N-rich carbon coated CoSnO₃ derived from in situ construction of a Co–MOF with enhanced sodium storage performance

Abstract

In this study, N-rich carbon coated interconnected CoSnO₃ nanoboxes (CoSnO₃–NCs) with controllable amounts of N-rich carbon ranging from 7.5% to 24.6% were firstly prepared by the carbonization of CoSnO₃–MOFs. Impressively, the preparation of CoSnO₃–MOFs has been realized for the first time by directly utilizing CoSnO₃ as the precursor under solvent-free conditions, bridging between the cobalt(II) ion in the skeleton of CoSnO₃ and the 2-methylimidazole. Interestingly, the growth of these CoSnO₃–MOFs can be manipulated by changing the amount of 2-methylimidazole used, resulting in a tunable N-rich carbon content. Furthermore, the electrochemical storage behavior of these materials for SIBs was initially explored. Compared with the pure CoSnO_3, the as-resulted CoSnO₃–NC materials showed largely enhanced sodium storage performance with a reversible capacity of 493.9 mA h g⁻¹ at a current density of 0.1 A g⁻¹ after 100 cycles. Moreover, the as-prepared materials showed an excellent high rate storage performance with a remarkable capacity of 273.8 mA h g⁻¹ at 1 A g⁻¹ after 1000 cycles. This work provides a new approach for constructing Co–MOFs as well as providing an efficient N-rich carbon-coating route, which may be expanded to other Co-based oxides and can greatly expand the development of new species of Co-based MOFs.

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1. Introduction

In the last few decades, metal–organic frameworks (MOFs), a new family of crystalline porous materials made up of metal ions and interconnected organic linkers, have been widely investigated for their tunable porosities, various structural topologies, and functionalities.^1–6 Their metallic ions mainly include the divalent and trivalent ions of 3d transition metals (Zn, Co, Fe, Ti, Ni, etc.), and the organic linkers primarily incorporate the organic carboxylic acid class and imidazole class.^7–16 Amongst these metals, cobalt is a very tempting candidate owing to the high catalytic activity of Co species.¹⁷ ZIF-67, one of the most widely used Co-based MOFs composed of Co²⁺ ions and 2-methylimidazole, is broadly utilized in many areas, such as gas separation, catalysis, and energy storage.^17–20 Currently, the reactions for synthesizing Co-based MOFs are carried out in solution including N,N-dimethyl formamide, ethanol, and water.^8,17,21,22 ZIF-67 thin films obtained by dissolving Co(NO₃)₂·6H₂O and 2-methylimidazole in methanol solutions delivered exceptionally long-lived excited states, exhibiting excellent photocatalytic performance.²³ However, if the synthetic process can be achieved without any solvent, the separation of the product will be greatly simplified, and the process can become more meaningful from the perspective of economic and environmental protection. Furthermore, the morphology characteristics of the Co-based MOFs are relatively unitary when synthesised through hydrothermal methods. The Co-based MOFs obtained from the traditional precursor (Co(NO₃)₂·6H₂O) only possess a single metal element, Co, limiting their potential application. In order to solve this problem, recently, a core–shell ZIF-67@ZIF-8 containing both Zn/Co elements was obtained via the growth of ZIF-8 at the surface of a ZIF-67 seed in methanol.²⁴ Although the material showed excellent performance for the semi-hydrogenation of acetylene, materials with a more simple preparation method and a homogeneous distribution of the bimetal element are urgently needed. Hence, it will be great to build the bridging between the Co-based double metal oxide and the organic ligands. Given this, new methods to construct Co-based MOFs and the development of new species of Co-based MOFs will be fascinating.

