Analogous graphite carbon sheets derived from corn stalks as high performance sodium-ion battery anodes

Abstract

Biomass derived carbon (BMC) materials have attracted much attention due to their high performance and abundant sources. Herein, analogous graphite carbon sheets (AGCS) from corn stalks have been synthesized via a simple high temperature carbonization and expansion process. A morphology study showed that the obtained carbon sheets retained the natural cross-sectional honeycomb-like shape and the longitudinal hollow tubular array structure of the corn stalks, which provided abundant macropores and micropores to facilitate sodium ion transportation and electrolyte diffusion. X-ray diffraction analysis showed that the interlayer spacing of the carbon sheets is larger (0.384 nm) than that of graphite (0.335 nm), which allowed sodium ion to be de/intercalated. When used as an anode for sodium ion batteries, the sample carbonized at 1200 °C (AGCS-1200) showed a better sodium ion storage performance than that carbonized at 900 °C (BMC-900). AGCS-1200 exhibits a stable reversible capacity of 231 mA h g⁻¹ after 200 cycles at 0.25C (1C = 200 mA g⁻¹), while the capacity value for BMC-900 was 162 mA h g⁻¹ after 100 cycles. What’s more, a better rate capability for AGCS-1200 (232, 136 mA h g⁻¹) than that of BMC-900 (125, 68 mA h g⁻¹) at rates of 1C and 5C, respectively, is demonstrated. Significantly, the AGCs-1200 anode shows an excellent long-term cycling stability, which delivers a capacity of 42 mA h g⁻¹ after 2000 charge–discharge cycles at a very high rate of 15C.

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Introduction

Electrical energy storage (EES) at national grid scale plays a significant role in addressing the issue of climate change by integrating a wide variety of renewable energy sources. Among the diverse EES devices, lithium ion batteries (LIBs) are the most promising technology for portable electronic devices and electrical vehicles (EVs) due to their high energy/power density and long cycle life.^1,2 Considering the large scale application of LIBs in grid energy storage, it would be threatened by an unbalanced lithium source distribution and high cost.^3,4 Sodium ion batteries (SIBs) are made from abundant earth sources of Na and represent an attractive technology for the next generation of energy storage.^5,6 However, the problem with SIBs is the larger size of the Na ion (99 pm) compared to that of the Li ion (59 pm), which leads to problems such as slower ion diffusion, a larger volume change and structure pulverization during charge and discharge cycles.^7,8 This issue for SIBs needs to be addressed through the discovery of suitable sodium ion intercalation materials. Recently, researchers have developed alloying metals,^9,10 metal oxides,^6,11 metal oxide–carbon hybrid materials,^12,13 inorganic intercalation compounds,¹⁴ etc. as potential anode materials for SIBs. However, each class of these materials suffers a huge volume change or a high over potential due to certain intrinsic limitations of the material itself.

Biomass derived carbon materials for EES applications have drawn much attention due to their excellent electrochemical performance, environmentally friendly characteristics and scalable synthesis methods. Various carbonaceous materials derived from biomasses have been synthesized for EES, which showed a good performance. For example, Wu et al. synthesized hard carbon from apple biowaste,¹⁵ which delivered a capacity of 245 mA h g⁻¹ after 80 cycles at a current density of 20 mA g⁻¹ and a capacity of 112 mA h g⁻¹ at a current density of 200 mA g⁻¹. Meng et al.¹⁶ have reported a bio-carbon material from harmful algal blooms, which delivered a capacity of 230 mA h g⁻¹ after 60 cycles at a current density of 20 mA g⁻¹ and a capacity of 135 mA h g⁻¹ at a current density of 100 mA g⁻¹. Despite the successes, the selected biomass sources used in these studies are very limited in nature, which could not meet the requirement of a low cost for grid scale EES applications.

As a by-product of corn production, corn stalks are an important and abundant biomass from the system of corn production. It is estimated that the annual total of biomass residues in China exceeds 700 million tons, among which corn stalks contribute up to 220 million tons.¹⁷ However, the current applications of corn stalks are primarily restricted to forage, fertilizer, fuel products and other fields. What’s more, large quantities of corn stalks are burned directly, which usually results in severe air pollution events (Fig. 1).^17,18 Therefore, it is urgent to search for a better way to make good use of the corn stalks and transform them into high value-added products. As with wood, corn stalk is likewise rich in cellulose and lignin, with a high content of up to 39.5% cellulose and 20.1% lignin in their rinds.¹⁹ Furthermore, the lignin in stalk rinds has a highly cross-linked three-dimensional framework in order to form a protective layer around the cellulose.²⁰ Such a feature of the corn stalk rinds being cellulose and lignin rich makes them an excellent biomass source for the synthesis of carbon materials with a good electrochemical performance.²¹

Fig. 1 Illustration of corn stalks and their different utilizations.

