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Pitch-derived N-doped porous carbon nanosheets with expanded interlayer distance as high-performance sodium-ion battery anodes
Fuel Processing Technology 177 (2018) 328–335
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Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc
Research article
Pitch-derived N-doped porous carbon nanosheets with expanded interlayer distance as high-performance sodium-ion battery anodes ⁎,1
Mingyuan Hao1, Nan Xiao
, Yuwei Wang, Hongqiang Li, Ying Zhou, Chang Liu, Jieshan Qiu
T
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State Key Lab of Fine Chemicals, Liaoning Key Lab for Energy Materials and Chemical Engineering, PSU-DUT Joint Center for Energy Research, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Coal tar pitch Soft carbon Carbon nanosheets Nitrogen doping Sodium-ion battery
Soft carbons with high conductivity have potential advantages as high-performance sodium-ion battery anodes, however, they suffer from small interlayer distance and low hereoatom content. In this work, porous carbon nanosheets (PCNS1000) with tunable microstructure, pore structure and chemical composition were prepared from coal tar pitch through a two-step process involving NaCl template method and NH3 treatment. PCNS1000 possesses an expanded interlayer distance of 3.82 Å, mesopore structure with average pore size of 5.7 nm, and a high nitrogen content of 4.17 wt%. Moreover, PCNS1000 retains soft carbon feature to a certain extent and consequent relative high conductivity. Benefiting from its appropriate structure and chemical composition, PCNS1000 exhibits a high reversible capacity of 270 mAh g−1 at 100 mA g−1 in sodium-ion half cells and excellent rate capability of 124 mAh g−1 at large current of 10 A g−1. After an ultralong charge-discharge cycling of 10,000 times, a high capacity retention of 86% was achieved, indicating its excellent electrochemical stability. The full cell assembled by Na3V2(PO4)3 cathode and PCNS1000 anode delivers a high capacity of 265 mAh g−1 at 100 mA g−1, with a good cycling ability of 89% capacity retention after 100 cycles at 1.0 A g−1.
1. Introduction Driven by the increasingly serious energy crisis and environmental pollution, a variety of renewable and clean energy resources such as wind, solar and tide are vigorously pursued worldwide to replace traditional fossil fuels. Unfortunately, these new energy resources are intermittent in time and dispersive in space. From a practical point of view, it is important to reserve the on-peak electricity and releasing the stored energy during the off-peak period [1,2]. In order to integrate these renewable energies into the electrical grid, matching large-scale energy storage systems (ESSs) are in urgent need. To meet the requirements for large-scale ESSs, several principles need to be considered: (1) excellent stability to endure long-term repeated chargedischarge cycles; (2) good rate performance for the very fast energy harvest and the large current output; (3) low material and manufacturing cost for large-scale utilization [3–7]. Among the various emerging energy storage technologies, lithium-ion batteries (LIBs) have been successfully used as power sources for portable electronic devices and electric vehicle due to their high energy density, long cycle life and reasonable charge potential profile [8]. However, the low abundance and uneven distribution of lithium greatly restrict LIB's prospects in
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large-scale power storage applications. Compared with lithium, sodium possesses similar electrochemical properties but much more abundant in earth's crust. Therefore, sodium-ion batteries (SIBs) have been expected to be an alternative for future stationary large-scale ESSs [9]. Nevertheless, the radius of Na+ ion is much larger than that of Li+ (1.02 Å vs 0.76 Å in radius) so that it cannot simply select electrode materials, e.g. graphite, for SIBs from those for LIBs [10–13]. Therefore, to find appropriate host materials meeting all the above requirements is crucial to realize the practical application of SIBs in large-scale ESSs. For the past few years, great efforts have been devoted to pursue high performance SIBs electrodes. In the case of anode materials, metal oxides [14], metal (e.g. Sn, Sb) [15], layered metal sulfides [16,17] and non-graphitizing carbons [18,19] have been investigated. Although metal and their sulfides exhibit high capacity, their cycling stability is poor due to the large volume expansion and pulverization of electrode materials. Besides, the high price and complicated preparation process also restrict their application in EESs. As one promising category, nongraphitizing carbons including hard carbons and soft carbons have been widely studied due to their eco-friendliness, abundance, thermal stability, and high sodium storage capacity [20,21]. Benefiting from their intrinsic large interlayer distance and ample lattice defects, hard
Corresponding authors at: No. 2 Ling Gong Road, High Technology Zone, Dalian 116024, China. E-mail addresses: [email protected] (N. Xiao), [email protected] (J. Qiu). Mingyuan Hao and Nan Xiao contributed equally to this work.
https://doi.org/10.1016/j.fuproc.2018.05.007 Received 4 February 2018; Received in revised form 4 May 2018; Accepted 4 May 2018 0378-3820/ © 2018 Elsevier B.V. All rights reserved.
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oxalic acid (V2O5: oxalic acid = 1:4, molar ratio) was also added in order to reduce V5+ to V3+. The mixture was ball milled for 8 h in Ar atmosphere and then dried at 80 °C for 3 h. Finally, the mixture was heated at 800 °C for 24 h in Ar atmosphere to yield NVP.
carbons exhibit high capacity and long lifespan as SIBs anode. In general, the hard carbons are commonly prepared by pyrolysis of macromolecular organic precursors, such as carbohydrates or synthetic polymers [19,22]. These highly disordered hard carbons usually show poor electrical conductivity, which is adverse for the charge transfer during charge/discharge process, resulting in an unsatisfied rate performance [21]. On the other hand, soft carbons rich in sp2 carbon show much higher electronic conductivity than hard carbons, which contributes to enhance their rate performance [20,23–25]. However, every coin has its two sides, the small interlayer distance and low level of heteroatom of soft carbon result in a low reversible capacity (e.g., ≈100 mA h g−1 at 75 mA g−1) and poor rate performance compared to hard carbon which is detrimental to large-scale ESSs [26]. It is generally accepted that enhancing the heteroatom content and interlayer distance can improve the electrochemical performance of carbon anodes for SIBs. Therefore, an effective strategy to synthesize soft carbons with suitable interlayer distance and hereoatom doping level for high performance SIB anodes is highly demanded. In this work, high-performance carbon anode was prepared through a two-step process including template carbonization in Ar using coal tar pitch as soft carbon precursors and followed by heat treatment in NH3/ Ar. The first-step was designed to form a two-dimensional (2D) soft carbon intermediate with good electrical conductivity and the secondstep was intended to introduce appropriate N-dopant and enlarge the soft carbons' interlayer distance. NaCl particles which can be easily removed by water washing are used as template in the first step making this synthesis more environmentally friendly and industrializable. The materials' 2D morphology, high specific surface area and appropriate mesoporous structure benefit electrolyte penetration and shorten Na+ diffusion distance. Moreover, the doped heteroatoms and enlarged interlayer distance further enhance the reversible storage of Na+. Based on the above excellences, this novel carbon material shows high capacity, good rate performance and ultralong life span, which is believed to be a suitable power device for large-scale ESSs.