Sodium-ion batteries (SIBs) have been deemed as the most hopeful alternative for potential application in large-scale energy storage by considering their low cost and low environmental impact.^25–39 Up to now, most MOF materials could not be directly employed as an anode for SIBs because of their poor electronic conductivity. In order to better utilize their advantages for SIBs, MOFs were usually employed as precursors to prepare electrochemically active materials by in situ carbonization, such as MOF-based derivative porous carbons, metal oxides, and metal oxide/carbon.^8,10,40–42 Notably, the volume expansion of the metal oxide during the discharge/charge processes can be effectively released and the electronic conductivity is also improved by introducing a carbon matrix.^9,41,42 Hierarchical hollow NiO/Ni/graphene was prepared from the carbonization of an Ni–MOF, and a capacity of about 190 mA h g⁻¹ at a current density of 0.2 A g⁻¹ was obtained after 200 cycles.⁴³ Although some progress has been achieved for MOF-based anode materials, the MOF-based derivative materials are mainly acquired by a simple carbonization of several classic MOFs, meaning that the types of MOF-based anode material are limited. Furthermore, the electrochemical performance of these materials for SIBs cannot be effectively further evaluated, showing that their application in power storage is still at an early stage in comparison to their colourful usage in the fields of catalysis and drug delivery. CoSnO₃, a good anode material for LIBs, has not yet been explored as an electrode material for SIBs.⁴⁴ Considering that the current approaches for the preparation of carbon coatings are relatively complex and the carbon amounts cannot be effectively regulated, a new approach was suggested as shown in Scheme 1.

Scheme 1 The design process for the preparation of CoSnO₃–MOFs and CoSnO₃–NCs.

In this paper, a series of CoSnO₃–NC nanobox materials with controllable carbon content were acquired by the in situ carbonization of CoSnO₃–MOFs, showing a controllable N-rich carbon content ranging from 7.5% to 24.6%. The CoSnO₃–MOFs were prepared for the first time via the chemical bonding method using the bridge role of Co²⁺ under solvent-free conditions (Scheme 1). Also, the storage behavior of the CoSnO₃–NC material for SIBs was firstly explored. Overall, our work provides a new approach to construct Co-based MOFs and offers a carbon coating method for the Co-based oxides, greatly enhancing the electrochemical storage performance for SIBs with a high reversible capacity of 493.9 mA h g⁻¹ at 0.1 A g⁻¹ after 100 cycles.

2. Experimental section

2.1 Materials synthesis

2.1.1 Preparation of CoSnO₃–MOF. The synthesis of CoSnO₃ nanoboxes was based on a reported method with some modifications. Typically, 10 mL ethanol (Alfa Aesar, Ar) solution containing 0.522 g SnCl₄ was dropwise added into a 60 mL aqueous solution composed of 0.0476 g CoCl₂·6H₂O (Alfa Aesar, 95%) and 0.588 g sodium citrate (Alfa Aesar, 98%) by a separating funnel under vigorous stirring. After 5 min, 10 mL aqueous solution containing 4 g NaOH (Shanghai Titan Scientific Co., Ltd. 95%) was dropwise added to it. After magnetic stirring for 1 h, 40 mL NaOH aqueous (8 M) solution was dropwise added into the above obtained solution. The product was gained by centrifugal separation and was washed by water three times. After being dried at 80 °C overnight under vacuum, CoSn(OH)₆ was acquired. Finally, the CoSnO₃ nanoboxes were obtained by annealing CoSn(OH)₆ in Ar flow at 300 °C for 2 h with a heating ramp of 2 °C min⁻¹. 120 mg CoSnO₃ and 120 mg 2-methylimidazole (Alfa Aesar, 95%) were ground into a powder and transferred into an 80 mL autoclave with a Teflon-lined interior. After heating at 200 °C for 24 h, a lavender coloured powder was obtained. The as-obtained solid was marked as CoSnO₃–MOF-120. For comparison, CoSnO₃–MOFs prepared using 60 mg and 240 mg 2-methylimidazole were denoted as CoSnO₃–MOF-60 and CoSnO₃–MOF-240, respectively.

2.1.2 Preparation of CoSnO₃–NC. The CoSnO₃–NC nanoboxes were prepared via the carbonization of a series of CoSnO₃–MOFs in Ar atmosphere. Specifically, CoSnO₃–NC materials derived from CoSnO₃–MOF-60, CoSnO₃–MOF-120, and CoSnO₃–MOF-240 under 500 °C were separately recorded as CoSnO₃–NC-60, CoSnO₃–NC-120, and CoSnO₃–NC-240, respectively.

