Free 3D Nanocomposite Secondary Battery Anodes
Volume 11 · No. 47 – December 16 2015
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47/2015 High Full-Electrode Basis Capacity Template-Free 3D Nanocomposite Secondary Battery Anodes P. V. Braun and co-workers
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Anodes
High Full-Electrode Basis Capacity Template-Free 3D Nanocomposite Secondary Battery Anodes Jinyun Liu, Junjie Wang, Jinwoo Kim, Hailong Ning, Zeng Pan, Sean J. Kelly, Eric S. Epstein, Xingjiu Huang, Jinhuai Liu, and Paul V. Braun* Secondary batteries are now widely used for applications ranging from portable electronics to electric and hybrid electric vehicles,[1–3] which coupled with the demand for ever increasing power density, energy density, and cycle life, and have led to an exceptional amount of research on new materials and structures for electrochemical energy storage.[4–7] We for example have focused on 3D scaffolded electrodes for high power[8,9] and energy.[10–12] While our deterministically structured anode and cathode electrode concepts[8–12] offer some performance advantages, the required metallic scaffold is heavy, and results in a lower than desired gravimetric energy density. Here, using Fe3O4 as a high capacity active material system, we show a route to scaffold-free 3D structured electrodes with high gravimetric energy densities. In the commercially dominant carbon/ transition-metal oxide (e.g., LiCoO2) batteries, the low theoretical capacity of the carbon-based anode (372 mAh g−1) serves to limit the energy density and cycling stability.[13,14] Transition metal oxides, e.g., iron oxides, tin oxide, cobalt oxides, and nickel oxide, have received broad attention since the pioneering work by Tarascon and co-workers.[15] On the basis of a conversion mechanism (Fe3O4 + 8Li+ + 8e− ↔ 3Fe0 + 4Li2O),[16] Fe3O4 has a theoretical capacity of ≈926 mAh g−1, is naturally abundant, low-cost, and non-toxic,[17] making it of particular interest. However, similar to other transition metal oxides (as well as some other high capacity anodes such as silicon), Fe3O4 undergoes a large volume change (≈180%) during lithiation– delithiation, resulting in poor cycling stability due to cracking of the active phase.[18] To address the propensity for cracking, nanostructuring and coating with a protection layer,[19] and
Dr. J. Y. Liu, Prof. X. J. Huang, Prof. J. H. Liu Research Center for Biomimetic Functional Materials and Sensing Devices Institute of Intelligent Machines Chinese Academy of Sciences Hefei, Anhui 230031, P. R. China Dr. J. Y. Liu, J. J. Wang, Dr. J. Kim, Dr. H. L. Ning, Dr. Z. Pan, S. J. Kelly, E. S. Epstein, Prof. P. V. Braun Department of Materials Science and Engineering Frederick Seitz Materials Research Laboratory Beckman Institute for Advanced Science and Technology University of Illinois at Urbana-Champaign Urbana, IL 61801, USA E-mail: [email protected] DOI: 10.1002/smll.201502538 small 2015, 11, No. 47, 6265–6271
to provide electron and ion transport pathways, and space for the volume changes during cycling, 3D scaffolded electrodes, such as using scaffolds based on macroporous carbon or metal foams[20,21] have been considered. However, as already mentioned,[8–12] the scaffolds reduce the electrode capacity. Here, we report a scaffold-free composite anode consisting of about 5 nm diameter Fe3O4 nanoparticles integrated with carbon, fabricated as shown in Figure 1. Briefly, a SiO2 inverse opal, fabricated from a polystyrene (PS) opal, was used as a template for a one-pot solvothermal Fe3O4/C synthesis. After carbonization, the SiO2 scaffold was etched, forming a 3D template-free Fe3O4/C composite. The carbon stabilizes the Fe3O4 nanoparticles and probably the solid-electrolyte interphase (SEI) layer, enhancing the Coulombic efficiency, and also provides an efficient electron transport pathway. Because no additional scaffold is needed, and the mass fraction of carbon is less than 10%, the full electrode capacity is high. Figure 2a shows the SiO2 inverse opal obtained after thermal removal of the PS opal template (Figure S1, Supporting Information). Using a solvothermal growth, employing ferrocene as an iron and carbon source, a 3D SiO2@Fe3O4/C precursor was fabricated. As shown in Figure 2b and Figure S2 (Supporting Information), an about 100 nm thick composite layer deposits on the scaffold uniformly throughout the electrode without clogging the pores connecting the cavities. Subsequently, carbonization was performed at 500 °C under argon (see Figure 2c,d). The SiO2 scaffold was etched using a hydrothermal treatment in an ammonia solution, forming a 3D template-free Fe3O4/C composite (Figure 2e,f and Figure S3, Supporting Information). The electrode structure is retained throughout the process, leaving evidence of voids where the SiO2 scaffold previously was. An obvious gap between two Fe3O4/C composite layers can be found in high magnification cross-sectional images (Figure S3b,d, Supporting Information), which was the location of the etched SiO2 layer. Energy-dispersive X-ray spectroscopy confirms SiO2 removal (Figure S4, Supporting Information). In transmission electron microscopy (TEM) (Figure 2g), a compact inverse opal structure that includes interconnected voids associated with the removed SiO2 can be observed. Higher magnification TEM (Figure 2h) indicates the composite layer consists of 5 nm diameter nanoparticles integrated within a carbon matrix. The crystalline nature of the nanoparticles is confirmed by lattice-resolved TEM (Figure S5, Supporting Information). The carbon content of
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Figure 1. Schematic of the scaffold-free 3D Fe3O4/C anode fabrication.
