Low cost and green preparation process for -Fe …...Low cost and green preparation process for...

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Low cost and green preparation process for α-Fe 2 O 3 @gum arabic electrode for high performance sodium ion batteries Li Xu, a,b Hansinee Sitinamaluwa, c Henan Li, a,c Jingxia Qiu, a Yazhou Wang, b Cheng Yan, c Huaming Li,* ,a Shouqi Yuan a and Shanqing Zhang* ,b a Institute for Energy Research, School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, P. R. China. E-mail: [email protected] b Centre for Clean Environment and Energy, Griffith School of Environment, Gold Coast Campus, Griffith University, QLD 4222, Australia. E-mail: [email protected] c School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, Brisbane, QLD 4001, Australia

Transcript of Low cost and green preparation process for -Fe …...Low cost and green preparation process for...

Page 1: Low cost and green preparation process for -Fe …...Low cost and green preparation process for α-Fe 2 O 3 @gum arabic electrode for high performance sodium ion batteries Li Xu,a,b

Low cost and green preparation process for α-Fe2O3@gum

arabic electrode for high performance sodium ion

batteries

Li Xu,a,b

Hansinee Sitinamaluwa,c

Henan Li,a,c

Jingxia Qiu,a

Yazhou Wang,b

Cheng Yan,c

Huaming Li,*,a

Shouqi Yuana

and Shanqing Zhang*,b

aInstitute for Energy Research, School of Chemistry and Chemical Engineering, Jiangsu

University, Zhenjiang 212013, P. R. China. E-mail: [email protected]

bCentre for Clean Environment and Energy, Griffith School of Environment, Gold Coast

Campus, Griffith University, QLD 4222, Australia. E-mail: [email protected]

cSchool of Chemistry, Physics and Mechanical Engineering, Queensland University of

Technology, Brisbane, QLD 4001, Australia

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Abstract

Conventional electrode manufacturing processes for lithium ion batteries involve the use of

toxic organic solvents (such as N-methyl-2-pyrrolidone, NMP). A low cost and green

preparation process for high performance electrodes for sodium ion batteries (SIBs) is

important to address simultaneously the environmental and health risks of production

processes and the shortage of lithium metal. Herein, gum arabic (GA), which is a non-toxic

biodegradable biopolymer, is used as a water soluble binder to design a water-based electrode

preparation process to fabricate α-Fe2O3 electrodes (i.e., α-Fe2O3@GA electrode). The α-

Fe2O3@GA electrode demonstrates better mechanical properties and binding capability than

that of the α-Fe2O3 electrode with poly(vinylidene fluoride) (PVDF) as the binder (α-

Fe2O3@PVDF electrode). Due to these merits, a higher rate and cycling performance of the

α-Fe2O3@GA electrode are achieved compared with the α-Fe2O3@PVDF electrode when

both electrodes are used for SIBs' application. The α-Fe2O3@GA electrode demonstrates high

initial discharge and charge capacities of 2437 and 1102 mA h g-1

at the current density of 0.2

A g-1

. The α-Fe2O3@GA electrode maintains a high reversible discharge capacity of 492 mA

h g-1

at the current density of 5 A g-1

after 500 cycles with a fading rate of 0.08% per cycle

after the first cycle, which indicates a superior cycling performance. The outstanding

performance of the resultant SIBs suggests that the green fabrication process of the α-

Fe2O3@GA electrode would play a critical role in the future battery industry.

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1. Introduction

It has been reported that 80% of the annual global energy consumption (4.1 ×1020

J) is

derived from the combustion of non-renewable fossil fuels including natural gas, coal, and

oil.1 On account of the imminent shortage of non-renewable fossil fuels, the excessive

reliance on fossil fuels has caused severe environmental problems and become a heavy

burden to the current and future society. In response to this problem, the requirements for

sustainable energy are swiftly growing. To cope with the ever-increasing energy demand, it is

the holy-grail in the field of sustainable energy to develop high-efficiency energy storage

technologies with high safety, low cost, high energy density, and high power density.2

Among the energy storage technologies, lithium ion batteries (LIBs) have been widely

utilized as a good choice in hybrid electric vehicles and portable electronic devices by virtue

of their high energy density and long life span.3 However, the large-scale applications of

LIBs in energy storage will be curtailed due to the rarity, uneven distribution and high price

