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High Temperature Carbonized Grass as a High Performance Sodium Ion Battery Anode Fang Zhang, Yonggang Yao, Jiayu Wan, Doug Henderson, Xiaogang Zhang, and Liangbing Hu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12542 • Publication Date (Web): 05 Dec 2016 Downloaded from http://pubs.acs.org on December 7, 2016

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ACS Applied Materials & Interfaces

High Temperature Carbonized Grass as a High Performance Sodium Ion Battery Anode Fang Zhang1,2, Yonggang Yao2, Jiayu Wan2, Doug Henderson2, Xiaogang Zhang1, Liangbing Hu2,* 1

College of Materials Science and Technology, University of Aeronautics and Astronautics,

Nanjing, Jiangsu Province, 210016 2

Department of Materials Science and Engineering, University of Maryland College Park,

College Park, Maryland, 20742 Email: [email protected]

ACS Paragon Plus Environment

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ABSTRACT: Hard carbon is currently considered the most promising anode candidate for room temperature sodium ion batteries because of its relatively high capacity, low cost, and good scalability. In this paper, switchgrass as a biomass example was carbonized under an ultra-high temperatures, 2050 °C, induced by Joule heating to create hard carbon anodes for sodium ion batteries. Switchgrass derived carbon materials intrinsically inherit its three-dimensional porous hierarchical architecture, with an average interlayer spacing of 0.376 nm. The larger interlayer spacing than that of graphite allows for the significant Na ion storage performance. Compared to the sample carbonized under 1000 °C, switchgrass derived carbon at 2050 °C induced an improved initial Coulombic efficiency. Additionally, excellent rate capability and superior cycling performance are demonstrated for the switchgrass derived carbon due to the unique high temperature treatment. KEYWORDS: hard carbon, high-temperature carbonization, biomass material, sodium ion battery, long Cycling


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1. INTRODUCTION Electrical energy storage in grid scale plays a significant role in addressing the climate change by integrating a wide variety of renewable energy sources. Among diverse energy storage technologies, batteries possess a number of desirable features, including flexible power and energy properties, high-energy conversion efficiency and simple maintenance.1-3 Sodium ion has similar electrochemistry to lithium ion, and sodium ion batteries (SIBs) are made from earth abundant, widely available sodium containing resource, which are expected to be a low cost alternative to lithium ion batteries (LIBs). Furthermore, SIBs are capable of satisfactory operation at room temperature by discovering suitable sodium ion intercalation materials.4-12 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, larger volume change and structure damage upon charge and discharge cycles.13, 14 Thus, it is impossible to simply adapt the current Li ion battery electrode materials to SIBs technology. For example, graphite (theoretical capacity of 372 mAh g-1)

for LIBs anode only shows electrochemical

sodiation/desodiation capacity of < 35 mAh g-1.15, 16 Recent theoretical calculation indicates that the small interlayer distance of 0.34 nm cannot accommodate the large sodium ion to intercalate into graphite.17 Extensive effort has been made in developing anode materials for SIBs, including inorganic intercalation compounds,18-20 organic compounds,21-23 or alloying metal/metal oxides.24-28 However, such of these materials usually exhibit certain intrinsic drawbacks, such as a high over-potential associated with conversion or alloying reaction and poor cycle performance. Carbon material is a dominant electrode material for electrochemical energy storage device due to its desirable electrochemical attributes and low cost.14,


