A simple preparation of porous graphene nanosheets containing onion-like nano-holes with favorable high-rate Li-storage performance

Abstract

Porous graphene nanosheets (PGNs) containing onion-like nano-holes were simply prepared using copper nitrite as template and phenolic resin as carbon source by graphitization at 2800 °C. Based on the investigations of morphology and structure, PGNs were used as anode materials for lithium-ion batteries, and it was found that this PGN electrode exhibits a stable voltage plateau, endurable Li-storage capability and outstanding rate performance of 258 mA h g⁻¹ at 1 A g⁻¹ and 105 mA h g⁻¹ at 10 A g⁻¹ after 700 cycles, superior to those of other carbon anodes with a low voltage plateau. The reasons are discussed and explained in detail.

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1. Introduction

Graphene based materials have gained great attention all over the world since the first report on free-standing graphene published by Geim in 2004.^1–7 Among these materials, porous graphene nanosheets (PGNs) have become one of the hotspots in the graphene research field due to their peculiar physicochemical properties such as tunable pore distribution, large specific surface area, good adsorption properties, and ultra-high charge-carrier mobility.^8–12 Porous structures in carbon materials have been proved to advance the electrochemical behaviours by supplying more active sites and facilitating ion diffusion rate.^13–15 In the previous reports, PGNs were considered as promising candidates for high-rate energy storage devices such as lithium ion batteries (LIBs) and supercapacitors since their suitable porous structure can significantly enhance the rate performances by shortening the transport paths and increasing the contact interface between electrolyte and electrode materials.^16–19 However, great obstacles also exist so far for the preparation and application of PGNs. Firstly, PGNs were generally synthesized by activating graphene or graphene oxide with traditional agents like KOH, or using SiO₂ and polystyrene microspheres as templates to produce porous structure, or etching carbon on graphene sheets with MnO₂.^16,20–22 The basic process was to synthesize graphene nanosheets by common ways like Staudenmaier or Hummers method, which need excessive acid, alkali and strong oxidant.^23,24 These processes are usually un-environmentally friendly, complex and difficult to control. Secondly, large specific surface area could not be fully utilized thus the corresponding high theoretical capacity barely achieved when applied for energy-storage devices.^22,25 Thirdly, no stable voltage plateau during charge/discharge processes is an urgent problem for pure PGNs to be used as the anode materials for LIBs.^9,26

Herein, a new and simple route was put forward for the gram-scale synthesis of high crystal few-layered PGNs by graphitizing (2800 °C for 1 h) hierarchical porous carbon nanosheets (HPCS) from phenolic-formaldehyde (PF) resin precursor. PGNs have abundant onion-like mesopores penetrated on the surface, and exhibit the thickness of ca. 1–10 nm and the specific surface area of 117 m² g⁻¹. When used as the anode material for LIBs, stable voltage plateau was clearly observed at 0.01–0.2 V in the discharge process. Moreover, the PGNs performed endurable Li-storage capability, especially high-rate ability, suggesting their huge potential for high power electric sources.

2. Experimental

2.1 Synthesis procedures

Thermoplastic phenolic-formaldehyde resin (300–500 polymeric units) and copper nitrate (Cu(NO₃)₂·3H₂O, analytical pure grade) were mixed in a Cu/C atomic ratio of 1/4 in the presence of hexamethylene-tetramine (a common curing agent for the cross linking reaction of linear phenolic resins²⁷). The mixture was solidified at 150 °C for 4 h and further carbonized at 600 °C for 6 h in a nitrogen gas atmosphere to get copper embedded carbon nanosheets (CECN). CECN were treated by hydrogen peroxide and then hydrochloric acid to remove Cu nanoparticles. After vacuum filtration and drying, HPCS were obtained.²⁸ Thereafter, graphitization was carried out at 2800 °C for 1 h to obtain the final products—PGNs. Pure graphene derived from Hummers method was also graphitized under the same condition in order to compare the electrochemical behaviors with those of PGNs. The contrast sample was denoted as Graphene-2800.

