Nitrogen and Phosphorus Dual-Doped Graphene Aerogel Confined

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Nitrogen and phosphorous dual-doped graphene aerogel confined monodisperse iron phosphide nanodots as an ultrafast and long-term cycling anode material for sodium-ion batteries Yaping Wang, Qi Fu, Chuan Li, Huanhuan Li, and Hua Tang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03561 • Publication Date (Web): 15 Oct 2018 Downloaded from http://pubs.acs.org on October 17, 2018

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ACS Sustainable Chemistry & Engineering

Nitrogen and phosphorous dual-doped graphene aerogel confined monodisperse iron phosphide nanodots as an ultrafast and long-term cycling anode material for sodium-ion batteries Yaping Wang,†,‡ Qi Fu,† Chuan Li,† Huanhuan Li,*,§and Hua Tang*,† †

School of Material Science & Engineering, Jiangsu University, 301 Xuefu Road, Jingkou District,

Zhenjiang 212013, P. R. China ‡

Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Nankai

University, 94 Weijin Road, Nankai District, Tianjin 300071, P. R. China §

Automotive Engineering Research Institute, Jiangsu University, 301 Xuefu Road, Jingkou

District, Zhenjiang 212013, P. R. China AUTHOR INFORMATION Corresponding Author * [email protected] * [email protected]

ACS Paragon Plus Environment

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ABSTRACT: Transition metal phosphides have recently gained much interest as anodes for sodium-ion batteries (SIBs). However, their intrinsic volume change during Na ion uptake/release leads to poor cycling stability and limited rate performance. To solve this problem, a unique hybrid architecture of iron phosphide nanodots bound on 3D phosphorus-doped graphitic nitrogen-rich graphene (FeP/NPG) is obtained from the phosphidation of NH2-rich reduced graphene oxide (rGO) decorated Fe2O3. Monodispersed FeP nanodots integrating with 3D NPG networks and high content of graphitic N not only induce fast Na ion/electron transfer kinetic and excellent structural stability during long-term cycling, but also enhance the capacitive contribution. These features of FeP/NPG result in high-performance sodium storage. A high reversible capacity of 613 mAh g -1 is achieved at 50 mA g-1. Also, excellent rate capability of 422 mAh g-1 and 349 mAh g-1 is observed at 1 A g-1 and 3 A g-1, respectively. More importantly, an ultra-stable capacity of 378 mAh g-1 at 1 A g-1 can be obtained upon long-term cycling. It shall be possible to extend this strategy for fabricating other transition metal phosphide based anodes for advanced SIBs.

KEYWORDS: sodium ion batteries, anode materials, iron phosphide, N,P-codoped graphene

INTRODUCTION It is widely regarded that sodium-ion batteries (SIBs) are the ideal alternatives to lithium-ion batteries (LIBs), especially for large-scale energy storage, due to the earth abundance and low cost of sodium sources as well as the similarities between Na and Li ions such as chemical properties and electrochemical behaviors in electrode materials.1-10 Unfortunately, because the radius of Na ion is much larger than that of Li ion, greater volume variation and lower specific capacity would occur in SIBs than in LIBs when analogous electrode materials are employed. 11-15 Indeed, some


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attractive electrode materials in LIBs such as graphite anode exhibit extremely poor sodium storage performance.16 Thus, it is an essential ingredient to develop novel electrode materials with high charge/discharge capacity and good cycling stability. In recent years, phosphorus has drawn much attention as an anode material for SIBs on account of its high theoretical specific capacity (2596 mAh g-1) and relatively low redox potential (~0.4 V vs Na/Na+).17-24 Nevertheless, its low electrical conductivity (~10 -14 S cm-1) and dramatic volume expansion (~490 %)25 during Na ion uptake/release lead to poor cycling stability and limited rate performance. Combining P with conductive transition metals to form transition metal phosphides (TMPs) has proved to be a feasible approach for improving the electrical conductivity and reducing the volume change.26-29 However, the volume expansion has not been completely overcome and the diffusion kinetic of Na ions in TMPs is relatively poor.26-29 Further investigations proposed that integrating nanostructured TMPs with a carbonaceous matrix could effectively enhance sodium storage, which may benefit from shortened diffusion pathway of Na ions, alleviated volume change, and ameliorated electron transport through conductive carbon networks. 30-37 For example, the porous reduced graphene oxide (rGO)@CoP@FeP composite reported by Yin et al.35 showed excellent cycling stability with a capacity of 456.2 mAh g-1 after 200 cycles, together with good rate capability. MoP/carbon nanorods prepared by Ji et al. 36 exhibited outstanding long-term cycling performance and attractive rate capability. Even so, TMP-based materials with satisfactory electrochemical performance have rarely been achieved up to now. 30-37 Introduction of heteroatoms such as N, P, S and B into carbonaceous materials has attracted great interest for enhancing the sodium storage performance over the past several years.38-48 In this respect, the most commonly used heteroatom is N, which is feasible for more Na ions insertion from the introduced “topological defects”.41-43 Moreover, the diverse doping types of N can tuning