the composite is about 9% by weight by thermogravimetric analysis (Figure S6, Supporting Information). X-ray diffraction (Figure S7, Supporting Information) indicates that the nanoparticles are magnetite phase Fe3O4 (JCPDS card No. 98-0294). As calculated from the Sherrer equation,[22] the average size of Fe3O4 nanoparticles is about 5.8 nm, agreeing with the TEM observed dimensions. We suspect that the Fe3O4 nanoparticle–carbon composite forms roughly as follows. First, ferrocene molecules electrostatically adsorb on the surface of the silica.[23] As the temperature increases, the ferrocene decomposes to form iron oxide species, which are further oxidized to Fe3O4 by H2O2 in the system,[24] and which additional ferrocene continue to adsorb on the surface. Concurrently, the cyclopentadienyl groups in ferrocene decompose, forming a carbon layer on the Fe3O4 particles, preventing the small Fe3O4 nanoparticles from growing into larger particles. Cyclic voltammetry (CV) curves for the first three cycles of an ≈8 µm thick Fe3O4/C composite anode are shown in Figure 3a. In the first cycle, three peaks in the cathodic process are observed at about 1.58, 1.17, and 0.65 V, which can be attributed to formation of LixFe3O4 (x < 1), further reduction of LixFe3O4 to Li2Fe3O4, and the final reduction of Li2Fe3O4 to Fe0,[25–27] respectively. Accompanied by the reduction of Li2Fe3O4, an irreversible reaction occurs, probably due to SEI formation.[28] In subsequent cycles, the cathodic peaks were positively shifted compared to the first cycle. In the anodic process, a broad peak was recorded from 1.1 to 2.0 V, corresponding to the oxidation of Fe0 to Fe3+.[29,30] After the first cycle, the CV curves nearly overlap, indicating good reversibility. Figure 3b shows the first, second, third, 50th, and 100th discharge–charge curves of the Fe3O4/C composite anode cycling at a current density of 800 mA g−1. Three plateaus (≈1.6, ≈1.2, and ≈0.7 V) in the first discharge profile correspond to the cathodic peaks in the CV curve. The
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Fe3O4/C composite gives initial discharge and charge capacities of ≈1233 and 858 mAh g−1 (electrode basis, including Fe3O4 and carbon), respectively, along with a Coulombic efficiency of about 70% (Figure 3c). In the second cycle, the discharge and charge capacities decreases to 964 and 844 mAh g−1, respectively, still a little higher than electrode theoretical capacity (876 mAh g−1) calculated from mass fractions of Fe3O4 (91%) and carbon (9%), which is perhaps caused by SEI formation. The Coulombic efficiency in the third cycle exceeds 90%. After about 10 cycles, the capacity becomes stable, and the Coulombic efficiency rises from 98% to 99%. After 100 cycles, the electrode-based discharge and charge capacities are about 780 and 770 mAh g−1, respectively, far exceeding the theoretical capacity of a graphite anode, and exceeding the reported capacities of a number of both pure Fe3O4 and Fe3O4-related composite systems.[31–33] Calculated from the electrode geometrical dimensions (≈1.4 cm × 0.7 cm × 8 µm), the electrode volume is ≈7.84 cm2 µm. The mass of the Fe3O4/C nanocomposite is typically ≈1.07 mg. This results in discharge and charge volumetric capacities of about 1064 and 1051 mAh cm−3, respectively, which are considerably higher than the actual volumetric capacity (≈300 mAh cm−3)[34] of a commercial graphite-based anode (theoretical volumetric capacity 837 mAh cm−3). We suspect that through structure optimization to increase the active material loading, such as by optimizing the 3D scaffold geometry, for example by increasing the ratio of connecting neck size to the void diameter of the silica scaffold as we demonstrated previously,[9] it may be possible to achieve an even higher volumetric capacity based on this material system. The performance at increasing current densities from 200 to 4000 mA g−1 followed by 200 mA g−1 cycles is shown in Figure 3d (discharge–charge curves are presented in Figure S8, Supporting Information). At the highest current density of 4000 mA g−1, the electrode gives a discharge capacity
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over 10 cycles, showing a small decay of ≈9% compared to the initial cycles at the same current density. The voltage hysteresis of the 8 µm thick Fe3O4/C anode is about 1 V (Figure 3e) and appears stable during cycling at rates ranging from 200 to 4000 mA g−1, suggesting that the hysteresis is not due to a kinetic phenomenon either within the Fe3O4 nanoparticle or due to ion or electron transport resistances. We attribute the good C-rate performance to the conductive carbon phase, which provides good pathways for electron transfer, the 3D porous structure, which enables rapid ion diffusion, and the nanostructured nature of the Fe3O4, which provides short solid-state diffusion distances. In order to characterize the state of the carbon, Raman spectroscopy was performed on the Fe3O4/C nanocomposites (Figure S9, Supporting Information), and peaks at about 1350 and 1582 cm−1, which can be assigned to the sp3- and sp2-bonded carbon,[35,36] respectively, are clearly observed, indicating the amorphous nature of the carbon in the Fe3O4/C nanocomposites.