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Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 22841−22850

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Achieving Ultrafast and Stable Na-Ion Storage in FeSe2 Nanorods/ Graphene Anodes by Controlling the Surface Oxide Dan Li, Jisheng Zhou,* Xiaohong Chen, and Huaihe Song* State Key Laboratory of Chemical Resource Engineering, Beijing Key Laboratory of Electrochemical Process and Technology for Materials, Beijing University of Chemical Technology, Beijing, 100029, P. R. China

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S Supporting Information *

ABSTRACT: Designing transitional metal selenides (TMSes) with superior rate and cyclic performance for sodium-ion storage remains great challenges. To achieve this task, the influence of surface oxides on Na-ion storage behavior of FeSe2 is investigated by designing FeSe2 with varying oxide content. It is found that surface oxide has an inhibitory effect on the activity of FeSe2. Smallsized FeSe2 on graphene with higher surface oxide content exhibits obviously inferior performance compared to large-sized FeSe2 with lower oxide content. By controlling oxide content, the prepared FeSe2 nanorods/graphene exhibits a high capacity of 459 mAh/g at 0.1 A/g and superior rate performance. Only 10% capacity decrease occurs with the increase in current density from 0.1 to 5 A/g. Even at 25 A/g (∼50 C), it delivers a capacity of 227 mAh/g with almost no decay after 800 cycles. The influence mechanism of surface oxide is investigated. The oxide can be converted to a sodiated shell with high mechanical strength and poor conductivity, which generates phase-transition resistance to suppress the sodiation of FeSe2 core, blocks the transfer of Na-ions and electrons in subsequent sodiation processes. Understanding the effect of surface oxide on Na-ion storage will be helpful in designing TMSes and other active materials. KEYWORDS: surface oxide, iron selenide, nanorod, oriented attachment growth, graphene, sodium-ion battery, anode

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INTRODUCTION Because of the abundance of sodium resources,1−3 many efforts have been devoted to developing emerging sodium-ion batteries (SIBs) to replace commercial lithium-ion batteries (LIBs).4−6 However, the radius of the sodium ion is approximately 50% larger than that of the lithium ion (1.02 Å for Na+ vs 0.76 Å for Li+),7−10 so the design of new anode materials with suitable crystal structures for the accommodation and migration of sodium ions has become an emerging topic.11,12 Recently, transition metal selenides (TMSes)13−16 have become promising candidates for Na-ion storage because of their large crystal lattice with fast ion transport and high conductivity, and their intermediates NaxMSe2 show superconductivity17 toward sodiation. Chen et al.18 first reported that FeSe2 microspheres can facilely exhibit a relatively high Na-ion storage capacity (ca. 447 mAh/g) and enhanced rate performance, showing clear advantages over their oxide and sulfide counterparts as anode materials for SIBs.19−22 Nevertheless, TMSes also suffer from obvious capacity fading and poor rate performance due to their large volume expansion with the insertion of Na ions.23,24 In this regard, a common strategy is to combine the TMSes with a carbon matrix to improve the cycling and rate performance.25,26 Nonetheless, this method has limitations for achieving the full potential improvements in their cycling stability and rate performance for SIBs. Achieving both high rate capabilities (a significant © 2018 American Chemical Society

capacity at 10 A/g) and a long cycle life (>300 cycles) remains a great challenge.27 Considering the micro or submicron size of most metal selenides synthesized in previous reports, a strategy for synthesizing ultrafine nanoparticles of TMSe28 was recently developed to enhance the diffusion kinetics and to buffer the volume changes to improve the Na-ion storage capacity in order to achieve a super high rate capability and cycling stability. In addition to the particle size, the energy storage features of the electrode materials for LIBs/SIBs strongly depend on their surface characteristics because the diffusion of Li/Na ions is influenced by the surface of the electrode materials.29−31 Therefore, attention must be paid to the influence of the surface features of TMSe on the Na-ion storage performance to improve the electrochemical properties of TMSes. Surface oxides are one of the most common surface defects found on various electrode materials. Similarly, metal oxides also usually exist on the surface of TMSes due to the exposure of the metal selenide to air or the incomplete selenization process because TMSes are generally prepared by the selenization of transitional metal oxides (TMOs).32−35 To date, investigations of the influence of surface oxides on some electrode materials Received: April 18, 2018 Accepted: June 8, 2018 Published: June 8, 2018 22841

DOI: 10.1021/acsami.8b06318 ACS Appl. Mater. Interfaces 2018, 10, 22841−22850

Research Article

ACS Applied Materials & Interfaces Scheme 1. Schematic Illustration for the Synthesis of FeSe2/GNS-400

