A floral variant of mesoporous carbon as an anode material for high performance sodium and lithium ion batteries

Abstract

The floral variant of mesoporous carbon was simply prepared by direct pyrolysis of zinc citrate followed by washing with dilute hydrochloric acid. The unique floral microstructure endows the carbon with ultrahigh reversible capacity, excellent cycle stability and superior rate performance as an anode material for both sodium ion batteries and lithium ion batteries. The floral variant of mesoporous carbon exhibits a reversible sodium storage capacity as high as 438.5 mA h g⁻¹ at a current density of 30 mA g⁻¹ and retains a value of 68.7 mA h g⁻¹ at an enhanced current density of 10 A g⁻¹. Moreover, the floral mesoporous carbon can deliver a tremendous reversible capacity up to 1370 mA h g⁻¹ at 50 mA g⁻¹ as an anode for lithium ion batteries. It can output a high reversible capacity of 222 mA h g⁻¹ even when being charged and discharged at 50 A g⁻¹. Based on the astounding capacity and rate performance, the floral variant of mesoporous carbon can be regarded as one of the most promising anode materials for both sodium-ion and lithium-ion batteries.

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1. Introduction

Advanced energy storage and conversion devices are indispensable for the utilization of renewable and clean energy sources such as wind and solar power. Lithium ion batteries (LIBs) with high energy density, high power density and long cycle life are the most successfully developed devices so far. They have been widely applied in mobile electronics and regarded as the most promising candidate for electric vehicles.¹ However, with the possibility of enormous demands on available lithium resources for large-scale energy storage system in future, disquiets on the supply and cost of LIBs have arisen due to the limited lithium reserves and their uneven distribution. In this situation, sodium ion batteries (SIBs), which were originally investigated in parallel with LIBs, have again attracted much attention recently because of the infinite sodium resources and their low cost.^1–4

Carbon materials have been widely used as electrode materials for batteries because of their high conductivity, good chemical stability and low cost. Graphite is one of the most popular anode materials for commercial LIBs. However, the theoretical lithium storage capacity of the graphite electrode is only 372 mA h g⁻¹, which cannot support the LIBs with high energy density. Furthermore, graphite electrode is inapplicable for SIBs for its negligible sodium storage capacity, probably due to the mismatch between the narrow graphite interlayer distances and the large sodium ions.² In contrast, hard carbon materials with large interlayer distance and disordered structure show very high capacities for both lithium and sodium storage, which can be regarded as the leading anode materials for SIBs as well as LIBs.^3,5–7 Generally, hard carbons prepared from sucrose, glucose and resorcinol-formaldehyde resin can deliver reversible sodium storage capacities of 150–300 mA h g⁻¹, but their cycle stability and rate performance are insufficient.^8–12

Recently, designing nanostructured morphology or/and hierarchically porous structure has been reported as an effective strategy to improve the electrochemical performance of carbon materials. The nanostructure of the carbon materials can provide some active sites for Na/Li ion storage to enhance the specific capacity. In addition, the diffusion distance of Na/Li ion in the carbon materials is decreased, so the rate capability can be improved.^13–16 For these concerns, three-dimensional (3D) floral variant of mesoporous carbon has attracted special attention due to their unique carbon sheet structure, high specific surface area and abundant interconnected pores. However, only a few approaches have been reported for the synthesis of 3D floral mesoporous carbon, all of which involve the tedious multi-step and costly processes.^17,18

Herein, we report a facile, sustainable strategy for the synthesis of the floral variant of mesoporous carbon (FMC) through direct pyrolysis of zinc citrate followed by washing with dilute HCl. Zinc citrate is the only raw material for the FMC preparation, which acts as the carbon precursor, the template of mesopores and the inducer of the floral morphology simultaneously. The unique microstructure enables the FMC exhibit ultrahigh reversible capacity, excellent cycle stability and superior rate performance as anode material for both SIBs and LIBs.

2. Experimental

2.1 Synthesis and characterization

The synthesis of the floral variant of mesoporous carbon is very simple. Zinc citrate (Zn₃(C₆H₅O₇)₂·2H₂O, AR), as the only raw material, was pyrolyzed at 800 °C for 2 h at a ramping rate of 10 °C min⁻¹ under flowing nitrogen (99.999%). After washed with dilute hydrochloric acid and distilled water, the floral variant of mesoporous carbon denoted as FMC was obtained.

