Dopamine-derived N-doped carbon encapsulating hollow Sn4P3 microspheres as anode materials with superior sodium storage performance

Accepted Manuscript Dopamine-derived N-doped carbon encapsulating hollow Sn4P3 microspheres as anode materials with superior sodium storage performance Erzhuang Pan, Yuhong Jin, Chenchen Zhao, Miao Jia, Qianqian Chang, Mengqiu Jia PII:

S0925-8388(18)32863-9

DOI:

10.1016/j.jallcom.2018.07.361

Reference:

JALCOM 47077

To appear in:

Journal of Alloys and Compounds

Received Date: 20 June 2018 Revised Date:

30 July 2018

Accepted Date: 31 July 2018

Please cite this article as: E. Pan, Y. Jin, C. Zhao, M. Jia, Q. Chang, M. Jia, Dopamine-derived N-doped carbon encapsulating hollow Sn4P3 microspheres as anode materials with superior sodium storage performance, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2018.07.361. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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ACCEPTED MANUSCRIPT Dopamine-derived N-doped carbon encapsulating hollow Sn4P3 microspheres as anode materials with superior sodium storage performance Erzhuang Pana, Yuhong Jinb*, Chenchen Zhaob, Miao Jiaa, Qianqian Changa, Mengqiu Jiaa* Beijing Key Laboratory of Electrochemical Process and Technology for Materials,

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a

Beijing University of Chemical Technology, Beijing 100029, China. b

Beijing Guyue New Materials Research Institute, Beijing University of Technology,

Beijing 100124, China.

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*Corresponding authors. E-mail: [email protected]. Tel: +86 10 64413808; [email protected], Tel: +86 10 67396288

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Abstract:

Sn4P3 is one of the most promising anode materials for sodium ion batteries (SIBs) owning to the alloying reaction of P and Sn with Na to form Na3P and Na15Sn4, which is beneficial to achieve a high specific capacity, especially for the high volumetric capacity (6650 mAh cm-3). However, the high capacities generate large volume

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changes, which pulverize the anode material, resulting in poor cycling stability. This restricts its practical applications for SIBs. Here, the dopamine-derived N-doped carbon encapsulating hollow Sn4P3 microspheres (hollow Sn4P3@C) composites are

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prepared by an in-situ self-polymerization of dopamine on the surface of hollow SnO2 microspheres followed by a carbonization process and a low temperature phosphorization using NaH2PO2 as P source. The results of the structural and

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morphological characterization can be demonstrated that as-prepared hollow Sn4P3@C composites are constructed by hollow Sn4P3 microspheres with the ultrathin N-doped carbon coating. The as-prepared samples are tested as the anode materials for SIBs. Compared with bared hollow Sn4P3 microspheres, hollow Sn4P3@C composites exhibit better electrochemical sodium storage performance. As a result, hollow Sn4P3@C composites deliver the first discharge and charge specific capacities of 840 and 587 mAh g-1 with a high initial coulombic efficiency of 70 % at a current density of 0.2 A g-1. Moreover, hollow Sn4P3@C composites display superior rate capabilities of 555, 438, 339, 239, 157 and 92 mAh g-1 at 0.2 A g-1, 0.5 A g-1, 1 A g-1, 1

ACCEPTED MANUSCRIPT 2 A g-1, 5 A g-1 and 10 A g-1, respectively. Meanwhile, hollow Sn4P3@C composites show a high discharge capacity of 372 mAh g-1 at 0.2 A g-1 after long 200 cycles. A detailed electrochemical kinetic analysis indicates that energy storage for Na+ in Sn4P3 is due to a pseudocapacitive mechanism. The good electrochemical sodium storage

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performance of as-prepared hollow Sn4P3@C composites may be attributed to the introduction of N-doped carbon coating and unique hollow structure. Therefore, our work provides a new structure model for the development of new anode materials for SIBs.

