Cellulose Acetate-Derived Nanocarbon

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Energy, Environmental, and Catalysis Applications

Electrospun Kraft Lignin/Cellulose Acetate Derived a Nano Carbon Network as an Anode for High-Performance Sodium-Ion Batteries Hao Jia, Na Sun, Mahmut Dirican, Ya Li, Chen Chen, Pei Zhu, Chaoyi Yan, Jun Zang, Jiansheng Guo, Jinsong Tao, Jiasheng Wang, Fangcheng Tang, and Xiangwu Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b13033 • Publication Date (Web): 03 Dec 2018 Downloaded from http://pubs.acs.org on December 5, 2018

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ACS Applied Materials & Interfaces

Electrospun Kraft Lignin/Cellulose Acetate Derived a Nano Carbon Network as an Anode for High-Performance Sodium-Ion Batteries Hao Jiaa,b, Na Suna, Mahmut Diricanb,*, Ya Lib,c, Chen Chenb, Pei Zhub, Chaoyi Yanb, Jun Zangb, Jiansheng Guoa , Jinsong Taod,*, Jiasheng Wange, Fangcheng Tange, and Xiangwu Zhangb,*

a

Key Laboratory of Textile Science and Technology, Ministry of Education, College of Textiles, Donghua University, Shanghai, 201620, China

b

Fiber and Polymer Science Program, Department of Textile Engineering, Chemistry and Science, North Carolina State University, Raleigh, NC 27695-8301, USA

c

Silk Institute, College of Materials and Textiles , Zhejiang Sci-Tech University, Hangzhou, Zhejiang, 310018, China

d

State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou, 510640, China

e

Guangzhou Lushan New Materials Co., Ltd, Guangzhou, 510530, China

*Corresponding authors: - E-mail: [email protected] - E-mail: [email protected] - E-mail: [email protected]

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Abstract An innovative nano carbon network material was synthesized from electrospun Kraft lignin (KL) and cellulose acetate (CA) blend nanofibers after carbonization at 1000 oC in nitrogen atmosphere and its electrochemical performance was evaluated as an anode material in sodium-ion batteries (SIBs). Apart from its unique network architecture, introduced carbon material possesses high oxygen content of 13.26%, wide interplanar spacing of 0.384 nm and large specific surface area of 540.95 m2∙g-1. The electrochemical test results demonstrate that this new nano carbon network structure delivers a reversible capacity of 340 mAh∙g-1 at a current density of 50 mA∙g-1 after 200 cycles and exhibits high rate capacity by delivering a capacity of 103 mAh∙g-1 at an increased current density of 400 mA∙g-1. The presented work rendered an innovative approach for preparing nano carbon materials for energy storage applications and could open up new avenues for novel nano carbon fabrication from green and environmentally friendly raw materials.

Keywords: Sodium-ion battery, nano carbon network, kraft lignin, cellulose acetate, electrospinning

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1. Introduction Recently, sodium-ion batteries (SIBs) have drawn rising attention and shown immense potential in the stationary energy storage applications.1-3 Compared with lithium-ion batteries (LIBs), which are commercially used in portable electrical devices, SIBs are greatly appropriate for large-scale renewable energy storage and can be considered as low-cost batteries due to significant abundance of sodium resource.4-7 Despite the great promise of SIBs, many critical properties, such as cycle life and reversible capacity, need to be improved for realization of their long-term development and practical application.8-9 Therefore, considerable amount of efforts have been taken for exploring proper electrode materials to improve the electrochemical performance of SIBs. As anodes for SIBs, various carbonaceous materials such as carbon black