29-32

Among various types of

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carbon materials, hard carbon shows attractive electrochemical features as an anode for SIBs, including considerable reversible capacity (200-300 mAh g-1) and low operating potential (the average is 0.3 V versus Na+/Na).33-37 However, the issue of low initial coulombic efficiency (CE) for hard carbon needs to be addressed. Much effort has been made to address this problem recently, such as surface modification to reduce the contact area of a carbon anode with electrolyte.38, 39 In addition, long-term cycle stability and rate performance of hard carbon for SIBs need to be improved as well. Recently, biomass derived carbon materials have attracted much attention due to their environmentally friendly characteristics and abundant resource.40-43 Switchgrass is known as a high yield biomass crop, which has been promoted as a major biomass source for biofuel.44 Additionally, studies have been conducted on the potential of using switchgrass to produce natural cellulose fibers for textiles.45 As with wood, switchgrass is likewise rich in cellulose, having a high content of up to 50% cellulose and around 21% cross-linked lignin in their stems.45 This feature makes the majority of the switchgrasses easy to carbonize. Grass in nature intrinsically has macroporous cellulose architecture in its leaves and stems, which naturally adsorbs ions and water to fulfill the metabolism process. Specifically, this function was performed by the large volume of hyaline cell and the thin but flexible cell wall in the grass stems. The SEM (scanning electron microscopy) image on cross section of switchgrass in Figure 1a reveals the macroporous architecture built up by cell walls to form an interconnected open framework. The SEM observation near the outer skin of grass stems shown in Figure1b highlights the parallel channel array structure in the longitudinal dimensions. Rich cellulose with such macroscopic structure makes switchgrass an ideal precursor to produce three-dimensional (3D) porous carbon material with attractive electrochemical properties.


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In our research, the stem of switchgrass was carbonized under an ultra-high temperature (2050 °C) to create hard carbon material. This carbonization temperature is much higher than others from other biomass toward SIBs applications.42, 43, 46, 47 To the best of our knowledge, this is the first grass derived anode material carbonized by such a high temperature of beyond 2000 °C for SIBs applications. Interestingly, the electrical conductivity of switchgrass derived carbon is improved with the increase of carbonization temperature while the surface area is decreased, which is ideal for a hard carbon anode. As expected, the carbon anode treated at 2050 °C shows an enhanced initial CE and rate capability compared to carbon prepared at 1000 °C. In addition, a high specific capacity of 200 mAh g-1 is delivered over a prolonged 800 charge/discharge cycles for the high temperature induced carbon anode. It is reasonable to believe that switchgrass can supply an abundant carbon source, allowing for facile production of 3D porous carbon material with an attractive sodium ion storage performance. This will make electrical energy storage more sustainable and cost-effective.

Figure 1. Hierarchical architecture of switchgrass stem. (a) SEM images of cross section; and (b) longitudinal section on the stem.


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2. EXPERIMENTAL SECTION 2.1. Materials Preparation. Dried switchgrass was obtained from our campus (University of Maryland, College Park), where grass stems were cut into small segments and washed with water/ethanol before drying at 60 °C. Dried grass stems were then loaded into a tubular furnace and carbonized at the temperature of 1000 °C for 2 hours under Argon, which was named GC1000. The yield of GC-1000 was calculated to be about 25% according to the weight loss of hard carbon before and after carbonization. GC-1000 was further treated with a Joule heating process by using a homemade device. Specifically, the GC-1000 rod (3 cm × 1.8 mm) was connected to copper electrodes by silver paste and was suspended on a glass holder (Figure S1). Both ends of the glass holder were extended by wires to an externally connected KEITHLEY power source. The Joule heating process was conducted in the Ar-filled glove box, where DC current was applied and the GC-1000 rod was heated up by Joule heating at a temperature at 2050 °C (GC2050).