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Keywords: Hollow Sn4P3 microspheres; Dopamine; N-doped carbon; Anode materials; Sodium ion batteries

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1. Introduction

Sodium ion batteries (SIBs) have been considered as a promising alternative for lithium ion batteries (LIBs) due to the very low cost of sodium resource [1-3]. However, it should be noted that SIBs have been restricted in the commercial application due to their low energy density, short cycle life and unsatisfied rate

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capability [4, 5]. Specifically, traditional electrode materials used in commercial LIBs cannot be directly applied in SIBs, which is attributed to larger and heavier sodium ion compared with lithium ion [6, 7]. Therefore, extensive efforts have been devoted

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to finding the new and suitable electrode materials with the high capacity and structural integrity along with the fast rate performance and long cycling stability for SIBs [8, 9].

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Among all possible anode materials, transition metal phosphides (TMPs), such as FeP [10, 11], NiP3 [12], CoP [13], Zn3P2 [14], CuP2 [15], Se4P4 [16], have attracted a great attention due to their high theoretical capacity, low intercalation potentials for Na+. Compared with different TMPs, Sn4P3 has been considered as an ideal anode material for SIBs due to ultrahigh theoretical volumetric capacity (6650 mAh cm-3), good electrical conductivity (30.7 S cm-1) and low potential plateau for accommodating Na+ [17]. The pioneered work reported by Kim and coworkers firstly displayed the high capacity (718 mAh g-1) and long cycling stability (~100% of capacity retention over 100 cycles) of Sn4P3 anode materials for SIBs [18]. However, the rate performance of 2

ACCEPTED MANUSCRIPT Sn4P3 anode materials was not mentioned. In order to improve the performance of Sn4P3, it can be an effective strategy to optimize the structure of the anode material accompanied with surface carbon coating. As for the structure optimization, compared to the traditional nanoparticles, hollow structure has been widely used to modify the

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anode materials with the large volume change [19]. This structure leads to several advantages. Firstly, it can shorten the diffusion paths of electrolyte ions, which can increase the rate capability. Secondly, it provides a large surface area and a lot of active sites, which can enhance the electrochemical activity under the high rate.

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Furthermore, the enough void volume in hollow structures can release the volume strain during cycling, which is beneficial to improve the cycling stability. However,

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the synthesis of hollow Sn4P3 structure is a big challenge [20].

With the exception of the structure optimization, carbon coating is considered as an effective and cheap method to improve the performance of Sn4P3 anode materials because it has the good electronic conductivity to increase the electron transport and provides the good elastic buffer layer to effectively relieve the stress of volume

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change during cycling. Liu and coworkers prepared uniform yolk-shell Sn4P3@C nanospheres using a top-down phosphorized method with yolk-shell Sn@C nanosphere as a precursor [21]. Owing to this unique structure, the obtained yolk-shell

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Sn4P3@C nanospheres exhibited high reversible capacity, superior rate capability and long cycling stability. However, they displayed a very low initial Coulombic efficiency of 43.8%, which can be attributed thick carbon coating (~100 nm).

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Meanwhile, the prepared method is complicated and time-consuming. Moreover, as compared with bare carbon materials, N-doped carbon might further enhance both ion and electron diffusion, resulting in enhanced electronic and ionic conductivity [22-24]. However, despite the merits brought by N-doped carbon, to be best of our knowledge, there is lack of reports on the surface modification of Sn4P3 hollow microspheres with N-doped carbon. Herein, we report the first synthesis of N-doped carbon encapsulating hollow Sn4P3 microspheres (hollow Sn4P3@C). The hollow structure provides a large specific surface area, better access to the electrolyte, and shorter diffusion paths for both ions 3

ACCEPTED MANUSCRIPT and electrons. Meanwhile, the ultrathin carbon coating enhances the conductivity and facilitates transport of electrons. As expected, the as-prepared hollow Sn4P3@C composite could deliver a stable capacity for 200 cycles at a current density of 0.2 A g-1 accompanied with good rate performance, which can be attributed to the superiority of the hollow structure and the N-doped carbon coating.