The morphology of the carbon was observed by scanning electron microscopy (SEM; JSM-6701F) and transmission electron microscopy (TEM; JEOL-2100). The composition and crystallitic structure of the samples were characterized by powder X-ray diffraction (XRD) patterns, performed on a Bruker AXS D8 with Cu Kα radiation (λ = 0.1541 nm). Raman spectra was obtained with a Renishaw 1000 Raman spectrometer (514 nm). Nitrogen (77 K) adsorption/desorption measurement was performed on a Micromeritics ASAP 2460 to characterize the porosity parameters of the carbon. The specific surface area (S_BET) was calculated by the conventional BET (Brunauer–Emmett–Teller) method. The total pore volume (V_t) was calculated from the adsorbed N₂ amount at a relative pressure of 0.99. The pore size distribution was calculated by the density function theory (DFT) method using a carbon slit pore equilibrium model.

2.2 Electrochemical measurements

For electrochemical characterization, the slurry of the FMC (80 wt%), conductive agent (Super-P, 10 wt%), and binder (PVDF, 10 wt%) in NMP were coated onto copper foil. After being dried at 120 °C for 12 h under vacuum, the as-prepared electrodes were cut into pellets of 10 mm in diameter. The mass loading of FMC in each electrodes is about 0.8 g cm⁻². The pellets were used as the working electrodes to assemble the coin cells (2025-type) in an argon-filled glovebox (Mikrouna, H₂O, O₂ <0.1 ppm). For sodium ion cell, a Na foil was used as the counter electrode, glass fiber was used as the separator, and 1 mol L⁻¹ NaClO₄ dissolved in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (1 : 1 v/v) was used as the electrolyte. For lithium ion cell, a Li foil and Celgard 2300 was used as the counter electrode and the separator, respectively, and electrolyte was 1 mol L⁻¹ LiPF₆ dissolved in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1 : 1 v/v). Cyclic voltammetry (CV) was carried out using the CS350 electrochemical workstation at a scan rate of 0.1 mV s⁻¹. Galvanostatic charge/discharge measurements were performed on a Land BT2000 battery tester (Wuhan, China).

3. Results and discussion

The schematic for the preparation procedure of the FMC is illustrated in Fig. 1. Zinc citrate was used as precursor and pyrolyzed at 800 °C for 2 h under flowing nitrogen. During thermal pyrolysis, zinc citrate was pyrolyzed to carbon atoms and ZnO nanoparticles, which had self-assembled nano-ZnO/carbon composites with floral morphology. In the composites, ZnO nanoparticles are evenly dispersed in the carbon matrix. After washed with diluted HCl, the ZnO nanoparticles were removed and mesopores are left at the sites just now occupied by ZnO nanoparticles. As a result, the floral mesoporous carbon denoted as FMC was obtained. Clearly, the synthetic process of the FMC is very simple. The only raw material, zinc citrate, acts as carbon precursor, hard template and inducer of the floral morphology simultaneously.

Fig. 1 Schematic of the synthetic process of the floral mesoporous carbon.

As shown in the SEM images (Fig. 2a and b), the FMC exhibit flower-like morphology, and the particle size is estimated to be ∼20 μm with abundant large pores arisen from the stacking of the carbon nanosheets. HRTEM image of the FMC (Fig. 2c) exhibits abundant mesopores ranging from several to tens of nanometers, whose pore walls mainly consist of randomly orientated non-graphitic layers.

Fig. 2 (a and b) SEM and (c) HRTEM images of FMC; (d) nitrogen adsorption–desorption isotherms and (e) the pore size distribution of FMC.

To further investigate the porosity of FMC, nitrogen adsorption–desorption isothermal analysis is employed. As shown in Fig. 2d, the isotherms of the FMC are between type I and type IV according to the classification of IUPAC. The sharp adsorption increase and the distinct hysteresis loops at the wide relative pressure range of p/p₀ = 0.4–1 indicate the existence of abundant mesopores in the sample. The specific surface area of the FMC is 1382 cm² g⁻¹, as calculated by the BET method. The total pore volume of the FMC is found to be as high as 2.02 cm³ g⁻¹, among which the mesopore volume (1.88 cm³ g⁻¹) makes a contribution of 93.1%. It is confirmed that the mesopores are predominant in the pore structure of the FMC. As shown in the pore size distribution curves calculated by the conventional DFT method (Fig. 2e), the pores of the FMC mainly distribute in the range of 2–20 nm with an average pore size of 5.85 nm in diameter. The high specific surface area and abundant mesopores are expected to offer an ample electrode/electrolyte interface for ions or charge transfer and rapid diffusion.

