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Effect of surface modification on high-surface-area carbon nanosheets anode in sodium ion battery
Accepted Manuscript Effect of surface modification on high-surface-area carbon nanosheets anode in sodium ion battery Huanlei Wang, Wenhua Yu, Nan Mao, Jing Shi, Wei Liu PII:
S1387-1811(16)00089-5
DOI:
10.1016/j.micromeso.2016.02.003
Reference:
MICMAT 7565
To appear in:
Microporous and Mesoporous Materials
Received Date: 29 September 2015 Revised Date:
1 December 2015
Accepted Date: 1 February 2016
Please cite this article as: H. Wang, W. Yu, N. Mao, J. Shi, W. Liu, Effect of surface modification on highsurface-area carbon nanosheets anode in sodium ion battery, Microporous and Mesoporous Materials (2016), doi: 10.1016/j.micromeso.2016.02.003. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Effect of surface modification on high-surface-area carbon nanosheets anode in sodium ion battery Huanlei Wang, Wenhua Yu, Nan Mao, Jing Shi*, Wei Liu*
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Institute of Materials Science and Engineering, Ocean University of China, Qingdao 266100, China
*Corresponding authors. Tel: +86 532 66781906. Email: [email protected] (J. Shi),
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[email protected] (W. Liu).
Abstract
This paper reports a case study of the effect of nitrogen and oxygen functional groups of high-surface-area carbon nanosheets anode on electrochemical performance
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in sodium ion batteries. Hemp bast fiber is chosen to fabricate high-surface-area carbon nanosheets (2190 m2 g-1). After being treated with urea under hydrothermal condition, the as-prepared carbon nansheets are functionalized with nitrogen- and
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oxygen- containing groups on the surface. Both the as-obtained and functionalized
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carbon nanosheets are used as anodes for sodium ion batteries, and the effect of nitrogen and oxygen functional groups on the electrochemical performance is investigated. The results indicate that the surface area of functionalized carbon nanosheets is significantly reduced to 840 m2 g-1. However, both the carbon nanosheets display similar high capacity, rate capability and good cyclability. The electrodes deliver a capacity of 162/173 mAh g-1 at 1 A g-1 after 2000 cycles and retain a capacity of 49/62 mAh g-1 at 10 A g-1. Besides the ultrahigh surface area and
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ACCEPTED MANUSCRIPT dilated graphitic layer of porous carbon nanosheets, chemisorption through functional groups are also found to be responsible for the high capacity. Moreover, the functionalized carbon nanosheets exhibit elevated Coulombic efficiency. These results
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confirm that modifying the surface of carbons is a promising strategy to improve the electrochemical performance.
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Keywords: surface modification; biomass; porous carbon; anode; sodium ion
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batteries
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ACCEPTED MANUSCRIPT 1. Introduction In the field of electric energy storage, lithium ion batteries have been successfully utilized as a power source in portable devices and hybrid electric vehicles
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for several decades [1, 2]. Concerning the cost and limitation of lithium terrestrial reserves [3], it is necessary to explore low-cost new rechargeable battery systems to meet the growing demands for energy storage. Owing to the wide availability and low
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cost of sodium, sodium ion batteries turn to be the promising candidate for the
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large-scale energy storage systems. It is worth to note that sodium ions are approximately 55% larger in radius than lithium ions, which can lead to the difficult insertion/extraction of sodium ions into graphitic layers [4, 5]. Until now, compared to lithium ion batteries, sodium ion batteries still shows the disadvantages associated
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with low energy density and poor cycle life. However, recent study demonstrates that certain sodium ion battery electrode materials are approaching the efficiencies of lithium ion battery electrodes [6]. Thus, developing high-performance electrode
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materials with high capacity and long-term stability is essential for further
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development of sodium ion batteries. Carbonaceous material is a dominant candidate as electrode material for electric
energy storage devices [7-10]. The commercial graphite anode for lithium ion batteries is not suitable for sodium ion batteries [11], which is ascribed to the difficult insertion of sodium ions into graphitic layers. An efficient strategy to facilitate the sodium insertion and extraction is to expand the graphitic interlayer spacing [12], which helps to improve the electrochemical performance of the carbon electrodes.
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ACCEPTED MANUSCRIPT Alternative carbon anodes, including disordered carbon [13, 14], partially graphitic carbon [15-17], graphene [18, 19], and graphene hybrids [20, 21], all show improved electrochemical performance for sodium ion batteries. These materials with dilated
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graphitic layer either possess high surface area with abundant micro- and mesoporosity, or have “openness” sheet-like morphologies, which allows for fast solid-state diffusion kinetics.
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Another popular approach to improve the electrochemical performance of carbon
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anode for sodium ion batteries is to introduce heteroatoms in carbon structure [22]. The heteroatom doping can create a significant amount of defects, which may serve as additional active sites for sodium storage. Recent reported nitrogen doped carbons, including hollow carbon nanowires[12], porous nitrogen doped nanosheets[9],
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N-doped carbon nanofiber[23], nitrogen-doped carbon nanotubes[24], nitrogen-doped ordered mesoporous carbon[25] and nitrogen doped graphene foams[18], exhibited promising electrochemical properties. Besides, the oxygen-containing groups can also
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be functionalized for providing high electrochemical performance, which is related
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with the surface redox reaction between carbon-oxygen functional groups and alkali metal ions [26, 27]. This strategy is firstly utilized in lithium ion batteries, and can also be employed to develop high-performance sodium ion batteries. In this paper, we aims to study the synergistic effect of nitrogen- and
oxygen-containing surface functional groups of high-surface-area carbon nanosheets anode on its performance in sodium ion battery. The carbon nanosheets were derived from the biomass-hemp bast fiber [28], and biomass derived carbons receives
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ACCEPTED MANUSCRIPT extensive attention due to the low-cost character [29-33]. Hydrothermal treatment with urea resulted in an incorporation of a significant amount of nitrogen and oxygen in the surface of carbon nanosheets. This mild hydrothermal treatment was different
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from the conventional high temperature hydrolysis process performed in excess of 600 oC, and it was unique in introducing sufficient nitrogen and oxygen functional groups but not destroying the macroscopic morphology of carbon nanosheets. The
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surface modification by functional groups can reduce the surface area and improve the
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Coulombic efficiency without sacrificing the excellent electrochemical performance, suggesting the functional groups play a positive effect on enhancing the electrochemical property. The post-treatment method provides us a promising way to
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achieve high-performance carbon anode for sodium ion batteries.
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ACCEPTED MANUSCRIPT 2. Experimental 2.1. Synthesis of high-surface-area carbon nanosheets The carbon nanosheets (denoted as CNS) were prepared according to previously
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reported procedure [28, 34]. An aqueous dispersion (60 g L-1) of hemp bast fiber was placed in a stainless steel autoclave. The autoclave was heated up to 180 oC and maintained at the target temperature for 12 h. The resultant solid, denoted as
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hydrochar, was recovered by filtration, washed with distilled water and dried at 100 o
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C. The hydrochar material was thoroughly mixed with KOH at a weight ratio of 1:1
in an agate mortar. Then, the mixture was heated to 850 oC under argon atmosphere (5 o
C min-1), and followed by a “heat soak” time of 30 min at this temperature. The
resulting carbon nanosheets were then thoroughly washed with 10 wt% HCl and
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distilled water. Finally, the carbons were dried in an oven at 100 oC. 2.2. Synthesis of N,O-codoped carbon nanosheets Before the treatment with urea, the pretreatment of carbon nanosheets with nitric
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acid is to enhance the dispersivity of carbon nanosheets in aqueous solution. The
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carbon nanosheets were firstly refluxed in 50 wt% nitric acid at 60 oC for 4 h. Nitric acid treated carbon nanosheets and urea (the mass ratio of urea and carbon nanosheets is 300:1) were added to 30 mL of distilled water. After stirring for 1 h, the solution was sealed in the autoclave and maintained at 180 oC for 15 h. After that, the N,O-codoped carbon naosheets were washed thoroughly with distilled water to remove the residual inorganic species and dried at 100 oC. The obtained sample was labeled as CNS-U, where U represents the urea treatment.