2.2 Materials characterization

The crystalline phases of the above series of materials were detected using an X-ray diffractometer (XRD, Rigaku, Rint-2000) with a 0.154 nm Cu-Kα radiation. The morphology and composition of the as-obtained samples were recorded using scanning electron microscopy (SEM) (JSM 6400), transmission electron microscopy (TEM) (JEOL) and X-ray photoelectron spectroscopy (XPS, PerkinElmer, USA). The BET surface area and pore size distributions were measured using a Micromeritics ASAP 2020 analyzer. Thermogravimetric analysis (TGA) data were obtained using a thermal analysis instrument (NETZSCH STA449F3) with a heating rate of 5 °C min⁻¹ from room temperature to 600 °C in air. The electronic conductivities were collected using the standard four probe method at room temperature with pellet samples (FOUR-POINT PROBE METER, SDY-4D, China).

2.3 Electrochemical measurements

All the electrochemical measurements were tested using the half-cells (CR2016-type). The electrodes were prepared by uniformly dispersing a water slurry made up of active material, Super P (conductive carbon, TIMCAL), and binder carboxymethyl cellulose (CMC, Alfa Aesar) with a mass ratio of 14 : 3 : 3 onto Cu foil. After being dried at 90 °C for 12 h under vacuum, the CR2016-type half-cells were assembled in a Braun-glovebox in argon atmosphere. The mass loading of the active material is about 1.0–1.3 mg cm⁻². The electrolyte was composed of NaClO₄ (1 M) (Alfa Aesar, AR) and propylene carbonate (PC, Alfa Aesar, AR). The polypropylene film (Celgard 2400) and metallic sodium (Sinopharm Chemical Reagent Co., Ltd, 99.9%) were separately utilized as the separator and counter electrode for the SIBs. Cyclic voltammetry (CV) plots were recorded using a Solartron 1470 Multistat system and the galvanostatic charge/discharge tests were measured using an Arbin BT2000 instrument in the voltage range of 0.01 to 3.0 V (vs. Na/Na⁺). Electrochemical impedance spectroscopy (EIS) results were also obtained using a Solartron Analytical instrument under an AC voltage of 5 mV amplitude between 0.01 Hz and 100 kHz. Besides this, an equivalent circuit was utilized to fit the EIS results.

3. Results and discussion

3.1 Solvent-free synthesis and characterization of the CoSnO₃–MOF

As shown in Scheme 1, CoSnO₃ nanoboxes were chosen as the precursor of the Co²⁺-source, and 2-methylimidazole was utilized as the organic ligand. When heating up to 200 °C, 2-methylimidazole first melted and permeated the surface of CoSnO₃, and then was chemically bonded with Co²⁺, resulting in the formation of a surface Co-based MOF at the internal and external surface of the CoSnO₃ nanoboxes. The formation of the CoSnO₃–MOFs was confirmed by the X-ray diffraction (XRD) characterization presented in Fig. 1a. Apparently, unlike the amorphous structure of CoSnO₃, CoSnO₃–MOF-60, CoSnO₃–MOF-120, and CoSnO₃–MOF-240 exhibit the same characteristic peaks as the standard Co–MOF(ZIF-67).¹⁷ Note that the characteristic peaks in CoSnO₃–MOF-60 corresponding to ZIF-67 are not obvious, which was attributed to the incomplete growth of CoSnO₃–MOF resulting from the insufficient amount of 2-methylimidazole. Remarkably, the characteristic peak intensity of CoSnO₃–MOF-60, CoSnO₃–MOF-120 and CoSnO₃–MOF-240 became stronger with the higher amounts of 2-methylimidazole, showing that the growth degree of CoSnO₃–MOFs can be manipulated by the amount of 2-methylimidazole used. This also convincingly demonstrates the formation of the surface CoSnO₃–MOF, and this method may be superior to the traditionally uncontrolled hydrothermal preparation method. The N₂ adsorption–desorption isotherms and pore distributions of CoSnO₃ and CoSnO₃–MOF-120 are depicted in Fig. 1b and c. The surface area of CoSnO₃–MOF-120 is 526 m² g⁻¹, about ten times larger than that of CoSnO₃ (48.5 m² g⁻¹). Besides, the pore distribution of CoSnO₃–MOF-120 is centered at 1.16 nm, rather than at 9.33 nm like CoSnO₃, which demonstrates the formation of CoSnO₃–MOF from another point.

