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3D Hierarchical Framwork Assembled by Cobblestone-like CoSe2@C Nanospheres for Ultrastable Sodium Ions Storage Peng Ge, Hongshuai Hou, Sijie Li, Lanping Huang, and Xiaobo Ji ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01888 • Publication Date (Web): 11 Apr 2018 Downloaded from http://pubs.acs.org on April 11, 2018

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ACS Applied Materials & Interfaces

3D Hierarchical Framwork Assembled by Cobblestone-like CoSe2@C Nanospheres for Ultrastable Sodium Ions Storage Peng Ge,1,2 Hongshuai Hou,1,2,3 Sijie Li,2 Lanping Huang1* and Xiaobo Ji1,2,3 * 1

State Key Laboratory for Power Metallurgy, Central South University,

Changsha, 410083, China. 2

College of Chemistry and Chemical Engineering, Central South

University, Changsha, 410083, China. 3

Institute of Advanced Electrochemical Energy, Xi’an University of

Technology, Xi’an, 710048, China. ABSTRACT: Sodium ions batteries, as the promising commercial energy system, are restricted by its sluggish kinetic and low sodium ions storage. Metal-selenide possess well conductivity and capacity, but still suffers from the stacked problem and volume expansion. Significantly, CoSe2/C is successfully prepared with the assistance of citric acid as both chelating agent and carbon precursor, displaying that cobblestone-like nanospheres with the radii (< 25 nm) distribute uniformly in the carbon matrix. Expected that the established Co-O-C bonds enhance the stability of structure with faster ions shuttling. With the available electrolyte (NaCF3SO3/ diethylene glycol dimethyl ether) in a potential window range from 0.5 to 3.0 V, the as-obtained sample shows the ultra-long lifespan at 4.5 A g-1, retaining a capacity of 345 mAh g-1 after 10000 cycles. Utilized the detailed kinetic analysis, it is clear that the surface-controlled electrochemical behavior mainly contributes to the excellent large-current cycling stability and Na-storage capacity. The ex-situ results support that the crystal and morphology structure keep stable. The work is anticipated to 1

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enhance the in-depth understanding of CoSe2/C anode and supply a facile manner to obtain electrode materials for SIBs.

KEYWORDS: CoSe2/C, Co-O-C bond, sodium-ions batteries, ultra-long cycling stability, kinetics behaviors

1. INTRODUCTION Restricted by the large radius (0.102 nm) of sodium ions, the available electrode materials for sodium ions batteries (SIBs) with enormous commercial potential have captured numerous attentions.1-5 Learning from lithium ions batteries (LIBs), transition-metal based materials were deemed as the promising materials with high Na-storage capacity, derived from the conversion reaction.5-7 Note that the anions (Group VI, O, S, Se) in the compounds serve as important roles in electronic conductivity and electrochemical properties. Among these materials, metal-oxide possess the largest theoretical capacity, however, the generated compact SEI films in the reaction would hinder the Na-ions diffusion entrance, leading to the dissatisfied capacity.

8,9

Owing to the “shuttle effect” of polysulfide with the deteriorated

electrochemical properties, the breakthrough has not been achieved on ultralong-term cycles in metal-sulfide.10 Compared to the afore-mentioned materials, metal-selenide display the narrow band gap with good conductivity.11 Moreover, with the suitable electrolyte, the reaction energy barrier is lowered, facilitating faster electronic transferring. Although 2


