Air-Stable Porous Fe2N Encapsulated in Carbon Microboxes with

Aug 17, 2017 - (1-3) However, the energy and power densities of current commercial LiCoO2/graphite-based LIBs are insufficient, and the cost is still ...
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Letter pubs.acs.org/NanoLett

Air-Stable Porous Fe2N Encapsulated in Carbon Microboxes with High Volumetric Lithium Storage Capacity and a Long Cycle Life Yifan Dong,†,§ Bingliang Wang,§ Kangning Zhao,‡,§ Yanhao Yu,‡ Xudong Wang,‡ Liqiang Mai,§ and Song Jin*,† †

Department of Chemistry, ‡Department of Material Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706, United States § State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, P. R. China S Supporting Information *

ABSTRACT: The development of inexpensive electrode materials with a high volumetric capacity and long cycle-life is a central issue for large-scale lithium-ion batteries. Here, we report a nanostructured porous Fe2N anode fully encapsulated in carbon microboxes (Fe2N@C) prepared through a facile confined anion conversion from polymer coated Fe2O3 microcubes. The resulting carbon microboxes could not only protect the air-sensitive Fe2N from oxidation but also retain thin and stable SEI layer. The appropriate internal voids in the Fe2N cubes help to release the volume expansion during lithiation/delithiation processes, and Fe2N is kept inside the carbon microboxes without breaking the shell, resulting in a very low electrode volume expansion (the electrode thickness variation upon lithiation is ∼9%). Therefore, the Fe2N@C electrodes maintain high volumetric capacity (1030 mA h cm−3 based on the lithiation-state electrode volume) comparable to silicon anodes, stable cycling performance (a capacity retention of over 91% for 2500 cycles), and excellent rate performance. Kinetic analysis reveals that the Fe2N@C shows an enhanced contribution of capacitive charge mechanism and displays typical pseudocapacitive behavior. This work provides a new direction on designing and constructing nanostructured electrodes and protective layer for air unstable conversion materials for potential applications as a lithium-ion battery/capacitor electrode. KEYWORDS: Lithium-ion batteries, Fe2N, carbon encapsulation, high volumetric capacity, porous, conversion electrode

L

expansion, irreversible conversion process, large voltage hysteresis, and the unstable solid electrolyte interphase (SEI) layer during cycling processes. Such naturally formed SEI is usually fragile and easily breaks off and forms fluffy debris as the active content expands and contracts during lithiation and delithiation. This results in continuous consumption of the electrolyte and cyclable (“live”) lithium, and full-cell batteries with the high capacity conversion anodes often die quickly because of Li- and/or solvent exhaustion.15−17 Therefore, ensuring SEI stability at the electrolyte−electrode interface is critical for long-life lithium full cells. Among the various conversion electrode materials, earthabundant and inexpensive iron nitrides (Fe2N and Fe3N) have relatively low molecular weight and can transfer 2−3 electrons per formula unit as anode materials, resulting in a theoretical capacity up to 900 mA h g−1, 3 times larger than that of graphite.18−20 Metal nitrides, including iron nitrides and especially Li3N, are also well-known for the high Li ion diffusion due to the Li vacancies within their crystal structures.20,21

ithium ion batteries (LIBs) have dominated the power market of portable electronics due to the long cycle life, high capacity, high energy efficiency, and low self-discharge properties.1−3 However, the energy and power densities of current commercial LiCoO2/graphite-based LIBs are insufficient, and the cost is still too high for applications in the large-scale energy storage technologies (e.g., stationary and vehicle applications).2,4 The energy density of a battery electrode can be calculated following the equation:5 E = nFE o /M

(1)

where E is the energy density, n refers to the number of electrons transferred per formula unit of reactants, F is the Faraday constant, Eo is the electrochemical potential of the cell reaction, and M is the formula weight of the active materials. Accordingly, a higher energy density can be expected if more electrons are transferred per formula unit of the active material. The discovery of reversible multiple-lithium storage in metal oxide/fluoride conversion electrodes in the early 2000s opens up promising opportunities for high-energy-density and high volumetric capacity storage9−13 that does not depend on available interstitial sites in the intercalation electrodes. Instead, it is realized through heterogeneous conversion reactions.11,13,14 However, their practical applications have been deterred by the large volume © 2017 American Chemical Society

