Lithiation Behavior of Individual Carbon-Coated Fe3O4 Nanowire

Jan 25, 2017 - This irreversible phase conversion may be a major cause of capacity fading of the electrode in the first cycle. As for the Fe3O4 electr...
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Lithiation Behavior of Individual Carbon-Coated Fe3O4 Nanowire Observed by in Situ TEM Qingmei Su,*,† Shixin Wang,† Yanling Xiao,† Libing Yao,‡ Gaohui Du,*,‡ Huiqun Ye,† and Yunzhang Fang† †

Zhejiang Provincial Key Laboratory of Solid State Optoelectronic Devices, Zhejiang Normal University, Jinhua, 321004, China Institute of Physical Chemistry, Zhejiang Normal University, Jinhua, 321004, China

‡

S Supporting Information *

ABSTRACT: Fe3O4 nanowires, as a typical transition-metal oxide (TMO), are being considered as promising anodes for lithium ion batteries (LIBs) due to their 1D structure and high specific capacity. However, their underlying mechanism and electrochemical behaviors are still poorly understood. Here, the dynamic behavior and the electrochemical reaction of the carbon-coated Fe3O4 (Fe3O4@C) nanowire have been investigated directly through assembling a nanoscale LIBs inside transmission electron microscope (TEM). The in situ TEM results reveal that the Fe3O4 nanowires undergo cracking and fracturing upon the first lithiation, but the carbon coatings still embrace the oxide cores well after lithiation and play a role in maintaining the mechanical and electrical integrity. Meanwhile the lithiation process involves the conversion of Fe3O4 nanowires to Fe nanograins and the formation of Li2O along the lithium ions diffusion direction. The delithiated product is FeO rather than the original phase of Fe3O4 after the first delithiation process. This irreversible phase conversion may be a major cause of capacity fading of the electrode in the first cycle. As for the Fe3O4 electrode, about 78% of the capacity loss can be attributed to the irreversible phase reaction in the first cycle. During the subsequent lithiation-delithiation cycles, the Fe3O4 electrode shows a reversible conversion between Fe and FeO nanograins, accounting for the good reversibility of Fe3O4 anodes for LIBs. Our in situ results provide important insights into the electrochemical behavior and conversion mechanism of TMO-based anodes in LIBs and are helpful for designing LIBs with outstanding performance.

1. INTRODUCTION

hollow or porous nanostructures to moderate the serious volume changes,7,10 adding an elastomeric substrate,11 or encapsulating the electrode material inside a carbon shell.12 Carbon coating is one of the surface modification techniques to improve the electrochemical performances as it may serve as a barrier to protect the inner active material.13−15 Recently, carbon-coated FexOy are extensively studied as anode materials for LIBs due to their high specific capacity, good rate performance, and improved cycling performance,16−18 which can be attributed to the superior electrical conductivity and mechanical effect of the outer carbon layer. For instance, carbon-decorated Fe3O4 nanotubes,19 Fe3O4/carbon nanorods,20 and carbon@Fe3O4 nanospindles21 have been synthe-

Lithium ion batteries (LIBs) hold great promise for demanding applications on portable electronic devices and electric vehicles. The development of advanced LIBs requires electrode materials with superior energy density and good cyclability.1−3 In recent years, nanostructured TMOs are being considered as promising anodes for LIBs, based on a new conversion mechanism. These materials possess higher specific capacities than that of the conventional carbonaceous anodes widely used in LIBs.4−7 Among these TMO anode materials, 1D Fe3O4 nanowires have attracted considerable attention due to their low cost, higher specific capacities, and environmental friendliness.8 However, like other TMOs, Fe3O4 shows poor ionic and electronic conductivity, large volume change, and rapid capacity fading during cycles, resulting in disintegration of active materials from the current collector.9 It has been well demonstrated that many strategies have been employed to mitigate these adverse effects, such as preparing © 2017 American Chemical Society

Received: December 25, 2016 Revised: January 25, 2017 Published: January 25, 2017 3295

DOI: 10.1021/acs.jpcc.6b12973 J. Phys. Chem. C 2017, 121, 3295−3303

Article

The Journal of Physical Chemistry C

Figure 1. (a) TEM image of the prepared Fe3O4@C nanowires. (b) TEM image of an individual Fe3O4@C nanowire. (c) HRTEM image of the Fe3O4 core. (d) SAED pattern of the inner core, confirming the core is single crystal Fe3O4.