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the electronic structure for fast insertion/extraction of Na ions and transport of electrons.41-43 For example, pyridinic-N doping can generate individual or triple N pyridinic vacancies in a planar structure of graphene, which are useful for improving the capacity and the rate capability. 41-43 Density functional theory (DFT) computations revealed that graphitic-N doping can upshift the Fermi level in graphene, and thus can greatly enhance the electrical conductivity to achieve fascinating high-rate performance.49 However, the controllable N-doping of carbonaceous materials towards high performance sodium storage is always a challenging task. Another attractive heteroatom is P, which can introduce so-called “protrusions” to enlarge the interlayer distance and surface area for its much larger radius compared with C, as well as to boost electronic properties of carbonaceous materials for its lower electronegativity relative to C. 44-46 However, owing to the large difference between P and C, the doping amount of P in carbonaceous materials is relatively low, resulting in the limited effect of tuning the electrochemical performance. 44-46 In the light of the synergistic effect of multiple heteroatom doping, N and P dual-doping is a practicable way for significantly enhancing the electrochemical performance of carbonaceous materials.50, 51 A typical example is N,P-codoped carbon microspheres reported by Li et al., 50 which delivered high capacity, promising cycling stability and rate capability. Undoubtedly, those features for heteroatom doped carbonaceous materials are also beneficial for improving sodium storage of their composites.52, 53 For instance, MoSe2/N,P-codoped carbon nanosheets prepared by Yang et al.53 could facilitate the insertion/extraction of Na ions. It is deduced that N,P-codoped carbonaceous materials are the perfect decorations for TMP-based anodes, not only for their promising electrochemical activity, but also for the possibility of simultaneously achieving Pdoping and TMPs in a single synthetic process. For example, Ni2P/N,P-codoped carbon nanosheet composite prepared by Zhao et al.54 showed good cycling stability and rate capability. Nonetheless,


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there have been few reports on N,P-codoped carbon decorated TMPs with high-performance for SIBs till now.54 Graphene aerogel (GA) is a 3D architecture constructed from plenty of graphene sheets. The high specific surface area and interconnected structure of GA not only guarantee the effective absorption of metal ions55, but also favor fast transfer of Na ions and electrons56-58. Herein, we report an efficient synthetic strategy for fabricating 3D N,P-codoped graphene (NPG) confined iron phosphide nanodots via phosphidation of NH2-rich rGO supported Fe2O3. There are two advantages in this unique architecture. First, the uniform FeP nanodots chemically bonded on NPG could shorten the Na ion diffusion distance and suppress the aggregation of FeP upon repeated Na ion insertion/extraction. Second, 3D NPG with high graphitic-N contents could ensure more Na ions insertion and the fast Na ion and electron transport. As a result, the obtained FeP/NPG exhibits unprecedented sodium storage performance in terms of high capacity, impressive rate performance and long-term cycling stability.

EXPERIMENTAL Materials Preparation. Fe2O3/rGO was firstly synthesized as the precursor. Typically, 60 mg FeCl3•6H2O (Alfa Aesar, 97%) was slowly added into 20 ml aqueous graphene oxide suspension which contained 30 mg graphene oxide (GO, Suzhou TANFENG Tech Co., Ltd) by ultrasonication for 1 h. Subsequently, 40 μl ethylenediamine (EDA, Sinopharm Chemical Reagent Co., Ltd, 99%) was injected into the above suspension under ultrasonication for 30 min.59 The mixture was then transferred into a 50 ml glass vial, and heated at 90 0C for 10 h. After cooling, the resulted hydrogel was washed thoroughly with deionized water and freeze-dried overnight.


