Hollow carbon-shell/carbon-nanorod arrays for high performance Li-ion batteries and supercapacitors

Abstract

N-doped hollow hierarchical peanut-like carbon-shells with nanorod arrays on the surface and interconnected networks in the core are prepared, which display rich porosity, superhigh specific surface area and a high degree of graphitization. The unique composition and hierarchical structure of the carbon resulted in very promising electrochemical energy storage performance.

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Electrochemical storage devices with high energy and power performance are highly desired with the increasing development of portable electronic devices and large-scale energy power stations. Li-ion batteries (LIBs) and supercapacitors (e.g., electric double-layer capacitors, EDLCs), which offer the most promising approach for effective and fast storage of electrical energy, should be able to satisfy the ever increasing and urgent demands.^1–4 It is generally believed that the energy efficiency and power performance of electrodes depend significantly on a series of electrochemical phenomena, such as mass transport, charge transfer, and electrochemical reaction kinetics.^5–7 Therefore, one effective strategy to create high-performance LIBs and supercapacitors is to develop electrodes with large surface area, well-developed porous structures and high electrical conductivity, in which highways of both ion and electron are simultaneously constructed to enhance the electrochemical reaction kinetics in both the bulk electrode and the electrode/electrolyte interface.

Recently, porous carbon is expected to become the ultimate electrodes to bridge the energy density and power density, due to the open and flexible porous structure, large surface area, as well as excellent mechanical and electrochemical stability. The interconnected porous, hollow and/or defect structure can shorten solid-state ion diffusion length, provide fast ion transport channel, and offer large electrode/electrolyte interface for charge-transfer reaction.^8–13 Nevertheless, the high-performance applications of porous carbon materials are seriously hindered, where outstanding electronic conductivity is required. Taking this into account, modifying surface functionalities (such as N and B) in graphitic structure and increasing the degree of graphitization could be helpful.^11–16 The presence of heteroatoms in graphite structure can break the electroneutrality and change the Fermi level, resulting in the enhancement of electric conductivity and electrochemical reactivity.^17,18 Therefore, for the carbon materials, porous hierarchical structure and incorporation of heteroatoms are both desirable for high-performance applications. However, it is still difficult to control and produce a good combination of hierarchical structure, pore size, heteroatom-doping and high degree of graphitization.

Herein, we synthesized a novel N-doped hollow hierarchical peanut-like carbon-shell with nanorods arrays on the surface and interconnected network in the core (denoted as N-HPC), which displays rich porosity, superhigh specific surface area and large total pore volume. The prepared novel material has a high doping level of nitrogen and, a sufficient degree of graphitization. In this resulting structure, as represented schematically in Fig. 1, the carbon nanorods and interconnected networks attached on the hollow sphere can access on all sides to the electrolyte. This architecture is favorable for rapid ion-diffusion and charge-transfer from different directions, yielding the N-HPC with enhanced power capability. In addition, the synergistic effect between the N and O groups creates highly active sites all over the N-HPC surface, which significantly improves the electrochemical activity. When tested as electrode of LIBs and supercapacitors, the N-HPC exhibits an exceptional reversible capacity, outstanding rate capability and long cycling stability.

Fig. 1 Schematic conduction/diffusion of electrons and ions within the N-HPC.

Fig. 2 shows SEM and TEM images of the synthesized N-HPC. It can be seen from Fig. 2a and c that the N-HPC displays a peanut-like general frame with a hollow structure. The particle size is around 1–3 μm in diameter and 3–5 μm in length, with a shell thickness of 1–2 μm, as the histograms shown in Fig. S1.† The SEM and TEM images in Fig. 2b and d show the rough external surface morphology, and highlight that carbon nanorods arrays with a diameter of ∼30 nm grown on the particle surface. The lattice fringes of carbon nanorods (Fig. 2d inset, Fig. S2†) and the sharp peak located at ∼26° in XRD pattern (Fig. S3†), corresponding to the (002) plane of graphite, confirms the relatively high degree of graphitization of the N-HPC. These TEM and high-resolution TEM images (Fig. 2c and d, inset and Fig. S4†) clearly reveal that the porous structure is homogeneously distributed within the peanut-like shell and the nanorods, which consists of large quantities of defects between discontinuous graphene sheets as indicated by the arrows. To study the internal distribution of carbon networks in the N-HPC particles, the MnO@C templates (Fig. S5†) are heated in air at 310 °C to remove the carbon.¹⁹ The underlying porous structure revealed by the TEM image after carbon removal clearly demonstrated that carbon networks are interdispersed in the N-HPC, as represented schematically in Fig. S6.† The presence of carbon nanorods arrays, interconnected networks and defects may be favourable for rapid ion-diffusion and charge-transfer from different directions, and contributes to the improvement of electrochemical properties.^5,13

Fig. 2 (a and b) SEM images, and (c and d) TEM images of the N-HPC. The inset of (d) shows the distorted lattice fringes of graphene sheets.

