Î³-Fe2O3@CNTs Anode Materials for Lithium Ion Batteries Investigated

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γ‑Fe2O3@CNTs Anode Materials for Lithium Ion Batteries Investigated by Electron Energy Loss Spectroscopy Xiaoxin Lv,† Jiujun Deng,† Biqiong Wang,‡,§ Jun Zhong,† Tsun-Kong Sham,§ Xuhui Sun,*,† and Xueliang Sun*,‡ †

Soochow University-Western University Centre for Synchrotron Radiation Research, Institute of Functional Nano and Soft Materials Laboratory (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices, Soochow University, Suzhou 215123, China ‡ Department of Mechanical and Materials Engineering, University of Western Ontario, London, ON N6A 5B9, Canada § Department of Chemistry, University of Western Ontario, London, ON N6A 5B7, Canada S Supporting Information *

ABSTRACT: Atomic layer deposition was employed to deposit maghemite (γ-Fe2O3) nanoparticles on carbon nanotubes (CNTs) to prepare the γ-Fe2O3@CNTs composites, which exhibit a superior lithium storage performance as the anode of lithium ion batteries (LIBs). The high reversible capacity of 859.7 mA h/g was observed after 400 cycles at a current density of 500 mA/g. Even at the high current density of 10000 mA/g, the specific cyclic capacity of 464.4 mA h/g can still be obtained. Furthermore, electron energy loss spectroscopy results reveal that the Fe chemical state plays a critical role in the evolution of the capacity of γ-Fe2O3@CNTs composite anodes during the cycling process. The incomplete conversion of the chemical state in γ-Fe2O3 reduces the capacity, while the recovery of the chemical state of γ-Fe2O3 during the cycling process may cause the increase in capacity. This work provides insight into understanding the detailed working mechanism of transition metal oxides in LIBs, which helps in the design of electrode materials with promising lithium storage performance.

1. INTRODUCTION The rechargeable lithium ion battery (LIB) has been regarded as one of the most promising energy storage systems for highpower fields because of its high energy density, long lifetime, low self-discharge, and environmental benignity.1 Graphite is employed as the commercial anode material for LIBs at present. However, it suffers from a low theoretical capacity (372 mA h/ g) that limits the energy density of the battery.2,3 It is essential to develop a new anode from durable, low-cost, and environmentally benign materials with a high energy density to meet the desired requirements of high-power LIBs. In the past few decades, transition metal oxides such as Fe3O4,4,5 CuO,6,7 Fe2O3,8,9 and Co3O410,11 have been investigated as anode materials for LIBs because of their high theoretical capacity. Among these transition metal oxides, Fe2O3 has been considered as one of the most promising anode materials for LIBs because it is environmentally friendly, abundant, and inexpensive and has a relatively high theoretical capacity (1007 mA h/g).12,13 The mechanism of the reaction of Fe2O3 with lithium is an electrochemical conversion process (Fe2O3 + 6Li ↔ 2Fe + 3Li2O) in which one formula unit Fe2O3 can react © 2017 American Chemical Society

with six lithium atoms to form a material containing Fe clusters embedded in an amorphous Li2O matrix that can reversibly convert back to Fe2O3 during the charging process.14 A large volume change caused by the electrochemical conversion process leads to the pulverization of the electrode, resulting in poor cycling stability.15 Another main problem for Fe2O3 is poor conductivity that causes rapid capacity fading during the long cycling.16 Many strategies have been developed to address these problems.12−18 Among them, introducing carbon materials with Fe2O3 to form composites is a popular solution for improving the rate performance and cycling performance of Fe2O3 electrodes. For example, Lee et al. synthesized carboncoated porous γ-Fe2O3 microparticles that delivered a high reversible capacity of >900 mAh/g after 40 cycles at a current density of 2000 mA/g.8 Lou et al. obtained carbon-coated αFe2O3 hollow nanohorns on the CNT backbone that exhibited a very stable capacity retention of 800 mA h/g over 100 cycles Received: December 18, 2016 Revised: April 4, 2017 Published: April 4, 2017 3499

DOI: 10.1021/acs.chemmater.6b05356 Chem. Mater. 2017, 29, 3499−3506

Article

Chemistry of Materials

Figure 1. SEM images of γ-Fe2O3@CNTs composites (a) ALD-100, (b) ALD-300, and (c) ALD-500 and high-resolution TEM images of (d) ALD100, (e) ALD-300, and (f) ALD-500. Insets in panels d−f show the corresponding SAED patterns.

