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NiO/C nanocapsules with onion-like carbon shell as anodematerial for lithium ion batteries

Xianguo Liu a,b,*, Siu Wing Or b,*, Chuangui Jin a, Yaohui Lv a, Chao Feng a, Yuping Sun c

a School of Materials Science and Engineering, Anhui University of Technology, Ma’anshan 243002, PR Chinab Department of Electrical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kongc Center for Engineering Practice and Innovation Education, Anhui University of Technology, Ma’anshan 243002, PR China

A R T I C L E I N F O

Article history:

Received 4 January 2013

Accepted 5 April 2013

Available online 13 April 2013

0008-6223/$ - see front matter � 2013 Elsevihttp://dx.doi.org/10.1016/j.carbon.2013.04.014

* Corresponding authors: Fax: +86 555 231157E-mail addresses: liuxianguohugh@gmail

A B S T R A C T

The synthesis of NiO/C nanocapsules with NiO nanoparticles as the core and onion-like car-

bon layers as the shell is reported. The NiO/C nanocapsules deliver an initial discharge

capacity of 1689.4 mAh g�1 at 0.5 C and maintain a high reversible capacity of

1157.7 mAh g�1 after 50 cycles compared to the NiO nanoparticles of 383.5 mAh g�1. As an

anode material for lithium ion batteries, the NiO/C nanocapsules exhibit a remarkable dis-

charge capacity, a high rate charge–discharge capability and an excellent cycling stability.

The improvements are ascribed to the fact that the onion-like carbon shells not only can

provide enough voids to accommodate the volume change of NiO nanoparticles but also

can prevent the formation of solid electrolyte interface (SEI) films on the surface of the

NiO nanoparticles and hence the direct contact of Ni and SEI films upon lithium extraction.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Lithium ion batteries (LIBs), a fast-developing technology in

electric energy storage, are the dominant power source for a

wide range of portable electronic devices [1–10]. The current

commercial LIBs have been manufactured with LiCoO2 cath-

odes and carbon (graphite) anodes [1]. Because the capacity

of carbon anodes is limited by the composition of LiC6

(372 mAh g�1) [1,2], diverse anode materials have been devel-

oped to make more active spaces for lithium (Li) storage,

including carbon, group IV elements and transition metal oxi-

des [3–9]. Recently, the transition metal oxide NiO has been

intensely investigated due to its high theoretical capacity

(718 mAh g�1), low material cost and nontoxicity [11]. How-

ever, partial pulverization and electrode cracking often occur

in the NiO anodes during the repetitive cycling process with Li

ions, leading to the loss of electrical contact and the fading of

capacity [12–15].

er Ltd. All rights reserved

0, +852 23301544..com (X. Liu), eeswor@po

Continuous efforts on novel electrode materials with high

rate charge-discharge capability are important. These can be

achieved by the successful design and development of nano-

structured electrodes. There are two superiorities of the elec-

trodes with nanostructures: (1) their large surface area

endows them with high capacity and a favorable response

to high rate charge-discharge cycling due to the dramatic

increment of reaction sites and interfaces between the active

materials and the electrolyte, as the nanoscale ingredients

make Li ion diffusion much easier by effectively shorting

the diffusion length to grain size; (2) the interior space among

the microscale structures allows the volume variation upon

insertion/extraction of Li ions to be better accommodated

[16]. To date, NiO with various nanostructures such as nano-

particles, nanosheets and nanotubes have been fabricated

[17–19]. However, it is still a great challenge to maintain a

large reversible capacity combined with a high coulombic effi-

ciency, a long cycling life and a good rate capability of NiO

electrode materials [20]. To circumvent this problem, carbo-

.lyu.edu.hk (S.W. Or).

216 C A R B O N 6 0 ( 2 0 1 3 ) 2 1 5 – 2 2 0

naceous materials with high flexibility and electrical conduc-

tivity have been widely used as matrices for NiO anodes to

improve their Li ion storage properties. For example, NiO–C

and NiO–graphene materials with a high capacity and an im-

proved cyclability were achieved as anode materials for Li ion

batteries [20–25]. The core/shell-type nanostructure, named

by nanocapsules, is identified as a good way to markedly im-

prove the cycling behavior and kinetics of lithium intercala-

tion and de-intercalation in composites. Recently, there

have been a few reports about NiO–C core/shell nanostruc-

tured materials with amorphous carbon as the shell by means

of various kinds of chemical method [7,10,16,23,25]. As dis-

cussed in our previous works, compared with the amorphous

C shells, onion-like C shells can effectively improve the elec-

trical conductivity [26–28].