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ACCEPTED MANUSCRIPT 2.3. Material characterization Powder X-ray diffraction (XRD) pattern was performed on a Bruker D8 Advance X-ray diffractometer using Kα radiation. The chemical composition of the carbon
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samples was determined by X-ray photoelectron spectroscopy (XPS, Kratos Analytical) equipped with a monochromatic Al-Kα radiation. Nitrogen adsorption and desorption isothermal at 77K was conducted with an Autosorb-1 (Quantachrome)
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surface area- pore size analyzer. The morphologies and structures of carbons were
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examined using transmission electron microscopy (TEM, JEOL JEM-2100). Raman spectra were collected with a 532 nm laser under ambient conditions with a confocal micro-Raman system (LabRAM HR800) at room temperature. 2.4. Electrochemical measurements
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Electrochemical evaluations were performed using CR2032-type coin cells assembled in an argon-filled glove box with sodium metal as the counter electrode. As prepared
sample,
super
P
and
poly
(vinyl
difluoride)
dissolved
in
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N-methyl-pyrrolidone were mixed at a weight ratio of 80:10:10 to form a slurry,
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which was then spread on the stain steel spacer and dried at 120 oC under vacuum to obtain working electrodes. The working electrode and counter electrode were separated by a polyethene separator. An electrolyte consisting of a solution of 1M NaClO4 in a 1:1 volume ratio of ethylene carbonate and diethyl carbonate was used. Cyclic voltammetry (CV) experiments were carried out using a CHI 660 workstation at a scan rate of 0.1 mV s-1. The discharge-charge measurements were conducted in the voltage range of 0.01-3 V using a Land CT2001A battery test system.
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ACCEPTED MANUSCRIPT 3. Results and discussion The chemical bonging state of all types of surface groups was characterized by using XPS analysis. From the XPS survey (Fig. S1), it can be concluded that the
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surface of CNS and CNS-U mainly contains oxygen (B.E. ~530 eV) and nitrogen (B.E. ~ 400 eV) besides carbon (B.E. ~ 285 eV) with no other heteroatoms being observable. The total nitrogen and oxygen contents reached up to 4.18 at% and 15.06
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at% for CNS-U, as shown in Table 1. The high-resolution O1s and N1s XPS spectra
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of CNS and CNS-U are shown in Fig. 1. For oxygen, there are generally three types of oxygen functionalities: C=O quinone groups (O-I, ~531 eV), C-OH phenol groups and/or C-O-C ether groups (O-II, ~ 532 eV), and COOH carboxylic groups (O-III, ~ 535 eV) [35, 36]. In CNS, the O 1s spectrum reveals the presence of two main peaks
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(Fig. 1a), which can be assigned to O-I (2.22 at%) and O-II (4.48 at%) functionalities. However, for CNS-U (Fig. 1b), the contents of O-I (4.51 at%) and O-II (5.40 at%) groups are significantly increased, whereas the hydrothermal process also introduces
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the O-III group (5.15 at%) into the carbon surface. It is generally accepted that these
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oxygen-containing functional groups can result in the surface redox reaction between carbon-oxygen functional groups and sodium ions, especially through –C=O/-C-O-Na redox pair [16]. Besides, more nitrogen-containing functional groups can be observed on the surface after urea treatment (Fig. 1c and 1d), and it appears that urea treatment favors the formation of pyridinic nitrogen (N-6, ~398.4 eV, 1.03 at%), whereas the content of pyrrolic/pyridine nitrogen (N-5, ~400 eV, 3.15 at%) is significantly increased. It can be assumed that nitrogen doping can induce a large number of
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ACCEPTED MANUSCRIPT defects and add sodium insertion sites, thus enhancing the sodium storage capacity. The functional groups on the surface of carbon nanosheets, although important, can enhance the wettability of electrolyte if the surface is accessible for the electrolyte
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ions. This accessibility is usually related with the porosity of the materials. Nitrogen sorption measurements was used to evaluate the porous structures of CNS and CNS-U. Fig. 2a and 2b shows the nitrogen adsorption-desorption isotherms and the related
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pore size distributions obtained by density functional theory (DFT) method for CNS
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and CNS-U specimens. The calculated surface areas and pore volumes are summarized in Table 2. As Fig. 2a demonstrated, type I/IV isotherms could be found for CNS and CNS-U. However, the calculated surface area decreases from 2190 m2 g-1 for CNS to 840 m2 g-1 for CNS-U, whereas the total pore volume also decreases
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from 1.36 cm3 g-1 for CNS to 0.62 cm3 g-1 for CNS-U. This is mainly due to the blocking of some pores by surface functional groups. The pore size distributions of CNS and CNS-U are similar, which contain about 56% micropores and 44%
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mesopores. The slight change in weighted pore size and its distribution after the
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hydrothermal treatment with urea indicates that there are enough pathways for the transportation of electrolyte ions in CNS-U. It is generally accepted that sodium ions couldn’t be easily intercalated into
graphite with an interlayer pacing of 0.335 nm due to the high energy barrier to overcome [12]. So, it is necessary to prepare carbons with dilated graphitic interlayers to meet the demand for the smooth intercalation/extraction of sodium ions. In order to investigate the degree of graphitization for CNS and CNS-U, XRD analysis is
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ACCEPTED MANUSCRIPT performed with the patterns shown in Fig. 2c. For CNS and CNS-U, the diffraction peak at a 2θ value of ~ 24o can be assigned as the (002) reflection of graphitic layer-by-layer structure [12]. Such broad diffraction peak indicates the characteristic
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of disordered carbonaceous structures. From the 2θ degree of the (002) peak, the interlayer distance of graphitic layers is calculated to be ~0.37 nm, which is substantially larger than that of graphite and suitable for the intercalation of sodium
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ions. Based on the full width at half-maximum of the (002) diffraction peak, the
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calculated average domain size is about 0.82 nm, indicating the domains are composed of 2-3 stacked graphitic layers (0.82/0.37=2.22) [12]. The structure of CNS and CNS-U was further investigated by Raman spectroscopy. As shown in Fig. 2d, CNS and CNS-U exhibit broad disorder-induced D-bands (~ 1340 cm-1) and in-plane
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vibration G-bands (~ 1595 cm-1). The values of the integrated intensity of D- and G-bands could be obtained by fitting the spectra with Voigt function (Fig. S2) [8]. The calculated values of IG/ID can be used to index the degree of graphitic ordering in the
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carbons. As Table 2 demonstrates, the IG/ID value of CNS and CNS-U is 0.45 and 0.32,
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respectively, which indicates that the modification of functional groups by hydrothermal treatment can decrease the graphitic ordering caused by N, O-doping and the edge defects. The lack of the sharp and strong peaks in the spectral range of 2500-3500 cm-1 suggests the lack of thick graphitic ribbon structures [37], in consistent with the XRD analysis. Besides the porosity, the open architecture of carbon materials could also allow full access of the electrolyte to the active surfaces, minimizing high rate diffusional
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ACCEPTED MANUSCRIPT losses through the liquid. Similar to our previously reported results [28], CNS consists of highly interconnected carbon nanosheets (Fig. 3a). Compared with the previously reported activated carbons [38, 39], the obtained CNS morphologically resembles a
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macroscopically open sponge due to the combined hydrothermal carbonization and KOH activation. It is clear that the urea treatment cannot change the morphology of carbons, as shown in Fig. 3c. Fig. 3b and 3d presents the high resolution TEM
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micrographs of CNS and CNS-U, highlighting the porous structure with partially
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graphitic ribbons.
Cyclic voltammetry and galvanostatic discharge/charge cycling on both CNS and CNS-U samples, tested between 0.01 and 3.0 V versus Na/Na+, were investigated (Fig.4). Fig. 4a and 4b reproduces the CV curves of CNS and CNS-U at 0.1 mV s-1.
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The small reduction peak at ~0.45 V is observed in the first scan and disappeared in the subsequent scans. Due to the carbon’s high surface area, the formation of a SEI layer would occur at these potentials [40]. A pair of highly reversible
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oxidation/reduction peaks are observed below 0.1 V, which can be ascribed to
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insertion-extraction of sodium ions in the graphitic interlayers [9], similar to lithium insertion into graphite. A nearly rectangular curve is observed in the high potential range (i.e. 0.2-3.0 V), which can be attributed to the physisorption of sodium ions on the nanopores and the chemisorption happened at the surface functional groups and/or the edge and defects of graphitic layers[19, 41]. The similar CV curves for CNS and CNS-U indicates that both physisorption and chemisorption mechanisms exhibit capacitive-like behaviors [41].