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have been carried out. On one hand, surface oxides have been shown to act as a buffer layer to mitigate the volume effect and enhance the cycling ability of the electrode materials.36−40 For example, Mullins et al.36 improved the cycling stability of silicon electrodes by introducing a small amount of oxygen on the surface of silicon thin films. Xiong et al.37 proved that lithium ions can easily penetrate and react reversibly with the underlying Si nanowires when there is an oxide layer of a suitable thickness on the surface. On the other hand, the improvement in cycling stability was achieved with some sacrifice of the capacity. Cui et al. reported41 that the surface oxide formed a relatively strong shell around the Si core, mechanically limiting the diameter expansion and the extent of lithiation, thereby decreasing the capacity of the Si electrode. Liu et al.42 also found that the surface oxide layer on Si nanoparticles adversely affected their initial performance as electrodes by reducing the reversible capacity. However, the influence of surface oxide on the Na-ion storage performance of TMSes is still unclear. TMSes and TMOs differ greatly in their electrical conductivity, crystal structures, and bonding energy, etc. Thus, it can be deduced that the formation of a surface oxide will significantly affect the Na-ion storage properties of TMSes. To the best of our knowledge, the effects of surface oxide on the electrochemical performance of TMSe and, based on these effects, the ability to produce a high-performance anode material with an elaborate design have not yet been explored in the literature. In this work, FeSe2 uniformly anchored on the surface of graphene nanosheets (FeSe2/GNS) was synthesized through the selenization of iron oxide to form high-performance anode materials for SIBs. By controlling the selenization process, FeSe2 with various surface oxide contents can be obtained, providing a good opportunity to investigate the influence of the surface oxide on the Na-ion storage performance of FeSe2. It is found that surface oxide coatings generate a phase-transfer resistance in the underlying FeSe2, suppressing the extent of sodiation and resulting in poor Na-ion storage kinetics. Therefore, controlling the surface oxide on FeSe2 can greatly improve the capacity and achieve superior rate performances. Remarkably, the prepared FeSe2 nanorods/graphene composites with almost no surface oxide can deliver a high capacity of 459 mAh/g and exhibit an obviously superior rate performance: a negligible decrease in the capacity occurred with the increase in the current density from 0.1 to 5 A/g. This strategy for achieving high Na-ion storage performance anodes by controlling the surface oxide has the great potential to the design of other TMSes.

RESULTS AND DISCUSSION

Morphology and Structure of FeSe2/GNS. The FeSe2/ GNS composite was obtained through a simple selenization process using iron oxide combined with graphene nanosheets (referred to as FeOX/GNS) as precursors (Scheme 1). First, the FeOX/GNS composite was synthesized by a procedure described in our previous work using Fe(NO3)3 and GNSs as sources under ice bath condition.22 The as-prepared iron oxide was in the form of amorphous nanoparticles with an average diameter of 5 nm, which uniformly anchored on the graphene nanosheets. Then, FeOX/GNS was converted to a FeSe2/GNS composite by selenization using Se powder as a selenium source. When the composite is selenized at 400 °C for 12 h, FeSe2 nanorods with much less surface oxide are generated on the GNSs, which is assigned as FeSe2/GNS-400. The XRD pattern (Figure S1) confirms that all the peaks of the FeSe2/ GNS-400 match well with FeSe2 (JCPDS 82-0269). The SEM image (Figure 1a) shows that the FeSe2/GNS-400 composite possesses a curled morphology with a thin wrinkled structure and that the FeSe2 components are uniformly anchored on the surfaces of graphene. After further magnification (Figure 1b),

Figure 1. Low-magnification (a) and high-magnification (b) SEM images of as-prepared FeSe2/GNS composite synthesized at 400 °C for 12 h. (c) Elemental distributions of FeSe2/GNS-400 using the HAADF model: (i) HAADF-TEM image and corresponding elemental mappings of C (ii), O (iii), Fe (iv), and Se (v). 22842

DOI: 10.1021/acsami.8b06318 ACS Appl. Mater. Interfaces 2018, 10, 22841−22850

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Figure 2. Low-magnification and high-magnification TEM images and the corresponding selected area electron diffraction (SAED) patterns of asprepared FeSe2/GNS composites synthesized at 300 °C for 6 h (a, e), 300 °C for 12 h (b, f), 350 °C for 12 h (c, g), and 400 °C for 12 h (d, h). (i) Schematic illustration of the formation process of the surface oxide on FeSe2 and the formation mechanism of the rod-like structure of iron selenide.

