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1

Dealloyed Fe3O4 octahedra as anode material for lithium-ion

batteries with stable and high electrochemical performance

Shi Jiaa, b, Tingting Songc, Bingge Zhaoa, b, Qijie Zhaia, Yulai Gaoa, b,*

a Shanghai Key Laboratory of Modern Metallurgy and Materials Processing, Shanghai

University, 149 YanChang Road, 200072 Shanghai, P. R. China

b Laboratory for Microstructures, Shanghai University, 99 Shangda Road, 200444

Shanghai, P. R. China

c School of Mechanical and Mining Engineering, The University of Queensland,

Brisbane, QLD 4072, Australia

Abstract

Fe3O4 octahedra are synthesized by dealloying Al-15Fe (at.%) alloy ribbons in 5

mol·L-1 NaOH solution at 95±5 ć for 2 hours. The electrochemical properties of the

Fe3O4 octahedra as anode material for lithium-ion batteries are investigated. X-ray

diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray

spectroscopy (EDX) are employed to study the structural evolution. The results show

that the Al can be leached out from Al-15Fe ribbons consisting of α-Al (Fe) and

Al13Fe4 phases to obtain regular Fe3O4 octahedra. Galvanostatic charge-discharge

cycling of the Fe3O4 octahedra in half cell configuration with lithium at 50 mA·g-1

current density exhibits first discharge capacity of 1077 mAh·g-1, 16.3% higher than

* Corresponding author. Tel. : +86-21-56332144. E-mail address: ylgao@shu.edu.cn.

2

the theoretical capacity of Fe3O4. The cell shows stability at charge-discharge rate of

50 mA·g-1 and good capacity retention rate up to 38 cycles. The coulombic efficiency

of the Fe3O4 octahedra remains as nearly 100% except for the first cycle. Moreover,

dealloying strategy is suggested to be facile in large-scale production of superior

anode materials for lithium-ion batteries, indicating its promising prospect for

practical application.

Keywords: AlFe ribbons; dealloying; Fe3O4 octahedra; lithium-ion batteries; anode

material

1. Introduction

Dealloying, which is referred to a selective corrosion process, has been widely

used to fabricate nanoporous metals, during which the more active component is

dissolved away from the precursor yet the less active component is remained in

nanoporous microstructure [1]. Dealloying has a long history, e.g the Incan

civilization dealloyed copper from the surface of copper-rich Cu/Au alloys to create

an illusion of a pure gold artifact, the so-called process generally known as depletion

gilding [2]. In addition, variations of depletion gilding were independently developed

in Europe and used by medieval artisans [3-5]. In recent years, dealloying has been

paid renewed attentions attributing to the fact that certain particular systems exhibit

nanoporosity evolution upon dealloying [6]. Ding et al. [7, 8] has produced

nanoporous gold (NPG) with multiple pore size distribution through chemical

3

dealloying of Ag–Au alloys. Zhang et al. [9, 10] has used Al-Au film as the precursor

alloy, also employed dealloying way to fabricate NPG, which could reduce the loss of

precious metals. Song et al. [11-13] has systematically studied the dealloying

behavior of Al-Ag ribbons to produce nanoporous silver (NPS). Some other alloy

systems, such as Al–Cu [14, 15], Cu-Mn [16], Pt–Si [17] and Al–Pd [18], have also

been developed to produce nanoporous copper (NPC), nanoporous platinum (NPPt)

and nanoporous palladium (NPPd), respectively.

Recently, some researchers have found that dealloying of Al-Fe alloy (-1.662 V

standard electrode potential (SHE) for Al3+/Al and -0.4402 V SHE for Fe2+/Fe) in

NaOH solution did not result in nanoporous Fe as the above-mentioned alloy systems,

but octahedral Fe3O4 particles [19]. This finding opens up new opportunities for

dealloying approach in creating not only nanoporous metals but also metallic oxides

promising in wide applications like TiO2, Mn3O4 and Co3O4 [19]. Some other

methods had also been developed for the preparation of Fe3O4 particles, such as

hydrothermal process [20] and coprecipitation method [21]. However, their reaction

process are generally complex and hard to control [22, 23]. Compared with these

methods, dealloying strategy revealed evident advantages of simple processing,

shorter time consumption, smaller size obtained, nearly absolute yield, and was

applicable for large-scale synthesis [13, 24]. Our previous studies have found a

variety of reaction conditions (the concentration of OH-, the solution temperature and

dealloying time) could affect the dealloying results and dealloying Al-15Fe ribbons in

5 mol·L-1 NaOH solution for 2 hours at 95 ± 5 � is the optimal condition to acquire

4

regular Fe3O4 octahedra within the scope of our research works [25].

