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RGO with optimized SEI film and fast Li-ion diffusion
Journal Pre-proof An embedded heterostructure Fe2O3@α-FeOOH/RGO with optimized SEI film and fast Li-ion diffusion Meng Ma, Liyun Cao, Hui Qi, Kai Yao, Jianfeng Huang, Zhanwei Xu, Shaoyi Chen, Jiayin Li PII:
S0925-8388(19)32884-1
DOI:
https://doi.org/10.1016/j.jallcom.2019.151657
Reference:
JALCOM 151657
To appear in:
Journal of Alloys and Compounds
Received Date: 13 May 2019 Revised Date:
29 July 2019
Accepted Date: 30 July 2019
Please cite this article as: M. Ma, L. Cao, H. Qi, K. Yao, J. Huang, Z. Xu, S. Chen, J. Li, An embedded heterostructure Fe2O3@α-FeOOH/RGO with optimized SEI film and fast Li-ion diffusion, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.151657. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
An Embedded Heterostructure Fe2O3@α-FeOOH/RGO with Optimized SEI Film and Fast Li-ion Diffusion Meng Ma a, Liyun Cao a, *, Hui Qi a, Kai Yao a, Jianfeng Huang a, b, Zhanwei Xu a, Shaoyi Chen b and Jiayin Li a, b, *
a
School of Material Science and Engineering, Shaanxi Key Laboratory of Green Preparation and Functionalization for Inorganic Materials, Shaanxi University of Science and Technology, Xi’an, 710021, China. b
DINGXIN CELLS CO., LTD. Nantong, 226600, China.
Corresponding Author *[email protected]
*[email protected]
ABSTRACT: As an indispensable part of lithium-ion batteries (LIBs), the quality of solid-electrolyte interphase (SEI) influences directly the electrochemical performance. α-FeOOH with superior theoretical capacities, low cost, and environmental friendliness has been regarded as a promising anode of LIBs. In this work, we found that the hydroxyl groups on the surface of α-FeOOH bond with organic electrolytes that forming an inferior SEI layer contained excessive ROCO2Li, finally causing a poor Li+ transport. Thus, we construct a novel heterostructure of spindle-like α-FeOOH nanorods embedded in mulberry-like Fe2O3 sphere on reduced graphene oxide sheets (F@F/C) based on the dissolution-recrystallization mechanism to optimize the composition of SEI layers. The content of ROCO2Li is decreased as expected in the SEI of F@F/C electrode, which diminish the ionic transfer impedance in the interface and provide more alternative diffusion pathways. As expected, the heterostructural hybrid achieves an excellent electrode performance with a reversible capacity about 1800 mAh g−1 at 0.2 A g-1 after 300 cycles. Even cycled at a high current density of 1 A g-1, the hybrid also remains 1050 mAh g−1 after 600 cycles with a capacity decay rate of only 0.005% per cycle.
Keywords: SEI layer; ionic diffusion; iron oxide, heterostructural hybrid; Li ion battery
1. Introduction As a main commercial anode material for lithium-ion batteries (LIBs) , the theoretical capacity of graphite is only 372 mAh g−1, which is difficult to meet the high capacity requirements of electric vehicles and grid-scale energy storage [1-7]. Therefore, to further increase the energy density for demanding application, the development of new electrode materials for LIB becomes more urgent. Iron oxides / hydroxides have been widely researched as anodes for LIBs due to their features with attractive theoretical capacity, low cost, and environmental friendliness [8-11]. Among them, α-FeOOH has been regarded as one of promising anodes due to its high specific capacity (905 mAh g−1) [12-14]. However, it is rarely reported since α-FeOOH was first utilized as anode material for LIBs in 2009 [15-17]. In this work, we found that its surface with substantial hydroxyl groups always bond with organic electrolytes generating excessive ROCO2Li component. As we all know, the SEI can be characterized by a multi-layered structure formed by the heterogeneous stacking of small domains with distinct compositions [18-20]. In the SEI film formed by the electrolyte containing ethylene carbonate (EC) and dimethyl carbonate (DMC), the metastable ROLi, ROCO2Li (R is organic group) dominate in the outer region near the electrolyte, while the inner inorganic region near the electrode/SEI interface exist more stable components, such as Li2O, Li2CO3 and LiF etc [21]. Among them, the Li2CO3 component has a good chemical stability and ionic conductivity than ROCO2Li [22,23]. Thus, the regulation of Li2CO3 and ROCO2Li components in SEI is worth investigating to improve electrochemical properties, for example, reducing the alkyl lithium carbonate on the surface of graphite electrode by put Li2CO3 additives into the conventional electrolyte to rapidly form a stable SEI layer with a fast Li+ diffusion [24,25]. Analogously, how to optimize the quality of SEI to ameliorate the ionic conductivity of α-FeOOH electrodes need to be further development.
