FeCO3 Composite as a High-Performance

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C: Energy Conversion and Storage; Energy and Charge Transport

Ternary Fe2O3/Fe3O4/FeCO3 Composite as a HighPerformance Anode Material for Lithium-Ion Batteries Yu Huang, Yanwei Li, Renshu Huang, and Jinhuan Yao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b02132 • Publication Date (Web): 01 May 2019 Downloaded from http://pubs.acs.org on May 1, 2019

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The Journal of Physical Chemistry

Ternary

Fe2O3/Fe3O4/FeCO3

Composite

as

a

High-

Performance Anode Material for Lithium-Ion Batteries Yu Huang, Yanwei Li*, Renshu Huang, Jinhuan Yao* Guangxi Key Laboratory of Electrochemical and Magneto-chemical Functional Materials, College of Chemistry and Bioengineering, Guilin University of Technology, Guilin 541004, China

ABSTRACT Fe-based oxides have been recognized as one of the most promising anode materials for lithium-ion batteries (LIBs) due to their theoretical capacities, low cost, ecofriendliness, and natural abundance. However, poor cycling stability and low rate capability severely hindered their practical applications. Compared to single component material, multicomponent composites are more capable to achieve optimal electrochemical performance for electrode materials due to the synergetic effect. In this work, a novel ternary Fe2O3/Fe3O4/FeCO3 composite is fabricated by a facile hydrothermal method with FeCl2·4H2O and urea as raw materials. When evaluated as an anode material for LIBs, this ternary Fe2O3/Fe3O4/FeCO3 composite exhibits excellent rate capability (with reversible capacities of 624, 488, and 270 mAh g-1 at 1.0, 3.0, and 10.0 A g-1, respectively) and remarkably cycling stability (with a reversible

* Corresponding

authors.

E-mail addresses: [email protected] (Y. Li); [email protected] (J. Yao). 1

ACS Paragon Plus Environment

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capacity of 779 mA h g-1 after 300 cycles at 0.5 A g-1 and 410 mA h g-1 after 800 cycles at 3.0 A g-1), which are much superior to those of the single-component Fe2O3 material. It is found that the ternary Fe2O3/Fe3O4/FeCO3 composite possesses good structural integrity during cycling and fast electrochemical reaction kinetics, which result in the excellent long-term cycling stability and enhanced high-rate capability. The results may provide clues for the rational structural design of high-performance Fe-oxides anode materials for next-generation LIBs.

2


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1. INTRODUCTION To meet the ever-increasing demands of clean energy conversion and storage, developing high-performance energy storage devices have become an urgent topic. Lithium-ion batteries (LIBs) show great advantages as the sources of energy supply because of their of high energy density, long lifespan, negligible self-discharge, and environmental friendliness.1-5 Currently, LIBs are widely used in the field of portable consumer electronics, hybrid electric vehicles (HEVs), electric vehicles (EVs), and smart grids.6-9 In this context, there is a dramatic demand for LIBs with higher energy density, better power density, and longer cycling life. Unfortunately, the commercial graphite-based anode materials are far from satisfying this demand due to their limited theoretical capacity (372 mAh g-1), poor rate capability, and safety issues. Transition metal oxides (TMOs) are considered as one of the most competitive candidates for the potential anode materials of next-generation LIBs owing to their high theoretical specific capacity and resources abundance.10-13 More specifically, Fe-based oxides (Fe2O3 and Fe3O4) stand out from other candidates because of their high theoretical capacity (~1000 mAh g-1), low cost of production, eco-friendliness, natural abundance, and good safety.8,14-16 Despite of these intriguing features, Fe-based oxides anode materials suffer from poor cycling stability and low rate capability due to their intrinsic poor conductivity, sluggish Li+ diffusion kinetics, and huge volume changes during the lithiation and de-lithiation processes.17-19 Constructing multicomponent composites comprising different types of materials provides a new avenue for improving the electrochemical performance of electrode materials.11,20-23 For example, 3



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Luo et al.22 synthesized TiO2@Fe2O3 hollow nanostructures using atomic layer deposition (ALD), and the composite delivered a reversible specific capacity 530 mAh g-1 after 200 cycles at the current density of 0.2 A g-1; Hao et al.