2c and g). It is difficult to observe the surface oxide layers by HRTEM, but the weak diffraction rings belonging to Fe2O3 in SAED patterns (Figure 2c) and the diffraction peak of Fe2O3 at 31.3° in XRD (Figure S1b) could still be seen, indicating the existence of incompletely selenized iron oxide. Moreover, after sputtering the surface of the FeSe2/GNS-350 sample (Figure S3), the XPS spectra confirm that the intensity of the oxygen peaks decreases with increasing sputtering time, indicating that oxides have formed on the surface of FeSe2. The content of the oxide is approximately 0.61%, while the content of FeSe2 is 80.56% (Figure S2c and Table S1). At 400 °C for 12 h, the grain boundaries are eliminated, perfect rod-like structures are formed (Figure 2d and h), and the oxide content also decreases to only 0.09% (Figure S2d and Table S1). When the selenization temperatures are further improved to 450 and 500 °C (Figure S4), the phase compositions of obtained samples are changed to Fe7Se8 rather than FeSe2. Therefore, we choose the three samples obtained at 300, 350, and 400 °C to expand our discussion. On the basis of the evolution of the morphology and structure of FeSe2, FeSe2 rods are formed by the aggregation of ultrathin FeSe2 nanoparticles as the primary building blocks. The growth trend of FeSe2 is also confirmed by XRD (Figure S1a), in which the intensities of the diffraction peaks of FeSe2 are improved gradually with selenization. Therefore, it is reasonable to deduce that the formation of FeSe2 rods should be attributed to the Oriented Attachment (OA) mechanism.43 The formation process of the FeSe2 rods on GNSs by the OA mechanism can be schematically illustrated in Figure 2i. First, the process for selenization of iron oxide nanoparticles to form FeSe2 is a lengthy one. At lower temperatures and shorter times of selenization, it is difficult for iron oxide to fully selenized to FeSe2, so the residual oxide layer is partially wrapped around the FeSe2 core. Then, bare FeSe2 nano-

the rodlike structure of FeSe2 can be obviously distinguished with a diameter of approximately 50 nm and a length of 1 μm. The HAADF-STEM image and corresponding elemental mapping images (Figure 1c(i−v)) demonstrate the rod-like structure of FeSe2 in more detail, in which the Fe and Se elements are uniformly dispersed throughout the whole rod. Determination of the formation process of FeSe2 rods on GNSs is of great interest. For this purpose, samples obtained at various selenization temperatures and times are investigated. It can be found that the morphology and surface oxide content can vary widely with an increase in the temperature and time of the selenization process (Figure 2a−d). When the selenization is carried out at 300 °C for 6 h (Figure 2a), the obtained sample retains the nanoparticle morphology rather than forming rods, and the particles are dispersed evenly on the GNSs and have an average diameter of ∼8 nm. Incompletely selenized nanoparticles are also often observed. As shown in Figure 2e, it is clear that the amorphous Fe2O3 layer is partially wrapped around the FeSe2 core. Then, the selenization time is prolonged to 12 h at 300 °C, and the obtained sample is abbreviated as FeSe2/GNS-300. In the FeSe2/GNS-300, these particles start to aggregate by attaching to each other (Figure 2b), and the average size of the particles increases to ∼15 nm. The crystallinity of FeSe2 is noticeably improved. Meanwhile, the surface oxides around the particles obviously become thinner (Figure 2f). According to the TG curves (Figure S2a and b and Table S1), the content of the iron oxide is approximately 6.60%, while the content of FeSe2 is 72.68%, and the remaining content is graphene (the related calculation can be seen in the Supporting Information). When the reaction temperature is increased to 350 °C and maintains for 12 h, the obtained sample (referred to as FeSe2/GNS-350) achieves one-dimensional structures, which are composed of many aggregated nanoparticles with clear grain boundaries (Figure 22843

DOI: 10.1021/acsami.8b06318 ACS Appl. Mater. Interfaces 2018, 10, 22841−22850

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Figure 3. (a) XPS survey spectra of FeSe2/GNS-300, FeSe2/GNS-350, and FeSe2/GNS-400 and their O 1s (b) and Fe 2p (c) spectra. (d) The schematic illustration of the surface composition of these three composites.

Figure 4. Initial three discharge−charge curves for FeSe2/GNS-300 (a), FeSe2/GNS-350 (b), and FeSe2/GNS-400 (c) at 100 mA/g. (d) Cycling performance at 100 mA/g and corresponding Coulombic efficiency for FeSe2/GNS-300, 350, and 400. (e) Rate capability of FeSe2/GNS-300, 350, and 400. (f) Electrochemical impedance plots after three cycles at 100 mA/g. (g) Cycling performance for FeSe2/GNS-400 cycled at a current of 2 A/g, 5 A/g, 10 A/g and 25 A/g and the corresponding Coulombic efficiency.

particles move along the GNSs and tend to connect with each other to form FeSe2 clusters with many grain boundaries driven by a reduction in the surface energy. Noticeably, the attachment processes usually occur between the FeSe2 crystal planes, as seen by the HRTEM observations, due to that OA processes are more easily occurred in metal sulfides and selenides.43−45 Therefore, the attachment processes of FeSe2

wrapped by oxide shells usually occur between the FeSe2 crystal planes, as seen by the HRTEM observations. Thus, the iron oxides are continuously exposed and forced to stay at the surface of FeSe2 clusters due to the aggregation of the nanoparticles. Finally, perfect FeSe2 rods are formed by matching the misaligned lattice of nanoparticles via the rotation of FeSe2 nanoparticles and/or atom-to-atom reor22844