Over the past few decades, Fe3O4 has been widely investigated in many fields,

such as ferromagnetic�[26], biomedical�[27, 28], and catalysis applications�[29, 30]. It

is also a promising anode material in lithium ion batteries, with theoretical capacity of

926 mAh·g!1�[31, 32], higher than commercialized graphite anode (372 mAh·g!1)�[33].

Fe3O4 with various morphologies and sizes have been synthesized and used as anode

of lithium-ion batteries. Cheng et al.�[34] have fabricated rugated porous Fe3O4 thin

films as stable binder-free anode materials for lithium ion batteries which can supply

reversible capacity of 432 mAh·g!1 (~360 mAh·g!1 after 100th cycle at 1 A·g!1 rate).

Chen et al.�[35] have developed a porous hollow Fe3O4 particles that can deliver a

reversible capacity of 500 mAh·g!1 at 100 mA·g!1 rate (up to 50 cycles). Su et al.�[36]

have synthesized one-dimensional magnetic Fe3O4 nanowires that can exhibit 770

mAh·g!1 reversible capacity at 50 mA·g!1 rate (around 100 cycles). But the initial

capacity and cyclic stability of these Fe3O4 anodes can not be both excellent, and the

processes to obtain these Fe3O4 particles are complex and costly. Therefore,

developing a technique which can easily obtain Fe3O4 with excellent electrochemical

performance in large quantities is significant.

Attributed to the advantage of dealloying and melt-spinning technique (preparing

the precursor alloy ribbons), in this paper, we synthesized regular Fe3O4 octahedra by

dealloying Al-15Fe ribbons in a 5 mol·L-1 NaOH solution for 2 hours at 95 ± 5 �.

The as-prepared sample was used as anode material for lithium-ion batteries to test

the electrochemical performance.

5

2. Experimental section

2.1 Material preparation

The alloy ingot with nominal compositions of Al-15Fe (at.%) was prepared from

Al (purity, 99.999 wt.%) and Fe (purity, 99.99 wt.%) using an induction furnace. The

pre-alloyed ingots were remelted and rapidly solidified into ribbons using a single

roller melt-spinning method at a linear speed of 18.4 m·s-1 in an argon atmosphere.

Continuous ribbons with a cross section of (0.025 ~ 0.045) " 5 mm2 were prepared.

Dealloying of these rapidly solidified Al–15Fe alloy ribbons was carried out in 5

mol·L-1 NaOH solutions at 95 ± 5 ć. The solution was prepared from analytic

reagents and distilled water. After being dealloyed, the samples were rinsed with

distilled water and dehydrated alcohol six times with ultrasonic cleaning equipment.

Finally, the samples were dried in an electro-thermostatic blast oven (DHG-9076A) at

40 ć.

2.2 Structure characterization

The phases of the rapidly solidified Al–15Fe alloys and the as-dealloyed samples

were identified by X-ray diffraction (XRD, Rigaku D/Max-2200) in the 2θ range of

10–80° at a scanning rate of 2°·min-1, using Cu-Kα radiation (λ= 0.154056 nm). The

microstructures of the as-dealloyed samples were observed by scanning electron

microscopes (SEM, JSM-6700F combined with Phenom Pro X). The instrument

employed for elemental identification was an energy dispersive X-ray spectroscopy

(EDX, INCA by Oxford) coupled with the SEM.

6

2.3 Electrochemical testing

Electrochemical experiments were performed with coin cell (CR2025) using

lithium foil as counter and reference electrode. The working electrode was fabricated

by mixing the as-dealloyed Fe3O4 powders, acetylene black, and polyvinylidene

fluoride (PVDF) binder at a weight ratio of 80:10:10 in N-methyl-pyrrolidinone

(NMP) solvent. The resultant slurry was then uniformly pasted onto a copper foil

using a doctor blade method and then dried in a vacuum oven (DZF-6062) at 120 �

for 12 hours. The cells were assembled in an argon-filled glove box and a

Celgard2325 separator soaked with a 1 mol·L-1 LiPF6 ethylene carbonate-diethyl

carbonate (EC/DEC, 1:1 by volume) electrolyte solution was used. The cells were

discharged and charged at a current density of 50 mA·g-1 in a voltage range of 0-3 V

using a battery tester (LAND CT2001A).

