchical porousower-like NiCo2O4 in the voltage range 0–3.0 V vs. Li/Li+ and ata scan rate of 0.1 mV s�1. Three cathodic peaks were foundduring the rst cycle. The broad peak centered at �1.35 V canbe attributed to the destruction (or amorphization) of thecrystal structure and is easily distinguishable from the othercycles. The intense peak located at �0.95 V can be assigned tothe reduction of Co3+ and Ni2+ to metallic Co and Ni, respec-tively. While the minor peak at 0.73 V can be ascribed to theformation of a solid/electrolyte interface (SEI). There are twooxidation peaks centered at �1.4 V and �2.1 V in the followinganodic scan, which can be ascribed to the oxidation of themetallic Ni and Co to Ni oxides and Co oxides, respectively.37–39

The second cycle is characterized by distinctive redox peaks at�1.0 and �1.4/2.1 V, corresponding to the reduction/oxidationof the Ni oxides and Co oxides.30 From the second cycleonwards, the CV curves overlap very well, which indicates thegood reversibility of the electrochemical reactions. Based onthe above analysis and previous reports,30,39,40 the lithiuminsertion/extraction reactions for the as-prepared hierarchicalporous ower-like NiCo2O4 electrode are believed to proceedas follows:

NiCo2O4 + 8Li+ + 8e� / 2Co + Ni + 4Li2O (1)

Ni + Li2O 4 NiO + 2Li+ + 2e� (2)

Co + Li2O 4 CoO + 2Li+ + 2e� (3)

CoO + 1/3Li2O 4 1/3Co3O4 + 2/3Li+ + 2/3e� (4)

J. Mater. Chem. A, 2013, 1, 10935–10941 | 10939

Journal of Materials Chemistry A Paper

Publ

ishe

d on

03

July

201

3. D

ownl

oade

d by

RM

IT U

ni o

n 23

/08/

2013

16:

27:1

3.

View Article Online

Representative discharge–charge proles of the as-preparedhierarchical porous ower-like NiCo2O4 electrode at a currentdensity of 100 mA g�1 are shown in Fig. 4b. The initial dischargeand charge capacities are 1508.7 (�13.5 mol of Li) and 1056.5mA h g�1 (�9.5 mol of Li), respectively. The irreversible capacityloss during the rst cycle can be attributed to the formation of aSEI lm. Interestingly, even the initial charge capacity exceedsthe 8 mol of Li that can be theoretically delivered based on theconversion reaction (eqn (1)). A possible reason for this may bethe reversible formation/dissolution of a polymeric gel-like lmthat results from decomposition of the electrolyte.10,30 In addi-tion, it should be noted that a large deviation in potential existsbetween the charge and discharge curves. This phenomenonmay be due to the polarization related to ion transfer during thecycling process, which has been observed in other transitionmetal oxides.10,40

Fig. 4c shows the cycling behavior of the hierarchical porousower-like NiCo2O4 electrode at current densities of 100 and500 mA g�1 in the voltage range 0.005–3.0 V. The dischargecapacities obtained for the rst and second cycles at 100 mA g�1

are 1509 and 1146 mA h g�1, respectively. It is important to notethat this sample shows a marked increase in capacity aer theinitial 10 cycles, which arises from an activation process. Aer60 cycles at 100 mA g�1, the discharge capacity is retained at939 mA h g�1, corresponding to around 82% of the second cycledischarge capacity (1146 mA h g�1). Even at a high currentdensity of 500 mA g�1, a discharge capacity of 640 mA h g�1 isretained aer 60 cycles. In order to demonstrate the advantagesof the hierarchical porous ower-like NiCo2O4 for lithiumstorage, the cycling performance of NiCo2O4 with differentmorphologies, prepared in the absence of PVP or by addingdifferent amounts of PVP, was also investigated under identicaltest conditions (Fig. S8, ESI†). Notably, these samples exhibitpoor capacity retention when compared with the hierarchicalporous ower-like NiCo2O4. This demonstrates that having anoptimum structure (including morphology, inner structure, andcrystallinity) is benecial for improving electrochemicalperformance. More importantly, this also indicates that thehierarchical porous ower-like architecture is really useful forthe application of a NiCo2O4 anode in LIBs.

