Article pubs.acs.org/IC
Engineering Mesoporous Single Crystals Co-Doped Fe2O3 for HighPerformance Lithium Ion Batteries Huabin Kong, Chade Lv, Chunshuang Yan, and Gang Chen* MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin, Heilongjiang 150001, China S Supporting Information *
ABSTRACT: To achieve high-efficiency lithium ion batteries (LIBs), an effective active electrode material is vital. For the first time, mesoporous single crystals cobalt-doped Fe2O3 (MSCs Co−Fe2O3) is synthesized using formamide as a pore forming agent, through a solvothermal process followed by calcination. Compared with mesoporous single crystals Fe2O3 (MSCs Fe2O3) and cobalt-doped Fe2O3 (Co−Fe2O3), MSCs Co−Fe2O3 exhibits a significantly improved electrochemical performance with high reversible capacity, excellent rate capability, and cycling life as anode materials for LIBs. The superior performance of MSCs Co−Fe2O3 can be ascribed to the combined structure characteristics, including Co-doping and mesoporous single-crystals structure, which endow Fe2O3 with rapid Li+ diffusion rate and tolerance for volume change.
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INTRODUCTION Lithium ion batteries (LIBs) have been widely used in portable electronics products, and they are expected to be a promising power source in electric vehicles due to their high power density and long-term life.1 With rapidly increasing demand for improved energy storage devices, the development of LIBs with high energy density and outstanding cycling stability has become a crucial issue.2 To satisfy these urgent needs, massive efforts have been focused on searching for new highperformance electrode materials. Transition metal oxides (TMOs) gradually attracted researcher’s interest because of the high specific capacity and abundant resources. Unfortunately, drastic volume changes and mechanical strain occur during charge−discharge process, leading to severe capacity fading. Furthermore, intrinsically low conductivity of TMOs endow them with poor rate performances.3 To overcome these pivotal limitations, special endeavors have been devoted to the rational structure design to tailor properties of electrode materials. The mesoporous singlecrystals (MSCs), possessing high surface areas/pore volumes and superior structural stability, have become a conceptually new class of structure.4 Conceivably, MSCs can not only shorten the diffusion distances of Li+ but also provide a buffer area to relieve the volume expansion, resulting in better cycling stability.5 These intriguing merits will give them greater competition in LIBs. For example, Lou et al.6 have synthesized mesoporous single-crystal Si on Cu foam accompanying with the excellent cycling performance. Although the mesoporous structure can alleviate volume expansion, Fe2O3 still experiences the problem of poor conductivity.7 One of the most commonly used techniques is © 2017 American Chemical Society
to design Fe2O3-based composites with conducting materials, such as carbon materials.8 However, introducing too much carbon will significantly reduce the capacity of electrode material.9 Thus, it is crucial to find other approaches to improve the electrical conductivity. Recently, element doping exhibits considerable advantages to alter the intrinsic conductivity.10 It is reported that the doping of metal elements can improve the conductivity of TMOs (Fe2O3, Co3O4, and NiO).11 Tu et al.12 synthesized the Co-doped NiO nanoflake arrays, resulting in the enhanced conductivity via increasing the hole concentration. Thi et al.13 synthesized NiO nanoparticles and introduced Co 2+ in NiO, improving largely the conductivity. Therefore, the Co doping can improve the rate capability by enhancing electronic conductivity. However, as far as we know, few studies have been reported on the MSCs Codoped materials to settle the volume expansion and conductivity simultaneously as an anode material. Herein, a simple solvothermal method was designed to prepare mesoporous single crystals cobalt-doped Fe2O3 (MSCs Co−Fe2O3) by using formamide as a pore-forming agent. The Co2+ dopant substitutes Fe3+ and is uniformly distributed among Fe2O3, becoming incorporated into the Fe2O3 lattice. The combination of Co doping and MSCs offers many advantages, including introduced oxygen vacancies to enhance the specific capacity, provide a buffer area to relieve volume expansion/contraction, and shorten diffusion pathway for electrons. These advantages are beneficial to the huge improvement of electrochemical properties. When they are Received: January 3, 2017 Published: June 26, 2017 7642
DOI: 10.1021/acs.inorgchem.7b00008 Inorg. Chem. 2017, 56, 7642−7649
Article
Inorganic Chemistry evaluated as anode materials, the obtained unique MSCs Co− Fe2O3 exhibit superior electrochemical performance, suggesting their promising applications in LIBs.