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ACCEPTED MANUSCRIPT To test the rate performance, we charged and discharged CNS and CNS-U electrodes from 0.1 to 10 A g-1. Fig. 4c and 4d shows the charge-discharge profiles of CNS and CNS-U electrodes at 0.1 A g-1. The first charge-discharge cycle of CNS
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electrode reveals the initial discharge and charge capacities to be 1380 and 480 mAh g-1, showing an initial Coulombic efficiency of 34.8%. For CNS-U, the initial discharge and charge capacity is 990 and 445 mAh g-1, respectively. The initial
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Coulombic efficiency of CNS-U shows a notable improvement, which is increased to
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44.9%. The large capacity loss of the initial charge capacity is mainly attributed to the decomposition of the electrolyte and the formation of a dense SEI layer. For high-surface-area carbons, the SEI formation is considered to be the major contributor to early capacity loss [42, 43]. Obviously, CNS-U electrode with significantly reduced
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surface area shows a notable improvement in the initial Coulombic efficiency. However, the initial charge capacity of CNS-U is slightly lower than that of CNS, indicating the increased capacity from the redox reaction through functional groups
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and active sites on the surface of carbon can compensate the decreased capacity from
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the physisorption on the nanopores. The Coulombic efficiency of CNS-U is always higher than that of CNS during the first 10 cycles, and it can approach ~96 % after 10 cycles. This result certificates that low surface area is critical for the stabilization of SEI layer. Therefore, controlling the surface area of carbons is an efficient way to improve the Coulombic efficiency. The present work gives an example that the irreversible capacity loss can be reduced by modifying the surface of the carbon materials.
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ACCEPTED MANUSCRIPT As shown in Fig. 4e and 4f, when the current was increased from 0.1 to 1 A g-1, the CNS and CNS-U electrodes can deliver charge capacities of 191 and 162 mAh g-1. Even at a very high current density of 10 A g-1, a capacity of 62 and 49 mAh g-1 is still
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maintained for CNS and CNS-U electrodes, demonstrating the excellent rate performance. At high rates, the specific capacity of CNS-U is lower than that of CNS, which can be ascribed to the limitation of fast redox reactions. When the rate is turned
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back to 0.1 A g-1 after cycling at different rates, the specific capacity of 270 and 230
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mAh g-1 can be recovered for CNS and CNS-U. The observed capacity fading of CNS and CNS-U electrodes can be attributed to the incomplete stabilization of SEI film, the trapping of sodium ions between the graphitic interlayers and in nanopores, and irreversible reaction between sodium ions and the functional groups [9, 16, 19]. It
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should be pointed out that the capacity and rate capability obtained in the present work is comparable to or much better than those reported for other carbon-based anode materials [7-9, 12, 13, 15-17, 19, 20, 23, 40, 44-49].
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Fig. 5a shows the relatively long-term cycling performance for the CNS and
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CNS-U electrodes measured at 1 A g-1. The Coulombic efficiency increases dramatically upon cycling (Fig. 5b), reaching 88% for CNS and 95% for CNS-U after 10 cycles. The Coulombic efficiency of CNS-U is always higher than that of CNS before 600 cycles due to the low surface area character of CNS-U. There is large capacity loss before 200 cycles for both CNS and CNS-U electrodes. After 200 cycles, the capacity is stable. After 2000 cycles, the CNS and CNS-U can deliver a charge capacity of 172 and 163 mAh g-1 with the initial capacity retention of 50-60%. We
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ACCEPTED MANUSCRIPT believe that both the low Coulombic efficiency and fast degradation at the beginning cycles are related with the formation of SEI film, the irreversible trapping of sodium ions in defective sites and graphitic layers. Alternatively pores in the carbons may
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provide a benefit of buffering the sodiation-induced expansion/contraction during cycling. Such a good cycling performance of 2000 cycles has been seldom reported in previous investigations on anode materials for sodium ion batteries.
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In order to clarify the contribution from functional groups, a nitrogen-free carbon
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material was obtained by annealing of CNS at 1100 oC for 5h under Argon atmosphere. The obtained nitrogen-free sample was denoted as CNS-A, where A represents the annealing treatment. At the annealing temperature of 1100 oC, one might expect that some nitrogen/oxygen containing groups would be removed while
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the structure of CNS might be preserved [50]. As XPS analysis demonstrated (Fig. S3), the nitrogen can be hardly detected, and the oxygen content is decreased to 3.53 at%. Another thing worth mentioning is the C=O quinone groups, which is considered
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to be reacted with sodium ions, is disappeared. From Fig. S4, it is clear that CNS-A
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and CNS show similar nitrogen sorption isotherms and pore size distributions. The increased surface area and pore volume for CNS-A (SBET=2420 m2 g-1, Vt=1.52 cm3 g-1) can be ascribed to the removing of functional groups on the surface of carbons. For the electrochemical performance (Fig. S5), the initial discharge and charge capacity for CNS-A is 1140 and 385 mAh g-1 at 0.1 A g-1, and the capacity at 10 A g-1 is 45 mAh g-1. CNS-A exhibits lower capacity than that of CNS, which can be attributed to the removing of nitrogen/oxygen functional groups. Above 0.2 V vs.
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ACCEPTED MANUSCRIPT Na/Na+, the sodium storage comes from the physisorption of sodium ions on the nanopores and the chemisorption happened at the surface functional groups and/or the edge and defects of graphitic layers. Below 0.2 V vs. Na/Na+, the capacity mainly
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comes from the insertion of sodium between graphene layers [19]. We divided the capacity into two parts, as shown in Fig. S6. The highly developed porous structure facilitates
ion
diffusion,
leading
to
the
high
capacity
retention
from
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physi/chemisorption on the surface of carbons. However, the solid-state diffusion
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controlled intercalation mechanism results in the significantly reduced capacity at high charging/discharging rates. The capacity ratio for CNS/CNS-U/CNS-A between capacity above 0.2 V and capacity below 0.2 V vs Na/Na+ increased from 2.57/2.88/2.43 at 0.1 A g-1 to 3.64/4.03/3.03 at 1 A g-1 and 3.35/3.72/2.50 at 5 A g-1,
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further confirming the large contribution from physi/chemisorption at high rates. It is worth to note that the capacity ratio between capacity above 0.2 V and capacity below 0.2 V can be ranged as CNS-U > CNS > CNS-A, which is the same sequence of the
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content of nitrogen and oxygen in carbons, confirming the existence of functional
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groups is favorable for enhancing charge storage. The excellent electrochemical performance for CNS and CNS-U should be
attributed to the combined characteristics of the interconnected “openness” carbon nanosheets, functional groups, large surface area, and dilated graphitic interlayers. First of all, the interconnected carbon nanosheets with open structure makes the CNS and CNS-U favorable for the accessibility of the electrolyte. The ion diffusional limitation within the electrolyte is also important for high rate performance. There are
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ACCEPTED MANUSCRIPT abundant micropores and mesopores on the carbon nanosheets, and such open 2D porous structure is expected to offer fast ion access to the surface of carbon with short diffusion distances. Secondly, at relative high potential, the charge storage mechanism
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can be supposed to surface electro-adsorption/desorption. For CNS, the 2190 m2 g-1 of usable micro- and mesopores ensures large sodium storage. For CNS-U, the oxygen-/nitrogen-containing functional groups on the surface can accelerate the
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surface redox reactions. As Raman analysis demonstrated, the introduction of
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functional groups can induce a more disordered carbon structure, suggesting an increased amount of defects on the graphitic layers [51]. Such defects can also be used as active sites for chemisorption of sodium ions. The surface area of CNS-U is significantly lower than that of CNS, but the capacity of CNS-U is comparable to that
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of CNS, giving us the evidence that the surface modification is a promising method to improve sodium storage. Thirdly, the large interlayer spacing facilitates sodium ion transport and storage between graphitic layers, which is critical for the larger sodium
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ions. It is worth to emphasize that high surface area carbon with highly defective sites
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can enhance the sodium storage capacity with the sacrificing of Coulombic efficiency. Therefore, a balance should be made among porosity, graphitization, and surface modification for carbon materials in order to achieve superior electrochemical performance with high capacity, high Coulombic efficiency, and excellent rate capability.
