Carbon with Expanded and Well-Developed Graphene Planes

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Carbon with expanded and well-developed graphene planes derived directly from condensed lignin as a high-performance anode for sodium-ion batteries Dohyeon Yoon, Jieun Hwang, Wonyoung Chang, and Jaehoon Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14776 • Publication Date (Web): 08 Dec 2017 Downloaded from http://pubs.acs.org on December 11, 2017

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ACS Applied Materials & Interfaces

Carbon with expanded and well-developed graphene planes derived directly from condensed lignin as a high-performance anode for sodiumion batteries

Dohyeon Yoon,a Jieun Hwang,a Wonyoung Chang,b Jaehoon Kim*a,c

a

School of Mechanical Engineering, Sungkyunkwan University,

2066, Seobu-Ro, Jangan-Gu, Suwon, Gyeong Gi-Do 16419, Republic of Korea b

Center for Energy Convergence, Korea Institute of Science and Technology,

Hwarangno 14-gil 5, Seongbuk-gu, Seoul 02792, Republic of Korea c

Sungkyun Advanced Institute of Nano Technology (SAINT), Sungkyunkwan University,

2066, Seobu-Ro, Jangan-Gu, Suwon, Gyeong Gi-Do 16419, Republic of Korea.

*

To whom correspondence should be addressed. Tel.: + 82-31-299-4843, fax: + 82-31-290-5889,

e-mail: [email protected] (Prof. Jaehoon Kim)

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Abstract In this study, we demonstrate that lignin, which constitutes 30–40 wt% of the terrestrial lignocellulosic biomass and is produced from 2nd-generation biofuel plants as a cheap byproduct, is an excellent precursor material for sodium-ion battery anodes (NIBs). Because it is rich in aromatic monomers that are highly crosslinked by ether and condensed bonds, the lignin material carbonized at 1300 °C (C-1300) in this study has small graphitic domains with well-developed graphene layers, a large interlayer spacing (0.403 nm), and a high micropore surface area (207.5 m2 g-1). When tested as an anode in a NIB, C-1300 exhibited an initial coulombic efficiency of 68% and a high reversible capacity of 297 mAh g-1 at 50 mA g-1 after 50 cycles. The high capacity of 199 mAh g-1 at less than 0.1 V with a flat voltage profile and extremely low charge/discharge voltage hysteresis (< 0.03 V) make C-1300 a promising energy dense electrode material. In addition, C-1300 exhibited an excellent high-rate performance of 116 mAh g-1 at 2.5 A g-1 and showed stable cycling retention (0.2% capacity decay per cycle after 500 cycles). By comparing the properties of the lignin-derived carbon with oak-sawdustderived and sugar-derived carbons and a low-temperature carbonized sample (900 °C), the reasons for the excellent performance of C-1300 were determined to result from facilitated Na+ ion transport to the graphitic layer and the microporous regions that penetrate through the less-defective and enlarged interlayer spacings.

Keywords: Sodium ion batteries, Lignin, Carbon, Defect, Dilated interlayer spacing, Micropore


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Introduction Because of resource depletion and global warming caused by the extensive use of fossil fuels, the effective utilization of clean and renewable energy sources such as solar, wind, and wave power has received considerable attention. To integrate the intermittent production of electrical energy from renewable sources into the current electrical grid, the development of large-scale energy storage systems is essential for the efficient distribution and utilization of renewable energy. Although lithium-ion batteries (LIBs) are currently the most practical choice for portable electronic devices and electronic vehicles, the non-uniform terrestrial distribution and limited resources of lithium hinder the successful realization of grid-scale electrical energy storage systems.1-2 In this context, sodium-ion batteries (NIBs) are considered one of the most promising alternatives to LIBs because of the natural abundance, global distribution, and low cost of sodium.2-7 Despite these clear advantages, the absence of suitable anode electrode material to host Na+ ions is a major obstacle preventing the realization of NIBs.8 Graphite, which is commercially used for LIB anodes, is not appropriate for NIBs because the intercalation of Na+ ions into the graphite layers is difficult, resulting from the large ionic radius of Na. Consequently, graphite NIBs have extremely low capacities of ca. 30 mAh g-1.9-10 Consequently, the growing demand for grid-scale energy storage systems is driving the development of economically viable, low-cost, energy dense, and safe carbon-based anode materials for NIBs. Lignin, a low-cost byproduct generated from the wood and pulp industries and cellulosic ethanol/butanol plants, is the most abundant natural aromatic polymer. Lignin is found in trees and other plant-based lignocellulosic biomass, representing up to 40% of their weight. Because of its low cost, abundance, and global distribution, various techniques have been developed to convert lignin into valueadded chemicals and renewable fuels.11-14 In addition, the unique polymeric structure of lignin, which comprises aromatics rings and condensed linkages, makes it an excellent precursor resource for carbonbased materials for use in various applications, including activated carbons,15 bio-based polymers,16 and capacitors.17 In the context of low-cost energy storage applications, the use of lignin as the precursor for 3 ACS Paragon Plus Environment


