Materials Research Bulletin 47 (2012) 1987–1990

Template-directed preparation of two-layer porous NiO film via hydrothermalsynthesis for lithium ion batteries

Z. Chen *, A. Xiao, Y. Chen, C. Zuo, S. Zhou, L. Li

College of Chemistry and Chemical Engineering, Hunan University of Arts and Science, Changde, Hunan 415000, China

A R T I C L E I N F O

Article history:

Received 1 February 2012

Received in revised form 27 March 2012

Accepted 11 April 2012

Available online 17 April 2012

Keywords:

A. Oxides

A. Thin films

B. Energy storage

B. Electrochemical properties

A B S T R A C T

A two-layer porous NiO film is prepared by hydrothermal synthesis method through self-assembled

monolayer polystyrene spheres template. The substructure of the NiO film is composed of ordered close-

packed hollow-sphere array and the superstructure is made up of randomly NiO nanoflakes. The

electrochemical properties are measured by galvanostatic charge/discharge tests and cyclic voltam-

metric analysis (CV). As anode material for lithium ion batteries, the two-layer porous NiO film exhibits

high initial coulombic efficiency of 75%, high reversible capacity and rather good cycling performance.

The discharge capacity of the two-layer porous NiO film is 501 mAh g�1 at 0.5 C after 50 cycles. The two-

layer porous architecture is responsible for the enhancement of electrochemical properties.

� 2012 Elsevier Ltd. All rights reserved.

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Materials Research Bulletin

jo u rn al h om ep age: ww w.els evier .c o m/lo c ate /mat res b u

1. Introduction

Lithium ion batteries have attracted considerable attention dueto increasing demands for portable electronic devices [1]. Highlithium storage capacity, high coulombic efficiency, and longcycling life are still the major challenges for high-performancelithium ion batteries [2]. Over the past decades, great efforts havebeen dedicated to searching for alternative anode materials oflithium ion batteries for improving their energy density and safety.Among the numerous explored systems, 3d transition metal oxidessuch as nickel oxide (NiO), cobalt oxide (Co3O4), and copper oxide(CuO) have been demonstrated with high reversible capacities(about three times larger than those of graphite) at a relative lowpotential, which greatly spurs the rapid development in this field[3–6].

Since the pioneering work of Tarascon’s group [7], NiO isconsidered as one of the most promising anode candidates with atheoretic capacity of 718 mAh g�1. However, large irreversiblecapacity in the first cycle and poor capacity retention duringcharge/discharge cycling restrict its practical applications. Theundesirable performance is ascribed to the volume change andsubsequently particle pulverization during the charge/dischargeprocess. Extensive efforts have been dedicated to overcomingthese problems by either designing new types of active materialsor modifying traditional electrodes. An effective strategy is toconstruct NiO with high porous nanostructures possessing high

* Corresponding author. Tel.: +86 736 78186715; fax: +86 736 78186716.

E-mail address: xiaoanguo123@126.com (Z. Chen).

0025-5408/$ – see front matter � 2012 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.materresbull.2012.04.018

surface area, large surface-to-volume ratio, and favorable struc-tural stability. To date, lots of nanostructured NiO includingnanotube [8], nanowall [9,10], nanosheet [11], nanobowel [12],nanocone[13], and ordered mesoporous structure [14], have beenprepared by various methods and enhanced performances areobtained in these systems compared to the bulk counterparts dueto their fast electron/ion transfer and morphological stability.

In recent years, controlled synthesis of active material forlithium ion batteries by template technique has attractedtremendous interest [15]. Nano/microscaled materials withdifferent morphologies such as Fe3O4 nanorod [16], V2O5 nanotube[17], three-dimensional (3D) Co3O4 [4], and 2D CuO [18]nanostructures can be obtained through the template synthesismethod [15]. These template-synthesized materials normallyexhibit small crystalline size, high surface area, large surface-to-volume ratio, and favorable structural stability, leading toenhanced performance. Previously, Yuan et al. [12] reported a2D ordered NiO macrobowl array prepared by an electrodepositionmethod through monolayer polystyrene (PS) spheres template andits improved properties for lithium ion batteries. Besides, ourgroup reported a porous-structured NiO film prepared by achemical bath deposition through polystyrene spheres template[19]. In the present work, we report a two-layer 3D porous NiO filmvia hydrothermal synthesis method based on monolayer PSspheres template and apply it as anode material for lithium ionbatteries. The two-layer porous NiO film exhibits high initialcoulombic efficiency of 75%, high reversible capacity and rathergood cycling performance. The discharge capacity of the two-layerporous NiO film is 501 mAh g�1 at 0.5 C after 50 cycles. The NiOfilm electrodes exhibit noticeable electrochemical performance

80706050403020

Ni (200)

Ni (220 )

∀

∀

∀∀

(220)

(200)

Inte

nsi

ty /

a.u

.

