Low-temperature reduction–pyrolysis–catalysis synthesis of carbon nanospheres for lithium-ion batteries

Abstract

Carbon nanospheres were synthesized using a reduction–pyrolysis–catalysis process with the reaction of carbon tetrachloride and aluminum at low temperature (160 °C), which is much lower than that of traditional methods. The carbon nanospheres as an electrode for Li-ion batteries exhibited a high specific capacity of 719 mA h g⁻¹ at a rate of 0.2 A g⁻¹ (98.5% charge capacity retention) after 100 charge–discharge cycles.

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Carbon materials are mostly used as anode materials in commercial lithium ion batteries (LIBs) because of their outstanding kinetics and availability at low cost.^1–3 However, graphite has a low theoretical specific capacity (372 mA h g⁻¹) and low rate-capacity, and thus provides considerable opportunity for exploring new carbon-based anodes with high capacity and excellent cycling performances. Up to now, a large number of carbon materials have been investigated, such as amorphous (nanosized graphite crystallites) nanospheres,⁴ carbon nanotubes,⁵ graphenes⁶ etc. Among these alternatives, amorphous carbon nanospheres have been demonstrated to be attractive because of their high chemical activity and controllable degree of porosity, as well as graphitization for electrochemical applications.^7,8 Typical carbon nanosphere synthesis methods employ chemical vapor deposition or pyrolysis of the organic precursors.^9,10 However, these synthesis methods require complicated experimental processes and high temperatures, usually between 500 and 1000 °C. Recently, Yao et al.¹¹ and Choucair et al.¹² reported that chemical reduction of carbon precursors (C₂H₃Cl₃, C₂H₄Cl₂ and CCl₄ etc.), with the addition of transition metal catalysts (potassium or sodium), could significantly decrease the temperature required for the formation of carbon nanospheres to as low as ∼100 °C. However, the use of potassium or sodium during these reaction process are exceedingly dangerous (highly explosive) and thus industrially unfavorable for continuous production on a large scale. Therefore, it is desirable to shed light on developing a facile, economical and safe method to synthesize carbon nanospheres for high performance electrode materials.

In this work, we report a reduction–pyrolysis–catalysis method at a very low temperature (160 °C) to synthesize carbon nanospheres by using carbon tetrachloride (CCl₄) as the carbon precursor and aluminum (Al) as the reductant. The obtained carbon nanospheres with a high surface area exhibit excellent performances as the electrode material for LIBs. We thus believe that this facile, economical and safe synthesis process is scalable to an industrially feasible approach.

In a typical synthesis, 10 mL of CCl₄ (Aladdin, 99%) and 5 g of metal Al (Aladdin, 99.95%) were put into a stainless steel autoclave of 100 mL capacity (REMBE, Germany), and then 0.1 g of AlCl₃ (Aladdin, 98%) was added to the autoclave as a catalyst. Subsequently, the autoclave was heated for 12 h at 160 °C. After the reaction, the resulting products were washed with 1 M hydrochloric acid and deionized water. Then the black deposit was dried at 60 °C for 6 h. The yield of carbon nanospheres was about 69.3%.

The morphology and microstructure of the obtained carbon nanospheres were measured using field-emission scanning electron microscopy (FESEM, HITACHI SU70) and transmission electron microscopy (TEM, FEI JEM-2100). The X-ray diffraction (XRD) patterns were recorded using an X-ray powder diffraction microdiffractometer (XRD, PANalytical X’Pert Pro, Netherlands). The Raman spectra were recorded using OMNIC Dispersive Raman software (Thermo Fisher Scientific, USA) with a DXR laser operating at 532 nm with incident power of 10 mW. The surface area of the products was measured using the Brunauer–Emmett–Teller (BET) method on an AUTOSORB-iQ (Quantachrome, USA) using N₂ as the adsorbate gas.

The electrochemical measurements were carried out using the CR2032 coin cells with lithium foil as the counter and reference electrode, 1 mol L⁻¹ of LiPF6 solution in a mixture of ethylene carbonate, dimethyl carbonate and ethylmethyl carbonate (EC : DMC : EMC = 1 : 1 : 1 by volume ratio) as the electrolyte, and a polypropylene film (Celgard 2300) as a separator. The carbon nanospheres were mixed with super P carbon black and polyvinylidene fluoride (PVDF) with the weight composition of 80 : 10 : 10. The electrochemical properties were studied with a Land automatic battery tester (Wuhan, China). The rate capability was examined at different current densities from 0.1 A g⁻¹ to 10 A g⁻¹. The cyclic voltammogram scanned at 0.05 mV s⁻¹ between 0 and 3.0 V for 5 cycles on a Solartron 1470E Electrochemical Interface (Solartron Analytical, UK).

