Accepted Manuscript

Title: Pyro-Synthesis of Nanostructured Spinel ZnMn2O4/C asNegative Electrode for Rechargeable Lithium-Ion Batteries

Author: Muhammad Hilmy Alfaruqi Alok Kumar Rai VinodMathew Jeonggeun Jo Jaekook Kim

PII: S0013-4686(14)02274-9DOI: http://dx.doi.org/doi:10.1016/j.electacta.2014.11.066Reference: EA 23738

To appear in: Electrochimica Acta

Received date: 10-9-2014Revised date: 11-10-2014Accepted date: 11-11-2014

Please cite this article as: Muhammad Hilmy Alfaruqi, Alok Kumar Rai, Vinod Mathew,Jeonggeun Jo, Jaekook Kim, Pyro-Synthesis of Nanostructured Spinel ZnMn2O4/Cas Negative Electrode for Rechargeable Lithium-Ion Batteries, Electrochimica Actahttp://dx.doi.org/10.1016/j.electacta.2014.11.066

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Pyro-Synthesis of Nanostructured Spinel ZnMn2O4/C as Negative Electrode for Rechargeable Lithium-Ion Batteries

Muhammad Hilmy Alfaruqi1, Alok Kumar Rai1, Vinod Mathew, Jeonggeun Jo, Jaekook Kim*

Department of Materials Science and Engineering, Chonnam National University, 300

Yongbong-dong, Bukgu, Gwangju 500-757, Republic of Korea

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*Corresponding author. Tel: +82-62-530-1703; Fax: +82-62-530-1699.

E–mail address: jaekook@chonnam.ac.kr (Jaekook Kim)

1 These authors contributed equally to this work.

Abstract

ZnMn2O4/C nanoparticles are synthesized by one step polyol assisted pyro-synthesis for use

as the anode in rechargeable lithium ion batteries without any post heat treatment. The as-

prepared ZnMn2O4/C is tetragonal with a spherical particle size in the range of 10–30 nm.

Electrochemical measurements were performed using the as-prepared powders as the active

material for a lithium-ion cell. The nanoparticle electrode delivered an initial charge capacity ACCEPTED M

ANUSCRIPT

of 666.1 mAh g-1 and exhibited a capacity retention of ∼81% (539.4 mAh g-1) after 50 cycles.

The capacity enhancement in the as-prepared ZnMn2O4/C may be explained on the basis of

the polyol medium that enables to develop a sufficient carbon network that can act as

electrical conduits during electrochemical reactions. The carbon network appears to enhance

the particle-connectivity and hence improve the electronic conductivities.

Keywords: ZnMn2O4; Carbon; Electrochemical properties; Li-ion batteries

1. Introduction:

The latest technology products such as laptop computers, mobile phones, digital

cameras and electric vehicles require high-performance energy storage systems to power

them. Until today, lithium-ion batteries have been the most attractive energy storage systems

to satisfy the requirement, due to their high voltage, high energy density and light weight [1].

Commercially available lithium-ion batteries comprised of LiCoO2 as the cathode and

graphite as the anode. Graphite is being considered for use as the anode material for lithium-

ion batteries for several reasons, such as the fact that the Li+ ions can easily intercalate/de-

intercalate into/from the graphite structure, low irreversible capacity at the first cycle, a flat

and low potential profile, a long cycle life, abundant and relatively low cost [2]. However, in

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spite of its advantages, graphite has several drawbacks, such as its low energy density, low

theoretical capacity (372 mAh g-1), easy exfoliation and safety issues related to lithium

deposition [2, 3]. These drawbacks limit its application to electric vehicles, hybrid electric

vehicles and plug-in hybrid electric vehicles. Therefore, searching for new electrode materials

which can act as an alternative to graphite with a high specific capacity and excellent

electrical conductivity is an urgent task in building the next-generation high-power lithium-

ion batteries, in order to fulfill the ever-growing demand in various consumer electronic

devices.

Poizot et al. were the first to find that transition metal oxides undergo a reversible

conversion reaction, where the metal cation is reduced into transition metal nanoparticles

embedded into a matrix of Li2O upon discharge and are oxidized back into transition metal

oxides nanoparticles during subsequent charge [4, 5]. In addition, nanostructured metal

oxides such as manganese oxide [6], cobalt oxide [5], iron oxide [7] and tin oxide [8] have

been among the most widely investigated alternative anode materials for use in lithium ion

batteries over the last decade, because of their high specific capacity (500 to 1000 mAh g-1),

in comparison to conventional graphite anode (372 mAh g-1). Unfortunately, the huge

volume expansion/contraction during repetitive charge/discharge cycling processes, large

irreversible capacity loss, low capacity retention, severe aggregation during the cycling

process and poor electrical conductivity limit its application for high power lithium ion

batteries [9].