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2. Experimental section 2.1 Materials synthesis

Synthesis of hollow Sn4P3: Hollow SnO2 nanoparticles were synthesized by a

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template-free method [25]. Hollow Sn4P3 was synthesized by a low-temperature phosphidation reaction. 0.1 g hollow SnO2 and 0.5 g NaH2PO2 were hand milled and heated at 280

for 30 min at 5

min-1 in N2 atmosphere. After cooling down, Sn4P3

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was obtained by washing with dilute HCl solution (0.1 mol L-1) and deionized water. Synthesis of hollow Sn4P3@C: 200 mg prepared hollow SnO2 and 100 mg dopamine hydrochloride were dissolved into 200 mL of Tris-buffer solution (10 mM). After stirring for 3 h at room temperature, the products were collected via suction filtration and washed with deionized water and ethanol for several times, and dried at 60 °C

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overnight. Dopamine was used as the Nitrogen source. The as-prepared samples were heated at 600 °C for 2 h in N2 with a heating rate of 5 °C min-1 to transform into N-doped hollow SnO2@C. Hollow Sn4P3@C was synthesized by the same method with hollow Sn4P3. 0.1 g SnO2@C and 0.5 g NaH2PO2 were hand milled and heated at for 30 min at 5

min-1 in N2 atmosphere. After cooling down, Sn4P3@C was

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obtained by washing with dilute HCl solution (0.1 mol L-1) and deionized water.

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2.2 Characterization

The composition and crystalline structure of samples was characterized by powder X-ray diffraction (XRD), performed on Rigaku D/max-2500 with Cu K radiation (=1.54056 Å). X-ray photoelectron spectra (XPS) was conducted on ESCALAB 250 using Al Ka X-ray radiation.The specific Brunauer-Emmet-Teller (BET) surface area of hollow Sn4P3@C was determined by Micromeritics Model ASAP 2460. The structures and morphologies of samples were displayed by high-resolution transmission electron microscopy (HRTEM, JEOL JEM-3010). 2.3 Electrode preparation and electrochemical measurements 4

ACCEPTED MANUSCRIPT The electrochemical measurements were performed by CR2032 type coin cell using sodium as counter electrode. The mass ratio of active materials, Super-p and sodium alginate was 7:2:1. The slurries were coated on copper foil and dried out at 120 overnight in vacuum. Glass fiber was used as separator and a solution of 1.0 M

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NaClO4 in EC/DMC (v/v=1:1) with 5.0% FEC as the electrolyte was used for SIBs. Cyclic voltammetry (CV, 0.1 mV s-1, 0.01-3.0 V vs Na+/Na) and electrochemical impedance spectra (EIS, 5 mV, 100 kHz-0.01 Hz) were measured by electrochemical workstation (CS310). Cycling test was carried out by Neware CT3008 battery testing

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3. Results and discussion

Scheme 1. Schematic illustration of the formation of hollow Sn4P3@C. Scheme 1 shows the synthesis process of hollow Sn4P3@C involving three steps as

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illustrated. Firstly, a template-free method was used to synthesize hollow SnO2 nanoparticles. And then, hollow SnO2 nanoparticles were coated with dopamine , the hollow SnO2@C composites were obtained.

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hydrochloride. After heating at 600

Finally, hollow SnO2@C was transferred to hollow Sn4P3@C by a low-temperature phosphidation reaction.

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Fig. 1. XRD patterns of the hollow Sn4P3@C composite and hollow Sn4P3. Fig. 1 displays XRD results of as-prepared samples. The diffraction peaks of hollow Sn4P3 located at 28.8°, 30.3°, 31.5°, 44.5°, and 45.7° correspond to (015), (0012), (107), (0114), and (110) planes, respectively, which is in accordance with the rhombohedral Sn4P3 phase (JCPDS No. 73-1820) [26]. Meanwhile, the diffraction

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peaks of hollow Sn4P3@C composite are consistent with them of the single Sn4P3 phase, indicating the formation of the high purity of Sn4P3 in the composite. Moreover, these results prove that SnO2@C had been successfully transformed to Sn4P3@C by a

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low-temperature phosphidation reaction.

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Fib. 2. XPS spectra of (a) Sn3d, (b) P2p, (c) C1s and (d) N1s regions of the hollow Sn4P3@C composite.