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To prepare FeP/NPG, 50 mg Fe2O3/rGO was subjected to phosphidation by 250 mg NaH2PO2•H2O (Sigma-Aldrich, 99%) with 2 h heating at 300 0C under Ar atmosphere.60, 61. FeP/NPG was collected after cooling. Similar procedure was employed to obtain those samples for comparison. For other FeP/N,Pcodoped graphene nanocomposites, the only change was the mass ratio of FeCl3•6H2O to GO, which was expressed as X. They were denoted as FeP/NPG-X. Pure FeP and NPG were obtained in the absence of graphene oxide and FeCl3•6H2O, respectively. The characteristic results of pure FeP and NPG can be found in Supporting Information (Figure S1 and S2). Materials Characterization. The crystal structures of samples were measured by a Rigaku D/max2500PC X-ray diffractometer (XRD) with Cu Kα (λ=1.5418 Å). Raman spectra were performed on a DXR spectrometer with 532 nm laser excitation. The morphology of samples was obtained by filedemission scanning electron microscope (SEM, JEOL, JSM-7800F) and transmission electron microscope (TEM, JEOL, JSM-2100HR). Elemental mapping images were characterized by energy dispersive X-ray spectroscopy (EDS) (Oxford). Fourier transform infrared (FTIR) spectra were determined by a Thermo Scientific Nicolet iS50. X-ray photoelectron spectroscopy (XPS) was performed on a PHI 5000 VersaProbe. The thermal performance of FeP and FeP/NPG was evaluated on a NETZSCH STA 449C instrument under air atmosphere at a heating rate of 10 0C min-1. Electrochemical Measurements. The electrochemical tests were performed by using coin-type cells (CR2032). To prepare working electrodes, the active material, carbon black (Super P) and binder with a mass ration of 7:2:1 were firstly mixed together to form a slurry. The binder is carboxymethyl cellulose sodium dissolved in deionized water or polyvinylidene fluoride dissolved


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in N-Methyl pyrrolidone. The slurry was then uniformly pasted on Cu foil. The thickness is controlled as 150 µm. After that, the Cu foil was dried in vacuum at 80 0C for 2 h and 110 0C for 10 h, respectively, and was then punched into circular pieces with a diameter of 12 mm. The loading amount of the total material was about 0.90-1.00 mg cm-2. The test cells were consisted of working electrode, glass fiber (separator), sodium metal (counter and reference electrode), and electrolyte (1 mol/L of NaClO4 in ethylene carbonate/diethyl carbonate solution (1:1, v/v) with 5% fluoroethylene carbonate). The assembly of cells was operated in an argon-filled (99.999%) glovebox. Charge/discharge tests were conducted on a Neware BTS4000 Battery Tester between the voltage of 0.005 and 2.5 V (vs. Na/Na+). The specific charge/discharge capacities were calculated based on the mass of FeP/N,P-codoped graphene nanocomposites. Cyclic voltammogram (CV) was carried out on a CHI600E Electrochemical Workstation between 0 and 2.5 V (vs. Na/Na+). EIS was performed on a ZAHNER Zennium Electrochemical Workstation with the frequency range from 0.01 to 200 kHz.

RESULTS AND DISCUSSION In our work, FeP/N,P-codoped graphene nanocomposite prepared from 1:2 (mass ratio of graphene oxide (GO) to FeCl3•6H2O) is denoted as FeP/NPG, and the related precursor is expressed as Fe2O3/rGO. Thus, the fabrication process of FeP/NPG could be taken as an example to demonstrate the synthesis of FeP/N,P-codoped graphene nanocomposites, which is schematically illustrated in Figure 1. First, crumpled GO sheets with oxygenous functional groups (Figure S3a, b and c in the Supporting Information) could adsorb lots of Fe3+ on the surface. Meanwhile, because of the introduced ethylenediamine (EDA, a basic and weak reducing agent with two -NH2 groups in each molecule), GO sheets are reduced and functionalized with plenty of -NH2 groups. The following


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hydrothermal process induces the partly N doping and assembly of graphene into a 3D network. Simultaneously, uniform Fe2O3 nanodots are in situ nucleated and anchored on graphene sheets due to the hydrolysis of Fe3+ in the alkaline solution. The precursor of 3D Fe2O3/NH2-rich rGO is achieved after freeze-drying. The phosphidation treatment of the precursor with NaH 2PO2•H2O results in the transformation of Fe2O3 nanodots and 3D NH2-rich rGO networks into FeP nanodots and 3D phosphorus-doped graphitic nitrogen-rich graphene networks, respectively, and thus generates FeP/N,P-codoped graphene nanocomposite.