Beside these simple binary transition metal oxides, ternary metal oxides AB2O4

containing two transition metals (A and B) with spinel structures have also been proposed as

an interesting material, because of their low price, abundance, environmental friendliness and,

more importantly, the possible synergetic enhancement of the components in such mixed

metal oxides [10]. Among these ternary transition metal oxides series, recently ZnMn2O4 has

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been considered as one of the most attractive compounds, because of the low oxidation

(delithiation) potential of zinc and manganese nanoparticles: 1.2 V and 1.5 V, respectively,

which ultimately increase the battery output voltage compared to that provided by simple

binary metal oxides [11-13]. Furthermore, zinc and manganese are abundant, environmentally

friendly, and relatively inexpensive compared to nickel and cobalt. Interestingly, ZnMn2O4

can store Li+ ions through both conversion and alloying reactions, because Zn, one of the

products of the conversion reaction, further alloys with Li to form LiZn such as [11,14]:

OLiMnZneLiOZnMn 242 499 ++→++ −+ (1)

LiZneLiZn ↔++ −+ (2) −+ ++↔+ eLiMnOOLiMn 44222 2 (3)

−+ ++↔+ eLiZnOOLiZn 222 (4)

However, as in the case of other anodes, the poor electric conductivity and large volume

change of the ZnMn2O4 nanostructures during the electrochemical reaction lead to a fast

capacity decrease. However, a few attempts have been made to overcome the aforementioned

drawbacks of ZnMn2O4 by focusing on controlling the morphology accompanied with

synthesis of nano-sized particles and graphene based nanocomposites using expensive and

tedious methods [14-17]. Nanostructured materials have become a subject of great interest in

a variety of mechanical, electrical and optical applications, due to their unique physico-

chemical properties [18]. In the realm of rechargeable batteries, nanostructured electrodes

offer a large surface area and therefore tend to enhance the diffusion kinetics by reducing the

diffusion pathway for electronic and ionic transport. In addition, nanostructured materials

also facilitate the formation of a large contact area between the active materials and

electrolyte which, in turn, ensures a relatively greater amount of guest-ion insertion/de-

insertion. In addition, it is also well-known that the simultaneous carbon coating of

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nanostructured materials is an effective way to enhance the performance of secondary

batteries. These carbon coatings not only tend to improve the electrical conductivity and

prevent excessive particle growth, but also accommodate the volume changes of the electrode

during cycling, which can result in the improvement of the cycle life.

Therefore, we report here the facile and rapid synthesis of carbon coated ZnMn2O4

with a nanostructured morphology via the polyol assisted pyro-synthesis method without the

use of any conventional carbon agent. This method may provide a path way towards cost-

effective and simple strategies of nanomaterial production under very short reaction times.

The polyol here acts as a primary fuel to induce a flame which can instantly provide ultrahigh

thermal energy to the surroundings. While the polyol undergoes fast combustion (exothermic),

the precursors thermochemically decompose (endothermic) and nucleate under the oxygen-

limited atmosphere by the useful consumption of the thermal energy released during the

exothermic reaction [19, 20]. The entire process occurs dynamically within a few seconds.

The high energy generated and short reaction time facilitates rapid nucleation and suppresses

grain growth, which can in turn lead to the formation of highly crystalline ultrafine particles.

In addition, the use of polyol solvent, which also allows the formation of the carbon network

to improve the electrical connectivity between the particles, is also discussed. All these

factors appear to significantly influence the overall electrochemical performance of the as-

prepared ZnMn2O4/C anode at high current rates.

2. Experimental method:

2.1. Materials Synthesis:

The polyol-assisted pyro synthesis to obtain the ZnMn2O4/C nanoparticles is a

straightforward approach and is clearly illustrated in Scheme 1. Briefly, the nanostructured

ZnMn2O4/C was prepared by the pyro-synthesis method using zinc acetate

[Zn(CH3COO)2.2H2O, 98% Yakuri Pure Chemicals, Japan] and manganese acetate

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[Mn(CH3COO)2.4H2O, 97% Yakuri Pure Chemicals, Japan] as the initial precursors. The

starting precursors were dissolved in 80 ml of ethylene glycol [Daejung Chemicals, South

Korea] and stirred for 24 h at room temperature to obtain a homogenous solution. The

ethylene glycol used in the present synthesis acts not only as a solvent and reducing agent but

also a carbon source. Subsequently, 50 ml of liquid thinner was added to the solution for 30

minutes. After that, the final solution was poured onto a hot-plate preheated to 250 oC. To

induce the combustion process, the solution was ignited using an electric torch, resulting in

the rapid precipitation of highly crystalline nanoparticles [19, 20]. The powder was obtained

directly after the combustion process. The final product was thoroughly ground using an

agate mortar and directly used for the evaluation of the electrochemical performance without

any further heat treatment.