(approximately $5000 per ton) of lithium.4 Compared to lithium, sodium has a high natural

abundance (ranked 4th

among the most abundant elements) and is fairly inexpensive

(approximately $150 per ton).5 Situated below lithium in the periodic table, sodium exhibits

similar physical and chemical properties as lithium.6 Sodium is widely considered as a

promising alternative for lithium in large-scale energy storage systems,7 and great efforts

have been devoted to the exploration and development of sodium ion batteries (SIBs). Kim

and co-workers calculated that SIBs are ca. 10% less expensive in total cost than LIBs,

assuming the same energy density.8 The large ionic radius of Na

+ (Na

+ versus Li

+: 1.02

versus 0.76 Å) causes large volume expansion, large polarization and slow reaction kinetics,

which result in low rate capacities and a poor cycle life.10,11

Nevertheless, the superiority in

SIBs development is the similarities in the manufacturing process between NIBs and LIBs.9

Prior work on LIBs can provide a great guidance into the material selection for SIBs, which

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can significantly accelerate the technological advancement of SIBs.12

A multitude of efforts

have been devoted to explore suitable electrodes with fast reversible sodiation/desodiation

reaction kinetics and high rate capacities.10,13,14

In general, electrodes are commonly composed of electrode materials, conducting agents,

binders and current collectors. These electrode materials occupy about 80% of the mass in the

entire battery and directly determine the performance of the battery.15

As one of the mature

anode materials, transition metal oxides have captured a multitude of attention owing to their

high theoretical specific capacities (around 1000 mAh g-1

), which are almost three times as

much as that of graphite (372mAh g-1

).16

α-Fe2O3, as the most stable phase of iron oxide, is

considered as a promising active anode material on the basis of its abundant distribution, low

cost, environmental benignity, corrosion resistance, and high theoretical specific capacity

(~1007 mA h g-1

).17

Despite its great promise, the practical application of α-Fe2O3 is still

plagued by intrinsically low conductivity, agglomeration, large volume expansion and

pulverization upon charge/discharge, which eventually result in a low-rate capability and

short-term cyclability.18

Two common methods have been employed to circumvent these

problems. The first approach involves the use of nanomaterials with different dimensions and

morphologies to mitigate the mechanical stress due to volume expansion.19

The other

approach is coating or combining conductive materials (such as carbon nanotubes and

graphene) with α-Fe2O3.20

Essentially, another effective approach is to apply a suitable binder

in the electrode fabrication process for enhancing the adhesion between the active material,

conductive additives and current collector.21

The primary goal of the binder is to glue the electrode materials and conducting

agents on the current collector. Although binders only account for 2–5% of the mass in an

electrode, the electrochemical performances of the electrode strongly depend on the binder,

which can enhance the electrical contact between the active materials and conducting

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agents.15,22,23

The ideal binder for application in SIBs should have a low resistance, low cost,

high electrochemical stability, strong bonding strength, and be soluble in green solvents (such

as water).24

Due to the high electrochemical stability, bonding strength, and adhesion of

poly(vinylidene fluoride) (PVDF), PVDF is the most widely used binder for both cathodes

and anodes in the battery industry.25

However, the major drawback of PVDF is that it is

expensive (1.24 AUD per g from Sigma-Aldrich) and only soluble in highly toxic organic

solvents (such as N-methyl-2-pyrrolidone, NMP). NMP is volatile, flammable, explosive, and

expensive (around 1 AUD per mL from Sigma-Aldrich).23

As early as 2001, NMP has been

confirmed as a reproductive toxicant by the Californian Environmental Protection Agency.

Thus, it is compulsory by law to recycle it in the fabrication process of electrodes; however,

the recycling of NMP significantly increases production cost. In order to address this

problem, water-soluble binders have recently drawn much attention owing to their

nontoxicity, high stability, and low-cost.24

Gumarabic (GA) is a cheap, widely available (with

an annual production of over 1 million tons), non-toxic and biodegradable biopolymer from

the exudates of the Arabic Senegal and Acacia Seyal trees.26

GA is a complex mixture

consisting of polysaccharides and glycoproteins.27

Polysaccharides, which have a highly

branched structure, are composed of long carbohydrate monosaccharide units with a great

deal of hydroxyl groups and make up about 90 wt% of the total components of GA.28,29

That

is to say, GA is hydrophilic and enables the fabrication of electrodes in a green aqueous

process. Glycoproteins are high molecular weight proteins containing long oligosaccharide

chains covalently attached to polypeptide side-chains.26

The long spiral glycoprotein chain

behaves like fibers, which can enhance the strength, toughness and energy absorption of

composites.28

As an electrode binder, GA can enhance the tolerance of volume expansion of

electrode materials. Inspired by these properties, PVDF binders can be substituted for the cost

effective and green GA to fabricate high performance electrodes for SIBs.