The XRD pattern of the FMC (Fig. 3a) exhibits two broad diffraction peaks at around 21.06° and 43.38°, corresponding to the graphite (002) and (101) crystal planes, respectively, which is the characteristic of the amorphous carbon. The interlayer distance of the FMC is calculated to be 0.42 nm according to Bragg's equation, much larger than that of graphite (0.335 nm), which is beneficial to insert Na-ion.¹⁴ The diffraction peaks of the pyrolysis product before washing can be entirely indexed to the zincite phase ZnO (JPDS 36-1451), which can act as template for the mesopores of the FMC. The Raman spectrum of the FMC (Fig. 3b) presents a G-band at 1590 cm⁻¹ (related to graphitic carbon) and a D-band at 1334 cm⁻¹ (associated with defects). The intensity ratio of the G-band to the D-band (I_G/I_D) is 1.15, a relatively high value for the amorphous carbon, implying good electrical conductivity of the FMC.

Fig. 3 (a) XRD patterns of the intermediate product ZnO/C composites and the floral mesoporous carbon; (b) Raman spectra of the floral mesoporous carbon.

The FMC is expected to have excellent sodium storage performance due to its unique microstructure. As shown in Fig. 4a, the FMC shows typical voltage profiles of hard carbons between 0 and 3.0 V at a current density of 30 mA g⁻¹.^19,20 The discharge profile shows a large sloping curve, indicating the surface charge transfer/electro-adsorption/desorption mechanism are predominant in the sodium ion storage.¹⁶ The initial reversible capacity is as high as 438.5 mA h g⁻¹, much higher than 100–300 mA h g⁻¹ of the reported hard carbons.²¹ However, the FMC suffers high irreversible capacity in the first few cycles. The high irreversible capacity is a common phenomenon for the hard carbons with high surface area, which can be attributed to the electrolyte decomposition at the electrode/electrolyte interface.^22–24 After the initial adjustment cycles, the FMC shows good reversibility. The CV profiles are recorded in the first three cycles to survey the electrochemical process (Fig. 4b). The strong reduction peak at 0.5 V in the first cycle can be attributed to the electrolyte decomposition and the consequent formation of the SEI layer. Although the SEI formation results in the low coulombic efficiency of the FMC at first charge–discharge cycle,^25,26 it is actually advantageous for protecting the electrolyte from continuous decomposition on the carbon electrode. The CV profiles of the subsequent cycles almost overlap, consisting of reduction peaks at 0.01 V and rectangular shapes at high potential. It implies that the ultra-high capacity of the FMC can be attributed to the co-contribution of two factors: the inserted Na-storage in the expanded graphene interlayer, and the capacitive Na-storage in the abundant mesopores.

Fig. 4 (a) Charge/discharge curves at 30 mA g⁻¹, (b) cyclic voltammograms at 0.1 mV s⁻¹, (c) cycle stability at 50 mA g⁻¹ and (d) rate performance of the FMC as anode material for SIBs.

The cycle durability and high rate capability are of great importance for battery materials. After 200 cycles at 50 mA g⁻¹, the capacity of the FMC remains 236 mA h g⁻¹ (Fig. 4c), indicating excellent cycle stability. Fig. 4d shows the rate performance of the FMC at various current densities. The reversible capacities of 242.5, 201, 160, 122.3, and 98.3 mA h g⁻¹ are achieved at 0.1, 0.2, 0.5, 1 and 2 A g⁻¹, respectively. Even at very high current densities of 5 and 10 A g⁻¹, the FMC can deliver reversible capacities of 75.6 and 68.7 mA h g⁻¹, indicating excellent rate performance. After cycling at various rates, the specific capacity can still be recovered to 376.1 mA h g⁻¹ at 30 mA g⁻¹. The rate capability of the FMC are superior to (or comparable to) many other carbon-based materials, such as hollow carbon nanowires (149 mA h g⁻¹ at 0.5 A g⁻¹),¹⁴ carbon sphere (40 mA h g⁻¹ at 1.5 A g⁻¹)²⁷ and nitrogen-doped porous carbon fiber (72 mA h g⁻¹ at 10 A g⁻¹).²⁸