2.2. Materials Characterization:

To investigate the crystal structure of the as-prepared ZnMn2O4/C powder, its X-ray

powder diffraction (XRD) pattern was obtained by a Shimadzu X-ray diffractometer with Cu-

Kα radiation (λ = 1.5406 Å). The surface morphology and particles sizes were analyzed by

field-emission scanning electron microscopy and field-emission transmission electron

microscopy (FE-TEM, Philips Tecnai F20 at 200 keV, KBSI, Chonnam National University,

South Korea). The samples were soaked in ethanol and dispersed by ultrasonic vibration

before coating onto copper grids for FE-TEM examination. The EDX mapping of the sample

was done using an energy dispersive X-ray analyzer (EMAX Energy EX-200, Horiba)

attached to F20. The carbon content of as-prepared ZnMn2O4/C powder was determined by

CHN elemental analysis using Flash-2000 Thermo Fisher.

2.3. Electrochemical measurements:

The electrochemical properties of the as-prepared ZnMn2O4/C sample was evaluated

using lithium metal as the reference electrode by using a coin-type (CR-2032) half-cell. The

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electrodes were prepared by mixing 80 wt.% active material, 10 wt.% Super P as the

conducting agent and 10 wt.% PVDF as the binder in N-methyl-2-pyrrolidinone (Sigma

Aldrich) as the solvent to form a homogenous slurry. The slurry was coated on copper foil as

the current collector by the doctor blade technique and dried at 100 ºC in a vacuum oven for

12 h. Subsequently, the coating was pressed between stainless steel twin rollers to improve

the adhesion between the copper foil and active materials. The electrodes were cut into

circular disks and assembled as half-cells in a glove box. The electrolyte was comprised of a

solution containing 1 M LiPF6 in a mixture of ethylene carbonate and dimethyl carbonate

(molar ratio 1:1). The cells were kept overnight before the electrochemical measurements.

The discharge/charge measurements were performed at room temperature using BTS 2004H

(Nagano Keiki Co. Ltd., Ohta-ku, Tokyo, Japan). The cells were cycled in the voltage range

between 0.01 and 2.8 V vs. Li/Li +. Cyclic voltammetry (CV) measurements of the electrode

were performed between 0 and 2.8 V (versus Li+/Li) using a Bio Logic Science Instrument

(VSP 1075) at a scan rate of 0.1 mV S-1.

3. Results and Discussion:

3.1 Crystal structure and morphology

Fig. 1 shows the XRD pattern of the as-prepared ZnMn2O4/C nanoparticles. All of

diffraction peaks can be perfectly indexed to the characteristic diffractions peaks of tetragonal

ZnMn2O4 (JCPDS card no. 24-1133, space group I41/amd) with a spinel structure. The

relatively broad diffraction peaks suggest the nanocrystalline nature of the product.

The morphology of the as-prepared ZnMn2O4/C powder was studied by FE-SEM and

FE-TEM and the recorded images are displayed in Fig. 2. The FE-SEM image shown in Fig.

2 (a) illustrates that the ZnMn2O4/C powder is composed of uniform spherical nanoparticles

with sizes in the range of 10-30 nm. In order to understand the clear carbon distribution in the

as-prepared sample, FE-TEM images of ZnMn2O4/C were recorded and the results are shown

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in Fig. 2. Figs. 2 (b) and (c) show the FE-TEM images of the as-prepared ZnMn2O4/C at

different magnifications. The clear contrast observed in the images of Figs. 2 (b) and (c)

clearly appears to distinguish between the carbon coated particles and the carbon network

present in the sample, which is favorable for the fast electrical conduction and electrode

reactions. The carbon content estimated by the CHN elemental analysis is 8.7%, which

appears to be sufficient for the formation of a carbon layer and network. The relatively darker

locations appear to indicate the presence of carbon coated ZnMn2O4 nanoparticles with slight

agglomeration. It is well known that smaller nanoparticles aggregate into secondary particles,

which is probably due to their extremely small dimensions and high surface energies. Fig. 2

(d) clearly shows interplanar spacings of 3.02 Å and 4.88 Å for the (112) and (101) crystal

planes, respectively, which are well matched with the XRD result in Fig. 1. The well-defined

lattice fringes confirmed that the as-synthesized ZnMn2O4 nanoparticles possessed well-

pronounced crystalline structures.