23fabricated

Fe2O3/MnO2 composite, which delivered a high specific capacity of 706 mAh g-1 at 5 A g-1 after 2000 cycles. The improved lithium storage performance of those composites can be ascribed synergetic effect of the individual components and the unique hybrid structure. FeCO3, a new class of anode material for LIBs, has attracted much attention recently because of its abundant natural resources, low cost, easy preparation, and considerable theoretical capacity.24 In the present work, we fabricated a novel ternary Fe2O3/Fe3O4/FeCO3 composite by a very facile hydrothermal method with FeCl2·4H2O and urea as raw materials. When studied as an anode material for LIBs, this ternary Fe2O3/Fe3O4/FeCO3 composite shows high specific capacity, excellent cycling stability, and superior high-rate capability due to the synergetic effect of individual components, improved structural integrity, and enhanced electrochemical reaction kinetics during lithiation and de-lithiation processes. The results demonstrate the promising feature of this ternary Fe2O3/Fe3O4/FeCO3 composite as a novel high-performance anode material for LIBs.

2. EXPERIMENTAL SECTION 2.1 Synthesis of ternary Fe2O3/Fe3O4/FeCO3 composite

4


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The ternary Fe2O3/Fe3O4/FeCO3 composite was synthesized via a facile one-step hydrothermal method. Typically, 1.05 g FeCl2·4H2O and 1.58 g urea was dissolved in 70 mL distilled water. After sonicating for 2 h at room temperature, the mixed solution was transferred into a to 100 mL Teflon-lined stainless steel autoclave and maintained at 180 °C for 24 h in an oven. After reaction, the product was collected by filtration, washed thoroughly with distilled water, and freeze dried until the water was completely removed. For comparison, the single-component Fe2O3 was also prepared by the same method as described above but without using urea. 2.2 Physical characterizations The phase structure of the as-prepared samples was identified by X-ray diffraction (XRD) with X’Pert3 power diffractometer (Panalytical, X’Pert3 power) with Cu Kα radiation. The morphology of the samples was observed by filed-emission scanning electron microscopy (FESEM, Hitachi SU 5000) equipped with energy-dispersive Xray spectroscopy (EDS, Bruker QUANTAX system). Fourier transform infrared (FTIR) spectra were performed on a Thermo Nicolet NEXUS 470 spectrometer. The valent state of Fe, O, and C in the sample was determined by X-ray photoelectron spectroscopy (XPS, Thermo Electron Corp, ESCALAB 250Xi) with a monochromatic Al Kα radiation. 2.3 Electrochemical measurements The electrochemical performance of the ternary Fe2O3/Fe3O4/FeCO3 composite and single-component Fe2O3 was examined by using CR2016 coin-type cells with pure 5



lithium foil as counter electrode, Celgard 2400 polypropylene film as separator, and 1.0 M LiPF6 in ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC) (1: 1: 1 in volume ratio) as electrolyte. The working electrode was fabricated by mixing 70 wt% active material (ternary Fe2O3/Fe3O4/FeCO3 composite or singlecomponent Fe2O3), 20 wt% carbon black (Super P), and 10 wt% polyvinylidene fluoride (PVDF) with an appropriate N-methy pyrrolidone (NMP) solvent to form a slurry, then the slurry was uniformly coated on a Cu foil. The coated electrode was dried at 80 °C for 12 h in vacuum, and then punched into disks with a diameter of 15 mm as the working electrode. The loading mass of electrodes is ∼0.7 mg cm-2. CR2016type coin cells were assembled in an Ar-filled glove box (Mikrouna, Super 1220/750/900) containing less than 0.1 ppm H2O and O2. Galvanostatic discharge/charge tests were conducted in a voltage range of 0.01-3.0 V (vs. Li/Li+) using a multi-channel battery testing system (Neware, BTS-5V/10 mA, China) under various current densities. And the specific capacity of electrode was based on the mass of active material. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were carried out by a CHI760E electrochemical workstation (Chenhua, Shanghai, China). The CV was recorded in a potential range of 0.01-3.0 V (vs. Li/Li+) at various scan rates; the frequency range used in EIS test is from 105 to 10-2 Hz with an AC voltage amplitude of 5 mV. The full cell was assembled with the ternary Fe2O3/Fe3O4/FeCO3 composite as anode and commercial LiCoO2 as the cathode; the separator and electrolyte used in the full cell are the same as those of the ternary Fe2O3/Fe3O4/FeCO3 composite half-cells. The capacity ratio of the LiCoO2 cathode to 6