2.3. Material characterization The morphology of samples was characterized by a FEI Nova Nano SEM 450 field-emission scanning electron microscope (FESEM). FEI TF30 was employed to take transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) images. Raman spectra were recorded with a DXR Raman Microscope (Thermal Scientific), the laser excitation was 532 nm and the power was 1 mW. Xray diffraction patterns were recorded on an Empyrean (PANalytical B.V.) X-ray diffractometer (XRD) with Cu Kα radiation (λ = 1.5406 Å). The interlayer distance (d002) was calculated by XRD spectra using the Bragg equation. The pore structure was studied via N2 sorption isotherm using a physical adsorption instrument (Micromeritics ASAP 2020). Elemental analysis (EA) was performed with a vario EL III Element Analyzer (Elementar, German). X-ray photoelectron spectroscopy (XPS) was performed on Thermo ESCALAB 250XI with a monochromatic Al K X-ray source. 2.4. Electrochemical measurements The PCNS1000s, acetylene black, and polyvinylidene fluoride (PVdF) were mixed (7:2:1, mass ratio), then a slurry was formed with the addition of NMP. The slurry was uniformly coated on Cu substrates (9 μm in foil thickness) and dried at 100 °C under vacuum for 12 h. The active mass loading was controlled to about 0.85 mg cm−2. The volume of added electrolyte was controlled to about 150 μL. The cathode electrodes were made by coating NVP in the similar way on Al foil, the mass ratio of NVP to PCNS1000 was controlled around 4. Electrochemical performance was analyzed using CR2016 coin-type cells. The half and full cells were assembled in an argon-filled glovebox, with PCNS1000 as working electrode, glass fiber (Whatman GF/D) as separator, and sodium foils or NVP as the counter electrodes. 1 M NaClO4 in a 1:1 (v/v) mixture of ethylene carbonate and dimethyl carbonate was used as the electrolyte. Rate and cycling performance of the cells were conducted on a Land CT2001A battery tester between 0.01 and 2.80 V. Cyclic voltammetry (CV) was tested with a CHI 660A electrochemical workstation between the voltage window of 0.01 and 2.80 V at 0.1 mV s−1. Electrochemical impedance spectroscopy (EIS) was conducted on a Zahner-Elektrik IM6e electrochemical workstation by sweeping the frequency from 100 kHz to 10 mHz with AC amplitude of 5 mV.
2. Experimental 2.1. Synthesis of PCNS1000 Coal tar pitch was supplied by Ansteel, China. Elemental analysis shows that the coal tar pitch is composed of 92.2% C, 4.4% H, and 1.1% N (weight percent). The recrystallized NaCl salt with an average particle size of 10 μm was used as template (see Experimental section; Fig. S1, Supporting information). In a typical run, 1 g coal tar pitch was dissolved in 30 mL N-methyl-2-pyrrolidone (NMP) under violently stirring. 10 g NaCl template was added into the obtained solution. After stirred for 1 h, the mixture was evaporated at 205 °C to get rid of the solvent, followed by heating at 700 °C for 2 h in Ar atmosphere. After the removal of NaCl template with deionized water and dried in oven at 100 °C for 12 h, the intermediates were taken as CNS700. The prepared CNS700 was further heated at 1000 °C with a heating rate of 5 °C min−1 for 2 h in NH3/Ar (1:1 (v/v)) atmosphere with a flow rate of 150 mL min−1 to obtain the PCNS1000. The carbon yield of PCNS1000 is 35.1 wt%. As comparison, PCNS with various NH3 treatment temperatures were studied. The sample heated at 900 °C in NH3/Ar was named as PCNS900. The carbon yield of PCNS900 is 50.6 wt%. By raising the NH3 treatment temperature to 1100 °C, no sample was left due to the severe etching reaction. CNS1000 was prepared by carbonizing at 1000 °C for 2 h in Ar atmosphere. The carbon yield of CNS1000 is 55.2 wt%.
3. Results and discussion The morphology of PCNS1000 is studied by FESEM. As shown in Fig. S1b, the PCNS1000 contains rough carbon nanosheets, which are interconnected to form micron-sized macropores. The size of macropore is consistent with the dimension of NaCl template (Fig. S1a), indicating the templating and confining role played by these micron-sized salt particles in the formation of the 2D carbon nanosheets. The macropores can reserve the electrolyte which shortens the diffusion distance of sodium ion. Besides, the interconnected carbon nanosheets form three dimensional conductive network, which is expected to enhance the transport rate of electrons. The rapid electron transfer and facile ion transport are beneficial to boosting the rate performance of electrode materials. Higher magnification SEM image (Fig. 1a) shows a highly wrinkled and rough surface of PCNS1000. While in the case of the counterparts carbonized in Ar, CNS700 (Fig. S1c, d) and CNS1000 (Fig. 1d) possess a rather smooth surface. The TEM results further verify the effect of NH3 treatment on the microstructure of the carbons. The TEM image of PCNS1000 (Fig. 1b) reveals that they display a rough
2.2. Synthesis of NVP The Na3V2(PO4)3 (NVP) was synthesized by a one-step solid state reaction as follows: A stoichiometric ratio of V2O5 and NaH2PO4 was mixed in a ball milling vial. Sugar was added as carbon precursor and 329
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Fig. 1. SEM images of (a) PCNS1000 and (d) CNS1000; TEM images of (b) PCNS1000 and (e) CNS1000; HRTEM images of (c) PCNS1000 and (f) CNS1000.