[37,38] Since amorphous carbon has a resistivity of about 10−5 Ω m at 20 °C,[39] we calculate that any polarization of the electrode due to the electrode resistivity is minimal. For practical applications, electrodes considerably thicker than 8 µm are generally required. Commercial electrodes, for example, can exceed 100 µm thick. To evaluate if the approach discussed here can apply to thick electrodes, ≈100 µm thick Fe3O4/C anodes were prepared using a commercial disordered mesostructured Ni scaffold as a sacrificial template. The electrode fabrication route and the related material characterizations are shown in Figures S10 and S11 (Supporting Information). Importantly, the Fe3O4/C composite is present all the way through the thickness of the electrode. Figure 3f shows the discharge–charge curves of the 100 µm thick Fe3O4/C anode at a current density of 1000 mA g−1 (capacities and Coulombic efficiency are shown in Figure S12, Supporting Information). The electrode Figure 2. Images during the anode fabrication procedure. Top-view SEM images of a) the retains a full electrode-based capacity SiO2 inverse opal and b) the SiO2@Fe3O4/C precursor obtained after solvothermal growth. greater than 710 mAh g−1 (about twice c) Top-view and d) cross-sectional SEM images of the structure after carbonization. e) Top- of theoretical capacity of graphite anode) view and f) cross-sectional SEM images of the Fe3O4/C after removal of SiO2 scaffold. over 50 cycles. The capacity is slightly g,h) TEM images of the Fe3O4/C composite. lower than the thinner electrode system, perhaps because the initial template is exceeding 600 mAh g−1, and a Coulombic efficiency of 98.3%. disordered, which reduces the surface area per unit volume. When the current density was returned to 200 mA g−1, the Figure 4 shows the ordered inverse opal elecdischarge capacities returned to an average of 833 mAh g−1 trode after 100 discharge–charge cycles. The electrode retains a small 2015, 11, No. 47, 6265–6271
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Figure 3. Electrochemical properties of the Fe3O4/C composite. a) The first three CV curves over the potential range of 0.25 to 3.0 V versus Li+/Li at a 0.1 mV s−1 scan rate. b) Galvanostatic discharge–charge cycles of the anode at a current density of 800 mA g−1. c) Specific capacity and Coulombic efficiency over 100 cycles. d) The C-rate performance of the electrode over the range from 200 to 4000 mA g−1. e) Cycling curves of the Fe3O4/C anode at different current densities. f) The first, second, third, and 50th galvanostatic discharge–charge curves (at a current density of 1000 mA g−1) of a 100 µm thick Fe3O4/C composite anode fabricated using a commercial Ni scaffold as sacrificial template. All capacities above are on a full electrode basis.
profile close to its initial morphology before cycling, indicating the structure is robust even to the volume changes induced by cycling. In Figure 4a (high-magnification images in Figure S13a,b, Supporting Information), there is some fluffyappearing material coating the surface, which is perhaps related to SEI formation. Similar materials can also be found in cross-sectional SEM images (Figure 4b and Figure S13c,d, Supporting Information). As shown in the TEM images (Figure 4c), the Fe3O4/C composite remains well connected
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even after cycling. In the high-magnification TEM image (Figure 4d), a surface layer can be observed which we assume is the SEI layer. The Fe3O4 nanoparticles remain integrated tightly within the carbon matrix, separating them from the SEI layer, which probably improves capacity retention during cycles. Impedance spectroscopy (Figure S14, Supporting Information) indicates a stable surface chemistry and consistent Li ion diffusion kinetics for the Fe3O4/C composite. By looking carefully at Figure S15 (Supporting Information),
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Figure 4. Electrode structure after cycling. a) The top-view and b) the cross-sectional SEM images, c) low- and d) high-magnification TEM images of the template-free Fe3O4/C composite anode after charge-discharge over 100 cycles.
the volume change of Fe3O4/C composite during lithiation and delithiation can be observed. In particular, gaps can be seen between the nodules of the delithated Fe3O4/C composite in Figure S15f (Supporting Information), which are not present in the lithiated Fe3O4/C composite in Figure S15b (Supporting Information). The overall dimension of the electrode does not appear to change, suggesting that the volume expansion–contraction is buffered within the nanocomposite matrix. In summary, we have demonstrated a unique high fullelectrode basis capacity anode design for secondary batteries, which consists of a structurally robust 3D Fe3O4/C composite. No additional current collector scaffold and no bonding additives are required, resulting in an anode with both high gravimetric and volumetric capacities. In our system, the fabricated template-free Fe3O4/C anode provides a discharge capacity of ≈780 mAh g−1 on a full-electrode basis after 100 cycles at a current density of 800 mA g−1, and the Coulombic efficiency is steady at 98% to 99%. The volume fraction of the Fe3O4/C composite in full electrode (including voids) is ≈28%, resulting in a volumetric discharge capacity of about 1064 mAh cm−3, which is significantly greater than the volumetric capacity of a commercial graphite anode. As a demonstration of the commercial potential, an 100 µm thick Fe3O4/C anode was also fabricated through the presented template-free strategy, and it exhibited a full electrode-based capacity greater than 710 mAh g−1 (about twice of theoretical small 2015, 11, No. 47, 6265–6271
capacity of graphite anode) after 50 cycles at a current density of 1000 mA g−1.