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Herein, a facile and green electrode fabrication process is designed to enhance the

performance of SIBs (Scheme 1). The α-Fe2O3 electrode is prepared via a green GA-water

based electrode fabrication process for SIBs for the first time. GA is soluble in water,

indicating that the electrode fabrication process with GA as binder can refrain from

application of toxic NMP solvent. Owing to the addition of GA, α-Fe2O3 and conductive

additives can be uniformly dispersed with good contact in the electrodes, which contributes to

the charge transfer and enhances the rate performance of SIBs. Furthermore, the

glycoproteins of GA can form a strong network, which can immobilize the α-Fe2O3

nanoparticles and conductive additives on the current collector, and finally contribute to

improving the toughness of the electrode. The mechanical properties of the α-Fe2O3 electrode

are systematically researched via nanoscratch and nanoindentation tests. The α-Fe2O3

electrode with GA as the binder (α-Fe2O3@GA electrode) demonstrates better mechanical

properties and binding capability than that of the α-Fe2O3 electrode with PVDF as the binder

(α-Fe2O3@PVDF electrode). Compared with PVDF as the conventional binder, the GA

binder can effectively enhance the electrochemical performance of α-Fe2O3 electrodes in

SIBs.

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Scheme 1. Schematic of the green gum arabic-water based electrode fabrication

process.

2. Experimental

Material fabrication and characterization

All chemicals were of analytical grade and used as received without further purification.

Sodium hydroxide (NaOH), GA, PVDF, sodium carboxymethylcellulose (NaCMC) and poly

(acrylic acid) (PAA) were obtained from Sigma-Aldrich. The ionic liquid, 1-octyl-3-

methylimidazolium chloride ([Omim]Cl), was obtained from the Centre for Green Chemistry

and Catalysis, Lanzhou Institute of Chemical Physics, CAS. According to the literature,30,31

the iron ion-containing ionic liquid, 1-octyl-3-methylimidazolium tetrachlorideferrate(III)

([Omim]FeCl4), was synthesized with [Omim]Cl.

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α-Fe2O3 hollow microspheres were synthesized via a simple ionic liquid assisted

solvothermal method according to the report on the previous study by our group.32

In a

typical reaction, the appropriate ionic liquid [Omim]FeCl4 (16 mmol) was dissolved in 80 ml

absolute ethanol to form a homogeneous solution. Then, the appropriate amount of NaOH (24

mmol) was added into the abovementioned solution with vigorous stirring and continued for

30 min. The solution was then placed in a 100 mL Teflon-sealed autoclave and maintained at

140 oC for 24 h and then cooled to room temperature. The final product was separated by

centrifugation, washed with distilled water and absolute ethanol four times, and dried under

vacuum at 60 oC for 12 h before further use.

Electrode preparation

To fabricate the α-Fe2O3@GA electrode, a conventional slurry coating process was

applied. Briefly, GA was firstly dissolved in deionized water to 40 mg ml-1

to produce a

binder solution. α-Fe2O3, carbon black conductive additive and polymer binder were mixed

with a weight ratio of 7 : 1.5 : 1.5. The sonicated homogeneous slurry was uniformly coated

onto copper foil with an area of ca. 1.0 cm2. All the pasted Cu foil electrodes were dried at

60o C in a high vacuum oven overnight to assume the complete removal of water. The areal

mass loading was typically ca. 0.3 mg cm-2

(dried weight of α-Fe2O3). For comparison, α-

Fe2O3@PVDF electrodes were also prepared under the same conditions, and GA binder and

water solvent were replaced with PVDF binder and NMP solvent, respectively.

Electrochemical evaluation

The electrochemical behavior of the resultant α-Fe2O3 electrodes was tested in coin

cells with galvanostatic discharge–charge cycling tests at room temperature (ca. 23 oC) on an

LANDCT 2001A battery tester (Wuhan, China) in the voltage range from 0.001 to 2.5 V vs.

Na/Na+. All current density and specific capacity calculations are based on the active mass of

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α-Fe2O3. CR2032 coin-type cells were assembled in an argon-filled glove box (MBRAUN;

O2 and H2O content #0.1 ppm), using a glass fiber (GA-55) membrane as the separator,

sodium metal sheet as the counter electrode, and 1.0 M NaClO4 in ethylene carbonate

(EC)/propylene carbonate (PC) (1 : 1 by volume) as the electrolyte. A CHI 660D

electrochemical workstation (CHI Instrument, Shanghai, China) was used to perform CV

measurements at a scan rate of 0.1 mV s-1

and potential from 0.001 V to 2.5 V. EIS

measurements were conducted over the frequency range from 100 kHz to 10 mHz.