carbon-based anode materials as an alternative to graphite is a logical choice. However, lignin-based anodes for NIB applications have been relatively uninvestigated, and previous works have utilized mixtures of lignin and petroleum-derived chemicals as the precursors to carbon electrode materials.18-19 For example, Li et al. prepared an amorphous carbon material using the co-pyrolysis of petroleum pitch and lignin emulsions.18 The carbon prepared with a 1:1 ratio of pitch and lignin and calcined at 1400 °C exhibited a high reversible capacity of 254 mAh g-1 at 30 mA g-1 after 150 cycles, a high initial coulombic efficiency (CE) of 82%, and excellent cycling stability. Jin et al. synthesized nanofibrous carbon webs from polyacrylonitrile and lignin mixture using an electrospinning process.19 The carbonized material prepared with 1:1 ratio of polyacrylonitrile and lignin at 1400 °C delivered a high reversible capacity of 293 mAh g-1 at 20 mA g-1 after 10 cycles with an initial CE of 71%. Even though previous investigations have clearly demonstrated that lignin-based anodes are promising anodes for NIBs, the intrinsic nature of carbon materials derived only from lignin and their electrochemical properties in NIBs have not been discussed. Compared to anodes for NIBs, those for LIBs have been the subject of significant investigation. For example, Tenhaeff et al. prepared a fibrous carbon electrode from organosolv (Alcell) lignin using melt processing followed by oxidative stabilization and carbonization.20 The reversible capacity of the slurrycoated fibers prepared at 1000 °C was 350 mAh g−1 at a current density of 15 mA g−1. Wang et al. proposed fabricating carbon fibrous mats from organosolv lignin with added polyethylene oxide using electrospinning.21 The prepared anode delivered a reversible capacity of 445 mAh g−1 at 30 mA g−1. He et al. prepared lignosulfonate/polyaniline micro/nanospheres using in situ polymerization followed by carbonization.22 The composite electrode exhibited a reversible capacity of 431 mAh g−1 at 60 mA g−1. Zhang et al. synthesized hierarchical porous carbon from steam explosion lignin using KOH as an activation and pore-forming agent.23 The porous carbon delivered a reversible capacity of 470 mAh g−1 at 200 mA g−1, whereas a nonporous carbon electrode prepared without KOH activation delivered a lower reversible capacity of 180 mAh g−1. Chang et al. examined the effect of H2 reduction on the