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some capacity would be sacrificed on account of the upgraded sodiation/desodiation platforms, the well cycling stability could be largely enhanced.12-14 For instances, the excellent electrochemical properties (372 mAh g-1 after 2000 cycles at 1.0 A g-1) were exhibited by sphere FeSe2 with the utilization of the electrolyte NaCF3SO3/DEGDME (diethylene glycol dimethyl ether).15 The nanooctahedra NiSe2 through hydrothermal reaction displays stable cycling, showing 313 mAh g-1 after 4000 cycles at 5.0 A g-1.16 The urchin-like CoSe2 has been reported by Chen’group, delivering a Na-storage capacity of 410 mAh g-1 after 1800 cycles at 1.0 A g-1.17 However, the volume expansion with pulverization and weak dissolution of polyselenium should not be neglected, perhaps deteriorating the electrode samples, which could bring inferior electrochemical performances. Two controlled strategies were employed to further impart promotion, (1) reducing particles size can increase the contacting area with electrolyte, shorten the ions diffusion path and lower areal current density; (2) introducing the carbon layer could protect the direct erosion of electrolyte, alleviate the volume swelling and inhibit the pulverization.18 The solvothermal reaction or the complicated precursor were frequently used and it is obvious that the complex manner is not suitable for practical application. Thus, constructing 3D structure comprised of 0D metal-selenium/carbon is of importance through the facile fabricated process, which significantly optimizes the properties of target samples and realize the commercial value. Moreover, the introduction of M(metal)-O/N-C bonds act an significant role in imparting the improvement on the structure stability and conductivity.19-21 3



As the familiar complexing agent, citric acid has been widely used to obtain the uniform particles with carbon layer. Recent report demonstrated that the citric acid was pyrolyzed to form dual-carbon layer coating Na3V2(PO4)3.22 The Ag/TiO2 was also dispersed in the carbon due to the effective complexing agent.23 The plentiful -COOH groups would increase the ability of forming uniform complex and M-O-C bonds in the final product. In this paper, benefitting from the dual effect of critic as both chelating agent and carbon precursor, 3D framework assembled by 0D CoSe2/C with the small radii (< 25 nm) was successfully obtained, accompanying with the introduction of the Co-O-C bonds. The obtained sample delivers the ultralong-term stablility, keeping the capacity of 345 mAh g-1 after 10000 cycles at 4.5 A g-1 with the coulombic efficiency (98%). The excellent electrochemical properties can be attributed to effective stable structure, ether-based electrolytes and capacitive-controlled behaviors. 2. EXPERIMENTAL SECTION 2.1 Chemicals All the chemicals were purchased form Aladdin Chemical and used without any purification. 2.2 Synthesis of CoSe2/C structure In brief, 0.5 g critic, 1.0 g Co(NO3)2·6H2O (> 99%) and 1.0 g Se (> 99%) were mixing in agate mortar. Then, the materials were grinding adequately for 30 min for homogeneous mixing. The uniform mixing was placed in the porcelain boat, then transferred to the tube furnace. The stairing conditions was at 500 oC for 2h with 4


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heating rate of 5 oC min-1 at H2/Ar (5% : 95%). After the nature cooling, the product was milling with 30 min to obtain the final samples CoSe2/C. 2.3 Characterization of the materials The crystal structure were probed by the X-ray diffraction (XRD, Cu kα radiation at λ = 1.54 Å, Rigaku powder diffractometer (DMAXB) X-ray spectrometer, standard samples (LaB6 powder) and Raman spectra (Renishaw INVIA micro-Raman spectroscopy). Utilized the thermogravimetric analysis (TGA), the content was determined through 30 to 800 oC with 10 oC min-1 at atmosphere. The specific surface area and pore distribution were conducted by BET (Micromeritics, ASAP 2020). The morphology and detail internal structure were carried out by field emission electron microscopy (SEM, JSM-7600) and high-resolution transmission electron microscopy (HRTEM, JEM-2010F). The X-ray photoelectron (XPS) was characterized to analysis the valence state. 2.4 Electrochemical properties The obtained samples, CMC (carboxymethylcellulose sodium) and super carbon with the mass ratio of 70: 15: 15 were mixed uniformly in deionized water. Then, the slurry was painted in the Cu foil, further dried at 80 oC for 12 h. The foil was tailored to the plate with the diameter of 1 cm, and the active mass can reach to 1.0 mg - 1.5 mg. The counter-electrode was metallic sodium foil, and the electrolyte as 1 M NaCF3SO3 in DEGDME (diethylene glycol dimethyl ether). The CR2016 coin cells were assembled in the argon-filled glovebox. The galvanostatic discharge-charge was conducted by Land battery systems (5 V, 10 mA) in the potential from 0.5 V to 3.0 V 5



at different current densities. The CV (cyclic voltammetry) and EIS (electrochemical impedance spectroscopywere) were performed by MULTI AUTOLAB M204.