3. Results and discussion

3.1 Structure characterization

Fig. 1 shows the XRD patterns of the starting Al–15Fe ribbons and the

dealloying products obtained in a 5 mol·L-1 NaOH solution at 95 ± 5 ć for 2 hours

(The insertion is the image of Al-15Fe ribbon). The rapidly solidified Al–15Fe

precursor contains α-Al (JCPDS No. 65-2869), Al13Fe4 (JCPDS No. 50-0797) and a

small amount of α-Fe (JCPDS No. 65-4899). It was deemed that the small amount of

α-Fe phase was attributed to the regional segregation of Fe element in the preparation

7

process of the Al–15Fe alloy ingots and ribbons. The fact that the alloy is slightly rich

in aluminum can benefit the corrosion process by accelerating the reaction rate in

alkaline solution [19]. The diffraction peaks of the dealloying products mainly

correspond to Fe3O4 (JCPDS No. 89-0688) indicating Al in α-Al(Fe) and Al13Fe4

phases was removed and the remained Fe was oxidized into Fe3O4. A small amount of

α-Fe phase (JCPDS No. 65-4899) can be detected, produced in the rapid solidification

process. The general morphology of as-dealloyed Fe3O4 was observed by SEM (as

shown in Fig. 2(a, b)). Typical SEM images reveal that the as-dealloyed sample

shows a regular octahedral structure, which is different from the typical nanoporous

structure after being dealloyed [37]. An inset crystallographic diagram of a Fe3O4

octahedron is shown in Fig.2(a) and the inset SEM-BSE image in Fig. 2 (b) shows the

morphology of the α-Fe phase (shown by the yellow arrows) existed in the dealloying

products. The typical EDX result (corresponding to the area marked by an cross in Fig.

2(b)) is shown in Fig. 2(c) and only 0.44 at.% Al could be detected (The peak of the C

element was caused by carbon conductive adhesive), indicating that the dealloying

process was completed. The average edge length of the regular Fe3O4 octahedra was

evaluated as 550 ± 177 nm, as shown in Fig. 2(d) (acquired by counting the edge

length of all the Fe3O4 octahedra observed in Fig. 2(a)).

3.2 Electrochemical properties

The lithium storage capacity of the as-prepared Fe3O4 octahedra was evaluated

by electrochemical testing. Fig. 3(a) shows the discharge/charge profiles of Fe3O4

8

octahedra at 1st, 2nd, 10th, 20th and 38th cycles. The discharge/charge curves are

consistent with the previous report [38, 39]. As can be seen from Fig. 3(a), the voltage

variation during the initial discharge process have three distinct features: Firstly, the

discharge voltage shows a steep voltage drop from 1.87 V to 0.77 V, which can be

attributed to lithium ion insertion into the Fe3O4 particles as shown in Eq. (1) [38, 40,

41]. Then a long voltage plateau at around 0.77 V versus Li/Li+ was observed

corresponding to the conversion reaction that results the formation of Li2O and Fe as

shown in Eq. (2) [32, 42]. Finally, a sloping curve from 0.77 V down to the cutoff

voltage of 0.01 V, indicating the formation of a solid electrolyte interphase (SEI) film

[43]. The voltage plateau observed here is in good correspondence with the literature

reported before [42, 44-46].