Figure 1. Schematic illustration of the formation of FeP/NPG.

Figure 2a shows the XRD patterns of the precursor and the phosphidated product. Diffraction characteristics of α-Fe2O3 (JCPDS No. 33-0664) and graphene sheets (the broad peak of ~25) can be found in the precursor. However, after phosphidation, all the sharp peaks can be perfectly indexed to the orthorhombic FeP phase (JCPDS No. 65-2595). Meanwhile, the peak of graphene sheets is still observed. Thus, Fe2O3/rGO can be successfully transformed to FeP/NPG by the phosphidation. Raman spectra of Fe2O3/rGO precursor and FeP/NPG are illustrated in Figure 2b. Both samples show the D and G band of graphene. The G band centered at ~1590 cm -1 is attributed to the vibration of sp2-bonded carbon atoms in a two-dimensional hexagonal lattice, while the D


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band at ~1334 cm-1 is related to the defects and disorder in the hexagonal graphitic layers. 62 The calculated intensity ratio of D to G (ID/IG) of Fe2O3/rGO is higher than that of GO (Figure S3d), indicating the reduction of GO under hydrothermal condition. Moreover, ID/IG of FeP/NPG is 1.15. Thus, rGO in Fe2O3/rGO precursor can be further reduced during phosphidation. Moreover, in the FTIR spectra (Figure S4), the NH2-rich rGO could be observed in the Fe2O3/rGO precursor, which is turned into NPG after phosphidation. Furthermore, the calculated content of FeP in FeP/NPG is ~46.9 wt% (Figure S5).

Figure 2. (a) XRD patterns of Fe2O3/rGO precursor and FeP/NPG. (b) Raman spectra of Fe2O3/rGO precursor and FeP/NPG. The detailed structure features of FeP/N,P-codoped graphene nanocomposites are elucidated by SEM and TEM. SEM images (Figure S6) indicate that the loading percentage of FeP nanodots on NPG increases with the increase of initial FeCl3•6H2O. For FeP/NPG, all the graphene sheets are homogeneously covered by dozens of FeP nanoparticles, as shown in Figure 3a and 3b. Those FeP particles are spherical and monodispersed with an average size of ~15 nm, as shown in Figure 3c. It is obvious that the obtained FeP/NPG product well inherits the morphology of Fe2O3/rGO precursor (Figure S7). High-resolution TEM (HRTEM) image (Figure 3d) for FeP/NPG displays the interplanar spacing of 0.202 and 0.259 nm, which correspond to the (211) and (200) planes of


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FeP, respectively. Moreover, the elemental mapping images (Figure 3e) indicate the homogeneous distribution of Fe, P, N and C throughout the FeP/NPG sample.

Figure 3. (a) SEM, (b,c) TEM and (d) HRTEM images of FeP/NPG. (e) Elemental mapping images of FeP/NPG. Inset of (a) is the photograph of FeP/NPG. The chemical valence and composition of Fe2O3/rGO precursor and FeP/NPG were investigated by XPS. Figure 4a shows the full XPS profiles of Fe2O3/rGO precursor and FeP/NPG. Fe, N, C and O are observed in both samples, while P is only detected in FeP/NPG, further indicating the transformation of Fe2O3/rGO precursor to FeP/NPG. The detection of O in FeP/NPG should be attributed to the surface adsorption of oxygen from air and the residual oxygen-containing functional groups from GO, which is in line with FTIR spectra (Figure S4). The high-resolution Fe2p spectra for Fe2O3/rGO precursor and FeP/NPG are shown in Figure 4b. For Fe2O3/rGO precursor, two prominent peaks located at 724.5 and 711.1 eV are attributed to the Fe 3+ 2p1/2 and Fe3+ 2p3/2, while a satellite peak at 719.9 eV is characteristic of Fe 3+ in α-Fe2O3.63 In the Fe2p