interlayer distance of pitch-derived carbon by high temperature NH3 treatment can promote the electrochemical performance of carbons for SIBs. Raman spectra of the samples in Fig. 2d exhibit the G band at 1590 cm−1 and D band at 1350 cm−1, corresponding to the in-plane vibration of sp2 carbon atoms and disordered structures, respectively. For PCNS1000, The relative intensity (ID/IG) is 1.09, much higher than 0.97 of CNS1000, indicative of excess defects introduced by NH3 treatment which results in the domination of disordered structure [27,28]. The results of XRD and Raman further demonstrate that the NH3 treatment can enlarge the interlayer distance and create abundant defects in the PCNS1000, which can provide more open channels and active sites for Na+ insertion and consequentially improve the performance of carbon anodes. Fig. 2c present the nitrogen adsorption–desorption isotherms of the coal tar pitch-derived carbons. It can be clearly observed that the PCNS1000 shows a typical IV isotherm with a hysteresis loop at relative pressure P/Po between 0.4 and 1.0 The specific Brunner−Emmet−Teller (BET) surface area of PCNS1000 is 546 m2 g−1 much larger than that of CNS1000 (30 m2 g−1), indicating the effect of NH3treatment on the pore structure of carbons. The pore-size distribution (Fig. 2d) shows that PNCS1000 has abundant mesopores from 2 nm to 50 nm which occupy as much as 79.6% of total pore volume. The large BET area benefits the contact of electrolyte to anode surface, and appropriate mesoporous structure shortens the diffusion distance of Na+ [24,29,30]. The above results indicate that the NH3 treatment process can enlarge the interlayer distance, introduce defects as well as form appropriate mesopore into PCNS1000. The obtained carbons, however, still remain the typical soft carbon nature to a certain extent even after the NH3 treatment. These structural evolutions are expected to facilitate ion transport and fast electron transfer as well as provide abundant Na+ storage sites. Besides pore structure, the NH3 treatment also modified the carbons' elemental composition and surface chemistry which can be characterized by elemental analysis and XPS. The elemental analysis in Table S1 shows that the N content in CNS700 is 1.60 wt%, which is derived from the precursor coal tar pitch (N content 1.10 wt%). However, after NH3 treatment at 1000 °C, N content of PCNS1000 increased to 4.17 wt% which is much higher than that of CNS1000 (1.81%), indicating the N-
surface covered with wrinkles. As comparison, the CNS1000 possesses smooth surface, which is consistent with the SEM image. The difference of samples' surface topography indicates the etching role played by NH3. By NH3 etching, the carbon yield of PCNS1000 (35.1 wt%) is much lower than that of CNS1000 (55.2 wt%). Besides modifying the surface topography of the carbon sheets, the NH3 treatment also affects their microstructure and chemical composition significantly. HRTEM was used to characterize the microstructure of carbons with or without NH3 treatment. By comparing the HRTEM image of PCNS1000 (Fig. 1c) with that of CNS1000 (Fig. 1f), the effect of NH3 treatment on the microstructure of carbons is quite clear. In the case of PCNS1000, as shown in Fig. 1c, nanosized graphitic clusters with several carbon layers homogeneously distribute in carbon matrix, indicating that the PCNS1000 remains the typical soft carbon nature to a certain extent after NH3 treatment. However, these graphitic clusters in PCNS1000 are smaller and more poorly aligned than those in CNS1000, indicating that the NH3 treatment can impede the development of order structure and reduce the size of nanocrystalline clusters in some degree. Meanwhile, as shown in Fig. 1c, the interlayer distance of PCNS1000 is enlarged to 0.39 nm which is much larger than 0.35 nm of CNS1000 (Fig. 1f). The SEM and TEM results indicate that the NH3 treatment can tune the morphology and microstructure of carbons by introducing defects into carbons and enlarging the interlayer distance [27,28], by which more storage sites are created and the Na+ transfer resistance is decreased. XRD and Raman were carried out to gain more detailed microstructure of carbons. XRD patterns for PCNS1000 and CNS1000 are shown in Fig. 2a. Both samples exhibit peaks at around 24.0° and 43.6°, corresponding to the (002) and (100) reflections of carbon material, respectively. Compared with CNS1000, the (002) peak of PCNS1000 shifts to a lower angle with a wider distribution, indicating the enhanced interlayer distance and more disordered structure, which is in good agreement with the HRTEM images. Average interlayer spacing of PCNS1000 is calculated to be 0.382 nm according to the Bragg's Law, much larger than those of CNS1000 (0.343 nm) and graphite (0.335 nm). According to the previous report, interlayer distance which is greater than 0.37 nm allows reversible intercalation/deintercalation of sodium ion [5,6]. It is reasonable to expect that the enlarged 330
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Fig. 2. (a) XRD patterns and (b) Raman spectra of CNS1000 and PCNS1000; (c) N2 adsorption–desorption isotherms of PCNS1000 and CNS1000; (e) pore size distribution curves of PCNS1000 and CNS1000.
The sodiation/desodiation behavior of the PCNS1000 was investigated in 2016 coin-type cells. Fig. 4a shows the CV curves of PCNS1000 at a scanning rate of 0.1 mV s−1 in the voltage range of 0.01–2.8 V. In the first cycle, the cathodic peak at around 0.5 V can be due to the formation of a solid electrolyte interphase (SEI) film, and it disappears after the initial discharge. The peak at around 0.01 V represents the insertion process of Na+ between the carbon layers. After the first cycle, the CV curves almost overlap, indicating the stable reversibility of Na+ storage. Fig. 4b displays the charge-discharge profiles of the PCNS1000 anode in the working voltage window 0.01–2.8 V at 0.1 A g−1. The initial discharge capacity is 448 mAh g−1, while the initial charge capacity is 296 mAh g−1, corresponding to a coulombic efficiency of 66%. The PCNS1000 exhibits a specific capacity of 313 mAh g−1 for the third cycle and 270 mAh g−1 for the tenth cycle. As for the subsequent cycles, it can be stabilized at 265 mAh g−1. The capacity loss is due to the irreversible reaction between sodium and surface functional groups as well as the decomposition of electrolyte and inevitable formation of SEI film. Fig. 4c exhibits the rate performance of PCNS1000 and CNS1000 at the current densities between 0.1 and 10 A g−1. The PCNS1000 electrode delivers a discharge specific capacity of 302 mAh g−1 at 0.1 A g−1, then is stabilized at 270 mAh g−1. Even cycled at higher current densities of 0.2, 0.5, 1, 2, and 5 A g−1, high capacities of 228, 186, 167, 150, and 133 mAh g−1 can be still retained, respectively. In contrast, the CNS1000 sample delivered reversible capacities of 174, 137, 104, 91, 83 and 75 mAh g−1 at current densities of 0.1, 0.2, 0.5, 1, 2 and 5 A g−1, respectively. In particularly, even at an ultra-high discharge current density of 10 A g−1, the PCNS1000 still remains a capacity of 124 mAh g−1, corresponding to a capacity retention of about 46%. CNS1000, by contrast, remains capacity of 63 mAh g−1 at a current density of 10 A g−1,
doping role played by NH3 treatment. XPS was performed to analyze the elemental composition and bonding configurations of the prepared carbon samples. The XPS spectra shown in Fig. 3a indicate that both PCNS1000 and CNS1000 possess three peaks centred at 284.5 eV, 398.1 eV and 532.1 eV, which are identified as C 1 s, N 1 s and O 1 s, respectively. The N content in PCNS1000 is 4.3 at.%, much higher than 1.5 at.% of CNS1000. The high resolution N 1 s spectra of PCNS1000 (Fig. 3b) and CNS1000 (Fig. S2a) can be resolved into three peaks, pyridinic N (N-6, 397.9 eV), pyrrolic N (N-5, 399.6 eV), and quaternary N (N-Q, 400.7 eV). The area percentages of the three fitting peaks in PCNS1000 are successively 52.3%, 25.8% and 21.9%%, representing the content percentages of the corresponding three N pieces. For CNS1000, the content percentages of the corresponding three N species are 21.8%, 8.1% and 70.1%. The differences in N species proportion and N content are closely related with heat treatment atmosphere. The high-resolution C 1 s spectra in Fig. 3c further proves the existence of C]N, with a content of 28.4%. For CNS1000 (Fig. S2b) the content of C]N (13.6%) is much lower than that in PCNS1000. Fig. 3d schematically exhibits the different types of nitrogen configurations. The doped N atoms can create more defects and enhance the electronic conductivity of carbons by contributing extra electrons to the π-conjugated system. The electrochemical properties of carbon electrode can be significantly enhanced by the nitrogen-containing groups, especially pyridinic N [31–33]. The NH3 treatment can easily bring more pyridinic N and pyrrolic N which are expected to create some extra defects in the PCNS1000 which provides more active sites for Na+ storage [32–36]. That could be one of the reasons for the improved specific capacity of PCNS1000 compared with CNS1000. The doped N can enlarge the spacing distance of carbon material which promote reversible intercalation/deintercalation of sodium ion [37,38]. 331
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Fig. 3. (a) XPS spectrum of PCNS1000 and CNS1000; high-resolution XPS spectrum of (b) N 1s and (c) C 1s of PCNS1000; (d) Schematic configurations of the heteroatom doping in PCNS1000.
