Facile synthesis of selenium/potassium tartrate derived porous carbon composite as an advanced Li–Se battery cathode

Abstract

Porous carbon with a unique 3D structure has been prepared by the immediate carbonization of potassium tartrate. At an optimal carbonization temperature of 700 °C, the carbon mainly composed of micro- and small meso-porous structure has a Brunauer–Emmett–Teller (BET) specific surface area of 816.2 m² g⁻¹, and the element Se with an amorphous structure is uniformly encapsulated into the porous structure of carbon. The weight ratio of Se in the composite can reach ∼50%. As the Li–Se battery cathode, the composite shows a (2nd) reversible discharge capacity of 550.5 mA h g⁻¹ with an initial coulombic efficiency of 68.2% at 0.24C, and a discharge capacity of 485.3 mA h g⁻¹ can be retained after 80 cycles. Even at a high current density of 1.2C, the cell also delivers a stable discharge capacity of about 452.3 mA h g⁻¹. The good electrochemical performances of the as-prepared composite may be attributed to high specific surface area and small porous size.

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1. Introduction

Sustainable, highly efficient and environmentally friendly energy storage systems have attracted more and more attention accompanying the development of mobile devices and electric vehicles. Among these systems, lithium–sulfur (Li–S) batteries have been regarded as one of most promising systems due to their high theoretical specific capacity (1672 mA h g⁻¹), energy density (2600 W h kg⁻¹) and low cost.^1,2 However, the insulating nature of sulfur and high dissolubility of the polysulfide intermediates during cycling eventually result in low rate capability, low sulfur utilization and poor cycling stability.^3,4 Similarly, the same family element selenium (Se) can be used as a cathode for homogenous Li/Na–Se batteries. Importantly, the selenium has a high electronic conductivity (1 × 10⁻³ S m⁻¹), approximately 20 orders of magnitude greater than that of sulfur (1 × 10⁻²⁸ S m⁻¹), which suggests the homogeneous lithium–selenium (Li–Se) battery can possess better rate capability and higher selenium utilization. Furthermore, the carbonate-based electrolytes can be used for Li–Se batteries instead of the more expensive ether-based electrolytes used in Li–S batteries.^5–10

However, the electrochemical performances of pristine Se are poor compared with commercial lithium ion battery cathodes such as LiMn₂O₄ and LiCoO₂. In order to improve the electrochemical performances especially cycling stability and rate capability of Se, one of most effective route is to construct selenium/porous carbon composite. Therein, the carbon framework and porous structure can provide excellent electron channel, storage reasonable amount of Se, inhibit the dissolution of polyselenides, and buffer the volume change during cycling.^6,7,11–13 According to previous literatures, it can be found that the porous size of carbon substrate have a great effect on the electrochemical performance of Se/C composite, and the carbon with smaller porous size and larger specific surface area will possess better electrochemical performances.^14–18 For example, Li et al. have prepared nitrogen-doped carbon sponge composed of hierarchical microporous carbon layers from metal–organic framework. The Se is impregnated into 0.4–0.55 nm micropores of as-prepared carbon, and resultant composite can deliver a discharge capacity of 443.2 mA h g⁻¹ at the 200^th cycle with a coulombic efficiency up to 99.9% at 0.5C.¹⁹

Thus, it's a challenge to produce porous carbons with extremely large surface areas, high pore volumes and porosity made up of micropores (<2 nm) and small mesopores (2–5 nm) as carbon substance of Li–Se cathode.^14–19 An initial carbonization of organic compounds and subsequent chemical activation with certain materials (KOH, NaOH, K₂CO₃ or ZnCl₂ et al.) are usually used to preparation of porous carbon.^20–22 By comparison, a direct carbonization of organic salts such as calcium/potassium/sodium citric, potassium gluconate can obviously save experimental time and reduce cost, and the intermediate formed KOH and/or K₂CO₃ accompanying with carbonization can play a role as self-activation agent to produce microporous structure.^23–28 In the present study, the potassium tartrate hemihydrate (C₄H₄K₂O₆·0.5H₂O) is used as raw material to prepare porous carbon material, and the obtained carbon show overall 3D characteristic with inner microporous and small meso-porous structure. Then, the Se/carbon composite is obtained from melting diffusion route using above optimal prepared carbon as substrate. The structures, morphologies, porous properties and electrochemical performances of porous carbon and corresponding Se/C composite are clearly discussed in the text.