DOI: 10.1021/acsami.8b06318 ACS Appl. Mater. Interfaces 2018, 10, 22841−22850

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ACS Applied Materials & Interfaces

FeSe2/GNS-400 not only exhibits a greater capacity and higher Coulombic efficiency but also possesses an extraordinary rate capability compared to the other two samples (Figure 4e). When the current density changes from 100 mA/g to 5 A/ g (∼0.2 to 10 C), the reversible capacity of FeSe2/GNS-400 remains between 460 and 415 mAh/g with a decrease of only 10%. Even at the extremely high current density of 10 A/g (∼20 C), it still demonstrates a reversible capacity as high as 390 mAh/g, exhibiting ultrafast Na-ion storage (corresponding to less than 2.5 min for one discharge process). The capacity retention is also quite impressive: after high rates of charge− discharge, the reversible capacity recovers to 455 mAh/g at 100 mA/g. In contrast, the capacities of FeSe2/GNS-300 and FeSe2/GNS-350 are much lower than that of FeSe2/GNS-400 at the same rates. For FeSe2/GNS-300, at a high rate of 20 C, it could only deliver a reversible capacity of 123 mAh/g, which is just 30% of the capacity of FeSe2/GNS-400. The rate capability of FeSe2/GNS-400 (Table S4) is also obviously superior to that of FeSe2 in previous reports.25,34,35,54,55 For example, Kang et al.34 also observed surface iron oxides on iron selenide in FeSe2/carbon composites. When used as anode materials for SIBs, the FeSe2/carbon composites delivered reversible capacities of 400, 380, 360, 320, and 270 mAh/g at 0.2, 0.5, 1, 2, and 5 A/g, respectively, showing an obvious capacity fading with increasing current density. In addition to the excellent rate capability, FeSe2/GNS-400 also show good cycling stability at high current densities (Figure 4g). When FeSe2/GNS-400 is discharged/charged up to 800 cycles at 2, 5, 10, and 25 A/g (25 A/g ≈ 50 C), the reversible capacities remain at 461, 436, 430, and 227 mAh/g, respectively, with almost no decay compared to the initial reversible capacity. In some reports,18,26 FeSe2 also showed good rate performances, but the cycling stabilities at high rates were not satisfactory. For example, in a recent report,26 when a carbon-coated hollow FeSe2 nanosphere electrode was cycled at 1 A/g, the reversible capacity showed a noticeable decline starting from the 100th cycle. To reveal the reasons for excellent performance at high rates, we observed the morphology and structure of FeSe2/GNS-400 cycled at 0.1, 0.5, 5, and 10 A/g after two cycles, respectively. Interestingly, the FeSe2 still maintains the nanorod structure after cycled at various current densities, indicating its good structural stability (Figure S5a−d). Noticeably, a homogeneous SEI film can be observed on the FeSe2 naonrods, and the thickness of SEI film is maintained at 5 nm at various current densities, indicating the high stability of SEI film (Figure S5e−l). A stable SEI film on the electrode could effectively suppress the further decomposition of the electrolyte, reduce the side reactions, maintain the integrity of the electrode, and thus accelerate the diffusion of Na ions.56,57 Furthermore, the kinetic analyses (Figure S6a−c), based on CV measurements at different scan rates, indicate that Na-ion storage in the FeSe2/GNS-400 is a capacitive-controlled process. The capacitive contribution gradually increases as the scan rate increasing. At the scan rate of 1 mV/s, the capacitive-contribution ratio is as high as 93.5%. Both the capacitive-controlled Na-ion storage process and stable SEI film are the kinetic favorable factors to support excellent rate capability of FeSe2/GNS-400.18 Someone may believe that the inferior electrochemical performance of FeSe2/GNS-300 and FeSe2/GNS-350 should be attributed to the lower theoretical Na-ion storage capacity of iron oxide. To rule out this proposal, capacity contributors of iron oxide in the three samples was calculated (Table S3). In