Energy Dispersive X-ray (EDX) elemental mapping studies were also performed to

further confirm the presence of carbon in the sample and the results are displayed in Fig. 3.

The EDX mapping showed the homogeneous distribution of carbon (C) and other elements

(Zn, Mn and O) in the specific portion (dark field image) of the as-prepared ZnMn2O4/C

sample selected for the mapping studies, indicating the good quality of the chemical synthesis.

More importantly, the carbon signal was detected uniformly over the whole selected area,

validating our structural design and indicating that carbon is well distributed within the

sample.

3.2 Electrochemical performance

In order to verify the suitability of the as-prepared ZnMn2O4/C nanoparticles for use

as the anode material for lithium ion batteries, the electrochemical performance with respect

to Li+ ion insertion/extraction was investigated by CV and galvanostatic cycling. Fig. 4 (a)

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shows the 1st, 2nd, 5th, 10th and 20th cycles of CV at a scan rate of 0.1 mV s-1 in the voltage

range of 0 to 2.8 V. The first CV cycle is significantly different from the subsequent cycles,

which indicates that the reaction mechanism is different during the first lithium intercalation.

In the first cathodic scan, two obvious reduction peaks are observed at ~1.0 V, followed by an

intensive peak at ~0.2 V, respectively. The first broad peak located at ~1.0 V disappears in the

later cycles, which may be ascribed to the irreversible decomposition of the solvent in the

electrolyte to form a solid electrolyte interphase (SEI) film [9-10, 16]. The intensive peak at

~0.2 V may be due to the irreversible reduction of ZnMn2O4 with Li into Zn0 and Mn0

embedded in amorphous Li2O and further the reaction of the newly formed Zn0 with Li to

form Li-Zn alloy [9-10, 16]. The subsequent anodic scan shows a wide oxidation peak

ranging from 1.0 V ~1.8 V, corresponding to the oxidation of Mn and Zn to MnO (~ 1.2 V)

and ZnO (~1.5 V), respectively, along with the decomposition of the Li2O matrix [9]. From

the second cycle onward, the repeated reduction peaks shift to ~0.4 V, while the oxidation

peaks remain at the same position. The CV curves are similar to those of nanocrystalline

ZnMn2O4 obtained by different synthesis techniques [9-10, 16].

Fig. 4 (b) shows the discharge/charge curves of the as-prepared ZnMn2O4/C

nanoparticle electrode for the initial three cycles cycled between 0.01 and 2.8 V at a constant

current rate of 0.03 C. The first discharge curve starts from the open circuit voltage (~3.1 V)

and shows a continuous decrease in voltage with a small plateau at about ~0.7 V, then a wide

plateau at about ~0.3 V and a continuous slope to the lower cut off voltage of 0.01 V,

consuming an overall capacity of 1415.8 mAh g-1. The obtained capacity is much larger than

the theoretical capacity of bulk ZnMn2O4 (1008 mAh g-1) expected by the reduction of

ZnMn2O4 into Zn0 and Mn0 and further the formation of LiZn alloy, according to equation (1)

and (2) [11, 14, 22]. The first charge capacity is 666.1 mAh g-1, corresponding to the

dealloying of LiZn to Zn0 and the oxidation of metal Zn0 to ZnO and Mn0 to MnO [10]. The

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extra capacity in the first discharge profile may be attributed to the irreversible formation of a

SEI film at the ZnMn2O4/C crystal–electrolyte interface, electrolyte decomposition and the

formation of Li2O. In addition, as a result of the large volume expansion/contraction of the

ZnMn2O4/C nanocrystals during the charge/discharge process, the reversible capacity fades in

the initial few cycles. Therefore, it retained reversible charge capacities of 712.1 mAh g-1 and

663.4 mAh g-1 for the 2nd and 3rd cycles, respectively.

Fig. 4 (c) shows the cycling performance and corresponding Coulombic efficiency of

the as-prepared ZnMn2O4/C nanoparticle electrode cycled at a constant current rate of 0.03 C.

It can be clearly seen that during the initial 20 cycles, there is relatively fast capacity decay.