To further examine the porous structure of N-HPC, nitrogen adsorption–desorption isotherms are measured (Fig. 3a). The specific Brunauer–Emmett–Teller (BET) surface area of the activated carbon reaches as high as 1322.16 m² g⁻¹ with a total pore volume of 2.36 m³ g⁻¹. The pore size distribution (Fig. 3a, inset) is evident that the N-HPC possess both micropores peaked at 1.58 nm and mesopores peaked at 2.56 and 22 nm. These findings may be ascribed to the interconnected porous, hollow and defect structure caused by the removal of the MnO, which can be confirmed by the observation of TEM and HRTEM. Raman-spectrum analyses of the N-HPC in Fig. 3b indicate that the G-band has a greatly enhanced intensity. The intensity ratio (R = I_G/I_D) of the G-band to the D-band is ∼1.29, which is higher than that of amorphous carbon (0.95),¹¹ demonstrating a great increase in the degree of graphitic crystalline structure. The high degree of graphitization is also confirmed by XRD and HRTEM. X-ray photoelectron spectroscopy (XPS, Fig. 3c) of the sample confirms the successful incorporation of N atoms into carbon structure. A doping level of ∼7.6 wt% nitrogen in the N-HPC is revealed by combustion element analysis. XPS N1s spectrum of the obtained carbon is shown in Fig. 3d. Three components are resolved as pyridinic (N-6, ∼398.6 eV), pyrrolic (N-5, ∼400.3 eV), and graphitic (N-Q, ∼401.4 eV) type of N atoms, respectively.^18,20 Owning to the novel hierarchical structure, well-defined porous, N-doped and high degree of graphization, the N-HPC may exhibit unusual electrochemical properties in the application of LIBs and supercapacitors.

Fig. 3 (a) Nitrogen adsorption–desorption isotherms of N-HPC. The inset shows the pore size distribution. (b) Raman spectra, (c) total XPS spectrum and (d) N1s spectra of the N-HPC.

The electrochemical performance of N-HPC as a LIBs anode material is investigated in the voltage range 0.01–3 V versus Li⁺/Li. The cyclic voltammogram (CV) profiles of the N-HPC (Fig. 4a) recorded in the first four cycles are typical for carbonaceous anode materials.^12,21 During the second to the fourth charge–discharge cycle, there is no clear change, suggesting that the N-HPC electrode is stable with reversible Li-intercalation reactions. Fig. 4b shows the galvanostatic charge–discharge profiles of the N-HPC for different cycles at a rate of 0.1 A g⁻¹. The initial reversible charge capacity is as high as 1003 mA h g⁻¹, more than 2 times higher than the theoretical value of graphite (372 mA h g⁻¹). However, an irreversible capacity of about 387 mA h g⁻¹ is observed during the first cycle, which can be attributed to the irreversible consumption of the solid electrolyte interphase (SEI) layer, the irreversible loss of some Li storage sites, and other side reactions.^21,22

Fig. 4 The electrochemical performance of the as-fabricated N-HPC as LIBs anode. (a) Cyclic voltammograms (CV) at the scan rates of 0.1 mV s⁻¹; (b) charge–discharge curves and (c) cycling performance at the rate of 0.1 A g⁻¹; (d) rate capability at different current densities of the N-HPC.

The cycling performance of N-HPC electrodes at 0.1 A g⁻¹ is present in Fig. 4c. The N-HPC anode shows excellent cyclability in deep charge–discharge. The capacity becomes stable and reversible after the initial few cycles and then gradually increases to 1134.4 mA h g⁻¹ after 200 cycles, which can be ascribed to the activating process of the porous anode. Meanwhile, the Coulombic efficiency of the N-HPC increase from the initial of 72% to more than 98% after a few cycles. If the current density is raised up to 1.5 A g⁻¹, the initial reversible capacity drops down to 657 mA h g⁻¹, however, the cycling performance has a similar trends to the rate of 0.1 A g⁻¹ (Fig. S7†). Furthermore, after long-term cycling, the remaining hollow peanut-like structure demonstrates excellent mechanical and electrochemical stability (Fig. S8†). Notably, the N-HPC also shows remarkable high-power rate capability and cycling stability (Fig. 4d). The reversible capacities are 958, 768, 657, 613, and 559 mA h g⁻¹ at rates of 0.1, 0.8, 1.5, 3, and 4 A g⁻¹, respectively. Even at extremely high current density of 8 A g⁻¹, the reversible capacity of 435 mA h g⁻¹ can still be achieved, which is also higher than the theoretical capacity of graphite. When the rate is tuned back to 0.1 A g⁻¹ after cycling at different rates, the specific capacity can still recovered to 1180 mA h g⁻¹. We compared the BET surface area and electrochemical performance of the N-HPC with those of some carbon based materials as anodes for Li-ion batteries (Table S1†), the N-HPC shows an excellent rate capability and the highest rate-retention within these different carbonaceous materials.