Received: June 26, 2017 Revised: August 8, 2017 Published: August 17, 2017 5740

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is kept inside the carbon microcubes without breaking, resulting in a very low electrode volume expansion with ∼9% of the electrode thickness variation upon lithiation, much reduced from that of the Fe2N electrode (∼90%). Therefore, the Fe2N@C electrodes maintain a high volumetric capacity (1030 mA h cm−3 based on the lithiation-state electrode volume) comparable to silicon anodes,8,15,31 stable cycling performance (at 10 A g−1 with a capacity retention of over 91% for 2500 cycles), and excellent rate performance. It is also found that the kinetic of Fe2N@C electrode is governed by a pseudocapacitive behavior and the capacity contribution is mainly controlled by the capacitive behavior. Our design provides further insights in achieving high volumetric capacity and novel strategy to protect air unstable conversion electrode materials for potential applications as lithium-ion battery/capacitor electrode. The Fe2N@C microcubes were synthesized through an anion conversion from Fe2O3 microcubes in ammonia atmosphere. Figure 1a illustrates the synthetic process of the Fe2N@C microcubes. First, uniform Fe2O3 microcubes (Figure S1a and b) with an average size of 587 (±42) nm (Figure S2a) were synthesized through a facile and scalable hydrothermal method (see Materials and Methods in the Supporting Information for details) and used as the precursor. Then, the as-prepared Fe2O3 microcubes were directly coated with a conformal polydopamine (PDA) layer through the in situ polymerization of dopamine in a Tris buffer solution, forming a Fe2O3@PDA core@shell microcube structure (Figure S1c and d). These Fe2O3@PDA microcubes were then annealed in an NH3 atmosphere at 500 °C to enable the anion conversion38 from O2− to N3− (Figure 1c) as well as the carbonization of PDA. Because of the large volume contraction during the conversion of Fe2O3 crystal structure (unit cell volume: 300.6 Å3, density: 5.18 g cm−3) to Fe2N (unit cell volume: 87.2 Å3, density: 7.14 g cm−3), pores were readily formed between the carbon shell and the Fe2N core, as well as inside the Fe2N core. Figure 2a shows the SEM image of the Fe2N@C products, where uniform microcubes are observed with no other impurities

Therefore, they are promising for achieving high rate performance and as fast ion conductors for the emerging all solid-state Liion batteries.22 Furthermore, Fe2N could possibly exhibit pseudocapacitive behavior23,24 (i.e., ultrafast charge transfer from surface/subsurface regions) owing to the stable phase change.25 This provides an outstanding platform for simultaneously accomplishing both high energy output and large power density.1,26,27 The main limitation of the Fe2N electrode is the short cycling lifetime. Intensive lithium ion insertion induces a huge volume change, which gives rise to the pulverization of electrode as well as the frequent formation of insulating SEI layer.28−30 As a consequence, fast capacity fading is often observed. Moreover, Fe2N (and metal nitrides in general) could be easily oxidized in air to form an oxide layer on the surface that hinders Li transport, raising additional challenge for the manufacturing process. These issues severely hinder the practical applications of iron nitrides as battery electrode materials. Hollow carbon confinement has shown great promise in improving the cycling stability issue of silicon anodes and conversion electrode materials by effectively accommodating the electrode volume change and achieving a thin and stable SEI layer.30−35 However, the large void space will seriously decrease the volumetric specific capacity of anodes.5−8,36 The volumetric specific capacity of electrodes is perhaps even more important than the gravimetric specific capacity for many applications, especially large-scale automobile applications, because the space is often more limiting than the weight of the batteries.5−8 A high volumetric performance requires the combination of highperformance electrode materials with high density and an effective configuration of the different parts involved. Unfortunately, the current volumetric performance of different types of electric energy storage devices is still not satisfactory for practical applications.37 Therefore, the rational design of electrodes to ensure both high volumetric specific capacity and the structure stability is crucial.37 Herein, we report a successful design of air-stable porous Fe2N microcubes confined in the nitrogen-doped carbon microcubes (denoted as Fe2N@C) through a facile confined anion conversion method, as illustrated in Figure 1a. In our design, the resulting carbon microboxes could not only protect the airsensitive Fe2N from oxidation and keep it stable for over one month in the air but also retain a thin and stable SEI layer. The internal voids in the Fe2N cubes help to release the volume expansion during lithiation/delithiation processes, and the Fe2N