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EXPERIMENTAL SECTION
Preparation of the MSCs Co−Fe2O3. In the process of preparation, 0.270 g of FeCl3·6H2O (1 mmol) and 0.029 g of Co(NO3)2·6H2O (0.1 mmol) were dissolved in 10 mL of formamide and 20 mL of ethanol. The solution was stirred for 30 min and transferred into a 50 mL Teflon-lined stainless steel autoclave and kept at 180 °C for 12 h. Subsequently, the autoclave cooled to room temperature, and the precursor was collected by centrifugation, washed by deionized water several times, and dried at 80 °C for 12 h. Finally, the sample was thermally treated at 500 °C for 2 h. As a comparison, MSCs Fe2O3 and Co−Fe2O3 were obtained without adding cobalt source and the calcination process. Material Characterizations. The composition and purity of the material were characterized by X-ray diffraction (XRD) on Rigaku D/ max-2000 diffractometer with Cu Kα radiation. Raman spectra were tested on a Renishaw inVia micro-Raman spectroscopy system with a laser wavelength of 532 nm. The micromorphology was characterized by a HELIOS NanoLab 600i field emission scanning electron microscope (FE-SEM). The operating voltage was set to 20 kV, and the samples were prepared by dropping the pre-ultrasonic-dispersed (10 min) ethanol turbid liquid onto the chip of silicon. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) of the samples were performed on FEI Tecnai G2 F30 S-Twin operating at 300 kV. X-ray photoelectron spectroscopy (XPS) was accomplished using a Thermo Scientific ESCALAB 250Xi X-ray photoelectron spectrometer with a pass energy of 20.00 eV and an Al Kα excitation source (1486.6 eV). Brunauer−Emmett−Teller (BET) data were obtained with a Micromeritics ASAP 2020 (Accelerated Surface Area and Porosimetry System). Infrared (IR) test of the material was characterized on a Fourier transform infrared (FTIR) spectrometer (Shimadzu) in the range of 400−4000 cm−1. Electrochemical Measurements. The electrochemical evaluation was performed on 2025-type coin cells. The prepared electrode material, carbon black, and poly(vinyl difluoride) (PVDF) were mixed in a weight ratio of 80:10:10. Slurry of the mixture was stirred for 12 h and pasted on copper foil, followed by the electrode film being dried in vacuum at 100 °C for 12 h. The average mass loading of active materials was ∼1.0 mg cm−2. For LIBs, lithium foil was used as the counter electrode. LiPF6 (1 M) in a mixture of ethylene carbonate and diethyl carbonate (w/w = 1:1) was used as the electrolyte. Cell assembly was performed in glovebox. The cyclic voltammetry (CV) and electrochemical impendence spectroscopy (EIS) measurements were performed on CHI604C electrochemical workstation with a voltage range from 3.0 to 0.01 V at a scanning rate of 0.5 mV s−1. The galvanostatic charge/discharge measurements were performed on a battery testing system (NEWARE BTS-610) with a voltage of 0.01− 3.0 V.
Figure 1. Microstructure analysis of MSCs Co−Fe2O3. (a) SEM images, (c) TEM images, (b, d) HRTEM images, (e) SAED pattern, (f) HAADF-STEM images, and (g−i) elemental mapping of MSCs Co−Fe2O3.
morphology of material (Figure S1). Through observing TEM images of the MSCs Fe2O3 and Co−Fe2O3 (Figures S2 and S3), MSCs Fe2O3 possesses mesoporosity, while mesopores vanish in Co−Fe2O3, indicating that the mesopores are created during the calcination process. Brunauer, Emmett, and Teller (BET) measurement is performed to verify the mesoporous characteristic of sample. Figure 2 displays the nitrogen sorption isotherms of MSCs Co−Fe2O3 and Co−Fe2O3. The pore size distribution of MSCs Co−Fe2O3 (inset of Figure 2a) exhibits the average pore diameter is 2.8 nm, which affirms that mesoporous single crystals are characteristic of MSCs Co−Fe2O3. Without Co doping, MSCs Fe2O3 also possesses mesoporous single-crystals structure, and the average pore diameter is 2.2 nm (Figure S4). In addition, the inset of Figure 2b shows the average pore diameter of Co−Fe2O3 is 53.8 nm. In addition, attributed to the mesoporosity of MSCs structure, the surface area of MSCs Co−Fe2O3 is increased fivefold over that of Co−Fe2O3. On the basis of above results, we can safely draw the conclusion that the MSCs structure is successfully achieved, which not only might shorten the Li+ diffusion distance but also could provide a good accommodation for the volume changes, which may result in an excellent cycling stability. The crystallinity and phase composition is characterized by XRD. Figure 3a displays the XRD patterns of MSCs Co− Fe2O3, MSCs Fe2O3, and Co−Fe2O3. All the main peaks in the XRD patterns can be indexed to α-Fe2O3 (JCPDS No.79− 0007). No other additional diffraction peaks are detected for cobalt oxides or other impurities, indicating the highly purity of samples. It is found that the (104) diffraction peak position of MSCs Co−Fe2O3 slightly shifts to a larger 2θ value by comparing the enlarged XRD patterns (Figure 3b). It might be due to the incorporation of cobalt ion into the Fe2O3 lattices, resulting in the broadening of lattice fringes. With the aim of verifying above conjecture, the sample is characterized by energy-dispersive X-ray (EDX) mapping. As can be seen, the EDX mapping demonstrates the Co doping is realized with good distribution (Figure 1g−i). In addition, the element content of sample is analyzed by energy-dispersive spectroscopy (EDS; Figure S5) and inductively coupled plasma (ICP); the EDS indicates that the material contains three
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RESULTS AND DISCUSSION The morphology of mesoporous single-crystals cobalt-doped Fe2O3 (MSCs Co−Fe2O3) characterized by the SEM Figure 1a displays the sample is composed of ∼100 nm nanoparticles. Subsequently, the detailed structure of MSCs Co−Fe2O3 is further tested by TEM and HRTEM. Figure 1b,c clearly confirms that many mesopores are randomly distributed on the nanoparticles. Figure 1d shows clear fringes with a measured interplanar spacing of 0.368 nm, corresponding to the (012) plane of Fe2O3. From selected area electron diffraction (SAED) pattern of MSCs Co−Fe2O3 (Figure 1e), the diffraction spots prove the single-crystalline nature of the material. As a comparison, MSCs Fe2O3 and Co−Fe2O3 are obtained without adding cobalt source and the calcination process. It is found that Co doping and calcination process will not change the 7643
DOI: 10.1021/acs.inorgchem.7b00008 Inorg. Chem. 2017, 56, 7642−7649
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Inorganic Chemistry
Figure 2. Nitrogen adsorption/desorption isotherms and pore size distributions (inset) of samples (a) MSCs Co−Fe2O3 and (b) Co−Fe2O3.