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electrochemical performance of a lignin-derived carbon anode.24 At 74 mA g−1, the anode demonstrated a capacity of ca. 400 mAh g−1 with an improved initial CE. Even though these previous works have shown that lignin-based carbon anode materials exhibit excellent electrochemical performance after stabilization, the formation of composites and carbonization with polymers derived from petroleum resources are somewhat complex, and multi-step synthetic routes are required; therefore, it is difficult to implement these anodes for low-cost secondary battery applications. In addition, only limited lignin resources (e.g., organosolv lignin and lignosulfonate) have been investigated previously. In addition, the properties of lignin depend strongly on the source of lignocellulosic biomass (e.g., hardwood or softwood) and the delignification method (e.g., the lignosulfonate process, Kraft process, steam explosion process, organosolv process, or strong acid hydrolysis).11, 25-26 Currently, worldwide, many commercial-scale cellulosic ethanol or other types of biofuel plants are in operation or under construction, and some of these plants (e.g., BlueFire Renewables, and GS Caltex) use the strong acid hydrolysis process to separate the lignin from cellulose and hemicellulose. Therefore, it is critical to understand the physicochemical and electrochemical properties of carbon-based anode materials derived from the lignin generated in cellulosic biofuel plants. Herein, we explore a simple, scalable, and efficient route for the fabrication of lignin-based carbon anode materials using concentrated strong acid hydrolysis lignin (CSAHL) as a precursor. CSAHL was isolated from oak wood using a pilot-scale, concentrated sulfuric acid hydrolysis plant in use for the development of a cellulosic butanol plant in South Korea. After purification, the CSAHL was carbonized at 900 or 1300 °C under a N2 atmosphere. The effect of the heat-treatment temperature on the physicochemical and electrochemical properties of the carbonized lignin was investigated. When tested as an anode in NIBs, the lignin sample carbonized at 1300 °C exhibited an excellent reversible capacity (297 mAh g-1 at 50 mA g-1 after 100 cycles) with a clear voltage plateau at less than 0.1 V and a high rate performance (173 mAh g-1 at 1.0 A g-1 and 116 mAh g-1 at 2.5 A g-1). More importantly, at a continuous high charge–discharge rate of 2.5 A g-1, the C-1300 electrode delivered a stable capacity of 114 mAh g-1, even after about 500 cycles, with almost 100% CE. The electrochemical behavior of the carbon-based 5 ACS Paragon Plus Environment


anode derived from CSAHL makes it a promising active material for use in large-scale NIB applications. The relationship between the physicochemical and electrochemical properties of the samples carbonized at 900 and 1300 °C are discussed. The Li+ ion insertion/deinsertion of the lignin-derived electrodes was also investigated to understand possible differences in the Na+ and Li+ ion intercalation mechanisms. In addition, the carbon-based anode derived from CSAHL was compared to those derived from oak sawdust and sucrose octaacetate to gain an insight into possible dominant factors determining the electrochemical performance of the biomass-derived electrodes.

Experimental Materials The CSAHL used in this study was obtained from a pilot-scale delignification unit installed in South Korea. Oak sawdust, purchased from a local market in South Korea, was treated in an aqueous concentrated sulfuric acid solution to separate the lignin from the cellulose and hemicellulose. Sucrose octaacetate (SOA, C28H38O19, ≥97%) was purchased from Sigma-Aldrich (USA). Table S1 lists the results of proximate and ultimate analyses of the CSAHL, oak sawdust, and SOA used in this study. For the proximate analysis, the decomposition behavior of CSAHL was examined using thermogravimetric analysis (TGA) under an air atmosphere (Figure S1a). The measured inorganic (or ash) content in CSAHL was 6.3 wt%. High-purity nitrogen (99.99%) was purchased from the JC Gas Company (South Korea). Distilled and deionized (DDI) water was prepared using an AQUAMaxTM-Basic 360 water purification system equipped with a 0.22-µm filter (Young Lin Instrument Co., Ltd., South Korea). Polyvinylidene fluoride (PVDF, Kureha Chemical Industry Co., Japan), acetylene black (DENKA Co. Ltd., Japan), and 1-methyl-2-pyrrolidinone (NMP, purity of 98%, Alfa Aesar, USA) were used as received. Carbonization of CSAHL, oak sawdust, and SOA Prior to carbonization, the CSAHL was carefully washed three times in a 20 wt% aqueous KOH solution at 70 °C for 2 h followed by washing with a 1 M HCl aqueous solution at 60 °C for 15 h to remove the 6 ACS Paragon Plus Environment

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inorganic species present in the CSAHL. Figure S1b shows the TGA profiles of CSAHL before and after washing with KOH and HCl. After washing, the inorganic content decreased significantly from 6.3 to 0.5 wt%. The inorganic species in CSAHL and the carbonized samples are listed in Table S2. After rinsing with DDI water, the collected CSAHL was carbonized using a tubular furnace under a flow of N2 at 100 mL min−1. The temperature was ramped to 900 or 1300 °C at a heating rate of 10 °C min−1 using a model STF 16/180 tubular furnace (Carbolite Gero, UK). After reaching the experimentally desired temperature, CSAHL was carbonized for 6 h. The samples carbonized at 900 and 1300 °C are denoted C-900 and C1300, respectively. For comparison purposes, the oak sawdust and SOA were carbonized at 1300 °C under N2 flow for 6 h, denoted O-1300 and S-1300, respectively. Characterization The functional groups on the surfaces of the samples were characterized using a NICOLET iS10 Fouriertransform infrared (FT-IR) spectrometer (Thermo Electron Corp., USA). The phase structure of the samples was analyzed using X-ray diffraction (XRD, D/Max-2500V/PC Rigaku, Japan) with Cu Kα radiation generated at 40 kV and 50 mA. The average interlayer spacing (d) was calculated using the Bragg equation (Eq. 1), and the apparent crystallite thickness (Lc) and apparent layer-plane length parallel to the fiber axis (La) were calculated using the Scherrer equation (Eq. 2). =