Fig. 1 (a) The XRD patterns of the obtained samples. (b) and (c) N₂ adsorption–desorption isotherms and pore size distribution of the related samples. The SEM (d) and TEM (e) images of the CoSnO₃ nanoboxes. The SEM (f) and TEM (g–i) images of the CoSnO₃–MOF-120 nanoboxes.

SEM and TEM were employed to explore the morphology during the formation of CoSnO₃–MOF-120. As shown in Fig. 1d and e, the SEM and TEM images show that CoSnO₃ exhibits a nanobox shape with a shell width of about 30–40 nm, in which the inner cavities are obviously exposed by the sharp difference between the shells and their hollow interiors. Interestingly, as depicted in Fig. 1f–i, CoSnO₃–MOF-120 presents a very uniformly distributed nanobox shape, suggesting that the reaction between Co²⁺ and 2-methylimidazole does not destroy the original structure of CoSnO₃. Besides, a layer of membrane-like substance is clearly observed and well covers the surface of CoSnO₃, connecting the CoSnO₃ nanoboxes together. Note that the inner cavities are not apparent following the formation of the surface film.

The impact of different amounts of 2-methylimidazole on the morphology was also explored as shown in Fig. 2. A very thin layer of the membrane can be seen at the surface of CoSnO₃–MOF-60, when observed from the SEM and TEM images displayed in Fig. 2b, compared with CoSnO₃ in Fig. 2a. The thicknesses of the surface membrane in CoSnO₃–MOF-60 (Fig. 2b), CoSnO₃–MOF-120 (Fig. 2c), and CoSnO₃–MOF-240 (Fig. 2d) became thicker with the incremental amounts of 2-methylimidazole, indicating that the growth of the MOFs increased with more 2-methylimidazole. It should be noted that the CoSnO₃ nanoboxes are totally coated by the 2-methylimidazole layer in the CoSnO₃–MOF-120 and CoSnO₃–MOF-240 samples. Besides, unlike the relatively smooth surface of the CoSnO₃, the surface of CoSnO₃–MOFs becomes rougher and the inner cavities become less obvious. Entertainingly, considering the colourful colour features of the Co-based compounds, the colours of CoSnO₃–MOFs are displayed in Fig. S1,† exhibiting a change from dark grey (CoSnO₃–MOF-60) to purple (CoSnO₃–MOF-120), and lastly to bright purple (CoSnO₃–MOF-240), which means the MOFs are closer to the colour of ZIF-67 when the higher amounts of 2-methylimidazole are used. Furthermore, the BET specific areas of CoSnO₃, CoSnO₃–MOF-60, CoSnO₃–MOF-120, and CoSnO₃–MOF-240 are 48.5, 202.6, 526, and 806 m² g⁻¹ (Fig. S2†), respectively, indicating an increasing trend. The elemental mapping under TEM has been conducted to confirm the formation of ZIF-67. As shown in Fig. S3,† the C, N, Co, Sn, and O elements are uniformly distributed, proving that the formation of ZIF-67 is consistent throughout all the particles.

Fig. 2 The SEM and TEM images of CoSnO₃ (a), CoSnO₃–MOF-60 (b), CoSnO₃–MOF-120 (c), and CoSnO₃–MOF-240 (d).