Experimental Section PS Opal and SiO2 Inverse Opal Fabrication: The PS opals were fabricated on gold-coated glass substrates following our previous reports[9,40] with the indicated modifications. The glass was first cleaned with piranha (volume ratio of H2SO4 to H2O2 is 3:1) and then coated with 5 nm chromium and 60 nm gold by e-beam evaporation (Temescal, Inc). Caution, piranha is highly corrosive and potentially explosive. To modify the gold surface for PS opal growth, the gold-coated substrates were immersed in an aqueous solution of 3-mercapto-1-propanesulfonic acid, sodium salt (Sigma–Aldrich Corp.) for 2 h. Then 600 nm PS colloid spheres (Molecular Probes) were dispersed in Millipore water, forming a 0.2 wt% suspension. After drying with blown air, the pretreated substrates were placed vertically into vials containing the PS suspension (≈1.5 cm depth; the final opal structure is about 1.4 cm long) at 55 °C. To enlarge the pore size of the subsequent inverse opal and to enhance the bonding between PS spheres, the PS opal was sintered at 95 °C for 3 h. To fabricate the SiO2 inverse opal, first, SiO2 sol was prepared by mixing tetraethyl orthosilicate (TEOS, Sigma–Aldrich Corp.), ethanol, and a 0.1 M HCl solution (1:10:1 volume ratio) under stirring for 6 h at room temperature. The PS opal was held vertically, and the SiO2 sol was slowly dripped on it. The samples were then dried
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at room temperature for 24 h. This infiltration and drying process were repeated for twice. The obtained samples were sintered in air at 500 °C for 4 h using a heating ramp rate of 8 °C min−1, forming ≈8 µm thick SiO2 inverse opals. Fe3O4/C Composite Growth and Carbonization Treatment: In a typical solvothermal growth, via a process similar to our previous report,[41] 5 mmol ferrocene (Sigma–Aldrich Corp.) was dissolved in 35 mL acetone. Subsequently, 1.0 mL hydrogen peroxide (30 wt%) was added into the solution followed by ultrasonication for 30 min. The obtained solution was then transferred to a 50 mL Teflon-lined steel autoclave in which the SiO2 inverse opal coated substrate faced up. The autoclave was heated in an oven at 190 °C for 36 h. After cooling to room temperature naturally, the sample was removed and washed with ethanol and Millipore water followed by drying at 50 °C for 5 h. In the subsequent carbonization treatment, the sample was heated in Ar at 500 °C for 3 h under a ramp rate of 5 °C min−1. SiO2 Scaffold Removal: To etch the SiO2 scaffold from the sample, an ammonia solution-based hydrothermal treatment[42] was employed. Typically, 10 mL ammonium hydroxide solution (28%–30% NH3 basis, Sigma–Aldrich Corp.) was added into 25 mL Millipore water to form a diluted solution. The prepared solution was transferred to a 50 mL Teflon-lined steel autoclave, and a piece of SiO2@Fe3O4/C-coated substrate was put into the solution face up. The autoclave was sealed, heated at 150 °C for 8 h, and allowed to cool naturally to room temperature. The sample was then removed and thoroughly washed with ethanol and Millipore water followed by drying at 50 °C for 5 h. Thick Electrode Fabrication: Thick electrodes were formed started with a 3D mesoporous Ni template following a similar process as for the thin electrodes (see Supporting Information for fabrication details). Characterization: The fabricated samples were characterized using a Hitachi S-4800 SEM, a Hitachi S-4700 SEM equipped with an Oxford INCA energy-dispersive X-ray analyzer, a Philips X’pert MRD X-ray diffractometer with Cu K α radiation (1.5418 Å), and a JEOL 2010 LaB6 TEM operating at 200 kV. Elemental analysis was conducted on the Hitachi S-4700 SEM. The carbon content in the Fe3O4/C composite was analyzed using a Mettler-Toledo TGA 851e under an air atmosphere and a heating rate of 10 °C min−1 from 25 °C to 600 °C. Electrochemical Measurements: Electrochemical measurements of thin electrodes (≈8 µm) were conducted using two-electrode cells with a lithium metal counter and reference electrodes on Princeton Applied Research Model 273A and Biologic VMP3 potentiostats. An electrolyte consisting of 1 M of LiClO4 in a 1:1 mass ratio mixture of ethylene carbonate and dimethylene carbonate was used. All cells were assembled in an Ar-filled glove box. Both coin and larger format cells were studied with similar results; for the coin cells, the electrode indicate below was used. Electrochemical tests of the ≈100 µm thick electrodes were conducted using coin cells with the Fe3O4/C composite as the working electrode and a lithium metal foil as both the counter and reference electrodes on the same electrochemical workstation shown above. In coin cells, the electrolyte consists of 1 M LiPF6 dissolved in a 50:50 (w/w) mixture of ethylene carbonate and diethyl carbonate. A polypropylene microporous film was employed in coin cell as the separator. All electrode capacities were measured by a galvanostatic charge–discharge method. CV curves of the thin electrodes
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were recorded over the potential range of 0.25–3 V (vs. Li+/Li) at a scan rate of 0.1 mV s−1.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements The authors thank Dr. Hui Gang Zhang, Neil A. Krueger, and Jin Gu Kang for experimental assistance. This work was primarily supported by the US Department of Energy, Office of Science, Basic Energy Sciences, under Award no. DE-FG0207ER46471. X. Huang and J. Liu acknowledge support from the State Key Project of Fundamental Research for Nanoscience and Nanotechnology of China (Award no. 2011CB933700).