At present, some heterostructural designs of anode materials also have been applied to a few examples for the control of SEI formation, like the pomegranate-inspired Si-C structures [26], in which the carbon component functions as an electrolyte blocking layer to make thin, stable and spatially confined SEI form mostly outside the secondary particle. However, its coulombic efficiency decreases due to the high density of lithium trapping sites in amorphous carbon shell. So the carbon component needs to be replaced possibly by another active material that does not irreversibly trap large amounts of lithium. In this work, we take the lead in utilizing α-Fe2O3 to construct an embedded heterostructure for regulating the SEI formation of α-FeOOH. Based on the phase transformation between iron oxides and hydroxides under hydrothermal conditions [10], the heterostructure that spindle-like α-FeOOH nanorods embed in mulberry-like Fe2O3 spheres on reduced graphene oxide (RGO) sheets can be easily constructed. The Fe2O3 is inclined to form a stable SEI containing more Li2CO3, administering to reduce the relative content of ROCO2Li in the SEI of FeOOH. Meanwhile, the embedded heterostructure also provide more alternative ionic pathways. As a result, the ionic diffusion dynamics of Fe2O3@α-FeOOH/RGO hybrid is enhanced as expected with excellent cycling and rate capacities. 2. Experimental section 2.1 Material preparation All chemicals in this study have not been further treated. 0.040 g graphene oxide (GO) (0.8 mg mL-1) and 1.268 g FeCl2 4H2O (0.01 mol) were dispersed into 50 mL H2O under stirring for 2 h to form a homogeneous mixture. Then the obtained mixture was put into a 100 mL sealed Teflon-lined autoclave at 100 o C for 8 h. The prepared product was washed by centrifuge separation with DI water and ethanol for several times. At last, the sample was freeze-dried to obtain the Fe2O3@α-FeOOH/RGO
hybrid. Only changing the temperature to 80 and 120
o
C, the FeOOH/RGO and
Fe2O3/RGO samples can be obtained separately. 2.2 Characterizations Field emission scanning electron microscope (SEM, S-4800) and transmission electron microscopy (TEM, JEM-3010, at a 200 kV accelerating voltage) were used for observing the morphologies and microstructures of the samples. The X-ray diffraction (XRD, Bruker Discover 8 Diffractometer, Cu Ka radiation l ¼ 1.5406 Å) was used to characterize the crystal structure of samples. Fourier transform infrared transmission spectroscopy (FTIR, Bruker Vector-22 infrared spectrophotometer, KBr pellets) was carried out on the analysis of organic functional groups. The Raman measurement was used to detect the degree of graphitization of reductive graphene oxide (an InVia instrument with 532 nm diode-pumped solid-state lasers). X-ray photoelectron spectroscopy can investigate the surface composition of the samples (XPS, Axis Ultra spectrometer, Mg Ka source) and the SEI components of electrodes after cycling (ESCALAB 250Xi, sputter 60 seconds with a 2 KeV Ar+ beam). BET and BJH methods were used for the surface area and pore size distribution determination using N2 adsorption data. 2.3 Electrochemical measurements The electrochemical experiments were studied with coin cell (CR2032-type, half-cell) using Li foil as the counter electrode. 70 wt% active materials (FeOOH/RGO, Fe2O3@α-FeOOH/RGO, Fe2O3/RGO), 20 wt% conductive carbon Super P, 10 wt% carboxymethyl cellulose (CMC) and 10% the binder SBR with DI water were mixed to form a homogeneous slurry, which was evenly coated in a copper foil with a thickness of 15 µm and then dried at 80 oC in a vacuum oven for 12 h. And then the coated copper foil was sliced into circular electrodes with a diameter of about 1.4 cm and the mass loading 1.96 mg of active material. Reversible specific capacities were calculated based on the mass loading of active materials. The coin cells were
assembled in an Ar-filled glove box. The electrolyte was a mixture that 1.0 M LiPF6 dissolved in ethylene carbonate (EC) and diethyl carbonate (DMC) mixed solvent with a volume ratio of 1:1. The rate and cycle performance tests were performed in the cutoff voltage range between 0.01 and 3.00 V (vs. Li/Li+) on a Neware Cell Test System (Shenzhen, China). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) analysis were carried out on a VersaSTAT3 potentiostat at a scan rate of 0.2 mV s−1 in the voltage range between 0.01 and 3.00 V (vs. Li/Li+). 3. Results and discussion The general morphology and structure of as-prepared products under different hydrothermal condition are characterized by field emission scanning electron microscopy (SEM) and transmission electron microscopy (TEM) in Figure S1 and Figure 1. A schematic illustration is demonstrated to visually distinguish the structural features of the products at 80 o C and 100 o C. After hydrothermal reacted at 80 o C for 8 h, as-prepared sample displays the spindle-like nanorods ranges about 100~200 nm densely distributed at RGO sheets (Figure 1b-d). The lattice distance of spindle-like nanorods is 0.42 nm maps to the (110) lattice plane of α-FeOOH [27,28]. Rising temperature to 100 o C, the product shows a unique heterostructure that spindle-like nanorods about 100~200 nm embed in mulberry-like spheres about 300~400 nm on RGO sheets (Figure 1e-g). Raising the temperature to 120
o
C, FeOOH nanorods
completely convert to mulberry-like Fe2O3 nanospheres about 100~200 nm (Figure S2). The phase compositions of products are further confirmed by X-ray diffraction patterns (XRD) shown in Figure 2a. The diffraction pattern of spindle-like nanorods obtained at 80 o C exhibits three characteristic peaks centered at 21.2 o, 36.7 o and 41.2 o
, corresponding to (110), (111), (140) planes of goethite FeOOH respectively,
consistent with the TEM results and denoted as FOH/C. The sample at 100
o
C
displays a mixed iron oxide crystal structure including α-FeOOH and α-Fe2O3 due to the dissolution-recrystallization and incomplete transformation in two iron oxides
[29,30], which is designated as F@F/C. In the nanospheres obtained at higher temperature, the pure Fe2O3 is detected and no impurities, named as FO/C (Figure S3). The formation and phase transformation reactions of α-FeOOH and α-Fe2O3 are as follows [10]: Fe2+ + 2H2O → Fe(OH)2 + 2H+
(1)
4Fe(OH)2 + O2 → 4FeOOH + 2H2O
(2)
2FeOOH → Fe2O3 + H2O
(3)
Fourier transform infrared spectra (FTIR) in Figure S4 of FOH/C and F@F/C demonstrate abundant hydroxyl and carboxyl groups compared with FO/C [31-33]. The hydroxyl groups will make particles more active and facilitate to react with electrolyte. Furthermore, X-ray photoelectron spectroscopy (XPS) discriminate the chemical states of surficial various bonded elements between FOH/C and F@F/C. Full-survey-scan spectra in Figure S5 can clearly detect Fe, O, and C elements in two products. The Fe 2p XPS spectrum of FOH/C shows four characteristic peaks (Figure 2b, c). The two strong peaks at 709.5 and 723.1 eV are corresponding to Fe 2p3/2 and Fe 2p1/2 respectively. The weak and broad peaks at 717.5 and 732.0 eV are assigned to the shake-up satellites. In F@F/C, the peak positions of Fe 2p3/2 and Fe 2p1/2 obviously shift left to 708.3 and 722.1 eV, indicating the chemical state of Fe element has changed [34]. From high-resolution O1s spectra, the content of Fe-OH bond in F@F/C is acquired to 7.62%, remarkably lower than the 14.76% of FOH/C, indicating the unique embedded heterostructure evidently reduces the number of hydroxyl functional groups on the surface of α-FeOOH nanorods (Figure 2d, e). When employed products as anodes for LIBs, the cyclic voltammogram (CV) curves of FOH/C, F@F/C and FO/C electrodes are measured within a voltage window of 0.01 ~ 3 V at 0.2 mV s-1 (Figure 3a and Figure S6). In the CV curves of FOH/C, the first negative scan is characterized by three peaks centered at 1.78, 1.35, 0.55 and ~ 0 V. The peak at 1.78 V is corresponding to FeOOH to form LixFeOOH (x <1). The