Herein, naturally abundant corn stalks were adopted to synthesize analogous graphite carbon sheets (AGCS) toward SIB applications. The AGCS were synthesized using a simple high temperature carbonization and expansion procedure, which is desirable for taking full advantage of both the cellulose and lignin richness in the corn stalk rinds. Our experiments demonstrate that this tailored carbonization and expansion process not only enlarged the interlayer distance of the carbon sheets but also optimized the porosity in the carbon sheets, which finally facilitated electrolyte diffusion and de/intercalation of sodium ions to improve the electrochemical performance for SIB anodes. Significantly, corn stalks can act as an abundant and sustainable carbon source to allow the large scale synthesis of carbon materials with an attractive sodium ion storage performance. Furthermore, the synthesis process is scalable and cost-effective.

Experimental section

Synthesis of materials

Corn stalks were obtained from farmland in the North area in the Shanxi Province of China, where the stalks were divided into stalk piths and rinds. After being dried thoroughly in an oven at 110 °C for 24 h, the selected stalk rinds were mill-ground into a powder for the following experiment. A high temperature carbonization experiment was performed using a high temperature tube furnace with an inert atmosphere. Specifically, a certain amount of the dried stalk rind powder was put into a ceramic crucible that was loaded into the furnace and carbonized at a temperature of 900 °C (BMC-900) or 1200 °C (AGCS-1200), with a heating rate of 2 °C min⁻¹ under a nitrogen atmosphere (100 sccm). After being kept at the required temperature for 2 hours, the furnace was allowed to cool down naturally. The yields of the carbon sheets were estimated to be about 24% and 21%, respectively. After carbonization, the products were thoroughly washed with 0.1 M NaOH at 60 °C for 2 h and 0.1 M HCl at 40 °C for 4 h, and then rinsed with a large amount of distilled water to remove undesired ions. The obtained products were immersed in a mixed solution with concentrated H₂SO₄ and H₂O₂ in a volume ratio of 3 : 1 at 35 °C and stirred for 2 h. Then, a certain amount of distilled water was added into the solution and stirred for another 4 h at 35 °C. Finally, the products were thoroughly washed and dried for the following characterization and measurements (ESI, synthesis schematic diagram in Fig. S1†).

Material characterization

Thermal gravimetric analysis (TGA) was performed using a thermogravimetry analyzer (NETZSCH STA 409PC) with a heating rate of 10 °C min⁻¹ and a nitrogen flow rate of 20 ml min⁻¹. SEM images of the natural stalk rinds and carbonized products were obtained using a field emission scanning electron microscope (FESEM, LEO 1530 VP, Germany). TEM and HRTEM images were obtained using a transmission electron microscope (TEM, JEM-2100F) and a high-resolution transmission electron microscope (HR-TEM, JEOL-2010F, Japan) with an accelerating voltage of 200 kV. X-ray diffraction (XRD) was performed using a Bruker D8 Advance power X-ray diffractometer with Cu Kα radiation (λ = 0.15418 nm). Laser confocal micro-zone Raman spectroscopy (HORIBA LabRAM HR Evolution, Japan) was utilized to analyze the bonding characteristics and the existence of defects, using a 488 nm laser beam. The Brunauer–Emmett–Teller (BET) surface area was measured using nitrogen adsorption–desorption isotherms obtained at 77.521 K with an ASAP 2020 analyser (accelerated surface area and porosimetry system, Micrometrics, USA). The electrical conductivity of the carbon products was measured using a ST-2722 semiconductor resistivity of the powder tester at 10 MPa (Suzhou Jinge Electronic Co., Ltd).