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the ternary Fe2O3/Fe3O4/FeCO3 composite anode was designed to be ~1.2 based on their initial charge and discharge capacities in half cell. The capacity of the full cell was calculated based on the weight of active material in anode. The cycling performance of the full cell was measured at a current density of 200 mA g-1 (based on the weight of active material in anode) in the voltage range of 1.0-3.9 V.

3. RESULTS AND DISCUSSION XRD patterns of the as-prepared ternary Fe2O3/Fe3O4/FeCO3 composite and singlecomponent Fe2O3 sample are shown in Figure 1a. All the diffraction peaks of the singlecomponent Fe2O3 sample prepared by hydrothermal method with adding urea are consistent with the α-Fe2O3 (JCPDS NO.33-0664).25 By comparison, the addition of urea in the hydrothermal synthesis process generates two new phases indexing to Fe3O4 (JCPDS NO.65-3107)26 and FeCO3 (JCPDS NO.29-0696)27 in addition to the α-Fe2O3 (JCPDS NO.33-0664). That is, ternary Fe2O3/Fe3O4/FeCO3 composite can be synthesized with the assistance of urea by a facile hydrothermal method. The chemical structure of the ternary Fe2O3/Fe3O4/FeCO3 composite and sing-component Fe2O3 was also analyzed by FT-IR measurement and the results are shown in Figure 1b. The broad adsorption band from 3000 to 3600 cm-1 is assigned to the stretching and bending vibrations of adsorbed water molecules;28 the two absorption bands at 481 and 565 cm-1 can be ascribed to the Fe-O vibration in α-Fe2O3;29 the three absorption broads at 1392, 735, and 859 cm-1 can be attributed to the vibrations of CO32- anion in FeCO3.30-32 The FT-IR results are in good agreement with the XRD analysis. 7



Figure 1. (a)

XRD patterns, (b) FT-IR spectra, and (c) XPS survey spectra of the

ternary Fe2O3/Fe3O4/FeCO3 composite and single-component Fe2O3. Core-level XPS spectra of (d) Fe 2p, (e) C 1s, and (f) O 1s for the ternary Fe2O3/Fe3O4/FeCO3 composite and single-component Fe2O3.

To determine the valence state of Fe, O, C elements in the as-prepared ternary Fe2O3/Fe3O4/FeCO3 composite and single-component Fe2O3, XPS measurement was performed and the results are shown in Figure 1c-f. The peaks from Fe 2p, O 1s, and C 1s can be clearly detected in the wide-scan XPS spectrum (Figure 1c). The Fe 2p spectrum of the single-component Fe2O3 (Figure 1d) show two obvious peaks at 710.4 and 724.6 eV, which can be assigned to Fe 2p3/2 and Fe 2p1/2, respectively; the two shake-up satellite peaks at 718.7 and 733.1 eV are characteristic of Fe3+;32,33 The Fe 2p spectrum of the ternary Fe2O3/Fe3O4/FeCO3 composite (Figure 1d) appears two new peak at 709.3 and 715.2 eV, which correspond to the Fe2+ in FeCO3/Fe3O4 and the satellite peak of Fe2+, respectively.34 Figure 1e shows the C 1s spectra of the two 8