Owing to the hierarchical, porous structure and N-functionalities of N-HPC, it is valuable for supercapacitors applications as well. The electrochemical performance of N-HPC in a three-electrode supercapacitor setup is investigated. The nearly rectangular shape of CV curves at all sweep rates (Fig. 5a), and symmetric triangular shape of charge–discharge curves at various current densities (Fig. 5b), demonstrating the superior ion response and good capacitive properties of the N-HPC. It is found that the electrode material possess a high specific capacitance of 282 F g⁻¹ at 0.5 A g⁻¹. The surface area normalized capacitance is 21.3 μF cm⁻², which is consistent with the theoretical EDLC capacitance of carbon (10–25 μF cm⁻²).²³ In Fig. 5c, a sharp decrease in the capacitance is observed as the current densities rise from 0.5 to 1.0 A g⁻¹, but it maintains quite well under higher current densities, indicating the N-HPC exhibits a good capacitance retention capability. For example, a capacitance of 228 F g⁻¹ can be achieved at 15.0 A g⁻¹, which is 81.4% of the value at 0.5 A g⁻¹. The excellent retention capability (Table S2†), indication of high power density, reflects that the specific capacitance of N-HPC is not kinetically limited. This result may be owed to the novel hierarchical nanostructure and high electrical conductivity that grants effective accessibility for electrolyte and electron even at a higher speeds, making the material directly suitable for high-rate operation. The long-term charge–discharge behavior at a rate of 1.0 A g⁻¹ is present in Fig. 5d. It is clear that a specific capacitance of about 252 F g⁻¹ (above 98% of the initial capacitance) is still remained after 2000 cycles, which illustrates that the N-HPC displays a good cycling stability as the supercapacitors electrode material. Additionally, the similarity of the triangular shape (Fig. S9†) and electrochemical impedance spectra (EIS, Fig. S10†) at 1^st and 2000^th cycle, also indicates that the electrode possesses stable performance and good charge propagation.

Fig. 5 The electrochemical performance of the as-fabricated N-HPC for supercapacitor tested in 3-electrode cell in 1 M KOH. (a) CVs at different scan rates; (b) charge–discharge curves at different current densities; (c) specific capacitances at different current densities; (d) cycling performance at rate of 1 A g⁻¹ for the N-HPC.

The remarkable electrochemical performance of the N-HPC is the result of its novel nanostructure, rich porosity, N-doping, and high electrical conductivity. Carbon nanorods arrays and interconnected networks of the N-HPC are favorable for ion diffusion and charge transfer from different directions, which provide sufficient contact between active material and electrolyte for the rapid electrochemical reaction. The porous structure leads to high specific surface area that can reduce the diffusion length and allow a mass of ions transport from outside into the inner. The interconnected carbon networks strengthen the mechanical stability of the hollow structure and provide a continuous pathway for electron transport. Additionally, N-doping and abundant defects in the N-HPC can enhance the electrochemical reactivity and electronic conductivity, which additionally contributes to the excellent electrochemical performance.

In summary, N-doped hierarchical hollow peanut-like carbon-shell with carbon-nanorods arrays on the surface and interconnected network in the core has been successfully fabricated. The novel nanostructure contributes to a superhigh specific surface area and well-developed porous structure. The prepared material as LIBs anode and supercapacitors electrode exhibits excellent electrochemical performance with a good combination of high reversible specific capacity, outstanding rate capability and long cycling stability. Considering the wide applications of porous carbon materials in electrochemistry, we believe that this novel structure can be extended for enhancing the performance of different electrochemical systems.

Acknowledgements

This work was supported by the National Basic Research Program of China (973 Program) (2013CB934001), National Natural Science Foundation of China (51274017), National 863 Program of China (2012AA052201 and 2012AA110102), and International S&T Cooperation Program of China (2012DFR60530).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra14683b

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