Materials characterisation

X-Ray powder diffraction (XRD) data in the 2q range of 20-80o were collected on a Bruker

D8 diffractometer with high-intensity Cu-Ka radiation (l ¼ 1.54 Å). Surface images of the

composite electrode were collected on a JEOL 7001F field emission scanning electron

microscope (SEM) coupled with energy-dispersive X-ray. Nanoindentation and nanoscratch

tests were conducted using a Hysitron TI 950 nanoindentation system with a Berkovich

indenter (three-sided pyramidal tip with a radius of approximately 150 nm, and 142.3o total

included angles).

3. Results and Discussion

The XRD patterns of the α-Fe2O3@GA electrode, α-Fe2O3@PVDF electrode and pristine

α-Fe2O3 are shown in Fig. 1. Clearly, the XRD patterns of all the samples match the α-Fe2O3

hexagonal structure (JCPDS no. 33-0664), and no other iron-containing compounds are

observed, such as Fe3O4, γ-Fe2O3, and β-FeOOH. This indicates that the crystalline structure

of α -Fe2O3 is not altered when the α-Fe2O3 electrodes are prepared with GA and PVDF as

binders.

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Fig. 1. XRD patterns of the α-Fe2O3@GA electrode, α-Fe2O3@PVDF electrode and

pristine α-Fe2O3 powder.

The mechanical properties of the α-Fe2O3@GA and α-Fe2O3@PVDF electrodes were

measured by nanoscratch and nanoindentation tests. The nanoscratch test was conducted

under a normal force of 500 mN and the scratch length was 6 mm. As shown in Fig. 2A, the

friction coefficient of the α-Fe2O3@GA electrode is higher than that of the α-Fe2O3@PVDF

electrode. This indicates that GA possesses stronger binding capability than PVDF.

Compared with the α-Fe2O3@PVDF electrode, the friction coefficient of the α-Fe2O3@GA

electrode is steadier over the entire scratch length due to the improved homogeneity of the α-

Fe2O3@GA electrode.33

The 3D in-situ nanoscratch images of the electrodes obtained via

scanning probe microscopy (SPM) are shown in Fig. 2, which further display the

homogeneity of electrode materials. As shown in Fig. 2(C and D), the 3D in situ nanoscratch

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images of the α-Fe2O3@GA and α-Fe2O3@PVDF electrodes are very different after the

nanoscratch test. The depth of the scratch track of the α-Fe2O3@GA electrode (870 nm) is

much smaller than that of the α-Fe2O3@PVDF electrode (1410 nm), which indicates that the

deformation of the α-Fe2O3@GA electrode is much less when subjected to the same normal

force. The increased mechanical strength in the α-Fe2O3 electrodes with GA binder can

provide greater tolerance to volume change than that of the α-Fe2O3 electrode with PVDF

binder. During the charge/discharge process of SIBs, the appropriate mechanical strength will

benefit the overall stability and binding capability. To this end, nanoindentation tests were

conducted on the α-Fe2O3@GA and α-Fe2O3@PVDF electrodes with the loading forces of

500 mN and 1000 mN. The indentation depth of the α-Fe2O3@GA electrode (300 nm) is

smaller than that of the α-Fe2O3@PVDF electrode (700 nm) under 500 mN (Fig. 2B). The

indentation depths of the α-Fe2O3@GA and α-Fe2O3@PVDF electrodes increase to 425 and

1000 nm under 1000 mN (Fig. S1), respectively. Therefore, the hardness of the α-Fe2O3@GA

electrode is approximately two times greater than that of the α-Fe2O3@PVDF electrode.

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Fig. 2. (A) Friction coefficient of the α-Fe2O3@GA and α-Fe2O3@PVDF electrodes; (B)

load–displacement curves for the α-Fe2O3@GA and α-Fe2O3@PVDF electrodes under the

force of 500 mN; 3D in situ nanoscratch images of the α-Fe2O3@GA electrode (C) and α-

Fe2O3@PVDF electrode (D) after nanoscratch tests.

The mechanical properties of these two electrodes in electrolyte environment were also

investigated by nanoscratch and nanoindentation tests. The friction coefficient of the α-

Fe2O3@GA electrode is almost the same as the dry condition; however, a significant decrease

is observed in the α-Fe2O3@PVDF electrode (Fig. S2A). The 3D in situ nanoscratch indicates

that the scratch depth in the α-Fe2O3@GA electrode is smaller compared with that in the α-

Fe2O3@PVDF electrode after electrolyte uptake (Fig. S2C and D). The nanoindentation depth

under 500 mN in the α-Fe2O3@PVDF electrode is 60% deeper than that in the α-Fe2O3@GA

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electrode after electrolyte uptake (Fig. S2B). All these results indicate that the α-Fe2O3@GA

electrode has better mechanical strength and stability than the α-Fe2O3@PVDF electrode.