In our present work we design a new type of NiO/C nano-

capsules with onion-like C shells, in which onion-like C shells

not only can provide enough volume to accommodate the vol-

ume change of NiO but also can effectively prevent the forma-

tion of solid electrolyte interface (SEI) films on the surface of

NiO nanoparticles and the aggregation of NiO nanoparticles.

The nanocapsules presented here are simple and may lead

to broad applications. The electrochemical performances of

NiO/C nanocapsules with onion-like C shells as an anode

for Li ion batteries are investigated in detail. NiO/C nanocap-

sules with a highly reversible capacity, an excellent cycle per-

formance and a high safety may be attractive as anodes in Li

ion batteries.

.u.)

2. Experimental

2.1. Synthesis

A modified arc-discharge method was used in synthesis and

has been described in detail elsewhere [26–28]. In brief, bulk

Ni on a water-cooled copper crucible was used as the anode.

The cathode was a carbon needle. After the chamber was

evacuated, 1.6 · 104 Pa of pure argon, 0.4 · 104 Pa of hydrogen

and 40 ml liquid ethanol were introduced into the chamber.

The arc discharge current was maintained at 80 A for 0.5 h.

The partial pressure of ethanol increased with time. When

the arcing time reached 0.5 h, the pressure of the chamber

was as high as 1 atmospheric pressure because of the

decomposition of ethanol and the expansion of the gas with

increasing temperature. After passivation for 8 h in argon,

the products were collected from depositions on the top of

the water-cooled chamber. To prepare the NiO/C nanocap-

sules and NiO nanoparticles, the products were annealed

at 300 and 500 �C for 2 h in air, respectively.

20 30 40 50 60 70 80

NiO(c)

(b)NiO

Inte

nsit

y(a

2 theta (degree)

(a)Ni

Fig. 1 – X-ray diffraction patterns of (a) as-prepared products

by arc-discharge method and (b) products annealed at 300 �Cin air and (c) products annealed at 500 �C in air.

2.2. Characterization

The composition and phase purity of the as-synthesized sam-

ples were analyzed by X-ray diffraction (XRD) at 40 kV voltage

and 50 mA current with Cu Ka radiation (k = 1.5418 A). Trans-

mission electron microscopy (TEM) and high-resolution

TEM (HRTEM) images were obtained on a JEOL JEM-2010 trans-

mission electron microscope at an acceleration voltage of

200 kV.

Electrochemical experiments were carried out by using

standard CR2032 type coin cells. The working electrodes were

prepared by mixing the NiO/C nanocapsules/NiO nanoparti-

cles, carbon black and poly(vinyl difluoride) (PVDF) at a weight

ratio of 80:10:10 and by pasting on a pure Cu foil. A metallic Li

foil was employed as the counter electrode, while 1 M LiPF6 in

ethylene carbonate (EC)–dimethyl carbonate (DMC) (1:1 in vol-

ume) was used as the electrolyte. The cells were assembled in

an argon-filled glove box. The galvanostatic charge–discharge

tests were performed on a land battery program-control test

system (Wuhan, China) between 0 and 3.2 V at room temper-

ature. The cyclic voltammetry (CV) test was carried out on an

electrochemical workstation (Model 2273, Princeton Applied

Research, USA) in the potential window of 0–3.2 V (vs. Li/Li+)

at a scan rate of 0.5 mV s�1.