2.2 Characterization

The samples were characterized by scanning electron microscope (SEM) (ZEISS SUPRATM 55 field emission microscope), Hitachi H-800 transmission electron microscope (TEM) operating at 200 kV, high-resolution transmission electron microscopy (HRTEM) (JEOL-3010) and X-ray diffraction (XRD) (Rigaku D/max-2500B2+/PCX system) using Cu Kα radiation (λ = 1.5406 Å) over the range of 5–90° (2θ) at room temperature. The thickness of PGNs was obtained under ambient condition using an atomic force microscope (AFM, Veeco NanoScope 3D). Raman spectroscopy was carried out using a 532 nm laser (Aramis, Jobin Yvon). Nitrogen sorption isotherm was measured with ASAP2020 (Micromeritics, USA). Before sorption measurements, the samples were degassed at 300 °C for 6 hours. Specific surface areas were estimated according to the Brunauer–Emmett–Teller (BET) model, and pore size distributions were calculated by the Density Functional Theory (DFT) method.

2.3 Electrochemical measurement

Electrochemical measurements were carried out by using 2032 coin-type cells. The working electrodes were prepared by mixing the active material (PGNs and Graphene-2800), acetylene black, and poly(vinyldifluoride) (PVDF) at a weight ratio of 8 : 1 : 1 and pasting the mixture onto foam nickel. Pure lithium sheet was used as the counter electrode. The electrolyte consisted of a solution of 1 M LiPF₆ in ethylene carbonate/dimethyl carbonate (1 : 1 by volume). The cells were assembled in an argon-filled glove box with the concentrations of moisture and oxygen below 1 ppm. The electrochemical performance was tested at various rates in the voltage range of 0.01–2.5 V. The cyclic voltammetry and the electrochemical impedance spectral (EIS) measurements were carried out on a CHI 660B electrochemical working station. For the cyclic voltammetric measurements, the sweep rate was 0.1 mV s⁻¹, and the potential range was 0.01–2.5 V. For the EIS measurements, the frequency range was from 100 kHz to 10 mHz.

3. Results and discussion

The formation of PGNs is illustrated as Fig. 1. CECN with the thickness of ca. 40 nm were obtained after the solidification and carbonization of the PF resin/Cu(NO₃)₂ mixture. The most of formed Cu nanoparticles with diameter of 10–30 nm were embedded or encapsulated inside the carbon nanosheets while the Cu nanoparticles on the surface were also coated by thin carbon layers. Abundant mesopores with corresponding size and dehiscent carbon nano-bubbles (Fig. 2a) on the original position of Cu nanoparticles were left after fully removal of Cu by H₂O₂/HCl solution.²⁸ The subsequent graphitization process leads to a densification of the amorphous carbon flakes as well as the crystallinity enhancement. As a result, graphene-like structure was formed with the decoration of penetrated polygonal onion-like nano-holes.

Fig. 1 Schematic diagram for the formation of PGNs.

Fig. 2 (a) SEM image of HPCS; (b–d) SEM, (e) TEM, (f, g) HRTEM and (h) AFM images of PGNs.

The SEM images of PGNs (Fig. 2b and c) reveal a curled morphology consisting of crumpled paper-like structure. The thickness of PGN is ca. 1–10 nm, corresponding to about 2–20 layer stacking of the monatomic graphene sheets. The thickness of HPCS decrease and the mesopores on HPCS shrink due to the high-temperature graphitization, resulting in a stress-induced orientation towards the mesopores as shown in Fig. 1 (indicated by the red arrows), which will lead to the wrinkle of obtained porous graphene nanosheets. Shrinked and collapsed mesopores can be observed on PGN in Fig. 2d. Graphitization may remove structural defects, and thereby cause the structure change from separated smooth carbon nanosheets to coalescent rippled graphene nanosheets. The thin carbon layers are clearly seen to be penetrated by large amount of polygonal pores with the average diameter of ca. 20 nm. The black stripes displayed in the TEM image (Fig. 2e) are the crinkles and textures on the rugged nanosheets, corresponding to the nanoribbon structures shown in the HRTEM image of Fig. 2f. The mesopores on PGN are polygonal onion-like nano-holes, which may be from the shrinked mesopores by graphitization (Fig. 2g). On the other hand, the crystallinity of PGN increases after high-temperature treatment, so the boundaries of inwardly invaginate mesopores exhibit concentric circles. It should be indicated that, when hard carbons such as phenolic resin-based carbons are graphitized at high temperatures, the nanoribbon-like carbon structure is also presented and the concentric centre structures are usually observed.^29–36 However, different from this case, our similar hollow onion-like structures are from the porous edges exposed at the outside surface of graphene nanosheets. The AFM image (Fig. 2h) shows a thickness of 1.484 nm for a typical PGN.