In addition, the compatibility and dispersion of the GA binder were compared with those

of the PVDF binder. The surface morphologies of the α-Fe2O3@GA and α-Fe2O3@PVDF

electrodes were obtained via SEM imaging and the related elemental mappings (Fig. 3). The

SEM image and related elemental mappings of the α-Fe2O3@GA electrode indicate a better-

distributed contact between α-Fe2O3 and conductive carbon black on the surface of the α-

Fe2O3@GA electrode. In contrast, the α-Fe2O3 particles and conductive carbon black are

agglomerated on the surface of the α-Fe2O3@PVDF electrode. This demonstrates that GA has

better dispersion compared with that of PVDF. The α-Fe2O3 electrode prepared with the GA

binder is able to form a strong network between the α-Fe2O3 particles and conductive carbon

black, which can probably reduce the resistance between adjacent α-Fe2O3 particles and

promote charge transfer, and thus result in outstanding electrochemical performances.25,34

Fig. 3. SEM images and elemental mappings of the α-Fe2O3@GA (A) and α-

Fe2O3@PVDF electrodes (B).

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The α-Fe2O3@GA and α-Fe2O3@PVDF electrodes were tested as anodes of SIBs. The

electrochemical properties of the α-Fe2O3@GA electrode were first evaluated via cyclic

voltammetry (CV) measurements for three cycles in the potential range of 0.001–2.5 V (vs.

Na/Na+) at a scan rate of 0.1 mV s

-1 (Fig. 4A). During the initial cathodic process of the first

cycle, the weak peak at about 1.0 V is associated with sodium ion insertion into the Fe2O3

crystal to form NaxFe2O3 and the reversible reduction reaction of Fe2O3 to form Fe0.19

The

broad peak at about 0.4 V is assigned to the irreversible formation of a solid electrolyte

interface (SEI) film and some side reactions between the electrode materials and electrolyte.35

During the anodic process of the first cycle, the weak anodic peaks at about 0.7 and 1.40 V

are associated with the two-step oxidation of Fe0 (Fe

0 / Fe

2+ and Fe

2+/Fe

3+, respectively).

36 On

account of the heavy mass, large size, and poor mobility of the sodium ion, it is difficult for

strong redox peaks to emerge.17

The diversity between the initial and subsequent scan cycles

is attributed to irreversible processes in the first cycle due to the formation of an SEI film.

The CV curves overlap well after the first cycle, which indicates the superior stability of the

α-Fe2O3@GA electrode for repeated electrochemical reactions. The CV curves of the α-

Fe2O3@PVDF electrode are also shown in Fig. S3(A). The stability of the α-Fe2O3@PVDF

electrode is weaker compared with the α-Fe2O3@GA electrode. Fig. 4B depicts the

representative charge–discharge voltage profiles of the α-Fe2O3@GA electrode at 0.2 A g-1

between 0.001 and 2.5 V. The α-Fe2O3@GA electrode demonstrates high initial discharge

and charge capacities of 2437 and 1102 mA h g-1

. In the first cycle, the irreversible capacity

is mainly due to the decomposition of the electrolyte to form an SEI film,18

which is

consistent with the CV result. The initial capacity of the α-Fe2O3@GA electrode far exceeds

the theoretical capacity of α-Fe2O3 (~1007 mA hg-1

), which can be attributed to the formation

of the SEI film and further Na+ storage via a pseudocapacitance process by virtue of the

charge separation at the metal/Na2O phase boundary.37

The α-Fe2O3@PVDF electrode shows

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lower initial discharge and charge capacities (708 and 434 mA h g-1

, respectively) (Fig. S3B).

The rate capabilities of the α-Fe2O3@GA and α-Fe2O3@PVDF electrodes are illustrated in

Fig. 4C. The α-Fe2O3@GA electrode delivers a high reversible capacity of 547 mA h g-1

at a

current density of 5 A g-1

. The reversible capacity of 372 mA h g-1

is also exhibited at a high

current density of 15 A g-1

. 99% of the original capacity (540 mA h g-1

) recovers when the

current density reverts to 5 A g-1

. Moreover, the excellent coulombic efficiency of the α-

Fe2O3@GA electrode is ca. 99% (Fig. 4D). The α-Fe2O3@PVDF electrode only delivers a

low reversible capacity of 12 mA h g-1

at 5 A g-1

(Fig. 4C). The reversible capacities of the α-

Fe2O3@PVDF electrode have no difference at high current densities (10 and 15 A g-1

).