3. RESULTS AND DISCUSSION

Figure. 1 The chemical structure properties of CoSe2/C: (a) XRD patterns, (b) Raman spectra, (c) TG/DSC curves, (d) XRD patterns of residual products, (e) N2 adsorption isotherms, (f) FTIR spectra, (g) the crystalline structure and ions shuttling. As shown in Fig. 1a, the XRD patterns of the as-obtained CoSe2/C and C were displayed, which are indexed to the standard PDF#88-1712, revealing that the sample belongs to the pure cubic trogtalite CoSe2, Pa-3(205) space group, a = b = c = 5.8593 6


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Å .24 And the peaks located at 30.48o, 34.19o, 37.57o, 43.65o, 51.70o, 56.59o, 58.93o were noticed. As displayed in Table S1, they are corresponding to (200), (210), (211), (220), (311), (023) and (321) lattice planes, respectively, indicating the well crystalline.25 According to the Scatter’s equation (Dhkl=Kλ/βcosθ, K = 0.9, λ = 1.5406

Å),26-28 the rough size of crystalline grains was about 10 nm. Moreover, the peaks of carbon was not found in the XRD pattern of CoSe2/C, which is ascribed to the high peak intensity of CoSe2. Raman spectra was performed to further investigate the detail crystal structure and the existed carbon. The D peak appeared at 1364 cm-1 and the G peak located 1596 cm-1 are associated with sp3 and sp2-hydried carbon, and ID/IG is about 0.85, revealing the high graphitic degree.29-31 The peak at 673 cm-1, marked as A1g, is the charactertic peak of CoSe2.32,33 In order to confirm the content of carbon, the tested TG/DSC curves and the XRD pattern of residual product was exhibited in Fig.1c, d. At the temperature of 403 oC, the CoSe2 was transformed into CoSe with the sublimation of SeO2. And the carbon was converted to CO2 at 429 oC. When the temperature increased to 591 oC, the CoSe was decomposed into Co3O4 with the leaving

of

SeO2.

Based

on

the

equation

(the

content

of

carbon

=

1-3WCo3O4×MCoSe2/MCo3O4), the content of CoSe2 is about 82.6 % and the content of carbon was 17.4 %. The internal valence bonds of CoSe2/C was preliminary evaluated by FTIR spectra in Fig. 1f and S1. Compared to that of cobalt acetylacetonate (blue lines) containing the Co-O-C bond at ~ 600 cm-1 , it is found that the same peak of the target materials appeared, indicating the formation of Co-O-C bond, allowing the stable structure and facile electron transfer.19,20,34 In Fig. 1e, the process of the 7



electrons transferring from carbon to the CoSe2 crystal lattice was displayed. The Co-O-C as the bridge could facilitate faster ions shuttling. Thus, this bond was further investigated as following.

Figure. 2 XPS spectrum of CoSe2/C: (a) full spectrum, (b) Co 2p, (c) Se 3d, (d) C 1s, (e) O 1s. Further analyzing the valence bonds of the CoSe2/C through XPS, the detailed chemical interaction between CoSe2 and carbon matrix were found in Fig. 2. Clearly, 8