2.1.2. Preparation of Fe3O4@C Nanowires. Fe3O4@C nanowires were synthesized by the CVD process, which was performed at 450 °C under the flow of C2H2 and Ar in a volume ratio of 1:4 (100 sccm). A total of 0.20 g of Fe2O3 nanowires was spread on a quartz plate and was then placed into a tube furnace. After reaction for 30 min, the chamber was cooled to room temperature under Ar as carrier gas to obtain the final Fe3O4@C nanowires. 2.2. In Situ Electrochemical Experiments. The in situ nanobattery observations were conducted inside a TEM (JEOL JEM-2100F) with a Nanofactory TEM-STM holder. Such an in situ TEM study enables the real-time imaging of electrochemical reaction of an individual Fe3O4@C nanowire during lithiation and delithiation. A few Fe3O4@C nanowires were attached to an Au rod as the working electrode. Bulk Li metal was scratched by a shaped W tip and used as the counter electrode; a layer of Li2O was used as solid electrolyte. The Au and W wires were mounted on STM-TEM holder, which was sealed in a home-built bag filled with dry argon. The Li2O covered Li electrode came into contact with an individual Fe3O4@C nanowire by manipulating the piezo-driven stage with nanometer precision. Once a contact was made, the negative or positive potentials were applied to initiate the lithiation and delithiation, respectively. The EELS measurement was carried out using a Gatan Enfina parallel electron energy loss spectrometer. The whiteline intensity ratio (IL3/IL2) was calculated by integrating the background subtracted intensities using the standard AE−r model for both L3 and L2 edges and then dividing them to get the ratio, which was analyzed to determine the oxidation state changes of Fe elements during the lithiation and delithiation processes.

sized to improve the electrochemical performance of Fe3O4 electrode. Although the electrochemical performance of carbon-based composite has been improved, the consequences of carbon coating on electrochemical behaviors have not been studied.22−25 Moreover, Fe3O4 suffers from a dramatic capacity loss in the first cycle owing to the serious volume change and crack formation, but the issue of how cracks initiate and evolve is still unclear,26 and no experimental results provide details on the microstructure evolution in electrode as well as its influence on the capacity fading. So a fundamental understanding of their electrochemical behaviors during lithiation/delithiation may help develop strategies to mitigate the large volume change and fracture accumulation during cycles and thus design anode materials with high capacity and good retention for LIBs.27,28 So an in-depth understanding on what happens to Fe3O4@C nanowire anode in the electrochemical cycles is critically important for improving the capacity and lifetime of LIBs. We give in situ dry-cell nanobattery studies on the electrochemical behaviors and conversion mechanism of Fe3 O 4@C nanowires during lithiation and delithiation processes inside TEM. The microstructure and phase evolutions of Fe3O4@C nanowires during the cyclings were monitored by TEM and the corresponding techniques, such as electron diffraction (ED) and electron energy spectroscopy (EELS). This present study achieves a thorough understanding on the electrochemical behaviors and conversion mechanism of Fe3O4 anode, which is envisaged to be helpful for optimum design of anode materials for LIBs with excellent performance.

2. EXPERIMENTAL SECTION 2.1. Preparation of Fe3O4@C Nanowires. 2.1.1. Preparation of Fe2O3 Nanowires. FeCl3·6H2O (2.0 mmol) and Na2SO4 (2.5 mmol) were homogeneously dispersed in deionized water (100 mL). Then 80% of the above mixed solution was transferred into a 100 mL autoclave, which was maintained at 60 °C for 12 h. After the autoclave was cooled to room temperature, the products were filtered, washed with distilled water and ethanol for several times, and finally dried at 60 °C for 12 h to obtain Fe2O3 nanowires.