10,

carbon spheres 11, carbon fibers 12, and porous carbon 13, have been studied and some of them present excellent and stable electrochemical performance. However, the precursor materials used in those studies, such as polyacrylonitrile (PAN) and polyvinyl alcohol (PVA), depend on the petroleum resources, which are relatively costly and environmentally unsustainable.14 Considering emerging energy and resource concerns, plant derived renewable resources should be considered as an alternative to substitute the synthetic polymers.15 Hu et al. fabricated cellulose-derived carbon nanocrystals (CNCs) as anode material for SIBs and achieved a high reversible capacity of 340 mAh∙g-1 at a current density of 100 mA∙g-1.16 Nevertheless, more eco-friendly electrode materials still need to be explored and tested. Herein, lignin is an obviously attractive candidate for nano carbon synthesis as it is the second most abundant and inexpensive natural biopolymer on earth. Among numerous types of lignin materials processed by different pulping techniques, kraft lignin (KL) is the most commonly available one. However, as demonstrated in the previous research studies, pure KL derived porous carbon does not show ideal electrochemical performance in SIBs due to its relative large micro structure.17 Hence, the key point to broaden its application in SIBs is to realize the desirable nanometer structure, which would greatly facilitate sodium ion 3



transport and storage. It should be noted that electrospinning technology is one of the most popular and efficient methods to fabricate electrode materials possessing controllable nano structures, especially nanofibers. In battery application, electrospun nanofibers are demonstrated to have shorter diffusion paths compared to the commonly applied powder materials and faster intercalation kinetics relying on their high specific surface area.18 For SIBs, electrospun nanofibers could offer a relatively large number of sodium insertion sites, and a decreased charge-transfer resistance at the interface between the electrolyte and active electrode materials. However, it is unfortunately that pure KL solution could not be processed into nanofibers at any concentration by electrospinning technique. This is mainly because KL has a relatively low molecular weight and high polydispersity indexes, thus it lacks of chain structures and/or molecular entanglements.19 As a consequence, the current application of KL as raw material for electrode fabrication is extremely limited, mainly due to the confined structural design. In this study, another green material, cellulose acetate (CA), was selected as the complementary carbon precursor to KL because of its high availability and low cost.20 More importantly, it is found that these two biopolymers could interact very well to enable the preparation of precursor nanofibers by electrospinning, which can then be further converted into carbon materials of controlled nano structure by thermal treatment in inert environment. In light of the aforementioned phenomenon, we demonstrate a novel and simple approach to prepare nano carbon network by pyrolysis of electrospun KL/CA nanofibers in nitrogen atmosphere. The KL/CA derived nano carbon network was evaluated as an anode material in SIBs for the first time. Electrochemical results showed that the electrospun KL/CA derived nano carbon network anode (E-KL/CA-C) delivered a high reversible capacity of 293 mAh∙g-1 in the first cycle and 340 mAh∙g-1 in the 200th cycle at 50 mA∙g-1. In addition, E-KL/CA-C demonstrated excellent rate capability in SIBs. Therefore, the presented work rendered an innovative approach for preparing nano carbon materials for energy storage applications and could open up new avenues for fabrication of novel nano carbons from 4


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green and environmentally friendly raw materials.

2. Experimental 2.1. Preparation of porous carbon material KL with a molecular weight cutoff of 1,000-3,900 was precipitated from black liquor of bleachable-grade pulp. Cellulose acetate (average Mn ∼ 30,000 by GPC, 39.8 wt% acetyl), acetone, and N, N-dimethylcyclohexylamine were purchased from SigmaAldrich and used without further treatment. The fabrication process of electrospun KL/CA derived nano carbon network is shown in Figure 1. Firstly, electrospun KL/CA blend nanofibers were fabricated. To be specific, KL and CA (1:1) with a total concentration of 8 wt.% was dissolved in acetone and N, N-dimethylcyclohexylamine blend solvent with 2:1 volume ratio. The electrospinning of precursor solution was kept under a steady voltage of 20 kV, with a nozzle to collector distance of 20 cm and a flow rate of 0.75 ml∙h-1. Finally, the as-spun KL/CA nanofibers were carbonized at 1000 ˚C in nitrogen for 1h with a heating rate of 2 ˚C∙min-1 to abtain an electrospun KL/CA derived nano carbon which was denoted as E-KL/CA-C. For comparison, electrospun pure CA nanofibers were also prepared can thermally-treated to form carbon using the same procedure, and the carbon prepared from these pure CA nanofibers was denoted as E-CA-C.