cycling stability of both two samples is mostly due to the 2D nanosheets structure [41–43]. Fig. 4e shows the long cycling life of PCNS1000. After activated at 100 mA g−1 for the first 20 cycles, a charge capacity of 144 mAh g−1 can be delivered at 2 A g−1. After an ultra-long 10,000 cycles, stable capacity retention of 86% can still be retained, corresponding to a capacity loss as low as 0.0014% per cycle. Therefore, PCNS1000 derived from coal tar pitch, a low cost by-product of coking industry, exhibits the superior sodium storage capability and extremely excellent cycle stability. For comparison, PCNS900 with lower temperature NH3 treatment were also synthesized. The carbon yield of PCNS900 (50.6 wt%) is much larger than that of CNS1000 (35.1 wt.) indicating a deficient etching reaction at low NH3 treatment temperature. XRD patterns for PCNS900 and PCNS1000 are compared in Fig. S4a. The average interlayer spacing of PCNS900 is calculated to be 0.349 nm according to the Bragg's Law, much smaller than that of PCNS1000 (0.382 nm) due to the deficient etching reaction. The small interlayer distance restricts the intercalation/deintercalation of sodium ion resulting in a much lower specific capacity of PCNS900 than that of PCNS1000 (Fig. S4b). The enhanced capacity, superior cycling ability and good rate capability of PCNS1000 can be ascribed to the unique structure as a result of its soft carbon precursor and preparation process. Firstly, the interconnected 2D nanosheet structure and soft carbon feature supplies a short ion diffusion distance and fast electrons transport network.
corresponding to a capacity retention of only 35%. When the current is turned back to 0.1 A g−1, the specific capacity can be recover up to 269 mAh g−1 indicating the excellent robustness of the electrode for , Na+ storage. The EIS of PCNS1000 and CNS1000 are shown in Fig. S3a. The small diameter of the semicircle in the medium frequency region indicates the low charge transfer resistance of both PCNS1000 and CNS1000. It is noteworthy that charge transfer resistance of the PCNS1000 is much smaller than that of previously reported hard carbon anodes [39,40], which can be attributed to the remained soft carbon nature of PCNS1000. However, the sodium-ion diffusion coefficients of PCNS1000 and CNS1000 calculated according to eq. (S1) are 9.23 × 10−12 cm2 S−1 and 9.89 × 10−13 cm2 S−1, respectively. The sodium-ion diffusion coefficient of PCNS1000 is 9.3 times higher than that of CNS1000, suggesting good Na+ transport properties of PCNS1000. This amazing rate performance can be attributed to its superior structure including the enlarged interlayer distance, the abundant active sites [25], the appropriate mesopore and its soft carbon nature. Furthermore, PCNS1000 possesses much higher specific capacities than that of CNS1000 anode at all the measured current densities which can be attributed to the synergistic effect of expanded interlayer distance and N doping. At the cycling current of 500 mA g−1, both of the two carbon anodes exhibit stable capacities (Fig. 4d). PCNS1000 still exhibits a high specific capacity of 176 mAh g−1 after 1000 cycles corresponding to a high capacity retention of 93%. The excellent
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Fig. 4. (a) CV curves of PCNS1000 between 0.01 and 2.8 V at 0.1 mV s−1; (b) Galvanostatic discharge-charge profiles of PCNS1000 at 100 mA g−1; (c) Rate capability of the electrodes at different current densities; (d) Cycling performance of the electrodes at 0.5 A g−1; (e) Long-term cycling stability of PCNS1000 at 2 A g−1.
and 104 mAh g−1, respectively, can remain for use. After cycled at 2A g−1 for 100 times, high capacity retention of 89% can still be achieved (shown in Fig. 5b), indicating its great practical use potential. These capacity values in this work are much higher than most of the reported carbon-based anode in full cells (as summarized in Fig. 5c) [18,20,23,44]. The full cell could be charged to 3.7 V, enabling to light a red light-emitting diode (Fig. 5d).
Secondly, the suitable mesoporous structure greatly benefits the accessibility of the electrolyte and rapid ion diffusion/transport. Thirdly, the enhanced interlayer distance allows the reversible intercalation/ deintercalation of sodium ion in PCNS1000, overcoming the kinetic limitations. Finally, the doped N can also give rise to the improved capacities. The superior electrochemical performance of the PCNS1000 in halfcell encourages us to further measure its performance in full cells. NVP was chosen as the cathode material for its high voltage plateau and stability, which delivered 93.4 mAh g−1 at 0.1 A g−1 and a plateau of 3.4 V (Fig. S5a). It is noteworthy that even at a large charge-discharge current of 2 A g−1, the NVP cathode can be stably cycled over 950 times (Fig. S5b). As shown in Fig. 5a, in the voltage window of 0.5–3.9 V, the PCNS1000//NVP full cell delivers a high dischange capacity of 265 mAh g−1 at 0.1 A g−1 (calculated based on the anode mass, the same as below), with a voltage plateau around 2.7 V. At the current of 0.2, 0.5, 1.0 and 2.0 A g−1, stable discharge capacities of 228, 153, 125
4. Conclusions In summary, N-doped porous carbon nanosheets, which are prepared by a two-step process, exhibits high performance when used as sodium-ion battery anode. The results of this study show that NH3 treatment is a pivotal factor for controlling microstructure and chemical composition of pitch-derived carbons. Through NH3 treating, PCNS1000 exhibits a large interlayer distance of 3.82 Å, abundant mesoporous structure, and high nitrogen doping content. Combining its 333
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Fig. 5. (a) Rate performance of NVP//PCNS1000 full cell at current densities from 0.1 A g−1 to 2 A g−1; (b) cycling stability of the full cell at 2 A g−1; (c) rate capacity of PCNS1000 and other carbon-based SIBs anodes; (d) optical image of a red LED lit by the full cell.
soft carbon nature, PCNS1000 exhibits high reversible capacity (270.0 mAh g−1 at 0.1 A g−1), improved rate capacity and extremely long lifespan (10,000 times with a capacity loss of 0.0014% per cycle) in sodium-ion battery half cells. When extended to full cell of NVP// PCNS1000, it exhibits high reversible capacity of 265 mAh g−1 at 0.1 A g−1 and high average potential of 2.7 V.