3. Results and discussion

In Fig. 1, XRD patterns show the phase components of prod-

ucts prepared by the modified arc-discharge method and

the subsequent annealing in air for 2 h at 300 and 500 �C,

respectively. It can be seen that the XRD pattern of the prod-

ucts prepared by the modified arc-discharge method matches

only the face-centered cubic Ni structure. There are no Ni-

oxide peaks detectable in the XRD pattern of the products pre-

pared by the modified arc-discharge method due to the pro-

tective C shells. It should be noted that there is no evidence

for pure C indicating its small amount (less than 3%) in the

products [26]. Furthermore, it is also difficult to detect the

XRD pattern of the C shells because breaking down the peri-

odic boundary condition (translation symmetry) along radial

direction [26]. The XRD patterns of the products annealed at

300 and 500 �C can be indexed as a single phase of NiO, with

no evidence for the existence of other oxides such as Ni15O16

and Ni2O3. It is known that the NiO phase is the only stable

phase in air. With an increase in the annealing temperature

up to 500 �C, there are gradual increase in intensity and nar-

row in diffraction peaks, indicating the growth of NiO

nanoparticles.

The morphology and size distribution of the products pre-

pared by the modified arc-discharge method and the subse-

quent annealing in air for 2 h at 300 and 500 �C are shown

in Fig. 2. Most of the Ni/C nanocapsules, NiO/C nanocapsules

and NiO nanoparticles are irregular spheres with a narrow

Fig. 2 – TEM images of (a) Ni/C nanocapsules, (c) NiO/C

nanocapsules and (e) NiO nanoparticles; the corresponding

HRTEM images of (b) Ni/C nanocapsules, (d) NiO/C

nanocapsules and (f) NiO nanoparticles.

0 400 800 1200 16000.0

0.5

1.0

1.5

2.0

2.5

3.0

30th

30th

1st

1st

Capacity mAh g( )-1

Pot

enti

al

V v

s. L

i/Li

()

+

NiO/CNiO

Fig. 3 – The galvanostatic charge–discharge curves of NiO/C

nanocapsules and NiO nanoparticles for the 1st and 30th

cycles.

C A R B O N 6 0 ( 2 0 1 3 ) 2 1 5 – 2 2 0 217

distribution of diameters. Fig. 2(a) shows Ni/C nanocapsules

with the diameters of 10–40 nm, which is larger than that

(10–30 nm) of NiO/C nanocapsules (Fig. 2(c)) and is smaller

than that (20–80 nm) of NiO nanoparticles (Fig. 2(e)). Further-

more, the HRTEM images, as shown in Fig. 2(b) and (d), clearly

indicate that the Ni/C nanocapsules and NiO/C nanocapsules

own a ‘core/shell’ type structure and the inner nanoparticles

cores are encapsulated by an onion-like C cages. The lattice

plane spacing of the shells is about 0.34 nm, corresponding

to the (002) plane of graphite [26–28]. The shells of Ni/C nano-

capsules are about 3.2 nm thick in Fig. 2(b), while the shells of

NiO/C nanocapsules are about 1.5 nm thick in Fig. 2(d). In

Fig. 2(f), the HRTEM image observation shows that the NiO

nanoparticles do not have the core/shell-type structure. This

can be explained by the fact that the onion-like C shells no

longer act as a barrier to effectively prevent the oxidation of

Ni cores because of the oxidation during the annealing pro-

cess, which leads to the reduction of the size of the NiO/C

nanocapsules. The Ni nanoparticle cores are oxidized into

NiO with an increase in the annealing temperature. When

the annealing temperature reaches 500 �C, the onion-like C

shells are completely depleted. The volume change from Ni

to NiO leads to an increase in the size of the NiO nanoparti-

cles. As shown in Fig. 2(b), it is worthy noted that there is a

hollow structure between Ni nanoparticles and onion-like C

shells, which can provide enough void volume to accommo-

date the volume change of the Ni nanoparticles during the

oxidation process and that of NiO nanoparticles during the

cycling process [3].

The electrochemical performance of NiO/C nanocapsules

and NiO nanoparticles as anode materials for Li ion batteries

was investigated. Fig. 3 displays the galvanostatic discharge–

charge curves for the 1st and 30th cycles of the NiO/C nano-

capsule and NiO nanoparticle electrodes measured at a cur-

rent density of 0.5 C (1 C = 718 mAh g�1). During the 1st

charge–discharge cycle, the discharge and charge capacities

of NiO/C nanocapsule electrode are 1689.4 and

1196.0 mAh g�1, respectively. The irreversible capacity loss is

493.4 mAh g�1 with an initial coulombic efficiency of 70.8%.