3.2 Characterization of the CoSnO₃–NC materials

After carbonization in Ar atmosphere, a series of CoSnO₃–NC materials were obtained. As shown in Fig. S4,† CoSnO₃–NC-120 exhibits the characteristic peaks of amorphous CoSnO₃ (between 30 and 40°) and amorphous carbon (around 24° and 43°) resulting from the carbonization of the organic linkers.⁴⁴ The nitrogen sorption isotherm of CoSnO₃–NC-120 (Fig. 3a) shows a typical type IV hysteresis loop, proving the coexistence of the mesopores and micropores,⁴⁵ which can allow better electrolyte wetting. The surface area of CoSnO₃–NC-120 is 58.84 m² g⁻¹, which is far less than that of CoSnO₃–MOF-120, and this was attributed to the carbonization of 2-methylimidazole. Moreover, the surface areas of CoSnO₃–NC-60 and CoSnO₃–NC-240 are 50.2 and 62.8 m² g⁻¹ (Fig. S5†), respectively, which might be ascribed to their different carbon content derived from the organic precursor. The mesopores and micropores of CoSnO₃–NC-120 are mainly centered at 3.9 nm and 1.4 nm, respectively (Fig. 3b). Besides, as shown in Fig. S5,† CoSnO₃–NC-60 and CoSnO₃–NC-240 present a similar nitrogen sorption isotherm to CoSnO₃–NC-120 (type IV), indicating that they possess similar pore structures. Furthermore, the carbon content of the CoSnO₃–NC materials was measured using the TGA method. The carbon content of CoSnO₃–NC-60 (Fig. 3c), CoSnO₃–NC-120 (Fig. 3d), and CoSnO₃–NC-240 (Fig. 3e) was 7.5%, 14.6%, and 24.6%, respectively, which can effectively improve the electronic conductivity of the CoSnO₃ nanoboxes. Furthermore, the conductivity value of CoSnO₃, CoSnO₃–NC-60, CoSnO₃–NC-120, and CoSnO₃–NC-240 was 8.6 × 10⁻⁷, 8.8 × 10⁻⁴, 7.5 × 10⁻³ and 2.6 × 10⁻³ S cm⁻¹, respectively, also showing an elevation in conductivity.

Fig. 3 (a) The N₂ adsorption–desorption isotherms of the obtained sample, and (b) the pore size distribution of CoSnO₃–NC-120. TGA curves of (c) CoSnO₃–NC-60, (d) CoSnO₃–NC-120, and (e) CoSnO₃–NC-240. (f) to (i) are the XPS survey and N1s, Co 2p, and Sn 3d spectra of the CoSnO₃–NC-120 sample, respectively.

To further characterize the element species and the element valences of the as-obtained samples, XPS experiments were implemented and the results are depicted in Fig. 3f–i. The XPS survey of CoSnO₃–NC-120 (Fig. 3f) showed that the sample consisted of Co, Sn, C, N, and O elements. For comparison, the XPS surveys of CoSnO₃, CoSnO₃–NC-60, and CoSnO₃–NC-240 are displayed in Fig. S6a.† According to the XPS results, CoSnO₃, CoSnO₃–NC-60, CoSnO₃–NC-120, and CoSnO₃–NC-240 show a surface nitrogen content of 0.0%, 2.4%, 4.56% and 7.6%, respectively, exhibiting a controllable and high surface nitrogen content, and this can enhance the electronic conductivity of the as-obtained materials and further optimize their electrochemical performance.⁴⁶ Fig. S6b† shows the C 1s XPS spectrum, and the three peaks located at 284.8, 286.5, and 288.3 eV can be attributed to the sp² or sp³ hybridized carbon (C–C or C=C), the C–O or C–N bond, and the C=O bond, respectively.⁴⁷ From the N1s spectrum depicted in Fig. 3g, the three peaks centered at 398.5, 400.4, and 400.8 eV may be separately related to pyridinic N, pyrrolic N, and graphitic N, respectively.¹⁹ The high-resolution XPS spectrum of Co 2p (Fig. 3h) exhibits two major peaks around 780.9 and 796.9 eV accompanied by a broad satellite peak, and these were assigned to Co 2p_3/2 and 2p_1/2 derived from Co²⁺,^48,49 which indicates that the valence of Co²⁺ almost remains unchanged during the carbonization process. The Sn 3d peak in the survey scan can be allocated into two parts (Fig. 3i), and the peaks located at 486.3 and 495.1 eV are separately attributed to Sn 3d_5/2 and Sn 3d_3/2 which is well in accordance with the electronic state of Sn⁴⁺.⁴⁹