second small peak at 1.52 V can be ascribed to the formation of Li1+xFeOOH (x < 1). A short and broad cathodic peak at 0.55 V is observed, which associates with the conversion reaction to form metallic Fe (0) and SEI layers. The small peak is then observed at 0 V probably corresponding to the further formation of SEI layers and interfacial storage [11]. The detailed lithium storage reactions of FOH/C are as follows [27a]: FeOOH + Li+ + e- ↔ LiFeOOH
(4)
LiFeOOH + xLi+ + xe- ↔ Li(1+x)FeOOH
(5)
Li1+xFeOOH + (2-x)Li+ + (2-x)e- ↔ Fe + Li2O + LiOH
(6)
The first negative scan of FO/C is characterized by only one peak centered at 0.65 V. This very sharp cathodic peak is ascribed to the Li+ insertion and the occurrence of cubic Li2Fe2O3, as described in Equations 4, 5 and 6. The curves have a high degree of coincidence after two cycles, indicating that the conversion reaction of FO/C is highly reversible. The lithium storage mechanism of FO/C can be demonstrated by the following reactions [27b]: Fe2O3 + xLi+ + xe- → LixFe2O3
(7)
LixFe2O3 + (2-x)Li+ + (2-x)e- → Li2Fe2O3
(8)
Li2Fe2O3 + 4Li+ + 4e- ↔ 2Fe + 3Li2O
(9)
Notably, the curve shape of F@F/C hybrid is similar to a combination of FO/C and FOH/C, which indicating its electrochemical lithium storage mechanism is the superposition of two active components: the intercalation and conversion reactions, the formation/deformation of SEI layers and interfacial storage [35,37]. Meanwhile, the final stable redox peak positions of FO/C and FOH/C are located at 0.814 V and 0.721 V respectively, and this voltage difference may have preventive effect on grain pulverization during cycling. When the F@F/C electrode discharge to 0.814 V, the
conversion reaction of external Fe2O3 occurs, and the FeOOH remain stable without reaction at this potential. Subsequently discharging to 0.721 V, the internal FeOOH nanorods transform to elemental iron, while the external unresponsive Fe2O3 sphere limit its expansion space and avoid the utterly destroy of electrodes. So it can be predicted that the external Fe2O3 could protect the structure of FeOOH nanorods throughout the process of lithiation/ delithiation. In the charge/discharge curves (Figure S7), it should be noted that the voltage windows of FO/C and FOH/C with the increase of cycling time are changed obviously, which illustrates these electrodes all suffer varying degrees of polarization. However, the feeble variation of voltage window of F@F/C demonstrates the polarization phenomenon are weakened due to the promoted ionic diffusion kinetics. Cyclic performance of the F@F/C electrode are tested at 0.2 A g-1, it delivers a high initial discharge capacity of 1412.1 mAh g-1 with the initial coulombic efficiency of 78.0%. After 300 cycles deep charge/discharge, it still remains the reversible capacity about 1860 mAh g-1 (Figure S8a). The promising rate capability was further tested between a voltage window of 0.01 ~ 3 V, the hybrid shows the typical discharge capacities of 842, 804, 748, 686 and 489 mAh g-1 at the current of 0.2, 0.5, 1, 2 and 5 A g-1 (Figure 3c). Impressively, its specific capacity is approximate to the double of FOH/C at high current density of 5 A g-1 (Figure 3d). When the current density is back to 0.2 A g-1, it still maintain 240 cycles about 956 mAh g-1 (Figure S8b), indicating the embedded structure can withstand rapid lithiation/ delithiation process. Even at a high current density of 1 A g-1, the F@F/C still holds prominent reversible capacity of ~ 1050 mAh g-1 over 600 cycles with capacity decay rate of only 0.005% per cycle, better than FO/C and FOH/C electrodes (Figure 3e and Figure S7c). The initial capacity of FOH/C sample is relatively higher than FO/C, but its cycling stability is not as satisfactory as the latter. The long-cycling performance of F@F/C integrates the advantages of both FO/C and FOH/C. In comparison with the FeOOH
and Fe2O3-based anodes reported to date, F@F/C hybrid is a promising anode for LIBs with high reversible capacity and long cycling-life (Figure 4 and Table S1). Electrochemical impedance spectroscopy (EIS) tests can be used to analyze the kinetics of charge transfer reactions and lithium ion diffusion in the anode electrodes [37,38]. The FOH/C and F@F/C electrodes after 10 and 300 cycles are given by Nyquist plots in Figure 5. The typical randles circuits for cycled cells fitting by Nyquist plots are shown in Figure S9, and the resistance values of relative components after 300 cycles are listed in Table S2. As the number of cycles increases, the semicircles at high frequency of three anodes both gradually grow larger, implying the value of charge transfer resistance (Rct) and solid electrolyte interphase resistances (RSEI) increase, especially for FOH/C. After 300 cycles, the value of RSEI in FOH/C is 101 Ω obviously lower than the 25.59 Ω of FOH/C, indicating the Li+ diffusion kinetics in interface is facilitated availably during cycling due to the participation of Fe2O3. Furthermore, we also quantificationally estimate Li+ diffusion coefficient (DLi+) to describe the ionic diffusion kinetic according to the following equations [38]: Zre = Rb + Rct + σwω-1/2 DLi+ = R2T2 / 2A2n4F4Co2σw2
(10) (11)
Where Zre is the real part of electrical impedance; Rb is the bulk resistance of battery; Rct is the Faradic charge-transfer resistance and σw is the Warburg coefficient; ω is angular frequency; R is ideal gas constant; T is absolute temperature; A is the specific surface area of the electrode; F is Faraday constant; n is the molar amount of transfer charge; Co is the volume molar concentration of Li+. According to Formula 10, Warburg coefficient (σw) is negative related to the Li-ion diffusion coefficient (DLi+) in electrodes. The Zre as a function of ω-1/2 to calculate the value of σw of FOH/C and F@F/C electrodes after 10 and 300 cycles are shown in
Figure 5c, d. After 300 cycles, the calculated σw for F@F/C is evidently smaller than FOH/C, explaining the Li+ diffusion in F@F/C is accelerated as expected, which is consistent with the conclusion of EIS test. Throughout the cycling, the σw of F@F/C hybrid always keeps smaller, implying that it maintains faster ion diffusion all the time. By SEM, TEM and XPS tests, we further investigate the morphology, microstructure and chemical composition of the electrodes after cycling in order to explore the cause of fast ionic diffusion kinetics in F@F/C. After carefully examined many areas of electrode, the cycled FOH/C electrode has been almost covered with a thick SEI layer. However, the F@F/C electrode exhibits a porous morphology with uniformly distributed particles, which is more conducive to the infiltration of electrolyte and ionic diffusion (Figure 6). The sensitive XPS for detecting the surface elements is effective to identify the tiny phase transition of superficial SEI layers in Figure 7 and Figure S10. All electrodes are etched 60s for removing the surface SEI film to obtain the internal chemical states. The contents of ROCO2Li and Li2CO3 components before and after etching are listed in the Table 1.