Being used as anode material for LIBs, the FMC presents similar electrochemical behaviour (Fig. 5a and b) to the sodium storage. Furthermore, it shows much enhanced performance in LIBs compared to SIBs as lithium ion has a smaller size. The FMC displays a high initial reversible capacity up to 1370 mA h g⁻¹ at a current density of 50 mA g⁻¹, nearly four times higher than theoretical capacity of commercial graphite (372 mA h g⁻¹). Besides, the value is far above that of most other pyrolytic carbon materials, ranging from 200 to 900 mA h g⁻¹.²⁹ Moreover, a high irreversible capacity of 1649.7 mA h g⁻¹ is measured in the first cycle, which results from the SEI formation of FMC and trap lithium ions in this SEI film and/or from irreversible lithium insertion into special positions.⁶ After the first few cycles, the cycling capacity of the FMC becomes stable and reversible with the coulombic efficiency above 95%. As shown in Fig. 5c, the initial charge capacity of the FMC at 100 mA g⁻¹ is 983 mA h g⁻¹, which gradually increases to 1100 mA h g⁻¹ subsequently and then retains the value stably for 250 cycles with coulombic efficiency near 100%, indicating good cycle durability of the FMC. The rate performance of the FMC is shown in Fig. 5d. It can deliver capacities of 643, 546, 540, 426, 379 mA h g⁻¹ at the current densities of 0.5, 1, 2, 5, 10 A g⁻¹, respectively. Even at the current density of as high as 20 and 50 A g⁻¹, the FMC can output the capacities of up to 294 and 222 mA h g⁻¹, respectively. Its excellent rate capability is much superior to some other high-rate carboneous anode materials, such as carbon nanoparticles (383.4 mA h g⁻¹ at 3.72 A g⁻¹),³⁰ porous graphitic carbon nanosheets (115 mA h g⁻¹ at 11.2 A g⁻¹),³¹ porous graphene (199 mA h g⁻¹ at 20 A g⁻¹),³² and nitrogen-doped porous carbon nanofiber webs (226 mA h g⁻¹ at 20 A g⁻¹).³³

Fig. 5 Charge/discharge curves at 50 mA g⁻¹ (a), cyclic voltammograms at 0.1 mV s⁻¹ (b), cycle performances at 100 mA g⁻¹ (c) and rate performance (d) of the FMC as anode materials for LIBs.

The FMC exhibits ultrahigh capacity, excellent cycle stability and superior rate capability in both sodium and lithium ion storage, indicating a promising anode material for SIBs and LIBs. The outstanding electrochemical performance can be ascribed to its unique microstructure. Firstly, the large interlayer distance (0.42 nm) facilitates sodium/lithium ion storage and transport between the graphene layers, especially for the large sodium ions,³⁴ while the high surface area contributes large capacitive storage of sodium/lithium ions, making the FMC exhibits ultrahigh Na/Li-storage capacities. Secondly, the large interlayer distance and developed mesoporous structure are helpful to overcome the volume change problem during charge–discharge process. Thus, the stable Na⁺/Li⁺ insertion/extraction and the consequent excellent cycle performance are ensured. Finally, the floral morphology and developed mesopores in the FMC can shorten the Na⁺/Li⁺ diffusion distance and provide continuous electron conduction pathways, resulting in the superior rate capability.

4. Conclusion

The floral variant of mesoporous carbon has been simply prepared through direct pyrolysis of zinc citrate. The zinc citrate acts as the carbon precursor, the template and the inducer of the floral morphology simultaneously. The as-obtained carbon possesses floral morphology, developed mesoporous structure and large interlayer distance, and exhibits ultrahigh reversible capacity, excellent cycle stability and superior rate capability as anode materials for both Na-ion batteries and Li-ion batteries. The outstanding electrochemical performance combined with the simple and sustainable preparation procedure implies the floral mesoporous carbon may be one of the most promising anode materials for next generation batteries. Furthermore, the unique microstructure of the FMC are expected to has broad applications in many other energy conversion and storage devices such as supercapacitors, lithium–sulfur batteries, lithium–air batteries and fuel cells.

Acknowledgements

This work was financially supported by the National Key Basic Research and Development Program of China (2015CB251100), the National Natural Science Foundation of China (51572011, 21073233, 51202083) and the Fundamental Research Funds for the Central Universities (buctrc201410).

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