3. RESULTS AND DISCUSSION The morphology and microstructure of the prepared Fe3O4@C nanowires are given in Figure 1. Figure 1a is a low magnification TEM image of the obtained Fe3O4@C nanowires. Many Fe3O4@C nanowires with diameter of ∼180−240 nm are observed. Figure 1b is a typical TEM image of an individual Fe3O4@C nanowire. It confirms the core−shell structure of carbon-coated Fe3O4 nanowire, suggesting the 3296

DOI: 10.1021/acs.jpcc.6b12973 J. Phys. Chem. C 2017, 121, 3295−3303

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The Journal of Physical Chemistry C

Figure 2. Snapshots of microstructural evolution of a part of a Fe3O4@C nanowire during the first lithiation process with the potential of −1.0 V. (a−g) Screen shot of Fe3O4@C nanowire captured from videos S1−S3 in the Supporting Information. (h) TEM image of Fe3O4@C nanowire after full lithiation reaction.

reaction near the outer shell of the Fe3O4@C nanowire proceeds faster than that at the core, resulting in the formation of a lithiated shell with an unreacted crystalline Fe3O4 core, namely a “V-shaped” reaction front in the Fe3O4 nanowire. The unreacted Fe3O4 core exhibits a tapering shape, with its diameter decreasing in the direction from the reaction front toward the point of initial contact. For the nanowire shown in Figure 2, a nanocrack formed at the lithiation front (Figure 2b). The crack continues to grow as the lithiation front progresses along the Fe3O4@C nanowire (Figure 2c−g and videos S1−S3 in the Supporting Information). After the full lithiation process, the crack length increases into 2.4 μm, and the original Fe3O4 nanowire along with the carbon shell is separated into two subnanowires by the crack, but the carbon coating is still continuous and tightly adjoined to the oxide core, which is beneficial for the morphological maintenance and the improvement of the electrochemical performance. The lithiation behavior of the Fe3O4@C nanowire was also significantly different from that of carbon nanotube-coated Co9S8 nanowire,29 in which a weak radial change occurred and the lithiated part was extruded out of the open end of the carbon nanotube,

presence of a thin and continuous carbon layer around the Fe3O4 nanowire with a thickness of ∼39 nm. The HRTEM image of the core is given in Figure 1c. It indicates that the core is single-crystalline with lattice fringes of 2.9 and 4.9 Å, corresponding to the (220) and (111) planes of cubic Fe3O4 along [−110] zone axis. The structure of the core is further confirmed by the selected-area electron diffraction (SAED) pattern given in Figure 1d, in accordance with the (111), (220), and (11−1) planes of cubic Fe3O4 along the [−110] zone axis. The first lithiation process of the Fe3O4@C nanowire is illustrated by a series of time-resolved TEM images in Figure 2, which were captured from videos S1−S3 in the Supporting Information. Prior to lithiation, the Fe3O4@C nanowire has a uniform diameter, as shown in Figure 2a. When a bias of −1.0 V was applied, lithium ions were electrochemically driven from Li2O/Li (the lower left tip) to lithiate the Fe3O4@C nanowire. The lithiation process of this Fe3O4@C nanowire is shown in Figure 2b−g. A reaction front (pointed out by red arrows in images b−g of Figure 2) propagates progressively along the axis of the Fe3O4 nanowire, resulting in an obvious volume expansion. During this lithiation reaction, the electrochemical 3297

DOI: 10.1021/acs.jpcc.6b12973 J. Phys. Chem. C 2017, 121, 3295−3303

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The Journal of Physical Chemistry C

Figure 3. Time-resolved TEM images of individual Fe3O4@C nanowires with a potential of −3.0 V. (a) TEM image of a pristine Fe3O4@C nanowire. (b) TEM image of the corresponding fully lithiated Fe3O4@C nanowire, showing the formation of severe crack and fracture. (c−h) Time sequence of the electrochemical lithiation process of a Fe3O4@C nanowire with the potential of −3.0 V. The other scale bars are 0.2 μm.