ientations by the migration of grain boundaries or dislocations.46−48 XPS is a robust analytical technology for the detection of the chemical states of elements at a sub-5 nm depth surface. XPS survey spectra (Figure 3a) reveal variations in the surface oxide composition on three FeSe2/GNS samples synthesized at different temperatures. The three samples are all composed of C, O, Fe, and Se elements. Noticeably, the intensities of the oxygen peaks decrease with increasing selenization temperature, which reflects the reduced oxygen content on the surface of the composites. The O 1s spectra (Figure 3b) of the samples synthesized at different temperatures are mainly composed of three peaks at 533.6, 532.3, and 530.7 eV. The peaks at 533.6 and 532.3 eV should be attributed to the epoxy C−O groups in the graphene and Se−O, respectively.49,50 Compared with other peaks, the intensities of the peaks at 530.7 eV belonging to Fe2O351 show an obviously weaker trend with higher selenization temperatures. The Fe 2p3/2 spectra shown in Figure 3c also confirm the results from the O 1s spectra. According to previous reports,26,51 the Fe 2p3/2 core levels located at 707 eV should be derived from FeSe2, while the Fe 2p3/2 core levels located at 711 eV should be correspond to Fe2O3. As the selenization temperature increase, the FeSe2 peaks enhance, while the Fe2O3 peaks weaken gradually. To more intuitively illustrate the change in the surface oxides, we estimated the content of Fe2O3 by the ratio of the peak areas of Fe2O3 and FeSe2 in the Fe 2p3/2 spectra, and the calculation results (Figure 3d) show that the content of oxide on the sample surface decrease from 91% in FeSe2/GNS-300 to 19% in FeSe2/GNS-400. However, based on the TG analysis, the content of oxide in the whole samples decrease from 6.60% in FeSe2/GNS-300 to 0.09% in FeSe2/GNS-400. Therefore, XPS confirms again that oxides are concentrated on the surface of the samples. Ultrafast and Stable Na-Ion Storage Performance of FeSe2/GNS-400. For small particle sizes in FeSe2/GNS, such as in the FeSe2/GNS-300 and FeSe2/GNS-350 samples, a superior electrochemical performance as anode materials for SIBs is expected.28,52 However, beyond all expectations, the actual test results show that the rod-like FeSe2/GNS-400 with the largest size exhibits the best Na-ion storage performance. In the discharge−charge curves shown in Figure 4, the first discharge capacity and reversible capacity of FeSe2/GNS-400 are 493 and 459 mAh/g at 0.1 A/g, respectively. However, the reversible capacities of FeSe2/GNS-300 and FeSe2/GNS-350 are 230 and 384 mAh/g, respectively, which are obviously lower than that of FeSe2/GNS-400. Additionally, FeSe2/GNS400 shows a very high Coulombic efficiency (93.1%), far exceeding those of FeSe2/GNS-300 (85.2%) and FeSe2/GNS350 (89.9%). The irreversible capacity, on one hand, is caused by the irreversible formation of a solid electrolyte interphase (SEI) film on the surface of the electrode. By simulating the Nyquist plots (Figure 4f and Table S2), we find that the Rf (SEI film resistance) of FeSe2/GNS-400 (6.29 Ω) is much lower than those of FeSe2/GNS-300 (27.02 Ω) and FeSe2/ GNS-350 (7.39 Ω). On the other hand, the irreversible capacity is affected by the surface, which leads to irreversible capacity loss due to the formation of Na2O.53 The irreversible capacity loss should be the lowest in FeSe2/GNS-400 because it has the lowest surface oxide content of the samples measured. Accordingly, the irreversible capacity loss in FeSe2/GNS-400 is very low, thus showing a high Coulombic efficiency. 22845

DOI: 10.1021/acsami.8b06318 ACS Appl. Mater. Interfaces 2018, 10, 22841−22850

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ACS Applied Materials & Interfaces our previous work,22 the reversible capacities of GNS and iron oxide are ∼200 and 253 mAh/g, respectively. Therefore, on the basis of the mass content of each component (Table S1), it can be calculated that the capacity contributed by graphene and iron oxide are 37.4 and 0.2 mAh/g, so iron selenide contributes ∼92% of the capacity in FeSe2/GNS-400. Similarly, capacity contributions of iron oxide in the FeSe2/ GNS-300 and FeSe2/GNS-350 can be calculated to be ∼16.7 and 1.5 mAh/g, respectively. Therefore, the contributed capacities of iron selenide in the FeSe2/GNS-300 and FeSe2/ GNS-350 can also be up to as high as 75% and 90%, respectively. (Related calculation processes can be seen in the Supporting Information.) Therefore, we can completely ignore the capacity contribution of the surface oxide. Thus, it is reasonable to deduce that the surface oxide plays a great role in inhibiting the electrochemical activity of FeSe2. To verify this inference, FeSe2/GNS-400 was reoxidized in air at 100 °C for 1 h. Interestingly, the FeSe2 rods decompose again into particles with an average diameter of 100 nm after oxidation (Figure S7). The XRD pattern (Figure S1) confirms that diffraction peaks are still indexed to FeSe2, but the intensities of the peaks are obviously decreased, indicating a decrease in the particle size, which is consistent with the TEM investigation. However, the elemental mapping images indicate that the oxygen is distinctly distributed on the particle. In conjunction with the relevant XRD and XPS (Figures S1 and S8) analyses, we find an increase in surface oxides on the FeSe2/GNS-400 after treatment. Interestingly, the reversible capacity obviously decreases from 459 mAh/g before oxidation to 362 mAh/g after oxidation (Figure S9). In addition, the rate performance after oxidation is also poorer than that before oxidation. These results confirm that the electrochemical performance of FeSe2 is very sensitive to the surface oxide. Influence Mechanism of Surface Oxide on the Na-Ion Storage Behavior of FeSe2/GNS. The surface oxide does have an inhibitory effect on the electrochemical activity of FeSe2, but the mechanism has not been widely studied. To explore this, cyclic voltammetry (CV) measurement and ex situ TEM were carried out to reveal the influence of the surface oxide on the sodium-ion storage behavior. Figure 5e shows the CV curve of the FeSe2/GNS-400 during the first cycle. There are three reduction peaks at approximately 0.51/0.70/1.32 V and three oxidation peaks at approximately 1.52/2.04/2.32 V that can be observed in the first cycle of the CV curve, which is consistent with a previous report on an FeSe2 anode for SIBs.25 The three reduction peaks form in response to the generation of NaxFeSe2, FeSe, and Na2Se, as well as Fe0 and Na2Se. Among them, the reduction peak at 1.32 V is related to the formation of the SEI films due to the decomposition of the electrolyte.26,58 Meanwhile, the three oxidation peaks (at 1.52/ 2.04/2.32 V) form in response to the generation of FeSe, the intermediates NaxFeSe2, and the final reaction product FeSe2, respectively. An oxidation peak at approximately 1.8 V appears in the curve of FeSe2/GNS-300 (Figure 5a), which belongs to the oxidation reaction of the surface oxides.22 In Figure 5f, the second cycle and third cycle are highly overlapped, suggesting the good cycling stability of the electrodes. A distinct reduction peak at ∼1.8 V occurs, which is mainly caused by the formation of ultrafine FeSe2 nanocrystals after the first discharge/charge cycle, resulting in a shift in the reduction peak to a higher voltage.26 By comparing the CV curves of these three compounds, the influences of surface oxide on the electrochemical activity of