The elemental composition of the obtained hollow Sn4P3@C was measured by XPS. Fig. 2a-d show the XPS core-level spectra of the main elements. From Fig. 2a, two

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peaks located at 487.6 and 496.1 eV correspond to the binding energies of Sn 3d5/2 and Sn 3d3/2 spin orbital splitting photoelectrons for the as-obtained sample,

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respectively, which are in accordance with the elemental valence of Sn3+ [27]. As shown in Fig. 2b, the peaks at 130 and 130.9 eV are ascribed to the binding energies of P 2p3/2 and P 2p1/2, respectively [28]. Moreover, the peak at 134.1 eV could be assigned to the oxidized phosphate species on sample surface during the XPS measurement [29]. The XPS spectrum of C1s (Fig. 2c) was fitted with three components. The peaks at 284.6, 285.3 and 287.6 eV are attributed to the C-C, C-N and C-C=O functional groups, respectively[30]. The presence of C-N functional group confirms that the nitrogen elements are doped into the carbon matrix. The N1s spectrum (Fig. 2d) is fitted into three peaks which are located in 399.2, 400.2 and 7

ACCEPTED MANUSCRIPT 401.5 eV, corresponding to N1 (pyridinic N), N2 (pyrrolic N), and N3 (graphitic N) [31], respectively. These results further confirm that the formation of Sn4P3@C

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composite.

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Fig. 3. (a) N2 adsorption-desorption isotherms and pore-size distributions (inset) of the hollow Sn4P3@C and Sn4P3@C; (b) TGA curves of hollow Sn4P3@C. The porous structure and specific surface area of the hollow Sn4P3@C composite are examined by N2 adsorption-desorption isotherms. Fig. 3a shows nitrogen adsorption-desorption isotherms and the corresponding pore-size distribution [32] of

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as-prepared hollow Sn4P3@C composite and hollow Sn4P3. It can be noted that the isotherms display the characteristic type IV with a broad hysteresis loop, indicating the mesoporous structure of as-prepared sample. Moreover, as observed from Fig. 3a (inset), the average pore diameter of hollow Sn4P3@C is about 4.35 nm, further

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suggesting the mesoporous structure of hollow Sn4P3@C. The BET specific surface area and pore volume are 46.8 m2 g-1 and 0.05 cm3 g-1, respectively. As for hollow

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Sn4P3, the specific surface area and pore volume are 21.3 m2 g-1 and 0.07 cm3 g-1, respectively. These results demonstrate that the mesoporous structure and high surface area of hollow Sn4P3@C composite can provide more contact area and channels for the diffusion of sodium ion. In Fig. 3b, the small weight loss below 200

is

attributed to the removal of adsorbed water on the surface of Sn4P3@C. The rapid weight loss from 400 to 600

is ascribed to the carbon component in Sn4P3@C

composite has been burned completely. So, the mass content of carbon and Sn4P3 is about 21 % and 79 %, respectively.

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Fig. 4 (a, d) TEM images of hollow SnO2; (b, e) TEM images of hollow SnO2@C; (c, f) TEM images of hollow Sn4P3@C; (g) High-resolution TEM image and (h) the SAED pattern of hollow Sn4P3@C.

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Morphology, microstructure and the formation process of the hollow Sn4P3@C composite are characterized by TEM and HRTEM images. As shown in Fig.4a and

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4d, it can be observed that SnO2 nanoparticle has a hollow structure. After a dopamine-derived carbon coated on hollow SnO2 nanoparticles, it can be shown hollow structure of SnO2 nanoparticle can be maintained. Moreover, a thin carbon coating appears on the surface of hollow SnO2 nanoparticles (Fig.4b and 4e). Finally, from Fig. 4 (c-f), we can clearly observe hollow structure and carbon coating of Sn4P3@C nanoparticles, maintaining a very similar shape and structure to the hollow SnO2@C nanoparticles. This indicates that the structure of the precursor can be maintained well during the process of preparing hollow Sn4P3@C by low-temperature phosphidation reaction. Meanwhile, Hollow structure Sn4P3 nanoparticle is uniformly 9

ACCEPTED MANUSCRIPT coated by a dopamine-derived carbon layer of about 20 nm (Fig. 4g). Moreover, the HR-TEM image in Fig. 4g clearly shows two marked d-spacings of 0.31 and 0.33 nm, corresponding to the (015) and (012) planes of rhombohedral Sn4P3, respectively. The SAED pattern of the hollow Sn4P3@C composites, as shown in Fig. 4h, corresponds

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well with the (015), (107) and (110) planes of Sn4P3, which are well matched to the

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XRD results.