Figure 2. (a, b) SEM images and (c) low-resolution and (d) highresolution TEM images with electron diffraction pattern (inset in d), (e) EDS mapping of Fe2N@C.

or broken carbon shell. There was only a very slight increase in average size from the 593 (±50) nm for the PDA coated Fe2O3 microcubes to 597 (±49) nm (Figure S2). A further magnified SEM image in Figure 2b shows the transparent nature of carbon shell indicating the thin and conformal coating of PDA-derived carbon. To better characterize the inside core structure, the Fe2N@C microcubes are subjected to focused ion beam (FIB) milling to break the carbon shell and expose the inner core (Figure S3). The core is found to be microporous with a pore

Figure 1. (a) Schematic illustration of the formation process of Fe2N@ C microcubes. (b) Fe2N@C can resist the oxidation in the air and maintain high phase purity. (c) Crystal structures from Fe2O3 to Fe2N through the anion conversion from O2− to N3−. Red, blue, and black spheres represent Fe, O, and N atoms, respectively. 5741

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Figure 3. (a) PXRD patterns of Fe2N@C and Fe2N (* mark the peaks from Fe3O4). Mössbauer spectra of (b) Fe2N@C and (c) Fe2N at room temperature. High-esolution (d) N 1s XPS and (e) C 1s XPS spectra of Fe2N@C. (f) Fe 2p XPS spectra of Fe2N@C and Fe2N.

diameter of ∼30 nm. The TEM image (Figure 2c) reveals the vast contrast between the Fe2N core and the carbon shell, confirming the uniform and thin carbon coating. The inner particles remain cube-like structure but become segregated from the outer shell with pores that formed between the core and shell, as well as inside the core. Each particle is detached from each other with no apparent aggregation. The sharp contrast within the cores in the TEM image (Figure 2c) also confirms the porous nature of the inner core. High-resolution TEM reveals the carbon shell thickness is ∼6 nm. The SAED pattern (inset of Figure 2d) reveals the single-crystalline nature of the inside core. Further, the EDS mapping (Figure 2e) shows the uniform distribution of Fe, N, C, and a small amount of O elements in the Fe2N@C microcubes, confirming the conformal carbon coating nature on iron nitride cores. We also synthesized Fe2N microcubes without carbon shell using similar anion conversion reaction (see Materials and Methods in the Supporting Information for details) as a comparison (Figure S4). To probe the crystal structures, powder X-ray diffraction (PXRD) patterns of the Fe2N and Fe2N@C samples were recorded and presented in Figure 3a. Three major sharp and characteristic peaks at 40.2, 42.9, and 56.6° were observed for the Fe2N@C sample. These peaks can be assigned to the (0 0 2), (1̅ 1̅ 1), and (1̅ 1̅ 2) diffraction peaks of ideal εFe2N (JCPDS Card No. 01-072-2126, space group P3m1, a = 2.76 Å, c = 4.41 Å). However, they are also similar to those from the ideal ε-Fe3N (JCPDS Card No. 00-049-1663, space group P6322, a = 2.695 Å, c = 4.362 Å). The very similar lattice constants make it difficult to distinguish the ε-Fe3N and ε-Fe2N phases according to the diffraction patterns alone (Figure S5).31 Both ε-Fe3N and ε-Fe2N structures result from the different nitrogen arrangements on the hexagonal close-packed (HCP) octahedral interstices.32,33 Such structures with a large number of vacancies can have a range of stoichiometry under the same nominal formula of ε-Fe3N due to the variable occupancy of the N sites.31 Thus, the ε-Fe3N phase also crystallizes in the HCP crystal structure with the space group P6322, or P312 with two formula units. There is also an orthorhombic ξ-Fe2N phase, which only exists at high temperature (over 600 °C) or high pressure32 and can be eliminated by PXRD in our case.39−43 In addition, it is well-known that the metal nitrides are easily oxidized even upon mere exposure to air by forming surface