Figure 5. CV curves at a scan rate of 0.1 mV/s between 0.01 and 2.8 V vs Na+/Na for the first cycle (a, c, e) and the second and third cycle (b, d, f) of FeSe2/GNS-300, FeSe2/GNS-350, and FeSe2/GNS-400, respectively.

FeSe2 are clear. Beyond the first cycle, FeSe2/GNS-400 retains three reduction peaks at approximately 0.47/0.72/1.83 V and three oxidation peaks at approximately 1.53/2.04/2.32 V, and the potential range of the conversion reaction for FeSe2/GNS400 is ∼1.85 V (Figure 5f). For comparison, the FeSe2/GNS300 retains two reduction peaks at approximately 0.64/1.95 V and two oxidation peaks at approximately 1.53/2.04 V (Figure 5b), while FeSe2/GNS-350 retains two reduction peaks at approximately 0.72/1.83 V and three oxidation peaks at approximately 1.53/2.04/2.32 V (Figure 5d). The potential ranges of FeSe2/GNS-300 and FeSe2/GNS-350 are ∼1.40 and 1.60 V, respectively. It is obvious that the phase transformation of FeSe2/GNS-400 during the reactions occurs over a much broader potential range than the phase transformation of the other two composites, which is a clear indication of the lower stress/stain during the phase transformation of the FeSe2/ GNS-400, according to a previous report.59 The high phasetransformation resistances cause weaker redox current peaks for FeSe2/GNS-300 and FeSe2/GNS-350 than those for FeSe2/GNS-400.59 To understand the origins of the stress/strain and resistance, it is important to recognize how Na reacts with surface iron oxides and the iron selenide cores. As we know, Na reacts with Fe2O3 to form Na2O compounds and Fe0, while it reacts with FeSe2 to form Na2Se compounds and Fe0. The Fe−O and Na− O bonds in the sodiated oxide shell are much stronger than the Fe−Se and Na−Se bonds in the FeSe2 core due to their greater ionicity.60 Therefore, it is reasonable to deduce that a relatively strong sodiated oxide shell would mechanically limit the expansion of the FeSe2 core by inducing compressive stress to generate a phase-transition resistance, according to Cui et al.41 To further verify the inhibitory effect of the surface oxides on the electrochemical activity of FeSe2, ex situ TEM investigations were carried out to analyze the volume variations 22846

DOI: 10.1021/acsami.8b06318 ACS Appl. Mater. Interfaces 2018, 10, 22841−22850

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Figure 6. Effect of the oxide layer on the diameter expansion of iron selenide before and after the first sodiation at 0.5 V vs Na/Na+. TEM images of FeSe2/GNS-300 (a, b), FeSe2/GNS-350 (c, d), and FeSe2/GNS-400 (e, f) before and after sodiation at 0.5 V vs Na/Na+ and the corresponding diameter statistics.