2θ/ degree

(111

)

Ni (111)NiO

Fig. 1. XRD pattern of two-layer porous NiO film.

Z. Chen et al. / Materials Research Bulletin 47 (2012) 1987–19901988

with good cycle life and high capacity due to the highly porousstructures.

2. Experimental

All solvents and chemicals were of reagent quality and wereused without further purification. The monodispersed PS sphereswith particle sizes of 500 nm in diameter were purchased from AlfaAesar Corporation. They were well dispersed in deionized waterand prepared as a suspension with concentration of 2.5 wt% beforefabricating colloidal monolayers.

Clean nickel foil with a size of 2.5 � 2.5 cm2 was used as thesubstrate. The fabrication process of monolayer PS spheres

Fig. 2. SEM images of (a) top and side views of the monolayer PS spheres template (side v

(side view presented in inset); (d) TEM image of individual NiO nanoflake (SAED patte

template has been already described in previous works[10,12,20]. In a typical synthesis of precursor film, 5 mmol ofnickel nitrate (Ni(NO3)2) and 25 mmol urea (CO(NH2)2) weredissolved in 50 ml distilled water to form homogeneous solution.Then the homogeneous solution was transferred into Teflon-linedstainless steel autoclave liners. After that, the template electrodeabove was immersed into the reaction solution. The liner wassealed and maintained at 120 8C for 1 h in an electric oven. Thereactions for precursor film involved in the hydrothermal synthesiscould be illustrated as follows:

H2NCONH2þ H2O ! 2NH3þ CO2 (1)

NH3�3H2O ! NH4þ þ OH� (2)

Ni2þ þ 2OH� ! NiðOHÞ2 (3)

Afterwards, the sample was immersed in toluene for 24 h toremove the PS spheres template. Finally, the as-prepared samplewas annealed at 400 8C for 1.5 h. The loading weight of the NiO isapproximately 0.9 mg cm�2.

The morphology and microstructure of the sample werecharacterized by a field emission scanning electron microscopy(FESEM, Hitachi S-4700), transmission electron microscopy (TEM,JEM 200 CX 200 kV), and X-ray diffraction (XRD, Philips PC-APDwith Cu Ka radiation).

Test coin cells were assembled in an argon-filled glove boxusing the nickel-supported NiO films as working electrode, Li foilas counter-electrode, polypropylene film as separator, and anelectrolyte of 1 M LiPF6 in a 50:50 (w/w) mixture of ethylenecarbonate and diethyl carbonate. The galvanostatic charge–discharge tests were conducted on LAND battery program-controltest system from 0.02 to 3.0 V (versus Li/Li+) at room temperature(25 � 1 8C). Cyclic voltammetry (CV) tests were carried out using the

iew presented in inset); (b) and (c) top and side views of two-layer porous NiO film

rn presented in inset).

3.02.52.01.51.00.50.01.0

0.8

0.6

0.4

0.2

0.0

-0.2

-0.4

-0.6

-0.8

-1.0

Cathodic process

Curr

ent

den

sity

/ m

A c

m-2

Potential (vs. Li+/Li) / V

0.85 V

1.32 V

1.56 V2.34 VAnodic process

Fig. 4. CV curve of the two-layer porous NiO film at a scanning rate of 0.1 mV s�1 at

the second cycle.

Z. Chen et al. / Materials Research Bulletin 47 (2012) 1987–1990 1989

CHI660C electrochemical workstation at a scanning rate of0.1 mV s�1.

3. Results and discussion

Fig. 1 shows the XRD pattern of the as-annealed film. Except forthree representative peaks from Ni foil, all the other diffractionpeaks can be indexed to cubic NiO, which is consistent with thevalue in the standard card (JCPDS 47-1049). SEM image of the as-fabricated monolayer PS spheres template is presented in Fig. 2a.PS spheres are well-organized into an ordered monolayer arraywith a hexagonal close-packed alignment on the substrate.Interestingly, the as-synthesized NiO film exhibits a two-layerporous structure. The upper part is randomly porous net-like NiOnanoflakes with a thickness of 25 nm and the lower part is close-packed monolayer NiO hollow-sphere with a diameter of about500 nm (Fig. 2b and c). Combining these results, it is reasonablethat the NiO film with a two-layer porous structure could besuccessfully prepared via hydrothermal synthesis method basedon monolayer PS spheres template. The individual flake form thefilm presents a smooth appearance (Fig. 2d). In addition, alldiffraction rings in the selected area electronic diffraction (SAED)pattern of the nanoflake can be indexed to the cubic NiO phase(JCPDS 47-1049), indicating that the NiO film is polycrystalline innature.