at a current density of 500 mA/g.19 Because of the unique structure and high electrical conductivity, CNTs have been widely used to form hybrids with electrode materials to improve their electrochemical performance, particularly enhancing the conductivity and better buffering the volume expansion of electrode materials during cycling.20−23 In this study, we chose CNTs as the substrate to prepare γ-Fe2O3@ CNTs composites by depositing maghemite nanoparticles on CNTs via atomic layer deposition (ALD). Because CNTs not only can improve the conductivity but also can better buffer the volume expansion of Fe2O3 electrode materials, the prepared γFe2O3@CNTs composite anodes show good rate capability and cycling performance. Atomic layer deposition (ALD) has been regarded as an important technique for synthesizing materials because of its good capacities in sequential and self-limiting surface reactions.24 One ALD cycle includes four steps: (a) supply a metal precursor for the first self-terminating reaction, (b) purge to evacuate the nonreacted precursor and gaseous byproducts, (c) supply the O precursor of oxygen or H2O for the second self-terminating reaction, and (d) purge again. Recently, ALD has attracted a great deal of attention for application in LIBs because of its atomic layer control and excellent conformality, such as synthesizing electrode materials25−28 and a solid electrolyte,29−31 and modifying the interface between the electrodes and electrolyte.32−34 Because of its unique characteristics, ALD has been regarded as a powerful method for growing metal oxides that can have their size controlled and be well distributed on a supporting substrate.

ALD in a Savannah 100 system (Cambridge Nanotech Inc.). The source temperatures of the two precursors, ferrocene (FeC10H10, 98%, Sigma-Aldrich) and oxygen, were set at 130 °C and room temperature, respectively. The size and density of the γ-Fe2O3 nanoparticles on MWCNTs were controlled by different ALD cycles (100, 300, and 500), which were denoted as ALD-100, ALD-300, and ALD-500, respectively. Characterization of the Material. The morphology and microstructure of γ-Fe2O3@CNTs composites were characterized by scanning electron microscopy (SEM) (Zeiss-supra55) and transmission electron microscopy (TEM) (FEI Quanta FRG 200F, operating at 200 kV) equipped with selected area electron diffraction (SAED). Electron energy loss spectroscopy (EELS) (Gatan Inc., equipped with a transmission electron microscope) was employed to reveal the chemical and phase composition changes of the individual sample under TEM observation. X-ray diffraction (XRD) (PANalytical, Zmpyrean) and X-ray photoelectron spectroscopy (XPS) (Kratos AXIS UltraDLD) were also used to investigate the crystal structure and chemical state of the samples, respectively. The content of γ-Fe2O3 in the sample was measured using thermogravimetric analysis (TGA) (Mettler Toledo, TGA1) at a heating rate of 10 °C/ min from room temperature to 800 °C. Electrochemical Measurements. The electrochemical performance of all the electrodes was tested in CR2032 coin type half-cells. The electrodes were prepared by mixing the active materials, poly(vinylidene fluoride) (PVDF), and Super P carbon in a 70:20:10 weight ratio. The CR2032 coin type half-cells were assembled in an Ar-filled glovebox (Mbraun, Labstar) with Li foil (counter and reference electrodes), a separator (Celgard 2325), and an electrolyte [1 M solution of LiPF6 in ethylene carbonate (EC), diethyl carbonate membrane (DEC), and dimethyl carbonate (DMC), 1:1:1 by volume]. The galvanostatic discharge−charge experiments were performed at different current densities from 100 to 10000 mA/g in the voltage range of 0.01−3 V using a LAND CT2001A battery test system. Cyclic voltammetry (CV) curves were measured between 0.01 and 3.00 V at a scan rate of 0.1 mV/s.

2. EXPERIMENTAL SECTION Preparation of γ-Fe2O3@CNTs Composites. Multiwalled carbon nanotubes (MWCNTs, Sigma-Aldrich, 40−60 nm) were treated with nitric acid at 90 °C for 2 h, filtered and washed with deionized water, and finally dried at 80 °C overnight. The pretreated MWCNTs were used as substrates for γ-Fe2O3 nanoparticle deposition at 350 °C by 3500


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Figure 2. (a) X-ray diffraction patterns of the ALD-100, ALD-300, and ALD-500 samples, pure CNTs, and standard γ-Fe2O3. (b) Thermogravimetric analysis curves of the ALD-100, ALD-300, and ALD-500 samples.