∼15 nm. The FeSe2/GNS-350 initially possesses a similar particle diameter to that of FeSe2/GNS-300, but after the first sodiation, the average diameter of FeSe2/GNS-350 is as high as ∼40 nm, which is ∼2.7 times higher than its original size (Figure 6c and d). Compared to FeSe2/GNS-300 and FeSe2/ GNS-350, FeSe2/GNS-400 endures the largest volume variation. After sodiation, it changes from nanosized (50 nm) rods to large micron-sized (0.3 μm) agglomerates that is more than 6 times its initial sizes. (Figure 6e and f). The larger the variation in size for electrode materials is, the higher their sodiation degree is.41 Therefore, ex situ TEM analyses experimentally reveals that the surface oxide shell generates an inhibitory effect on the

before and after the first sodiation. First, three samples, namely, FeSe2/GNS-300, FeSe2/GNS-350, and FeSe2/GNS400, were dispersed on the Cu grids and imaged by TEM, and their sizes and structures were recorded. Second, the grids with the samples were inserted into electrochemical half cells with Na metal foils as the counter electrodes, and they were discharged from the open circuit voltage to 0.5 V vs Na/Na+ at 100 mA/g. Finally, the grids were removed and washed, and the sodiated samples were imaged again by TEM. Figure 6 shows the TEM images of the three samples as well as the corresponding diameter statistics of the FeSe2 before and after sodiation. Among them, the average diameter of FeSe2/GNS300 (Figure 6a and b) do not obviously change, remaining 22847

DOI: 10.1021/acsami.8b06318 ACS Appl. Mater. Interfaces 2018, 10, 22841−22850

Research Article

ACS Applied Materials & Interfaces

powder was purchased from Alfa Aesar Chemical Reagent Company. All of the chemicals used were AR grade. All the water used was deionized water (18.2 Ω·cm−1) Synthesis of Samples. The FeSe2/GNS composite was obtained through a selenization reaction of FeOX/GNS. FeOX/GNS composite was synthesized by a reaction between Fe(NO3)3 and NaBH4 under the condition of ice bath, followed by oxidation in the air at 250 °C for 3 h, as described in our previous work.22 In the experiment, 3.3 mmol of graphene (0.040 g) and 0.8 mmol of Fe(NO3)3·9H2O (0.336 g) were sonicated in 20 mL of ethanol with an atom ratio of 4:1 for 30 min with a sonicator at 80 W to form a homogeneous solution. After the alcohol was evaporated at 90 °C under the strong stirring, the mixture of Fe(NO3)3 and GNS was added to the solution of NaBH4 (1 M) to react for 4 h in an ice bath under the strong stirring. Then, amorphous iron oxide/GNS composites can be obtained by centrifugal separation and subsequent drying at 50 °C for 1 h under vacuum. To obtain pure iron oxide, amorphous iron oxide/ GNS composites were oxidized in further in the air at 250 °C for 3 h. The selenization of FeOX/GNS was carried out in a quartz-tube oven. FeOX/GNS (0.050 g) and 0.6 g of Se powders were loaded in a covered alumina boat and placed in the overn. Then, the FeSe2/GNS composites were obtained by annealing the FeOX/GNS composite in a Ar flow mixturing with 15% H2. For comparison, the FeSe2/GNS composites with various surface oxide can be prepared by adjusting the annealing temperature in a range from 300 to 400 °C and annealing time from 6 to 12 h. Structural and Physical Characterization. The morphology and structure of the obtained composites were characterized by scanning electron microscopy (SEM, ZEISS SUPRATM 55 field emission microscope). X-ray diffraction (XRD) was performed by a Rigaku D/max-2500B2+/PC system using Cu Kα radiation (λ = 1.5406 Å) over the range of 5−90° (2θ) at room temperature. Highangle annular dark-field scanning transmission electron microscopy (HAADF-TEM) images and corresponding high-resolution transmission electron microscopy (HRTEM) images were measured by a TECNAI G2 F30. Elemental dispersive spectroscopy (EDS) mappings were carried out on an electron microscope (TECNAI G2 F30) to determine the distribution of O, Se, and Fe in the composites. Thermogravimetric analysis (TGA) was measured by an STA449C produced by the Netzsch Company in Germany at a heating rate of 5 °C/min from room temperature to 900 °C. X-ray photoelectron energy spectra (XPS) were recorded using a twin anode Al Kα X-ray source with a 30 eV pass energy and a 0.5 eV step size over an area of 2 mm × 2 mm on the sample. Electrochemical Characterization. The electrochemical properties of the samples were characterized with a CR2025 type cell. Pure Na foil was used as the counter electrode, a glass fiber served as the separator, and one molar NaSO3CF3 solution in diglyme was used as the electrolyte. The working electrode was prepared by mixing 80 wt % of the active materials, 10 wt % of super P and 10 wt % of sodium carboxymethylcellulose (CMC) in distilled water to form a slurry, which was then spread onto the Cu foil and subsequently dried at 120 °C for 12 h under vacuum. The mass of the active materials in every electrode was controlled in the range of 1.0−1.5 mg. The cells were discharged and charged in a voltage window from 0.5 to 2.9 V using a Land battery tester (CT2001A, China) at room temperature. Cyclic voltammetry (CV) and electrochemical impedance spectral (EIS) measurements with a frequency range from 100 kHz to 10 mHz were tested on an electrochemical workstation (ZAHNER ZENNIUM).