2. Experimental

2.1 Synthesis of potassium tartrate derived porous carbon

All of reagents are analytical grade and were used as received. 5–10 g of potassium tartrate was loaded in a porcelain boat, and then directly heated to a target temperature (i.e., 600, 700 or 800 °C) in a horizontal tube furnace at a heating rate of 5 °C min⁻¹. The sample was kept at this temperature for 2 h under flowing Ar atmosphere. After cooling to room temperature, the obtained black powder was washed with diluted hydrochloric acid until pH comes to 7, subsequent deionized water and ethanol. Finally, the black carbon was dried at 80 °C under air. The C-700 denoted in the following text indicates the sample was obtained at 700 °C, and C-600 and C-800 were similar.

2.2 Synthesis of Se/microporous carbon composite

0.2 g of above carbon and 0.3 g of selenium powder (Aladdin China) was well mixed in an agate mortar using a small amount of ethanol as assistant. After the evaporation of ethanol, the Se/carbon composite was prepared using melting-diffusion route, which was heated at 300 °C for 6 h in a horizontal tube furnace under flowing Ar atmosphere.

2.3 Structural characterization

The crystal phases were recorded on a powder X-ray diffractormeter (DX-2700, Dandong) with Cu-Kα radiation (40 kV, 30 mA) and 0.06 degree per s in the 2θ range of 10 and 80°. The morphologies, surface structures and energy dispersive X-ray spectrometer (EDS) mapping were observed using JEOL JSM-7500F scanning electron microscope (SEM, 5 kV). Thermogravimetric analysis (TGA, DSC200PC) was conducted to determine the selenium content in the composite. N₂ adsorption/desorption measurements were performed using a gas adsorption/desorption instrument (Micromeritics, USA) at 77 K.

2.4 Electrochemical measurement

All the electrochemical measurements were carried out using coin cells (CR 2016) at room temperature (22–28 °C). To preparation of working electrode, active materials (i.e., Se/carbon composite), binder sodium alginate and conductive agent acetylene carbon were well ground in an agate mortar at a weight ratio of 7 : 2 : 1. The resulting mixtures were slurried by deionized water, pasted onto aluminium foils, dried at 80 °C for about 5 h under air, and then cut into discs with a diameter of 14 mm. Polymer (Celgarad 2400) and commercial LBC 301 LiPF₆ solution (Shenzhen CAPCHEM) were used as separator and electrolyte, respectively, and the coin cells were assembled in an argon-filled glove box. Galvanostatic cycling tests were carried out on a Neware CT-3008 battery test system within a voltage region of 1.0–3.0 V (vs. Li⁺/Li) at different current densities. Cyclic voltammetry (CV) studies were conducted on a Modula XM Electrochemical Workstation (Solartron, UK) at a scanning rate of 0.1 mV s⁻¹ between 1.0 and 3.0 V.

3. Results and discussion

The influences of carbonization temperature on the structures, specific surface area and porous sizes of potassium tartrate derived carbon are studied firstly, as shown in Fig. S1 and S2.† The XRD patterns and Raman spectrums of different carbon are presented in Fig. S1,† and each sample has two broad peaks nearby 24° and 43° (Fig. S1a†), which can be indexed to amorphous and graphitized structure, respectively. Similar, the co-existence of two peaks located at ∼1350 (disordered carbon, D band) and ∼1575 cm⁻¹ (graphitized carbon, G band) can be found (Fig. S1b†). Generally speaking, the intensity ratios of G band to D band can be used to measure the graphitized level. Herein, the estimated peak intensity ratio of 600, 700 or 800 °C sample is 0.274, 0.310 or 0.330, respectively. It has an increasing trend with elevated calcination temperature, suggesting the increasing temperature may help to improve the graphitized level.^15,29 The N₂ adsorption–desorption isotherms and pore size distribution of different carbon are revealed in Fig. S2,† giving the estimated BET specific surface area of C-600, C-700 or C-800 is 148.2, 816.2 or 729.9 m² g⁻¹, respectively. Interestingly, the pore size distribution of each sample is the range from 0 to 5 nm, which should be attributed to the self-activation of potassium-contained compound. Moreover, the C-700 possess a larger pore volume (microporous volume) of 0.3972 cm³ g⁻¹ (0.2824 cm³ g⁻¹) compared with other two carbon samples, meaning more selenium can be encapsulated into the porous structure of C-700. The yield of porous carbon from potassium tartrate at 700 °C can come to 56.4% after experiencing the carbonization, washing and drying.