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the peaks of Co2p, C1s, Se3d, O1s were displayed in the full spectrum of sample, confirming the existence of above-mentioned elements. The high resolutions of C, Se, O elements were probed to evaluate the chemical states. It is found that the peak of Co 2p is comprised of Co2p1/2 and Co2p3/2 approximately located at 792 eV, 777 eV, which contain the Co-Se and Se-Co-Se bonds.25,32,33 And the Se 3d peak can be deconvoluted into five peaks Se3d5/2, Se3d3/2, Co3p3/2, Co3p1/2 and SeO2, which mainly appeared at 54.3 eV, 55.3 eV, 57.4 eV, 58.8 eV and 60.4 eV. Among them, the Co-Se bonds are indexing into the peak at 54.3 eV.32 The existing slight SeO2 is ascribed to that some Se on the surface was oxidized to some extent. Importantly, according to the previous work,20,35,36 the peak at 285 eV is associated with Co-O-C, and those at 283, 286, 286 eV are relation with C-C, C-O, C=O bonds in Fig. 2d. Meanwhile, the Co-O-C bond at 532.8 eV is also found in the spectrum of O1s, and other peaks are C-O at 535 eV, C=O at 531 eV and the adsorbed O2 from atmosphere at 530 eV. This XPS analysis can effectively verity the existence of Co-O-C bonds.

9



Figure. 3 The morphology structure of CoSe2/C: SEM images at different magnifications (a, b, c), TEM images (d, e), HRTEM images (f) and SAED images (g) of CoSe2/C and carbon, mapping images (h), EDX spectrum (i). 10


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0D to 3D hierarchical cobalt selenide framework was confirmed in Fig. 3a, b. Clearly, some porous were observed in the sample, facilitating the infiltration of electrolyte, thereby shortening the ions diffusion paths.37,38 Shown by the SEM images in Fig. 3c, it is found that unique framework is comprised of 0D cobblestone-like nanospheres, which effectively increase the active sites of the target sample. The nanospheres were dispersed homogeneously in carbon matrix in Fig. 3d. And the radius of nanopsheres (< 25 nm) are clear in Fig. 3e, which is beneficial for that the CoSe2 reacted with Na+.33,39 In Fig. 3f, the clear graphitic lattice space is about 0.34 nm and the lattice fringe of CoSe2 is 0.26 nm, corresponding to (210) facet (JCPDs: 88-1712). In comparison of the SAED images about carbon, the diffraction rings of CoSe2/C are indexed to (200), (211), (311) for CoSe2, meanwhile, that of graphitic planes (002) are exhibited.24 The elements of Co, Se, C, O were distributed uniformly in Fig. 3i. Clearly, the atomic of Co : Se is 1 : 2, further verifying that the target product CoSe2 is obtained. As the afore-mentioned discussion, the corresponding formation was proposed in Figure. 4. With the increased temperature, citric acid were firstly molten. Benefitting from its complexing function, the Co2+ ions were contacted with the citric acid to form -COO-Co. Subsequently, 5% H2 render some Co2+ reduced to Co particles which were distributed uniformly in complexing agent. Accompanying with the improvement of temperature, the citric acid was carbonized to form 3D framework structure, and the porous were created by the leaving of some oxygen. Meanwhile, the Se with H2 was changed to H2Se, further reacting with Co to obtain CoSe2/C. 11



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Figure. 4 The formation mechanism of CoSe2/C.

Table. 1 The reaction process of CoSe2/C.

Process

Discharging

Charging

Chemical equation

Mechanism

S1

CoSe2 + xNa- + xe- → NaxCoSe2

Intercalation

S2

NaxCoSe2+ (2 - x) Na+ + (2 - x) e- → CoSe +Na2Se

Conversion

S3

CoSe + 2Na+ +2e- → Co + Na2Se

Conversion

S4

Co + 2Na2Se → NaCoSe 2 +(4 - x) Na+ + (4 - x) e-

Inverse conversion

S5

NaxCoSe2 → CoSe2 + xNa+ + xe-

Inverse conversion

12


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Figure. 5 The electrochemical properties of CoSe2/C and C: (a) cycling stability and (b) the sodiation/desodiation curves at the current density of 0.5 A g-1. (c) rate capability at various current densities and (d) the discharge/charge platforms. (e) ultralong-term cycling stability at large current density of 4.5 A g-1 ， (f) the 13