samples.36,37 The high-resolution O 1s and Fe 2p XPS spectra of the ALD-100, ALD-300, and ALD-500 samples are presented in panels c−f of Figure S2, respectively. The O 1s spectra of all three samples show one peak at 530.2 eV that results from γ-Fe2 O3 and another peak at 531.8 eV, corresponding to the CO groups due to the defects from CNTs.38,39 For Fe 2p XPS spectra (shown in Figure S2f), two main peaks at 710.8 and 725 eV corresponding to Fe 2p3/2 and Fe 2p1/2, respectively, accompanied by two weak satellites at ∼718.9 and ∼733 eV, respectively, demonstrate that the chemical state of Fe belongs to Fe2O3 in all of these samples rather than Fe3O4.40,41 The cyclic voltammetry (CV) measurements were performed to identify the electrochemical reactions of ALD-100, ALD-300, and ALD-500 as the LIB anodes, and the initial three cycles are shown in Figure 3. In the first cycle, both ALD-300 and ALD500 samples (panels c and e of Figure 3, respectively) exhibit two cathodic peaks at 1.02 and 0.79 V that can be ascribed to electrochemical reaction of lithium with γ-Fe2O3 in different steps and the decomposition of the electrolyte.42 Two anodic peaks at 1.64 and 1.87 V corresponding to the oxidation of Fe0 to Fe2+ and further oxidation to Fe3+ are also observed. In the subsequent cycles, the cathodic peak at 1.02 V disappeared and the cathodic peak at 0.79 V shifts slightly to a higher voltage, and the intensity of the 0.79 V peak shows an obvious decrease compared with that in the first cycle. All of these changes can be attributed to the capacity loss, and some irreversible processes occurred in the first cycle. The difference between the CV curves of these three samples is possibly due to their different composition. For the ALD-100 sample, the Fe2O3 content is lower than that in the ALD-300 and ALD-500 samples, and thus, the intensity of the peaks that stand for Fe2O3 reaction with lithium is different. A pair of peaks at 0.01 and 0.15 V related to lithium intercalation and deintercalation of CNTs is observed in the first cycle in the ALD-100 sample (shown in Figure 3a).40 Two cathodic peaks also exist at 0.75 and 0.66 V, which correspond to intercalation of lithium into γFe 2 O 3 and the formation of a SEI layer on CNTs, respectively.43−45 In the following cycles, the main cathodic peak at 0.75 V shifts slightly to a higher voltage and the intensity of this cathodic peak obviously decreases compared with that of the first cycle, which also demonstrates the capacity loss and that some irreversible processes occurred in the first

3. RESULTS AND DISCUSSION The γ-Fe2O3@CNTs composites were synthesized through a simple ALD method using CNTs as the substrate. After ALD deposition, γ-Fe2O3 nanoparticles were obtained on the CNTs. The morphology of γ-Fe2O3 deposited on CNTs is the nanoparticle because the substrate and its surface modification could determine the morphology of Fe2O3 loaded via ALD.35 The SEM images of the ALD-100, ALD-300, and ALD-500 samples are shown in panels a−c of Figure 1, respectively. As shown in the SEM images, the γ-Fe2O3 nanoparticles are uniformly dispersed on the CNTs. The density of the γ-Fe2O3 nanoparticles increases with the number of ALD cycles. The morphology and structure of the ALD-100, ALD-300, and ALD-500 samples were further characterized by TEM and SAED. The low-magnification TEM image in Figure S1 shows the γ-Fe2O3 nanoparticles uniformly distributed on the CNTs. The high-resolution TEM images and corresponding SAED data are shown in Figure 1d−f. It is obvious that the size of the γ-Fe2O3 nanoparticles increases with the number of ALD cycles. The average sizes of the γ-Fe2O3 nanoparticles of ALD100, ALD-300, and ALD-500 samples are approximately 2, 7, and 12 nm, respectively. All corresponding SAED patterns of these three samples confirm the existence of crystalline γFe2O3. Figure 2a shows the XRD patterns of the ALD-100, ALD300, and ALD-500 samples and pure CNTs. The peaks at 30.56°, 33.57°, 35.83°, 57.75°, and 63.12° are indexed to the (220), (310), (311), (511), and (440) planes of tetragonal γFe2O3 (JCPDS Card No. 39-1346), respectively. There is no γFe2O3 peak in the ALD-100 sample, indicating the content of Fe2O3 in this sample is too low to be detected by XRD. The contents of γ-Fe2O3 in the composites were estimated by TGA in air as shown in Figure 2b. The weight percentages of γ-Fe2O3 in ALD-100, ALD-300, and ALD-500 are 13.15, 37.46, and 51.19%, respectively, which are consistent with the SEM results. The compositions and valence states of ALD-100, ALD-300, and ALD-500 samples were characterized by XPS spectra. As shown in Figure S2a, the survey spectra of all samples show the peaks for Fe, O, and C, demonstrating their presence in all of these samples. The C 1s spectra of these three samples are also presented in Figure S2b, and all three spectra show one peak at 284.5 eV corresponding to graphitic carbon of CNTs in these 3501