A high rate capability is an important parameter for mate-rials in LIBs, mainly because it reduces the discharge–chargetime in practical applications. We also investigated the rateperformance, as shown in Fig. 4d. By virtue of its uniquestructure, the hierarchical porous ower-like NiCo2O4 shows anexcellent cycling response to the continuously varied currentdensity. Even at a high rate of 2 A g�1, a capacity of �420 mA hg�1 could still be retained, which is comparable to that ofcommercially used graphite (372 mA h g�1). Moreover, withreference to the rate performance, an enhancement wasobtained compared with previous work on NiCo2O4 anodes,wherein only �390 mA h g�1 was maintained, even at a lowcurrent of 1.6 A g�1.30 When the current density is reduced backto 250 mA g�1, an average discharge capacity of �680 mA h g�1

could be recovered. This good rate capability demonstrates thathierarchical porous ower-like NiCo2O4 has great potential as ahigh-rate anode material in LIBs.

10940 | J. Mater. Chem. A, 2013, 1, 10935–10941

It is well known that the cycling stability and rate perfor-mance are highly related to the interfacial charge-transferprocess and Li+ diffusion. To gain further insight into thetransport kinetics of the electrochemical reaction process, wemeasured the electrochemical impedance spectra (EIS) of thehierarchical porous ower-like NiCo2O4 aer different cycles,which has been proven to be an important and useful tool forinvestigating the kinetics of Li insertion electrodes. Theimpedance spectra (Fig. 5) consist of two partially overlappedsemicircles in the high and medium frequency regions andinclined lines in the low-frequency domains. Usually, highfrequency semicircles can be ascribed to a combination of thesurface lm resistance and the charge-transfer impedance(Rsf+ct), while the linear region corresponds to the Warburgdiffusion process (W), reecting the solid state diffusion of Liinto the bulk of the active materials. Notably, the lowest value(16 U) of the combined surface lm resistance and the charge-transfer impedance (Rsf+ct) is exhibited in the fresh cell, furtherconrming the formation of a SEI lm aer the rst discharge–charge process. In subsequent cycles, Rsf+ct increases steadily,reaching 66 U in the 20th cycle (Fig. S9, ESI†). It is worthpointing out that the impedance increases slowly during thecycling process, which is a normal phenomenon that occurs inmany anode materials.10 Moreover, only small changes could beobserved in the impedance plots, implying a relatively expedientdiffusion of Li+ and electrons as the cycle number increased.Furthermore, these results also demonstrate the stability of thehierarchical porous ower-like NiCo2O4, which ensures bettercapacity retention and is a further indication of the advantagesof this hierarchical porous ower-like structure.

To the best of our knowledge, this is the rst reportdescribing the utilization of hierarchical porous ower-likeNiCo2O4 as an anode for LIBs. More importantly, a high speciccapacity with enhanced capacity retention and an excellentrate capability are achieved in the present work through engi-neering of the microstructure. It is believed that the superiorelectrochemical performance of our NiCo2O4 material may berelated to several aspects of the unique hierarchical porousower-like structure. Specically, the open space betweenneighboring nanoplates allows for easy diffusion of the elec-trolyte (Scheme 2a), ensuring that every nanoplate can take partin the electrochemical reaction because every nanoplate is incontact with electrolyte. Furthermore, the micro-sized hierar-chical structure composed of nano-sized subunits effectivelyincreased the NiCo2O4/electrolyte contact area, facilitating fasttransportation of Li+ and electrons, resulting in a better ratecapability (Scheme 2b). Finally, the structural strain and volumechanges during cycling could also be accommodated effectivelydue to the sufficient number of pores on the surface of thenanoplates, thus improving the cycling stability.

Conclusions

In summary, uniform hierarchical porous ower-like NiCo2O4

has been synthesized using a facile hydrothermal approachfollowed by calcination in air. It is found that PVP plays a keyrole, not only in controlling the formation of the delicate

This journal is ª The Royal Society of Chemistry 2013

Paper Journal of Materials Chemistry A

Publ

ishe

d on

03

July

201

3. D

ownl

oade

d by

RM

IT U

ni o

n 23

/08/

2013

16:

27:1

3.