voltage-time curves from 6000 to 6010 loops. The detailed Na-storage behaviors of CoSe2/C were performed as anode for SIBs in Fig. 5. The tested potential range was from 0.5 to 3.0V, which is ascribed to that (1) few capacity was existed in the voltage between 0and 0.5V, (2) some side reactions would be evoked at low potential, resulting in the poor cycling stability.40 Based on the previous report,33 the capacity of bare CoSe2 is only about 300 mAh g-1 at 0.5 A g-1. Surprisingly, as exhibited in Fig.5a, the as-prepared sample shows the initial discharge/charge capacity of 545 and 467 mAh g-1 at 0.5 A g-1, yielding a coulombic efficiency (CE) of 85.6 %. These advantages may be due to unique nano-structure, bring about the adequate active sites. The capacity of 46 mAh g-1 was found for the pure carbon from citric acid. Note that the high initial CE is in favor of the practical application. From 2nd to 75th cycles, owing to the active process and side reactions of electrode, the decreased part was displayed. Subsequently, the stable cycling is obvious and no fading can be found form 100th to 500th cycles, delivering a capacity of 428 mAh g-1 with a CE of 100.3 %, resulted from the designed significant structure and the introduction of carbon with Co-O-C bonds. It is clear that a couple of oxidation/reduction platforms were located at 2.1 and 1.1 V, agreeing well with the CV curves in Fig. 3S. The peak at 1.3 eV was found at first CV curves, which is attributed to the formation of SEI film. Moreover, three discharge platforms were found at 1.6, 1.1, 0.7 V and two charge platforms were suited at 1.5, 1.8V for 100th, 200th, 500th, demonstrating the stable reaction of electrode as listed by the corresponding reaction process (S1, S2, S3, S4, S5) in Table 1, which have been 14


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demonstrated by previous report.41 The rate capability was evaluated at various current density of 0.2, 0.4, 0.8, 1.6, 3.2 and 6.4 A g-1, keeping capacities of 484, 447, 380, 335, 304 and 296 mAh g-1. When the current density comes back to 2.0 A g-1, the capacity could recover to 355 mAh g-1, implying its good reversibility. In addition, the excellent rate property was confirmed by the stable platforms at stepwise current densities in Fig.5d, revealing the lower pulverization. Impressively, after 10000 cycles at 4.5 A g-1, the Na-storage capacity still remains 345 mAh g-1 with ~ 100% retention based on 1000th cycle, suggesting the ultralong-term cycling stability and enormous practical promising, which is better than the previous reports in Table 2. And the bare CoSe2 with graphene displayed the relative low capacity of 280 mAh g-1 after 1000 cycles at 4.5 mAh g-1 in Fig. S2. In Fig. 5f, the stable platforms from 6000th to 6010th loops, and two sodiation/desodiation processes were completed in 1127s at 4.5 A g-1. The advantages of the electrode samples were concluded to effective framework structure, uniform carbon matrix and stable Co-O-C bonds.

15



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Table. 2 The comparsion of CoSe2 and other anodes with remarkable cycling stability employed in SIBs. Materials

CoSe2

CoSe2@ CoSe2

Manner

Electrolyte

hydrothermal

NaCF3SO3

method

DEGDME

Calcining ZIF-67

NaClO4

PGC/CNTs CoSe2@

Current

Cycles

Capacity

Density

(n)

(mAh g-1)

Voltage

Ref

(V)

(A g-1)