The XRD pattern of PGNs is given in Fig. 3a. The peak located at 26.5° is the (002) diffraction peak in graphitic materials. The sharp and intense characteristic indicates the highly ordered layered stacking structure of PGNs. Noticeably, a broad and dispersive peak ranging from 15–35° is also clearly observed in XRD pattern, demonstrating the existence of amorphous or turbostratic carbon in PGNs even after high temperature graphitization. This phenomenon should be ascribed to large amount of disordered regions and edges induced by abundant polygonal mesopores on highly curved graphene flakes. The Raman spectrum of PGNs given in Fig. 3b can further help to deeply understand their porous property of the highly ordered few layer stacking structure. The sharp G-band located at 1580 cm⁻¹ demonstrates that the carbon atoms are densely packed in long-range honeycomb conjugated type in one graphene layer, while the appearances of D-band at 1350 cm⁻¹ and the D′-band at 1620 cm⁻¹ indicate the disordered carbon from edge sites, amorphous regions or dislocations. What's more, the obtained PGNs perform intensive and sharp 2D-band (2700 cm⁻¹), whose intensity is even stronger than that of the G-band, further proving the few layer ordered stacking structure in PGNs.^37,38 PGNs possess a specific surface area of 117 m² g⁻¹, with a micropore area of 55.2 m² g⁻¹ and an external surface area of 61.8 m² g⁻¹. The nitrogen adsorption isotherms of PGNs in Fig. 3c exhibit characteristics of type-II, manifesting the existence of certain amount of micropores and abundant macropores reflected from the adsorption in high pressure. The inconspicuous hysteresis loop shown as the inset of Fig. 3c demonstrates the presence of small amounts of mesopores. The micropores were generated by the pyrolysis of PF resin during carbonization³⁹ and their content was decreased about 77.5% after graphitization (micropore area of 245.4 m² g⁻¹ for HPCS²⁸), suggesting most of micropores disappeared by graphitization. Mesopores were also shrinked and the pore size decreased to ca. 10 nm from HRTEM images. Quantities of macropores could be ascribed to the coalescence of graphene nanosheets (Fig. 3d).

Fig. 3 (a) XRD pattern and (b) Raman spectrum of PGN. (c) Nitrogen adsorption–desorption isotherms of PGN and (d) its pore size distribution using DFT equation.

High-temperature heat treatments always bring about the diminishment of surface functional groups and the decrease of surface area.^40,41 The shrinkage of mesopores decrease the material's adsorption quantities of N₂, but the formed onion-like nano-holes should endow PGNs anode with superior high-rate performance for LIBs because they will cut down the diffusion path, provide more Li-storage sites and enhance the transmission rate of Li⁺, as described in the following part.

The cyclic voltammetry curve of PGNs (Fig. 4a) exhibits two apparent peaks at 0.5–0.8 V and 0.01–0.2 V in the first reduction process. The former peak should be attributed to the formation of SEI film and irreversible Li-insertion into superfine pores, while the latter peak corresponds to the reversible insertion of Li into the graphene layers and the nanocavities. The obvious oxidation peak at 0.2–0.3 V shown in charge process can be ascribed to the Li-extraction. Compared with traditional hard carbon anodes, PGN exhibits obvious Li-insertion/extraction peak centred at low potential instead of gradually rised/declined Li-storage capacity along with the potential, which can be ascribed to the high graphitization degree of carbon and the shrink of pore structures. Although this is a common phenomenon in charge–discharge processes of graphitized carbon materials, PGN shows higher capacity and much more favorable high-rate performance than other graphitized carbon anodes.^42–44

Fig. 4 (a) Cyclic voltammetry curve, (b) galvanostatic discharge/charge curves, and (c) cycling performance of PGN electrode at a current density of 50 mA g⁻¹. (d) High-rate performance of PGN electrode at 50 mA g⁻¹, 1–6 A g⁻¹ and 10 A g⁻¹.

Galvanostatic charge–discharge experiments were carried out at different current densities in order to characterize the electrochemical performance of PGN. The charge–discharge voltage profiles at a current density of 50 mA g⁻¹ for the first three cycles are shown in Fig. 4b. The initial discharge (lithium insertion) capacity is 968 mA h g⁻¹ and the reversible capacity is 430 mA h g⁻¹. The curves show stable voltage plateaus at 0.01–0.2 V possessing the specific capacity of 245 mA h g⁻¹, which is quite different from traditional hard carbons and graphene electrodes.^9,26,41,45 The capacity contribution of PGN mainly centres at low potential, which endows the battery assembled with this anode material with stable charge and discharge behaviours, high battery voltage and steady energy output.