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Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc
Research article
Pitch-derived N-doped porous carbon nanosheets with expanded interlayer distance as high-performance sodium-ion battery anodes ⁎,1
Mingyuan Hao1, Nan Xiao
, Yuwei Wang, Hongqiang Li, Ying Zhou, Chang Liu, Jieshan Qiu
T
⁎
State Key Lab of Fine Chemicals, Liaoning Key Lab for Energy Materials and Chemical Engineering, PSU-DUT Joint Center for Energy Research, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Coal tar pitch Soft carbon Carbon nanosheets Nitrogen doping Sodium-ion battery
Soft carbons with high conductivity have potential advantages as high-performance sodium-ion battery anodes, however, they suffer from small interlayer distance and low hereoatom content. In this work, porous carbon nanosheets (PCNS1000) with tunable microstructure, pore structure and chemical composition were prepared from coal tar pitch through a two-step process involving NaCl template method and NH3 treatment. PCNS1000 possesses an expanded interlayer distance of 3.82 Å, mesopore structure with average pore size of 5.7 nm, and a high nitrogen content of 4.17 wt%. Moreover, PCNS1000 retains soft carbon feature to a certain extent and consequent relative high conductivity. Benefiting from its appropriate structure and chemical composition, PCNS1000 exhibits a high reversible capacity of 270 mAh g−1 at 100 mA g−1 in sodium-ion half cells and excellent rate capability of 124 mAh g−1 at large current of 10 A g−1. After an ultralong charge-discharge cycling of 10,000 times, a high capacity retention of 86% was achieved, indicating its excellent electrochemical stability. The full cell assembled by Na3V2(PO4)3 cathode and PCNS1000 anode delivers a high capacity of 265 mAh g−1 at 100 mA g−1, with a good cycling ability of 89% capacity retention after 100 cycles at 1.0 A g−1.
1. Introduction Driven by the increasingly serious energy crisis and environmental pollution, a variety of renewable and clean energy resources such as wind, solar and tide are vigorously pursued worldwide to replace traditional fossil fuels. Unfortunately, these new energy resources are intermittent in time and dispersive in space. From a practical point of view, it is important to reserve the on-peak electricity and releasing the stored energy during the off-peak period [1,2]. In order to integrate these renewable energies into the electrical grid, matching large-scale energy storage systems (ESSs) are in urgent need. To meet the requirements for large-scale ESSs, several principles need to be considered: (1) excellent stability to endure long-term repeated chargedischarge cycles; (2) good rate performance for the very fast energy harvest and the large current output; (3) low material and manufacturing cost for large-scale utilization [3–7]. Among the various emerging energy storage technologies, lithium-ion batteries (LIBs) have been successfully used as power sources for portable electronic devices and electric vehicle due to their high energy density, long cycle life and reasonable charge potential profile [8]. However, the low abundance and uneven distribution of lithium greatly restrict LIB's prospects in
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1
large-scale power storage applications. Compared with lithium, sodium possesses similar electrochemical properties but much more abundant in earth's crust. Therefore, sodium-ion batteries (SIBs) have been expected to be an alternative for future stationary large-scale ESSs [9]. Nevertheless, the radius of Na+ ion is much larger than that of Li+ (1.02 Å vs 0.76 Å in radius) so that it cannot simply select electrode materials, e.g. graphite, for SIBs from those for LIBs [10–13]. Therefore, to find appropriate host materials meeting all the above requirements is crucial to realize the practical application of SIBs in large-scale ESSs. For the past few years, great efforts have been devoted to pursue high performance SIBs electrodes. In the case of anode materials, metal oxides [14], metal (e.g. Sn, Sb) [15], layered metal sulfides [16,17] and non-graphitizing carbons [18,19] have been investigated. Although metal and their sulfides exhibit high capacity, their cycling stability is poor due to the large volume expansion and pulverization of electrode materials. Besides, the high price and complicated preparation process also restrict their application in EESs. As one promising category, nongraphitizing carbons including hard carbons and soft carbons have been widely studied due to their eco-friendliness, abundance, thermal stability, and high sodium storage capacity [20,21]. Benefiting from their intrinsic large interlayer distance and ample lattice defects, hard
Corresponding authors at: No. 2 Ling Gong Road, High Technology Zone, Dalian 116024, China. E-mail addresses: [email protected] (N. Xiao), [email protected] (J. Qiu). Mingyuan Hao and Nan Xiao contributed equally to this work.
https://doi.org/10.1016/j.fuproc.2018.05.007 Received 4 February 2018; Received in revised form 4 May 2018; Accepted 4 May 2018 0378-3820/ © 2018 Elsevier B.V. All rights reserved.
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oxalic acid (V2O5: oxalic acid = 1:4, molar ratio) was also added in order to reduce V5+ to V3+. The mixture was ball milled for 8 h in Ar atmosphere and then dried at 80 °C for 3 h. Finally, the mixture was heated at 800 °C for 24 h in Ar atmosphere to yield NVP.
carbons exhibit high capacity and long lifespan as SIBs anode. In general, the hard carbons are commonly prepared by pyrolysis of macromolecular organic precursors, such as carbohydrates or synthetic polymers [19,22]. These highly disordered hard carbons usually show poor electrical conductivity, which is adverse for the charge transfer during charge/discharge process, resulting in an unsatisfied rate performance [21]. On the other hand, soft carbons rich in sp2 carbon show much higher electronic conductivity than hard carbons, which contributes to enhance their rate performance [20,23–25]. However, every coin has its two sides, the small interlayer distance and low level of heteroatom of soft carbon result in a low reversible capacity (e.g., ≈100 mA h g−1 at 75 mA g−1) and poor rate performance compared to hard carbon which is detrimental to large-scale ESSs [26]. It is generally accepted that enhancing the heteroatom content and interlayer distance can improve the electrochemical performance of carbon anodes for SIBs. Therefore, an effective strategy to synthesize soft carbons with suitable interlayer distance and hereoatom doping level for high performance SIB anodes is highly demanded. In this work, high-performance carbon anode was prepared through a two-step process including template carbonization in Ar using coal tar pitch as soft carbon precursors and followed by heat treatment in NH3/ Ar. The first-step was designed to form a two-dimensional (2D) soft carbon intermediate with good electrical conductivity and the secondstep was intended to introduce appropriate N-dopant and enlarge the soft carbons' interlayer distance. NaCl particles which can be easily removed by water washing are used as template in the first step making this synthesis more environmentally friendly and industrializable. The materials' 2D morphology, high specific surface area and appropriate mesoporous structure benefit electrolyte penetration and shorten Na+ diffusion distance. Moreover, the doped heteroatoms and enlarged interlayer distance further enhance the reversible storage of Na+. Based on the above excellences, this novel carbon material shows high capacity, good rate performance and ultralong life span, which is believed to be a suitable power device for large-scale ESSs.