96%

0.4-3.0

5.0

2000

386

201740

75%

0.001-3

0.2

100

420

201742

80%

0.5-3.0

1.0

1000

390

201733

94%

0.5-3.0

1.0

1800

410

201741

37%

0.01-3.0

5.0

7000

210

201543

49%

0.01-3.0

6.7

10000

116

201844

/

0.15-2.5

2.1

10000

163

201745

57%

0.01-3.0

1.6

10000

104

201746

42%

0.01-3.0

10

10000

110

201747

85.6%

0.5-3.0

4.5

10000

345

The work

EC:DEC/PEC Calcining ZIF-67

CNTs

NaCF3SO3 DEGDME

Urchin-like

hydrothermal

NaCF3SO3

CoSe2

method

DEGDME

Electrospinning

NaClO4

N-doped carbon

CE

EC:DEC/PEC TiO2

hydrothermal

NaClO4

method

PC/PEC

NaVSnO4

Sol-gel manner

NaClO4 EC:DEC/PEC

MoP

Calcining

NaClO4 PC/PEC

TiO2/MoO2

CoSe2/C

hydrothermal

NaClO4

method

EC:DEC/PEC

Facile Calcining

NaCF3SO3 DEGDME

16


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Figure. 6 Kinetic analysis of CoSe2/C: (a) CV curve at small scan rates (0.1, 0.3, 0.5, 0.7 mV s-1), (b) CV curve at large scan rate (1.5, 3.0, 5.0, 7.0 mV s-1), (c) Plots of ν1/2 vs. I used for calculating the diffusion coefficient. (d) the liner of ν1/2 vs. I / ν1/2 for evaluating capacitive-controlled behaviors, (e) the liner of log (ν) vs. log (I) obtained form CV curve, (f) the plots of log (ν) vs. log (I) at 0.1 - 7.0 mV s-1, (g) the ratio of pseudo-capacitive capacity at 0.1 mV s-1, (h) Contribution ratio of capacitive vs. 17



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diffusion at stepwise scan rate. The kinetic analysis of CoSe2/C through CV curves were explored to understand the in-depth reason of the outstanding electrochemical performances. As shown in Fig. 6a, b for CV curves at 0.1, 0.3, 0.5, 0.7, 1.5, 3.0, 5.0 and 7.0 mV s-1, the weak peak at 1.6 V was included in the distinct oxidation peak at 1.7 V, meanwhile, three reduction ones were obvious at each curves, greatly coinciding with the discharge-charge platforms. And the similar shapes of each curves again verified the weak polarization. According to the previous reports,48-53 the equations as following were used to explore the kinetic behavior in detail.

Ip = 2.69×105 n3/2AD1/2v1/2CNa+

(1)

i = avb

(2)

log (i) = blog(v) + log(a)

(3)

I(V) = k1v + k2v1/2

(4)

In Equation 1, the n is the transform electrons in the reaction, the A is the area contracting with electrolyte, D is the diffusion coefficient, CNa+ is about the concentration of sodium ions, v and Ip are scan rate and the relative peaks current. The Ip/v1/2 can be fitting to 2.19, -1.31 for oxidation and reduction peaks, and the corresponding DNa+ is calculated to 1.03 × 10-12 and 0.37 × 10-12 cm2 s-1. The average diffusion coefficient can reach to 0.7× 10-12 cm2 s-1, revealing the fast ions shuttling. Benefitting from the Dunn’s work,50,51,54 the energy-storage types were clear, containing

Faradic

process

(pesudocapacitive

behaviors,

diffusion-controlled

behaviors) and non-Faradaic process (electric double layer). The equation 3 comes 18