The morphologies of the CoSnO₃–NC materials were also marked by SEM and TEM characterization, and the results are depicted in Fig. 4. It is clearly observed that the nanobox shape of CoSnO₃–MOF is well kept after carbonization at 500 °C. Compared to the unobvious carbon layer in CoSnO₃–NC-60 (Fig. 4a), the carbon layer in CoSnO₃–NC-120 (Fig. 4b) can be easily noted. Moreover, the carbon layer is too thick to easily observe the CoSnO₃ nanoboxes in CoSnO₃–NC-240 (Fig. 4c) according to the SEM images, and these observations are similar to those of the corresponding CoSnO₃–MOFs. These nitrogen-rich carbon coated structures can largely enhance the electronic conductivity, effectively ease the volume expansion, and thus avoid the structure collapse during the charge/discharge processes. Besides, note that the CoSnO₃–NC nanoboxes are stacked by countless nanoparticles with a carbon layer covering their surface according to the TEM images. Also, note that the carbon layers are obviously observed in the HRTEM images and become thicker with the more 2-methylimidazole used, showing controllable carbon content of the CoSnO₃–NC material.

Fig. 4 The SEM and TEM images of CoSnO₃–NC-60 (a), CoSnO₃–NC-120 (b), and CoSnO₃–NC-240 (c).

3.3 Electrochemical performance tests

Fig. 5a/S7a† show the charge/discharge cycling performances of the as-obtained samples for SIBs. The first discharge/charge capacities of CoSnO₃, CoSnO₃–NC-60, CoSnO₃–NC-120, and CoSnO₃–NC-240 were 927.2/558.5, 899.2/526, 879.2/494.7, and 733.8/442.9 mA h g⁻¹ at a current density of 100 mA g⁻¹ within a voltage range of 0.01 to 3.0 V. The capacity loss in the initial cycle might be attributed to the unavoidable formation of a solid electrolyte interface (SEI) layer, some irreversible side reactions between the electrolyte and the sodium species, electrolyte decomposition, and the irreversible insertion process, which is a common phenomenon for most of the SIB anode materials.^{44,46,47,50,51} Moreover, CoSnO₃, CoSnO₃–NC-60, CoSnO₃–NC-120, and CoSnO₃–NC-240 separately displayed a capacity of 218.3, 293.1, 493.9, and 399.9 mA h g⁻¹ at 100 mA g⁻¹ after 100 cycles. The capacity of CoSnO₃–NC-60 gradually decreased with the cycling numbers, and this may be attributed to the fact that there was insufficient carbon to completely coat the CoSnO₃ particles. CoSnO₃–NC-120 and CoSnO₃–NC-240 were stable after 100 cycles. However, the capacity of CoSnO₃–NC-240 was lower than that of CoSnO₃–NC-120 due to the high carbon content, showing that a carbon content of 14.6% is the optimal coating. Overall, the big difference in cycling performance after 100 cycles is attributed to the different amounts of carbon element, which can effectively enhance the electronic conductivity and maintain the structural stability through the carbon-wrapped and interconnected carbon structures.

Fig. 5 (a) The discharge capacities of the corresponding samples, and (b) the rate capability of the as-obtained products. (c) The first five CV curves and (d) the discharge/charge profiles of the half-cells utilizing CoSnO₃–NC-120 as the electrode, and (e) the long cycling life tests of the half-cells utilizing CoSnO₃–NC-120 as the electrode at a high current density of 1 A g⁻¹.

Importantly, no noticeable morphology changes were detected for the CoSnO₃–NC-120 anode after 100 cycles at a current density of 100 mA h g⁻¹ (Fig. 6), which confirms that the carbon layer can effectively stabilize the structure and remit the large volume change during the insertion/extraction of sodium ions.

Fig. 6 (a) and (b) The SEM images of CoSnO₃–NC-120 after cycling for 100 cycles.