=

,

(1)

(2)

In Eqs. 1 and 2, θ is the Bragg angle of the peaks (degrees), λ is the wavelength of the X-rays used (Cu Kα radiation: 0.154 nm), and β is the full-width at half-maximum of the XRD peak. For calculating Lc and Lc, the β and 2θ values for (002) and (100) peaks of the carbon materials, respectively, and the corresponding form factor K values of 0.90 and 1.84, respectively, were used.19 Raman spectra were collected using a LabRAM HR800 confocal Raman microscope (HORIBA Jobin-Yvon, USA). The morphology of the samples was observed using a JEOL JSM-7500F field-emission scanning electron 7 ACS Paragon Plus Environment


microscope (FE-SEM, JEOL Inc., USA). High-resolution transmission electron microscope (HR-TEM) images were taken using a Tecnai-G2 HR-TEM (FEI Co. Ltd., OR, USA) operated at 200 kV. X-ray photoelectron spectroscopy (XPS) was performed using a PHI 5000 Versa Probe spectrometer (ULVACPHI Inc., Japan). The Brunauer–Emmett–Teller (BET) surface area and the microstructural properties were determined using a BELSORP-Mini II apparatus (BEL Japan Inc., Japan). The chemical composition of the samples was estimated using X-ray fluorescence spectrometry (XRF, S4 Pioneer, Bruker AXS GmbH, USA). Elemental analysis (EA) of the lignin and bio-oil were conducted with a Vario EL cube elemental analyzer equipped with a TCD detector (Elementar Analysensysteme GmbH, Germany). The combustion tube and the reduction tube temperature were maintained at 1150 °C and 850 °C, respectively. The oxygen content was analyzed using a TCD detector in O-mode with a pyrolysis tube at 1170 °C. The electrochemical measurements were performed in CR2032-type coin-cell configurations. The working electrode was composed of 70 wt% active material, 10 wt% acetylene black as a conducting agent, and 20 wt% PVDF as a binder. After blending in N-methyl-2-pyrrolidone (NMP), the slurry was cast uniformly on Cu foil. The electrode film was dried on a heating board at 80 °C for 24 h to remove the solvent. The dried electrode film was then punched into 14-mm-diameter discs (area of 1.54 cm2 and active material loading of 1.18–1.23 mg cm-2) and weighed. The test cells were fabricated using the composite anode and metallic sodium or lithium metal as a counter-electrode, which were separated by a glass microfiber filter (GF/B, Whatman, UK) and a microporous polypropylene separator (Celgard 2500, Celgard LLC, USA). The electrolyte for the NIBs was 1 M NaClO4 dissolved in ethylene carbonate (EC)/propylene carbonate (PC)/dimethyl carbonate (DMC) solvent (volume ratio of EC:PC:DMC = 9:9:2). The electrolyte for LIBs was 1 M LiPF6 dissolved in ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) solvent (volume ratio of EC:DMC:EMC = 1:1:1). The cells were assembled in a glove box filled with high-purity Ar gas. The NIB cells were galvanostatically charged and discharged over a voltage range of 0.005–2.5 V (hereafter, versus Na/Na+) and the LIBs cells charged and discharged over 0.005–3.0 V (hereafter, versus Li/Li+) using a WBCS 3000 model battery 8 ACS Paragon Plus Environment

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test system (WonATech Corp., South Korea) at room temperature. For the rate performance measurements, the current density was varied from 0.05 to 50 A g−1. Cyclic voltammetry (CV) tests of the cells were performed using a model ZIVE MP1 potentiostat analyzer (WonATech Corp.) at a scanning rate of 0.5 mV s−1 between 0.005 and 2.5 V for the first 10 charge–discharge cycles. Electrochemical impedance spectroscopy (EIS) tests were performed using a ZIVE MP1 impedance analyzer (WonATech Corp., Korea) in a frequency range of 100 kHz to 10 mHz at room temperature.