and no crack and fracture was observed during the first lithiation. The reason is that the mechanical strength of the graphitized carbon nanotube is ∼130 GPa, while the strength of the amorphous carbon layer is only ∼3 GPa.30 We also monitored the lithiation process of another Fe3O4@C nanowire as given in the Supporting Information, movie S4. It reveals the same morphological evolution as we discussed above. He et al. reported the appearance of LiFe3O4 phase owing to the intercalation of lithium ions during the initial lithiation,31 whereas the LiFe3O4 phase has not been observed during the in situ TEM study in this work. The reason is that the lithiation rate in our in situ TEM experiment is larger than 1 C (926 mA g−1); the intercalation process could be bypassed at high discharge rate owing to the enhancements in reaction kinetics.31 The magnified TEM images (Figure S1 captured form movie S5 in the Supporting Information) show the migration of the reaction front through the Fe3O4@C nanowire during the first lithiation process. The boundary between the lithiated and unlithiated parts is obvious, and the radial expansion and crack formation of the lithiated Fe3O4@C nanowire reveal the

insertion of Li+ ions into the carbon shell and Fe3O4 core. The carbon shell and Fe3O4 core with the original sizes of 41 and 118 nm (marked in Figure S1e) expand to 47 and 171 nm (Figure S1h), corresponding to a volume expansion of Fe3O4 core of about 110.9% during the first lithiation process. This relatively small initial volume expansion of Fe3O4@C nanowire is likely to be ascribed to the mechanical confinement of the surface carbon layer.19,30,32 The rate performance is significant for LIB electrode. It was found that the electrode may evolve in different electrochemical behaviors with varied current densities.33 So the electrochemical behavior of another Fe3O4@C nanowire under a larger bias of −3.0 V (corresponding to a large current density) is represented in Figure 3 and movie S6 in the Supporting Information. Figure 3a,b shows both the Fe3O4 nanowire and carbon shell undergo severe crack after full lithiation with the potential of −3.0 V, which is much more violent than the reaction at −1.0 V. Figure 3c−h depicts the time-resolved TEM images of the lithiation process of this Fe3O4@C nanowire. As given in Figure 3c, the Fe3O4@C nanowire was selected and recorded as the beginning, and the diameter is 191 nm. The 3298

DOI: 10.1021/acs.jpcc.6b12973 J. Phys. Chem. C 2017, 121, 3295−3303

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The Journal of Physical Chemistry C

Figure 4. Structure and phase characterization of a Fe3O4@C nanowire after the first full lithiation process. (a) TEM image of an initial Fe3O4@C nanowire. (b) TEM image of the Fe3O4@C nanowire after full lithiation. (c) The magnified TEM image of lithiated Fe3O4@C nanowire in (b). (d) HRTEM image of the lithiated Fe3O4@C nanowire. (e) ED pattern and (f) EELS spectrum of the fully lithiated electrode.

in active materials, so the electrodes generally show a decreased reversible capacity at high reaction rates. Furthermore, the drastic formation of cracks and fractures in the lithiated electrode at high reaction rate can induce severe damage to the electrode, leading to irreversible capacity loss in real battery. It is worth noting that carbon shells still embrace the fractured Fe3O4 cores as one body in both cases. The carbon coating is still continuous and plays a role in maintaining the mechanical and electrical integrity of the nanowire electrode during the electrochemical reaction. On the contrary, it is imaginable that the pure Fe3O4 would be pulverized and partial electrode would be electrochemically inactive due to the loss of electrical contact between particles without the assistance of carbon coating, leading to rapid capacity fading. Although carbon coating cannot completely suppress the volume expansion and fracture of oxide cores, it is beneficial for the maintenance of electrical conductivity and the improvement of the electrochemical performance. The detailed structure and phase characterization of a Fe3O4@C nanowire after the first full lithiation are presented in Figure 4. Figure 4a is a TEM image of an individual Fe3O4@C nanowire; it can be clearly seen that the Fe3O4 nanowire is encapsulated within the carbon layer completely and the carbon coating with a size of 31.1 nm. In contrast, this Fe3O4@C nanowire shows serious expansion and severe fracture after full lithiation (Figure 4b). Moreover, the carbon layers expand its size from 31 to 35 nm, suggesting the lithium ions are inserted into carbon layer. The TEM image marked by the red box in Figure 4b is further magnified and shown in Figure 4c. Obviously, the Fe3O4 nanowire has converted to a uniform