The initial Coulombic efficiency was around 47.04%. The relatively poor initial Coulombic

efficiency, which has been a typical disadvantage for high-capacity metal oxide based

negative electrode materials, can be mainly ascribed to the incomplete conversion reaction

and irreversible lithium loss due to the formation of an SEI film on the surface of the active

material and electrolyte decomposition during the discharge process [12]. A similar

phenomenon is also observed in other oxide systems during the initial few cycles, due to the

formation of an unstable SEI layer on the metal particles and the lack of intimate electrical

contact between the electrode materials and current collector [10]. Thereafter, the electrode

exhibits almost stable cycling performance and maintains a capacity of around 539.4 mAh g-1

after 50 cycles with a Coulombic efficiency of almost ~99%. It is reasonable to suggest that

the small nanoparticle size, which increased the contact area between the active material and

the electrolyte, is one of the reasons for the high reversible capacity of the ZnMn2O4/C

nanoparticle electrode. The smaller nanoparticles allowed quicker lithium-ion intercalation

and deintercalation, due to the short distances for lithium-ion transport within the particles.

Most importantly, the enhanced electrochemical performance of the ZnMn2O4/C electrode

probably originates from the existence of a carbon network, which imparts good electronic

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conductivity to the electrode materials. In addition, the obtained value is comparable to those

of previously reported ZnMn2O4 nanoparticles synthesized by tedious and multistep synthesis

techniques [10-11, 21].

In addition to the high capacity and cyclability, the rate capability was also checked at

various current rates between 0.03 C to 3.0 C and the results are shown in Fig. 4 (d). It is

obvious that the capacity decreases with increasing current rate. The as-prepared ZnMn2O4/C

nanoparticle electrode exhibits average charge capacities of 666.1, 505.4, 433.1, 365.7, 300.1,

234.4, 161.5 and 121.9 mAh g-1 at current rates of 0.03 C, 0.06 C, 0.12 C, 0.24 C, 0.48 C,

0.97 C, 1.9 C and 3.0 C, respectively. More importantly, the charge capacity of the as-

prepared ZnMn2O4/C nanoparticle electrode can also recover around 80% of its initial charge

capacity value when the current rate is returned back to 0.03 C, indicating the better cycling

performance of the present nanoparticle electrode at various current rates. However, the

obtained charge capacity value at high current rates is comparable than those reported for

ZnMn2O4 nanoparticles synthesized by the hydrothermal method, but the synthesis adopted in

the present study is very cost-effective, novel and simple [11].

4. Conclusions:

In summary, we have successfully synthesized ZnMn2O4/C nanoparticles through the

polyol-assisted pyro synthesis novel method for the first time without any post heat treatment

and used as a lithium storage anode for lithium ion batteries. The resultant nanoparticles are

spherical with sizes in the range of 10-30 nm. As anode material, the ZnMn2O4/C

nanoparticle electrode exhibits a high reversible charge capacity of 666.1 mAh g-1 at 0.03 C

and in spite of that it also retained a reversible capacity of 539.4 mAh g-1 over 50 cycles

under the same current rate and 121.9 mAh g-1 under the high current rate of 3.0 C. It is

anticipated that the combined factors of a high specific surface area, good electrical contact

between the particles, easier lithium ion diffusion, surface carbon coating and the presence of

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carbon networks within the sample will contribute to the enhanced electrode performance of

ZnMn2O4/C in lithium batteries.

Acknowledgments:

This work was supported by the Global Frontier R&D Program (2013-073298) on

Center for Hybrid Interface Materials (HIM) funded by the Ministry of Science, ICT &

Future Planning.

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Figure Captions

Scheme 1: Schematic diagram of the pyro-synthesis of ZnMn2O4/C anode. Figure 1: X-ray powder diffraction pattern of as-prepared ZnMn2O4/C sample obtained by the polyol-assisted pyro synthesis method. Figure 2: (a) FE-SEM image and (b and c) FE-TEM images at different magnifications and (d) corresponding HR-TEM image of as-prepared ZnMn2O4/C sample. Figure 3: Elemental mapping images of as-prepared ZnMn2O4/C sample prepared by polyol-assisted pyro synthesis method; dark-field image and the elemental distributions. Figure 4: Electrochemical performance of as-prepared ZnMn2O4/C nanoparticles electrode. (a) Cyclic voltammograms at a scan rate of 0.1 mV S-1. (b) Discharge/charge voltage profiles at the rate of 0.03 C. (c) Cyclability and Coulombic efficiency plots between 0.01 and 2.8 V vs. Li/Li + at the constant rate of 0.03 C. (d) The C-rate capability at various current rates between 0.03 C and 3.0 C.

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