Results and Discussion Lignin has a polymeric structure of aromatic rings, which can serve as a potential precursor for the synthesis of carbon-based anode materials for NIBs. CSAHL has two major chemical bonds: ether linkages (β–O–4, α–O–4, 4–O–5) and condensed linkage (β–5, 5–5, β–1, β–β). In addition, various types of oxygen functionalities (methoxy, hydroxyl, phenolic, and carbonyl groups) are present in CSAHL. The relative amounts of chemical linkages and oxygen functionalities in lignin depend strongly on the type of wood (softwood or hardwood) and lignin isolation methods (e.g., Kraft process, lignosulfonate process, organosolv process, steam explosion, or strong acid hydrolysis).11, 25-26 The CSAHL used in this study exhibits a high degree of condensation because condensed linkages form during strong acid delignification, as confirmed by the nitrobenzene oxidation method.14 Figure 1a shows the FT-IR spectra of CSAHL and the carbonized samples. The CSAHL contains various types of oxygen functionalities including hydroxyl (v–C–OH at ca. 3400 cm−1, including vibrations originating from COOH and H2O), carbonyl (v–C=O at 1712 cm−1), carboxyl (v–COO− at 1612 cm−1), and ethers/alcohols (v–C–O–C and v–C–OH at 1050–1200 cm−1). In addition, the CSAHL retains the symmetric CH3 stretching at 2974 cm−1, asymmetric CH2 stretching at 2927 cm−1, symmetric CH2 stretching at 2887 cm−1, aromatic ring C=C vibrations at 1504 and 1461 cm−1, and C–H out-of-plane vibrations at 882 cm−1. After carbonization at 900 °C, the intensity of the peaks associated with the oxygen functionalities (hydroxyl, carbonyl, and ether/alcohol) were reduced significantly, indicating that some degree of deoxygenation had occurred. At 1300 °C, the intensity of the oxygenated peaks further 9 ACS Paragon Plus Environment


decreased, but the OH group at and COO– group at 1612 cm−1 persisted. As listed in Table 1, the oxygen content decreased significantly, from 37.6 (CSAHL) to 3.25 (C-900) and to 2.15 wt.% (C-1300), and, at the same time, the carbon content increased from 54.09 (CSAHL) to 90.69 (C-900) and to 95.64 wt.% (C1300) after carbonization. This result clearly demonstrates that high-temperature calcination is very effective for removing most of the oxygen functionalities present in CSAHL. XPS was employed to investigate the chemical functionalities further, as shown in Figures 1b–d. The atomic ratios of carbon to oxygen (C1s/O1s) estimated from the XPS survey scan are listed in Table 1. When the calcination temperature was increased from 900 to 1300 °C, the C1s/O1s ratio increased significantly, from 14.6 to 17.4, and this trend agrees well with the EA results. The C 1s spectra were deconvoluted using six types of carbon in different chemical environments:27-28 sp2 graphitic carbon (284.5 eV), hydroxyl (C–OH, 285.7 eV), ether and epoxy (C–O/epoxy, 286.6 eV), carbonyl (C=O, 287.6 eV), carboxyl (O=C–O, 288.7 eV), and the sp2 graphitic carbon satellite (π-π*, 290.1 eV). Most of the oxygen functionalities present in CSAHL were removed by the high-temperature calcination. The dominant chemical bonding species in C-900 and C-1300 was sp2 carbon, indicating the formation of a graphitic structure. The area percentages of the peaks in the C 1s spectra of CSAHL and the carbonized samples were quantified, and the results are listed in Table S3. In CSAHL, the major bonding species were sp2 carbon (67.7%), hydroxyl (C–OH, 13.1%), and C–O/epoxy (10.9%) groups. After carbonization, the C-900 and C-1300 samples showed similar chemical bonding species: 70–73% sp2 carbon, 13–14% hydroxyl, 3–4% ether and epoxy, 5% carbonyl, and 2–4% carboxyl groups and ca. 4% π-π* shake-up. The crystal structure of the carbonized samples was characterized using XRD, and the results are shown in Figure 2. The XRD pattern of CSAHL exhibited a sharp peak at 2θ = 22.3° with an interlayer spacing of 0.397 nm. The C-900 and C-1300 samples exhibited broad peaks centered at 2θ = 21.4 and 22.0°, respectively, which are associated with the (002) crystal plane of stacked graphene sheets. In addition, both samples exhibited a relatively small peak at 2θ = ca. 44°, which can be attributed to the (100) planes of sp2-hybridized hexagonal carbons. The values of the interlayer spacing, Lc, and La, 10 ACS Paragon Plus Environment