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samples. The C 1s peak at 284.8 eV of the surface adventitious carbon is used as the reference for calibration; the peak at 288.2 eV corresponds to C=O.35,36 For ternary Fe2O3/Fe3O4/FeCO3 composite, the C 1s peak locating at 289.7 eV can be attributed the CO32- in FeCO3.24,37 The O 1s spectrum of the single-component Fe2O3 (Figure 1f) shows two peaks at 529.9 and 531.4eV, which relate to Fe-O bonds and adsorbed hydroxyl groups/water molecules, respectively.38 For the ternary Fe2O3/Fe3O4/FeCO3 composite, the O 1s peak at 532.2 eV corresponds to the CO32- in FeCO3.24 The XPS results are in accordance with the XRD and FT-IR results. These findings confirms that the phase structure and component of the product can be tuned by introducing urea in the hydrothermal synthesis process. According to the reactants, a possible formation process of the ternary Fe2O3/Fe3O4/FeCO3 composite by hydrothermal method can be deduced. When the temperature exceeds 80 °C, urea reacts with H2O and decomposes to CO2 and NH3 (Equation (1)).39 The NH3 can partially reduce Fe3+ to Fe2+. The NH3 and CO2 combine with H2O and produce OH- (Equation (2)) and CO32- (Equation (3)), respectively. The hydrolysis of CO32- can also generate OH- (Equation (5)).40 The formation of OH(Equation (2) and (5)) provides alkaline conditions for the precipitation of Fe3+/Fe2+ (Equation (6) and (7)). Finally, Fe2O3 and Fe3O4 are formed by dehydration of Fe(OH)3 and FeⅡFe2Ⅲ(OH)8 (Equation (8) and (9)). The combination of Fe2+ and CO32- forms FeCO3 precipitation (Equation (4)).27,41 (NH2)2CO + H2O  2NH3 + CO2

(1)

NH3 + H2O  NH4+ + OH-

(2) 9



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CO2 + H2O  CO32- + 2H+

(3)

CO32- + Fe2+  FeCO3-↓

(4)

CO32- + H2O  HCO3- + OH-

(5)

Fe3+ + 3OH-  Fe(OH)3

(6)

Fe2+ + 2Fe3+ + 8OH-  FeⅡFe2Ⅲ(OH)8↓

(7)

2Fe(OH)3  α-Fe2O3 + 3H2O

(8)

FeⅡFe2Ⅲ(OH)8  Fe3O4 + 4H2O

(9)

Figure 2 presents the SEM images of the as-prepared ternary Fe2O3/Fe3O4/FeCO3 composite and single-component Fe2O3. It can be seen that the single-component Fe2O3 is composed of the conglomeration of irregular polyhedron and some spherical nanoparticles (Figure 2a,b). By contrast, the ternary Fe2O3/Fe3O4/FeCO3 composite shows a more complex hybrid morphology, which consist of small nanoparticles, irregular polyhedrons, and rhombohedras (Figure 2c-f). EDS analysis was performed to identify the chemical composition of the small nanoparticles, irregular polyhedrons, and rhombohedras in ternary Fe2O3/Fe3O4/FeCO3 composite (Figure S1). The rhombohedras show strong C signal and the atomic ratio of Fe: C: O (17.37: 16.91: 65.72) is very close to 1: 1: 3 (Figure S1a), implying they are mainly FeCO3; for the small nanoparticles and irregular polyhedrons, the C signal is very weak (the atomic ratio of Fe: C: O is 28.16: 9.31: 62.53 in the small nanoparticles and 29.39: 7.21: 63.40 in the irregular polyhedrons) (Figure S1b,c), suggesting they are mainly Fe2O3 and Fe3O4. From the EDS mapping image (Figure S2), it can be found that these rhombohedra particles have a high C content, which also indicates that these particles 10


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are FeCO3 phase. This ternary heterostructure of Fe2O3/Fe3O4/FeCO3 may provide an enhanced inner electric field at the interface of Fe2O3, Fe3O4, and FeCO3 particles, which would improve the electron transfer and Li+ diffusion in the composite during the discharge/charge process.42-44 The tap density of the ternary Fe2O3/Fe3O4/FeCO3 composite was measured to be 1.80 g cm-3 (based the ratio of the mass and the volume of powders). This value is significantly higher than the previously reported Fe-based anode materials (Table S1). The high tap density of ternary Fe2O3/Fe3O4/FeCO3 composite can be attributed to the severe agglomeration of micron-sized large particle as well as the nano-sized small particles filled in the void space (Figure 2c). Since larger tap

density

could

provide

higher

volumetric-energy-density,

this

ternary

Fe2O3/Fe3O4/FeCO3 composite could be a promising high volumetric capacity anode material for LIBs (Table S1).