Before etching, the surface C1s spectra in all electrodes both show
four states: C−C, ROCO2Li, C-O and Li2CO3 (Figure 7c, d). According to previous reports [39,40], Li-alkyl carbonates, produced by EC-DMC electrolyte react with Li ions, always generate at SEI interface in contact with electrolyte. Li2CO3 usually exists in the interior SEI layer near electrodes. The content of ROCO2Li component in FOH/C electrode is account for 20.57% almost twice as much of F@F/C (10.27%) and FO/C (10.76%) because more active hydroxyl groups in FeOOH can react easily with electrolyte to generate more ROCO2Li. The content ratios of ROCO2Li to Li2CO3 in F@F/C electrodes is 0.33, significantly less than the 0.66 of FOH/C, certifying the Li-alkyl carbonates in hybrid has been reduced as expected. After etching, the peaks which belong to the ROCO2Li disappear and the content of Li2CO3 decrease for all electrodes, indicating the exposure of inner chemical state.
Moreover, the Fe-O bonds appear in O1s spectra also confirm the removal of outside organic components of SEI and the exposure of inner oxide particles (Figure 7e, f). The lithium-ion conductivity and stability of Li-alkyl carbonates are both lower than Li2CO3, thus the strategy ameliorating the SEI quality by control the components content will improve the battery performance [41,42]. The possible ionic diffusion mechanism in embedded structure is illustrated in the schematic Figure 8. By constructing the embedded heterostructure, the relative content of Li2CO3 and ROCO2Li components in the SEI is ameliorated to improve the Li+ diffusion kinetics and structural stability of F@F/C. Moreover, owing to the absent of SEI at the junction of two active materials, Li ions are facilitate to transfer from Fe2O3 sphere to FeOOH rods, reducing ionic diffusion barriers thus providing more ionic transport pathways. 4. Conclusion In summary, the mulberry-like Fe2O3 sphere @ spindle-like α-FeOOH nanorods / reduced graphene oxide (F@F/C) hybrid has been fabricated based on the dissolution-recrystallization mechanism. The as-prepared hybrid with a unique embedded architecture can effectively reduce the content of ROCO2Li components in the SEI layer to decrease the ionic transfer impedance in the interface and providing alternative diffusion paths. In addition, the external Fe2O3 spheres can limit the expansion space of internal α-FeOOH nanorods to avoid destroy of the electrode. Finally, the hybrid exhibits excellent cycle and rate capacities. At 0.2 A g-1, the F@F/C remains a high reversible capacity of 1800 mAh g−1 after 300 cycles. Even at a high current of 1 A g-1, it still shows an excellent long-cycling life about 1050 mAh g−1 over 600 cycles. Besides, such a strategy of ameliorating SEI film to improve the ionic diffusion kinetics by heterostructure design could provide useful information for constructing more beneficial anode materials. ASSOCIATED CONTENT
Supporting Information. More structural images of F@F/C; SEM, TEM and XRD of the contrastive sample FO/C; FTIR and XPS spectrogram of three samples; CV and galvanostatic discharge-charge curves of the FOH/C and FO/C; Cycling performances of the FO/C and F@F/C; the XPS survey of three composites; the table of performances comparison of Fe2O3 and FeOOH-based electrodes for LIBs recently. AUTHOR INFORMATION Corresponding Authors *[email protected] *[email protected] Notes The authors declare no competing financial interest. REFERENCES [1] J.M. Tarascon, M. Armand, Nature 414 (2001) 359-367. [2] Y. Zeng, M. Yu, M. Yue, P. Fang, Y. Tong, Adv. Energy Mater. 6 (2016) 1601053. [3] Y. Wang, G. Cao, Adv. Mater. 20 (2010) 2251-2269. [4] M. Li, Y. Wang, C. Liu, H. Gao, W. Dong, Electrochim. Acta 67 (2012) 187–193. [5] L. Wang, G. Zhu, W. Yu, J. Zeng, X. Yu, Q. Li, H. Xie, Sol. Energy Mater. and Sol. Cells. 184 (2018) 1-8. [6] T. Soltani, A. Tayyebi, B.-K. Lee, Sol. Energy Mater. and Sol. Cells. 185 (2018) 325-332. [7] J. Yang, L. Tang, R. Bao, L. Bai, Z. Liu, B. Xie, M. Yang, W. Yang, Sol. Energy Mater. and Sol. Cells. 174 (2018) 56-64.
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Table 1. The content distribution of ROCO2Li and Li2CO3 components in three delithiated electrodes before and after etching in the end of 10 cycling. The before and after etching represent the surface and internal chemical states of the electrodes. The ratio of ROCO2Li to Li2CO3 in FOH/C is maximal, almost twice as much as F@F/C and FO/C.
Etching
Before
After
Components
FOH/C %
F@F/C %
FO/C %
ROCO2Li
20.57
10.27
10.76
Li2CO3
30.84
30.37
33.62
ratio
0.66
0.33
0.32
Li2CO3
12.09
19.33
17.19
a
b
c
d
e
f
g
Figure 1. (a) Schematic illustration of the synthesis procedures. (b) SEM and (c, d) TEM images of the FOH/C. (e) SEM and (f, g) TEM images of the F@F/C.
a
b
d
c
e
Figure 2. (a) XRD patterns for FOH/C and F@F/C samples synthesized at 80 and 100 o C. (b, c) High-resolution Fe2p and (d, e) O1s XPS spectra of the F@F/C and FOH/C.
a
c
b
d
e
Figure 3. (a) Cyclic voltammetry curves of the F@F/C between a voltage window of 0.01~3 V at a scan rate of 0.2 mV s-1. (b) Galvanostatic discharge/charge voltage profiles of the F@F/C at current density of 0.2 A g-1 for the different cycles. (c) Typical discharge-charge profiles at different current density of the F@F/C. (d) Rate performances at different specific currents of F@F/C and FOH/C electrodes. (e) Long cycling performances of the F@F/C and FOH/C at 1 A g-1.
Figure 4. Performance comparisons of several recent works related to FeOOH and Fe2O3-based anodes and our F@F/C for LIBs at high current density of 1A g-1.
a
b
c
d
Figure 5. Nyquist plots of FOH/C and F@F/C electrodes after (a) 10 and (b) 300 cycles at the current density of 1 A g-1. Zre as a function of ω-1/2 in the semi-infinite region for calculation of Warburg coefficient (σw) for all the samples after (c) 10 cycles and (d) 300 cycles.
a
c
b
d
Figure 6. SEM and TEM images with different magnification of (a, b) FOH/C and (c, d) F@F/C electrodes after 300 cycles.
Figure 7. The survey spectroscopy of delithiated (a) FOH/C and (b) F@F/C electrodes and their high-resolution (c, d) C1s, (e, f) O1s XPS spectra in the ending of 10 cycles before and after etching 60 s. All red curves belong to the states after etching.
Figure 8. Schematic illustration of lithium ion diffusion processes in FOH/C and F@F/C electrodes. Li+ are more likely to be transported into FeOOH from the membrane junction without SEI layer.
1. We rationally construct a heterostructure of spindle-like α-FeOOH nanorods embed in mulberry-like Fe2O3 sphere based on dissolution-recrystallization mechanism. 2. Embedded architecture optimizes the proportion of Li2CO3 and ROCO2Li in solid-electrolyte interphase to diminish the ionic transfer impedance in the interface and provide more alternative diffusion pathways, thus improving ionic diffusion kinetics significantly.