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2.2. Physical Characterization. SEM images of switchgrass and carbonized products were taken with a Hitachi SU-70 field emission scanning electron microscope. High-resolution transmission electron microscopy (HRTEM) images were taken on a JEOL 2100F transmission electron microscope at an accelerating voltage of 200 kV. The radiation emitted by the high temperature grass carbon rod was detected by an optical fiber (400 µm diameter) carefully aligned to maximize sample radiation intensity and connected to a spectrometer (Ocean Optics). The emission spectra were recorded by a scientific CMOS camera, using a 550 nm narrow-pass filter to restrict the images to a representative visible wavelength. X-ray diffraction (XRD) patterns were collected with a Bruker D8 advance by using Cu Kα radiation (λ = 1.5406 Å). Raman spectra were recorded using a Labram Aramis Raman spectrometer with a 633 nm He-Ne laser source. Nitrogen adsorption/desorption measurements were performed with a Micromeritics Tristar II 3020 analyzer at 77 K. 2.3. Battery Assembly and Electrochemical Measurements. Electrochemical measurements were performed using CR2025 coin cells with Na metal as the counter electrode. The working electrode was prepared by mixing 80% active material, 10% carbon black (Super-P) and 10% poly(vinylidenedifluoride) in N-methylpyrrolidone to form a slurry, which was coated on a copper foil using a doctor blade. The coated copper foil was dried at 100 °C in a vacuum oven overnight. The mass loading of active material is around 0.5 mg cm-2. The electrolyte was 1.0 M NaClO4 dissolved in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) solvent with a volume ratio of 1:1. Cyclic voltammetry tests were conducted on a Biologic VMP-3 electrochemical potentiostat. Galvanostatic charge-discharge and long term cycle performance measurements were performed on a LAND-CT 2100A battery test system. All electrochemical measurements were conducted at room temperature.


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3. RESULTS AND DISCUSSION The morphology of carbonized switchgrass under high temperature was first investigated by SEM. The SEM observation confirms that the switchgrass derived carbon has similar morphology regardless of carbonization temperature. Even when the carbonizing temperature was increased from 1000 to 2050 °C, the macroscale carbon frameworks did not collapse. The hollow 3D structure with linked macropores are clearly seen (Figure 2a), which is similar to the macroporous features of grass before carbonization. High magnification SEM image in Figure 2b shows that numerous voids on the carbon walls were generated after high temperature carbonization. Such an open hollow framework was constructed by extremely thin carbon walls which were derived from hyaline in the grass. The carbon walls in the carbon GC-2050 displayed a thickness between 150 and 200 nm (Figure S2). The unique open structure is especially helpful for facilitating electrolyte penetration and ion diffusion. SEM image of GC-2050 shown in Figure 2c revealed no significant difference from the grass precursor in the longitudinal section after higher temperature carbonization. Similar parallel channel structures were observed in the carbon framework. The open pores on the channel wall were confirmed by the zoomed-in observation in Figure 2d, which is in accordance with the result in Figure 2b. The intrinsic macro scale openness of the grass derived carbon is one substantial feature for high material utilization in Na ion storage. HRTEM was carried out to investigate the microstructure of the grass derived carbon. Figure 2e and 2f present the HRTEM images obtained from GC-1000 and GC-2050, respectively. The structure of GC-1000 sample carbonized under lower temperature in Figure 2e consists of tiny graphitic domain, which assembles to form a lot of disordered microvoids. The contrasting HRTEM image of GC-2050 displays more ordered structure as will be demonstrated in XRD and Raman results (Figure 2f).


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Figure 2. (a) Low magnification SEM image from cross section of GC-1000; (b) Corresponding higher magnification SEM image of GC-1000; (c) Low magnification SEM image from longitudinal section of GC2050 (vertical lines show the array structure); (d) Corresponding higher magnification SEM micrograph of GC-2050 (dashed circles highlight the micro-/nanoscale holes for ion transport); (e) HRTEM image of GC1000; and (f) HRTEM image of GC-2050.

The high temperature carbonization (2050 °C) with Joule heating was conducted by a homemade device. At high power input (140 W), the temperature of carbonized grass was high and bright light emission was observed. Figure 3a shows the bright lighting of grass derived GC1000 carbon rod (3 cm × 1.8 mm) suspended on glass holders. The measured current and resistance change of carbonized grass during the Joule heating carbonization process is shown in Figure S3. With an increased driving voltage, the measured current increased linearly (Figure S3a), indicating that at a high driving voltage, the resistance decreased and resulted in the linear increase of measured current. It is further confirmed that the resistance of grass carbon decreased