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Received: August 23, 2015 Published online: October 19, 2015
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Anodes
High Full-Electrode Basis Capacity Template-Free 3D Nanocomposite Secondary Battery Anodes Jinyun Liu, Junjie Wang, Jinwoo Kim, Hailong Ning, Zeng Pan, Sean J. Kelly, Eric S. Epstein, Xingjiu Huang, Jinhuai Liu, and Paul V. Braun* Secondary batteries are now widely used for applications ranging from portable electronics to electric and hybrid electric vehicles,[1–3] which coupled with the demand for ever increasing power density, energy density, and cycle life, and have led to an exceptional amount of research on new materials and structures for electrochemical energy storage.[4–7] We for example have focused on 3D scaffolded electrodes for high power[8,9] and energy.[10–12] While our deterministically structured anode and cathode electrode concepts[8–12] offer some performance advantages, the required metallic scaffold is heavy, and results in a lower than desired gravimetric energy density. Here, using Fe3O4 as a high capacity active material system, we show a route to scaffold-free 3D structured electrodes with high gravimetric energy densities. In the commercially dominant carbon/ transition-metal oxide (e.g., LiCoO2) batteries, the low theoretical capacity of the carbon-based anode (372 mAh g−1) serves to limit the energy density and cycling stability.[13,14] Transition metal oxides, e.g., iron oxides, tin oxide, cobalt oxides, and nickel oxide, have received broad attention since the pioneering work by Tarascon and co-workers.[15] On the basis of a conversion mechanism (Fe3O4 + 8Li+ + 8e− ↔ 3Fe0 + 4Li2O),[16] Fe3O4 has a theoretical capacity of ≈926 mAh g−1, is naturally abundant, low-cost, and non-toxic,[17] making it of particular interest. However, similar to other transition metal oxides (as well as some other high capacity anodes such as silicon), Fe3O4 undergoes a large volume change (≈180%) during lithiation– delithiation, resulting in poor cycling stability due to cracking of the active phase.[18] To address the propensity for cracking, nanostructuring and coating with a protection layer,[19] and
Dr. J. Y. Liu, Prof. X. J. Huang, Prof. J. H. Liu Research Center for Biomimetic Functional Materials and Sensing Devices Institute of Intelligent Machines Chinese Academy of Sciences Hefei, Anhui 230031, P. R. China Dr. J. Y. Liu, J. J. Wang, Dr. J. Kim, Dr. H. L. Ning, Dr. Z. Pan, S. J. Kelly, E. S. Epstein, Prof. P. V. Braun Department of Materials Science and Engineering Frederick Seitz Materials Research Laboratory Beckman Institute for Advanced Science and Technology University of Illinois at Urbana-Champaign Urbana, IL 61801, USA E-mail: [email protected] DOI: 10.1002/smll.201502538 small 2015, 11, No. 47, 6265–6271
to provide electron and ion transport pathways, and space for the volume changes during cycling, 3D scaffolded electrodes, such as using scaffolds based on macroporous carbon or metal foams[20,21] have been considered. However, as already mentioned,[8–12] the scaffolds reduce the electrode capacity. Here, we report a scaffold-free composite anode consisting of about 5 nm diameter Fe3O4 nanoparticles integrated with carbon, fabricated as shown in Figure 1. Briefly, a SiO2 inverse opal, fabricated from a polystyrene (PS) opal, was used as a template for a one-pot solvothermal Fe3O4/C synthesis. After carbonization, the SiO2 scaffold was etched, forming a 3D template-free Fe3O4/C composite. The carbon stabilizes the Fe3O4 nanoparticles and probably the solid-electrolyte interphase (SEI) layer, enhancing the Coulombic efficiency, and also provides an efficient electron transport pathway. Because no additional scaffold is needed, and the mass fraction of carbon is less than 10%, the full electrode capacity is high. Figure 2a shows the SiO2 inverse opal obtained after thermal removal of the PS opal template (Figure S1, Supporting Information). Using a solvothermal growth, employing ferrocene as an iron and carbon source, a 3D SiO2@Fe3O4/C precursor was fabricated. As shown in Figure 2b and Figure S2 (Supporting Information), an about 100 nm thick composite layer deposits on the scaffold uniformly throughout the electrode without clogging the pores connecting the cavities. Subsequently, carbonization was performed at 500 °C under argon (see Figure 2c,d). The SiO2 scaffold was etched using a hydrothermal treatment in an ammonia solution, forming a 3D template-free Fe3O4/C composite (Figure 2e,f and Figure S3, Supporting Information). The electrode structure is retained throughout the process, leaving evidence of voids where the SiO2 scaffold previously was. An obvious gap between two Fe3O4/C composite layers can be found in high magnification cross-sectional images (Figure S3b,d, Supporting Information), which was the location of the etched SiO2 layer. Energy-dispersive X-ray spectroscopy confirms SiO2 removal (Figure S4, Supporting Information). In transmission electron microscopy (TEM) (Figure 2g), a compact inverse opal structure that includes interconnected voids associated with the removed SiO2 can be observed. Higher magnification TEM (Figure 2h) indicates the composite layer consists of 5 nm diameter nanoparticles integrated within a carbon matrix. The crystalline nature of the nanoparticles is confirmed by lattice-resolved TEM (Figure S5, Supporting Information). The carbon content of
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Figure 1. Schematic of the scaffold-free 3D Fe3O4/C anode fabrication.