The NiO nanoparticle electrode has lower discharge and

charge capacities of 1235.9 mAh g�1 and 846.2 mAh g�1,

respectively, for the 1st charge–discharge cycle, together with

a smaller initial coulombic efficiency of 68.5%. These values

are much higher than the theoretical capacity of 718 mAh g�1

of pure NiO electrode and can be attributed to the formation

of SEI films on the NiO/C nanocapsule and NiO nanoparticle

electrodes during the discharge process [5,11]. The theoretical

capacity of 718 mAh g�1 is predicted by the conversion reac-

tion mechanism NiO + 2Li! Ni + Li2O and calculated by the

number of transferred electronics in the reaction. However,

this is just an estimation. Actually, the phenomenon has been

reported several times, yet the measured capacity was higher

than the theoretical capacity [11,29]. Since the capacity of a

battery depends on the reaction of all the available materials,

a material with a hollow structure and a larger surface area

would display a higher reversible capability exceeding the

theoretical capacity because Li+ ions stored in the interfaces

and pores of the material could take part in the interfaces

and pores [29]. However, during the subsequent cycles, the

discharge plateaus shift from �0.4 to �0.8 V, while the charge

plateaus have no obvious changes and are located at around

1.5 and 2.5 V. The reversible capacity of the NiO/C nanocap-

sule electrode at the 30th cycle is 1110.4 mAh g�1, which is

higher than that of the NiO nanoparticle electrode of

606.4 mAh g�1. The coulombic efficiency of the 30th cycle of

the NiO nanocapsule electrode is about 74.0%, reflecting a

much better cycling performance. These results may be due

to the suppression of the side reactions between the NiO

nanoparticles and the electrolyte and an optimization of the

SEI films after the coating of the onion-like C shells [5]. The

effect become more apparent after the 30th cycle.

In order to further understand the effect of the onion-like

C shells on the electrochemical performance of the NiO nano-

218 C A R B O N 6 0 ( 2 0 1 3 ) 2 1 5 – 2 2 0

particles, CV tests were carried out in this work. Usually, a ser-

ies of irreversible reactions would occur during the first dis-

charge process such as the decomposition of electrolyte and

the formation of SEI film, resulting in a large initial irrevers-

ible capacity and a low coulombic efficiency [5,30]. Fig. 4

shows the CV curves of the NiO nanoparticle and NiO/C nano-

capsule electrodes in the potential range of 0–3.0 V at a scan-

ning rate of 0.5 mV s�1. Both electrodes have similar CV

curves. For the first cathodic scan, there is a strong peak at

0.25–0.5 V, corresponding to the decomposition of NiO into

Ni (NiO + 2Li! Ni + Li2O). The formation of amorphous Li2O

and SEI films can cause an irreversible capacity loss. With

increasing the number of cycles, this peak becomes broader

and weaker and shifts to about 0.79 V for the NiO electrode

and 0.75 V for the NiO/C electrode as a result of the conver-

sion of NiO into Ni. The small reduction peak found at around

1.20 V indicates to the formation of SEI films [11,31–33]. In the

subsequent cycles, the oxidation peak of NiO and NiO/C elec-

trodes located at about 1.5 V is associated with the partial

decomposition of the SEI films while the main oxidation peak

located at around 2.5 V is originated from the formation of

NiO [11,32–34]. For the NiO and NiO/C electrodes the intensity

of the cathodic peaks decreases in the subsequent scanning

cycles due to the irreversible reaction (NiO + 2Li! Ni + Li2-

O)and the formation of the SEI films [17]. For the NiO elec-

trode, the reduction peaks are located at 1.22 and 0.79 V,

while the oxidation peaks are detected at 1.52 and 2.57 V.