oxides.18,44 For the Fe2N without carbon protection, two peaks at 34.6° and 62.6° corresponding to Fe3O4 (JCPDS Card No. 00001-1111, space group Fd3̅m, a = 8.37 Å) emerge, suggesting the surface oxidation of Fe2N (Figure 3a). To further identify the exact phase as well as the oxidation product accurately, we used Mössbauer spectrometry to study the different types of Fe environments in Fe2N@C and Fe2N samples (Figure 3b and c, respectively). At room temperature, the spin relaxation time of superparamagnetic nanoparticles is in the order of 10−11 to 10−12 s, much shorter than the nuclear sensing time (10−8 s), and the sextet collapses into a singlet or doublet, and thus, the Fe2N@C spectrum shows only a doublet indicating the superparamagnetic nature.42 The spectral areas of the two doublets are related to the relative ratios of the two iron sites. Thus, the pure paramagnetic spectrum of ζ-Fe2N1−z has to be fitted with two subspectra for the Fe−III and Fe−II sites.45 The formation of Fe−III sites is associated with the additional nitrogen neighbors of iron which cause an increase of the isomer shift. For Fe2N@C, the peak area ratio of Fe−III and Fe−II sites is 30.4 : 62.3, indicating that z is 0.23 and the formula is Fe2N0.77. Additionally, only a minor impurity of oxidation product (Fe3O4) is observed (below 2%) by calculating the area of other minor peaks. Note that the Fe2N@C sample for the spectrum was left in air at room temperature for one month before data collection, which confirms the excellent stability in air. In contrast, the Fe2N sample without carbon shell displays a spectrum that is split into seven peaks (Figure 3c), which suggests multiple phases are in the sample. The proportion of oxidation product of the Fe2N reached almost 50%. These results further shows the excellent air stability of Fe2N@C. The surface chemistry of the Fe2N@C and Fe2N samples was further studied by X-ray photoemission spectroscopy (XPS). As shown in Figure S6a, typical C, N, O, and Fe signals were all detected in Fe2N@C and Fe2N samples. Compared with pristine Fe2N, Fe2N@C has stronger C and N peaks and weaker O peak, indicating the conformal carbon coating. Three types of nitrogen were identified after deconvoluting the high-resolution N 1s XPS spectrum of Fe2N@C (Figure 3d), namely, pyridinic type at 398.3 eV, pyrrolic type at 400.1 eV, and graphitic at 401.0 eV.46,47 The majority (78%) of nitrogen in Fe2N@C belongs to the pyridinic type, which could facilitate the Li-ion intercalation into such carbon layers.47 The carbon XPS spectrum (Figure 3e) is 5742

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Figure 4. CV curves of (a) Fe2N@C and (b) Fe2N electrodes in the first 3 cycles at 0.1 mV s−1. (c) Rate performance of Fe2N@C and Fe2N. Note the gravimetric specific capacity is calculated based on active materials. (d) b-value vs potential curves for Fe2N@C and Fe2N, the capacitive contribution by currents of (e) Fe2N@C and (f) Fe2N.

Fe2N + 3Li+ + 3e− ↔ 2Fe + Li3N

then deconvoluted and is mainly composed of graphitic carbon with a few oxygen functional groups, which suggests some oxidation on the surface of the carbon microboxes and also give rise to the weak O 1s peak (Figure S6a) and weak oxygen EDS signal seen in Figure 2e. It should be noted that the C−N accounts for 22% of carbon. The Fe XPS spectrum for Fe2N shows that the binding energies of Fe 2p3/2 and Fe 2p1/2 are 711.8 and 725.2 eV,48 indicating the formation of Fe3O4 on the surface, and only a small shoulder peak is suspected to be associated with iron nitride for Fe2N (Figure 3f). It should be noted that no XPS peaks associated with Fe are observed for Fe2N@C, indicating the microcube surface is completely covered by carbon and no opening of the carbon shell can be detected, which further suggests the conformal coating nature of the carbon shell. To determine the carbon content, thermogravimetric analysis (TGA) of Fe2N@C was carried out in air at a heating rate of 5 °C/min, showing the remaining mass percentage of Fe2O3 at about 117% (Figure S7). Considering that Fe:N ratio is 2:0.77, the amount of carbon for the Fe2N@C sample is estimated to be 10 wt %. The electrochemical performance of the Fe2N particles and porous Fe2N@C microcubes was investigated by assembling them into CR2016 coin cells with lithium foil as the counter electrode and cycling the cells with a cutoff voltage window of 0.01−3.0 V (see details in Materials and Methods in the Supporting Information). According to the previous reports,25,29 Fe2N undergoes the conversion reaction based the reaction below:

(2)

Figure 4a and b shows the comparison of the CV curves for porous Fe2N@C microcubes and Fe2N particles, respectively. The peak areas in the CV curve of porous Fe2N@C microcubes are higher than those of Fe2N particles, indicating the higher capacity of Fe2N@C microcubes. The CV curves of Fe2N@C microcubes in the following two cycles overlap quite well indicating the excellent reversibility. In contrast, the peak currents for Fe2N keep decreasing in the following two cycles, suggesting inferior reversibility. Additionally, the rate performance was evaluated. The Fe2N@C microcubes display an excellent rate capability and present approximately a reversible capacity of 567, 536, 526, 500, 474, 450, 404, and 356 mA h g−1 at different current densities of 0.1, 0.2, 0.5, 1.0, 2.0, 5.0, and 10.0 A g−1, respectively (Figure 4c). Furthermore, upon reducing the current density back to 0.1 A g−1, the electrode delivers a specific discharge capacity of about 573 mA h g−1, which is a retention of 99.6% and shows very little capacity decay. Clearly, this result indicates that the Fe2N@C microcubes can maintain higher capacities at high current densities over Fe2N in addition to exhibiting a good recovery performance, which is the necessary electrode characteristic for realizing high-power LIBs. To examine the possible reasons for the improved rate capability, the electrochemical kinetics was investigated. Both CV curves at different scan rates after first cycle (Figure S8) show a pair of broad peaks with increasing scan rate. Noticeably, both the cathodic and the anodic peaks shift merely with increasing 5743

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Figure 5. (a) Cycling stability of Fe2N@C and Fe2N electrodes at 100 mA g−1, (b) charge−discharge curves of Fe2N@C and Fe2N in the first three cycles, (c) long-term cycling performance for Fe2N@C at 10 A g−1, insets are the SEM images of the microcubes after 50 cycles and 2500 cycles. Note that the results are presented as volumetric capacities here. (d) Schematic illustrations of the different behaviors of Fe2N and Fe2N@C electrodes during electrochemical lithiation/delithiation. The yellow coating represents the SEI layer.

the current is divided into diffusion controlled and capacitive controlled through the following equation:23,24

scan rate, especially for the Fe2N@C microcubes, which is one of the typical features of pseudocapacitive material. Thus, to provide more insight about the charge storage mechanism in the Fe2N@C microcubes, we investigated the kinetics of the Fe2N materials using analytical approaches that provide quantitative information about differences between diffusion controlled and capacitive charge storage processes. A related analysis can be performed regarding the behavior of the peak current by assuming the current, i obeys a power-law relationship with the sweep rate (i = aνb),49 where a is a constant and ν is the sweep rate. The b-value in the above equation is therefore an estimate of the type of charge storage occurring in the material: if b is 0.5, the current is diffusion-controlled; if b is 1, the current is capacitive in nature. As shown in Figure 4d, the b-value analysis was performed for both samples in comparison using the CV data between 0.5 and 2.5 V (vs. Li/Li+). In the whole voltage range, the b-value of Fe2N@C approaches 1, even in the catholic peak area (∼1.4 V) of the Fe2N@C, and is always higher than that of Fe2N. Specifically, at the cathodic peak area (∼1.6 V) of Fe2N, the b-value extends toward 0.5, while the value of the Fe2N@C is around 0.9, indicating that the charge storage behavior of Fe2N@ C is mostly dominated by capacitive behavior which is quite different from Fe2N. Thus, to further determine the potential regions where the capacitive contributions occur in the CV plots,

i = k1v + k 2v1/2

(3)