Electrochemical measurements

Typically, the anode electrode was prepared by mixing 80 wt% active material, 10 wt% acetylene black, and 10 wt% polyvinylidene fluoride (PVDF) in N-methyl-2-pyrrolidinone (NMP) via a slurry coating technique. The ground slurries were uniformly spread onto thin copper foils, then dried under vacuum at 110 °C for 24 h. The coin cells (CR2032) were assembled in an argon-filled glove box, in which the oxygen content and moisture content were kept below 0.1 ppm. Sodium metal foil was utilized as the counter electrode, glass microfiber filters were utilized as the separator, and 1 M NaClO₄ in a mixed solvent of 1 : 1 (v/v) ethylene carbonate (EC) and diethyl carbonate (DEC) was utilized as the electrolyte. The coin cells were galvanostatically discharged/charged between 0.01 and 3 V (vs. Na/Na⁺) using a CT2001A Land battery measurement systems instrument (LAND Electronic Co.) at room temperature. All the cyclic voltammetry (CV) measurements were performed using a CHI 770D workstation at a sweeping rate of 0.1 mV s⁻¹ in a potential range from 0.01 to 3.0 V. Electrochemical impedance spectroscopy (EIS) measurements were obtained using the same workstation with an AC amplitude of 5 mV from 1 MHz to 0.01 Hz.

Results and discussion

The morphologies of the corn stalk rinds before and after carbonization were investigated using SEM. Fig. 2a and b show the SEM images of the corn stalk rinds. They display a honeycomb-like shape with linked macropores in the cross section image and a hollow tubular array structure in the longitudinal section image. Comparison of the SEM images of AGCS-1200 (Fig. 2c and d) and BMC-900 (Fig. 2e and f) indicated that the carbonized products retain the natural hollow and open pore structure of the corn stalks. Furthermore, both of the corn stalk derived carbon materials show a similar morphology, regardless of the carbonization temperature. It is worthwhile to note that high temperature carbonization generated a large number of pores on the tubular wall of the carbon framework (marked with circles). Although the morphologies are similar, the tubular wall of the open carbon framework becomes thinner (marked by the arrows) with an elevated carbonization temperature. Some of these features not only help with diffusion and penetration of the electrolyte, but also relieve the mechanical strain during charge/discharge cycles and provide more electrochemical active sites. Additionally, AGCS-1200 in Fig. 2g shows a lamellar structure with more loose graphite flakes, which can be seen in Fig. 2h. Such loose graphite flakes will highly promote de/intercalation of the larger sodium ions.²² In contrast, the specimen of BMC-900 in Fig. 2i did not show a morphology of analogous graphite flakes. To elucidate the microstructure of AGCS-1200, high-resolution transmission electron microscopy (HRTEM) analysis was performed and the results are shown in Fig. 3a and b. It consists of tiny graphite microcrystallites with parallel graphite layers, which could provide effective electron transmission to improve the rate capability of SIBs.^23,24 Thermal gravimetric analysis (TGA) was used to study the thermal decomposition of the corn stalks under a nitrogen atmosphere. The carbonization process of the corn stalks mainly resulted from pyrolysis of the cellulose, lignin and hemicellulose. It can be seen from Fig. 3c that the TGA curve exhibits an obvious mass loss in the temperature range from 200 to 800 °C. It has been reported that cellulose usually decomposes from 200 to 400 °C, along with the decomposition of hemicellulose. However, lignin decomposes in a broad temperature range of 150–750 °C, and the decomposition mainly occurs at a higher temperature.²⁵ We believed that the carbonization of the lignin-rich corn stalks is beneficial for graphitic ordering of the parallel-laminated graphite layers, which will be further confirmed by XRD and Raman characterization.

Fig. 2 SEM images of the (a) cross section and (b) longitudinal section of stalk rinds; (c) cross section and (d) longitudinal section of AGCS-1200; (e) cross section and (f) longitudinal section of BMC-900; (g) carbon sheets of AGCS-1200. (h) TEM image of AGCS-1200 and (i) SEM image of BMC-900.

Fig. 3 (a) and (b) High-resolution TEM (HRTEM) images of the AGCS-1200, (c) TG profile of the stalk rinds for carbonization under a nitrogen atmosphere.