Fig. 1a shows the X-ray diffraction patterns of the as-prepared products. The broadening peaks at 26.1° and 43°, which can be assigned to the (002) and (101) planes of hexagonal graphite (JCPDS no. 751621), suggest a low graphitization degree of the products. Furthermore, the graphitic structure of the products could be further demonstrated by Raman spectroscopy (Fig. 1b). Two characteristic peaks around 1339 and 1591 cm⁻¹ that could be ascribed to the D-band and the G-band originate from the defects and disorder in carbon materials, and from the stretching mode of the in-plane sp² C–C bonds of graphite-based materials, respectively. Moreover, the intensity ratio of the D-band to G-band (I_D/I_G), which could reflect the degree of disorder in graphite-like materials, is about 0.98, further confirming the low graphitization degree of the products.

Fig. 1 Phase determination (XRD pattern (a) and Raman spectrum (b)) of the as-obtained carbon nanospheres.

The morphology and microstructure of the as-prepared carbon nanospheres were demonstrated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), as is indicated in Fig. 2a and b, respectively. The products exhibit an interconnected spherical morphology with diameters ranging from 50 to 100 nm, as shown in the SEM and TEM images. The selected area electron diffraction (SAED) as shown in the inset of Fig. 2c indicates the amorphous like structure of the carbon nanospheres, which is consistent with the XRD results. From the high-resolution TEM (HRTEM) image shown in Fig. 2c, the expansion lattice spacing of the (002) plane (0.38 nm) compared to that of graphite (0.335 nm) also confirms the low graphitization degree of the carbon nanospheres.

Fig. 2 (a) SEM image and (b) TEM image of the as-obtained carbon nanospheres. (c) High resolution TEM (HRTEM) image of the edge, the arrows show the intercalated layers. The inset shows the selected-area electron diffraction (SAED) pattern. (d) Nitrogen adsorption/desorption isotherms (inset shows the corresponding pore-size distribution curve) of the as-obtained carbon nanospheres.

To demonstrate the surface area of the as-obtained carbon nanospheres, the nitrogen absorption/desorption isotherms were measured. As is shown in Fig. 2d, the isotherms show a typical IV-type curve and the relative surface area of the as-obtained carbon nanospheres is as high as 226.9 m² g⁻¹, according to the Brunauer–Emmett–Teller (BET) theory. Moreover, the pore size distribution curve shown in the inset of Fig. 2d indicates that there are mesopores with detectable sizes of 38 nm present in the as-obtained carbon nanospheres. We speculated that these mesopores, that could also be observed by HRTEM, might be produced in the process of removal of the residual Al and the byproducts of Al₂O₃ dispersed in the carbon nanospheres.^13,14 The isotherms of desorption and adsorption are not closed, this might be attributed to some of the N₂ molecules which are not desorbed from the surface of the sample after the adsorption process.

We investigated the electrochemical characteristics of the obtained carbon nanospheres to explore their potential application in LIBs. As is shown in Fig. 3a, the first charge–discharge cycle curve of the carbon nanosphere electrode delivers a reversible capacity of 1470 mA h g⁻¹, which is significantly higher than the theoretical value of graphite (372 mA h g⁻¹). However, there was an irreversible capacity of 669 mA h g⁻¹ and a plateau at approximately 0.75 V of the galvanostatic charge–discharge curves during the first cycle of the electrode, which could be attributed to the formation of a solid electrolyte interface (SEI) film on the surface of the carbonaceous electrode and the irreversible insertion of lithium into special positions such as the vicinity of the residual H atoms of the anode.^15,16 These could be further confirmed by the cyclic voltammetry (CV) curve of the first cycle in Fig. 3b. The unobvious plateau in Fig. 3a might be because the recorded rate (0.32 C) is much higher than that in Fig. 3b (0.06 C). There is a broad reduction peak at 0.8–1.3 V (disappeared after first cycle),¹⁷ which corresponds to the formation of the SEI film and irreversible insertion of lithium into the anode. Meanwhile, the three sharp oxidation peaks indicate a fast formation rate of the SEI film on the surface of the as-obtained carbon nanospheres. After the first cycle, the near overlap of the CV curves indicates a good capacity retention and the irreversible capacity losses of the material are mainly due to the formation of the SEI film during the first cycle.