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estimated using the center of the peaks and Eqs. 1 and 2, are listed in Table 2. The interlayer spacing of the graphene sheets in the CSAHL-derived carbonized samples decreased from 0.415 nm (C-900) to 0.403 nm (C-1300) with an increase in the carbonization temperature. This indicates that some degree of structural development from disorder to short-range order occurred during the high-temperature carbonization. The interlayer spacing values are larger than those of graphite (0.335 nm), coke prepared at 1000–1350 °C (0.344 nm),29-30 and most materials carbonized from a variety of precursors (e.g., banana peel,31 peat moss,32 sugar,33 pitch/lignin,18 argan shell,34 shaddock peel,35 hydrogen-enriched reduced graphene oxide,36 polyacrylonitrile,37 and corn cob38). As shown in Figure S2 and listed in Table 2, the interlayer spacing in C-1300 is larger than those of O-1300 (0.387 nm) and S-1300 (0.395 nm). The average thickness of the graphitic layers (Lc) of C-900 is 0.88 nm, indicating that approximately 2.12 pseudographitic domains were stacked in the sample. In the C-1300 sample, the domains were composed of approximately 2.23 stacked layers. In addition, the length of the graphitic domain (La) increased from 2.98 to 3.44 nm with increasing carbonization temperature. The increases in Lc and La with increasing carbonization temperature indicate that the number of graphene sheets and the graphitization degree increased with temperature.39 The values of La and Lc of C-1300 are slightly lower compared to those of O-1300 and S-1300, implying a lower degree of graphitic domain formation when CSAHL was used as the carbon precursor. The larger interlayer spacing and smaller graphitic domains of C-1300 than those of O-1300 and S-1300 could be attributed to the rigid aromatic-rich structure of CSAHL. During the heattreatment, the gases produced by the cleavage of the oxygen functionalities (CO2, CO, CH4, etc.) in CSAHL might induce the effective exfoliation of the carbon-based material.40-41 The recalcitrant nature of the monoaromatics and some of the condensed linkages (β–5, 5–5) could hinder the formation of polynuclear aromatics and, thus, the broadening of the graphitic domains upon heating. The large spacing between the basal planes and the small graphitic domains can facilitate Na+ ion diffusion, improving the diffusion kinetics.42-43 As discussed in the previous section, even after CSAHL was purified by washing with KOH and HCl before calcination, some impurities remained in the carbonized samples; a library search in the XRD 11 ACS Paragon Plus Environment


program indicated that C-900 might contain Si and C-1300 may contain CaS. It is not clear what caused the formation of Si and CaS during calcination, but carbothermal reduction during carbonization under an inert condition may be responsible for the formation of Si (from silica) and CaS (Ca2+ + SO43− + 2C → CaS + 2CO2).44 The carbon structure as a function of the carbonization temperature was examined using Raman spectroscopy, and the results are shown in Figure 3. The Raman spectrum of CSAHL (Figure S3a) did not exhibit characteristic amorphous or graphitic carbon bands at 800–2000 cm−1. The Raman spectra of the carbonized samples were deconvoluted into four peaks;45-47 as shown in Figures 3b and S3b–c, the use of four Gaussian bands resulted in a satisfactory Raman spectrum fitting with minimal fitting errors. The G band at 1594 cm−1 originates from the in-plane C–C bond stretching vibrations of sp2 hybridized graphitic carbon atoms with E2g symmetry, whereas the D1 band at 1341 cm−1 originates from the vibrations of disordered carbon atoms at the edges of graphite sheets or defective graphitic structure with A1g symmetry. Although the origin of the D3 and D4 bands at ca. 1500 and 1200 cm−1 is rather unclear, these bands have been observed previously in activated carbon cloth48 and soot.47 Short-range sp3 carbon vibration of amorphous carbon and the disordered graphitic lattice could be responsible for the D3 and D4 bands, respectively.47 The area percentage of the D3 and G peaks could serve as an indicator of the degree of graphitization.47, 49-50 As shown in Figure 3c, C-900 exhibited a lower degree of graphitization with a larger amorphous carbon region as compared to C-1300. In addition, the D3 to G area ratio of C-1300 (0.8) is much smaller than those of O-1300 (1.4) and S-1300 (1.0), indicating that the CSAHL-derived carbon exhibited a much higher degree of graphitic carbons as compared to the oak-sawdust-derived and SOA-derived carbons (Figures S3b–d). Therefore, the rich aromatic structure associated with lignin and the high calcination temperature resulted in a high degree of graphitization. The morphologies of the carbonized samples were examined using SEM and HR-TEM, and the results are shown in Figures 4a and b. The SEM images of C-900 and C-1300 indicate that the macroscopic morphology of the carbonized samples did not change much from that of CSAHL (Figure S4a); that is, all samples consist of carbon particles 1–10 µm in size that were aggregated to some degree. 12 ACS Paragon Plus Environment