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spectra of FeP/NPG, two peaks located at 722.4 and 711.3 eV are ascribed to Fe 3+ 2p1/2 and Fe3+ 2p3/2 in FeP, respectively.64 In addition, a peak at 707.5 eV is related to Fe0 2p3/2, which could also be observed in the previous reported FeP.65 Further XPS investigations of O1s and C1s for Fe2O3/rGO precursor shows the existence of Fe2O3 and graphene with residual oxygenous functional groups (Figure S8a and S8b). As contrast, the C1s spectra of FeP/NPG could be deconvoluted into four peaks, as illustrated in Figure 4c. The dominating peak centered at 284.6 eV is attributed to graphitic C. The peaks related to C-O and C=O66 become very weaker in intensity as compared with those in Fe2O3/rGO precursor. No peak of O-C=O could be detected. Thus, the phosphidation leads to the deep reduction of rGO in the precursor into NPG. This is essential for ensuring the high electrical conductivity of FeP/NPG. In addition, the peak of 285.6 eV is assigned to C-N/C-P,50, 51 suggesting the N/P doping (or dual-doping) in graphene. In the P2p spectrum of FeP/NPG (Figure 4d), three types of P species are observed. The characteristic peaks of P2p3/2 and P2p1/2 of FeP could be observed at 129.7 and 130.7 eV,67 respectively, also confirming the generation of FeP. In addition, P-C could be observed at 133.1 eV, further demonstrating the P doping in graphene.44 The other peak at 134.1 eV corresponds to the oxidized P from air contact. The N1s spectrum of FeP/NPG in Figure 4e can be fitted by three peaks at 398.1, 399.2 and 400.5 eV, which are well consistent with pyridinic-N, pyrrolic-N and graphiticN, respectively.41-43,

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The calculated contents are successively 9.28%, 19.48% and 71.24%

(Figure 4f). Comparing these results with the N1s spectrum of Fe2O3/rGO precursor and calculated ratios of different N species (Figure S8c and S8d), we can deduce that most of -NH2 groups in the precursor are directly converted into graphitic-N through phosphidation, although the reason is still under investigation. Therefore, the N-doping in this synthesis is highly controllable. As mentioned above, graphitic-N is very beneficial to enhancing the electrical conductivity of


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graphene.49 Thus, the high content of graphitic-N in FeP/NPG may result in the remarkable rate performance. In addition, from the EDS spectrum of NPG (Figure S9), it is confirmed that P can be doped into graphene during phosphidation.

Figure 4. (a) XPS profiles and (b) Fe2p spectra of of Fe2O3/rGO precursor and FeP/NPG. (c) C1s, (d) P2p and (e) N1s spectra of FeP/NPG. (f) Calculated ratios of different types of N species in FeP/NPG. The unique morphologic and structural features of FeP/NPG motivated us to evaluate it as an anode material for SIBs. Optimization of the electrochemical performance of FeP/NPG electrodes was firstly conducted with different binders, as illustrated in Figure S10. It can be seen that FeP/NPG electrodes with carboxymethyl cellulose sodium (CMC) has much better cycling stability than that with polyvinylidene fluoride (PVDF). This may be attributed to the crosslinked


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structure with CMC binder, which has better tolerance for the generated mechanical stresses during the volume variation.26, 69 Then, the cyclic performance of FeP/NPG electrode is compared with those of pure FeP and NPG electrodes, as shown in Figure 5a. The FeP electrode shows terrible capacity decay, as the initial discharge capacity of 751 mAh g -1 rapidly decreases to 126 mAh g -1 after 20 cycles. In comparison, the NPG electrode displays good capacity retention with gradual capacity loss after 4 cycles. It delivers a discharge capacity of 260 mAh g-1 after 50 cycles. More interestingly, The FeP/NPG electrode exhibits the best cyclic performance. The discharge capacity remains stable from the 8th cycle onward. A high value of 613 mAh g -1 is obtained after 50 cycles, corresponding to ~95% of the 8th discharge capacity. The superior electrochemical behaviours of the FeP/NPG electrode are also confirmed by its enhanced electron transfer kinetics, evidenced by the electrochemical impedance spectroscopy (Figure S11). Further comparison of the cycling performance of the FeP/NPG electrode with other FeP/N,P-codoped graphene nanocomposite electrodes of different mass ratios of GO to FeCl3•6H2O is illustrated in Figure S12. It can be seen that the FeP/NPG electrode exhibits higher reversible charge/discharge capacity and better cycling stability than other nanocomposite electrodes at 50 mA g-1. Thus, it seems that an appropriate ratio of FeP to NPG plays an essential role in simultaneously enhancing the reversible charge/discharge capacity and cyclic stability. The excellent cyclic performance of the FeP/NPG electrode is better than those of most of the reported transition metal phosphide based anodes (Table S1).