Fig. 5b presents the rate performance of the as-obtained anode materials measured at current densities of 0.1, 0.2, 0.4, 0.8, 1.6, 3.2, and 5.0 A g⁻¹. As expected, CoSnO₃–NC-120 delivers a superior rate performance with a reversible capacity of 519.2, 472.8, 403.7, 322.8, 249.1, 197.3, and 168.1 mA h g⁻¹ at 0.1, 0.2, 0.4, 0.8, 1.6, 3.2, and 5.0 A g⁻¹, respectively. Furthermore, CoSnO₃–NC-240 also shows a relatively good rate performance when compared with that of CoSnO₃and CoSnO₃–NC-60. The capacities of CoSnO₃–NC-120 and CoSnO₃–NC-240 can recover to 512.6 and 421.7 mA h g⁻¹, respectively, nearly equaling their initial capacities, which indicates the high stability of the material under high-rate charge/discharge processes. Comparing with the poor performance of CoSnO₃, the excellent rate performance might be assigned to the carbon layer on the surface which helps to maintain structural integrity, increasing the contact area between the active material and the electrolyte, and buffering the volume expansion.

The first five cyclic voltammetry (CV) curves of the CoSnO₃–NC-120 electrode were plotted in the voltage range of 0.01 to 3.0 V under 0.1 mV s⁻¹ for sodium-ion storage. As depicted in Fig. 5c, the reduction of the peak intensities between the initial cycle and the subsequent cycles suggests an irreversible capacity loss in the first discharge process, which might be connected to the formation of a solid–electrolyte interface (SEI) film on the surface of the electrodes, and to some irreversible side reactions. Importantly, after the second cycle, the CV curves of the CoSnO₃–NC-120 active material virtually overlap with the succeeding cycles, which demonstrates a good stability and cycling performance of CoSnO₃–NC-120 during the uptake and release of sodium ions. Fig. 5d shows the galvanostatic charge–discharge profiles of CoSnO₃–NC-120 under the 1st, 5th, 20th and 50th cycles at 100 mA g⁻¹. CoSnO₃–NC-120 exhibited a discharge capacity of 879.2, 506.3, 499.2, and 490.1 mA h g⁻¹ under the 1st, 5th, 20th and 50th cycles at 100 mA g⁻¹, respectively, which is far better than the performance of CoSnO₃ with corresponding discharge capacities of 927.2, 461.7, 394.2, and 296.4 mA h g⁻¹ (Fig. S7b†). Besides, like in most metal-oxide electrodes,^42,52,53 no obvious plateau-like features were observed in any of the discharge/charge profiles and the discharge profiles of CoSnO₃–NC-120 reflect a long slope below 1.5 V. More importantly, CoSnO₃–NC-120 delivered a superior long-term cycling stability with a high capacity of 273.8 mA h g⁻¹ after 1000 cycles at 1 A g⁻¹ (Fig. 5e), and this can be attributed to the stable structure derived from the interconnected carbon layer. This value is better than the capacities of most of the reported metal-oxide electrodes for sodium ion storage.

The electrochemical impedance spectroscopy (EIS) of the half-cells employed CoSnO₃, CoSnO₃–NC-60, CoSnO₃–NC-120 and CoSnO₃–NC-240 as anodes, and were measured after 5 cycles at a certain voltage of 0.8 V. As shown in Fig. 7a, the semicircle in the high frequency region and the inclined line in the low frequency region of the Nyquist plots can be separately related to the charge transfer resistance and the sodium-ion diffusion in the electrode.⁵⁴ The charge transfer impedances (R₃) of CoSnO₃, CoSnO₃–NC-60, CoSnO₃–NC-120, and CoSnO₃–NC-240 after 5 discharge/charge processes are 4860, 2640, 1380, and 1180 Ω, separately, which may be attributed to the N-rich carbon coated and interconnected structure. Besides, the charge transfer resistance of CoSnO₃–NC-120 under different cycles is depicted in Fig. 7b. The charge transfer impedance of the fresh cells was 920 Ω. The charge transfer impedance increased to 1380 Ω after 5 cycles, which may be attributed to the formation of the SEI film. Subsequently, the charge transfer impedance decreased from 1380 to 326 Ω when the cycling numbers increased from 5 to 50 cycles, and this can be ascribed to the remnant Na⁺ stemming from the irreversible process and the enhanced wettability of the electrode.^9,55 The comparison of CoSnO₃–NC-120 to other metal oxides for sodium storage performance reported in previous reports is provided in Table 1. The outstanding electrochemical performances should be ascribed to the N-rich carbon wrapped and interconnected structures, which can efficiently modify the electronic conductivity, increase the mechanical stabilities, and relieve the volume expansion.