To examine the porosity of AGCS-1200 and BMC-900, nitrogen isotherm adsorption/desorption measurements were carried out. It can be seen that both of the carbon samples show typical type I/IV isotherms with a hysteresis loop, implying the existence of micro-, meso- and macropores. In detail, the type I isotherms show a significant upward trend below a relative pressure of 0.45, suggesting the sample is rich in micropores. The presence of a clear hysteresis hoop above the relative pressure of 0.45 (P/P₀ = 0.45–0.95) indicates the existence of very large mesopores. Furthermore, the adsorption curve ascends very steeply owing to capillary condensation in the mesopores. In addition, both isotherms show an evident upwards trend at the highest relative pressures (P/P₀ = 0.95–1.0), indicating the existence of macropores. On the basis of IUPAC suggestions, hysteresis loops should be separated into four types.²⁶ It is obvious that the hysteresis loops in Fig. 4a accord with type-H4 loop characteristics due to adsorption/desorption in lamellar materials with narrow slit-like pores.²⁷ The pore size distribution profiles in Fig. 4b reveal that the most prevalent pore diameter is circa 3.8 nm for both carbon specimens. The Brunauer–Emmett–Teller (BET) specific surface area for AGCS-1200 and BMC-900 was determined to be 27.4 and 847.6 m² g⁻¹, respectively. The significant reduction in the specific surface area on going from BMC-900 to AGCS-1200 is due to improvement in the degree of crystallization of the graphite microcrystallites, which will be demonstrated in the XRD results.

Fig. 4 (a) Nitrogen adsorption/desorption isotherms of AGCS-1200 and BMC-900 (the inset), (b) the pore diameter distribution of AGCS-1200 and BMC-900 (the inset shows partial enlargement of the details), (c) XRD patterns of the AGCS-1200 and BMC-900 specimens, and (d) Raman spectra of AGCS-1200 and BMC-900.

Fig. 4c shows X-ray diffraction (XRD) patterns of both the carbon specimens. They all exhibit two broad diffraction peaks. The broad peaks at 2θ = 23.15° are triggered by reflections of the parallel-laminated graphite layers from the (002) crystal planes, while the peaks at 2θ = 43.23° are attributed to the (101) crystal planes of sp²-C.²⁸ The intensity of the (002) peak of AGCS-1200 is greater, which reveals that the c-axis length (L_c) of the graphitic domains has increased.²⁹ The higher intensity of the (101) peak in AGCS-1200 indicates an increase in the degree of graphitization. The interlayer distance of the graphite sheets of AGCS-1200 from the (002) peak was calculated to be 0.384 nm using the Bragg equation (0.372 nm for BMC-900), which is larger than that of the graphite d-spacing (0.335 nm). This will be greatly helpful in benefiting de/intercalation of the larger sodium ions.³⁰ The average thickness and width of the graphitic domain, which are the c-axis length (L_c) and a-axis distance (L_a), were calculated using the full width at half maximum (FWHM) values of (002) at 2θ = 23.15° and (101) at 2θ = 43.23°. The average thickness of BMC-900 and AGCS-1200 was calculated to be about 0.71 and 1.07, respectively. Thus, the number of laminated layers in the graphite domain for AGCS-1200 and BMC-900 was 2–3 and 1–2, respectively. The average thickness tends to a higher value with an increased carbonization temperature, suggesting that much more graphitic domains were formed in AGCS-1200. Highly crystallized graphite usually generates very large parallel-laminated graphite layers. An empirical R-factor value could also be used to evaluate the proportion of parallel-laminated graphite layers in the analogous graphite carbon materials.³¹ The R-factors of AGCS-1200 and BMC-900 were calculated to be 2.92 and 2.51 based on the (002) crystal planes, respectively (an illustration for determination of the R-factor is in Fig. S2 of the ESI†). The higher R-factor for AGCS-1200 indicates that very large parallel-laminated graphite layers are easily formed at a higher temperature. The numerous parallel-laminated graphite layers of AGCS-1200 should show better electrochemical behavior, which will be confirmed through electrochemical characterization. Fig. 4d shows Raman spectra of AGCS-1200 and BMC-900. Both samples exhibit a distinct broad disordered D-band at about 1355 cm⁻¹ and an in-plane bond-stretching motion G-band at about 1590 cm⁻¹. The intensity ratio of the D- to G-bands can be used to evaluate the degree of crystalization of the graphite. The ratio of I_D/I_G for BMC-900 and AGCS-1200 was calculated to be about 0.91 and 0.98, respectively. According to the literature, the increase in the I_D/I_G ratio on going from BMC-900 to AGCS-1200 might be indicative of an amorphous state shifting into a graphitic structure with more edges and numerous defects.³² In addition, AGCS-1200 shows intense second index Raman peaks characteristic of 2D and D + G, which could also be a demonstration of enhancement of the number of highly microcrystalline 2D graphite domains with more defects.³² This result agrees well with the XRD and HRTEM results. We believe that the presence of an intense 2D band is highly connected with an improvement in the electrical conductivity of the carbon materials. The electrical conductivity of AGCS-1200 was determined to be about 165.4 S cm⁻¹, which is obviously higher than that of BMC-900 (54.5 S cm⁻¹). The excellent electrical conductivity of the corn stalk derived carbon materials will greatly contribute to a high performance of the sodium ion battery anode.