Figure 3. (a) XRD patterns and (b) enlargement of the (311) diffraction peak positions in the range of 2θ = 32° to 35° of MSCs Co−Fe2O3, MSCs Fe2O3, and Co−Fe2O3.
Scheme 1. Schematic Illustration of the Proposed Formation Mechanism of MSCs Co−Fe2O3
from the CN triple bond converting to CO2 and NH3 escaping from the surface of nanoparticles. Apart from the enhanced surface area ascribed to the MSCs structure of MSCs Co−Fe2O3, the Co doping might introduce oxygen vacancies, which is also a favorable feature for improving the electrochemical performance. To prove above argument, XPS spectroscopy is performed. Figure 4a exhibits a widely scan XPS spectrum of MSCs Co−Fe2O3, which displays the presence of Fe 2p, Co 2p, O 1s, and C 1s. The characteristic peak in the inset of Figure 4b is assigned to the Co 2p characteristic peak of CoO, confirming the Co (II) oxidation state in MSCs Co−Fe2O3.14 From the Fe 2p spectra (Figure 4b), two obvious characteristic peaks of Fe 2p3/2 and Fe 2p1/2 can be observed at 708.5 and 722.4 eV, respectively. And the separation of 2p doublet is ∼14.0 eV.15 Two satellites can also be clearly observed, which is in accordance with previous work for Fe2O3.16 Figure 4c mainly includes two components, namely, peak A and peak B. The peak B suggests that the O ions have neighboring atoms with their full complement, and
elements, namely, Fe, Co, O. The result of ICP shows the mass percent of Fe and Co is ∼67.00% and 3.50%. As shown above, we employ a facile method to achieve the MSCs structure and Co doping. Therefore, a plausible mechanism responsible for the formation of MSCs Co− Fe2O3 should be proposed for further inspiration in fabricating other electrode materials with MSCs and metal doping. As illustrated in Scheme 1, in the first step, ethanol coordinated with Fe 3+ to produce Fe 2 O 3 crystal nucleus and Co 2+ substitutes some Fe3+ incorporating into the Fe2O3 lattice. In the following secondary growth stage, the Fe2O3 crystal nucleus is gradually growing. In addition, the formamide changes into hydrocyanic acid by dehydration. Subsequently, the coordination effect of hydrocyanic acid and iron atoms weaken the strength of CN triple bond, resulting in the peak position of CN triple bond shift to a lower wavenumber (Figure S6). Third, the Fe2O3 crystal nucleus continues to grow and eventually form nanoparticles. In the last stage, during the calcination of Co−Fe2O3, the mesopores emerge stemmed 7644
DOI: 10.1021/acs.inorgchem.7b00008 Inorg. Chem. 2017, 56, 7642−7649
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Figure 4. (a) XPS wide spectrum of MSCs Co−Fe2O3, the inset is the Co 2p spectrum, (b) Fe 2p XPS spectra, and (c) O 1s XPS spectra and (d) Raman spectra of MSCs Co−Fe2O3 and MSCs Fe2O3.
Figure 5. Electrochemical performances of MSCs Co−Fe2O3, MSCs Fe2O3, and Co−Fe2O3. (a) CV curves in the first three cycles at a scan rate of 0.5 mV s−1. (b) Charge−discharge profiles at a current density of 200 mA g−1. (c) Cycling performance at a current density of 200 mA g−1. (d) Rate capability at a current density from 100 to 1000 mA g−1. All the measurements are performed in the potential range of 0.01−3.0 V vs Li/Li+.