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ACCEPTED MANUSCRIPT 4. Conclusions Surface modification can be used as an efficient strategy to improve the performance of sodium ion batteries. Carbon nanosheets with high surface area (2190
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m2 g-1) and dilated graphitic layers deliver a high capacity of 192 and 62 mAh g-1 at 1 and 10 A g-1. The surface area was largely decreased to 840 m2 g-1, and abundant nitrogen- and oxygen- containing functional groups were detected in carbon
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nanosheets after urea treatment. However, N, O-doped carbon nanosheets show
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similar electrochemical performance with the as-prepared carbon nanosheets, suggesting the pseudocapacitive effect by surface redox reaction of functional groups and graphitic defects. The carbon nanosheets with and without urea treatment can deliver a charge capacity of 163 and 172 mAh g-1 with the initial capacity retention of
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50-60% after 2000 cycles at 1 A g-1. Moreover, the initial Coulombic efficiency of urea treated carbon nanosheets is increased from 34.8 % to 44.9 % owing to the decreased surface area. This work clearly indicates that carbon materials with suitable
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porous structure, heteroatom-doping and graphitic structure can be used as high
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performance anode materials for sodium ion batteries.
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ACCEPTED MANUSCRIPT Acknowledgement This work was funded by National Natural Science Foundation of China (No. 21471139,
and
51402272),
China
Postdoctoral
Science
Foundation
(No.
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2014M560581 and and 2015T80747), Shandong Province Outstanding Youth Scientist Foundation Plan (No. BS2014CL024), Qingdao Postdoctoral Science Foundation, Seed Fund from Ocean University of China and Fundamental Research
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Appendix A. Supplementary data
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Funds for the Central Universities.
Supplementary data associated with this article can be found, in the online version, at
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doi:******.
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ACCEPTED MANUSCRIPT [49] H.W. Song, N. Li, H. Cui, C.X. Wang, Nano Energy 4 (2014) 81-87. [50] W.Y. Tsai, R. Lin, S. Murali, L.L. Zhang, J.K. McDonough, R.S. Ruoff, P.L. Taberna, Y. Gogotsi, P. Simon, Nano Energy 2 (2013) 403-411.
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[51] C. Bommier, T. W. Surta, M. Dolgos, X.L. Ji, Nano Lett. 15 (2015) 5888-5892.
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ACCEPTED MANUSCRIPT Figure captions Fig. 1 High-resolution XPS O1s spectra of (a) CNS and (b) CNS-U samples. High-resolution XPS N1s spectra of (c) CNS and (d) CNS-U samples. Fig. 2 (a) Nitrogen adsorption-desorption isotherms of CNS and CNS-U. (b) Pore size
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distributions calculated from the adsorption isotherms using DFT method. (c) XRD patterns of CNS and CNS-U specimens. (d) Raman spectra of CNS and CNS-U.
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Fig. 3 Low-magnification TEM micrographs of (a) CNS and (c) CNS-U, highlighting the sheet-like structures. High-resolution TEM micrographs of (b) CNS and (d)
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CNS-U, demonstrating the porous and partially graphitic structures.
Fig. 4 Electrochemical performance of CNS and CNS-U as anodes for sodium ion battery. CV curves of (a) CNS and (b) CNS-U electrodes between 0.01 and 3 V at a scan rate of 0.1 mV s-1. Galvanostatic charge/discharge profiles of (c) CNS and (d) CNS-U at a current density of 0.1 A g-1. Rate capability of (e) CNS and (f) CNS-U
capacity).
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electrodes at various current densities (solid, discharge capacity; hollow, charge
Fig. 5 (a) Cycling performance and (b) related Coulombic efficiency of CNS and
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CNS-U anodes, at a current density of 1 A g-1.
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ACCEPTED MANUSCRIPT Table 1 Relative surface concentrations of carbon, nitrogen and oxygen species obtained by fitting the XPS spectra.
6.70 15.06 24.58
N-5 (%)
O 1s O-I (%) O-II (%)
O-III (%)
100 75.42
33.08 29.92
39.45
66.92 30.63
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CNS 92.35 0.95 CNS-U 80.76 4.18
N 1s N-6 (%)
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Sample C N O (at%) (at%) (at%)
Table 2 Physical parameters for CNS and CNS-U. SBETa (m2 g-1)
Vtb (cm3 g-1)
Dmeanc (nm)
CNS
2190
1.36
1.65
CNS-U
840
0.62
1.74
Pore vol in cm3 g-1 d and (pore vol%)
d002 (nm)
IG/ID e
V<2 nm
V>2 nm
0.64 (55.7)
0.51 (44.3)
0.37
0.45
0.27 (56.3)
0.21(43.7)
0.37
0.32
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Sample
Surface area was calculated with Brunauer-Emmett-Teller (BET) method.
b
The total pore volume was determined at a relative pressure of 0.98-0.99.
d
The weighted mean pore size was calculated from: D mean
n
∑ = ∑
i =1 n
di vi
.
v i =1 i
The volume of pores smaller than 2 nm (V<2 nm) and pores larger than 2 nm (V>2 nm)
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c
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a
was obtained by DFT analysis. e
ID and IG are the integrated intensities of D- and G-band.
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ACCEPTED MANUSCRIPT Fig. 1
b
CNS-U O 1s
542
540
538
536
534
532
530
528
542
526
540
536
534
532
530
528
526
396
394
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d
CNS-U N 1s
Intensity (a.u.)
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CNS N 1s
Intensity (a.u.)
538
Binding Energy (eV)
Binding Energy (eV)
c
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Intensity (a.u.)
CNS O 1s
Intensity (a.u.)
a
406
404
402
400
398
396
394
408
406
404
402
400
398
Binding Energy (eV)
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Binding Energy (eV)
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b 0.08 dV/dD Pore Volume (cm g nm )
-1
800
CNS CNS-U
0.06
600
adsorption desorption
CNS CNS-U
400 200 0 0.0
0.2
0.4
0.6
0.8
0.04
0.02
0.00 1
1.0
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c
CNS
30
40
50
60
80
500
CNS CNS-U
1000 1500 2000 2500 3000 3500 4000 -1 Raman Shift (cm )
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2 theta (degrees)
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Intensity (a.u.)
CNS-U
G-band D-band
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Intensity (a.u.)
d
20
10
Relative Pressure (P/Po)
Relative Pressure (P/Po)
10
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3
3
-1
-1
a 1000 Quantity Adsorbed (cm g )
Fig. 2
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b
c
d
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a
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Fig. 3
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ACCEPTED MANUSCRIPT Fig. 4 0.50
0.50
b
0.25 -1
Current (A g )
-1
Current (A g )
0.25 0.00 1st 2nd 5th 10th
-0.25 -0.50 -0.75 0.0
0.5
1.0
1.5
2.0
2.5
0.00
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a
1st 2nd 5th 10th
-0.25 -0.50 -0.75 0.0
3.0
0.5
1.0
+
3.0
d
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Voltage (V vs. Na /Na)
2.0
+
+
Voltage (V vs. Na /Na)
3.0 2.5
2.5 1st 2nd 5th 10th
1.5 1.0 0.5
200
400
600
800
2.0
3.0
1st 2nd 5th 10th
1.5 1.0 0.5
0
1000 1200 1400
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0
200
400
0.1 A g
-1
-1
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0.2 A g -1 0.5 A g -1 1Ag -1 2Ag -1 5Ag -1 10 A g
300
0.1 A g
-1
0
0
20
40
1000 1200 1400
1500 1200
-1
-1
900
f Capacity (mAh g )
1200
800
Capacity (mAh g )
Capacity (mAh g )
e 1500
600
-1
-1
Capacity (mAh g )
2.5
0.0
0.0
600
2.0
Voltage (V vs. Na /Na)
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c
1.5
+
Voltage (V vs. Na /Na)
900 600
0.1 A g
-1
-1
0.2 A g -1 0.5 A g -1 1Ag -1 2Ag -1 5Ag -1 10 A g
300
0.1 A g
-1
0 60
Cycle Number
80
100
0
20
40
60
80
100
Cycle Number
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ACCEPTED MANUSCRIPT Fig. 5
a 500
300 200 100 0
100 80 60
CNS CNS-U
40 20 0
0
500
1000
1500
2000
Cycle Number
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CNS-discharge CNS-charge CNS-U-discharge CNS-U-charge
-1
0
500
1000
1500
2000
Cycle Number
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Capacity (mAh g )
400
Coulombic Efficiency (%)
b 120
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ACCEPTED MANUSCRIPT Highlights
Nitrogen and oxygen co-doped carbon nanosheets can be prepared by using urea treatment. The as-obtained and functionalized carbon nanosheets exhibit excellent electrochemical performance. post-treatment
method
provides
a
promising
way
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The
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high-performance carbon-based anode for sodium ion batteries.