The electrochemical performances of the as-prepared two-layerporous NiO film are evaluated by standard method based on NiO/Licells. Fig. 3 shows the discharge/charge curves of the two-layerporous NiO film electrode measured between 0.02 and 3.0 V versusLi/Li+ at 0.1 C rate (1C = 718 mA g�1). The long plateau at 0.63 V inthe first discharge process is due to the conversion from NiO to Ni.The discharge plateau becomes higher in the second cycle. Thetwo-layer porous NiO film shows a first discharge capacity of932 mAh g�1, which is higher than the theoretical value(718 mAh g�1). The extra capacity could be attributed to theformation of solid electrolyte interphase (SEI) in the first dischargeprocess. This phenomenon happens in other 3d transition metaloxides including CuO, Co3O4 and FeO [4,5,21,22]. The first chargecapacity of the two-layer porous NiO film is approximately703 mAh g�1, much less than that of the first discharge. Theirreversible capacity loss between the first discharge and charge isattributed to the incomplete decomposition of both of the SEI andLi2O [1]. The corresponding initial coulombic efficiency is about75%, higher than that of the power electrode (65%) [23], andcomparable to other highly porous NiO films [3,12].

To understand the electrochemical process in detail, the CVprofile of the two-layer porous NiO film electrode is recorded in

100080060040020000.0

0.5

1.0

1.5

2.0

2.5

3.02nd

2nd

1st

Capacity / mAh g-1

Vo

ltag

e /

V (vs. L

i+/L

i)

1st

Fig. 3. The discharge/charge curves for the two-layer porous NiO film.

Fig. 4. In the cathodic process, the main reduction peak at 0.85 Vand weak reduction peak at 1.32 V correspond to the reduction ofNiO to metallic Ni nanoparticles and the formation of a partiallyreversible SEI layer [3,12,24], respectively. Wang and coworkersreported that SEI layer had already started above 2.0 V [25], andTarascon and co-workers also observed the formation of SEI layerby ex situ TEM at the sloping potential range of 1–0.02 V [26].During the following anodic process, the oxidation peak at 1.56 V isassociated with the partial decomposition of the SEI layer, andanother oxidation peak at 2.34 V corresponds to the decompositionof Li2O leading to the formation of NiO [3,12,24]. The peaks in theCV curve are consistent with the plateaus or sloping potentialranges in the voltage-capacity profiles.

Fig. 5 shows the capacity retention properties of the two-layerporous NiO film electrode. The two-layer porous NiO film is foundto sustain and deliver 570 mAh g�1 at 0.1 C, 501 mAh g�1 at 0.5 C,and 409 mAh g�1 at 2 C after 50 cycles, corresponding to 79%, 70%,57% of the theoretical value, respectively, demonstrating thecapability for a high cycling rate. These values are comparable tothose obtained from other hierarchically porous NiO films[3,12,14], and higher than the single-layer porous NiO film witha capacity of about 300 mAh g�1 at 2 C after 50 cycles [27]. Theenhanced performance is ascribed to the following morphologicalbenefits. First, the free-standing NiO nanoflakes with open spacesallow easier electrolyte penetration to every part of the particleand provide shorten diffusion paths for both electrons and lithiumions within the oxides. Second, the macropores in the monolayer

504030201000

200

400

600

800

1000

0.1 C

0.5 C

2 C

Cycle number

Cap

acit

y /

mA

h g

-1

Fig. 5. Cycling performances of the two-layer porous NiO film at different rates.

Z. Chen et al. / Materials Research Bulletin 47 (2012) 1987–19901990

hollow-sphere array can absorb and strongly retain electrolyte,ensuring efficient contact between active materials and electro-lytes. Third, the porous structure provides larger surface area andmore active sites for electrochemical reactions. These features areparticularly helpful for high rate applications, resulting in betterelectrochemical performance.

4. Conclusions

In summary, a two-layer porous NiO film has been successfullyprepared via hydrothermal synthesis method based on monolayerPS spheres template. As anode of lithium ion batteries, the two-layer porous NiO film exhibits high reversible capacity and rathergood cycling performance. The enhanced electrochemical perfor-mance is attributed to highly porous structure, which facilitatesthe contact between electrolyte and the oxide surface and providesshorter diffusion length for lithium ion and electrons, leading tohigher electrochemical activity.

Acknowledgement

This work was supported by the construct program of the keydiscipline in Hunan province (Applied Chemistry).

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