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Figure 5. (a) Comparison of cycle performance of FeP/NPG, FeP and NPG electrodes at 50 mA g-1. (b) Charge/discharge curves of FeP/NPG electrode. (c) Rate capability of FeP/NPG electrode at the current densities ranging from 0.05 to 3 A g-1. (d) Charge/discharge profiles of FeP/NPG electrode at different current densities. (e) Long-term cycling performance and related coulombic efficiency of FeP/NPG electrode at a current density of 1 A g-1.

The CV profiles of FeP/NPG electrode are illustrated in Figure S13. The scan rate is 0.1 mV s1.

A broad peak observed at 0.60 V in the first cathodic scan is attributed to the sodiation and

formation of the solid electrolyte interface (SEI) film.26, 36 During the following anodic scan, two oxidation peaks located at 0.90 and 1.83 V are related to the formation of FeP. Accordingly, two peaks detected at 1.00 and 0.23 V on the cathodic scan are associated with the electrochemical reduction of FeP to Fe.26 As compared with the reported FeP electrode,26 shift of the peak position to a higher potential is found, which results from the structural or textural arrangement during the


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initial discharge processes.70, 71 Furthermore, the CV curves of the 2nd and 3rd cycles are almost superimposed, suggesting the high reversibility of FeP/NPG electrode. Figure 5b shows the galvanostatic charge/discharge profiles of FeP/NPG electrode at 50 mA g 1

between 0.005 and 2.5 V (vs. Na/Na+). The first discharge and charge capacity are 919 and 720

mAh g-1, respectively, leading to a coulombic efficiency of ~78%. The irreversible capacity loss is mainly attributed to the formation of SEI films on the surface of electrode materials during the initial discharge process.26-36 After 10 cycles, the coulombic efficiency remains higher than 99% and the charge/discharge profiles are almost overlapped during the subsequent cycles, indicating the high reversible sodiation/desodiation of FeP/NPG electrodes. These results are consistent with the CV curves. To further evaluate the potential applications of FeP/NPG electrode in high-power SIBs, the rate capability is illustrated in Figure 5c. Specific capacities of 665, 627, 575, 495, 422 and 349 mAh g-1 can be observed at current densities of 0.05, 0.1, 0.2, 0.5, 1 and 3 A g -1, respectively. Moreover, when the current is back to 0.05 A g-1, a reversible capacity of 647 mAh g-1 is achieved, showing high reversibility. Figure 5d shows the charge/discharge profiles of FeP/NPG electrode at different current densities. It can be seen that the polarization becomes more serious with increasing the current density. Fortunately, the columbic efficiencies are always higher than 98%, also confirming the high reversibility of FeP/NPG electrode. Another attractive feature of FeP/NPG electrode is the excellent long-term cycle stability under a current density of 1 A g -1, as shown in Figure 5e. A reversible discharge capacity of 422 mAh g -1 can be obtained after 6 cycles. It remains at 378 mAh g-1 after 700 cycles, corresponding to ~90% of the 6th discharge capacity. Capacity loss of each cycle is only 0.014%. In addition, the columbic efficiencies of ~100% can be obtained during prolonged cycling, further suggesting the high reversibility of sodium uptake/release in


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FeP/NPG electrode. Furthermore, the TEM images of the cycled electrode show the characteristics of graphene supported FeP nanoparticles and interplanar spacing of FeP (Figure S14). Also, the Fe, P, C and N are uniformly distributed throughout the cycled electrode (Figure S15). These indicate the perfect structure stability of FeP/NPG. In addition, the outstanding rate and long-term cycling capabilities of FeP/NPG electrode are better than those of almost all the previously reported transition metal phosphide based anodes (Table S1 and Figure S16). To better understand the merits of FeP/NPG electrode, the reaction kinetic is analyzed by CV, as illustrated in Figure 6a. Similar cathodic and anodic behaviours can be observed under different sweep rates. According to the previous report,72-76 the peak current (i) and the scanning rate (ν) should obey the following equations: i= aνb logi = blogν + loga