For the NiO/C electrode, the reduction peaks are found at

1.20 and 0.75 V, while the oxidation peaks are seen at 1.46

and 2.54 V. Obviously, polarization of the NiO/C electrode is

reduced compared to the NiO electrode, indicating an

0.0 0.5 1.0 1.5 2.0 2.5 3.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

Cur

rent

(m

A)

Potential V vs. Li/Li )( +

1st2nd30th

0.46 V

0.79 V

1.22 V

2.57 V1.52 VNiO

0.0 0.5 1.0 1.5 2.0 2.5 3.0

-1.5

-1.0

-0.5

0.0

0.5

1.02.54 V1.46 V

1.20 V

0.75 V

0.44 V

Potential V vs. Li/Li( )+

Cur

rent

(m

A)

NiO/C

1st2nd30th

(a)

(b)

Fig. 4 – CV curves of (a) NiO nanoparticles and (b) NiO/C

nanocapsules for the 1st, 2nd and 30th cycles in the

potential range of 0–3.0 V and at a scanning rate of

0.5 mV s�1.

enhancement of reversibility during cycling. This is because

the high electronic conductivity of the onion-like C shells in

the NiO/C nanocapsules is beneficial to the diffusion of lith-

ium ions [5]. An electrochemical impedance spectroscopy

(EIS) test was also carried out to prove the positive effect of

the onion-like C shells on the NiO nanoparticles. Fig. 5a

shows the Nyquist plots of the NiO/C nanocapsule and NiO

nanoparticle electrodes at room temperature in the frequency

range of 100 kHz to 10 MHz in the first cycle. Both electrodes

have similar Nyquist plots, comprising a semicircle compo-

nent at high frequency representing the charge transfer resis-

tance and a linear component at low frequency indicating the

lithium diffusion process within the electrodes [34]. The inter-

cepts on the real axis can be considered as the combined

resistance of the ionic resistance of the electrolyte, the intrin-

sic resistance of the active materials and the contact resis-

tance at the active material/current collector interface [17].

Fig. 5(b) presents the Nyquist plots of the electrodes after

the 10th cycle. It is seen the NiO/C nanocapsule electrode

has a smaller semicircle than the NiO nanoparticle one. This

suggests that the NiO/C nanocapsule electrode has a smaller

electrochemical reaction resistance and that the onion-like C

shells is beneficial to improving the conductivity of the elec-

trode and enhancing the reaction kinetics [5]. However, the

combined resistance values of NiO/C nanocapsule and NiO

nanoparticle electrodes are at least one order of magnitude

higher than those in conventional electrolytes [35]. This is still

a big disadvantage of using ionic liquids as the electrolyte in

Li ion batteries at room temperature. The study presented

here is just the preliminary results. On-going studies in our

group include optimizing the surface area and carbon content

and selecting different ionic liquids to improve the ionic

conductivity.

Fig. 6 shows the cycling performance of the NiO/C nano-

capsule and NiO nanoparticle electrodes between 0 and

3.2 eV at 0.5 C. The NiO/C nanocapsule electrode delivers an

initial discharge capacity of 1689.4 mAh g�1 and exhibits a

good capacity retention without an obvious capacity fading.

It still retains a capacity of 1157.7 mAh g�1 after 50 cycles,

while the NiO nanoparticle electrode fades quickly and only

delivers a discharge capacity of 383.5 mAh g�1 after 50 cycles.

These results reveal that the NiO/C nanocapsule electrode

with onion-like C shells is beneficial to enhancing the pene-

tration of the electrolyte and reducing the charge transfer

resistance at the active material/electrolyte interface.

0 200 400 600 800 1000 12000

200

400

600

800

1000

1200

0 100 200 3000

100

200

300

z''(o

hm)

Z'(ohm)

NiO/CNiO

Z''(

ohm

)

Z'(ohm)

NiO/CNiO

(a) (b)

Fig. 5 – Nyquist polts of NiO/C nanocapsule and NiO

nanoparticle electrodes for (a) the 1st cycle and (b) the 10th

cycle.

0 10 20 30 40 50300

600

900

1200

1500

1800

Dis

char

ge C

apac

ity

( )

mA

h g-1

Cycle Number

NiO/CNiO

Fig. 6 – Cycling performance of NiO/C nanocapsule and NiO

nanoparticle electrodes at 0.5 C.

0 10 20 30 40 50 60 700

400

800

1200

1600

2000

Cycle Numbers

Dis

char

ge C

apac

ity

mA

h g

()

-1

0.2 C

2 C

1 C

0.5 C0.2 C

NiO/CNiO

Fig. 7 – Discharge capacity of NiO/C nanocapsule and NiO

nanoparticle electrodes at different discharge–charge rates

(0.2, 0.5, 1 and 2 C).