As depicted in Fig. 4c, PGN shows good cycling stability at the current density of 50 mA g⁻¹ and maintain 373 mA h g⁻¹ after 50 cycles. Attractively, PGN possess high specific capacity under high-rate charge–discharge processes (Fig. 4d). At the current density of 1 A g⁻¹ (about 3C), the initial reversible capacity reaches 258.4 mA h g⁻¹ and decreases a little to 237 mA h g⁻¹ after 100 cycles with only 0.08% capacity loss per cycle. The capacities at 2 A g⁻¹, 3 A g⁻¹ and 4 A g⁻¹ are 185.6 mA h g⁻¹, 161 mA h g⁻¹ and 140 mA h g⁻¹, respectively. When the current density increases to 6 A g⁻¹, its reversible capacity can maintain 121.7 mA h g⁻¹. Even after 700 cycles, a capacity of 105.6 mA h g⁻¹ can be remained at 10 A g⁻¹. The specific capacities of PGN at high current densities are much higher than those of graphite and graphene electrode^42,46,47 and are comparable with the results of Fan and Fang.^9,26 When the current density was returned to 1 A g⁻¹, a capacity of 294.2 mA h g⁻¹ is re-obtained, even higher than the capacity at the beginning 1 A g⁻¹. And when back to 50 mA g⁻¹, the capacity is increased to 494.6 mA h g⁻¹, also higher than tested at 50 mA g⁻¹ directly, suggesting the high structural and cyclic stabilities of PGN anode.

The lithium storage performance of pure graphene graphitized under the same condition was investigated to compare with that of PGNs (Fig. S1†). Although exhibiting similar voltage plateau at low potential, Graphene-2800 delivers a lower reversible capacity of 300 mA h g⁻¹ after 50 cycles and unsatisfied rate performance with only 40 mA h g⁻¹ maintained at 1 A g⁻¹. The unfavorable performances should be imputed to the agglomeration of graphene layers, since the thickening of graphene sheets decreased the electrochemical sites for lithium storage and was adverse to the diffusion of ions (Fig. S2†).

In order to characterize the transport kinetics for the electrochemical property of PGN, EIS is measured at the voltage of ca. 1.8 V. As depicted in Fig. 5a, the depressed semicircles in the high frequency regions of the Nyquist plots at high current density, being assigned to the SEI film and contact resistance, show a rather small diameter compared with that at 50 mA g⁻¹, indicating that PGN exhibits lower electric resistance under large current charge and discharge. This may be ascribed to the activation effect of pores by huge current and the irreversible insertion of Li during the cyclic process which improves the conductivity of porous nanomaterials.^48,49 (The EIS is modelled by an equivalent circuit shown in Fig. 5b.)

Fig. 5 (a) AC impedance spectra of PGNs electrodes after cycling at different current densities; (b) Randles equivalent circuit for the PGNs electrodes.

The outstanding high-rate performance should be attributed to the special porous graphene structure and good electrical conductivity of the nanosheets. For one aspect, the onion-like nano-holes on the nanosheets can cut down the diffusion paths of Li⁺, and is in favor of a rapid charge–discharge process and improving the rate of diffusion. As depicted in Fig. 6, Li⁺ can insert into the graphene layers from both the crystallized mesopores and the edge/planes of nanosheets. In addition, the mesopores may also adsorb Li⁺ and provide Li-storage capacity.⁵⁰ For another aspect, the graphitized nanosheets with onion-like pores increase the contact areas between electrode and electrolyte which is beneficial for the high-rate performance.

Fig. 6 Schematic diagram of Li-storage mechanism in PGNs.

4. Conclusions

Novel porous graphene nanosheets with onion-like nano-holes were prepared by introducing PF resin as carbon resources for the first time. PGNs exhibit stable voltage plateau at 0.01–0.2 V and excellent high-rate performance when used as anode materials for LIBs, giving great application potential for a new-generation of industrial anode materials. This work also provides a brand new and environmental friendly method for graphene preparation.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51202009 and 51272016), and Foundation of Excellent Doctoral Dissertation of Beijing City (YB20121001001).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra13905a

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