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from equation 2, and when b is close to 1, indicating the capacitive-controlled behaviors, and the b approaches to 0.5, suggesting the diffusion-controlled behaviors. Clearly, in Fig. 6e, the b value of Peak1 ~ Peak 4 is 0.82, 0.78, 0.80 and 0.79, respectively, which demonstrate that the electrochemical properties for CoSe2/C was dominated by the pesudocapacitive behaviors. When the scan rate increased to 7.0 mV s-1, it is found that the slope of Peak 1 stabilize 0.80 in Fig. 6f. In Fig. 6c, the well liner relationship between ν1/2 and I/ν1/2 is observed, and the slope k1, intercept k2 can be obtained for calculating the ratio of capacitive behaviors. Based on the equation 4,

k1v is relation with the capacitive-controlled contributions and k2v1/2 is associated with insertion behavior. Fig. 6g displays the detail-fitting curve for pseudo-capacitive contribution at 0.1 mV s-1, agreeing well with the previous report.33 Taking increased scan rates into consideration, the capacitive contributions is rising, about 67% at 0.1 mV s-1, 75% at 0.3 mV s-1, 80% at 0.5 mV s-1, 81% at 0.7 mV s-1, 92% at 1.5 mV s-1, 94% at 3.0 mV s-1, 95% at 5.0 mV s-1 and 96% at 7.0 mV s-1. These enhanced capacitive behaviors disclosed that the capacitive behaviors contribute to major capacity at large current density, perhaps derived from the significant merits of structure.

19



Figure. 7 Electrochemical impedance analysis of CoSe2/C: (a) Nyquist plots at undischarged condition, (b) the corresponding equivalent circuit, (c) Nyquist plots after different cycling, (d) plots of w1/2 vs. -Z’’, (e) in-situ Nyquist plots at different discharge conditions and (f) at different charge conditions, (g) various platform vs. phase transform. 20


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The electrochemical impedance spectroscopy (EIS) was conducted to analyze the internal impedance and in-situ EIS was performed to investigate the phase transform accompanying with slight variation of resistance. The Nyquist plots at undischarged condition and the relative fitting liner were shown in Fig. 7a, and the relative equivalent circuit was exhibited in Fig. 7b. The curves can be divided into two parts: (1) the semicircle in high-frequency is about Rs (the resistance of materials in the cell, containing electrode, electrolyte, separation), Rf (the impedance of SEI films), Rct (the resistance of charge-transfer); (2) the line in low-frequency is about Zw (warbug impedance, the Na+-diffusion in target sample). Obviously, the small Rct of CoSe2/C is 39 Ω, which is due to the high conductivity of the as-obtained sample. After various cycles (10th, 30th, 60th, 100th, 200th), it is found that Rct almost keep unchanged at 34 Ω, revealing stable electrochemical reaction. According to the Equation 5, 6, 7 as following, the diffusion coefficient from the line in low-frequency could be obtained.55,56 ω = 2πf

（ 5）

Zre = Rs + Rct + Rf + σω-1/2

（ 6）

D = 0.5 R2T2/S2n4F4C2σ2

（ 7）

Fitting the Zre and ω-1/2 is to gain the Warbug factor σ about 51, 47, 45, 43 at 10th, 30th, 60th , 100th clcles, which is used for calculating that their DNa+ are 5.3 × 10-14, 6.3 × 10-14, 6.8 × 10-14, 7.4 × 10-14 cm2 s-1. The DNa+ differed from the analysis, which is ascribed to the different calculating models. With the proceeding cycling, their DNa+ 21



were increasing, which is ascribed to the slight residual Co from the incomplete reconversion reaction in the electrode. In-situ EIS was carried out to evaluate the reaction process at 500th cycle as displayed in Table 1. When discharged, at 2.5V, no sodium ions inserted into the samples. With the lowing voltage, the Rct is decreasing, which is attributed to that the pure Co, and Na2Se could provide effective with matrix high conductivity, facilitating the ions transferring. When charged, the resistance is increasing and returned to the pristine resistance, suggesting the high reversible and outstanding kinetic. The analysis of EIS demonstrated that the small resistance of the target product acts an important role in the electrochemical reaction.