Clearly, the α-Fe2O3@GA electrode exhibits excellent rate performances, which indicate fast

reaction kinetics.

The long-term cycling performance of the α-Fe2O3@GA and α-Fe2O3@PVDF electrodes

was also tested at a current density of 5 A g-1

, as shown in Fig. 5. The α-Fe2O3@GA

electrode delivers an initial discharge capacity of 792 mA h g-1

, which is ca. 80% of the

theoretical capacity (1007 mA h g-1

) and much higher than that of the α-Fe2O3@PVDF

electrode (554 mA h g-1

). It can be observed that the α-Fe2O3@GA electrode maintains the

high reversible discharge capacity of 492 mA h g-1

after 500 cycles with an outstanding

coulombic efficiency (~100%), which indicates a superior cycling performance with a fading

rate of 0.08% per cycle after the first cycle. The α-Fe2O3@GA electrode exhibits an

outstanding cycling performance. The question is whether the structure of the α-Fe2O3@GA

electrode can tolerate the strain associated with Na+ insertion. As revealed in Fig. S4, the

surface morphology of the α-Fe2O3@GA electrode is very well retained after 500 cycles,

which to a certain extent explains the stable cycling. It is interesting that the Na-ion storage

performance of the α-Fe2O3@GA electrode does not significantly change at different mass

loadings (Fig. S5). NaCMC and PAA, as aqueous binders, have been widely used in SIBs. α-

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Fe2O3@NaCMC and a-Fe2O3@PAA electrodes have been also prepared, replacing GA

binder with NaCMC and PAA binder, respectively. The friction coefficients of the α-

Fe2O3@NaCMC and α-Fe2O3@PAA electrodes are lower compared with that of the a α-

Fe2O3@GA electrode (Fig. 2A and S6). This indicates that PAA and NaCMC possess weaker

binding capabilities than GA, which thus cause the poor cycling performance of the α-

Fe2O3@NaCMC and α-Fe2O3@PAA electrodes (Fig. 5). The reversible discharge capacity of

the α-Fe2O3@PVDF electrode also quickly fades with the following cycles (Fig. 5); it can

only deliver a reversible discharge capacity of ca. 31 mA h g-1

after 100 cycles. The GA

binder exhibits better capacity retention than the PVDF binder. Fig. S7 and S8 show the

electrochemical impedance spectra (EIS) for the α-Fe2O3@GA and the α-Fe2O3@PVDF

electrodes measured after different cycles at a current density of 5 A g-1

. The Nyquist plots of

the two electrodes show a semicircle at the high-medium-frequency region before cycling.

Note that the diameter of the semicircle for the α-Fe2O3@GA electrode is smaller than that of

the α-Fe2O3@PVDF electrode (355.6 Ω vs. 570.4 Ω). This demonstrates that the α-

Fe2O3@GA electrode possesses outstanding electrical conductivity and small charge-transfer

resistance. The well-distributed contact between α-Fe2O3 and conductive carbon black

benefits sodium-ion diffusion. The GA binder can enhance the interfacial properties of the

electrode and promote charge transfer at the electrode/electrolyte interface, which eventually

result in an outstanding electrochemical performance.25,38,39

After the first cycle, the charge

transfer resistance (Rct) of the α-Fe2O3@GA electrode decreases to 242 Ω and subsequently

reduces to 170.8 Ω after 10 cycles. In the following cycles, the Rct of the α-Fe2O3@GA

electrode is stabilized at about 160 Ω. The Rct of the α-Fe2O3@PVDF electrode during

cycling is unstable (Fig. S8). The stabilization of Rct of the α-Fe2O3@GA electrode confirms

that the sodium-ion diffusion and electron transfer are stabilized after extensive cycling,

which contributes to the enhanced cycling performance of the α-Fe2O3@GA electrode.

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Several recent studies related to α-Fe2O3-based anodes for SIBs are summarized in Fig.

6.18,19,35,40–45

Compared with other Fe2O3-based electrodes, the α-Fe2O3@GA electrode

exhibits a high reversible capacity and excellent cycling performance after 500 cycles at the

high current density of 5 A g-1

. The outstanding electrochemical performance of the α-

Fe2O3@GA electrode can be attributed to the application of GA binder. The “fiber in

concrete” provides strong binding capability, reinforced mechanical strength and flexibility.28

GA can enhance the mechanical properties of the electrode, which results in improved

tolerance to volume expansion in the α-Fe2O3 anode during the charge/discharge process. GA

also contributes to the optimum contact between α-Fe2O3 and conductive carbon black, which

can promote charge transfer and result in outstanding electrochemical performances for the α-

Fe2O3@GA electrode for SIBs.