S0925-8388(19)32884-1
DOI:
https://doi.org/10.1016/j.jallcom.2019.151657
Reference:
JALCOM 151657
To appear in:
Journal of Alloys and Compounds
Received Date: 13 May 2019 Revised Date:
29 July 2019
Accepted Date: 30 July 2019
Please cite this article as: M. Ma, L. Cao, H. Qi, K. Yao, J. Huang, Z. Xu, S. Chen, J. Li, An embedded heterostructure Fe2O3@α-FeOOH/RGO with optimized SEI film and fast Li-ion diffusion, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.151657. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
An Embedded Heterostructure Fe2O3@α-FeOOH/RGO with Optimized SEI Film and Fast Li-ion Diffusion Meng Ma a, Liyun Cao a, *, Hui Qi a, Kai Yao a, Jianfeng Huang a, b, Zhanwei Xu a, Shaoyi Chen b and Jiayin Li a, b, *
a
School of Material Science and Engineering, Shaanxi Key Laboratory of Green Preparation and Functionalization for Inorganic Materials, Shaanxi University of Science and Technology, Xi’an, 710021, China. b
DINGXIN CELLS CO., LTD. Nantong, 226600, China.
Corresponding Author *[email protected]
*[email protected]
ABSTRACT: As an indispensable part of lithium-ion batteries (LIBs), the quality of solid-electrolyte interphase (SEI) influences directly the electrochemical performance. α-FeOOH with superior theoretical capacities, low cost, and environmental friendliness has been regarded as a promising anode of LIBs. In this work, we found that the hydroxyl groups on the surface of α-FeOOH bond with organic electrolytes that forming an inferior SEI layer contained excessive ROCO2Li, finally causing a poor Li+ transport. Thus, we construct a novel heterostructure of spindle-like α-FeOOH nanorods embedded in mulberry-like Fe2O3 sphere on reduced graphene oxide sheets (F@F/C) based on the dissolution-recrystallization mechanism to optimize the composition of SEI layers. The content of ROCO2Li is decreased as expected in the SEI of F@F/C electrode, which diminish the ionic transfer impedance in the interface and provide more alternative diffusion pathways. As expected, the heterostructural hybrid achieves an excellent electrode performance with a reversible capacity about 1800 mAh g−1 at 0.2 A g-1 after 300 cycles. Even cycled at a high current density of 1 A g-1, the hybrid also remains 1050 mAh g−1 after 600 cycles with a capacity decay rate of only 0.005% per cycle.
Keywords: SEI layer; ionic diffusion; iron oxide, heterostructural hybrid; Li ion battery
1. Introduction As a main commercial anode material for lithium-ion batteries (LIBs) , the theoretical capacity of graphite is only 372 mAh g−1, which is difficult to meet the high capacity requirements of electric vehicles and grid-scale energy storage [1-7]. Therefore, to further increase the energy density for demanding application, the development of new electrode materials for LIB becomes more urgent. Iron oxides / hydroxides have been widely researched as anodes for LIBs due to their features with attractive theoretical capacity, low cost, and environmental friendliness [8-11]. Among them, α-FeOOH has been regarded as one of promising anodes due to its high specific capacity (905 mAh g−1) [12-14]. However, it is rarely reported since α-FeOOH was first utilized as anode material for LIBs in 2009 [15-17]. In this work, we found that its surface with substantial hydroxyl groups always bond with organic electrolytes generating excessive ROCO2Li component. As we all know, the SEI can be characterized by a multi-layered structure formed by the heterogeneous stacking of small domains with distinct compositions [18-20]. In the SEI film formed by the electrolyte containing ethylene carbonate (EC) and dimethyl carbonate (DMC), the metastable ROLi, ROCO2Li (R is organic group) dominate in the outer region near the electrolyte, while the inner inorganic region near the electrode/SEI interface exist more stable components, such as Li2O, Li2CO3 and LiF etc [21]. Among them, the Li2CO3 component has a good chemical stability and ionic conductivity than ROCO2Li [22,23]. Thus, the regulation of Li2CO3 and ROCO2Li components in SEI is worth investigating to improve electrochemical properties, for example, reducing the alkyl lithium carbonate on the surface of graphite electrode by put Li2CO3 additives into the conventional electrolyte to rapidly form a stable SEI layer with a fast Li+ diffusion [24,25]. Analogously, how to optimize the quality of SEI to ameliorate the ionic conductivity of α-FeOOH electrodes need to be further development.
At present, some heterostructural designs of anode materials also have been applied to a few examples for the control of SEI formation, like the pomegranate-inspired Si-C structures [26], in which the carbon component functions as an electrolyte blocking layer to make thin, stable and spatially confined SEI form mostly outside the secondary particle. However, its coulombic efficiency decreases due to the high density of lithium trapping sites in amorphous carbon shell. So the carbon component needs to be replaced possibly by another active material that does not irreversibly trap large amounts of lithium. In this work, we take the lead in utilizing α-Fe2O3 to construct an embedded heterostructure for regulating the SEI formation of α-FeOOH. Based on the phase transformation between iron oxides and hydroxides under hydrothermal conditions [10], the heterostructure that spindle-like α-FeOOH nanorods embed in mulberry-like Fe2O3 spheres on reduced graphene oxide (RGO) sheets can be easily constructed. The Fe2O3 is inclined to form a stable SEI containing more Li2CO3, administering to reduce the relative content of ROCO2Li in the SEI of FeOOH. Meanwhile, the embedded heterostructure also provide more alternative ionic pathways. As a result, the ionic diffusion dynamics of Fe2O3@α-FeOOH/RGO hybrid is enhanced as expected with excellent cycling and rate capacities. 2. Experimental section 2.1 Material preparation All chemicals in this study have not been further treated. 0.040 g graphene oxide (GO) (0.8 mg mL-1) and 1.268 g FeCl2 4H2O (0.01 mol) were dispersed into 50 mL H2O under stirring for 2 h to form a homogeneous mixture. Then the obtained mixture was put into a 100 mL sealed Teflon-lined autoclave at 100 o C for 8 h. The prepared product was washed by centrifuge separation with DI water and ethanol for several times. At last, the sample was freeze-dried to obtain the Fe2O3@α-FeOOH/RGO
hybrid. Only changing the temperature to 80 and 120
o
C, the FeOOH/RGO and