To investigate the electrochemical performance of the corn stalk derived carbon materials for SIB anodes, cyclic voltammetry (CV) and galvanostatic charge/discharge measurements were performed. Fig. 5a and b show the initial three CV curves of BMC-900 and AGCS-1200 obtained with a scan rate of 0.1 mV s⁻¹ and a potential range of 0.01–3 V. In the first scan, AGCS-1200 shows a narrow reduction peak around 0.5 V, while a reduction peak across a wide potential range of 0.01 to 1.0 V is present for BMC-900. These peaks might correspond to decomposition of the electrolyte and the formation of a solid electrolyte interface (SEI) on the surface of the carbon anode.³³ In the following CV scans, AGCS-1200 shows extremely sharp cathodic and less sharp anodic peaks between 0.01 and 0.15 V. The cathodic peaks could be ascribed to sodium ion insertion into the microvoid sites between the graphite domains in the materials,^23,30 while the anodic peaks correspond to sodium ion extraction. Extraction of the sodium ions results in less sharp anodic peaks, compared to the extremely sharp cathodic peaks in the CV curves for AGCS-1200, suggesting more sodium ion extraction occurs.³⁴ The CV profiles for AGCS-1200 are similar to those previously reported for hard carbon materials. In contrast, BMC-900 shows a broad cathodic peak and sharp anodic peaks than those of AGCS-1200, implying more sodium ion insertion and less sodium ion extraction. The subsequent two CV curves for both kinds of material show good repeatability, which reveals a good reversibility during sodium ion insertion/extraction. Meanwhile, Fig. 5c and d show the galvanostatic charge/discharge curves for the first, second, third and tenth cycles of BMC-900 and AGCS-1200 at 0.25C (1C = 200 mA g⁻¹). During the first discharge/charge cycle, BMC-900 exhibits discharge and charge capacities of 511.6 and 211.8 mA h g⁻¹, respectively, with an initial coulombic efficiency (CE) of 41.4%. The discharge and charge capacities for AGCS-1200 are 611.9 and 321.9 mA h g⁻¹, respectively, with an initial CE of 52.6%. Notably, BMC-900 and AGCS-1200 still maintain high discharge capacities of about 196 and 313 mA h g⁻¹ after the 10th cycle. In addition, a broad slope below about 1.0 V appears for both BMC-900 and AGCS-1200. The profile of the slopes and the plateau regions correspond to the pairs of broad and sharp peaks in the CV curves, respectively. The large initial irreversible capacity is primarily attributed to the formation of a SEI layer, electrolyte decomposition, as well as the irreversible trapping of sodium ions at analogous graphite lamellas and/or some surface functional group(s),^23,35,36 which agrees well with the CV results.

Fig. 5 Initial three cyclic voltammetry curves for (a) BMC-900 and (b) AGCS-1200 obtained at a scan rate of 0.1 mV s⁻¹; galvanostatic charge/discharge cycling results for (c) BMC-900 and (d) AGCS-1200 at 0.25C (1C = 200 mA g⁻¹).