1318 cm−1 is ascribed to a magnon−magnon scattering effect, which arises from the interaction of magnon−magnon.20 Compared with MSCs Fe2O3, the band of MSCs Co−Fe2O3 with slight shift indicates that a part of Fe3+ ions are substituted by Co2+. The bond length of Co−O bond gets longer than Fe− O band, leading to the bond length of neighbor Co−O bond being shortened. Therefore, to maintain electrical neutrality, some oxygen vacancies are created because of the fluctuation of chemical valence. The defect equation of Co doped Fe2O3 is as follows:
the peak A means that O ions are in oxygen-deficient regions.17 The ratio between peak A and peak B is increased, indicating that there are more oxygen vacancies in MSCs Co−Fe2O3 compared to MSCs Fe2O3. More oxygen vacancies will be conducive to enhance the discharge capacity of Fe2O3 as anode materials for LIBs. Raman spectra are also performed for characterizing the doped lattice position of cobalt ions. As showed in Figure 4d, both samples show seven characteristic bands.18 Among them, the A1g and E1g are assigned to symmetric stretching Fe−O vibration mode. The IR-active Eu mode at 659 cm−1 generally reflects local disorder in the lattice of Fe2O3.19 And the peak at 7645
DOI: 10.1021/acs.inorgchem.7b00008 Inorg. Chem. 2017, 56, 7642−7649
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Figure 6. Electrochemical performances of electrode made of the as-formed MSCs Co−Fe2O3, MSCs Fe2O3, and Co−Fe2O3. (a) Nyquist plots after 200 discharge/charge cycles. (b) The relationship between Zre and ω−1/2 in the low-frequency region. (c) The corresponding equivalent circuit. 2Co2 +
Fe2O3 ⎯⎯⎯⎯⎯→ 2Co′Fe + V ··O + 2OO
rise of the capacity may be attributed to the refinement of nanoparticles. As shown in Figure S10, MSCs Co−Fe2O3 nanoparticles gradually refine with the increase of the number of cycles. The refinement of nanoparticles may increase the specific surface area of electrode material, which will provide a larger reaction interface and improve the electrochemical performance.25 Moreover, decreased size of MSCs Co−Fe2O3 nanoparticles will lead to a shorter electronic and ionic transport length.26 In a word, the huge change in the morphology of the sample after discharge/charge cycles explains the capacity increases for the MSCs Co−Fe2O3 electrode. After 150 cycles, the structure and morphology of MSCs Co−Fe2O3 were almost unchanged, leading to stable discharge capacity. However, the capacity of Co−Fe2O3 does not have the process of rising, which may be due to the absence of mesopores in Co−Fe2O3, which leads to the structure being destroyed during charge/discharge process. Compared with MSCs Fe2O3, the reversible capacity of MSCs Co−Fe2O3 has been significantly improved, which can be ascribed to more oxygen vacancies being generated after Co doping, providing more active sites. Figure S11 shows even at a higher current density of 500 mA g−1, MSCs Co−Fe2O3 still has a high discharge capacity of 1073 mAh g−1. The specific capacity of MSCs Co−Fe2O3 is higher than theoretical capacity of Fe2O3, which could be ascribed to the redox reaction of Fe2O3 and CoO. In addition, excess Li can be accommodated at the interfaces of MSCs Co−Fe2O3, resulting in an increase of the total amount of Li stored. After charging/discharging 210 cycles, the rate performance of three electrodes is further investigated at differdent current densities. The MSCs Co−Fe2O3 exhibits excellent capacity retention with discharge capacities of 1277, 1200, 1046, and 874 mA g−1 at current densities of 100, 200, 500, and 1000 mA g−1, respectively (Figure 5d). Compared to MSCs Fe2O3 and Co−Fe2O3, the discharge capacity of MSCs Co−Fe2O3 has been obviously improved. Furthermore, when the current density is reversed to 100 mA g−1, the discharge capacity of 1280 mA g−1 is regained, which is almost equal to the initial discharge capacity, indicating that MSCs Co−Fe2O3 has an outstanding rate performance as electrode material. For further evaluating the diffusion of Li+, the EIS is performed for MSCs Co−Fe2O3, MSCs Fe2O3, and Co−Fe2O3
(1)
We next test the electrochemical performances of the MSCs Co−Fe2O3, MSCs Fe2O3, and Co−Fe2O3. To evaluate the cyclic performance, the cyclic voltammetry (CVs) analysis is performed to investigate the electrochemical processes in a voltage range of 0.01−3.0 V versus Li+/Li at a scan rate of 0.5 mV s−1, and the result is displayed in Figure 5a. In the first cycle, it exhibits one cathodic peak at 0.38 V, which corresponds to the Li+ insertion into Fe2O3, the formation of Li2O, and the electrolyte decomposition to form solid electrolyte interphase (SEI) films.21 The anodic peak at 1.71 V is the two-step oxidation process from Fe0 to Fe2+ and from Fe2+ to Fe3+. The cathodic peak shifts to 1.63 V in the following cycles. In addition, the distance between cathode peak and anode peak of MSCs Co−Fe2O3 is smaller compared with MSCs Fe2O3 (Figure S8), indicating that MSCs Co−Fe2O3 has a better reversibility in the process of charge/discharge. The charge/discharge profiles with different cycles of MSCs Co−Fe2O3 over the voltage range from 0.01 to 3.00 V at a current density of 200 mA g−1 are shown in Figure 5b. The first cycle charge/discharge profile displays a high discharge capacity of 1552 mA g−1 and charge capacity of 