2.3. Material characterization The morphology of samples was characterized by a FEI Nova Nano SEM 450 field-emission scanning electron microscope (FESEM). FEI TF30 was employed to take transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) images. Raman spectra were recorded with a DXR Raman Microscope (Thermal Scientific), the laser excitation was 532 nm and the power was 1 mW. Xray diffraction patterns were recorded on an Empyrean (PANalytical B.V.) X-ray diffractometer (XRD) with Cu Kα radiation (λ = 1.5406 Å). The interlayer distance (d002) was calculated by XRD spectra using the Bragg equation. The pore structure was studied via N2 sorption isotherm using a physical adsorption instrument (Micromeritics ASAP 2020). Elemental analysis (EA) was performed with a vario EL III Element Analyzer (Elementar, German). X-ray photoelectron spectroscopy (XPS) was performed on Thermo ESCALAB 250XI with a monochromatic Al K X-ray source. 2.4. Electrochemical measurements The PCNS1000s, acetylene black, and polyvinylidene fluoride (PVdF) were mixed (7:2:1, mass ratio), then a slurry was formed with the addition of NMP. The slurry was uniformly coated on Cu substrates (9 μm in foil thickness) and dried at 100 °C under vacuum for 12 h. The active mass loading was controlled to about 0.85 mg cm−2. The volume of added electrolyte was controlled to about 150 μL. The cathode electrodes were made by coating NVP in the similar way on Al foil, the mass ratio of NVP to PCNS1000 was controlled around 4. Electrochemical performance was analyzed using CR2016 coin-type cells. The half and full cells were assembled in an argon-filled glovebox, with PCNS1000 as working electrode, glass fiber (Whatman GF/D) as separator, and sodium foils or NVP as the counter electrodes. 1 M NaClO4 in a 1:1 (v/v) mixture of ethylene carbonate and dimethyl carbonate was used as the electrolyte. Rate and cycling performance of the cells were conducted on a Land CT2001A battery tester between 0.01 and 2.80 V. Cyclic voltammetry (CV) was tested with a CHI 660A electrochemical workstation between the voltage window of 0.01 and 2.80 V at 0.1 mV s−1. Electrochemical impedance spectroscopy (EIS) was conducted on a Zahner-Elektrik IM6e electrochemical workstation by sweeping the frequency from 100 kHz to 10 mHz with AC amplitude of 5 mV.
2. Experimental 2.1. Synthesis of PCNS1000 Coal tar pitch was supplied by Ansteel, China. Elemental analysis shows that the coal tar pitch is composed of 92.2% C, 4.4% H, and 1.1% N (weight percent). The recrystallized NaCl salt with an average particle size of 10 μm was used as template (see Experimental section; Fig. S1, Supporting information). In a typical run, 1 g coal tar pitch was dissolved in 30 mL N-methyl-2-pyrrolidone (NMP) under violently stirring. 10 g NaCl template was added into the obtained solution. After stirred for 1 h, the mixture was evaporated at 205 °C to get rid of the solvent, followed by heating at 700 °C for 2 h in Ar atmosphere. After the removal of NaCl template with deionized water and dried in oven at 100 °C for 12 h, the intermediates were taken as CNS700. The prepared CNS700 was further heated at 1000 °C with a heating rate of 5 °C min−1 for 2 h in NH3/Ar (1:1 (v/v)) atmosphere with a flow rate of 150 mL min−1 to obtain the PCNS1000. The carbon yield of PCNS1000 is 35.1 wt%. As comparison, PCNS with various NH3 treatment temperatures were studied. The sample heated at 900 °C in NH3/Ar was named as PCNS900. The carbon yield of PCNS900 is 50.6 wt%. By raising the NH3 treatment temperature to 1100 °C, no sample was left due to the severe etching reaction. CNS1000 was prepared by carbonizing at 1000 °C for 2 h in Ar atmosphere. The carbon yield of CNS1000 is 55.2 wt%.
3. Results and discussion The morphology of PCNS1000 is studied by FESEM. As shown in Fig. S1b, the PCNS1000 contains rough carbon nanosheets, which are interconnected to form micron-sized macropores. The size of macropore is consistent with the dimension of NaCl template (Fig. S1a), indicating the templating and confining role played by these micron-sized salt particles in the formation of the 2D carbon nanosheets. The macropores can reserve the electrolyte which shortens the diffusion distance of sodium ion. Besides, the interconnected carbon nanosheets form three dimensional conductive network, which is expected to enhance the transport rate of electrons. The rapid electron transfer and facile ion transport are beneficial to boosting the rate performance of electrode materials. Higher magnification SEM image (Fig. 1a) shows a highly wrinkled and rough surface of PCNS1000. While in the case of the counterparts carbonized in Ar, CNS700 (Fig. S1c, d) and CNS1000 (Fig. 1d) possess a rather smooth surface. The TEM results further verify the effect of NH3 treatment on the microstructure of the carbons. The TEM image of PCNS1000 (Fig. 1b) reveals that they display a rough
2.2. Synthesis of NVP The Na3V2(PO4)3 (NVP) was synthesized by a one-step solid state reaction as follows: A stoichiometric ratio of V2O5 and NaH2PO4 was mixed in a ball milling vial. Sugar was added as carbon precursor and 329
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Fig. 1. SEM images of (a) PCNS1000 and (d) CNS1000; TEM images of (b) PCNS1000 and (e) CNS1000; HRTEM images of (c) PCNS1000 and (f) CNS1000.