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Figure 1. Schematic illustration of the fabrication procedure of E-KL/CA-C. Nevertheless, as pure KL cannot be directly electrospun into nanofibers, KL derived carbon powder (KL-C) was prepared by dissolving KL in the same solvent and pyrolysis under same conditions after drying at 60 °C overnight. Similarly, KL/CA derived carbon powder (KL/CA-C) was also prepared as control sample using the same procedure of KL-C. 2.2. Structural and morphological characterization The morphology of E-KL/CA-C, E-CA-C, KL-C and KL/CA-C was detected by field emission scanning electron microscopy (FE-SEM, FEI VERIOS 460 L) and transmission electron microscopy (TEM and HRTEM, Hitachi HF 2000, accelerating voltage 200 kV). The phase and lattice structure identiﬁcation were conducted by using wide angle X-ray diffractometer (WAXD, Rigaku Smartlab) with Cu Kα radiation between 2θ angles from 10° to 60°. The surface area of the materials was obtained using the Brunauer-Emmett-Teller (BET) method by N2 adsorption–desorption isotherms on an ASAP 2020 (Micromeritics Instrument, USA) at 77 K. X-ray photoelectron spectroscopy (XPS, PHI5700 ESCA system, USA) was used to execute elemental analysis and identify surface functional groups at room temperature with a Kratos Analytical spectrometer and monochromatic Mg Kα X-ray source. Raman spectroscopy (Renishaw Raman 2000, USA) was used to detect the graphitic carbon structure and the presence of defects on carbon materials using a 514-nm laser beam. Moreover, elemental composition was determined by a Perkin Elmer 2400 Series II CHNS/O Elemental Analyzer. 2.3. Electrochemical characterization Electrochemical performance of the electrodes was evaluated in 2032 type coin cells. The as-prepared carbon material was mixed with Super-P and sodium alginate with a mass ratio of 75:15:10 to prepare a slurry where water was used for dissolving sodium alginate. The slurry was then casted on copper foil and dried under vacuum at 80 °C overnight to form the electrodes. The average mass loading of the anodes was around 1.0 mg∙cm-2. The half-cells were assembled in an Ar-filled glovebox with glass 6


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fiber mat (GE healthcare) as the separator and sodium metal (Na, Sigma–Aldrich) as the counter electrode. The electrolyte used was 1 M NaClO4 in ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:2 by weight) with 10 wt.% fluoroethylene carbonate (FEC) additive and the electrolyte amount was kept at 100 μL per cell. The electrochemical performance of carbon materials was tested on a LADN-CT 2001A battery system at room temperature. The potential range of charge-discharge cycles was controlled between 0.01 and 2.5 V vs. Na+/Na. Additionally, cyclic voltammetry (CV, 0.1∙mV s-1) and electrochemical impedance spectroscopy (EIS, 100 kHz–0.01 Hz, 10 mV) measurements were performed on a Gamry Reference 600 workstation. The reproducibility of each as-prepared anode was assessed by conducting all electrochemical measurements on at least ten samples, and their specific capacity was calculated based on the weight of the active material (each carbon material).

3. Results and Discussion The morphologies of electrospun KL/CA nanofibers and CA nanofibers were first examined by FESEM. Normally, KL cannot be electrospun into nanofibers because of the lack of chain structures and/or molecular entanglements caused by its relatively low molecular weight and high polydispersity index. However, as shown in Figure S1, KL can be successfully electrospun into nanofibers after mixing with CA. Notably, the mean diameter of electrospun KL/CA nanofibers (500 nm) was larger than that of electrospun CA nanofibers (400 nm). After carbonization, the four different carbon materials (E-KL/CA-C, KL/CA-C, KL-C and E-CA-C) presented different morphologies (Figure 2). The E-KL/CA-C carbon network derived from KL/CA blend precursor nanofibers still maintained portion of the fibrous structure due to the mixture of KL and CA in the precursor. More specifically, although the intersections of electrospun KL/CA nanofibers turned into pie-shaped carbon, the fibers between the intersections preserved their fibrous structure in the diameter of around 300 nm and formed a continuous network structure with the pie-shaped carbon. In contrast, the ECA-C presented a film structure while CA-C and KL/CA-C exhibited typical hard carbonaceous morphologies. Hence, the combination of electrospinning and the 7



subsequent thermal treatment process was demonstrated as an effective method to fabricate the nano carbon network of E-KL/CA-C, possessing a mixture of nanofibers and particles, from KL/CA blend precursor nanofibers. More significantly, the porous nature of this nano carbon network could lead to increased specific surface area, thereby resulting in enhanced electron transfer and improved Na kinetics.21 Compared to other three carbon particle materials, the increased reaction sites of E-KL/CA-C in their microstructure would also bring improved sodium storage performance.