Fe2O3/RGO samples can be obtained separately. 2.2 Characterizations Field emission scanning electron microscope (SEM, S-4800) and transmission electron microscopy (TEM, JEM-3010, at a 200 kV accelerating voltage) were used for observing the morphologies and microstructures of the samples. The X-ray diffraction (XRD, Bruker Discover 8 Diffractometer, Cu Ka radiation l ¼ 1.5406 Å) was used to characterize the crystal structure of samples. Fourier transform infrared transmission spectroscopy (FTIR, Bruker Vector-22 infrared spectrophotometer, KBr pellets) was carried out on the analysis of organic functional groups. The Raman measurement was used to detect the degree of graphitization of reductive graphene oxide (an InVia instrument with 532 nm diode-pumped solid-state lasers). X-ray photoelectron spectroscopy can investigate the surface composition of the samples (XPS, Axis Ultra spectrometer, Mg Ka source) and the SEI components of electrodes after cycling (ESCALAB 250Xi, sputter 60 seconds with a 2 KeV Ar+ beam). BET and BJH methods were used for the surface area and pore size distribution determination using N2 adsorption data. 2.3 Electrochemical measurements The electrochemical experiments were studied with coin cell (CR2032-type, half-cell) using Li foil as the counter electrode. 70 wt% active materials (FeOOH/RGO, Fe2O3@α-FeOOH/RGO, Fe2O3/RGO), 20 wt% conductive carbon Super P, 10 wt% carboxymethyl cellulose (CMC) and 10% the binder SBR with DI water were mixed to form a homogeneous slurry, which was evenly coated in a copper foil with a thickness of 15 µm and then dried at 80 oC in a vacuum oven for 12 h. And then the coated copper foil was sliced into circular electrodes with a diameter of about 1.4 cm and the mass loading 1.96 mg of active material. Reversible specific capacities were calculated based on the mass loading of active materials. The coin cells were
assembled in an Ar-filled glove box. The electrolyte was a mixture that 1.0 M LiPF6 dissolved in ethylene carbonate (EC) and diethyl carbonate (DMC) mixed solvent with a volume ratio of 1:1. The rate and cycle performance tests were performed in the cutoff voltage range between 0.01 and 3.00 V (vs. Li/Li+) on a Neware Cell Test System (Shenzhen, China). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) analysis were carried out on a VersaSTAT3 potentiostat at a scan rate of 0.2 mV s−1 in the voltage range between 0.01 and 3.00 V (vs. Li/Li+). 3. Results and discussion The general morphology and structure of as-prepared products under different hydrothermal condition are characterized by field emission scanning electron microscopy (SEM) and transmission electron microscopy (TEM) in Figure S1 and Figure 1. A schematic illustration is demonstrated to visually distinguish the structural features of the products at 80 o C and 100 o C. After hydrothermal reacted at 80 o C for 8 h, as-prepared sample displays the spindle-like nanorods ranges about 100~200 nm densely distributed at RGO sheets (Figure 1b-d). The lattice distance of spindle-like nanorods is 0.42 nm maps to the (110) lattice plane of α-FeOOH [27,28]. Rising temperature to 100 o C, the product shows a unique heterostructure that spindle-like nanorods about 100~200 nm embed in mulberry-like spheres about 300~400 nm on RGO sheets (Figure 1e-g). Raising the temperature to 120
o
C, FeOOH nanorods
completely convert to mulberry-like Fe2O3 nanospheres about 100~200 nm (Figure S2). The phase compositions of products are further confirmed by X-ray diffraction patterns (XRD) shown in Figure 2a. The diffraction pattern of spindle-like nanorods obtained at 80 o C exhibits three characteristic peaks centered at 21.2 o, 36.7 o and 41.2 o
, corresponding to (110), (111), (140) planes of goethite FeOOH respectively,
consistent with the TEM results and denoted as FOH/C. The sample at 100
o
C
displays a mixed iron oxide crystal structure including α-FeOOH and α-Fe2O3 due to the dissolution-recrystallization and incomplete transformation in two iron oxides
[29,30], which is designated as F@F/C. In the nanospheres obtained at higher temperature, the pure Fe2O3 is detected and no impurities, named as FO/C (Figure S3). The formation and phase transformation reactions of α-FeOOH and α-Fe2O3 are as follows [10]: Fe2+ + 2H2O → Fe(OH)2 + 2H+
(1)
4Fe(OH)2 + O2 → 4FeOOH + 2H2O
(2)
2FeOOH → Fe2O3 + H2O
(3)
Fourier transform infrared spectra (FTIR) in Figure S4 of FOH/C and F@F/C demonstrate abundant hydroxyl and carboxyl groups compared with FO/C [31-33]. The hydroxyl groups will make particles more active and facilitate to react with electrolyte. Furthermore, X-ray photoelectron spectroscopy (XPS) discriminate the chemical states of surficial various bonded elements between FOH/C and F@F/C. Full-survey-scan spectra in Figure S5 can clearly detect Fe, O, and C elements in two products. The Fe 2p XPS spectrum of FOH/C shows four characteristic peaks (Figure 2b, c). The two strong peaks at 709.5 and 723.1 eV are corresponding to Fe 2p3/2 and Fe 2p1/2 respectively. The weak and broad peaks at 717.5 and 732.0 eV are assigned to the shake-up satellites. In F@F/C, the peak positions of Fe 2p3/2 and Fe 2p1/2 obviously shift left to 708.3 and 722.1 eV, indicating the chemical state of Fe element has changed [34]. From high-resolution O1s spectra, the content of Fe-OH bond in F@F/C is acquired to 7.62%, remarkably lower than the 14.76% of FOH/C, indicating the unique embedded heterostructure evidently reduces the number of hydroxyl functional groups on the surface of α-FeOOH nanorods (Figure 2d, e). When employed products as anodes for LIBs, the cyclic voltammogram (CV) curves of FOH/C, F@F/C and FO/C electrodes are measured within a voltage window of 0.01 ~ 3 V at 0.2 mV s-1 (Figure 3a and Figure S6). In the CV curves of FOH/C, the first negative scan is characterized by three peaks centered at 1.78, 1.35, 0.55 and ~ 0 V. The peak at 1.78 V is corresponding to FeOOH to form LixFeOOH (x <1). The
second small peak at 1.52 V can be ascribed to the formation of Li1+xFeOOH (x < 1). A short and broad cathodic peak at 0.55 V is observed, which associates with the conversion reaction to form metallic Fe (0) and SEI layers. The small peak is then observed at 0 V probably corresponding to the further formation of SEI layers and interfacial storage [11]. The detailed lithium storage reactions of FOH/C are as follows [27a]: FeOOH + Li+ + e- ↔ LiFeOOH
(4)
LiFeOOH + xLi+ + xe- ↔ Li(1+x)FeOOH
(5)
Li1+xFeOOH + (2-x)Li+ + (2-x)e- ↔ Fe + Li2O + LiOH
(6)
The first negative scan of FO/C is characterized by only one peak centered at 0.65 V. This very sharp cathodic peak is ascribed to the Li+ insertion and the occurrence of cubic Li2Fe2O3, as described in Equations 4, 5 and 6. The curves have a high degree of coincidence after two cycles, indicating that the conversion reaction of FO/C is highly reversible. The lithium storage mechanism of FO/C can be demonstrated by the following reactions [27b]: Fe2O3 + xLi+ + xe- → LixFe2O3
(7)
LixFe2O3 + (2-x)Li+ + (2-x)e- → Li2Fe2O3
(8)
Li2Fe2O3 + 4Li+ + 4e- ↔ 2Fe + 3Li2O
(9)