(1) (2)

where both a and b are adjustable parameters. Especially, when b is equal to 0.5 or 1, the electrochemical reaction is controlled by diffusion or capacitance behaviour, respectively.72-76 For FeP/NPG electrode, the cathodic and anodic b values are 0.72 and 0.79, respectively (Figure 6b), indicating that the electrochemical reaction is under a mixed control of both behaviors. Further analysis of CV was made to quantitatively determine the capacitive contribution according to the following equation:72-76 i(V) = k1ν+k2ν1/2

(3)

It means that the capacitive (k1ν) and diffusion-controlled (k2ν1/2) current is combined to generate the total current response (i) at a related voltage (V). Figure 6c shows the typical capacitive current (shaded region) to the total current at 2.0 mV s -1. A dominating capacitive contribution can be clearly observed. This may be attributed to the high surface area and excellent


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conductivity of 3D phosphorus-doped graphitic nitrogen-rich graphene. The calculated capacitive contribution at different scan rates is demonstrated in Figure 6d. It can be seen that the contribution increases with increasing the scan rate (e.g., 45.2% at 0.2 mV s -1, 57.7% at 0.4 mV s-1, 61.5% at 0.6 mV s-1, 67.2% at 0.8 mV s-1, 72.9% at 1.0 mV s-1 and 81.2% at 2.0 mV s-1). This means the big contribution of capacitive process at high current densities greatly enhances the charge/discharge capacities of FeP/NPG electrode.5, 53

Figure 6. (a) CV profiles of FeP/NPG electrode at different scan rates from 0.2 to 2.0 mV s-1. (b) A linear relationship between log (sweep rate) and log (peak current). (c) Typical capacitive contribution of FeP/NPG electrode calculated from CV profiles. (d) Capacitive contributions of FeP/NPG electrode under different sweep rates.

CONCLUSIONS


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In summary, FeP/NPG nanocomposites are successfully obtained from the phosphidation of NH2rich reduced graphene oxide supported Fe2O3. High content of graphitic-N in FeP/NPG is achieved through the directional transformation of -NH2 in the precursor. Moreover, monodispersed FeP nanodots are uniformly embedded into a 3D NPG network. These two merits induce fast Na + and electron transfer kinetics, and excellent structural stability of FeP/NPG nanocomposite electrodes during sodiation/desodiation. When evaluated as an anode material for SIBs, this architecture shows outstanding sodium storage performances in terms of high reversible discharge capacity, excellent rate capability, and superb stability over long-term cycles. The fast Na ion storage in FeP/NPG nanocomposites is associated with the remarkable capacitive contribution. This work may offer an alternative way to rationally design advanced transition metal phosphide based anodes for high-performance SIBs.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Supplementary details of SEM and TEM images, FTIR spectra, TG curves, XPS profiles, Raman spectra, XRD patterns, elemental mapping images, and electrochemical performance.

Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT This work was supported by NSFC (51672113 and 21501071), Six Talents Peak Project of Jiangsu Province (2015-XNYQC-008 and 2016-XNYQC-003), and Special Funds for the Transformation of Scientific and Technological Achievements in Jiangsu Province (BA2016162).

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DOI

10.1038/ncomms7929.


29


(75)

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Su, D. W.; Kretschmer, K.; Wang, G. X. Improved Electrochemical Performance of Na-

Ion Batteries in Ether-Based Electrolytes: A Case Study of ZnS Nanospheres. Adv. Energy Mater. 2016, 6 (2), 1501785, DOI 10.1002/aenm.201501785. (76)

Yang, L. C.; Li, X.; He, S.; Du, G. H.; Yu, X.; Liu, J. W.; Gao, Q. S.; Hu, R. Z.; Zhu, M.

Mesoporous Mo2C/N-doped carbon heteronanowires as high-rate and long-life anode materials for Li-ion batteries. J. Mater. Chem. A 2016, 4 (28), 10842-10849, DOI 10.1039/c6ta03083a.

SYNOPSIS

A unique iron phosphide/nitrogen and phosphorous dual-doped graphene hybrid with ultrafast and long-term cycling sodium storage performance.


30

Nitrogen and Phosphorus Dual-Doped Graphene Aerogel Confined

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