Figure 2. SEM images of E-KL/CA-C (a), KL/CA-C (b), KL-C (c) and E-CA-C (d). Figure 3 shows HRTEM images of the cross-sectional view of E-KL/CA-C, KL/CA-C, KL-C and E-CA-C, in which the layered structures of these materials were apparently detected by the alternating bright and dark contrasts. According to calculation results, E-KL/CA-C exhibited wider interlayer distances (0.391 ± 0.002 nm) and longer channels than other three carbon materials, which is greatly favorable for sodium ion transport. The carbon materials prepared in this work were amorphous, and hence no ordered structure can be observed in these HRTEM images. The specific Brunauer–Emmett–Teller (BET) surface area and pore volume results of E-KL/CA-C, KL/CA-C, KL-C and E-CA-C are listed in Table 1. Among the four studied carbon 8


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materials, E-KL/CA-C exhibited the largest BET specific surface area of 540.95 m2∙g1,

with a pore volume of 0.27 cm3∙g-1. The BET specific surface areas of KL/CA-C, KL-

C and E-CA-C were 398.42, 163.69, and 83.28 m2∙g-1, respectively, while their pore volumes were 0.17, 0.08, and 0.04 cm3∙g-1. It is understandable that the higher porosity and larger surface area of E-KL/CA-C were originated from its well-designed microstructure. More significantly, these characters of E-KL/CA-C could provide better contact with the electrolyte, which would effectively facilitate the reaction with sodium in the battery system.10

Figure 3. HRTEM images of E-KL/CA-C (a), KL/CA-C (b), KL-C (c) and E-CA-C (d).

Table 1. Physical parameters of E-KL/CA-C, KL/CA-C, KL-C and E-CA-C. Samples

Surface area (m2∙g-1) 9


Pore volume (cm3∙g-1)


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E-KL/CA-C

540.95

0.27

KL/CA-C

398.42

0.17

KL-C

163.69

0.08

E-CA-C

83.28

0.04

Structures of the introduced carbon materials were further investigated by Raman spectroscopy and the results are demonstrated in Figure 4a. Two separate characteristic bands, i.e., D-band peak at 1343 cm-1 and G-band peak at 1589 cm-1, were observed for each carbon material, and these two peaks could be corresponded to disordered or defect carbon structure and graphitic crystalline structure, respectively.22 The relative intensity ratio (i.e., R-value) of D-band to G-band represents the degree of disorder in the carbon structure. According to calculation results, R-value of E-KL/CA-C (1.10) was lower than those of KL/CA-C, KL-C, and E-CA-C, which were 1.89, 1.19, and 1.20, respectively. Hence, it was demonstrated that E-KL/CA-C possessed a relatively higher graphitic crystalline structure.

Figure 4. (a) Raman spectra and (b) XRD patterns of E-KL/CA-C, KL/CA-C, KL-C and E-CA-C. XRD was also performed to characterize the structures of the as-prepared carbon materials. As presented in Figure 4b, two broad peaks at around 22.2° and 43.9° were observed for E-KL/CA-C, KL/CA-C, KL-C and E-CA-C. To be specific, these two peaks are corresponded to the (002) and (100) diffraction modes, presenting a disordered carbonaceous structure.23 However, it is remarkable that there was a slight 10