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continuously with higher input power (Figure S3b). The light emission from by the high temperature grass carbon was collected by an optical fiber (400 um diameter). In the process of measurement, the optical fiber was carefully aligned to the maximum intensity of the radiation for the accuracy. After heating to the designed temperature and equilibrated for 1 minutes to obtain fully thermal equilibrium state, the thermal radiation spectrum was recorded at the electrical power of 140 W, as shown in Figure 3b. The obtained spectrum was then fitted to gray body radiation equation (eq 1), which has two free parameters: T and γ for every spectrum.48-51

Bλ (λ , T ) = γε gray

2hc 2 5

λ

1 e

hc / λk BT

−1

(1)

Where kB is Boltzman’s constant, h is Planck’s constant, c is the speed of light, and scaling constant γ is introduced for fitting. To examine the phase and structure change of carbonized grass under high temperature, X-ray diffraction (XRD) measurements for the grass derived carbon were performed. Both two samples show almost identical twin broad peaks around 2θ = 23.6° and 43.6° (Figure 3c), corresponding to the (002) diffraction of the graphitic layer-by-layer structure and (101) diffraction for graphite.52 Note that the diffraction peak in GC-2050 sample shows higher intensity, especially for the (101) peak diffraction, indicating a higher degree of graphite crystallinity. From the (002) peak, the average graphene interlayer distance is calculated to be 0.376 nm based on Scherrer equation.17 The thickness and average width of graphite domain, which is the c-axis length (Lc) and a-axis distance (La) in the graphite lattice, are calculated by using the full width at half maximum (FWHM) value of (002) at 2θ = 23.6° and (101) at 43.6°. According to the FWHM value in XRD result, the average thicknesses of the graphitic domain in the samples GC-1000 and GC-2050 are calculated to be 1.3 and 1.8 nm, respectively. Thus, the stacked graphene layers in graphitic domain for carbon specimens of GC-1000 and GC-2050 are 3-4 (1.3/0.376 = 3.46)


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and 4-5 (1.8/0.376 = 4.79), respectively. Accordingly, the average width of graphitic domain for GC-1000 and GC-2050 is 2.1 nm and 4.9 nm, respectively. The above analysis indicates that the disordered graphitic structure in the carbon material of GC-1000 becomes more ordered with an increased carbonization temperature of 2050 °C. Figure 3d shows the Raman spectra of grass derived carbons. Two samples exhibit broad disorder-induced D-bands (≈1350 cm-1) and in-plane vibrational G-bands (≈1600 cm-1).53 A decrease in the intensity ratio ID/IG (from 1.07 to 1.04) is observed from carbon sample GC-1000 to GC-2050. This result has further confirmed the progressively ordered structure in GC-2050, which is in accordance with the XRD analysis. In addition, the presence of the intense 2D peak at ≈2690 cm-1 might be an evidence of 2D graphene domains of high crystallinity in GC-2050.54, 55

It was reported that the intensity ratio of the 2D to G band can be used to characterize the

stacking of graphene layers in the crystalline domain.56, 57 In our experiment, the ratio of I2D to IG in GC-2050 is calculated to be 0.76, which revealed that the crystalline graphite domain in GC2050 could be few-layered graphene.58, 59 As will be discussed, the decreased ID/IG value and the presence of intense 2D in GC-2050 are distinctly connected with an improved electronic conductivity. In order to examine the porosity of grass derived carbon, nitrogen adsorptiondesorption measurements were conducted and the results are shown in Figure 3e and 3f. Both carbon samples show IV-type isotherms, which revealed a mesoporous and microporous structure for the carbon materials. The Brunauer-Emmett-Teller (BET) specific surface area of GC-1000 is calculated to be 619.8 m2 g-1. Due to an improvement of the degree of graphitic crystallinity, the specific surface area of GC-2050 is decreased significantly to be 23.1 m2 g-1. It can be seen from Figure 3f that both carbon samples show a broad pore distribution varying from 1.7 to 110 nm. The enlarged view of pore size distribution in Figure 3f suggests that there is a


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large quantity of mesopores and micropores in GC-2050. Therefore, although the specific surface area of grass derived carbon decreased after Joule heating, the nature of its porosity is retained.