Notably, the curve shape of F@F/C hybrid is similar to a combination of FO/C and FOH/C, which indicating its electrochemical lithium storage mechanism is the superposition of two active components: the intercalation and conversion reactions, the formation/deformation of SEI layers and interfacial storage [35,37]. Meanwhile, the final stable redox peak positions of FO/C and FOH/C are located at 0.814 V and 0.721 V respectively, and this voltage difference may have preventive effect on grain pulverization during cycling. When the F@F/C electrode discharge to 0.814 V, the
conversion reaction of external Fe2O3 occurs, and the FeOOH remain stable without reaction at this potential. Subsequently discharging to 0.721 V, the internal FeOOH nanorods transform to elemental iron, while the external unresponsive Fe2O3 sphere limit its expansion space and avoid the utterly destroy of electrodes. So it can be predicted that the external Fe2O3 could protect the structure of FeOOH nanorods throughout the process of lithiation/ delithiation. In the charge/discharge curves (Figure S7), it should be noted that the voltage windows of FO/C and FOH/C with the increase of cycling time are changed obviously, which illustrates these electrodes all suffer varying degrees of polarization. However, the feeble variation of voltage window of F@F/C demonstrates the polarization phenomenon are weakened due to the promoted ionic diffusion kinetics. Cyclic performance of the F@F/C electrode are tested at 0.2 A g-1, it delivers a high initial discharge capacity of 1412.1 mAh g-1 with the initial coulombic efficiency of 78.0%. After 300 cycles deep charge/discharge, it still remains the reversible capacity about 1860 mAh g-1 (Figure S8a). The promising rate capability was further tested between a voltage window of 0.01 ~ 3 V, the hybrid shows the typical discharge capacities of 842, 804, 748, 686 and 489 mAh g-1 at the current of 0.2, 0.5, 1, 2 and 5 A g-1 (Figure 3c). Impressively, its specific capacity is approximate to the double of FOH/C at high current density of 5 A g-1 (Figure 3d). When the current density is back to 0.2 A g-1, it still maintain 240 cycles about 956 mAh g-1 (Figure S8b), indicating the embedded structure can withstand rapid lithiation/ delithiation process. Even at a high current density of 1 A g-1, the F@F/C still holds prominent reversible capacity of ~ 1050 mAh g-1 over 600 cycles with capacity decay rate of only 0.005% per cycle, better than FO/C and FOH/C electrodes (Figure 3e and Figure S7c). The initial capacity of FOH/C sample is relatively higher than FO/C, but its cycling stability is not as satisfactory as the latter. The long-cycling performance of F@F/C integrates the advantages of both FO/C and FOH/C. In comparison with the FeOOH
and Fe2O3-based anodes reported to date, F@F/C hybrid is a promising anode for LIBs with high reversible capacity and long cycling-life (Figure 4 and Table S1). Electrochemical impedance spectroscopy (EIS) tests can be used to analyze the kinetics of charge transfer reactions and lithium ion diffusion in the anode electrodes [37,38]. The FOH/C and F@F/C electrodes after 10 and 300 cycles are given by Nyquist plots in Figure 5. The typical randles circuits for cycled cells fitting by Nyquist plots are shown in Figure S9, and the resistance values of relative components after 300 cycles are listed in Table S2. As the number of cycles increases, the semicircles at high frequency of three anodes both gradually grow larger, implying the value of charge transfer resistance (Rct) and solid electrolyte interphase resistances (RSEI) increase, especially for FOH/C. After 300 cycles, the value of RSEI in FOH/C is 101 Ω obviously lower than the 25.59 Ω of FOH/C, indicating the Li+ diffusion kinetics in interface is facilitated availably during cycling due to the participation of Fe2O3. Furthermore, we also quantificationally estimate Li+ diffusion coefficient (DLi+) to describe the ionic diffusion kinetic according to the following equations [38]: Zre = Rb + Rct + σwω-1/2 DLi+ = R2T2 / 2A2n4F4Co2σw2
(10) (11)
Where Zre is the real part of electrical impedance; Rb is the bulk resistance of battery; Rct is the Faradic charge-transfer resistance and σw is the Warburg coefficient; ω is angular frequency; R is ideal gas constant; T is absolute temperature; A is the specific surface area of the electrode; F is Faraday constant; n is the molar amount of transfer charge; Co is the volume molar concentration of Li+. According to Formula 10, Warburg coefficient (σw) is negative related to the Li-ion diffusion coefficient (DLi+) in electrodes. The Zre as a function of ω-1/2 to calculate the value of σw of FOH/C and F@F/C electrodes after 10 and 300 cycles are shown in
Figure 5c, d. After 300 cycles, the calculated σw for F@F/C is evidently smaller than FOH/C, explaining the Li+ diffusion in F@F/C is accelerated as expected, which is consistent with the conclusion of EIS test. Throughout the cycling, the σw of F@F/C hybrid always keeps smaller, implying that it maintains faster ion diffusion all the time. By SEM, TEM and XPS tests, we further investigate the morphology, microstructure and chemical composition of the electrodes after cycling in order to explore the cause of fast ionic diffusion kinetics in F@F/C. After carefully examined many areas of electrode, the cycled FOH/C electrode has been almost covered with a thick SEI layer. However, the F@F/C electrode exhibits a porous morphology with uniformly distributed particles, which is more conducive to the infiltration of electrolyte and ionic diffusion (Figure 6). The sensitive XPS for detecting the surface elements is effective to identify the tiny phase transition of superficial SEI layers in Figure 7 and Figure S10. All electrodes are etched 60s for removing the surface SEI film to obtain the internal chemical states. The contents of ROCO2Li and Li2CO3 components before and after etching are listed in the Table 1.
Before etching, the surface C1s spectra in all electrodes both show
four states: C−C, ROCO2Li, C-O and Li2CO3 (Figure 7c, d). According to previous reports [39,40], Li-alkyl carbonates, produced by EC-DMC electrolyte react with Li ions, always generate at SEI interface in contact with electrolyte. Li2CO3 usually exists in the interior SEI layer near electrodes. The content of ROCO2Li component in FOH/C electrode is account for 20.57% almost twice as much of F@F/C (10.27%) and FO/C (10.76%) because more active hydroxyl groups in FeOOH can react easily with electrolyte to generate more ROCO2Li. The content ratios of ROCO2Li to Li2CO3 in F@F/C electrodes is 0.33, significantly less than the 0.66 of FOH/C, certifying the Li-alkyl carbonates in hybrid has been reduced as expected. After etching, the peaks which belong to the ROCO2Li disappear and the content of Li2CO3 decrease for all electrodes, indicating the exposure of inner chemical state.