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difference in the exact position [2 Theta (°0)] of (002) peaks indexed for each carbon material and the position of (002) peak indexed for E-KL/CA-C was the lowest (22.65o) among all four studied carbon materials, as shown in Table 2. On the other hand, corresponding interplanar spacing of E-KL/CA-C was the widest as 0.384 nm, while those of KL/CA-C, KL-C and E-CA-C were 0.372, 0.379, 0.364 nm, respectively. According to the simulation analysis in the previous report, it is hard for Na ions to intercalate into the interlayer spacing of graphite less than 0.335 nm. Hence, the larger interplanar spacing of E-KL/CA-C is favorable for sodium insertion and storage between parallel graphene layers. Table 2. Positions of (002) peak and interlayer spacings of E-KL/CA-C, KL/CA-C, KL-C and E-CA-C. Samples

Position of (002) peak

Interlayer spacing (nm)

E-KL/CA-C

22.65

0.384

KL/CA-C

23.39

0.372

KL-C

22.88

0.379

E-CA-C

23.85

0.364

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Figure 5. High-resolution XPS O 1s spectra of E-KL/CA-C (a), KL/CA-C (b), KL-C (c), E-CA-C (d). XPS analysis was performed to study the surface composition and chemical state of E-KL/CA-C, KL/CA-C, KL-C and E-CA-C. As shown in Figure S2, the survey spectra of as-prepared materials only demonstrated the presence of C and O due to the extremely low content of H and N in each material. The C and O contents of E-KL/CAC, KL/CA-C, KL-C and E-CA-C are shown in Table 3. It is worth noting that EKL/CA-C had higher oxygen content than KL/CA-C, KL-C and E-CA-C. Regarding the O1s of each material (Figure 5), the peaks located at 531.3 eV and 532.5 eV could correspond to C=O and C-O bonds. The peak observed at 533.6 eV was associated to the contribution of C-OH functionality. The following peak at 535.1 eV represents surface chemisorbed oxygen groups. Based on the XPS results (Figure 5), it is notable that the atomic percentages of single and double bonds between oxygen and carbon of E-KL/CA-C were the highest (52.13% and 19.23%, respectively) among all materials studied while its surface chemisorbed oxygen content was only 4.67% (the lowest among all materials). In contrast, the C=O and C-O bond contents of E-CA-C were the lowest, only at 27.27% and 7.47%, respectively, while its surface chemisorbed oxygen content was as the highest as 36.62%. It should be noted that the existence of oxygen containing groups, especially C=O, could greatly contribute to Na ion storage.24-25 The conversion from C=O to C-O upon sodiation has been demonstrated to be reversible. By contrast, contribution of the other functional groups to sodium storage is limited. Hence, the higher C=O bond contents of E-KL/CA-C would lead to higher energy storage capacity. Table 3. Elemental analysis result of E-KL/CA-C, KL/CA-C, KL-C and E-CA-C. Samples

Elements (wt.%) C

H

N

O

E-KL/CA-C

85.95

0.35

0.44

13.26

KL/CA-C

89.59

0.13

0.99

9.29

KL-C

90.25

0.23

1.04

8.48

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E-CA-C

88.23

0.04

0.35

11.38

3.2. Electrochemical performance Electrochemical behavior of the E-KL/CA-C electrode was first investigated by CV at a scanning rate of 0.1 mV∙s-1 between 0.01 and 2.5 V. Figure 6a shows the discharge (Na insertion)/charge (Na extraction) curves of the E-KL/CA-C electrode for the first three cycles. In the first cycle, the first reduction peak at 1.03 V could be assigned to irreversible reaction of the electrolyte with surface functional groups.26 Another broad irreversible peak at 0.75 V in the first cathodic process could be attributed to the decomposition of electrolyte and formation of a solid electrolyte interphase (SEI) layer.27 During the subsequent two cycles, these two reduction peaks disappeared, leading to a low initial Coulombic efficiency (CE) for E-KL/CA-C electrode. Low initial CE performance has reported for most SIB electrodes, which is mainly due to the formation of SEI and other irreversible reactions caused by the high specific surface area, porosity, defects, and functional groups.28 Meanwhile, there was a pair of redox peaks at a lower potential (at about 0.04 V and 0.13 V), which was corresponded to the sodiation/desodiation processes of the hard carbon.29 In the second cycle, the cathodic peaks observed between 0.63 V and 0.92 V were mainly attributed to the alloying process of Na ions with surface carbon-oxygen functional groups and the corresponding anodic peak at 1.29 V could be ascribed to the dealloying transformation.30 Specifically, the redox reaction between carbon-oxygen functional groups and Na could be expressed as ―C = O + 𝑁𝑎 + + 𝑒 ― ↔ ― 𝐶 ― 𝑂 ― 𝑁𝑎