Figure 3. (a) Digital photograph of switchgrass derived carbon under Joule heating; (b) Light emission spectra of switchgrass derived carbon under Joule heating process. The temperature was fitted according to black body radiation equation; (c) Representative XRD patterns and (d) Raman spectra of GC-1000 and GC-2050; (e) N2


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adsorption/desorption isotherms and (f) the pore size distribution curve of GC-1000 and GC-2050 (inset is the enlarged view of GC-2050).

To characterize the electrical conductivity of grass carbon before and after Joule heating, fourprobe I/V measurements were performed on the carbon (Figure 4a). The calculated values of electrical conductivity for the grass carbonized at 1000 and 2050 °C are 12.45 and 25.86 S cm-1, respectively (based on the entire cross section area). As shown in Figure 4b, the cell walls in the macropore structure are extremely thin and flexible with a large proportion of the area covered by pores. The electrical conductivity based on the actual carbon area is expected to be much higher. The real area coverage can be estimated approximately based on the projected area in Figure 4b. The average thickness of the cell wall is estimated to be about 2 µm (Figure S2. High magnified SEM image of cross section). Supposing the pore size is uniform with regular shape and the average inner pore diameter is 50 µm, the average outer pore diameter will be 52 µm. Thus, the percentage of pore area coverage in the carbon material is estimated to be 92%. Therefore, the conductivities with the real area of the cross section for grass carbonized at 1000 and 2050 °C are estimated to be 156 S cm-1 and 323 S cm-1, respectively, which are much higher than the conductivity values of hard carbon in literature.60 We believed that the excellent electron conductivity caused by Joule heating of the carbon accelerates the electron transport along the array structure in the material which in turn greatly contributes to the high performance of the SIB anode as shown in Figure 4c.


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Figure 4. (a) Photo image of carbon rod for 4-probe measurement; (b) SEM image of cross section from GC2050, highlighting the hollow structure formed by extremely thin cell wall; (c) SEM image of longitudinal section from GC-2050, highlighting the parallel array structure of channel; (d) 4-probe I/V measurements of GC-1000 and (e) GC-2050. (Inset is the schematic image of 4-probe measurement on grass derived carbon rod)

To investigate the electrochemical performance of grass carbon as a SIBs anode, cyclic voltammetry (CV) and galvanostatic discharge/charge measurements were performed. Both of two carbon anodes (GC-1000 and GC-2050) show similar CV curves as shown in Figure 5a, and 5b. In the first CV scan, two broad cathodic peaks appear at ~0.6 and 0.4 V for the sample of GC-1000 and GC-2050, which generally corresponds to the sodiation and the formation of solidelectrolyte interphase (SEI) layer on the carbon surface.35 In the following CV scans, a pair of sharp peaks at a low potential region (0.01-0.2 V) was observed for both carbon samples. In addition, a pair of weak humps was observed over a wide voltage range (0.2-1.2) in both