the composite is about 9% by weight by thermogravimetric analysis (Figure S6, Supporting Information). X-ray diffraction (Figure S7, Supporting Information) indicates that the nanoparticles are magnetite phase Fe3O4 (JCPDS card No. 98-0294). As calculated from the Sherrer equation,[22] the average size of Fe3O4 nanoparticles is about 5.8 nm, agreeing with the TEM observed dimensions. We suspect that the Fe3O4 nanoparticle–carbon composite forms roughly as follows. First, ferrocene molecules electrostatically adsorb on the surface of the silica.[23] As the temperature increases, the ferrocene decomposes to form iron oxide species, which are further oxidized to Fe3O4 by H2O2 in the system,[24] and which additional ferrocene continue to adsorb on the surface. Concurrently, the cyclopentadienyl groups in ferrocene decompose, forming a carbon layer on the Fe3O4 particles, preventing the small Fe3O4 nanoparticles from growing into larger particles. Cyclic voltammetry (CV) curves for the first three cycles of an ≈8 µm thick Fe3O4/C composite anode are shown in Figure 3a. In the first cycle, three peaks in the cathodic process are observed at about 1.58, 1.17, and 0.65 V, which can be attributed to formation of LixFe3O4 (x < 1), further reduction of LixFe3O4 to Li2Fe3O4, and the final reduction of Li2Fe3O4 to Fe0,[25–27] respectively. Accompanied by the reduction of Li2Fe3O4, an irreversible reaction occurs, probably due to SEI formation.[28] In subsequent cycles, the cathodic peaks were positively shifted compared to the first cycle. In the anodic process, a broad peak was recorded from 1.1 to 2.0 V, corresponding to the oxidation of Fe0 to Fe3+.[29,30] After the first cycle, the CV curves nearly overlap, indicating good reversibility. Figure 3b shows the first, second, third, 50th, and 100th discharge–charge curves of the Fe3O4/C composite anode cycling at a current density of 800 mA g−1. Three plateaus (≈1.6, ≈1.2, and ≈0.7 V) in the first discharge profile correspond to the cathodic peaks in the CV curve. The
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Fe3O4/C composite gives initial discharge and charge capacities of ≈1233 and 858 mAh g−1 (electrode basis, including Fe3O4 and carbon), respectively, along with a Coulombic efficiency of about 70% (Figure 3c). In the second cycle, the discharge and charge capacities decreases to 964 and 844 mAh g−1, respectively, still a little higher than electrode theoretical capacity (876 mAh g−1) calculated from mass fractions of Fe3O4 (91%) and carbon (9%), which is perhaps caused by SEI formation. The Coulombic efficiency in the third cycle exceeds 90%. After about 10 cycles, the capacity becomes stable, and the Coulombic efficiency rises from 98% to 99%. After 100 cycles, the electrode-based discharge and charge capacities are about 780 and 770 mAh g−1, respectively, far exceeding the theoretical capacity of a graphite anode, and exceeding the reported capacities of a number of both pure Fe3O4 and Fe3O4-related composite systems.[31–33] Calculated from the electrode geometrical dimensions (≈1.4 cm × 0.7 cm × 8 µm), the electrode volume is ≈7.84 cm2 µm. The mass of the Fe3O4/C nanocomposite is typically ≈1.07 mg. This results in discharge and charge volumetric capacities of about 1064 and 1051 mAh cm−3, respectively, which are considerably higher than the actual volumetric capacity (≈300 mAh cm−3)[34] of a commercial graphite-based anode (theoretical volumetric capacity 837 mAh cm−3). We suspect that through structure optimization to increase the active material loading, such as by optimizing the 3D scaffold geometry, for example by increasing the ratio of connecting neck size to the void diameter of the silica scaffold as we demonstrated previously,[9] it may be possible to achieve an even higher volumetric capacity based on this material system. The performance at increasing current densities from 200 to 4000 mA g−1 followed by 200 mA g−1 cycles is shown in Figure 3d (discharge–charge curves are presented in Figure S8, Supporting Information). At the highest current density of 4000 mA g−1, the electrode gives a discharge capacity
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over 10 cycles, showing a small decay of ≈9% compared to the initial cycles at the same current density. The voltage hysteresis of the 8 µm thick Fe3O4/C anode is about 1 V (Figure 3e) and appears stable during cycling at rates ranging from 200 to 4000 mA g−1, suggesting that the hysteresis is not due to a kinetic phenomenon either within the Fe3O4 nanoparticle or due to ion or electron transport resistances. We attribute the good C-rate performance to the conductive carbon phase, which provides good pathways for electron transfer, the 3D porous structure, which enables rapid ion diffusion, and the nanostructured nature of the Fe3O4, which provides short solid-state diffusion distances. In order to characterize the state of the carbon, Raman spectroscopy was performed on the Fe3O4/C nanocomposites (Figure S9, Supporting Information), and peaks at about 1350 and 1582 cm−1, which can be assigned to the sp3- and sp2-bonded carbon,[35,36] respectively, are clearly observed, indicating the amorphous nature of the carbon in the Fe3O4/C nanocomposites.