Thus, the structures of C-700 and resultant Se/C-700 composite have been clearly studied. Fig. 1 reveals the XRD patterns and Raman spectrums of C-700, pure Se and Se/C-700 composite, and the sharp diffraction peaks of commercial pure Se can be indexed to the standard trigonal Se (JCPDS: 06-0362). Interestingly, the diffraction peaks of formed Se/C-700 composite are similar to the carbon material, and without any Se characteristic can be detected (Fig. 1a), suggesting the uniformly encapsulation of Se into pores of potassium tartrate derived carbon, accompanying with the phase transformation of Se from the crystalline to amorphous.^14–18 This result also can be proved by Raman spectrums in Fig. 1b. The pure Se only presents a Raman peak of 236.1 cm⁻¹, which can be indexed to its chain-like structure. The potassium tartrate derived carbon displays two Raman peaks of 1377.0 (D band) and 1590.5 cm⁻¹ (G band), which can be assigned to disorder area and sp² graphitized structure of carbon, respectively (Fig. 1b). The as-prepared Se/carbon composite only inherit the Raman peak of carbon substrate, similar to previous literatures, suggesting the Se may possess the amorphous structure.^15,29

Fig. 1 XRD patterns and Raman spectrums of C-700, pure Se and Se/C-700 composite.

N₂ adsorption–desorption isotherms and pore size distribution of C-700 and Se/C-700 composite are comparatively studied in Fig. 2. The potassium tartrate derived carbon can possess a high (Brunauer–Emmett–Teller) BET surface area of 816.2 m² g⁻¹ and total porous volume of 0.3972 cm³ g⁻¹ (microporous volume: 0.2824 cm³ g⁻¹) from type I and III isotherms in Fig. 2a.^15,17 Importantly, it can be found that the pore distribution of the material is located at area of micro- (<2 nm) and small meso-pore (2–4 nm) (Fig. 2b), which is key to be used as the high-performance carbon substrate.^14–19 As shown in Fig. 2a, the BET specific surface area dramatically decreases from 816.2 to 3.7 m² g⁻¹ after formation of Se/C composite. Correspondingly, the porous volume of Se/C composite has been changed to 0.048 cm³ g⁻¹, indicating the Se mainly occupy the microporous and small mesoporous structure of porous carbon. A small amount of remaining porous structure can be used to buffer the volume change during cycling.⁶

Fig. 2 (a) N₂ adsorption–desorption isotherms and (b) pore size distribution of C-700 and Se/C-700 composite. The inserted is the pore size distribution of 0–12 nm.

The morphologies and surface structures of potassium tartrate derived carbon and corresponding Se/C composite are observed by SEM, as shown in Fig. 3. The as-prepared carbon with surface macropore shows unique 3D architectural structure (Fig. 3a), which can facilitate the mass transport and provide feasible channel access for molten selenium into the micropores or small mesopores of carbon.¹² The surface of this carbon is smooth, and without any macropore or mesopore can be observed (Fig. 3b). After compositing with Se, the overall morphology of carbon has not been changed (Fig. 3c), further suggesting the Se should be uniformly distribute into porous structure of carbon substrate. The smooth surface characteristic is also unchanged except a small amount of nanoparticles, which may be attributed to the carbonization segment or Se nanoparticles (Fig. 3d). Also, the TEM and HR-TEM images of as-prepared Se/C composite are revealed in Fig. S3.† From the TEM image (Fig. S4a†), it can be found the Se/carbon has similar morphology with pristine carbon shown in Fig. 3a. Partial graphitized phenomenon of graphite lattice fringes indicated by cycle can be observed in the marginal of composite (Fig. S4b†). Importantly, without crystalline Se can be found in the HR-TEM, further proving the Se with amorphous structure was confined in the pore of porous carbon.

Fig. 3 SEM images of as-prepared (a and b) C-700 and (c and d) Se/C-700 composite.

In order to further study the distribution property of Se in the porous carbon, the EDS mapping analysis is revealed in Fig. 4a–c. It can be clearly seen that the C and Se presents a homogenous distribution in pore of Se/C composite (Fig. 4b and c), which is a huge vindication that Se was confined in the pore of porous carbon.^14–18 Besides, the O element reflection also can be detected in Fig. S4,† which may be attributed to the incomplete carbonation of potassium tartrate. From the XPS spectrums of O_1s and C_1s (Fig. S5†), Only a peak located ∼533.0 eV can be found (Fig. S5a†), which can be indexed to the O 1s spectra, indicating the co-existence of O element in porous carbon. The C 1s spectrum is fitted to two peaks of 284.8 and 286.9 eV (Fig. S5b†), and the former data should be C–C bond, originated from the framework of porous carbon. The later peak may be the C–O bond (286.5 eV) according to previous literature,¹² proving the surface oxygen combines with carbon by C–O bond instead of C=O bond. Interestingly, the minor existence of O can promote to form Se/C composite by strong interaction between O and Se element as reported by Ye et al.,¹¹ as well as improve the hydrophilicity of electrode materials, which is essential for the preparation of electrode using water soluble binder (i.e., sodium alginate). Thermogravimetric analysis (TGA) was performed in a N₂ flow to confirm the true content of Se in Fig. 5. A minor weight loss from room temperature to 200 °C may attribute to the loss of moisture and adsorbed water. Subsequently, the weight loss from 200 to about 580 °C should be the evaporation of loading Se, and the estimated true Se content is ∼50% according to this curve. The loading loading density of Se in this porous carbon is not low compared with other literatures.^13,29

Fig. 4 EDS mapping of (a) selected area, (b) C and (c) Se mapping, (d) TGA curves of Se/C-700 composite from room temperature to 700 °C.