Fe3O4@C nanowire expands its diameter from 191 to 326 nm within 58.5 s, and a crack is formed on the outer carbon layer (as indicated by an arrow in Figure 3d). Continuous volume expansion and crack formation of the lithiated Fe3O4@C nanowire are also observed in Figure 3e; at 96.0 s the Fe3O4@C nanowire expands from 191 to 346 nm in diameter. After lithiation for 138.0 s, a long crack is formed and a subnanowire is peeled off from the pristine Fe3O4@C nanowire, as shown in Figure 3f. Then the nanowire undergoes intensive cracking and fracturing with the reaction. Figure 3g,h displays the appearance of a spate of fractures on the lithiated nanowire marked by the black arrows. Meanwhile, the volume expansion of the Fe3O4@ C nanowire propagates quickly along the reaction front. The drastic formation of cracks and fractures in the lithiated nanowire finally leads to severe damage of the nanowire. Our in situ TEM results with the potential of −1.0 and −3.0 V demonstrate that the electrochemical behaviors (lithiation mechanism) of Fe3O4@C nanowire are different in terms of the reaction rates. The electrochemical reaction with a large potential (−3.0 V) is violent, presenting serious cracks and fractures in the nanowire electrode. The electrochemical reaction with a low potential (−1.0 V) is relatively peaceful, and the Fe3O4@C nanowire is only separated into two subnanowires by the crack. It suggests that a small reaction rate is beneficial for the morphological maintenance of electrode material in the lithiation process by weakening the formation of cracks and fractures. The obviously large volumetric expansion at higher potential is ascribed to the formation of serious cracks and fractures. The cracks and fractures can cut down on the Li+ diffusion (lithiation reaction) 3299

DOI: 10.1021/acs.jpcc.6b12973 J. Phys. Chem. C 2017, 121, 3295−3303

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The Journal of Physical Chemistry C

Figure 5. Morphology and phase changes of Fe3O4@C nanowire during the first two cycles with the potential of −1.0/+3.0 V for lithiation/ delithiation, respectively. (a, a1, and a2) TEM, HRTEM images, and SAED pattern of the pristine Fe3O4@C nanowire electrode. (b−e) TEM image of Fe3O4@C nanowire in the first two cycles. (b1−e1) The magnified TEM images of the electrode in (a−e). (b2−e2) The corresponding ED patterns of the electrode in b−e.

with the valence state of zero,35 which suggests the transition from Fe3O4 to Fe0, in agreement with the ED results. In a real battery, a solid-electrolyte interface (SEI) layer is formed on anode materials as a result of the decomposition of liquid electrolyte during discharge.37,38 Because solid Li2O was used as electrolyte in our in situ TEM experiments, only a layer of Li2O was observed instead of a real SEI layer on the electrode materials. The microstructure evolution of a Fe3O4@C nanowire during the first lithiation is presented in Figure S2(b−i) in the Supporting Information. We can clearly see that the surface of the Fe3O4@C nanowire was coated by a layer of Li2O with a thickness of 5 nm after full lithiation (Figure S2(i)), which is similar to other in situ TEM experiments.33−35 In situ TEM experiments were performed to study the repeated microstructure and phase evolutions induced by lithiation-delithiation cyclings as presented in Figure 5. During the first two cycles, the potentials of −1.0 and +3.0 V were alternately applied on Fe3O4@C nanowire to realize the

compound with many ultrafine nanograins of 2−3 nm embedded in the matrix. Figure 4d is a HRTEM image of the lithiated Fe3O4 nanowire recorded from the region of the red box in Figure 4c. The lattice spacings of the circled nanograins are measured to be 2.0 Å, in accordance with the (110) plane of the cubic phase of Fe (JCPDS no. 87-0722). The ED pattern of the lithiated electrode in Figure 4e further confirms the microstructure. All the diffraction rings can be indexed as Fe and Li2O phases. It confirms the lithiation reaction leads to the formation of Li2O and Fe during the reduction of Fe3O4 nanowires (Figure 4e). Also, some strong arcs formed in the ED rings during the first lithiation suggest the presence of preferentially orientated nanograins in the particle after lithiation, which is similar to the lithiation of RuO2 and Fe2O3.34,35 EELS spectrum can be used to investigate the valence states of Fe.36 The corresponding EELS spectrum of the Fe3O4 core after lithiation process is shown in Figure 4f. The L3/L2 intensity ratio of Fe elements is 2.6, in accordance 3300