Fig. 3 Electrochemical characterizations of the as-obtained carbon nanospheres. (a) The initial five galvanostatic discharge–charge curves at a rate of 0.2 A g⁻¹. (b) The initial five cyclic voltammetry (CV) curves at a scan rate of 0.05 mV s⁻¹. (c) The cycling performance at a rate of 0.2 A g⁻¹. (d) The rate performance at different rates.

The cycling performance of the carbon nanospheres is presented in Fig. 3c, revealing a good cycling performance and reversibility. The coulombic efficiency increases from 42% for the initial cycle up to 98% after a few cycles. After 100 cycles, the electrode still maintains a specific reversible capacity of 719 mA h g⁻¹. Furthermore, the carbon nanosphere electrode shows an excellent rate performance at various charge–discharge rates, as indicated in Fig. 3d. The cell is first cycled at 0.1 A g⁻¹ for 10 cycles, then with a stepwise increase of discharge–charge rates to 10 A g⁻¹. The reversible capacities are 860, 642, 602, 531, 456, 353 and 294 mA h g⁻¹ at higher current rates of 0.1, 0.2, 0.5, 1, 2, 5 and 10 A g⁻¹, respectively. After cycling at increasing current rates of 10 A g⁻¹ until the 70^th cycle, the capacity of the electrode could return back to about 877 mA h g⁻¹ at a current density of 0.1 A g⁻¹. The increased capacity might be attributed to the stability of our electrode material during the charge–discharge process, and the activation of electroactive materials took place in the beginning cycles. These results are superior to those of commercial graphite and other carbon-based materials, such as graphenes,^18–21 carbon nanofibers,²² porous carbons,^23,24 hollow structures,^16,25 and nanotubes⁵ and their composites.^26–29

The highly reversible capacity, excellent cyclic performance and rate performance of the carbon nanospheres show a promising potential anode material for high-performance LIBs. The outstanding electrochemical performance of the as-obtained carbon nanospheres might be attributed to their unique structures as well as the large specific surface area. Firstly, the high specific surface area of the as-obtained carbon nanospheres is very beneficial for the access of liquid electrolyte into the interior of the bulk electrode and provides a large quantity of active sites for rapid and efficient transport of lithium ions. Furthermore, the interconnected structures of the carbon nanospheres form a continuous conductive network, which gives rise to the high electronic conductivity and is thus very beneficial for improving the rate performance of the electrode. And the larger d-spacing of the (002) plane could facilitate the intercalation/extraction of Li into the graphitic shells, leading to an enhanced reversible capacity.³⁰ Finally, the abundant mesopores not only provide more lithium storage sites, contributing to the high reversible capacity, but also the extra space to alleviate the volumetric change of the electrode during the Li insertion/extraction process, beneficial to the structural integrity and stability of the electrode during long cycling operation.¹⁵

Conclusions

In summary, carbon nanospheres could be simply synthesized by a reduction–pyrolysis–catalysis process with the reaction of carbon tetrachloride and aluminum at low temperature (160 °C). The as-obtained nanospheres used as the anode for Li-ion batteries exhibited a high specific capacity of 719 mA h g⁻¹ at a rate of 0.2 A g⁻¹ (after 100 charge–discharge cycles) and excellent cycling and rate performances, which might be ascribed to their unique structures with high specific surface areas and mesopores. Our study offers a promising, industrially feasible approach for large-scale production of high-performance carbon-based anode materials for rechargeable lithium-ion batteries.

Acknowledgements

This work was supported by the Postdoctoral Science Foundation of China (Grant no. 2015M570504), the National Natural Science Foundation of China (Grant no. 51371186) and the Zhejiang Province Key Science and Technology Innovation Team (Grant no. 2013TD02). W.H. thanks the Project of the Ningbo 3315 International Team, the “Strategic Priority Research Program” of the Chinese Project Academy of Science (Grant no. XDA09010201) and the Zhejiang Province Key Science and Technology Innovation Team (Grant no. 2013PT16) for their support.

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Footnote

† These authors contributed equally to this work.

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