(1)

Hence, the higher content of C=O bond contents in E-KL/CA-C makes this reaction peak more distinct and results in higher sodium storage. Moreover, it is notable that the CV curves almost overlapped each other after the first scan, indicating that the capacity decay mainly occurred in the first cycle and subsequently the electrode showed stable sodiation/desodiation processes. Considering the CV curves of KL/CA-C, KL-C and E-CA-C (Figure S2), they all exhibited similar reactions at the same peaks. However, it is important to point out that the peaks corresponding to sodiation/desodiation 13



processes of surface functional groups for E-KL/CA-C were the most distinctive, suggesting a higher reversible capacity of this electrode.

Figure 6. CV curves of E-KL/CA-C electrode between 0.01 and 2.5 V at a potential sweep rate of 0.1 mV∙s−1 (a), and corresponding Galvanostatic charge-discharge profiles at 1st, 2nd, 5th and 150th cycles (b). Galvanostatic charge-discharge tests of E-KL/CA-C anode were performed between 0.01 and 2.5 V (vs Na+/Na) at a constant current rate of 50 mA∙g−1 (corresponding to a 0.2 C rate). The charge/discharge profiles for the 1st, 2nd, 100th and 150th cycles of E-KL/CA-C are shown in Figure 6b. In the first cycle, specific discharge and charge capacities of E-KL/CA-C were recorded as 555 and 290 mAh∙g1,

respectively, with an initial CE of 52%. The large capacity loss in the initial cycle

was attributed to the partial decomposition of the electrolyte on the surface of active sites (such as edges, defects and functional groups) to form SEI film as discussed above.31 Besides, an obvious discharge plateau at a low voltage range of 0-0.1 V was ascribed to the intercalation of Na ion into graphitic layers and nanopores, which indicated that E-KL/CA-C possessed a disorderedly assembled graphite structure and sufficient “nanocavities” in hard carbon. In the 2nd cycle, the specific charge capacity of E-KL/CA-C anode rose up to 296 mAh∙g-1 while corresponding CE increased to 78%. Increase in the capacity after the first cycle might be due to the slow activation process of E-KL/CA-C anode in SIBs. It is notable that the discharge-charge curves of the 5th and 150th cycles almost overlapped, indicating an outstanding electrochemical reversibility. Moreover, as shown in Figure S3, KL/CA-C, KL-C and E-CA-C 14


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possessed similar discharge-charge curves but demonstrated much lower capacities. The cycling performance of E-KL/CA-C, KL/CA-C, KL-C and E-CA-C was evaluated at 50 mA∙g−1. As shown in Figure 7a, all four electrodes exhibited a capacity increasing trend in the initial cycles due to the slow anode activation process. After certain number of cycles, the capacity increasing trend slowed down, and their cycling performance became stable. However, there was a significant difference in reversible capacity of each carbon electrode. It is notable that, among all materials studied, EKL/CA-C delivered the highest reversible capacity. Specifically, the reversible capacity of E-KL/CA-C was around 290 mAh∙g−1 in the first cycle, and then it gradually increased to 340 mAh∙g−1 after 60 cycles and maintained similar capacities during the ongoing cycles. In contrast, the stable reversible capacity of E-CA-C was only around 68 mAh∙g−1 due to its small interlayer spacing. Moreover, the stable reversible capacities of KL-C and KL/CA-C were similar, both at about 190 mAh∙g−1. It was well demonstrated that abundant chemisorbed oxygen-containing groups and appropriate interlayer spacing of carbon anodes provided numerous transmission micro-channels for Na ions, which increased the available spaces for Na ion storage leading to a high reversible capacity. Additionally, it was found that the first cycle CEs of all four studied anodes were relatively low, namely at around 40-60%. Similar results were reported for some other carbon based anode materials, which can be assigned to the formation of solid electrolyte interphase on the electrode surface.8, 32 After 10-20 cycles, CEs of EKL/CA-C and KL-C approached to 99%, while they were only around 96% and 92% for KL/CA-C and E-CA-C, respectively.