Moreover, the Fe-O bonds appear in O1s spectra also confirm the removal of outside organic components of SEI and the exposure of inner oxide particles (Figure 7e, f). The lithium-ion conductivity and stability of Li-alkyl carbonates are both lower than Li2CO3, thus the strategy ameliorating the SEI quality by control the components content will improve the battery performance [41,42]. The possible ionic diffusion mechanism in embedded structure is illustrated in the schematic Figure 8. By constructing the embedded heterostructure, the relative content of Li2CO3 and ROCO2Li components in the SEI is ameliorated to improve the Li+ diffusion kinetics and structural stability of F@F/C. Moreover, owing to the absent of SEI at the junction of two active materials, Li ions are facilitate to transfer from Fe2O3 sphere to FeOOH rods, reducing ionic diffusion barriers thus providing more ionic transport pathways. 4. Conclusion In summary, the mulberry-like Fe2O3 sphere @ spindle-like α-FeOOH nanorods / reduced graphene oxide (F@F/C) hybrid has been fabricated based on the dissolution-recrystallization mechanism. The as-prepared hybrid with a unique embedded architecture can effectively reduce the content of ROCO2Li components in the SEI layer to decrease the ionic transfer impedance in the interface and providing alternative diffusion paths. In addition, the external Fe2O3 spheres can limit the expansion space of internal α-FeOOH nanorods to avoid destroy of the electrode. Finally, the hybrid exhibits excellent cycle and rate capacities. At 0.2 A g-1, the F@F/C remains a high reversible capacity of 1800 mAh g−1 after 300 cycles. Even at a high current of 1 A g-1, it still shows an excellent long-cycling life about 1050 mAh g−1 over 600 cycles. Besides, such a strategy of ameliorating SEI film to improve the ionic diffusion kinetics by heterostructure design could provide useful information for constructing more beneficial anode materials. ASSOCIATED CONTENT
Supporting Information. More structural images of F@F/C; SEM, TEM and XRD of the contrastive sample FO/C; FTIR and XPS spectrogram of three samples; CV and galvanostatic discharge-charge curves of the FOH/C and FO/C; Cycling performances of the FO/C and F@F/C; the XPS survey of three composites; the table of performances comparison of Fe2O3 and FeOOH-based electrodes for LIBs recently. AUTHOR INFORMATION Corresponding Authors *[email protected] *[email protected] Notes The authors declare no competing financial interest. REFERENCES [1] J.M. Tarascon, M. Armand, Nature 414 (2001) 359-367. [2] Y. Zeng, M. Yu, M. Yue, P. Fang, Y. Tong, Adv. Energy Mater. 6 (2016) 1601053. [3] Y. Wang, G. Cao, Adv. Mater. 20 (2010) 2251-2269. [4] M. Li, Y. Wang, C. Liu, H. Gao, W. Dong, Electrochim. Acta 67 (2012) 187–193. [5] L. Wang, G. Zhu, W. Yu, J. Zeng, X. Yu, Q. Li, H. Xie, Sol. Energy Mater. and Sol. Cells. 184 (2018) 1-8. [6] T. Soltani, A. Tayyebi, B.-K. Lee, Sol. Energy Mater. and Sol. Cells. 185 (2018) 325-332. [7] J. Yang, L. Tang, R. Bao, L. Bai, Z. Liu, B. Xie, M. Yang, W. Yang, Sol. Energy Mater. and Sol. Cells. 174 (2018) 56-64.
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Table 1. The content distribution of ROCO2Li and Li2CO3 components in three delithiated electrodes before and after etching in the end of 10 cycling. The before and after etching represent the surface and internal chemical states of the electrodes. The ratio of ROCO2Li to Li2CO3 in FOH/C is maximal, almost twice as much as F@F/C and FO/C.
Etching
Before
After
Components
FOH/C %
F@F/C %
FO/C %
ROCO2Li
20.57
10.27
10.76
Li2CO3
30.84
30.37
33.62
ratio
0.66
0.33
0.32
Li2CO3
12.09
19.33
17.19
a
b
c
d
e
f
g
Figure 1. (a) Schematic illustration of the synthesis procedures. (b) SEM and (c, d) TEM images of the FOH/C. (e) SEM and (f, g) TEM images of the F@F/C.
a
b
d
c
e
Figure 2. (a) XRD patterns for FOH/C and F@F/C samples synthesized at 80 and 100 o C. (b, c) High-resolution Fe2p and (d, e) O1s XPS spectra of the F@F/C and FOH/C.
a
c
b
d
e
Figure 3. (a) Cyclic voltammetry curves of the F@F/C between a voltage window of 0.01~3 V at a scan rate of 0.2 mV s-1. (b) Galvanostatic discharge/charge voltage profiles of the F@F/C at current density of 0.2 A g-1 for the different cycles. (c) Typical discharge-charge profiles at different current density of the F@F/C. (d) Rate performances at different specific currents of F@F/C and FOH/C electrodes. (e) Long cycling performances of the F@F/C and FOH/C at 1 A g-1.
Figure 4. Performance comparisons of several recent works related to FeOOH and Fe2O3-based anodes and our F@F/C for LIBs at high current density of 1A g-1.
a
b
c
d
Figure 5. Nyquist plots of FOH/C and F@F/C electrodes after (a) 10 and (b) 300 cycles at the current density of 1 A g-1. Zre as a function of ω-1/2 in the semi-infinite region for calculation of Warburg coefficient (σw) for all the samples after (c) 10 cycles and (d) 300 cycles.
a
c
b
d
Figure 6. SEM and TEM images with different magnification of (a, b) FOH/C and (c, d) F@F/C electrodes after 300 cycles.
Figure 7. The survey spectroscopy of delithiated (a) FOH/C and (b) F@F/C electrodes and their high-resolution (c, d) C1s, (e, f) O1s XPS spectra in the ending of 10 cycles before and after etching 60 s. All red curves belong to the states after etching.
Figure 8. Schematic illustration of lithium ion diffusion processes in FOH/C and F@F/C electrodes. Li+ are more likely to be transported into FeOOH from the membrane junction without SEI layer.
1. We rationally construct a heterostructure of spindle-like α-FeOOH nanorods embed in mulberry-like Fe2O3 sphere based on dissolution-recrystallization mechanism. 2. Embedded architecture optimizes the proportion of Li2CO3 and ROCO2Li in solid-electrolyte interphase to diminish the ionic transfer impedance in the interface and provide more alternative diffusion pathways, thus improving ionic diffusion kinetics significantly.