1115 mAh g−1 for the MSCs Co−Fe2O3. By contrast, we also characterized the electrochemical performance of MSCs Fe2O3 and Co−Fe2O3 (Figure S9), which indicated the initial discharge of MSCs Co− Fe2O3 has a significant improvement.22 The cycling performance of MSCs Co−Fe2O3, MSCs Fe2O3, and Co−Fe2O3 is examined by extended charge/discharge tests. Figure 5c exhibits the cycle stability of material at a current density of 200 mA g−1. All of the samples show a quick decline in discharge capacity from 1119, 978, and 697 mAh g−1 in the second cycle to 614, 330, and 99 mAh g−1 by the 25th cycle, respectively. The initial capacity loss could be stemmed from the irreversible formation of the SEI layer on the electrode surface.23 The ensuing capacity decline could be ascribed to the unstable SEI layer caused by repetitive volume change. The unstable SEI layer may fracture, exfoliate, and reform. Hence, the consumption of lithium ions and electrolyte during repetitive formation of SEI layers significantly reduced the reversible capacity of electrode material.24 Subsequently, the 7646
DOI: 10.1021/acs.inorgchem.7b00008 Inorg. Chem. 2017, 56, 7642−7649
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material with MSCs structure and Co doping exhibit superior electrochemical performance.
electrodes. The Nyquist plots of three samples are exhibited in Figure 6a. The Nyquist plots include a semicircle in the highmedium frequency region and an oblique straight line in the low-frequency region, which corresponds to the charge-transfer process and the mass transfer of Li+, respectively.27 It is observed that the semicircle diameter of MSCs Co−Fe2O3 and Co−Fe2O3 is much smaller than that of MSCs Fe2O3 from the Nyquist plots, demonstrating faster charge-transfer impedances of MSCs Co−Fe2O3 and Co−Fe2O3 compared with MSCs Fe2O3. This indicates that the electron transfer and ion diffusion have been improved. In addition, the conductivity of Fe2O3 also has been significantly enhanced. Thus, it results in an outstanding improvement in the rate performance. The diffusion coefficient (D) of Li+ can be calculated according to the following equation:28 Zre = RD + RL + σω−1/2
(2)
D = 0.5(RT /An2F 2Cσ )2
(3)
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CONCLUSIONS In summary, using formamide as a pore-forming agent, mesoporous single crystals cobalt-doped Fe2O3 (MSCs Co− Fe2O3) are successfully synthesized by facile solvothermal method and a calcination process. This unique nanostructure delivers a high reversible capacity and excellent long-term cycling stability (1222 mAh g−1 after 200 cycles at a current density of 200 mA g−1), outstanding rate capability (1073 mAh g−1 at a current density of 500 mA g−1). The greatly promoted electrochemical performance of MSCs Co−Fe2O3 can be attributed to its advantageous MSCs structure, high crystallinity and the cationic doping of Co into the Fe2O3 lattice, which guarantee shorter Li+ diffusion pathway, sufficient pore volume for accommodating the volume variation and enhanced charge transfer kinetics. It can be expected that our rational structure design will be beneficial to the development of LIBs.
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Herein, R is the gas constant, A is the surface area, T is the absolute temperature, n is the number of electrons per molecule oxidized, C is the concentration of Li+, F is the Faraday constant, σ is the Warburg impedance coefficient, which is related to Zre, ω is the angular frequency in the low-frequency region, and RD and RL are the diffusion resistance and liquid resistance, respectively. The relationship of three samples between Zre and ω−1/2 in the low-frequency region is shown in Figure 6b. It is shown that the Warburg impedance coefficient of MSCs Co−Fe2O3, MSCs Fe2O3, and Co−Fe2O3 are 31.0, 51.5, and 59.6 Ω cm2 s−1/2, respectively. It indicates mesoporous structure is beneficial to enhance the diffusion coefficient of Li+. An equivalent circuit model (Figure 6c) is used to explain the electrochemical impedance spectroscopy (EIS) of three electrodes. Rs, RSEI, and Rct represent the resistance of the electrolyte, the resistance of the SEI film, and the chargetransfer resistance, respectively. Table 1 shows both Rs, RSEI,
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00008. SEM images of Co−Fe2O3 and MSCs Fe2O3, TEM images, HRTEM images and SAED pattern of Co− Fe2O3 and MSCs Fe2O3, BET results of MSCs Fe2O3, EDS results of MSCs Co−Fe2O3, FTIR spectra of MSCs Co−Fe2O3 and Co−Fe2O3O, CV curves of Co−Fe2O3 and MSCs Fe2O3, charge−discharge profiles of Co− Fe2O3 and MSCs Fe2O3, TEM images of MSCs Co− Fe2O3 electrode after 200 charge/discharge cycles, cycle performance of MSCs Co−Fe2O3 at 500 mA g−1, and Performance comparison of different morphology of Fe2O3 anode material (PDF)
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Table 1. Electrochemical Impedance Parameters of the MSCs Co−Fe2O3, MSCs Fe2O3, and Co−Fe2O3 samples
Rs (Ω)
RSEI (Ω)
Rct (Ω)
MSCs Co−Fe2O3 (50th) Co−Fe2O3 (50th) MSCs Fe2O3 (50th)
3.149 4.143 5.536
23.498 54.273 89.396
80.761 124.94 185.15
ASSOCIATED CONTENT
S Supporting Information *
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Gang Chen: 0000-0003-1502-0330 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21471040).