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to
achieve
S1387-1811(16)00089-5
DOI:
10.1016/j.micromeso.2016.02.003
Reference:
MICMAT 7565
To appear in:
Microporous and Mesoporous Materials
Received Date: 29 September 2015 Revised Date:
1 December 2015
Accepted Date: 1 February 2016
Please cite this article as: H. Wang, W. Yu, N. Mao, J. Shi, W. Liu, Effect of surface modification on highsurface-area carbon nanosheets anode in sodium ion battery, Microporous and Mesoporous Materials (2016), doi: 10.1016/j.micromeso.2016.02.003. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Effect of surface modification on high-surface-area carbon nanosheets anode in sodium ion battery Huanlei Wang, Wenhua Yu, Nan Mao, Jing Shi*, Wei Liu*
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Institute of Materials Science and Engineering, Ocean University of China, Qingdao 266100, China
*Corresponding authors. Tel: +86 532 66781906. Email: [email protected] (J. Shi),
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[email protected] (W. Liu).
Abstract
This paper reports a case study of the effect of nitrogen and oxygen functional groups of high-surface-area carbon nanosheets anode on electrochemical performance
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in sodium ion batteries. Hemp bast fiber is chosen to fabricate high-surface-area carbon nanosheets (2190 m2 g-1). After being treated with urea under hydrothermal condition, the as-prepared carbon nansheets are functionalized with nitrogen- and
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oxygen- containing groups on the surface. Both the as-obtained and functionalized
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carbon nanosheets are used as anodes for sodium ion batteries, and the effect of nitrogen and oxygen functional groups on the electrochemical performance is investigated. The results indicate that the surface area of functionalized carbon nanosheets is significantly reduced to 840 m2 g-1. However, both the carbon nanosheets display similar high capacity, rate capability and good cyclability. The electrodes deliver a capacity of 162/173 mAh g-1 at 1 A g-1 after 2000 cycles and retain a capacity of 49/62 mAh g-1 at 10 A g-1. Besides the ultrahigh surface area and
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ACCEPTED MANUSCRIPT dilated graphitic layer of porous carbon nanosheets, chemisorption through functional groups are also found to be responsible for the high capacity. Moreover, the functionalized carbon nanosheets exhibit elevated Coulombic efficiency. These results
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confirm that modifying the surface of carbons is a promising strategy to improve the electrochemical performance.
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Keywords: surface modification; biomass; porous carbon; anode; sodium ion
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batteries
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ACCEPTED MANUSCRIPT 1. Introduction In the field of electric energy storage, lithium ion batteries have been successfully utilized as a power source in portable devices and hybrid electric vehicles
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for several decades [1, 2]. Concerning the cost and limitation of lithium terrestrial reserves [3], it is necessary to explore low-cost new rechargeable battery systems to meet the growing demands for energy storage. Owing to the wide availability and low
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cost of sodium, sodium ion batteries turn to be the promising candidate for the
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large-scale energy storage systems. It is worth to note that sodium ions are approximately 55% larger in radius than lithium ions, which can lead to the difficult insertion/extraction of sodium ions into graphitic layers [4, 5]. Until now, compared to lithium ion batteries, sodium ion batteries still shows the disadvantages associated
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with low energy density and poor cycle life. However, recent study demonstrates that certain sodium ion battery electrode materials are approaching the efficiencies of lithium ion battery electrodes [6]. Thus, developing high-performance electrode
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materials with high capacity and long-term stability is essential for further
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development of sodium ion batteries. Carbonaceous material is a dominant candidate as electrode material for electric
energy storage devices [7-10]. The commercial graphite anode for lithium ion batteries is not suitable for sodium ion batteries [11], which is ascribed to the difficult insertion of sodium ions into graphitic layers. An efficient strategy to facilitate the sodium insertion and extraction is to expand the graphitic interlayer spacing [12], which helps to improve the electrochemical performance of the carbon electrodes.
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ACCEPTED MANUSCRIPT Alternative carbon anodes, including disordered carbon [13, 14], partially graphitic carbon [15-17], graphene [18, 19], and graphene hybrids [20, 21], all show improved electrochemical performance for sodium ion batteries. These materials with dilated
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graphitic layer either possess high surface area with abundant micro- and mesoporosity, or have “openness” sheet-like morphologies, which allows for fast solid-state diffusion kinetics.
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Another popular approach to improve the electrochemical performance of carbon
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anode for sodium ion batteries is to introduce heteroatoms in carbon structure [22]. The heteroatom doping can create a significant amount of defects, which may serve as additional active sites for sodium storage. Recent reported nitrogen doped carbons, including hollow carbon nanowires[12], porous nitrogen doped nanosheets[9],
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N-doped carbon nanofiber[23], nitrogen-doped carbon nanotubes[24], nitrogen-doped ordered mesoporous carbon[25] and nitrogen doped graphene foams[18], exhibited promising electrochemical properties. Besides, the oxygen-containing groups can also
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be functionalized for providing high electrochemical performance, which is related
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with the surface redox reaction between carbon-oxygen functional groups and alkali metal ions [26, 27]. This strategy is firstly utilized in lithium ion batteries, and can also be employed to develop high-performance sodium ion batteries. In this paper, we aims to study the synergistic effect of nitrogen- and
oxygen-containing surface functional groups of high-surface-area carbon nanosheets anode on its performance in sodium ion battery. The carbon nanosheets were derived from the biomass-hemp bast fiber [28], and biomass derived carbons receives
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ACCEPTED MANUSCRIPT extensive attention due to the low-cost character [29-33]. Hydrothermal treatment with urea resulted in an incorporation of a significant amount of nitrogen and oxygen in the surface of carbon nanosheets. This mild hydrothermal treatment was different
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from the conventional high temperature hydrolysis process performed in excess of 600 oC, and it was unique in introducing sufficient nitrogen and oxygen functional groups but not destroying the macroscopic morphology of carbon nanosheets. The
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surface modification by functional groups can reduce the surface area and improve the
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Coulombic efficiency without sacrificing the excellent electrochemical performance, suggesting the functional groups play a positive effect on enhancing the electrochemical property. The post-treatment method provides us a promising way to
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achieve high-performance carbon anode for sodium ion batteries.
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ACCEPTED MANUSCRIPT 2. Experimental 2.1. Synthesis of high-surface-area carbon nanosheets The carbon nanosheets (denoted as CNS) were prepared according to previously
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reported procedure [28, 34]. An aqueous dispersion (60 g L-1) of hemp bast fiber was placed in a stainless steel autoclave. The autoclave was heated up to 180 oC and maintained at the target temperature for 12 h. The resultant solid, denoted as
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hydrochar, was recovered by filtration, washed with distilled water and dried at 100 o
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C. The hydrochar material was thoroughly mixed with KOH at a weight ratio of 1:1
in an agate mortar. Then, the mixture was heated to 850 oC under argon atmosphere (5 o
C min-1), and followed by a “heat soak” time of 30 min at this temperature. The
resulting carbon nanosheets were then thoroughly washed with 10 wt% HCl and
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distilled water. Finally, the carbons were dried in an oven at 100 oC. 2.2. Synthesis of N,O-codoped carbon nanosheets Before the treatment with urea, the pretreatment of carbon nanosheets with nitric
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acid is to enhance the dispersivity of carbon nanosheets in aqueous solution. The
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carbon nanosheets were firstly refluxed in 50 wt% nitric acid at 60 oC for 4 h. Nitric acid treated carbon nanosheets and urea (the mass ratio of urea and carbon nanosheets is 300:1) were added to 30 mL of distilled water. After stirring for 1 h, the solution was sealed in the autoclave and maintained at 180 oC for 15 h. After that, the N,O-codoped carbon naosheets were washed thoroughly with distilled water to remove the residual inorganic species and dried at 100 oC. The obtained sample was labeled as CNS-U, where U represents the urea treatment.