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Figure 3. Cyclic voltammetry (CV) curves of the (a) ALD-100, (c) ALD-300, and (e) ALD-500 samples. Charge−discharge voltage profiles of the (b) ALD-100, (d) ALD-300, and (f) ALD-500 samples at a current density of 100 mA/g.

cycle. After the second cycle, the peak intensity of all of these three samples is almost unchanged, indicating no significant capacity loss and good stability of these anode materials. Panels b, d, and f of Figure 3 present the charge−discharge voltage profiles of the three samples at a current density of 100 mA/g between 0.01 and 3.0 V, which are in good agreement with the CV results. A plateau located around 0.8 V corresponds to the electrochemical reaction of Fe2O3 with lithium. For the ALD-

100 sample (shown in Figure 3b), the plateau at 0.12 V is related to the reaction of CNTs with lithium, corresponding to the peak in the CV curves. In the subsequent cycles, the capacities of the second cycle and third cycle are almost the same for all three samples, indicating the good reversibility of these three samples. Figure 4 shows the electrochemical performance of ALD100, ALD-300, and ALD-500. In this work, the total weight of 3502


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Figure 4. Electrochemical performance of the ALD-100, ALD-300, and ALD-500 samples: (a) long cycling performance at a current density of 500 mA/g and Coulombic efficiency and (b) rate performance at different current densities.

γ-Fe2O3 and CNTs was used as the weight of the active material. The electrochemical performance of ALD-100, which is poorer than those of the samples of ALD-300 and ALD-500, is similar to that of pure CNTs (shown in Figure S3) because of the smaller amount of γ-Fe2O3 in the ALD-100 sample. Figure 4a presents the long cycling performance and Coulombic efficiency of ALD-100, ALD-300, and ALD-500 at a current density of 500 mA/g between 0.01 and 3.0 V. The first specific discharge capacities of the ALD-300 and ALD-500 samples are 858.5 and 913.1 mA h/g with Coulombic efficiencies of 66.28 and 69.31%, respectively. Even after 400 cycles, the reversible discharge capacities of the ALD-300 and ALD-500 samples remain at 615 and 859.7 mA h/g with capacity retention of 72 and 94%, respectively, whereas the commercial Fe2O3 nanoparticles show a sharp decrease after 10 cycles at a current density of 50 mA/g.46 Interestingly, in the long cycling performance, the capacities of ALD-300 and ALD-500 show a similar increasing trend during the first dozen cycles. The previous studies attributed this phenomenon to the growth of a polymeric gel-like film (PGF) and the valence states changes of the iron.47−50 The rate capabilities of ALD-100, ALD-300, and ALD-500 were also obtained as shown in Figure 4b. At a current density of 100 mA/g, ALD-300 and ALD-500 show high discharge capacities of 1402.3 and 1653.4 mA h/g, respectively. Even at a high current density of 10000 mA/g, the discharge capacities of the ALD-300 and ALD-500 samples remain at 400.4 and 464.4 mA h/g, respectively. The good electrochemical performance of the γ-Fe2O3@ CNTs composites can be attributed to the unique nanostructure of this composite. On one hand, the larger interlayer spacing of the CNTs can boost the level of transmission of Li ion. Moreover, the CNTs form a conductive network that can greatly improve the conductivity of the Fe2O3 nanoparticles. They also have good flexibility and a stable tubular structure that can effectively accommodate the change in volume of Fe2O3 nanoparticles during cycling. On the other hand, the distributed Fe2O3 nanoparticles on CNTs can enhance the specific surface area of this composite, resulting in an enlarged contact area between the composite of the electrode and electrolyte. Accordingly, the small size of the Fe2O3 nanoparticles improves the stabilization of its structure, relieving its volume change during cycling. To improve our understanding of the working mechanism of the γ-Fe2O3 anode in the cycling process, we chose the ALD-