interlayer distance of pitch-derived carbon by high temperature NH3 treatment can promote the electrochemical performance of carbons for SIBs. Raman spectra of the samples in Fig. 2d exhibit the G band at 1590 cm−1 and D band at 1350 cm−1, corresponding to the in-plane vibration of sp2 carbon atoms and disordered structures, respectively. For PCNS1000, The relative intensity (ID/IG) is 1.09, much higher than 0.97 of CNS1000, indicative of excess defects introduced by NH3 treatment which results in the domination of disordered structure [27,28]. The results of XRD and Raman further demonstrate that the NH3 treatment can enlarge the interlayer distance and create abundant defects in the PCNS1000, which can provide more open channels and active sites for Na+ insertion and consequentially improve the performance of carbon anodes. Fig. 2c present the nitrogen adsorption–desorption isotherms of the coal tar pitch-derived carbons. It can be clearly observed that the PCNS1000 shows a typical IV isotherm with a hysteresis loop at relative pressure P/Po between 0.4 and 1.0 The specific Brunner−Emmet−Teller (BET) surface area of PCNS1000 is 546 m2 g−1 much larger than that of CNS1000 (30 m2 g−1), indicating the effect of NH3treatment on the pore structure of carbons. The pore-size distribution (Fig. 2d) shows that PNCS1000 has abundant mesopores from 2 nm to 50 nm which occupy as much as 79.6% of total pore volume. The large BET area benefits the contact of electrolyte to anode surface, and appropriate mesoporous structure shortens the diffusion distance of Na+ [24,29,30]. The above results indicate that the NH3 treatment process can enlarge the interlayer distance, introduce defects as well as form appropriate mesopore into PCNS1000. The obtained carbons, however, still remain the typical soft carbon nature to a certain extent even after the NH3 treatment. These structural evolutions are expected to facilitate ion transport and fast electron transfer as well as provide abundant Na+ storage sites. Besides pore structure, the NH3 treatment also modified the carbons' elemental composition and surface chemistry which can be characterized by elemental analysis and XPS. The elemental analysis in Table S1 shows that the N content in CNS700 is 1.60 wt%, which is derived from the precursor coal tar pitch (N content 1.10 wt%). However, after NH3 treatment at 1000 °C, N content of PCNS1000 increased to 4.17 wt% which is much higher than that of CNS1000 (1.81%), indicating the N-
surface covered with wrinkles. As comparison, the CNS1000 possesses smooth surface, which is consistent with the SEM image. The difference of samples' surface topography indicates the etching role played by NH3. By NH3 etching, the carbon yield of PCNS1000 (35.1 wt%) is much lower than that of CNS1000 (55.2 wt%). Besides modifying the surface topography of the carbon sheets, the NH3 treatment also affects their microstructure and chemical composition significantly. HRTEM was used to characterize the microstructure of carbons with or without NH3 treatment. By comparing the HRTEM image of PCNS1000 (Fig. 1c) with that of CNS1000 (Fig. 1f), the effect of NH3 treatment on the microstructure of carbons is quite clear. In the case of PCNS1000, as shown in Fig. 1c, nanosized graphitic clusters with several carbon layers homogeneously distribute in carbon matrix, indicating that the PCNS1000 remains the typical soft carbon nature to a certain extent after NH3 treatment. However, these graphitic clusters in PCNS1000 are smaller and more poorly aligned than those in CNS1000, indicating that the NH3 treatment can impede the development of order structure and reduce the size of nanocrystalline clusters in some degree. Meanwhile, as shown in Fig. 1c, the interlayer distance of PCNS1000 is enlarged to 0.39 nm which is much larger than 0.35 nm of CNS1000 (Fig. 1f). The SEM and TEM results indicate that the NH3 treatment can tune the morphology and microstructure of carbons by introducing defects into carbons and enlarging the interlayer distance [27,28], by which more storage sites are created and the Na+ transfer resistance is decreased. XRD and Raman were carried out to gain more detailed microstructure of carbons. XRD patterns for PCNS1000 and CNS1000 are shown in Fig. 2a. Both samples exhibit peaks at around 24.0° and 43.6°, corresponding to the (002) and (100) reflections of carbon material, respectively. Compared with CNS1000, the (002) peak of PCNS1000 shifts to a lower angle with a wider distribution, indicating the enhanced interlayer distance and more disordered structure, which is in good agreement with the HRTEM images. Average interlayer spacing of PCNS1000 is calculated to be 0.382 nm according to the Bragg's Law, much larger than those of CNS1000 (0.343 nm) and graphite (0.335 nm). According to the previous report, interlayer distance which is greater than 0.37 nm allows reversible intercalation/deintercalation of sodium ion [5,6]. It is reasonable to expect that the enlarged 330
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Fig. 2. (a) XRD patterns and (b) Raman spectra of CNS1000 and PCNS1000; (c) N2 adsorption–desorption isotherms of PCNS1000 and CNS1000; (e) pore size distribution curves of PCNS1000 and CNS1000.
The sodiation/desodiation behavior of the PCNS1000 was investigated in 2016 coin-type cells. Fig. 4a shows the CV curves of PCNS1000 at a scanning rate of 0.1 mV s−1 in the voltage range of 0.01–2.8 V. In the first cycle, the cathodic peak at around 0.5 V can be due to the formation of a solid electrolyte interphase (SEI) film, and it disappears after the initial discharge. The peak at around 0.01 V represents the insertion process of Na+ between the carbon layers. After the first cycle, the CV curves almost overlap, indicating the stable reversibility of Na+ storage. Fig. 4b displays the charge-discharge profiles of the PCNS1000 anode in the working voltage window 0.01–2.8 V at 0.1 A g−1. The initial discharge capacity is 448 mAh g−1, while the initial charge capacity is 296 mAh g−1, corresponding to a coulombic efficiency of 66%. The PCNS1000 exhibits a specific capacity of 313 mAh g−1 for the third cycle and 270 mAh g−1 for the tenth cycle. As for the subsequent cycles, it can be stabilized at 265 mAh g−1. The capacity loss is due to the irreversible reaction between sodium and surface functional groups as well as the decomposition of electrolyte and inevitable formation of SEI film. Fig. 4c exhibits the rate performance of PCNS1000 and CNS1000 at the current densities between 0.1 and 10 A g−1. The PCNS1000 electrode delivers a discharge specific capacity of 302 mAh g−1 at 0.1 A g−1, then is stabilized at 270 mAh g−1. Even cycled at higher current densities of 0.2, 0.5, 1, 2, and 5 A g−1, high capacities of 228, 186, 167, 150, and 133 mAh g−1 can be still retained, respectively. In contrast, the CNS1000 sample delivered reversible capacities of 174, 137, 104, 91, 83 and 75 mAh g−1 at current densities of 0.1, 0.2, 0.5, 1, 2 and 5 A g−1, respectively. In particularly, even at an ultra-high discharge current density of 10 A g−1, the PCNS1000 still remains a capacity of 124 mAh g−1, corresponding to a capacity retention of about 46%. CNS1000, by contrast, remains capacity of 63 mAh g−1 at a current density of 10 A g−1,
doping role played by NH3 treatment. XPS was performed to analyze the elemental composition and bonding configurations of the prepared carbon samples. The XPS spectra shown in Fig. 3a indicate that both PCNS1000 and CNS1000 possess three peaks centred at 284.5 eV, 398.1 eV and 532.1 eV, which are identified as C 1 s, N 1 s and O 1 s, respectively. The N content in PCNS1000 is 4.3 at.%, much higher than 1.5 at.% of CNS1000. The high resolution N 1 s spectra of PCNS1000 (Fig. 3b) and CNS1000 (Fig. S2a) can be resolved into three peaks, pyridinic N (N-6, 397.9 eV), pyrrolic N (N-5, 399.6 eV), and quaternary N (N-Q, 400.7 eV). The area percentages of the three fitting peaks in PCNS1000 are successively 52.3%, 25.8% and 21.9%%, representing the content percentages of the corresponding three N pieces. For CNS1000, the content percentages of the corresponding three N species are 21.8%, 8.1% and 70.1%. The differences in N species proportion and N content are closely related with heat treatment atmosphere. The high-resolution C 1 s spectra in Fig. 3c further proves the existence of C]N, with a content of 28.4%. For CNS1000 (Fig. S2b) the content of C]N (13.6%) is much lower than that in PCNS1000. Fig. 3d schematically exhibits the different types of nitrogen configurations. The doped N atoms can create more defects and enhance the electronic conductivity of carbons by contributing extra electrons to the π-conjugated system. The electrochemical properties of carbon electrode can be significantly enhanced by the nitrogen-containing groups, especially pyridinic N [31–33]. The NH3 treatment can easily bring more pyridinic N and pyrrolic N which are expected to create some extra defects in the PCNS1000 which provides more active sites for Na+ storage [32–36]. That could be one of the reasons for the improved specific capacity of PCNS1000 compared with CNS1000. The doped N can enlarge the spacing distance of carbon material which promote reversible intercalation/deintercalation of sodium ion [37,38]. 331
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Fig. 3. (a) XPS spectrum of PCNS1000 and CNS1000; high-resolution XPS spectrum of (b) N 1s and (c) C 1s of PCNS1000; (d) Schematic configurations of the heteroatom doping in PCNS1000.