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Figure 7. Cycling and CE performance at a current density of 50 mA∙g-1 (a), rate performance (b), Randles–Sevcik plots obtained from the voltammetric data (c) and EIS spectra (d) of E-KL/CA-C, KL/CA-C, KL-C and E-CA-C. Rate capability of each anode was investigated at increased current densities from 50 to 500 mA∙g−1 and the results are shown in Figure 7b. It is remarkable that reversible capacities of E-KL/CA-C at different current densities were all much higher than those of other three anodes. Specifically, the reversible capacities of E-KL/CA-C were 326, 280, 205, and 143 mAh∙g−1 at current densities of 50, 100, 200, and 500 mA∙g−1, respectively. When the current density reduced back to 50 mA∙g−1 after cycling at elevated current densities, its capacity increased back to 339 mAh∙g−1 and indicated an excellent reversibility. This outstanding rate performance could be attributed to the large surface area, large interlayer spaces and nano network structure of E-KL/CA-C, which permits the organic electrolyte to enter the interior of electrode and provides sufficient penetration pathways to allow fast diffusion of Na ions. To further explain the extremely high rate performance of E-KL/CA-C, their CV tests at different scan rates of 0.1, 0.2, 0.5, 1 and 2 mV∙s−1 were performed (illustrated in Figure S5). The dependence of the anodic peak currents on the square root of the 16


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scan rate (ν1/2) is presented in Figure 7c. Obviously, the anodic peaks of carbon anodes showed a linear relationship with the square root of the scan rate, indicating their reaction rate against sodium is diffusion-controlled and the semi-infinite diffusion of Na+ into the carbon anode could be explained by the following classical RandlesSevchik equation33, 𝑖𝑝 = (2.69 × 105)𝑛3/2𝑆𝐷1/2𝐶𝑣1/2

(2)

Hence, the peak current value (ip) depended on the charge transfer number (n), electrode area (S), diffusion coefficient of Na ion (D), concentration of sodium ions(C), and potential scan rate (ν). Moreover, as the entire electrode materials were prepared and measured in the same conditions, the abovementioned Randles-Sevchik equation is further simplified as 𝑖𝑝 = 𝐴𝐷1/2𝐶𝑣1/2

(3)

where A can be regarded as a constant for all the batteries and AD1/2 represents the apparent diffusion coefficient of Na ion against the electrodes.33 In this case, the value of AD1/2 could be calculated as the slope by fitting peak line of each anode. Based on the fitting results, E-KL/CA-C exhibited the highest apparent diffusion coefficient of Na ion compared to all other anodes. It was demonstrated that the porous structure of E-KL/CA-C significantly shortened the Na ion transport pathway, which was the most probable reason of the efficient diffusion of Na ion. The accelerated Na ion diffusion behavior of E-KL/CA-C improved the sodiation/desodiation reaction kinetics and consequently enhanced the sodium storage ability as well as rate performance. The EIS measurement of E-KL/CA-C, KL/CA-C, KL-C and E-CA-C was subsequently conducted to examine their kinetic difference as illustrated in Figure 7d. It is well known that the charge transfer resistance is highly related to the semicircle in the high-medium frequency region in EIS spectra.34 Notably, the charge transfer resistance of E-KL/CA-C was the lowest among all studied carbon materials and was recorded as 180 Ω, as compared to 210 Ω, 384 Ω and 682 Ω for KL/CA-C, KL-C, ECA-C respectively. Hence, the outstanding electrical conductivity of E-KL/CA-C, which was originated from its continuous conductive nano network structure, would be 17