Figure 2. SEM images of (a,b) the as-prepared single-component Fe2O3 sample and (c-f) the ternary Fe2O3/Fe3O4/FeCO3 composite.

11



The lithium storage behaviors of the ternary Fe2O3/Fe3O4/FeCO3 composite was initially investigated by Cyclic voltammetry for the first fourth cycles at a scan rate of 0.1 mV s-1 (Figure 3a). During the first cathodic scan, a shoulder reduction peak at ~0.75 V and an intense reduction at 0.38 V are observed, which correspond to the reduction of Fe3+/Fe2+ (in Fe2O3/Fe3O4) to Fe0 and Fe2+ (in FeCO3) to Fe0, as well as the formation of solid electrolyte interface (SEI) film.37,45 In the first anodic scan, a shoulder peak at 1.2 V and a broad peak at 1.69 V can be successively assigned to the reversible oxidation of Fe0 to Fe2+ and Fe3+.46 In the second CV cycle, both the reduction and oxidation peaks shift to higher voltage and the intense reduction peak at 0.38 V disappears, which are caused by the structure reconstruction and the irreversible reaction for the formation of SEI film in the first cycle.28 After the second cycle, subsequent CV curves well overlap with each other, suggesting a good reversibility of the lithiation/de-lithiation reactions. For comparison, the CV profiles of the singlecomponent Fe2O3 was also studied under the same test condition and the result is shown in Figure S3a. Unlike ternary Fe2O3/Fe3O4/FeCO3 composite, the single-component Fe2O3 show only one intense reduction peak at 0.75 V in the first CV cycle, which is due to the reduction of Fe2O3 (Fe3+) to Fe0, the formation of amorphous Li2O, and concurrent formation of SEI film.10 The broad oxidation peak in the first CV cycle can be ascribed to the oxidation of Fe0 to Fe3+ (Fe2O3) during de-lithiation process.47 In the subsequent CV cycles, a couple of broad reduction/oxidation peaks located at 0.93/1.65 V are observed, correspond to the reversible reaction of Fe3+/Fe0 for the lithiation/delithiation processes. 12


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Figure 3b shows the typical discharge/charge curves of the ternary Fe2O3/Fe3O4/FeCO3 composite electrode in selected cycles at a current density of 0.5 A g-1. The first discharge curve exhibits an obvious extended potential plateau from 0.82 to 0.71 V and then a short potential plateau at 0.47 V followed by a sloping plot to the cut-off voltage of 0.01 V, which is in good agreement with the CV result (Figure 3a). The discharge and charge capacities in the first cycle are 1356 and 860, respectively, corresponding a coulombic efficiency of 63%. The large irreversible capacity loss in the first cycle can be mainly attributed to the formation of SEI film and electrolyte degradation.28 Subsequent discharge/charge curves show similar features and almost overlap with each other, suggesting the good cycling stability. In contrast, the single-component Fe2O3 electrode shows only one long discharge plateau from 0.65 to 0.5 V and then a sloping plot down to 0.01 V in the first discharge curve (Figure S3b); with the increase of cycle number, the voltage hysteresis between charge and discharge profiles increases, demonstrating its inferior cycling performance as compared to the ternary Fe2O3/Fe3O4/FeCO3 composite. Figure 3c compares the cycling performance of the ternary Fe2O3/Fe3O4/FeCO3 composite and single-component Fe2O3 electrodes at a current density of 0.5 A g-1 for 300 cycles. Obviously, the ternary Fe2O3/Fe3O4/FeCO3 composite shows a significantly higher reversible capacity than the single-component Fe2O3, which can be attributed to the synergistic effects of Fe2O3/Fe3O4/FeCO3.48-50 After 300 cycles, the ternary Fe2O3/Fe3O4/FeCO3 composite delivers a stable specific capacity of 779 mAh g-1, much higher than that (470 mAh g-1) of the single-component Fe2O3. The cycling performance of the ternary Fe2O3/Fe3O4/FeCO3 composite is 13