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ACCEPTED MANUSCRIPT 2.3. Material characterization Powder X-ray diffraction (XRD) pattern was performed on a Bruker D8 Advance X-ray diffractometer using Kα radiation. The chemical composition of the carbon
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samples was determined by X-ray photoelectron spectroscopy (XPS, Kratos Analytical) equipped with a monochromatic Al-Kα radiation. Nitrogen adsorption and desorption isothermal at 77K was conducted with an Autosorb-1 (Quantachrome)
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surface area- pore size analyzer. The morphologies and structures of carbons were
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examined using transmission electron microscopy (TEM, JEOL JEM-2100). Raman spectra were collected with a 532 nm laser under ambient conditions with a confocal micro-Raman system (LabRAM HR800) at room temperature. 2.4. Electrochemical measurements
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Electrochemical evaluations were performed using CR2032-type coin cells assembled in an argon-filled glove box with sodium metal as the counter electrode. As prepared
sample,
super
P
and
poly
(vinyl
difluoride)
dissolved
in
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N-methyl-pyrrolidone were mixed at a weight ratio of 80:10:10 to form a slurry,
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which was then spread on the stain steel spacer and dried at 120 oC under vacuum to obtain working electrodes. The working electrode and counter electrode were separated by a polyethene separator. An electrolyte consisting of a solution of 1M NaClO4 in a 1:1 volume ratio of ethylene carbonate and diethyl carbonate was used. Cyclic voltammetry (CV) experiments were carried out using a CHI 660 workstation at a scan rate of 0.1 mV s-1. The discharge-charge measurements were conducted in the voltage range of 0.01-3 V using a Land CT2001A battery test system.
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ACCEPTED MANUSCRIPT 3. Results and discussion The chemical bonging state of all types of surface groups was characterized by using XPS analysis. From the XPS survey (Fig. S1), it can be concluded that the
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surface of CNS and CNS-U mainly contains oxygen (B.E. ~530 eV) and nitrogen (B.E. ~ 400 eV) besides carbon (B.E. ~ 285 eV) with no other heteroatoms being observable. The total nitrogen and oxygen contents reached up to 4.18 at% and 15.06
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at% for CNS-U, as shown in Table 1. The high-resolution O1s and N1s XPS spectra
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of CNS and CNS-U are shown in Fig. 1. For oxygen, there are generally three types of oxygen functionalities: C=O quinone groups (O-I, ~531 eV), C-OH phenol groups and/or C-O-C ether groups (O-II, ~ 532 eV), and COOH carboxylic groups (O-III, ~ 535 eV) [35, 36]. In CNS, the O 1s spectrum reveals the presence of two main peaks
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(Fig. 1a), which can be assigned to O-I (2.22 at%) and O-II (4.48 at%) functionalities. However, for CNS-U (Fig. 1b), the contents of O-I (4.51 at%) and O-II (5.40 at%) groups are significantly increased, whereas the hydrothermal process also introduces
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the O-III group (5.15 at%) into the carbon surface. It is generally accepted that these
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oxygen-containing functional groups can result in the surface redox reaction between carbon-oxygen functional groups and sodium ions, especially through –C=O/-C-O-Na redox pair [16]. Besides, more nitrogen-containing functional groups can be observed on the surface after urea treatment (Fig. 1c and 1d), and it appears that urea treatment favors the formation of pyridinic nitrogen (N-6, ~398.4 eV, 1.03 at%), whereas the content of pyrrolic/pyridine nitrogen (N-5, ~400 eV, 3.15 at%) is significantly increased. It can be assumed that nitrogen doping can induce a large number of
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ACCEPTED MANUSCRIPT defects and add sodium insertion sites, thus enhancing the sodium storage capacity. The functional groups on the surface of carbon nanosheets, although important, can enhance the wettability of electrolyte if the surface is accessible for the electrolyte
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ions. This accessibility is usually related with the porosity of the materials. Nitrogen sorption measurements was used to evaluate the porous structures of CNS and CNS-U. Fig. 2a and 2b shows the nitrogen adsorption-desorption isotherms and the related
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pore size distributions obtained by density functional theory (DFT) method for CNS
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and CNS-U specimens. The calculated surface areas and pore volumes are summarized in Table 2. As Fig. 2a demonstrated, type I/IV isotherms could be found for CNS and CNS-U. However, the calculated surface area decreases from 2190 m2 g-1 for CNS to 840 m2 g-1 for CNS-U, whereas the total pore volume also decreases
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from 1.36 cm3 g-1 for CNS to 0.62 cm3 g-1 for CNS-U. This is mainly due to the blocking of some pores by surface functional groups. The pore size distributions of CNS and CNS-U are similar, which contain about 56% micropores and 44%
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mesopores. The slight change in weighted pore size and its distribution after the
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hydrothermal treatment with urea indicates that there are enough pathways for the transportation of electrolyte ions in CNS-U. It is generally accepted that sodium ions couldn’t be easily intercalated into
graphite with an interlayer pacing of 0.335 nm due to the high energy barrier to overcome [12]. So, it is necessary to prepare carbons with dilated graphitic interlayers to meet the demand for the smooth intercalation/extraction of sodium ions. In order to investigate the degree of graphitization for CNS and CNS-U, XRD analysis is
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ACCEPTED MANUSCRIPT performed with the patterns shown in Fig. 2c. For CNS and CNS-U, the diffraction peak at a 2θ value of ~ 24o can be assigned as the (002) reflection of graphitic layer-by-layer structure [12]. Such broad diffraction peak indicates the characteristic
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of disordered carbonaceous structures. From the 2θ degree of the (002) peak, the interlayer distance of graphitic layers is calculated to be ~0.37 nm, which is substantially larger than that of graphite and suitable for the intercalation of sodium
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ions. Based on the full width at half-maximum of the (002) diffraction peak, the
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calculated average domain size is about 0.82 nm, indicating the domains are composed of 2-3 stacked graphitic layers (0.82/0.37=2.22) [12]. The structure of CNS and CNS-U was further investigated by Raman spectroscopy. As shown in Fig. 2d, CNS and CNS-U exhibit broad disorder-induced D-bands (~ 1340 cm-1) and in-plane
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vibration G-bands (~ 1595 cm-1). The values of the integrated intensity of D- and G-bands could be obtained by fitting the spectra with Voigt function (Fig. S2) [8]. The calculated values of IG/ID can be used to index the degree of graphitic ordering in the
EP
carbons. As Table 2 demonstrates, the IG/ID value of CNS and CNS-U is 0.45 and 0.32,
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respectively, which indicates that the modification of functional groups by hydrothermal treatment can decrease the graphitic ordering caused by N, O-doping and the edge defects. The lack of the sharp and strong peaks in the spectral range of 2500-3500 cm-1 suggests the lack of thick graphitic ribbon structures [37], in consistent with the XRD analysis. Besides the porosity, the open architecture of carbon materials could also allow full access of the electrolyte to the active surfaces, minimizing high rate diffusional
10
ACCEPTED MANUSCRIPT losses through the liquid. Similar to our previously reported results [28], CNS consists of highly interconnected carbon nanosheets (Fig. 3a). Compared with the previously reported activated carbons [38, 39], the obtained CNS morphologically resembles a
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macroscopically open sponge due to the combined hydrothermal carbonization and KOH activation. It is clear that the urea treatment cannot change the morphology of carbons, as shown in Fig. 3c. Fig. 3b and 3d presents the high resolution TEM
SC
micrographs of CNS and CNS-U, highlighting the porous structure with partially
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graphitic ribbons.
Cyclic voltammetry and galvanostatic discharge/charge cycling on both CNS and CNS-U samples, tested between 0.01 and 3.0 V versus Na/Na+, were investigated (Fig.4). Fig. 4a and 4b reproduces the CV curves of CNS and CNS-U at 0.1 mV s-1.
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The small reduction peak at ~0.45 V is observed in the first scan and disappeared in the subsequent scans. Due to the carbon’s high surface area, the formation of a SEI layer would occur at these potentials [40]. A pair of highly reversible
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oxidation/reduction peaks are observed below 0.1 V, which can be ascribed to
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insertion-extraction of sodium ions in the graphitic interlayers [9], similar to lithium insertion into graphite. A nearly rectangular curve is observed in the high potential range (i.e. 0.2-3.0 V), which can be attributed to the physisorption of sodium ions on the nanopores and the chemisorption happened at the surface functional groups and/or the edge and defects of graphitic layers[19, 41]. The similar CV curves for CNS and CNS-U indicates that both physisorption and chemisorption mechanisms exhibit capacitive-like behaviors [41].