[37,38] Since amorphous carbon has a resistivity of about 10−5 Ω m at 20 °C,[39] we calculate that any polarization of the electrode due to the electrode resistivity is minimal. For practical applications, electrodes considerably thicker than 8 µm are generally required. Commercial electrodes, for example, can exceed 100 µm thick. To evaluate if the approach discussed here can apply to thick electrodes, ≈100 µm thick Fe3O4/C anodes were prepared using a commercial disordered mesostructured Ni scaffold as a sacrificial template. The electrode fabrication route and the related material characterizations are shown in Figures S10 and S11 (Supporting Information). Importantly, the Fe3O4/C composite is present all the way through the thickness of the electrode. Figure 3f shows the discharge–charge curves of the 100 µm thick Fe3O4/C anode at a current density of 1000 mA g−1 (capacities and Coulombic efficiency are shown in Figure S12, Supporting Information). The electrode Figure 2. Images during the anode fabrication procedure. Top-view SEM images of a) the retains a full electrode-based capacity SiO2 inverse opal and b) the SiO2@Fe3O4/C precursor obtained after solvothermal growth. greater than 710 mAh g−1 (about twice c) Top-view and d) cross-sectional SEM images of the structure after carbonization. e) Top- of theoretical capacity of graphite anode) view and f) cross-sectional SEM images of the Fe3O4/C after removal of SiO2 scaffold. over 50 cycles. The capacity is slightly g,h) TEM images of the Fe3O4/C composite. lower than the thinner electrode system, perhaps because the initial template is exceeding 600 mAh g−1, and a Coulombic efficiency of 98.3%. disordered, which reduces the surface area per unit volume. When the current density was returned to 200 mA g−1, the Figure 4 shows the ordered inverse opal elecdischarge capacities returned to an average of 833 mAh g−1 trode after 100 discharge–charge cycles. The electrode retains a small 2015, 11, No. 47, 6265–6271
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Figure 3. Electrochemical properties of the Fe3O4/C composite. a) The first three CV curves over the potential range of 0.25 to 3.0 V versus Li+/Li at a 0.1 mV s−1 scan rate. b) Galvanostatic discharge–charge cycles of the anode at a current density of 800 mA g−1. c) Specific capacity and Coulombic efficiency over 100 cycles. d) The C-rate performance of the electrode over the range from 200 to 4000 mA g−1. e) Cycling curves of the Fe3O4/C anode at different current densities. f) The first, second, third, and 50th galvanostatic discharge–charge curves (at a current density of 1000 mA g−1) of a 100 µm thick Fe3O4/C composite anode fabricated using a commercial Ni scaffold as sacrificial template. All capacities above are on a full electrode basis.
profile close to its initial morphology before cycling, indicating the structure is robust even to the volume changes induced by cycling. In Figure 4a (high-magnification images in Figure S13a,b, Supporting Information), there is some fluffyappearing material coating the surface, which is perhaps related to SEI formation. Similar materials can also be found in cross-sectional SEM images (Figure 4b and Figure S13c,d, Supporting Information). As shown in the TEM images (Figure 4c), the Fe3O4/C composite remains well connected
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even after cycling. In the high-magnification TEM image (Figure 4d), a surface layer can be observed which we assume is the SEI layer. The Fe3O4 nanoparticles remain integrated tightly within the carbon matrix, separating them from the SEI layer, which probably improves capacity retention during cycles. Impedance spectroscopy (Figure S14, Supporting Information) indicates a stable surface chemistry and consistent Li ion diffusion kinetics for the Fe3O4/C composite. By looking carefully at Figure S15 (Supporting Information),
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Figure 4. Electrode structure after cycling. a) The top-view and b) the cross-sectional SEM images, c) low- and d) high-magnification TEM images of the template-free Fe3O4/C composite anode after charge-discharge over 100 cycles.
the volume change of Fe3O4/C composite during lithiation and delithiation can be observed. In particular, gaps can be seen between the nodules of the delithated Fe3O4/C composite in Figure S15f (Supporting Information), which are not present in the lithiated Fe3O4/C composite in Figure S15b (Supporting Information). The overall dimension of the electrode does not appear to change, suggesting that the volume expansion–contraction is buffered within the nanocomposite matrix. In summary, we have demonstrated a unique high fullelectrode basis capacity anode design for secondary batteries, which consists of a structurally robust 3D Fe3O4/C composite. No additional current collector scaffold and no bonding additives are required, resulting in an anode with both high gravimetric and volumetric capacities. In our system, the fabricated template-free Fe3O4/C anode provides a discharge capacity of ≈780 mAh g−1 on a full-electrode basis after 100 cycles at a current density of 800 mA g−1, and the Coulombic efficiency is steady at 98% to 99%. The volume fraction of the Fe3O4/C composite in full electrode (including voids) is ≈28%, resulting in a volumetric discharge capacity of about 1064 mAh cm−3, which is significantly greater than the volumetric capacity of a commercial graphite anode. As a demonstration of the commercial potential, an 100 µm thick Fe3O4/C anode was also fabricated through the presented template-free strategy, and it exhibited a full electrode-based capacity greater than 710 mAh g−1 (about twice of theoretical small 2015, 11, No. 47, 6265–6271
capacity of graphite anode) after 50 cycles at a current density of 1000 mA g−1.