22


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Figure. 8 The ex-situ exploration of CoSe2/C electrode 500 loops: (a) CV curves, (b) XRD patterns, (c) Raman spectra, (d) and (e) SEM images at charged 3.0 V, (f) and (g) TEM images at charged 3.0 V. In the sodiation/desodiation process, the stable crystalline structure and morphology is of importance for the electrode. The explorations of CoSe2/C electrode after 500 loops were probed through ex-situ technologies. In Fig. 8a, with the comparison of CoSe2/C after 3 cycles at 1.5 mV S-1, the peaks of CV curves after 500 23



cycles become sharper, indicating that the phase transition occurred in a narrow potential, which is beneficial for the jarless electrochemical properties. In Fig. S4, the CV curves from 501th to 508th were displayed, it is found that their shape keep same, indicating the stable electrochemical reaction. Furthermore, its XRD pattern coincides well with the pristine one, demonstrating its stable crystal structure, but the intensity of peaks is weaker, revealing the lowered crystalline. According to the discussion about Fig. 1a, the peaks moving towards high angels and the lowered full wave at half maximum demonstrate the deducing particles. In the Raman spectra, the peaks intensity of PVDF is almost similar, and that of CoSe2/C is weaken, which may be due to the lowered crystalline, the formation of SEI film and the reducing of particles.33 That is consistence with from the analysis of ex-situ XRD. Clearly, with the protection of carbon matrix, the shape of CoSe2 nanospheres keeps not changed after 500 cycles in Fig. 8d, e. Note that the morphology of CoSe2/C have some change but still keep similar sphere structure and placed in the carbon matrix, meanwhile, slight Co grains were produced in Fig. 8g. The spacing lattice of CoSe2 is found to keep stable. Moreover, the lattice distance (0.193nm) is indexing to the (111) facet of standard Co (JCPDs Card: 88-2325). In the illustration of Fig. 8g, the diffraction rings are indexing to (200), (211), (311) planes of standard CoSe2 (JCPDs: 88-1712) and (111) facets of Co. It can be concluded that the target sample display excellent stability of structure and morphology. 4. CONCLUSIONS In summary, the effective 3D hierarchical selenium cobalt framework assembled 24


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by 0D CoSe2@C cobblestone-like nanospheres was obtained through significant mixing and sintering. The citric acid supplies plentiful oxygen-containing functional groups, inducing the formation of uniform carbon matrix and Co-O-C bonds, thereby imparting improvement on the electrochemical properties. Utilized the adaptive electrolyte (NaCF3SO3/DEGDME), the ultra-long lifespan was shown at large current density of 4.5 A g-1 between 0.5 and 3.0V, delivering a capacity of 345 mAh g-1 after 10,000 cycles. In spite of upgrading potential platform, the excellent cycling stability is of important for the commercial application. Ex-situ characterization demonstrated the stable crystal structure and morphology texture after fast sodiation/desodiation process. In addition, the detailed kinetic analysis for CV and in-situ EIS further reveal the ions transferring process and conversion mechanism. It is anticipated that this paper can enhance the in-depth understanding of CoSe2/C with Co-O-C bonds and provide significant manner to obtain CoSe2/C as anodes for SIBs.

■ ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.///// Raw FTIR data of cobalt acetylacetonate and CoSe2/C, cycling performance of CoSe2/graphene, CV curves of CoSe2/C at 0.1 mV s-1, CV curves of CoSe2/C from 501th to 508th at 1.5 mV s-2, data information of standard XRD peaks. AUTHOR INFORMATION 25



Corresponding Author * Email address: [email protected]; [email protected] ; Tel: +86 731-88879616; Fax: +86 731-88879616.

ACKNOWLEDGMENT This work was ﬁnancially supported by This work was financially supported by National Key Research and Development Program of China (2017YFB0102000), National Natural Science Foundation of China (51622406, 21673298 and 21473258), National Postdoctoral Program for Innovative Talents (BX00192), China Postdoctoral Science Foundation (2017M6203552), Young Elite Scientists Sponsorship Program by CAST (2017QNRC001), Innovation Mover Program of Central South University (2017CX004, 2018CX005), and Hunan Provincial Science and Technology Plan (2017TP1001).

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