Solving k1 and k2 gives the capacitive and diffusion contributions to the current. For both Fe2N@C and Fe2N, the amount of charge stored due to both diffusion and surface capacitive limited processes at 1 mV s−1 is presented in Figure 4e and f, respectively. The best capacitive materials have minimal diffusion contributions even at slow sweep rates, and that is what we observe here for Fe2N@C. Around 83% of the total current or the capacity originates from capacitive behavior (the shaded region in Figure 4e). Even in the peak region the capacity comes mostly from capacitive behavior, which is consistent with the result of b-value-potential plot. In contrast, merely 35% capacity of the Fe2N electrode is controlled by capacitive behavior, and the majority is diffusion controlled, especially in the peak region (Figure 4f). Clearly the carbon microboxes are critical for the pseudocapacitive behavior observed for the Fe2N@C samples. To study these nonintercalation electrodes (including alloying and conversion type electrodes) in detail, the electrode thickness of the pristine, lithiated electrodes (discharged to 0.01 V), and delithiated electrodes (charged to 3 V) is evaluated (Figure S9). The Fe2N@C electrode shows a much smaller thickness increase (from 11 to 12 μm, ∼9%) compared to Fe2N electrode (from 10 5744

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the carbon shell shows some flexibility in adapting to the large volume expansion. This phenomena is quite consistent with the SEM result in Figure S14 where there are a lot of humps in the carbon shell. On the other side, Figure S13c shows the Region 2 where the carbon layer is much smoother. The ex situ SEM images of the Fe2N@C microcubes after different cycles (50 and 2500 cycles) are shown in the inset of Figure 5c. Furthermore, the SEM image and EDS mapping of Fe2N@C microcubes at 50 and 2500 cycles in Figures S14 and S15 confirms that Fe2N remain stable inside the carbon microcube. In contrast, the SEM image and EDS mapping of the Fe2N particles at 50 and 2500 cycles in Figures S16 and S17 show that Fe2N are confined in an amorphous layer which is suspected to be the SEI layer. Clearly, the Fe2N@C microcube structure is retained quite well with no obvious increase in size. Therefore, we can conclude that the Fe2N@C microcubes were able to withstand the repeated volume expansion and contraction and remain intact during the long-term cycling. This suggests that, as illustrated in Figure 5d, the stable porous core−shell structure with an adequate internal space and good mechanical stability can effectively buffer the internal stress induced by volume variation of the electrode materials during cycling while maintaining a high volumetric capacity. Furthermore, the SEI layer is now formed outside of the carbon shell of the microcubes and will not be damaged during the cycling. Together with the free space design to accommodate the volume expansion of Fe2N, this synergistic effect allows these Fe2N@C microcube structures to successfully overcome the two major problems of conversion-type anodes, the instability of the SEI layer and the mechanical degradation caused by the large volume changes without significant sacrifice of the volumetric capacity. In conclusion, we have developed a facile confined anion conversion method to obtain nanostructured porous Fe2N@C microcubes. The Fe2N@C microcubes can resist the oxidation and keep Fe2N stable in the air. The well-designed Fe2N@C electrode displays very little volume expansion (∼9%) due to the readily formed confined space. An appropriate amount of internal voids in the Fe2N cubes is just enough to release the volume expansion during lithiation/delithiation processes. In this way, a high volumetric capacity of 1030 mA h cm−3 (based on the lithiation electrode volume) is still achieved at a current density of 100 mA g−1, which is comparable to silicon anodes; at the same time, excellent high-rate cyclability is also obtained with a capacity retention of 91% after 2500 cycles even at 10 A g−1, which is rarely achieved among metal nitride electrodes. Kinetic analysis reveals that the Fe 2 N@C shows an enhanced contribution of capacitive charge mechanism and displays typical pseudocapacitive behavior, enabling the high rate performance for Fe2N@C microcube electrode. Our work not only overcomes the major challenges facing iron nitride conversion electrode materials but also provides a new and general direction on designing nanostructured electrodes with protective layer for airunstable conversion type electrode materials with high volumetric storage capacity, high rate capabilities, and cyclability.