C A R B O N 6 0 ( 2 0 1 3 ) 2 1 5 – 2 2 0 219

Another excellent property associated with the NiO/C

nanocapsule electrode is its high discharge capability. Fig. 7

demonstrates the discharge capability of the NiO/C nanocap-

sule and NiO nanoparticle electrodes at different discharge-

charge rates. It can be found that the discharge capacity of

NiO/C nanocapsule electrode remains stable and decreases

regularly with an increase in discharge rate. The initial dis-

charge capacity of the NiO/C nanocapsule electrode at the

discharge-charge rates of 0.2, 0.5, 1 and 2 C are 1891.6, 1626,

1408.9, and 1105.6 mAh g�1, respectively. On the contrary,

the NiO presents a discharge capacity of only 1435, 1083.3,

860.9, and 558.8 mAh g�1 at the same rate. When the current

density is decreased from 2 to 0.2 C, the discharge capacity

(1700.6 mAh g�1) can be recovered. Additionally, after another

30 cycles at the 0.2 C rate the NiO/C electrode still delivers a

discharge capacity of 1645 mAh g�1, which corresponds to a

96.7% capacity retention of the initial discharge capacity of

1700.6 mAh g�1. These results demonstrate that the NiO/C

nanocapsule electrode has a good electrochemical reversibil-

ity and a high structural stability. Therefore, this novel NiO/C

nanocapsule electrode, which possesses the attractive char-

acteristics of high reversible capacity, good cucability and

high rate capability, is promising for negative electrodes in

Li ion batteries. The good electrochemical performance of

the NiO/C nanocapsule electrode is ascribed to the introduc-

tion of onion-like C shells in the core/shell-type structure.

In our work, the onion-like C shells on the NiO nanoparticles

are believed to have three major roles. First, C itself is an elec-

tronic conductor, which ensures good electrical contact of

NiO with the current collector and enhances the charge trans-

fer/Li+ transport. With the full and uniform coating of C, elec-

trons can easily reach all the positions where Li+ ion

intercalation takes place. This feature is particularly helpful

when the battery is cycled at high currents, and also offers

much more conductive pathways, which are very helpful to

the capacity, cyclability, and rate capability [7,8]. Second the

onion-like C shells act as a structural buffer to greatly allevi-

ate the volume variation of the NiO nanoparticle cores [9]. The

pure NiO structures are easily to be destroyed during the dis-

charge–charge process with large volume changed. For the

NiO/C nanocapsule electrode the preservation of the struc-

ture during the Li+ insertion/extraction processes helps to

keep the electrical continuity. Third, the SEI films formed on

the C shells might be more uniform and stable than on the

NiO nanoparticles, which is favorable for the structural stabil-

ity of the NiO/C nanocapsule electrode.

4. Conclusions

The present work provided a method for the synthesis of a

new type of NiO/C nanocapsule electrode with onion-like C

shells and NiO nanoparticle cores. The proposed NiO/C nano-

capsule electrode was first prepared by a modified arc-dis-

charge method and by annealing at 300 �C for 2 h in air. As

an anode material for Li ion batteries, the NiO/C nanocapsule

electrode exhibited a remarkable discharge capacity, a high

rate charge–discharge capability and an excellent cycling sta-

bility because the onion-like C shells not only can provide en-

ough voids to accommodate the volume change of

encapsulated NiO nanoparticles but also can prevent the for-

mation of SEI films on the surface of NiO nanoparticles and

hence the direct contact of Ni and SEI films upon lithium

extraction. The NiO/C nanocapsule electrode delivered an ini-

tial discharge capacity of 1689.4 mAh g�1 at 0.5 C and main-

tained a high reversible capacity of 1157.7 mAh g�1 after

50 cycles. The performance was much higher than the NiO

nanoparticles (383.5 mAh g�1). Thus, the present study can

shed light on the utility of onion-like C shells to improve

the electrochemical performance and safety of a variety of

transition-metal oxide nanoparticles.

Acknowledgements

This study was supported partly by the National Natural Sci-

ence Foundation of China (Grant Nos. 51201002 and

51071001), by the Research Grants Council of the HKSAR Gov-

ernment (Grant No. PolyU 5236/12E), and by The Hong Kong

Polytechnic University (Grant Nos. G-YK59, and 4-ZZ7L).

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