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The HR-TEM images of C-900 and C-1300 show that well-stacked, graphite-like layers in the turbostratic carbon domains. A clear space between the graphitic layers, which corresponds to the spacing between the (002) basal planes of graphene, was observed in the TEM images. The fringe distances of the interlayers decreased slightly from 0.42 to 0.40 nm as the calcination temperature was increased from 900 to 1300 °C, which is in good agreement with the XRD results. A high degree of graphitization may induce strong interaction between the basal planes, resulting in the more compact texture of the C-1300 sample. In addition, the graphitic layers were shifted and folded with respect to each other, forming micropores between the stacked layers with some degree of pore connectivity. To investigate the textural properties of C-900 and C-1300, N2 adsorption–desorption isotherms were determined, as shown in Figures 4c and d, respectively. The extremely small BET surface area of CSAHL (Figure S4b) indicates that an almost negligible number of nano-to-mesopores were present in the lignin sample. After calcination at 900 °C, the BET surface area increased substantially to 604.6 m2 g−1; the produced gases (CO2, CO, CH4, etc.) during calcination might cause the exfoliation of the carbon structure, contributing to the increase in the specific surface area.28, 40-41 C-900 exhibited a typical Type I isotherm (inset in Figure 4c), indicating a microporous structure. Most of the pores were less than 1 nm in size, indicating the extensive formation of micropores ( S-1300 (218 mAh g-1) > O-1300 (187 mAh g-1), while the reversible capacities at the low flat voltage region are in order of C-1300 (199 mAh g-1) > S-1300 (128 mAh g-1) ≈ 19 ACS Paragon Plus Environment


O-1300 (127 mAh g-1) at 50 mA g-1 after 50 cycles. This suggests that the carbon precursors play an important role in determining the physicochemical and electrochemical properties of the carbonized samples. As listed in Table 2, one noticeable difference between C-1300 and O-1300/S-1300 is the degree of disorder in the carbon structure; that is, the D3/G ratio of C-1300 (0.8) is much smaller than those of O-1300 (1.4) and S-1300 (1.0). In addition, the interlayer spacing and micropore surface area of C-1300 were much larger than those of O-1300 and S-1300. Therefore, in the C-1300 electrode, it is much easier for Na+ ions to penetrate the well-developed, less defective graphitic layers with larger interlayer spacing, and the transported Na+ ions could more easily remain in the micropore sites compared to those of O1300 and S-1300. The better graphitization also contributes to the higher initial CE of the C-1300 electrode compared to those of O-1300 and S-1300.