Fig. 4 (A) Cyclic voltammetry curves of the α-Fe2O3@GA electrode; (B) voltage capacity

profiles of the α-Fe2O3@GA electrode at a current density of 0.2 A g-1

; (C) rate cycling

Page 18: Low cost and green preparation process for -Fe …...Low cost and green preparation process for α-Fe 2 O 3 @gum arabic electrode for high performance sodium ion batteries Li Xu,a,b

capacities of the α-Fe2O3 @GA and α-Fe2O3 @PVDF electrodes at different current densities;

and (D) coulombic efficiency of the α-Fe2O3 @GA electrode.

Fig. 5 (A) Cyclic performance of the α-Fe2O3 @GA, α-Fe2O3 @PVDF, α-Fe2O3 @PAA

and α-Fe2O3 @NaCMC electrodes at a current density of 5A g-1

; and (B) charge–discharge

curves of the α-Fe2O3@GA electrode at a current density of 5 A g-1

.

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Fig. 6. Specific capacity versus cycle number for Fe2O3-based electrodes.

4. Conclusions

In summary, a high performance α-Fe2O3 electrode as an anode for SIBs is fabricated with

GA as the binder via a green water-based electrode fabrication process. The α-Fe2O3@GA

electrode demonstrates better mechanical properties and binding capability than that of the α-

Fe2O3@PVDF electrode. GA can uniformly disperse and strongly contact the α-Fe2O3 and

conductive additives, which result in enhanced charge transfer in the α-Fe2O3 electrode. GA

can also enhance the tolerance of volume expansion in the α-Fe2O3 electrode. Thus, the α-

Fe2O3@GA electrode exhibits a high rate performance and excellent cycling performance.

The α-Fe2O3@GA electrode demonstrates high initial discharge and charge capacities of

2437 and 1102 mA h g-1

at a current density of 0.2 A g-1

. A reversible capacity of 372mA h g-

1 is also exhibited at a high current density of 15 A g

-1. The α-Fe2O3@GA electrode maintains

the high reversible discharge capacity of 492 mA h g-1

at a current density of 5 A g-1

after 500

cycles with a fading rate of 0.08% per cycle after the first cycle, which indicates a superior

cycling performance.

Page 20: Low cost and green preparation process for -Fe …...Low cost and green preparation process for α-Fe 2 O 3 @gum arabic electrode for high performance sodium ion batteries Li Xu,a,b

Acknowledgement

This study has been financially supported by the National Natural Science Foundation of

China (no. 21476098, 21506081 and 21506077), Jiangsu University Scientific Research

Funding (15JDG048), University Natural Science Research of Jiangsu (15KJB530004), the

Jiangsu Province Postdoctoral Science Foundation (1501026B), Chinese Postdoctoral

Foundation (2016M590420 and 2016M591787) and a Project Funded by the Priority

Academic Program Development of Jiangsu Higher Education Institutions.

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Page 25: Low cost and green preparation process for -Fe …...Low cost and green preparation process for α-Fe 2 O 3 @gum arabic electrode for high performance sodium ion batteries Li Xu,a,b

Supplemental Information

A low cost and green preparation process of -Fe2O3 @gum

arabic electrode for high performance sodium ion battery

Li Xua,b

, Hansinee Sitinamaluwa c, Henan Li

a,c, Jingxia Qiu

a, Yazhou Wang

b, Cheng Yan

c, Huaming

Lia,*

, Shouqi Yuana, Shanqing Zhang

b,*

a Institute for Energy Research, School of Chemistry and Chemical Engineering,

Jiangsu University, Zhenjiang 212013, P. R. China

b Centre for Clean Environment and Energy, Griffith School of Environment,

Gold Coast Campus, Griffith University, QLD 4222, Australia

c School of Chemistry, Physics and Mechanical Engineering, Queensland

University of Technology, Brisbane, QLD 4001, Australia.