Fig. 5. (a) First three CV curves at a scan rate of 0.1 mV s−1 and (b)

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Charge-discharge curves for first three cycles at a current density of 0.2 A g-1 of the hollow Sn4P3@C composite for SIB. Fig. 5a displays the first three CV curves of 0.01-3.00 V (vs Na+/Na) at a scan rate of

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0.1 mV s-1 of the hollow Sn4P3@C composite. In the first cathode scan, a small peak at ~1.0 V is related to the formation of a solid electrolyte interphase (SEI). A shoulder

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follows at ~0.2 V could be mainly assigned to the Na-insertion of Sn4P3 (Sn4P3 + Na+ → Na15Sn4 + Na3P) [33]. The anodic peaks at 0.38 V and 0.65 V is associated with the desodiation of Na15Sn4 (Na15Sn4 → 4Sn + 15Na+ + 15e−) and Na3P (Na3P → P + 3Na+ + 3e−), respectively [34]. In the reserved scan, the hollow Sn4P3@C electrode displays two pairs of the redox peaks at 0.37/0.25V and 0.68/0.50V which due to the good reversible conversion reaction of P and Sn elements with Na+ [35, 36]. And then, the nearly overlapped CV curves indicate the excellent cycle performance of the hollow Sn4P3@C composite. Fig. 5b shows the first three galvanostatic discharge/charge curves of the hollow Sn4P3@C composite at a current density of 0.2 10

ACCEPTED MANUSCRIPT A g-1. The first discharge and charge process deliver specific capacity of 840 and 587 mAh g-1, with a high initial coulombic efficiency of 70 %. The loss capacity may be attributed to the formation of the SEI layer during the first discharge. The two potential plateaus at 0.3 V and 0.65 V correspond to two stepped Na storage reactions

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of P and Sn, separately [37].

Fig. 6. (a) Charge-discharge curves of hollow Sn4P3@C composite at different

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currentdensity from 0.2 to 10 A g-1; (b) Rate performances of hollow Sn4P3@C composite and hollow Sn4P3; (c) Charge-discharge curves of hollow Sn4P3@C

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composite at 0.2 A g-1; (d) Cycling performance of hollow Sn4P3@C composite and hollow Sn4P3 at 0.2 A g-1.

Fig. 6a demonstrates the discharge-charge curves of hollow Sn4P3@C composite at an increasing current densities from 0.2 to 10 A g-1 as anodes for SIB. Fig. 6b shows the rate capability of hollow Sn4P3@C and pure hollow Sn4P3. Compared with hollow Sn4P3, the rate capability of hollow Sn4P3@C composite is excellent. Moreover, the capacity of hollow Sn4P3@C composite could also be maintained at about 480 mAh g-1 when the current density goes back to 0.2 A g-1. In detail, the discharge capacities of the hollow Sn4P3@C composite at 0.2 A g-1, 0.5 A g-1, 1 A g-1, 2 A g-1, 5 A g-1 and 11

ACCEPTED MANUSCRIPT 10 A g-1 are 555, 438, 339, 239, 157 and 92 mAh g-1, respectively. Such good rate performance is mainly attributed to the special hollow structure and the protection of N-doped carbon shell [38]. The galvanostatic discharge-charge curves of hollow Sn4P3@C composite were measured within a voltage range of 0.01-3.0 V at a current density of 0.2 A g-1(Fig. 6c). With the increase of cycle times, the charge-discharge

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curves almost coincide, indicating the excellent long cycle stability [39]. As shown in Fig. 6d, it is obviously noted that hollow Sn4P3@C composite has a much better cycle performance than bared hollow Sn4P3. The first specific discharge and charge

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capacities of hollow Sn4P3@C composite are 840 and 587 mAh g-1. In the following cycles, the coulombic efficiency quickly increases to around 100% for hollow Sn4P3@C composite. And after 200 cycles at a current density of 0.2 A g-1, the

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specific discharge capacity is 372 mAh g-1, which is higher than the pure hollow Sn4P3 (24 mAh g-1). The rapidly decreasing capacity of hollow Sn4P3 may be attributed to its large volume expansion and pulverization [40]. On the contrary, hollow Sn4P3@C composite exhibits good cycle stability, which is due to the intact

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and conformal coating of N-doped carbon.