Conclusions In summary, we have demonstrated that concentrated strong acid hydrolysis lignin is a very promising carbon precursor for producing high-performance anode materials for NIBs. By taking advantage of its structure, which is rich in aromatic monomers that are highly crosslinked by ether and condensed bonds, the CSAHL sample carbonized at 1300 °C (C-1300) exhibited a higher micropore surface area (207.5 m2 g-1), a larger interlayer spacing (0.403 nm), and fewer defective graphene sheets with smaller graphitic domains than those of carbons derived from oak wood and sucrose octaacetate. These factors led to the facile penetration of Na+ ions into the graphitic layers and microporous region of C-1300 via the highenergy sites (e.g., defects, vacancies, and the edge of the turbostratic carbon region). The C-1300 electrode delivered a high reversible capacity of 297 mAh g−1 at 50 mA g−1 after 50 cycles (of which high flat voltage capacity of 199 mAh g-1 at voltages below 0.1 V with a low charge/discharge voltage hysteresis) and a high rate capacity of 116 mAh g−1 at a high charge–discharge rate of 2.5 A g-1 with only marginal capacity loss after 500 cycles. The results presented in this study demonstrate that the simple carbonization of lignin, which is a renewable, abundant, and low-cost byproduct generated by cellulosic


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biofuel plants, can provide a very promising alternative precursor for carbon-based anode materials for use in grid-scale NIB applications.

Supporting Information Characterization of CSAHL, C 1s peak deconvolution results, XRD patterns, Raman spectra and N2 adsorption-desorption isotherms of carbon materials, high-rate discharge-charge profiles, electrochemical performance of C-900 and C-1300. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgments This research was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (No. 2016R1A2B3008800). An additional support the Korea Institute of Science and Technology (KIST) Institutional Program (Project No. 2E26330/2E26292) are also appreciated.

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Figure captions Figure 1. (a) FT-IR spectra of CSHAL, C-900, and C-1300. High-resolution XPS spectra and peak deconvolution results for (b) CSAHL, (c) C-900, and (d) C-1300. Figure 2. XRD patterns of CSAHL, C-900, and C-1300. Figure 3. Raman spectra of (a) C-900 and (b) C-1300, and (c) area percentage of the G and D3 peaks of C-900 and C-1300. Figure 4. SEM and TEM images of (a) C-900 and (b) C-1300, and N2 absorption–desorption isotherms and micropore size distributions of (c) C-900 and (d) C-1300. Figure 5. Charge-discharge curves and differential capacity curves (a, b) CSAHL, (c, d) C-900, and (e, f) C-1300 measured at 0.005–2.5 V (vs. Na/Na+). Figure 6. (a) Cycling stability of CSAHL, C-900, and C-1300 at 50 mA g−1, (b) rate performance of CSAHL, C-900, and C-1300 at various current densities from 0.05 to 5.0 A g−1, (c) comparison of highrate performance, and (d) long-term cyclability of C-900 at 1 and 2.5 A g−1 in NIBs. Lofabad et al.,31 Hong et al.,54 Sun et al.,35 Ding et al.,32 Luo and Ji et al.,55 Jin and Wang et al.,37 Jin and Chong et al.,19 Li and Hu et al.,56 Prabakar et al.,57 Li and Huang et al.,58 Zhang et al.,59 Liu et al.,38 Figure 7. Nyquist plots for C-900 and C-1300 obtained in the rate capability test (a) before cycle, (b) after the first cycle, (c) after 100 cycles. Figure 8. (a) Galvanostatic 10th charge-discharge profiles of C-1300, O-1300 and S-1300, (b) cycling stability of C-1300, O-1300 and S-1300 at 50 mA g-1, and (c) rate performance of C-1300, O-1300 and S1300 at various current densities from 0.05 to 5.0 A g-1. (d-f) Nyquist plots of before cycle, after 1st cycle, and after 100 cycles of C-1300, O-1300 and S-1300 electrodes.


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Table 1 Chemical properties of CSAHL and the carbonized samples. XPS

EA N (wt%)

S (wt%)

H (wt%)

C (wt%)

O (wt%)

C/O ratio

C (at%)

O (at%)

C1s/O1s ratio

CSAHL

0.33

0.76

4.99

54.09

37.60

1.92

72.33

27.68

2.61

C-900

0.16

0.38

2.08

90.69

3.25

37.21

93.61

6.39

14.6

C-1300

0.03

1.31

N.D.a

95.64

2.15

59.31

94.56

5.44

17.4

O-1300

0.34

N.D.

N.D.

94.67

2.92

43.23

94.18

5.82

16.2

S-1300

N.D.

N.D.

2.98

97.24

2.12

62.46

94.65

5.35

17.7

a

Not detectable; under the detection limit of EA (

Carbon with Expanded and Well-Developed Graphene Planes

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