Fe3O4 + xLi+ + xe- == LixFe3O4 (1)

LixFe3O4 + (8 ! x)Li+ + (8 ! x)e- == 3Fe + 4Li2O (2)

The initial discharge process shows a total capacity of 1077 mAh·g-1 (the

position indicated by an arrow in Fig. 3(a)), 16.3% higher than the theoretical capacity

of Fe3O4 [31]. It is noted that this phenomenon has been reported for iron oxides

[47-49], which is usually ascribed to the formation of the SEI film and possible

interfacial lithium storage. Fig. 3(b) shows the cycle performance and coulombic

efficiency of the Fe3O4 electrode at a current density of 50 mA·g-1. The irreversible

capacity in the first cycle can be attributed to the decomposition of electrolyte and

formation of solid electrolyte interface [35]. The reversible capacity retention rate can

reach 33.9% over 38 cycles. Moreover, the Fe3O4 octahedra exhibit gradual capacity

9

fading after 29 cycles and its capacity attenuates just 5.2% from 29th cycle to 38th

cycle. The initial coulombic efficiency of the Fe3O4 octahedra is around 68%, and it

will quickly increase to 97% in the fifth one and remain as nearly 100% in the

following cycles. The poor coulombic efficiency of the first cycle is resulted from the

high irreversible capacity produced during the first discharge process. This is a typical

behavior for systems containing nano- or submicro-structured active materials that

have large electrode/electrolyte contact areas [47]. Liu et al. [50] employed Fe3O4

nanospheres as electrode materials of lithium-ion batteries and the initial discharge

capacity was up to 1600 mAh·g-1, but its capacity dropped about 80% only after 20

cycles (Correspondingly, the capacity retention rate is necessarily lower than 20% if it

carried out 38 cycles). Latorre-Sanchez et al. [51] had fabricated Fe3O4-CNT

composite and tested its electrochemical properties. And their testing results showed

that the reversible capacity reduced to 400 mAh·g-1 after 38 cycles (the capacity

retention rate is about 28.6%), although its initial discharge capacity could reach 1400

mAh·g-1. Obviously, the stability of the Fe3O4 octahedra anode is better than that of

other Fe3O4 particles or Fe3O4-based materials.

Though it needs an in-depth investigation to probe the mechanism for excellent

property of the dealloyed Fe3O4 octahedra as anode material for lithium-ion batteries,

yet the newly dealloyed Fe3O4 octahedra does display excellent initial discharge

capacity and high capacity retention. In addition, considering the facile preparation

process and possibility for large scale production, this kind of Fe3O4 is anticipated to

be an excellent anode material for lithium-ion batteries.

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4. Conclusions

Fe3O4 particles with the size of 550 ± 177 nm have been facilely fabricated by

the one-step dealloying of rapidly solidified binary Al-15Fe ribbons, which are

favorable for large-scale manufacture, in an alkaline solution under free corrosion

conditions. SEM analysis indicates that the submicro-sized Fe3O4 particles are in

regular octahedral structure. These Fe3O4 octahedra display a relatively high initial

discharge capacity of 1077 mAh·g!1 and reversible capacity retention of 33.9% over

38 cycles. The Fe3O4 octahedra exhibit gradual capacity fading after 29 cycles and its

capacity attenuated just 5.2% from 29th cycle to 38th cycle. The coulombic efficiency

of the Fe3O4 octahedra remained as nearly 100% except for the first cycle. The good

initial discharge capacity and high capacity retention of the submicron-sized Fe3O4

octahedra imply its good application prospect in the field of lithium-ion batteries.

Acknowledgements

This work is supported by the National Natural Science Foundation of China

(Grant Nos. 51171105 and 50971086) and the financial support from the 085 project

in Shanghai University is also acknowledged.

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16

Figure captions

Fig. 1. XRD patterns of the rapidly solidified Al-15Fe ribbons and the dealloying

products obtained in 5 mol·L-1 NaOH solution for 2 hours at 95 ± 5 ć. The

corresponding JCPDS information of the phases were also given in the figure. The

insertion is the photo of Al-15Fe ribbon.

Fig. 2. (a), (b) SEM images of the Fe3O4 samples after being dealloyed in 5 mol·L-1

NaOH solution for 2 hours at 95 ± 5 ć. (c) The corresponding EDX spectrum. (d)

Histogram of the particle size distribution for Fe3O4 octahedra. The inset in (a)

indicates the crystallographic diagram of a Fe3O4 octahedron. The inset in (b)

indicates the SEM-BSE image of the dealloying products.

Fig. 3. (a) The galvanostatic charge/discharge curves of the Fe3O4 electrode at a

current density of 50 mA·g-1. (b) Cycle performance and coulombic efficiency of the

Fe3O4 electrode at a current density of 50 mA·g-1.

17

Fig. 1

18

Fig. 2

19

Fig. 3