300 sample to perform an EELS study to reveal the chemical and phase composition changes of Fe2O3 in various steps during the cycling process. The EELS measurement under TEM observation can provide detailed chemical state information about the exact region of the sample, which could prevent the interference of the impurities in the sample. The characteristic features of the transition metals revealed by EELS are white lines (L2,3 edges). Because the L2,3 edges have local structures that are sensitive to the valence states of the metals, EELS has been widely used to investigate the valence states of the transition metals.51−53 The L2,3 edges of the transition metals correspond to the transition of 2p electrons to the unoccupied 3d states. The valence states of the transition metals can be investigated by both the white line ratio (L3/L2) and absolute energy positions.54−56 Many studies have demonstrated that the white line ratio (L3/L2) changes with the change in the valence state, which provides a possibility of identifying the oxidation state of the transition metals.52,57,58 The ALD-300 sample before cycling and samples after 1, 20, and 50 cycles in the charged state were chosen for the EELS experiments. Figure 5 shows the Fe-L2,3 edges of ALD-300 with different cycles at a current density of 500 mA/g. Table 1 summarizes the L3/L2 white line intensity ratios of the ALD-

Figure 5. EELS spectra of Fe-L2,3 edges of the ALD-300 sample before cycling and after different cycles at a current density of 500 mA/g. 3503



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Table 1. Fe-L2,3 Edge White Line Intensity Ratios and Corresponding Oxidation State of the ALD-300 Sample with Different Numbers of Cycles sample (ALD-300)

white line intensity ratio (L3/L2)

oxidation state

before cycling after 1 cycle after 20 cycles after 50 cycles

5.0 4.3 4.6 4.8

+352 +256,58 +2, +3 close to +3

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b05356. XPS data and low-magnification TEM images of the ALD-100, ALD-300, and ALD-500 samples and the electrochemical performance of pure CNTs (PDF)

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AUTHOR INFORMATION

Corresponding Authors

300 sample with different cycles, exhibiting a variation in the L3/L2 ratio of the sample after different numbers of cycles. The L3/L2 intensity ratio of the sample before cycling is 5.0, corresponding to the +3 oxidation state.52 After one cycle, the L3/L2 intensity ratio decreases to 4.3, corresponding to the +2 valence state in the FeO,57,59 indicating the sample cannot be restored to Fe2O3. After 20 cycles, the L3/L2 intensity ratio increases to 4.6 compared to that of the first cycle, indicating the increase in the amount of Fe3+ in this sample, which implies the transformation from Fe2+ to Fe3+. The L3/L2 intensity ratio of the sample after 50 cycles is 4.8, suggesting that the valence state of Fe is close to the +3 oxidation state, which implies more and more Fe can change back to Fe2O3. The low oxidation state of Fe (such as Fe2+) might reduce the capacity by trapping Li+ ions, and the recovery of the low oxidation state of Fe to Fe3+ would significantly increase the capacity.9 According to the results described above, we can conclude that in the first delithiation, the irreversible phase conversion is an important factor for the initial capacity fading and the recovery of the chemical state of γ-Fe2O3 during the cycling process might cause the capacity increase. The Fe-L2,3 white line energy position of the sample shown in Figure 5 shifts to a lower energy after the first cycle compared to the sample before cycling and after 20 and 50 cycles, and then the energy position after 50 cycles shifts back close to that of the sample before cycling. The white line positions directly correspond to the oxidation state of Fe, where the high energy position represents the high oxidation state. The results agree well with the white line intensity ratio (L3/L2).

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Xuhui Sun: 0000-0003-0002-1146 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by the Natural Science Foundation of China (NSFC) (Grants 91333112 and U1432249) and the Priority Academic Program Development of Jiangsu Higher Education Institutions. This project was also supported by the Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices and Collaborative Innovation Center of Suzhou Nano Science & Technology and sponsored by the Qing Lan Project.

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REFERENCES

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4. CONCLUSIONS We have prepared the γ-Fe2O3@CNTs composites via ALD. The γ-Fe2O3@CNTs composites as the anode of LIB exhibit superior lithium ion storage performance. The high reversible capacity of 859.7 mA h/g was achieved after 400 cycles at a current density of 500 mA/g. Even at a high current density of 10000 mA/g, the capacity of the sample can still reach 464.4 mA h/g. In addition, the working mechanism of γ-Fe2O3@ CNTs composite anodes during the cycling process was studied by EELS. The results show that the oxidation state of Fe affects the capacity evolution in which the incomplete phase conversion of the anode may reduce the capacity and the recovery of the chemical state of γ-Fe2O3 during the cycling process may cause an increase in capacity. EELS data also reveal that the irreversible phase conversion is an important factor for the initial capacity fading in the first cycle. These results will be useful for designing electrode materials with promising lithium storage performance and understanding the detailed working mechanism of transition metal oxides in LIBs. 3504


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Î³-Fe2O3@CNTs Anode Materials for Lithium Ion Batteries Investigated

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