and Rct at the 200th cycle of MSCs Co−Fe2O3 are significantly lower than MSCs Fe2O3 and Co−Fe2O3, which further demonstrates that MSCs structure and Co doping is favorable for Li+ transport. In view of the above analyses, the outstanding electrochemical performance of MSCs Co−Fe2O3 mainly originates from Co doping and MSCs structure, which are reflected in the following several points: (1) The improved rate capability originating from Co doping improves the electronic conductivity, and MSCs structure shortens the diffusion path so as to accelerate the diffusion rate of Li+. (2) The improved longterm cycling stability mainly due to MSCs structure can relieve the volume expansion/contraction in the process of charging/ discharging. (3) The promoted reversible capacity caused by the introduced oxygen vacancy provides more active sites, resulting in more Li+ storage space. Therefore, the electrode
REFERENCES
(1) (a) Peng, Z.; Freunberger, S. A.; Chen, Y. H.; Bruce, P. G. A reversible and higher-rate Li-O2 battery. Science 2012, 337, 563−566. (b) Etacheri, V.; Marom, R.; Elazari, R.; Salitra, G.; Aurbach, D. Challenges in the development of advanced Li-ion batteries: a review. Energy Environ. Sci. 2011, 4, 3243−3262. (c) Dunn, B.; Kamath, H.; Tarascon, J. M. Electrical Energy Storage for the Grid: A Battery of Choices. Science 2011, 334, 928−935. (d) Goodenough, J. B.; Kim, Y. Challenges for Rechargeable Li Batteries. Chem. Mater. 2010, 22, 587− 603. (2) (a) Liu, J.; Kopold, P.; Wu, C.; van Aken, P. A.; Maier, J.; Yu, Y. Uniform Yolk-shell Sn4P3@C Nanospheres as High-capacity and Cycle-stable Anode Materials for Sodium-ion Batteries. Energy Environ. 7647
DOI: 10.1021/acs.inorgchem.7b00008 Inorg. Chem. 2017, 56, 7642−7649
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Inorganic Chemistry Sci. 2015, 8, 3531−3538. (b) Liu, J.; Wen, Y.; Wang, Y.; et al. CarbonEncapsulated Pyrite as Stable and Earth-Abundant High Energy Cathode Material for Rechargeable Lithium Batteries. Adv. Mater. 2014, 26, 6025−6030. (c) Liu, J.; Wen, Y.; van Aken, P. A.; et al. Facile synthesis of highly porous Ni-Sn intermetallic microcages with excellent electrochemical performance for lithium and sodium storage. Nano Lett. 2014, 14, 6387−6392. (3) (a) Jiang, J.; Li, Y.; Liu, J.; Huang, X.; Yuan, C.; Lou, X. Recent advances in metal oxide-based electrode architecture design for electrochemical energy storage. Adv. Mater. 2012, 24, 5166−5180. (b) Devan, R. S.; Patil, R. A.; Lin, J.; Ma, Y. One-Dimensional MetalOxide Nanostructures: Recent Developments in Synthesis, Characterization, and Applications. Adv. Funct. Mater. 2012, 22, 3326−3370. (4) (a) Lou, X.; Deng, D.; Lee, J. Y.; Archer, L. A. Thermal formation of mesoporous single-crystal Co3O4 nano-needles and their lithium storage properties. J. Mater. Chem. 2008, 18, 4397−4401. (b) Wang, J.; Cao, F.; Bian, Z.; Leung, M. K.; Li, H. Ultrafine single-crystal TiOF2 nanocubes with mesoporous structure, high activity and durability in visible light driven photocatalysis. Nanoscale 2014, 6, 897−902. (c) Chen, W.; Jiang, H.; Hu, Y.; Dai, Y.; Li, C. Mesoporous single crystals Li4Ti5O12 grown on rGO as high-rate anode materials for lithium-ion batteries. Chem. Commun. 2014, 50, 8856−8859. (5) (a) Lin, L. D.; Xu, X. N.; Chu, C. X.; Yang, J.; et al. Mesoporous Amorphous Silicon: A Simple Synthesis of a High-Rate and Long-Life Anode Material for Lithium-Ion Batteries. Angew. Chem., Int. Ed. 2016, 55, 14063−14066. (b) Liu, H.; Bi, Z.; Sun, X.; Unocic, R. R.; Paranthaman, M. P.; et al. Mesoporous TiO2-B microspheres with superior rate performance for lithium ion batteries. Adv. Mater. 