cycling stability of both two samples is mostly due to the 2D nanosheets structure [41–43]. Fig. 4e shows the long cycling life of PCNS1000. After activated at 100 mA g−1 for the first 20 cycles, a charge capacity of 144 mAh g−1 can be delivered at 2 A g−1. After an ultra-long 10,000 cycles, stable capacity retention of 86% can still be retained, corresponding to a capacity loss as low as 0.0014% per cycle. Therefore, PCNS1000 derived from coal tar pitch, a low cost by-product of coking industry, exhibits the superior sodium storage capability and extremely excellent cycle stability. For comparison, PCNS900 with lower temperature NH3 treatment were also synthesized. The carbon yield of PCNS900 (50.6 wt%) is much larger than that of CNS1000 (35.1 wt.) indicating a deficient etching reaction at low NH3 treatment temperature. XRD patterns for PCNS900 and PCNS1000 are compared in Fig. S4a. The average interlayer spacing of PCNS900 is calculated to be 0.349 nm according to the Bragg's Law, much smaller than that of PCNS1000 (0.382 nm) due to the deficient etching reaction. The small interlayer distance restricts the intercalation/deintercalation of sodium ion resulting in a much lower specific capacity of PCNS900 than that of PCNS1000 (Fig. S4b). The enhanced capacity, superior cycling ability and good rate capability of PCNS1000 can be ascribed to the unique structure as a result of its soft carbon precursor and preparation process. Firstly, the interconnected 2D nanosheet structure and soft carbon feature supplies a short ion diffusion distance and fast electrons transport network.
corresponding to a capacity retention of only 35%. When the current is turned back to 0.1 A g−1, the specific capacity can be recover up to 269 mAh g−1 indicating the excellent robustness of the electrode for , Na+ storage. The EIS of PCNS1000 and CNS1000 are shown in Fig. S3a. The small diameter of the semicircle in the medium frequency region indicates the low charge transfer resistance of both PCNS1000 and CNS1000. It is noteworthy that charge transfer resistance of the PCNS1000 is much smaller than that of previously reported hard carbon anodes [39,40], which can be attributed to the remained soft carbon nature of PCNS1000. However, the sodium-ion diffusion coefficients of PCNS1000 and CNS1000 calculated according to eq. (S1) are 9.23 × 10−12 cm2 S−1 and 9.89 × 10−13 cm2 S−1, respectively. The sodium-ion diffusion coefficient of PCNS1000 is 9.3 times higher than that of CNS1000, suggesting good Na+ transport properties of PCNS1000. This amazing rate performance can be attributed to its superior structure including the enlarged interlayer distance, the abundant active sites [25], the appropriate mesopore and its soft carbon nature. Furthermore, PCNS1000 possesses much higher specific capacities than that of CNS1000 anode at all the measured current densities which can be attributed to the synergistic effect of expanded interlayer distance and N doping. At the cycling current of 500 mA g−1, both of the two carbon anodes exhibit stable capacities (Fig. 4d). PCNS1000 still exhibits a high specific capacity of 176 mAh g−1 after 1000 cycles corresponding to a high capacity retention of 93%. The excellent
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Fig. 4. (a) CV curves of PCNS1000 between 0.01 and 2.8 V at 0.1 mV s−1; (b) Galvanostatic discharge-charge profiles of PCNS1000 at 100 mA g−1; (c) Rate capability of the electrodes at different current densities; (d) Cycling performance of the electrodes at 0.5 A g−1; (e) Long-term cycling stability of PCNS1000 at 2 A g−1.
and 104 mAh g−1, respectively, can remain for use. After cycled at 2A g−1 for 100 times, high capacity retention of 89% can still be achieved (shown in Fig. 5b), indicating its great practical use potential. These capacity values in this work are much higher than most of the reported carbon-based anode in full cells (as summarized in Fig. 5c) [18,20,23,44]. The full cell could be charged to 3.7 V, enabling to light a red light-emitting diode (Fig. 5d).
Secondly, the suitable mesoporous structure greatly benefits the accessibility of the electrolyte and rapid ion diffusion/transport. Thirdly, the enhanced interlayer distance allows the reversible intercalation/ deintercalation of sodium ion in PCNS1000, overcoming the kinetic limitations. Finally, the doped N can also give rise to the improved capacities. The superior electrochemical performance of the PCNS1000 in halfcell encourages us to further measure its performance in full cells. NVP was chosen as the cathode material for its high voltage plateau and stability, which delivered 93.4 mAh g−1 at 0.1 A g−1 and a plateau of 3.4 V (Fig. S5a). It is noteworthy that even at a large charge-discharge current of 2 A g−1, the NVP cathode can be stably cycled over 950 times (Fig. S5b). As shown in Fig. 5a, in the voltage window of 0.5–3.9 V, the PCNS1000//NVP full cell delivers a high dischange capacity of 265 mAh g−1 at 0.1 A g−1 (calculated based on the anode mass, the same as below), with a voltage plateau around 2.7 V. At the current of 0.2, 0.5, 1.0 and 2.0 A g−1, stable discharge capacities of 228, 153, 125
4. Conclusions In summary, N-doped porous carbon nanosheets, which are prepared by a two-step process, exhibits high performance when used as sodium-ion battery anode. The results of this study show that NH3 treatment is a pivotal factor for controlling microstructure and chemical composition of pitch-derived carbons. Through NH3 treating, PCNS1000 exhibits a large interlayer distance of 3.82 Å, abundant mesoporous structure, and high nitrogen doping content. Combining its 333
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Fig. 5. (a) Rate performance of NVP//PCNS1000 full cell at current densities from 0.1 A g−1 to 2 A g−1; (b) cycling stability of the full cell at 2 A g−1; (c) rate capacity of PCNS1000 and other carbon-based SIBs anodes; (d) optical image of a red LED lit by the full cell.
soft carbon nature, PCNS1000 exhibits high reversible capacity (270.0 mAh g−1 at 0.1 A g−1), improved rate capacity and extremely long lifespan (10,000 times with a capacity loss of 0.0014% per cycle) in sodium-ion battery half cells. When extended to full cell of NVP// PCNS1000, it exhibits high reversible capacity of 265 mAh g−1 at 0.1 A g−1 and high average potential of 2.7 V.
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