View Article Online

hierarchical ower-like structure, but also in creating theuniformly distributed pores on each of the nanoplates. Whenevaluated as an anode material for LIBs, a reversible capacity of939 mA h g�1 at 100 mA g�1 could be retained aer 60 cycles,corresponding to around 82% of the second discharge capacity(1146 mA h g�1). Its superior rate capability makes ower-likeNiCo2O4 a promising material for high-rate anodes in LIBs. It isbelieved that the unique porous hierarchical architectureendows the as-prepared NiCo2O4 with these outstandingelectrochemical properties. Considering its excellent electro-chemical performance and simple and versatile preparationprocess, hierarchical porous ower-like NiCo2O4 might serve asa potential candidate for high-capacity anode materials in next-generation LIBs. In addition, this facile strategy may provide afeasible method for the preparation of other TMO functionalmaterials with hierarchical porous nano-/microstructures forapplication in energy storage devices.

Acknowledgements

The authors are thankful for the support and funding from theNational Research Foundation, Clean Energy Research Project(Grant number: NRF2009EWT-CERP001-036) and the TUMCREATE center for Electromobility.

Notes and references

1 T. L. Breen, J. Tien, S. J. Oliver, T. Hadzic and G. Whitesides,Science, 1999, 284, 948–951.

2 K. X. Yao, X. M. Yin, T. H. Wang and H. C. Zeng, J. Am. Chem.Soc., 2010, 132, 6131–6144.

3 H. Colfen and S. Mann, Angew. Chem., Int. Ed., 2003, 42,2350–2365.

4 G. X. Wang, H. Liu, J. Liu, S. Z. Qiao, G. M. Lu, P. Munroe andH. Ahn, Adv. Mater., 2010, 22, 4944–4948.

5 J. Haetge, P. Hartmann, K. Brezesinski, J. Janek andT. Brezesinski, Chem. Mater., 2011, 23, 4384–4393.

6 L. S. Zhong, J. S. Hu, H. P. Liang, A. M. Cao, W. G. Song andL. J. Wan, Adv. Mater., 2006, 18, 2426–2431.

7 J. Perez-Ramirez, C. Christensen, K. Egeblad,C. H. Christensen and J. C. Groen, Chem. Soc. Rev., 2008,37, 2530–2542.

8 Y. F. Cui, C. Wang, S. J. Wu, G. Liu, F. F. Zhang andT. M. Wang, CrystEngComm, 2011, 13, 4930–4934.

9 B. Guo, C. S. Li and Z. Y. Yuan, J. Phys. Chem. C, 2010, 114,12805–12817.

10 L. Hu, H. Zhong, X. R. Zheng, Y. M. Huang, P. Zhang andQ. W. Chen, Sci. Rep., 2012, 2, 986.

11 F. Y. Cheng, H. B. Wang, Z. Q. Zhu, Y. Wang, T. R. Zhang,Z. L. Tao and J. Chen, Energy Environ. Sci., 2011, 4, 3668–3675.

12 F. M. Courtel, H. Duncan, Y. Abu-Lebdeh and I. J. Davidson,J. Mater. Chem., 2011, 21, 10206–10218.

13 L. L. Li, S. J. Peng, Y. L. Cheah, J. Wang, P. F. Teh, Y. W. Ko,C. L. Wong and M. Srinivasan, Nanoscale, 2013, 5, 134–138.

14 Y. G. Li, P. Hasin and Y. Y. Wu, Adv. Mater., 2010, 22, 1926–1929.

This journal is ª The Royal Society of Chemistry 2013

15 L. F. Hu, L. M. Wu, M. Y. Liao and X. S. Fang, Adv. Mater.,2011, 23, 1988–1992.

16 H. L. Wang, C. M. B. Holt, Z. Li, X. H. Tan, B. S. Amirkhiz,Z. W. Xu, B. C. Olsen, T. Stephenson and D. Mitlin, NanoRes., 2012, 5, 605–617.