DOI: 10.1021/acs.jpcc.6b12973 J. Phys. Chem. C 2017, 121, 3295−3303

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The Journal of Physical Chemistry C Fe3O4 + 8Li+ + 8e− → 3Fe + 4Li 2O

lithiation and delithiation processes. Figure 5a is the TEM image of a representative Fe3O4@C nanowire with a diameter of 190 nm. The HRTEM image in Figure 5a1 exhibits clear lattice fringes of 4.9 Å, in accordance with (111) plane of Fe3O4. The corresponding FFT pattern of the Fe3O4 core is displayed in the inset in Figure 5a1, which can be well indexed as the (1̅1̅1), (002) and (111) planes of cubic phase of Fe3O4 along [1̅10] zone axis. The SAED pattern of the Fe3O4 core (Figure 5a2) agrees well with the FFT results. Figure 5b−e gives the repeated size changes of the Fe3O4@C nanowire in the first two cycles. The magnified TEM image of the electrode after the first lithiation given in Figure 5b1 suggests the Fe3O4@ C nanowire involves serious expansion and severe crack formation accompanying the size expansion from 190 to 241 nm, suggesting the lithium ions have been inserted into the Fe3O4 nanowire. Figure 5b2 presents the corresponding ED pattern of the electrode after first lithiation, in which the diffraction rings can be perfectly indexed to cubic Fe and Li2O. After the potential was reversed to +3.0 V for delithiation, the diameter of the Fe3O4@C nanowire decreases to 192 nm (Figure 5c1); it indicates that the lithium ions are extracted from the lithiated electrode. Figure 5c 2 presents the corresponding ED pattern of the Fe3O4@C electrode after the first delithiation, which reveals the delithiated product is FeO phase instead of the original Fe3O4 phase, which is well agreed with the previous results.39 It means the electrochemical reaction of Fe3O4 is irreversible in the first cycle. Furthermore, the second lithiation-delithiation cycle of the Fe3O4@C electrode was conducted, as shown in Figure 5d1,e1. The repeated expansion and contraction of the Fe3O4 nanowire is revealed. The marked size of the Fe3O4 nanowire increases to 240 nm in the second lithiation. Figure 5d2 shows the corresponding ED pattern of Fe3O4 electrode after the second lithiation, which suggests the products are Fe and Li2O, being the same with the first lithiation. After the second delithiation, the selected Fe3O4@C nanowire shrinks its size to 190 nm as displayed in Figure 5e1. It indicates that the morphological changes during the first two cycles are reversible. The ED pattern of the electrode shown in Figure 5e2 still agrees well with the FeO phase. In addition, EELS was conducted to reveal the valences of Fe in the first two cycles as displayed in Figure S3 in the Supporting Information, demonstrating the same phase conversion process. Our in situ TEM studies reveal that the electronic and ionic conductivity of Fe3O4@C nanowire is still high, and the entire Fe3O4@C nanowire can work well and maintain good electrochemical stability during cycling. Although the nanowire is easily fractured during the first lithiation process, carbon coating is favorable in maintaining the mechanical and electrical integrity of the electrode, and thus improves the electrochemical performance. Incorporating EELS analysis with TEM and ED results, we can conclude that the Fe3O4 undergoes irreversible microstructure change in the first cycle, which may be a major cause of severe capacity loss in the first discharge−charge process. The subsequent electrochemical reaction in the Fe 3 O 4 electrode is a reversible change between Fe and FeO nanograins. Therefore, the results presented in this experiment can well explain the large capacity loss in the first cycle, good reversibility and high Coulomb efficiency during the subsequent cycles for Fe3O4 anodes in LIBs.7,9,13 Briefly, the electrochemical reaction of Fe3O4 electrode in LIBs can be expressed as follows: During the first lithiation process:

During the following delithiation/lithiation processes: Fe + Li 2O ↔ FeO + 2Li+ + 2e−