11
ACCEPTED MANUSCRIPT To test the rate performance, we charged and discharged CNS and CNS-U electrodes from 0.1 to 10 A g-1. Fig. 4c and 4d shows the charge-discharge profiles of CNS and CNS-U electrodes at 0.1 A g-1. The first charge-discharge cycle of CNS
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electrode reveals the initial discharge and charge capacities to be 1380 and 480 mAh g-1, showing an initial Coulombic efficiency of 34.8%. For CNS-U, the initial discharge and charge capacity is 990 and 445 mAh g-1, respectively. The initial
SC
Coulombic efficiency of CNS-U shows a notable improvement, which is increased to
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44.9%. The large capacity loss of the initial charge capacity is mainly attributed to the decomposition of the electrolyte and the formation of a dense SEI layer. For high-surface-area carbons, the SEI formation is considered to be the major contributor to early capacity loss [42, 43]. Obviously, CNS-U electrode with significantly reduced
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surface area shows a notable improvement in the initial Coulombic efficiency. However, the initial charge capacity of CNS-U is slightly lower than that of CNS, indicating the increased capacity from the redox reaction through functional groups
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and active sites on the surface of carbon can compensate the decreased capacity from
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the physisorption on the nanopores. The Coulombic efficiency of CNS-U is always higher than that of CNS during the first 10 cycles, and it can approach ~96 % after 10 cycles. This result certificates that low surface area is critical for the stabilization of SEI layer. Therefore, controlling the surface area of carbons is an efficient way to improve the Coulombic efficiency. The present work gives an example that the irreversible capacity loss can be reduced by modifying the surface of the carbon materials.
12
ACCEPTED MANUSCRIPT As shown in Fig. 4e and 4f, when the current was increased from 0.1 to 1 A g-1, the CNS and CNS-U electrodes can deliver charge capacities of 191 and 162 mAh g-1. Even at a very high current density of 10 A g-1, a capacity of 62 and 49 mAh g-1 is still
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maintained for CNS and CNS-U electrodes, demonstrating the excellent rate performance. At high rates, the specific capacity of CNS-U is lower than that of CNS, which can be ascribed to the limitation of fast redox reactions. When the rate is turned
SC
back to 0.1 A g-1 after cycling at different rates, the specific capacity of 270 and 230
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mAh g-1 can be recovered for CNS and CNS-U. The observed capacity fading of CNS and CNS-U electrodes can be attributed to the incomplete stabilization of SEI film, the trapping of sodium ions between the graphitic interlayers and in nanopores, and irreversible reaction between sodium ions and the functional groups [9, 16, 19]. It
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should be pointed out that the capacity and rate capability obtained in the present work is comparable to or much better than those reported for other carbon-based anode materials [7-9, 12, 13, 15-17, 19, 20, 23, 40, 44-49].
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Fig. 5a shows the relatively long-term cycling performance for the CNS and
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CNS-U electrodes measured at 1 A g-1. The Coulombic efficiency increases dramatically upon cycling (Fig. 5b), reaching 88% for CNS and 95% for CNS-U after 10 cycles. The Coulombic efficiency of CNS-U is always higher than that of CNS before 600 cycles due to the low surface area character of CNS-U. There is large capacity loss before 200 cycles for both CNS and CNS-U electrodes. After 200 cycles, the capacity is stable. After 2000 cycles, the CNS and CNS-U can deliver a charge capacity of 172 and 163 mAh g-1 with the initial capacity retention of 50-60%. We
13
ACCEPTED MANUSCRIPT believe that both the low Coulombic efficiency and fast degradation at the beginning cycles are related with the formation of SEI film, the irreversible trapping of sodium ions in defective sites and graphitic layers. Alternatively pores in the carbons may
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provide a benefit of buffering the sodiation-induced expansion/contraction during cycling. Such a good cycling performance of 2000 cycles has been seldom reported in previous investigations on anode materials for sodium ion batteries.
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In order to clarify the contribution from functional groups, a nitrogen-free carbon
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material was obtained by annealing of CNS at 1100 oC for 5h under Argon atmosphere. The obtained nitrogen-free sample was denoted as CNS-A, where A represents the annealing treatment. At the annealing temperature of 1100 oC, one might expect that some nitrogen/oxygen containing groups would be removed while
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the structure of CNS might be preserved [50]. As XPS analysis demonstrated (Fig. S3), the nitrogen can be hardly detected, and the oxygen content is decreased to 3.53 at%. Another thing worth mentioning is the C=O quinone groups, which is considered
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to be reacted with sodium ions, is disappeared. From Fig. S4, it is clear that CNS-A
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and CNS show similar nitrogen sorption isotherms and pore size distributions. The increased surface area and pore volume for CNS-A (SBET=2420 m2 g-1, Vt=1.52 cm3 g-1) can be ascribed to the removing of functional groups on the surface of carbons. For the electrochemical performance (Fig. S5), the initial discharge and charge capacity for CNS-A is 1140 and 385 mAh g-1 at 0.1 A g-1, and the capacity at 10 A g-1 is 45 mAh g-1. CNS-A exhibits lower capacity than that of CNS, which can be attributed to the removing of nitrogen/oxygen functional groups. Above 0.2 V vs.
14
ACCEPTED MANUSCRIPT Na/Na+, the sodium storage comes from the physisorption of sodium ions on the nanopores and the chemisorption happened at the surface functional groups and/or the edge and defects of graphitic layers. Below 0.2 V vs. Na/Na+, the capacity mainly
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comes from the insertion of sodium between graphene layers [19]. We divided the capacity into two parts, as shown in Fig. S6. The highly developed porous structure facilitates
ion
diffusion,
leading
to
the
high
capacity
retention
from
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physi/chemisorption on the surface of carbons. However, the solid-state diffusion
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controlled intercalation mechanism results in the significantly reduced capacity at high charging/discharging rates. The capacity ratio for CNS/CNS-U/CNS-A between capacity above 0.2 V and capacity below 0.2 V vs Na/Na+ increased from 2.57/2.88/2.43 at 0.1 A g-1 to 3.64/4.03/3.03 at 1 A g-1 and 3.35/3.72/2.50 at 5 A g-1,
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further confirming the large contribution from physi/chemisorption at high rates. It is worth to note that the capacity ratio between capacity above 0.2 V and capacity below 0.2 V can be ranged as CNS-U > CNS > CNS-A, which is the same sequence of the
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content of nitrogen and oxygen in carbons, confirming the existence of functional
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groups is favorable for enhancing charge storage. The excellent electrochemical performance for CNS and CNS-U should be
attributed to the combined characteristics of the interconnected “openness” carbon nanosheets, functional groups, large surface area, and dilated graphitic interlayers. First of all, the interconnected carbon nanosheets with open structure makes the CNS and CNS-U favorable for the accessibility of the electrolyte. The ion diffusional limitation within the electrolyte is also important for high rate performance. There are
15
ACCEPTED MANUSCRIPT abundant micropores and mesopores on the carbon nanosheets, and such open 2D porous structure is expected to offer fast ion access to the surface of carbon with short diffusion distances. Secondly, at relative high potential, the charge storage mechanism
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can be supposed to surface electro-adsorption/desorption. For CNS, the 2190 m2 g-1 of usable micro- and mesopores ensures large sodium storage. For CNS-U, the oxygen-/nitrogen-containing functional groups on the surface can accelerate the
SC
surface redox reactions. As Raman analysis demonstrated, the introduction of
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functional groups can induce a more disordered carbon structure, suggesting an increased amount of defects on the graphitic layers [51]. Such defects can also be used as active sites for chemisorption of sodium ions. The surface area of CNS-U is significantly lower than that of CNS, but the capacity of CNS-U is comparable to that
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of CNS, giving us the evidence that the surface modification is a promising method to improve sodium storage. Thirdly, the large interlayer spacing facilitates sodium ion transport and storage between graphitic layers, which is critical for the larger sodium
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ions. It is worth to emphasize that high surface area carbon with highly defective sites
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can enhance the sodium storage capacity with the sacrificing of Coulombic efficiency. Therefore, a balance should be made among porosity, graphitization, and surface modification for carbon materials in order to achieve superior electrochemical performance with high capacity, high Coulombic efficiency, and excellent rate capability.