17 H. C. Chien, W. Y. Cheng, Y. H. Wang and S. Y. Lu, Adv.Funct. Mater., 2012, 22, 5038–5043.

18 Q. F. Wang, B. Liu, X. F. Wang, S. H. Ran, L. M. Wang,D. Chen and G. Z. Shen, J. Mater. Chem., 2012, 22, 21647–21653.

19 Z. Q. Liu, K. Xiao, Q. Z. Xu, N. Li, Y. Z. Su, H. J. Wang andS. Chen, RSC Adv., 2013, 3, 4372–4380.

20 G. Q. Zhang, H. B. Wu, H. E. Hoster, M. B. Chan-Park andX. W. Lou, Energy Environ. Sci., 2012, 5, 9453–9456.

21 G. Zhang and X. W. Lou, Adv. Mater., 2013, 25, 976–979.22 R. R. Salunkhe, K. H. Jang, S. W. Lee and H. Ahn, RSC Adv.,

2012, 2, 3190–3193.23 H. Jiang, J. Ma and C. Z. Li, Chem. Commun., 2012, 48, 4465–

4467.24 H. L. Wang, Q. M. Gao and L. Jiang, Small, 2011, 7, 2454–

2459.25 C. Z. Yuan, J. Y. Li, L. R. Hou, L. Yang, L. F. Shen and

X. G. Zhang, J. Mater. Chem., 2012, 22, 16084–16090.26 L. L. Li, S. J. Peng, Y. L. Cheah, P. F. Teh, J. Wang, G. Wee,

Y. W. Ko, C. L. Wong and M. Srinivasan, Chem. Eur. J.,2013, 19, 5892–5898.

27 B. Cui, H. Lin, J. B. Li, X. Li, J. Yang and J. Tao, Adv. Funct.Mater., 2008, 18, 1440–1447.

28 T. Y. Wei, C. H. Chen, H. C. Chien, S. Y. Lu and C. C. Hu, Adv.Mater., 2010, 22, 347–351.

29 Y. N. Nuli, P. Zhang, Z. P. Guo, H. K. Liu and J. Yang,Electrochem. Solid State Lett., 2008, 11, A64–A67.

30 J. F. Li, S. L. Xiong, Y. R. Liu, Z. C. Ju and Y. T. Qian, ACS Appl.Mater. Interfaces, 2013, 5, 981–988.

31 Y. L. Cheng, B. L. Zou, C. J. Wang, Y. J. Liu, X. Z. Fan, L. Zhu,Y. Wang, H. M. Ma and X. Q. Cao, CrystEngComm, 2011, 13,2863–2870.

32 Y. Borodko, S. E. Habas, M. Koebel, P. D. Yang, H. Freiand G. A. Somorjai, J. Phys. Chem. B, 2006, 110, 23052–23059.

33 X. Wang, X. L. Wu, Y. G. Guo, Y. Zhong, X. Q. Cao, Y. Ma andJ. N. Yao, Adv. Funct. Mater., 2010, 20, 1680–1686.

34 S. J. Peng, L. L. Li, P. N. Zhu, Y. Z. Wu, M. Srinivasan,S. G. Mhaisalkar, S. Ramakrishna and Q. Y. Yan, Chem.Asian J., 2013, 8, 258–268.

35 J. Wu, F. Duan, Y. Zheng and Y. Xie, J. Phys. Chem. C, 2007,111, 12866–12871.

36 Y. Y. Li, J. P. Liu, X. T. Huang and G. Y. Li, Cryst. Growth Des.,2007, 7, 1350–1355.

37 R. Qiao, X. L. Zhang, R. Qiu, J. C. Kim and Y. S. Kang, Chem.Eur. J., 2009, 15, 1886–1892.

38 L. Liu, Y. Li, S. M. Yuan, M. Ge, M. M. Ren, C. S. Sun andZ. Zhou, J. Phys. Chem. C, 2010, 114, 251–255.

39 S. L. Xiong, J. S. Chen, X. W. Lou and H. C. Zeng, Adv. Funct.Mater., 2012, 22, 861–871.

40 Y. J. Mai, S. G. Shi, D. Zhang, Y. Lu, C. D. Gu and J. P. Tu,J. Power Sources, 2012, 204, 155–161.

J. Mater. Chem. A, 2013, 1, 10935–10941 | 10941