to 19 μm, ∼90%). Additionally, upon delithiation process, the Fe2N electrode thickness only contracted a little from 19 to 17 μm, still much thicker than that of Fe2N@C electrode (∼11.5 μm). To more accurately represent and evaluate the electrode capacity, especially for these nonintercalation electrodes, we further use the volumetric capacity based on the lithiation-state electrode volume (highest thickness/volume during cycling) to represent the electrochemical performance (see Materials and Methods in the Supporting Information for the details of this calculation).8 We further evaluated the cycling stability of both samples at a lower current density of 100 mA g−1 (Figure 5a). The Fe2N@C microcubes deliver a higher volumetric capacity of 1295 mA h cm−3 with a sloping plateau at around 0.7 V than that of Fe2N particles (489 mA h cm−3, 0.6 V) in Figure 5b. The initial charge capacity of Fe2N@C microcubes is decreased to 822 mA h cm−3 after one cycle, corresponding to the first Coulombic efficiency (CE) of 74.5%, which is attributed to the likely irreversible formation of the initial solid electrolyte interphase layer on the surface, especially for the high surface area carbon. A clear difference in CEs after the following cycles for Fe2N@C microcube and Fe2N is shown in a magnified view in Figure S10. The CEs of Fe2N@C microcubes remain stable well above 99.0% after 15 cycles. In contrast, for Fe2N, the CEs are always lower, below 99.0% (Figure S10) indicating the instable SEI layer. After 50 cycles, Fe2N@C microcubes retained a volumetric capacity of 1030 mA h cm−3 which is comparable to silicon anode (see comparison in Figure S11), while Fe2N particles deliver a much lower capacity of 256 mA h cm−3. Furthermore, the electrochemical impedance spectra (EIS) of the Fe2N@C microcubes and Fe2N particles show a compressed semicircle from the high to medium frequency range of each spectrum, and a line inclined at approximately 45° in the low-frequency range (Figure S12). The compressed semicircle describes the charge transfer resistance (Rct) for these electrodes, and the inclined line is considered as Warburg impedance (ZW). In the equivalent circuit, Rs represents the Ohmic resistance of the electrode system, including the electrolyte and the cell components. Rct represents the resistance related to charge transfer, and CPE and Zw are the capacitance related to double layer, and Warburg impedance, respectively. The Rs values of the Fe2N@C microcubes and Fe2N particles are almost the same, while the Rct value of Fe2N@C microcubes (78 Ω) is lower than that of the Fe2N particles (235 Ω), suggesting an enhanced electrode kinetics. We have further evaluated the long-term cycling performance of the Fe2N@C microcubes at a very high current density of 10.0 A g−1 (Figure 5c). The Fe2N@C microcubes can still retain a reversible capacity of 555 mA h cm−3 after 2500 cycles with an average CE of around 98% which is rarely achieved among metal nitride electrodes (Table S1). The lithium storage properties of these Fe2N@C microcubes in terms of cycling stability and rate capability are superior to those of the reported iron nitride-based anodes. Ex situ SEM and TEM of the electrode materials after cycling were carried out to better understand the superior cycling behaviors. The TEM images of the Fe2N@C microcubes after lithiation to 0.01 V (Figure S13) show that the contrast in the microcube mostly disappears, indicating that internal voids as well as the voids between the shell and core disappeared, and it is very hard to distinguish the boundary between carbon shell and Fe2N core. The boundary is quite different on the two sides of the microcube and can be divided into two regions. Figure S13b shows the Region 1 where the carbon layer is not smooth anymore and large humps can be observed. This indicates that

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.7b02698. Details of the materials and methods, additional structural characterization of the products and electrodes, electro5745

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chemical characterization, EIS spectra, and comparison of electrode performance (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (S.J.). ORCID

Yifan Dong: 0000-0002-6431-7598 Kangning Zhao: 0000-0003-2916-4386 Xudong Wang: 0000-0002-9762-6792 Liqiang Mai: 0000-0003-4259-7725 Song Jin: 0000-0001-8693-7010 Author Contributions

Y.D., B.W., and K.Z. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported the National Science Foundation under grant no. DMR-1508558 to S.J. and by the State Key Laboratory of Advanced Technology for Materials and Processing at Wuhan University of Technology (2017-KF-3) to L.M. and S. J. Y.D. acknowledges the support from the China Scholarship Council. L.M. thanks the National Natural Science Foundation of China (51521001) and the National Natural Science Fund for Distinguished Young Scholars (51425204) for support.

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DOI: 10.1021/acs.nanolett.7b02698 Nano Lett. 2017, 17, 5740−5746