To evaluate the stability of BMC-900 and AGCS-1200 during long term charge/discharge cycling, cycle life measurements at a rate of 0.25C and 1C were carried out and the results are shown in Fig. 6a and b, respectively. An outstanding long-term cycling capability of the electrode material is important for future applications of SIBs. It can be seen from Fig. 6a that AGCS-1200 shows a much higher discharge capacity, of about 247 mA h g⁻¹, than that of 162 mA h g⁻¹ for the BMC-900 over 100 cycles at 0.25C. Even after 200 cycles of charge/discharge, AGCS-1200 still exhibits a high discharge capacity of about 231 mA h g⁻¹. When the rate is increased from 0.25C to 1C, similar results could be obtained. As shown in Fig. 6b, AGCS-1200 shows a higher discharge capacity, of about 144 mA h g⁻¹, compared to the value of 115 mA h g⁻¹ for BMC-900 over 300 cycles at a rate of 1C. So far, only a few reports have demonstrated that such high capacity values could be retained over 200 cycles under the same conditions.^35,37–39 Fig. 6c shows the rate performance of BMC-900 and AGCS-1200 at a variety of current densities from 0.25C to 10C. Initial reversible capacities of about 260, 232, 216 and 136 mA h g⁻¹ were obtained at 0.5C, 1C, 2.5C, and 5C for AGCS-1200, respectively. What’s more, a reversible capacity of about 100 mA h g⁻¹ still could be achieved at a high rate of 10C. To the best of our knowledge, only a few reports have shown a similar high capacity of over 100 mA h g⁻¹ at such a high current density.^37–42 In addition, BMC-900 shows initial reversible capacities of about 155, 125, 97 and 68 mA h g⁻¹ at a rate of 0.5C, 1C, 2.5C, and 10C, respectively. Furthermore, cycling at a very high rate of 15C, a reversible capacity of about 52 mA h g⁻¹ could be obtained, which is also higher than those of other reported results.^35,38 More importantly, when the current density was reverted back to the initial rate of 0.25C, reversible capacities of about 251 mA h g⁻¹ for AGCS-1200 and about 172 mA h g⁻¹ for BMC-900 could be obtained. Compared to BMC-900, the higher capacity value and better rate capability of AGCS-1200 are due to its highly crystalline graphite microcrystallites with numerous edge and defect sites. To study the electrochemical kinetics of both of the carbon SIB anodes in detail, electrochemical impedance spectroscopy (EIS) was performed before the discharge/charge cycle and the results are shown in Fig. 6d. The Nyquist plots of both the carbon anodes in (Fig. 6d) display a semicircle in the high frequency section, which corresponds to the resistance of charge transfer between the SEI/electrolyte interface. The sloping straight line in the low frequency section corresponds to the Warburg impedance of the sodium ion diffusion in the solid state of the electrode materials. The charge transfer resistance for AGCS-1200 was determined to be about 52.6 ohm, while the corresponding value for BMC-900 was about 865.8 ohms, which is much higher than that of AGCS-1200. This result indicates that AGCS-1200 has a better charge transport capability at the interface to support the electrochemical performance.

Fig. 6 Comparison of the cycling capacity retention of (a) BMC-900 and AGCS-1200 at 0.25C, and (b) BMC-900 and AGCS-1200 at 1C, (c) comparison of the rate performance of BMC-900 and AGCS-1200 at different current densities, (d) electrochemical impedance spectra of BMC-900 and AGCS-1200 before electrochemical test, (e) long-term cycling performance results for AGCS-1200 at 15C.

Considering the future high power application of SIBs, the long-term cycling capability for the anode under a high rate needs to be evaluated. As a result, we examined the stability of AGCS-1200 over 2000 cycles at a very high rate of 15C and the results are shown in Fig. 6e. After an initial activation over 5 cycles at 0.25C, AGCS-1200 showed a reversible capacity of around 106 mA h g⁻¹ at 15C. After 100 cycles, it still retained a reversible capacity of around 63 mA h g⁻¹. Then, finally, it delivered a reversible capacity of about 42 mA h g⁻¹ for the 2000th cycle.

Conclusions

In our study, we have demonstrated that the rinds of corn stalk could serve as an ideal biomass source for creation of SIB anodes with an attractive electrochemical performance. Based on the stalk rind characteristics of being cellulose and lignin rich, high temperature carbonization of the stalk rinds and a subsequent expansion procedure were used to synthesise analogous graphite sheets for SIB anode applications. The stalk rind derived carbon material retained the natural open and hierarchical pore channel structure of the corn stalks. The tailored carbonization and expansion process not only enlarged the interlayer spacing of the graphite domains in the carbon material but also generated abundant mesopores and micropores, which facilitated diffusion of the electrolyte and transportation of sodium ions, which allowed sodium ion de/intercalation to occur to improve the battery performance. As a result, AGCS-1200, which has highly crystallized graphite domains with numerous edge and defect sites, showed an outstanding sodium ion storage performance. The abundant biomass resource, scalable synthesis method, as well as excellent electrochemical performance will make corn stalk derived carbon materials competitive SIB anodes in the future.

Acknowledgements

This work was financially supported by the Open Fund of the Jiangsu Key Laboratory of Materials and Technology for Energy Conversion (No. MTEC-2015M0X), the Foundation of Graduate Innovation Center in the Nanjing University of Aeronautics and Astronautics (No. kfjj20160612) and the Priority Academic Program Development (PAPD) of the Jiangsu Higher Education Institutions.

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Footnote

† Electronic supplementary information (ESI) available: Schematic diagram of the synthesis, and an illustration. See DOI: 10.1039/c6ra22769d

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