The theoretical capacity of FeO is ∼744 mAh g−1, which is much lower than that of Fe3O4 of 926 mAh g−1 calculated assuming eight electrons transferred per formula weight. Our results suggest that there will be a theoretical capacity loss of 232 mAh g−1 resulting from the irreversible phase evolution in the first cycle. To further investigate the influence of the irreversible phase evolution on the capacity fading in the first cycle, we checked the initial capacity loss of pure Fe3O4 anode in LIBs in the literature. The results show that the capacity fading is 268 mAh g−1 for Fe3O4 nanowires at a current density of 0.1 C,40 312 mAh g−1 for self-assembled Fe3O4 nanoparticle clusters,41 314 mAh g−1 for single-crystalline mesoporous Fe3O4 nanorods.42 The average capacity fading in the first cycle is about 298 mAh g−1 for pure Fe3O4 electrode. From these results, we can estimate that about 78% of the capacity fading of Fe3O4 can be attributed to the irreversible phase conversion occurred in the first cycle. That is ∼22% of the capacity fading can be ascribed to other reactions (such as formation of SEI layer, electrode pulverization, and so on). The remarkable cycling performance and the good capacity retention after the first cycle result from the reversible conversion between Fe and FeO nanograins. Based on our in situ TEM results, several methods are suggested to improve the electrochemical performance of iron oxide materials in LIBs. First, Fe3O4 nanowire fracture seriously owing to the large strain induced by volume expansion during lithiation. Ultrathin Fe3O4 nanoparticles could be stable during electrochemical process because of the easy release of strain. Second, FeO can be used instead of Fe3O4 because FeO shows better reversibility as anode in LIBs. Third, graphitized carbon with superior mechanical strength is much better than the amorphous carbon. In the study of the electrochemical lithiation of Co9S8 nanowire-filled carbon nanotubes (CNTs), the closed CNT shells can well suppress the volume expansion of Co9S8 nanowires and prevent it from fracturing.29

4. CONCLUSIONS In summary, we have monitored the electrochemical behavior of individual Fe3O4@C nanowire using in situ TEM by constructing a nano-LIB device inside a TEM. The results show that the single-crystalline Fe3O4 nanowires convert to Fe nanograins embedded in Li2O matrix, accompanied by the formation of cracks and fractures during the first lithiation process. Moreover, the nanowire shrinks and converts into FeO nanograins rather than the original Fe3O4 phase in the delithiation process. Therefore, Fe3O4 undergoes irreversible phase transformation because of the occurrence of FeO (744 mAh g−1) instead of Fe3O4 (926 mAh g−1) in the first delithiation, which leads to a theoretical capacity loss of 232 mAh g−1. This irreversible phase conversion may be a major cause of capacity fading of the electrode in the first cycle. As for Fe3O4 electrode, about 78% of the capacity loss can be attributed to the irreversible phase reaction in the first cycle. The reversible conversion between FeO and Fe since the second cycle accounts for the good capacity retention of Fe3O4 anode in LIBs. In addition, the electrochemical behaviors are different with the lithiation rates. In any case the carbon coatings embrace the fractured nanowires well after reaction, 3301

DOI: 10.1021/acs.jpcc.6b12973 J. Phys. Chem. C 2017, 121, 3295−3303

Article

The Journal of Physical Chemistry C

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which is beneficial for the maintenance of mechanical and electrical integrity and the improvement of the electrochemical performance. The results presented by this experiment provide the direct evidence and a deep understanding of the electrochemical reaction mechanism and conversion behaviors of Fe3O4 based electrode in LIBs, and the experimental visualization provide significant insights on the optimum design of iron oxide anode materials to improve the capacity retention and cycling performance in LIBs.

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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.jpcc.6b12973. Movie showing the lithiation process of individual Fe3O4@C nanowire (AVI) Movie showing the lithiation process of individual Fe3O4@C nanowire (AVI) Movie showing the lithiation process of individual Fe3O4@C nanowire (AVI) Movie showing the lithiation process of individual Fe3O4@C nanowire (AVI) Movie showing the lithiation process of individual Fe3O4@C nanowire (AVI) Movie showing the lithiation process of individual Fe3O4@C nanowire (AVI) TEM images showing the lithiation process of individual Fe3O4@C nanowire (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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

Qingmei Su: 0000-0002-6662-7557 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 11574273 and 11504330), the Natural Science Foundation of Zhejiang Province, China (Nos. LQ15B01001 and LY16B030003), Zhejiang Provincial Science and Technology Key Innovation Team (No. 2011R50012), and Key Laboratory (No. 2013E10022).

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REFERENCES

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DOI: 10.1021/acs.jpcc.6b12973 J. Phys. Chem. C 2017, 121, 3295−3303

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DOI: 10.1021/acs.jpcc.6b12973 J. Phys. Chem. C 2017, 121, 3295−3303