electrochemical activity of the FeSe 2 core. A similar phenomenon was also observed in the antimony nanocrystals reported for Na-ion battery anodes.53 Kovalenko et al. found that the smallest Sb nanoparticles had the low capacity and inferior rate performance compared to the larger particles. They ascribed this phenomenon to the fact that Na2O, a poor conductor of Na ions, suppresses the reversible sodiation of the active core. The presence of a surface oxide has an inhibitory effect on many Na-ion active materials beyond the TMSes. The inhibitory effect of surface oxide on these materials is also quite complex. In addition to the phase-transformation resistance, a lower conductivity of Na2O for Na ions should also be considered, as described by Kovalenko et al.53 Na2O in the sodiated oxide shell should block the transfer of Na ions and electrons for subsequent sodiation, which would lead to electrode materials that are subjected to poor Na-ion storage kinetics. In this work, EIS analysis (Table S2) indicates that the value of the charge transfer resistance Rct is increased with the surface oxide content increasing. Notably, the Rct of FeSe2/ GNS-300 (25.91 Ω) is obviously higher than that of FeSe2/ GNS-400 (3.17 Ω). In other words, the surface oxide is an important factor in influencing the performance of TMSes and other Na-ion active materials. It is necessary to reduce or avoid the formation of surface oxides on these materials, especially for small-sized materials, which tend to be more easily oxidized.

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CONCLUSION In summary, FeSe2 uniformly anchored on graphene nanosheets was synthesized through the selenization of iron oxide to form high-performance anode materials for sodium-ion batteries. FeSe2 with various contents of surface oxides can be obtained by adjusting the selenization temperature and time. It is confirmed that surface oxides have an inhibitory effect on the electrochemical activity of FeSe2, resulting in a decrease in the capacity and rate performance. Therefore, by controlling the surface oxide content on FeSe2, a greatly improved capacity and superior rate performance can be achieved. Remarkably, the prepared FeSe2 nanorods/graphene composites with negligible surface oxide content can deliver a high capacity of 459 mAh/g and exhibit an obviously superior rate performance. The inhibitory effect of the surface oxides is attributed to the relatively strong sodiated oxide shell, which mechanically limits the expansion of the FeSe2 core by inducing compressive stress to generate a phase-transition resistance and suppresses the extent of sodiation, resulting in a decrease in the capacity and rate performance. However, the poor conductivity of the sodiated oxide shell would block the transfer of Na ions and electrons, leading to poor Na-ion storage kinetics of the electrode materials. Therefore, this work reveals an important factor in the enhancement of the performance of TMSes and other Na-ion active materials. The prevention of the formation of surface oxides on these materials is an efficient pathway toward designing advanced anode materials for high-performance Na-ion storage.

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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/acsami.8b06318.

EXPERIMENTAL SECTION

Materials. Iron(III) nitrate nonahydrate (Fe(NO3)3·9H2O, 98.5%) was purchased from Tianjin Guangfu Fine Chemical Industry Research Institute, China. Sodium borohydride (NaBH4, 98%) was purchased from Tianjin Hainachuan Science and Technology Development Company Ltd., China. Absolute alcohol (99.7%) was provided by Beijing Chemical Reagents Factory, China. Selenium

XRD patterns and TG curves of as-prepared of FeSe2/ GNS-300, FeSe 2 /GNS-350, FeSe 2 /GNS-400, and FeSe2/GNS-400 reoxidize; XPS survey spectra for the FeSe2/GNS-350 at different sputtering times from 0 to 22848

DOI: 10.1021/acsami.8b06318 ACS Appl. Mater. Interfaces 2018, 10, 22841−22850

Research Article

ACS Applied Materials & Interfaces

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100 s; XRD patterns of FeSe2/GNS selenized at 450 and 500 oC. TEM images of FeSe2/GNS-400 obtained by discharged/charged at 0.1, 0.5, 5, and 10 A/g after 2 cycles. CV curves of FeSe2/GNS-400 at different scan rates and log(i) vs log(v) plots at different redox states. Contribution ratio of the capacitive and diffusioncontrolled charge storage in FeSe2/GNS-400. SEM, TEM images, elemental distributions, XPS spectra, and electrochemical performances of FeSe2/GNS-400-reoxidize; relative mass percentage and contributed capacity of the FeSe2, Fe2O3, and GNS in samples; Randles equivalent circuit and the dynamical parameters of FeSe2/GNS-300, FeSe2/GNS-350, and FeSe2/GNS-400 after three cycles at 100 mA/g; and comparison of electrochemical performance of different iron selenide materials as anodes for sodium-ion batteries (PDF)

AUTHOR INFORMATION

Corresponding Authors

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

Jisheng Zhou: 0000-0003-4565-0835 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 (51572015 and 51272009), New Teachers’ Fund for Doctor Stations, Ministry of Education of China (20120010120004), and Foundation of Excellent Doctoral Dissertation of Beijing City (YB20121001001).

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