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superior to most other Fe-based oxides anode materials (Table S2). Figure 3d compares the rate capability of the ternary Fe2O3/Fe3O4/FeCO3 composite and the singlecomponent

Fe2O3

at

various

current

densities.

Apparently,

the

ternary

Fe2O3/Fe3O4/FeCO3 composite shows better rate capacity than the single-component Fe2O3. The ternary Fe2O3/Fe3O4/FeCO3 composite can deliver an average specific capacity of 762 mAh g-1 at 0.5 A g-1, 640 mAh g-1 at 1.0 A g-1, 502 mAh g-1 at 3.0 A g-1, 410 mAh g-1 at 5.0 A g-1, 350 mAh g-1 at 7.0 A g-1, and 270 mAh g-1 at 10.0 A g-1; when the current density is back to 0.5 A g-1, the reversible capacity can recover to the same level and display a stable specific discharge capacity of 681 mAh g-1 after 110 cycles; moreover, the specific capacity of the electrode can recover to its initial value of 680 mAh g-1 after twice successive sequence rate performance measurements. The ternary Fe2O3/Fe3O4/FeCO3 composite exhibits a stable discharge capacity of 855 mAh g-1 at the current density of 0.5 A g-1 for the next 140 cycles that followed the twice successive sequence rate performance measurements. These results confirm that the ternary composite structure can efficiently enhance the lithium-ions/electrons transportation for the electrochemical reaction of the anode material, and therefore deliver superior rate capability compared to the previously reported Fe-based anode materials (Figure S5). Figure 3e gives the long-term cyclability of the ternary Fe2O3/Fe3O4/FeCO3 composite electrode at the current density of 3 A g-1 for 790 cycles after activated at 0.5 A g-1 for 10 cycles. The electrode shows stable reversible capacity and high coulombic efficiency (nearly 100%) during the whole 10-800 cycles, and its specific discharge capacity maintains at 410 mAh g-1 after 800 cycles, suggesting the 14


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excellent long-term cycling stability at higher current density. For comparison, the single-component Fe2O3 delivers a specific discharge capacity of 293 mAh g-1 after 800 cycles (Figure S6).

Figure 3. (a) CV curves of the ternary Fe2O3/Fe3O4/FeCO3 composite at a scan rate of 0.1

mV

s-1.

(b)

Representative

discharge/charge

curves

of

the

ternary

Fe2O3/Fe3O4/FeCO3 composite in various cycles at 0.5 A g-1. (c) Cycling performance (0.5 A g-1) and (d) rate performance of the ternary Fe2O3/Fe3O4/FeCO3 composite and single-component Fe2O3 electrodes. (e) Long-term cycling performance of the ternary Fe2O3/Fe3O4/FeCO3 composite at 3 A g-1.

The

improved

conductivity

and

lithium-ions

diffusivity

of

the

ternary

Fe2O3/Fe3O4/FeCO3 composite are confirmed by EIS and CV analysis. Figue 4a gives the EIS plots of ternary Fe2O3/Fe3O4/FeCO3 composite and single-component Fe2O3 electrodes before and after 300 discharge/charge cycles (current density of 0.5 A g-1) at open-circuit potential. It can be seen that the charge-transfer resistance (Rct) of the two 15