2011, 23, 3450−3454. (6) Jing, S.; Jiang, H.; Hu, Y.; Li, C. Graphene supported mesoporous single crystal silicon on Cu foam as a stable lithium-ion battery anode. J. Mater. Chem. A 2014, 2, 16360−16364. (7) Yan, C.; Chen, G.; Zhou, X.; Sun, J.; Lv, C. Template-Based Engineering of Carbon-Doped Co3O4 Hollow Nanofibers as Anode Materials for Lithium-Ion Batteries. Adv. Funct. Mater. 2016, 26, 1428−1436. (8) (a) Mahmood, N.; Zhu, J. H.; Rehman, S.; Li, Q.; Hou, Y. L. Control over large−volume changes of lithium battery anodes via active−inactive metal alloy embedded in porous carbon. Nano Energy 2015, 15, 755−765. (b) Yan, N.; Wang, F.; Zhong, H.; Li, Y.; Chen, Q.; et al. Hollow porous SiO2 nanocubes towards high-performance anodes for lithium-ion batteries. Sci. Rep. 2013, 3, 1568−1573. (c) Zhang, H.; Zhou, L.; Yu, C. Highly crystallized Fe2O3 nanocrystals on graphene: a lithium ion battery anode material with enhanced cycling. RSC Adv. 2014, 4, 495−499. (9) Hu, L.; Yan, N.; Chen, Q.; Zhang, P.; Zhong, H.; Zheng, X.; et al. Fabrication based on the kirkendall effect of Co3O4 porous nanocages with extraordinarily high capacity for lithium storage. Chem. - Eur. J. 2012, 18, 8971−8977. (10) (a) Harunsani, M. H.; Oropeza, F. E.; Palgrave, R. G.; Egdell, R. G. Electronic and Structural Properties of SnxTi1−xO2 (0.0≤ x≤ 0.1) Solid Solutions. Chem. Mater. 2010, 22, 1551−1558. (b) Issac, I.; Scheuermann, M.; Becker, S. M.; Bardaji, E. G.; Adelhelm, C.; et al. Nanocrystalline Ti2/3Sn1/3O2 as anode material for Li-ion batteries. J. Power Sources 2011, 196, 9689−9695. (c) Kyeremateng, N. A.; Hornebecq, V.; Knauth, P.; Djenizian, T. Properties of Sn-doped TiO2 nanotubes fabricated by anodization of co-sputtered Ti-Sn thin films. Electrochim. Acta 2012, 62, 192−198. (11) (a) Li, G.; Xu, X.; Han, R.; Ma, J. Synthesis and superior electrochemical properties of shaggy hollow Zn-doped Fe2 O3 nanospheres for high-performance lithium-ion batteries. CrystEngComm 2016, 18, 2949−2955. (b) Torabi, M.; Sadrnezhaad, S. K. Electrochemical evaluation of nanocrystalline Zn-doped tin oxides as anodes for lithium ion microbatteries. J. Power Sources 2011, 196, 399−404. (c) Su, G.; Liu, Y.; Huang, L.; Lu, H.; Liu, S.; Li, L.; Zheng, M. Synthesis of hierarchical Mg-doped Fe3O4 micro/nano materials for the decomposition of hexachlorobenzene. Chemosphere 2014, 99, 216−223.
(12) Mai, Y.; Tu, J.; Xia, X.; Gu, C.; Wang, X. Co-doped NiO nanoflake arrays toward superior anode materials for lithium ion batteries. J. Power Sources 2011, 196, 6388−6393. (13) Thi, T. V.; Rai, A. K.; Gim, J.; Kim, J. High performance of Codoped NiO nanoparticle anode material for rechargeable lithium ion batteries. J. Power Sources 2015, 292, 23−30. (14) Petitto, S. C.; Marsh, E. M.; Carson, G. A.; Langell, M. A. Cobalt oxide surface chemistry: the interaction of CoO (100), Co3O4 (110) and Co3O4 (111) with oxygen and water. J. Mol. Catal. A: Chem. 2008, 281, 49−58. (15) Lian, L.; Hou, L.; Zhou, L.; Wang, L.; Yuan, C. Rapid lowtemperature synthesis of mesoporous nanophase ZnFe2O4 with enhanced lithium storage properties for Li-ion batteries. RSC Adv. 2014, 4, 49212−49218. (16) Liu, Y.; Xu, J.; Qin, X. Electrode activation via vesiculation: improved reversible capacity of γ-Fe2O3@C/MWNT composite anodes for lithium-ion batteries. J. Mater. Chem. A 2015, 3, 9682. (17) Hsieh, P. T.; Chen, Y.; Kao, K.; Wang, C. Luminescence mechanism of ZnO thin film investigated by XPS measurement. Appl. Phys. A: Mater. Sci. Process. 2007, 90, 317−321. (18) (a) Chernyshova, I. V.; Hochella, M. F., Jr.; Madden, A. S. Sizedependent structural transformations of hematite nanoparticles. 