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cathodic and anodic scans. The galvanostatic charge/discharge curves agree well with the CV results. Figure 5c and 5d show the representative voltage profiles of carbon anodes for the 1st, 2nd, 5th and 10th cycle at a current density of 50 mA g-1 in the potential range of 0.01-2 V versus Na/Na+. The initial Coulombic efficiencies (CE) for the GC-1000 and GC-2050 anode are calculated to be 42% and 64%, respectively. The initial irreversible capacity loss is mainly resulting due to the formation of SEI layer which is strongly coupled to the specific surface area of carbon material. Therefore, the improvement of the initial CE from the GC-1000 to the GC2050 anode is due to the dramatically decreased specific surface area when the grass is carbonized at a higher temperature. Compared to GC-1000, the capacity of GC-2050 in both sloping and plateau region reduced slightly due to a decrease of active sites of electrochemical adsorption/desorption to sodium ion that resulted from the decreases of specific surface area and defects when the temperature increased from 1000 to 2050 °C. Another interesting difference between GC-1000 and GC-2050 is their rate capabilities, as shown in Figure 5e. At a lower current rate of 0.05 and 0.1 A g-1, GC-1000 and GC-2050 show comparable capacities of 225 and 210 mAh g-1, respectively. With an increasing current density of 0.2, 0.5, 1 and 5 A g-1, GC-2050 shows less capacity decay with 160, 130, 120 and 110 mAh g-1 compared to the GC-1000 with 130, 90, 70 and 55 mAh g-1, which demonstrates the highly promising rate performance for switchgrass derived carbon. For a given current density in the range of 1-5 A g-1, GC-2050 can be on par with more open structures such as hollow carbon spheres, porous carbon and graphene nanosheets.61-63 As we discussed earlier in the text, the hierarchical pore structure with an extremely thin cell wall will facilitate the penetration of electrolyte into the surface of carbon thus effectively reducing the required Na ion diffusion distance. Additionally, the excellent electron conductivity generated by the high temperature


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carbonization contributes to the enhancement of rate capability especially for the carbon specimen of GC-2050. To evaluate the electrochemical stability of switchgrass derived carbon materials, long term cycling performance for GC-1000 and GC-2050 were examined. When cycled at a current rate of 25 mA g-1, a discharge specific capacity of 200 mAh g-1 for GC-1000 is retained over 400 cycles with a Coulombic efficiency of around 96% (Figure S4). By contrast, GC-2050 shows more impressive cycling stability, as shown in Figure 5f. When the electrode is cycled at a higher current rate of 50 mA g-1, a stable capacity of 200 mAh g-1 is retained over 800 cycles with a nearly 100% Coulombic efficiency after the first few cycles. To the best of our knowledge, such an excellent sodium ion storage performance of hard carbon for SIBs anode is rare, especially for the hard carbon without any activation procedures.


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Figure 5. Electrochemical performance of switchgrass derived carbon anode GC-1000 and GC-2050. Typical CV curves for the first five cycles of (a) GC-1000 and (b) GC-2050 at a scanning rate of 0.5 mV s-1 in the potential range of 0.01–2 V; Voltage profiles of (c) GC-1000 and (d) GC-2050 for the 1st, 2nd, 5th and 10th cycles (current rate: 50 mA g; potential range: 0.01–2 V); (e) Rate performances; (f) Long term cycling stability (current rate: 50 mA/g).


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4. CONCLUSION We have demonstrated that switchgrass, a naturally abundant grass could serve as an ideal precursor to create carbon material for high performance SIBs anode. The hard carbon with hierarchical porous architecture has been successfully synthesized through a tailored Joule heating process. A ultra-high carbonization temperature (2050 °C) induced by Joule heating leads to excellent electrical conductivity and a highly ordered pseudo-graphite structure with dilated interlayered spacing which allows for the significant Na ion storage performance. When used as an anode for Na-ion batteries, switchgrass carbonized at 2050 °C resulted in an evidently improved Coulombic efficiency (from 42 to 64%) compared to the specimen carbonized at 1000 °C. Joule heating induced GC-2050 also revealed an enhanced rate capability due to the improvement of electrical conductivity. Over 800 cycles, a superb capacity retention of high up to 87% and a Coulombic efficiency of nearly 100% after 10 cycles were demonstrated for the GC-2050. The outstanding Na ion storage performance, combined with the sustainable biomass source and scalable synthesis method makes switchgrass derived carbon an attractive SIBs anode material.

ASSOCIATED CONTENT Supporting information available: photograph and I-V and R-P measurements for Joule heating, magnified cross section SEM image of hard carbon, and cycling performance of GC-1000. ACKNOWLEDGEMENTS This work was supported as part of the Nanostructures for Electrical Energy Storage (NEES), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award number DESC0001160.


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