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electrodes is very similar before cycling. After 300 cycles at 0.5 A g-1, the combined charge transfer resistance and Li+ migrating resistance through SEI film (Rsf+ct) increases dramatically. The EIS plots were analyzed by the equivalent circuit shown in Figure 4b. After 300 cycles, the Rsf+ct values for the ternary Fe2O3/Fe3O4/FeCO3 composite and single-component Fe2O3 electrodes are 28.0 and 56.0 Ω, respectively, indicating the improved conductivity and enhanced charge-transfer kinetics of the ternary Fe2O3/Fe3O4/FeCO3 composite. To further understand the Li-ions diffusion kinetics of the ternary Fe2O3/Fe3O4/FeCO3 composite and single-component Fe2O3 electrodes, CV tests were performed at scan rates from 0.1 to 2.0 mV s-1 and the results are shown in Figure 4c,d. With the increase of scan rate, the oxidation peak moves gradually to high potential side, and the reduction peak moves to the low potential side, due to the growing polarization effect. It is well-known that the peak potential separation between oxidation and reduction peaks in CV curve is a measure of reversibility in the redox reaction;51 a smaller peak potential separation corresponds to better reversibility. By comparison, the potential separation of the ternary Fe2O3/Fe3O4/FeCO3 composite is smaller than the of the single-component Fe2O3 under the same scan rate, suggesting its superior electrochemical reaction reversibility. The Li+ diffusion coefficients (DLi+) of the two samples are calculated by using RandlesSevcik equation52-53: Ip=(2.69 × 105)n3/2ADLi+1/2CLi+v1/2

(10)

where Ip is peak current (A), n is the number of electrons involved in oxidation or reduction, A is the electrode area (cm2), DLi+ is the diffusion coefficient of Li+ (cm2 s16


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1),

CLi+ is the shuttle concentration (mol cm-3), and v is the scan rate (V s-1) in CV

measurement. Based on the CV data (Figure 4c,d), we can obtain four lines (Ip vs. v1/2) as shown in Figure 4e,f. According to the slopes of the lines and equation (10), the DLi+ of the two samples were calculated to be 1.72 × 10-10 mol cm-3 (oxidation peak) and 3.71 × 10-10 mol cm-3 (reduction peak) for the ternary Fe2O3/Fe3O4/FeCO3 composite, which is larger than those (5.40 × 10-11 mol cm-3 (oxidation peak) and 1.99 × 10-10 mol cm-3 (reduction)) of the single-component Fe2O3.

Figure 4. (a) Nyquist plots of the ternary Fe2O3/Fe3O4/FeCO3 composite and the singlecomponent Fe2O3 electrodes before cycling and after 300 cycles. (b) The equivalent circuit used for fitting the EIS plots. (c,d) CV curves of the ternary Fe2O3/Fe3O4/FeCO3 composite and the single-component Fe2O3 electrodes at various scan rates. (e,f) Relationship of the peak current (Ip) and the square root of scan rate (ν1/2) for the ternary Fe2O3/Fe3O4/FeCO3 composite and the single-component Fe2O3 electrode. Symbols and lines represent the experimental data and fitted linear lines, respectively.

17



To verify the structural stability, the morphology of ternary Fe2O3/Fe3O4/FeCO3 composite and single-component Fe2O3 electrodes before cycling and after 300 cycles at 0.5 A g-1 was observed by SEM. As shown in Figure 5a-c, before cycling, the hybrid morphology (small nanoparticles, irregular polyhedrons, and rhombohedras) can be clearly observed in the ternary Fe2O3/Fe3O4/FeCO3 composite electrode; after 300 cycles (Figure 5d-f), the initial morphologies of the small nanoparticles, irregular polyhedrons, and rhombohedras in the ternary Fe2O3/Fe3O4/FeCO3 composite are well maintained despite of the increased roughness on the surface of the active materials, indicating the robust structure stability of the ternary Fe2O3/Fe3O4/FeCO3 composite during long-term lithiation/de-lithiation processes. On the contrary, large cracks and severe pulverization of particles for the single-component Fe2O3 can be observed from the SEM images after 300 cycles (Figure S7), implying the severe structure degradation during repeated discharge/charge cycles.