Fig. 7 (a) The Nyquist plots of CoSnO₃, CoSnO₃–NC-60, CoSnO₃–NC-120, and CoSnO₃–NC-240 after five cycles and the inset is the equivalent circuit, and (b) the Nyquist plots of CoSnO₃–NC-120 with different cycle numbers.

Table 1 The storage performance comparison of CoSnO₃–NCs and other metal oxide anodes for SIBs reported in previous literatures

Anodes	Cycling performance (mA h g^−1)	Rate capability (mA h g^−1)	References
Carbon-confined SnO2	374 at 50 mA g^−1 after 100 cycles	∼307 at 0.2 A g^−1	56
		∼184 at 0.8 A g^−1
Nanostructured Co3O4	∼447 at 0.05 A g^−1 after 50 cycles	Not given	57
Co3O4 sheets/3D graphene	∼523.5 at 25 mA g^−1 after 50 cycles	∼82.3 at 0.5 A g^−1	58
CuO/Cu2O	∼415 at 0.05 A g^−1 after 50 cycles	∼273 at 0.5 A g^−1	42
	212 at 1 A g^−1 after 400 cycles	∼153.8 at 2.5 A g^−1
SnO2/reduced graphene oxide	∼333 at 0.1 A g^−1 after 150 cycles	∼125 at 1 A g^−1	59
Carbon coated rutile TiO2	∼175 at 84 mA g^−1 after 200 cycles	∼86.8 at 1.68 A g^−1	9
		∼70.6 at 3.36 A g^−1
Anatase TiO2@C	167.4 at 100 mA g^−1 after 110 cycles	125.5 at 0.5 A g^−1	60
		111.1 at 1 A g^−1
		88.9 at 2.5 A g^−1
MgFe2O4 hollow microboxes	135 at 50 mA g^−1 after 150 cycles	172 at 0.1 A g^−1	61
		85 at 1 A g^−1
Porous CoFe2O4 nanocubes	350 at 50 mA g^−1 after 50 cycles	329.7 at 0.2 A g^−1	62
		224 at 1 A g^−1
		175.5 at 2.5 A g^−1
CoSnO3–NCs	∼493.9 at 0.1 A g^−1 after 100 cycles	∼249.1 at 1.6 A g^−1	This work
	∼273.8 at 1 A g^−1 after 1000 cycles	∼197.3 at 3.2 A g^−1
		∼168.1 at 5 A g^−1

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4. Conclusion

In summary, a series of CoSnO₃–NC nanobox materials with controllable amounts of N-rich carbon have been prepared by regulating the growth of the CoSnO₃–MOFs under different amounts of 2-methylimidazole, in which the CoSnO₃ materials are well wrapped and interconnected by a N-rich carbon layer. Significantly, a simple and solvent-free approach for the synthesis of novel CoSnO₃–MOFs was provided through the chemical bonding between Co²⁺ and 2-methylimidazole. The CoSnO₃–MOFs present a nanobox shape with a layer substance consisting of a CoSnO₃–MOF covering it, and a high specific surface area. Furthermore, a superior sodium-ion storage performance of 493.9 mA h g⁻¹ at 0.1 A g⁻¹ can be acquired after 100 cycles and a reversible capacity of 273.8 mA h g⁻¹ is reserved at 1 A g⁻¹ after 1000 cycles. This work offers a new method to prepare Co–MOFs and a good N-rich carbon-coating route for Co-based oxides, which may also be suitable for use with other Co-based oxides.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the Young Elite Scientists Sponsorship Program by CAST (2017QNRC001), the National Postdoctoral Program for Innovative Talents (BX201600192), the National Natural Science Foundation of China (51622406, 21673298 and 21473258), the Innovation-Driven Project of Central South University (No. 2017CX004), the Fundamental Research Funds for the Central Universities of Central South University (2017zzts115 and 2017zzts454), the China Postdoctoral Science Foundation (2017M6203552), the National Key Research and Development Program of China (2017YFB0102000), and the Hunan Provincial Science and Technology Plan (2017TP1001).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ta10448k

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