1. Phase transition. Phys. Chem. Chem. Phys. 2007, 9, 1736−1750. (b) Cesar, I.; Sivula, K.; Kay, A.; Zboril, R.; Gratzel, M. Influence of feature size, film thickness, and silicon doping on the performance of nanostructured hematite photoanodes for solar water splitting. J. Phys. Chem. C 2009, 113, 772−782. (19) Bhowmik, R. N.; Vijayasri, G.; Ranganathan, R. Structural characterization and ferromagnetic properties in Ga3+ doped α-Fe2O3 system prepared by coprecipitation route and vacuum annealing. J. Appl. Phys. 2014, 116, 123905−123913. (20) (a) McCarty, K. F. Inelastic light scattering in α-Fe2O3: phonon vs magnon scattering. Solid State Commun. 1988, 68, 799−802. (b) Varshney, D.; Yogi, A. Structural and electrical conductivity of Mn doped hematite (α-Fe2O3) phase. J. Mol. Struct. 2011, 995, 157−162. (c) Ahmmad, B.; Leonard, K.; Kurawaki, J.; et al. Green synthesis of mesoporous hematite (a-Fe2O3) nanoparticles and their photocatalytic activity. Adv. Powder Technol. 2013, 24, 160−167. (21) (a) Han, F.; Li, D.; Li, W.; Lei, C.; Sun, Q.; Lu, A. Nanoengineered Polypyrrole-Coated Fe2O3@C Multifunctional Composites with an Improved Cycle Stability as Lithium-Ion Anodes. Adv. Funct. Mater. 2013, 23, 1692−1700. (b) Sun, B.; Horvat, J.; Kim, H. S.; Kim, W. S.; Ahn, J.; Wang, G. Synthesis of mesoporous α-Fe2O3 nanostructures for highly sensitive gas sensors and high capacity anode materials in lithium ion batteries. J. Phys. Chem. C 2010, 114, 18753− 18761. (22) Xu, X.; Cao, R.; Jeong, S.; Cho, J. Spindle-like mesoporous αFe2O3 anode material prepared from MOF template for high-rate lithium batteries. Nano Lett. 2012, 12, 4988−4991. (23) (a) Xu, G. L.; Li, J. T.; Huang, L.; Lin, W. F.; Sun, S. G. Synthesis of Co3O4 nano-octahedra enclosed by {111} facets and their excellent lithium storage properties as anode material of lithium ion batteries. Nano Energy 2013, 2, 394−402. (b) Ni, S. B.; Ma, J. J.; Lv, X. H.; Yang, X. L.; Zhang, L. L. The preparation of NiV3O8/Ni composite via an in situ corrosion method and its use as a new sort of anode material for Li-ion batteries. J. Mater. Chem. A 2014, 2, 8995−8998. (24) Sun, H.; Xin, G.; Hu, T. High-rate lithiation-induced reactivation of mesoporous hollow spheres for long-lived lithium-ion batteries. Nat. Commun. 2014, 5, 1 DOI: 10.1038/ncomms5526. (25) Ponrouch, A.; Palacín, M. R. Optimisation of performance through electrode formulation in conversion materials for lithium ion batteries: Co3O4 as a case example. J. Power Sources 2012, 212, 233− 246. (26) Li, X. W.; Qiao, L.; Li, D.; Wang, X. H.; Xie, W. H.; He, D. Y. Three-dimensional network structured α-Fe2O3 made from a stainless steel plate as a high-performance electrode for lithium ion batteries. J. Mater. Chem. A 2013, 1, 6400−6406. (27) (a) Riley, L. A.; Lee, S. H.; Gedvilias, L.; Dillon, A. C. Optimization of MoO3 nanoparticles as negative-electrode material in 7648
DOI: 10.1021/acs.inorgchem.7b00008 Inorg. Chem. 2017, 56, 7642−7649
Article
Inorganic Chemistry high-energy lithium ion batteries. J. Power Sources 2010, 195, 588−592. (b) Wang, Q.; Yu, B.; Li, X.; Xing, L.; Xue, X. Core-shell Co3O4/ ZnCo2O4 coconut-like hollow spheres with extremely high performance as anode materials for lithium-ion batteries. J. Mater. Chem. A 2016, 4, 425−433. (28) (a) Cui, Y.; Zhao, X.; Guo, R. Improved electrochemical performance of La0.7Sr0.3MnO3 and carbon co-coated LiFePO4 synthesized by freeze-drying process. Electrochim. Acta 2010, 55, 922−926. (b) Wang, Q.; Huang, Y.; Miao, J.; et al. Synthesis and electrochemical characterizations of Ce doped SnS2 anode materials for rechargeable lithium ion batteries. Electrochim. Acta 2013, 93, 120− 130.
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DOI: 10.1021/acs.inorgchem.7b00008 Inorg. Chem. 2017, 56, 7642−7649