Fig. 5 Electrochemical performances of Se/C-700 composite: (a) typical discharge charge curves, (b) CV curves of initial three cycles, (c) cycling stability at 0.24C and (d) rate capability at different current densities.

As Li–Se battery cathode, the electrochemical performances of various Se/C composite are studied in Fig. S6.† The Se/C-600, Se/C-700 and Se/C-800 deliver an initial discharge capacity of 222.5, 397.2 and 457.7 mA h g⁻¹, and charge capacity of 103.0, 270.7 and 226.3 mA h g⁻¹ (based on total Se/C composite) at 81 mA g⁻¹, correspondingly giving the initial coulombic efficiency of 46.3%, 68.2% and 49.4%, respectively. It can be clearly seen the Se/C-700 sample shows better electrochemical performances, and its true initial discharge/charge capacity can come to 794.4 and 541.4 mA h g⁻¹ based on net Se weight of composite, as shown in Fig. 5a. The electrochemical behaviour of Se/C composite is also studied by CV test, and the result is shown in Fig. 5b. The Se/C electrode only exhibits one reduction peak at 1.68 V and one oxidation peak at 2.05 V in the first CV cycle, indicating the one-step reversible reaction of Se to Li₂Se. For the 2nd CV cycle and thereafter, the only discrepancy is little position shift of the reduction peak, changing from 1.68 to 1.74 V, with a shoulder at ca. 1.60 V for the possible formation of Li₂Se₂ or Li₂Se_x.

Cycling stability of Se/C-700 composite is shown in Fig. S6b† and 5c, and it can be seen that the Se/C-700 composite has the best cycling performance than other two samples (Fig. S6b†). The 2^nd discharge capacity of Se/C-700 is 550.5 mA h g⁻¹, and a discharge capacity of 485.3 mA h g⁻¹ can be retained after 80 cycles. The capacity retention can come to 88.2%. More importantly, the discharge capacity in 20^th is 502.2 mA h g⁻¹, and an average capacity fade of 0.056% per cycle is observed in the range of 10^th–80^th cycle, combined with the excellent discharge charge curves in Fig. 5a, indicating the as-prepared composite has good cycling stability especially after initial 20 cycles.

The rate capacity retentions of this Se/C composite also are evaluated at different current densities as shown in Fig. 5d. As expected, the specific capacity decreases from 505.8 mA h g⁻¹ at 0.48C to 485.1 mA h g⁻¹ at 0.72C, even when the current density increases to 1.2C, the cell also delivers stable discharge capacity of about 452.3 mA h g⁻¹. Interestingly, when the current density goes back to 0.24C, the discharge capacity can reach a high value of 513.5 mA h g⁻¹. These results indicate the as-prepared Se/C composite are suitable for high-rate Li ions storage processes. A comparative structures and electrochemical performances of other similar Se/C composite are revealed in Tab. S1.† It can be seen that, the electrochemical performances of as-prepared Se/C composite is not bad, especially the cycling stability and rate capability, as well as initial coulombic efficiency. It should be emphasized that, an initial carbonization of organic compounds and subsequent chemical activation are necessary for the preparation of above carbon. In present work, the porous carbon is synthesized using a one-step route, and it's more facile and convenient.

4. Conclusions

We have developed a facile one-step carbonization of potassium tartrate and subsequent melting diffusion route to prepare the Se/C composite. The carbon (C-700) obtained from carbonization temperature of 700 °C possess a higher BET specific area and pore volume, and the corresponding Se/C composite has a Se loading density of ∼50 wt%. As Li–Se battery cathode, the composite delivers a reversible (2^nd) discharge capacity of 550.5 mA h g⁻¹ with initial coulombic efficiency of 68.2% at 0.24C, and a discharge capacity of 485.3 mA h g⁻¹ can be retained after 80 cycle. Perhaps, the potassium tartrate derived porous carbon can be used to encapsulate S or SeS_x as homogeneous Li–S or Li–SeS_x battery cathode, deserved to be conducted continuously.

Acknowledgements

The authors thank the financial supports from the Scientific Start Foundation of LongYan University (LB2014001), and from Provincial Science and Technology Department for Provincial Colleges and Universities Program (JK2015047).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra07837k

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