Fig. 7. Nyquist plots of hollow Sn4P3@C composite and hollow Sn4P3; Inset shows the equivalent circuit which can be used to fit the experimental data. Fig. 7 displays the Nyquist plots measured after 10 cycles of hollow Sn4P3@C and 12

ACCEPTED MANUSCRIPT hollow Sn4P3 electrodes. As shown in Fig. 7 (inset), Re corresponds to the electrolyte resistance, Rf corresponds to the solid electrolyte interface (SEI) layer resistance formed on the surface of the electrodes. Rct is the charge transfer resistance during the electrochemical reaction process [41]. The hollow Sn4P3@C electrode exhibits a

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much lower Rct (56.4 Ω) than that of hollow Sn4P3 electrode (79.2 Ω), indicating the fast electrons transfer during the discharge and charge processes [42]. The main reason is that carbon incorporation improves the electrical conductivity of as-prepared

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composites.

Fig. 8. (a) CV curves of the hollow Sn4P3@C composite at various scan rates,

from 0.2 to 1.0 mV s−1; (b) Determination of the b-value using the relationship between peak current to sweep rate; (c) CV curves of the hollow Sn4P3@C

composite with separation between total current (solid line) and capacitive currents (shaded regions) at 0.4 mV s-1; (d) Separation of contributions from capacitive and diffusion-controlled capacities at different sweep rates. We used a cyclic voltammetry-based analytical method to further study the electrochemical properties of hollow Sn4P3@C composite, explaining the contribution 13

ACCEPTED MANUSCRIPT of diffusion-controlled and capacitive in a total capacity [43, 44]. Fig. 8a displays the CV curves of the hollow Sn4P3@C composite at the scan rates from 0.2 to 1.0 mV s-1, showing the well preserved shapes with the increasing scan rate [45, 46]. Fig. 8b presents plot of log(i) versus log(ν) for both cathodic peak and anodic peak. Assuming that the current (i) obeys a power-law relationship with the scan rate (ν )

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leads to: i = aν b, where a and b are adjustable values [47-49]. The value of b is calculated by the slope of log(i) and log(ν), which is between 0.5 and 1.0. It is well known that the closer the value of b is to 1.0, the greater the proportion of capacitor

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behavior [50, 51]. Conversely, diffusion-controlled process dominates. Here in, the higher b value of the hollow Sn4P3@C composite electrode (0.93 and 0.91

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corresponding to cathodic peak and anodic peak, respectively) indicates that the total capacity process is controlled by both diffusion-controlled and capacitive where capacitive behavior dominates [52, 53]. In Fig. 8c, current response i at the corresponding voltage is separated from diffusion-controlled and capacitive contribution. The percentage of capacitive contribution for hollow Sn4P3@C is 67% at

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0.4 mV s-1. With the scan rate increase from 0.2 to 1.0 mV s-1, the contribution of capacitive is 58, 67, 78, 84 and 89 %, respectively (Fig. 8d). 4. Conclusions

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In summary, a novel nanostructure of hollow Sn4P3 microspheres coated with N-doped carbon has been successfully obtained. The as-prepared hollow Sn4P3@C composites exhibit the good rate capabilities and long cycling stability as anode

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materials for SIBs. The reversible capacity only decreases from 555 to 93 mAh g-1 when the current density increases from 0.2 to 10 A g-1. The discharge capacity can be maintained at 372 mAh g-1 at 0.2 A g-1 after 200 cycles. The enhanced electrochemical sodium storage performance is attributed to the hollow structure and N-doped carbon coating, which can shorten the ion diffusion pathways, increase the active sites, release the strain during charging/discharging processes, form the stable SEI film and increase the electrode electronic conductivity. References 14

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N-doped carbon encapsulating hollow Sn4P3 microspheres is prepared for the first time.


Hollow Sn4P3 microspheres are uniformly and conformally coated by N-doped carbon.

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As-prepared hollow Sn4P3@C composite exhibits long cycling stability.
As-prepared hollow Sn4P3@C composite exhibit high-rate capacity of 92 mAh g-1

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at 10 A g-1.