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ACCEPTED MANUSCRIPT 4. Conclusions Surface modification can be used as an efficient strategy to improve the performance of sodium ion batteries. Carbon nanosheets with high surface area (2190
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m2 g-1) and dilated graphitic layers deliver a high capacity of 192 and 62 mAh g-1 at 1 and 10 A g-1. The surface area was largely decreased to 840 m2 g-1, and abundant nitrogen- and oxygen- containing functional groups were detected in carbon
SC
nanosheets after urea treatment. However, N, O-doped carbon nanosheets show
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similar electrochemical performance with the as-prepared carbon nanosheets, suggesting the pseudocapacitive effect by surface redox reaction of functional groups and graphitic defects. The carbon nanosheets with and without urea treatment can deliver a charge capacity of 163 and 172 mAh g-1 with the initial capacity retention of
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50-60% after 2000 cycles at 1 A g-1. Moreover, the initial Coulombic efficiency of urea treated carbon nanosheets is increased from 34.8 % to 44.9 % owing to the decreased surface area. This work clearly indicates that carbon materials with suitable
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porous structure, heteroatom-doping and graphitic structure can be used as high
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performance anode materials for sodium ion batteries.
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ACCEPTED MANUSCRIPT Acknowledgement This work was funded by National Natural Science Foundation of China (No. 21471139,
and
51402272),
China
Postdoctoral
Science
Foundation
(No.
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2014M560581 and and 2015T80747), Shandong Province Outstanding Youth Scientist Foundation Plan (No. BS2014CL024), Qingdao Postdoctoral Science Foundation, Seed Fund from Ocean University of China and Fundamental Research
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Appendix A. Supplementary data
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Funds for the Central Universities.
Supplementary data associated with this article can be found, in the online version, at
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doi:******.
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ACCEPTED MANUSCRIPT [49] H.W. Song, N. Li, H. Cui, C.X. Wang, Nano Energy 4 (2014) 81-87. [50] W.Y. Tsai, R. Lin, S. Murali, L.L. Zhang, J.K. McDonough, R.S. Ruoff, P.L. Taberna, Y. Gogotsi, P. Simon, Nano Energy 2 (2013) 403-411.
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ACCEPTED MANUSCRIPT Figure captions Fig. 1 High-resolution XPS O1s spectra of (a) CNS and (b) CNS-U samples. High-resolution XPS N1s spectra of (c) CNS and (d) CNS-U samples. Fig. 2 (a) Nitrogen adsorption-desorption isotherms of CNS and CNS-U. (b) Pore size
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distributions calculated from the adsorption isotherms using DFT method. (c) XRD patterns of CNS and CNS-U specimens. (d) Raman spectra of CNS and CNS-U.
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Fig. 3 Low-magnification TEM micrographs of (a) CNS and (c) CNS-U, highlighting the sheet-like structures. High-resolution TEM micrographs of (b) CNS and (d)
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CNS-U, demonstrating the porous and partially graphitic structures.
Fig. 4 Electrochemical performance of CNS and CNS-U as anodes for sodium ion battery. CV curves of (a) CNS and (b) CNS-U electrodes between 0.01 and 3 V at a scan rate of 0.1 mV s-1. Galvanostatic charge/discharge profiles of (c) CNS and (d) CNS-U at a current density of 0.1 A g-1. Rate capability of (e) CNS and (f) CNS-U
capacity).
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electrodes at various current densities (solid, discharge capacity; hollow, charge
Fig. 5 (a) Cycling performance and (b) related Coulombic efficiency of CNS and
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CNS-U anodes, at a current density of 1 A g-1.
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ACCEPTED MANUSCRIPT Table 1 Relative surface concentrations of carbon, nitrogen and oxygen species obtained by fitting the XPS spectra.
6.70 15.06 24.58
N-5 (%)
O 1s O-I (%) O-II (%)
O-III (%)
100 75.42
33.08 29.92
39.45
66.92 30.63
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CNS 92.35 0.95 CNS-U 80.76 4.18
N 1s N-6 (%)
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Sample C N O (at%) (at%) (at%)
Table 2 Physical parameters for CNS and CNS-U. SBETa (m2 g-1)
Vtb (cm3 g-1)
Dmeanc (nm)
CNS
2190
1.36
1.65
CNS-U
840
0.62
1.74
Pore vol in cm3 g-1 d and (pore vol%)
d002 (nm)
IG/ID e
V<2 nm
V>2 nm
0.64 (55.7)
0.51 (44.3)
0.37
0.45
0.27 (56.3)
0.21(43.7)
0.37
0.32
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Sample
Surface area was calculated with Brunauer-Emmett-Teller (BET) method.
b
The total pore volume was determined at a relative pressure of 0.98-0.99.
d
The weighted mean pore size was calculated from: D mean
n
∑ = ∑
i =1 n
di vi
.
v i =1 i
The volume of pores smaller than 2 nm (V<2 nm) and pores larger than 2 nm (V>2 nm)
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c
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a
was obtained by DFT analysis. e
ID and IG are the integrated intensities of D- and G-band.
25
ACCEPTED MANUSCRIPT Fig. 1
b
CNS-U O 1s
542
540
538
536
534
532
530
528
542
526
540
536
534
532
530
528
526
396
394
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d
CNS-U N 1s
Intensity (a.u.)
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CNS N 1s
Intensity (a.u.)
538
Binding Energy (eV)
Binding Energy (eV)
c
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Intensity (a.u.)
CNS O 1s
Intensity (a.u.)
a
406
404
402
400
398
396
394
408
406
404
402
400
398
Binding Energy (eV)
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Binding Energy (eV)
26
ACCEPTED MANUSCRIPT
b 0.08 dV/dD Pore Volume (cm g nm )
-1
800
CNS CNS-U
0.06
600
adsorption desorption
CNS CNS-U
400 200 0 0.0
0.2
0.4
0.6
0.8
0.04
0.02
0.00 1
1.0
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c
CNS
30
40
50
60
80
500
CNS CNS-U
1000 1500 2000 2500 3000 3500 4000 -1 Raman Shift (cm )
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2 theta (degrees)
70
Intensity (a.u.)
CNS-U
G-band D-band
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Intensity (a.u.)
d
20
10
Relative Pressure (P/Po)
Relative Pressure (P/Po)
10
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3
3
-1
-1
a 1000 Quantity Adsorbed (cm g )
Fig. 2
27
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b
c
d
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a
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Fig. 3
28
ACCEPTED MANUSCRIPT Fig. 4 0.50
0.50
b
0.25 -1
Current (A g )
-1
Current (A g )
0.25 0.00 1st 2nd 5th 10th
-0.25 -0.50 -0.75 0.0
0.5
1.0
1.5
2.0
2.5
0.00
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a
1st 2nd 5th 10th
-0.25 -0.50 -0.75 0.0
3.0
0.5
1.0
+
3.0
d
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Voltage (V vs. Na /Na)
2.0
+
+
Voltage (V vs. Na /Na)
3.0 2.5
2.5 1st 2nd 5th 10th
1.5 1.0 0.5
200
400
600
800
2.0
3.0
1st 2nd 5th 10th
1.5 1.0 0.5
0
1000 1200 1400
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0
200
400
0.1 A g
-1
-1
EP
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0.2 A g -1 0.5 A g -1 1Ag -1 2Ag -1 5Ag -1 10 A g
300
0.1 A g
-1
0
0
20
40
1000 1200 1400
1500 1200
-1
-1
900
f Capacity (mAh g )
1200
800
Capacity (mAh g )
Capacity (mAh g )
e 1500
600
-1
-1
Capacity (mAh g )
2.5
0.0
0.0
600
2.0
Voltage (V vs. Na /Na)
SC
c
1.5
+
Voltage (V vs. Na /Na)
900 600
0.1 A g
-1
-1
0.2 A g -1 0.5 A g -1 1Ag -1 2Ag -1 5Ag -1 10 A g
300
0.1 A g
-1
0 60
Cycle Number
80
100
0
20
40
60
80
100
Cycle Number
29
ACCEPTED MANUSCRIPT Fig. 5
a 500
300 200 100 0
100 80 60
CNS CNS-U
40 20 0
0
500
1000
1500
2000
Cycle Number
RI PT
CNS-discharge CNS-charge CNS-U-discharge CNS-U-charge
-1
0
500
1000
1500
2000
Cycle Number
AC C
EP
TE D
M AN U
SC
Capacity (mAh g )
400
Coulombic Efficiency (%)
b 120
30
ACCEPTED MANUSCRIPT Highlights
Nitrogen and oxygen co-doped carbon nanosheets can be prepared by using urea treatment. The as-obtained and functionalized carbon nanosheets exhibit excellent electrochemical performance. post-treatment
method
provides
a
promising
way
SC
The
EP
TE D
M AN U
high-performance carbon-based anode for sodium ion batteries.
AC C
RI PT
to
achieve