Experimental Section PS Opal and SiO2 Inverse Opal Fabrication: The PS opals were fabricated on gold-coated glass substrates following our previous reports[9,40] with the indicated modifications. The glass was first cleaned with piranha (volume ratio of H2SO4 to H2O2 is 3:1) and then coated with 5 nm chromium and 60 nm gold by e-beam evaporation (Temescal, Inc). Caution, piranha is highly corrosive and potentially explosive. To modify the gold surface for PS opal growth, the gold-coated substrates were immersed in an aqueous solution of 3-mercapto-1-propanesulfonic acid, sodium salt (Sigma–Aldrich Corp.) for 2 h. Then 600 nm PS colloid spheres (Molecular Probes) were dispersed in Millipore water, forming a 0.2 wt% suspension. After drying with blown air, the pretreated substrates were placed vertically into vials containing the PS suspension (≈1.5 cm depth; the final opal structure is about 1.4 cm long) at 55 °C. To enlarge the pore size of the subsequent inverse opal and to enhance the bonding between PS spheres, the PS opal was sintered at 95 °C for 3 h. To fabricate the SiO2 inverse opal, first, SiO2 sol was prepared by mixing tetraethyl orthosilicate (TEOS, Sigma–Aldrich Corp.), ethanol, and a 0.1 M HCl solution (1:10:1 volume ratio) under stirring for 6 h at room temperature. The PS opal was held vertically, and the SiO2 sol was slowly dripped on it. The samples were then dried
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at room temperature for 24 h. This infiltration and drying process were repeated for twice. The obtained samples were sintered in air at 500 °C for 4 h using a heating ramp rate of 8 °C min−1, forming ≈8 µm thick SiO2 inverse opals. Fe3O4/C Composite Growth and Carbonization Treatment: In a typical solvothermal growth, via a process similar to our previous report,[41] 5 mmol ferrocene (Sigma–Aldrich Corp.) was dissolved in 35 mL acetone. Subsequently, 1.0 mL hydrogen peroxide (30 wt%) was added into the solution followed by ultrasonication for 30 min. The obtained solution was then transferred to a 50 mL Teflon-lined steel autoclave in which the SiO2 inverse opal coated substrate faced up. The autoclave was heated in an oven at 190 °C for 36 h. After cooling to room temperature naturally, the sample was removed and washed with ethanol and Millipore water followed by drying at 50 °C for 5 h. In the subsequent carbonization treatment, the sample was heated in Ar at 500 °C for 3 h under a ramp rate of 5 °C min−1. SiO2 Scaffold Removal: To etch the SiO2 scaffold from the sample, an ammonia solution-based hydrothermal treatment[42] was employed. Typically, 10 mL ammonium hydroxide solution (28%–30% NH3 basis, Sigma–Aldrich Corp.) was added into 25 mL Millipore water to form a diluted solution. The prepared solution was transferred to a 50 mL Teflon-lined steel autoclave, and a piece of SiO2@Fe3O4/C-coated substrate was put into the solution face up. The autoclave was sealed, heated at 150 °C for 8 h, and allowed to cool naturally to room temperature. The sample was then removed and thoroughly washed with ethanol and Millipore water followed by drying at 50 °C for 5 h. Thick Electrode Fabrication: Thick electrodes were formed started with a 3D mesoporous Ni template following a similar process as for the thin electrodes (see Supporting Information for fabrication details). Characterization: The fabricated samples were characterized using a Hitachi S-4800 SEM, a Hitachi S-4700 SEM equipped with an Oxford INCA energy-dispersive X-ray analyzer, a Philips X’pert MRD X-ray diffractometer with Cu K α radiation (1.5418 Å), and a JEOL 2010 LaB6 TEM operating at 200 kV. Elemental analysis was conducted on the Hitachi S-4700 SEM. The carbon content in the Fe3O4/C composite was analyzed using a Mettler-Toledo TGA 851e under an air atmosphere and a heating rate of 10 °C min−1 from 25 °C to 600 °C. Electrochemical Measurements: Electrochemical measurements of thin electrodes (≈8 µm) were conducted using two-electrode cells with a lithium metal counter and reference electrodes on Princeton Applied Research Model 273A and Biologic VMP3 potentiostats. An electrolyte consisting of 1 M of LiClO4 in a 1:1 mass ratio mixture of ethylene carbonate and dimethylene carbonate was used. All cells were assembled in an Ar-filled glove box. Both coin and larger format cells were studied with similar results; for the coin cells, the electrode indicate below was used. Electrochemical tests of the ≈100 µm thick electrodes were conducted using coin cells with the Fe3O4/C composite as the working electrode and a lithium metal foil as both the counter and reference electrodes on the same electrochemical workstation shown above. In coin cells, the electrolyte consists of 1 M LiPF6 dissolved in a 50:50 (w/w) mixture of ethylene carbonate and diethyl carbonate. A polypropylene microporous film was employed in coin cell as the separator. All electrode capacities were measured by a galvanostatic charge–discharge method. CV curves of the thin electrodes
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were recorded over the potential range of 0.25–3 V (vs. Li+/Li) at a scan rate of 0.1 mV s−1.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements The authors thank Dr. Hui Gang Zhang, Neil A. Krueger, and Jin Gu Kang for experimental assistance. This work was primarily supported by the US Department of Energy, Office of Science, Basic Energy Sciences, under Award no. DE-FG0207ER46471. X. Huang and J. Liu acknowledge support from the State Key Project of Fundamental Research for Nanoscience and Nanotechnology of China (Award no. 2011CB933700).
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Received: August 23, 2015 Published online: October 19, 2015
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