Page 18 of 32

extremely beneficial to promote the charge transfer reactions and was another main reason for its excellent electrochemical performance. The morphology changes of the four electrodes after 200 cycles were also investigated by SEM, after taken them out from tested cells and washed with a dimethyl carbonate solution. E-KL/CA-C could mostly preserve its original network structure with numerous pores, and no structural cracks were detected after long-term cycling tests. By contrast, other three electrodes exhibited partial fracture and were disintegrated into nanoparticles after repetitive sodium insertion and extraction processes. Furthermore, the electrochemical performance of E-KL/CA-C has been compared with literature data on carbon materials derived from relevant green raw materials (Table 4). Compared with carbon anodes prepared from other green raw materials, the as-prepared E-KL/CA-C exhibited higher capacity and better cycling performance at low current density, but comparable rate performance and Coulombic efficiency. It is, therefore, demonstrated that preparing nano carbon network is an effective way to use lignin in the anode application of SIBs. Table 4. Electrochemical performance comparison of E- KL/CA-C with carbon anodes reported in literature for SIBs. Voltage Precursor

range (V)

Our study Pitch and lignin35

Capacity (mAh∙g-1) (cycle number) Current density (mA∙g-1)

Capacity (mAh∙g-1) at high current density (mA g-1)

(cycle numbers)

293(1)/340(200)-50

143 (500)

99% (200)

0.115-2

254(1)/226(150)-30

162 (300)

97% (100)

0.01-2.7

292(1)-20

100(800)

lignin17 Biomass36

efficiency

0.01-2.5

Polyacrylonitri le and refined

Coulombic

0.01–3.0 360(1)/352(200)-50 18


90(500)

Nearly 100% (200) 99% (300)

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Cellulose nanocrystals16

0.01–2.5

Cellulose37

0.5-3

300(100)-100 360(1)/230(500)37.2

220(200)

184(1860)

Nearly 100% (400) Nearly 100% (500)

4. Conclusions In this work, we demonstrated a novel and simple approach to prepare nano carbon network by pyrolysis of electrospun KL/CA nanofibers precursors in inert environment. The sodium-ion battery assembled with this nano carbon network as the anode exhibited a high capacity as well as good cycling stability. To be specific, KL/CA derived nano carbon network showed a high reversible capacity of up to 293 mAh∙g-1 at a current density of 50 mA∙g-1 in the first cycle and 340 mAh∙g-1 over 200 cycles, as well as a superior rate capability. Besides, electrospinning process was demonstrated as an effective method to produce nano carbon network made of KL/CA, which possesses better electrochemical performance against sodium, when compared with the carbon material prepared by solution-casting method from the same raw material. To the best of our knowledge, the structural, electronic, and electrochemical properties of electrospun KL/CA-derived nano carbon network as SIB anode have not been reported. Thus, this work could open an avenue for using green and environmental-friendly raw materials in production of carbon nanomaterials and might have extremely beneficial effects in exploring the utilization of porous carbon material derived from KL and CA in the area of SIB anodes.

Associated Content Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XXX. SEM images, XPS spectra, cyclic voltammetry curves, and charge-discharge 19



curves, ex-situ SEM images (PDF).

Notes The authors declare no competing financial interest.

Acknowledgments Hao Jia and Na Sun contributed equally to this work. The research was supported by the Fundamental Research Funds for the Central Universities (grant number BCZD2018006) and the Special Project of International Scientific and Technological Cooperation in Guangzhou Development District (2017GH35). This work was also supported in part by the scholarship from China Scholarship Council (CSC) under the Grant CSC No. 201600090045. We also acknowledge the fund of Guangdong Province Industrial Sci.&Tech. projects (2017A010103006) and Guangzhou Industrial Sci.&Tech. projects (201804010368).

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Table of Contents Graphic

25



Figure 1 285x177mm (144 x 144 DPI)


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Figure 2 146x98mm (220 x 220 DPI)



Figure 3 447x400mm (144 x 144 DPI)


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Figure 4 293x121mm (144 x 144 DPI)



Figure 5 169x133mm (144 x 144 DPI)


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Figure 6 146x58mm (220 x 220 DPI)



Figure 7 221x161mm (144 x 144 DPI)


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Cellulose Acetate-Derived Nanocarbon

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