*Corresponding authors:

Prof. Huaming Li, email: [email protected]

Prof. Shanqing Zhang, email: [email protected]

Electronic Supplementary Material (ESI) for Journal of Materials Chemistry A.This journal is © The Royal Society of Chemistry 2017

Page 26: Low cost and green preparation process for -Fe …...Low cost and green preparation process for α-Fe 2 O 3 @gum arabic electrode for high performance sodium ion batteries Li Xu,a,b

0 200 400 600 800 10000

300

600

900

1200L

oad

(

N)

Indent depth (nm)

-Fe2O

3@GA

-Fe2O

3@PVDF

Fig. S1 Nanoscratch and indentation tests for -Fe2O3@GA and -Fe2O3@PVDF electrodes with

the force of 1000 μN.

Page 27: Low cost and green preparation process for -Fe …...Low cost and green preparation process for α-Fe 2 O 3 @gum arabic electrode for high performance sodium ion batteries Li Xu,a,b

0 1 2 3 4 5

0.2

0.3

0.4

0.5

0.6

Fri

ctio

n C

oef

fici

ent

Lateral Displacement (m)

-Fe2O

3@GA

-Fe2O

3@PVDF

(A)

0 200 400 600 800 10000

150

300

450

600

Lo

ad (

N)

Indent depth (nm)

-Fe2O

3@GA

-Fe2O

3@PVDF

(B)

Fig. S2 Nanoscratch tests (A) and nanoindentation tests (B) for -Fe2O3@GA and

-Fe2O3@PVDF electrodes after electrolyte uptake; 3D in-situ nanoscratch image of

-Fe2O3@GA (C) and -Fe2O3@PVDF (D) electrodes after electrolyte uptake by nanoscratch

tests.

Page 28: Low cost and green preparation process for -Fe …...Low cost and green preparation process for α-Fe 2 O 3 @gum arabic electrode for high performance sodium ion batteries Li Xu,a,b

0.0 0.5 1.0 1.5 2.0 2.5-0.6

-0.4

-0.2

0.0

0.2

Cu

rren

t (m

A)

Voltage (V)

(A)

0 500 1000 1500 2000 25000.0

0.5

1.0

1.5

2.0

2.5

Vo

ltag

e (V

)

Capacity (mAh g-1

)

1st Cycle

2nd

Cycle

3rd

Cycle

(B)

Fig. S3 (A) Cyclic voltammetry curves of the -Fe2O3@PVDF electrodes; and (B) voltage

capacity profiles of the -Fe2O3@PVDF electrodes at a current density of 0.2 A g-1

.

Page 29: Low cost and green preparation process for -Fe …...Low cost and green preparation process for α-Fe 2 O 3 @gum arabic electrode for high performance sodium ion batteries Li Xu,a,b

Fig. S4 SEM images of the α-Fe2O3@GA electrode before (A,B) and after (C,D) 500 cycles.

Page 30: Low cost and green preparation process for -Fe …...Low cost and green preparation process for α-Fe 2 O 3 @gum arabic electrode for high performance sodium ion batteries Li Xu,a,b

0 20 40 60 80 1000

200

400

600

800

Mass loading of 1 mg/cm2

Sp

ecif

ic C

apac

ity

(m

Ah

g-1

)

Cycle number

Mass loading of 0.3 mg/cm2

Fig. S5 Cyclic performance of the -Fe2O3@GA electrode at different mass loading.

Page 31: Low cost and green preparation process for -Fe …...Low cost and green preparation process for α-Fe 2 O 3 @gum arabic electrode for high performance sodium ion batteries Li Xu,a,b

0 1 2 3 4 50.35

0.40

0.45

0.50

0.55

0.60

Fri

ctio

n C

oef

fici

ent

Lateral Displacement (m)

-Fe2O

3@PAA

-Fe2O

3@NaCMC

Fig. S6 Nanoscratch tests for -Fe2O3@PAA and -Fe2O3@NaCMC electrodes

Page 32: Low cost and green preparation process for -Fe …...Low cost and green preparation process for α-Fe 2 O 3 @gum arabic electrode for high performance sodium ion batteries Li Xu,a,b

0 500 1000 15000

300

600

900

1200

1500

Pristine

1st cycle

10th cycle

50th cycle

100th cycle

250th cycle

500th cycle

-Z''

(Ohm

)

Z' (Ohm)

(A)

0 100 200 300 400 5000

200

400

600

Rct (

Ohm

)

Cycle number

(B)

Fig. S7 EIS of the -Fe2O3@GA electrode after different cycles.

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0 400 800 1200 16000

500

1000

1500

2000

2500

3000

Pristine

1st cycle

10th

cycle

50th

cycle

100th

cycle

-Z''

(Ohm

)

Z' (Ohm)

(A)

0 20 40 60 80 1000

150

300

450

600

(B)

Rct (

Ohm

)Cycle number

Fig. S8 EIS of the -Fe2O3@PVDF electrode after different cycles.