Figure 5. SEM images of the ternary Fe2O3/Fe3O4/FeCO3 composite (a-c) before cycling and (d-f) after 300 cycles at current density of 0.5 A g-1. 18


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To further evaluate the ternary Fe2O3/Fe3O4/FeCO3 composite for practical application, a full cell was assembled using this ternary Fe2O3/Fe3O4/FeCO3 composite electrode as the anode and commercial LiCO2 as the cathode. The full cell was cycled at 200 mA g-1 within the voltage window of 1.0-3.9 V for 40 cycles. As shown in Figure 6a, the first discharge capacity is 717 mAh g-1. With the increase of cycle number, the discharge capacity of the full cell decreases obviously. After 40 cycles, the discharge capacity is 316 mAh g-1, which is 44.1% of the first discharge capacity. The result indicates that the LiCoO2 cathode does not fit well with the anode for the full cell. The charge/discharge curves of the 1st, 2nd, 5th, 15th, and 30th cycle of the full cell are shown in Figure 6b. The capacity fading of the full cell may be caused by the following reasons: the mismatch of the current densities and voltages between the anode and cathode; an unreasonable capacity ratio of the anode and cathode (or the inferior cycling stability of the cathode); the irreversible loss of lithium due to SEI film formation and other side reactions; an inappropriate electrolyte.54-56 We believe that the full cell performance could be further improved by engineering the cell design, such as pre-lithiation of anode, optimizing the anode/cathode balance, adjusting the potential window, and changing the electrolyte composition.

19



Figure 6. (a) cycling performance and (b) Charge/discharge profiles of the full cell consisted of ternary Fe2O3/Fe3O4/FeCO3 composite anode and commercialized LiCoO2 cathode at a current density of 0.2 A g-1 in a voltage range of 1.0-3.9 V. The capacity was calculated based on anode mass.

4. CONCLUSIONS In summary, ternary Fe2O3/Fe3O4/FeCO3 composite with hybrid morophology (small nanoparticles, irregular polyhedrons, and rhombohedras) was successfully synthesized by a facile hydrothermal method with FeCl2·4H2O and urea as raw materials. Benefiting from the hybrid morphology and the synergistic effect of the three components and interfaces, this ternary Fe2O3/Fe3O4/FeCO3 composite exhibits a high reversible capacity (855 mAh g-1 at the current density of 0.5 A g-1), superior rate capability (with reversible capacities of 624 and 488 mA h g-1 at 1.0 and 3.0 A g-1, respectively), and excellent cycling stability (410 mAh g-1 at 3.0 A g-1 for 800 cycles), demonstrating the promising application as an anode material for next-generation LIBs. Compared to single-component Fe2O3, the ternary Fe2O3/Fe3O4/FeCO3 composite possesses much 20


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better structural integrity during long-term cycling, lower electrochemical reaction resistance, and faster lithium-ion diffusivity, which result in the superior lithium storage performance. The results may provide clues for the rational design of high-performance electrode materials for next-generation LIBs from the view point of multiphase/composition.

SUPPORTING INFORMATION The supplementary information provides EDS patterns and EDS mapping images of the as-prepared ternary Fe2O3/Fe3O4/FeCO3 composite, the CV and discharge/charge profiles of the single-component Fe2O3 sample, the typical discharge/charge profiles of the ternary Fe2O3/Fe3O4/FeCO3 composite and single-component Fe2O3 sample in rate performance test, comparison of the rate capability and cycling stability of the ternary Fe2O3/Fe3O4/FeCO3 composite and the previously reported Fe-based anode materials, SEM images of the single-component Fe2O3 electrode before cycling and after 300 cycles at a current density of 0.5 A g-1.

ACKNOWLEDGEMENTS The authors thank the financial supports from the National Natural Science Foundation of China (No. 51664012 and 51464009), Guangxi Natural Science Foundation of China (2017GXNSFAA198117 and 2015GXNSFGA139006), Guangxi Key Laboratory of Electrochemical

and

Magnetochemical

Functional

Materials

(EMFM20181102/EMFM20181117) and Innovation Project of Guangxi Graduate 21



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Education of China (YCSW2018159).

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