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Nitrogen-doped carbon derived from pre-oxidized pitch for surface dominated potassium-ion storage
Carbon 155 (2019) 601e610
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Nitrogen-doped carbon derived from pre-oxidized pitch for surface dominated potassium-ion storage Qing Sun, Deping Li, Jun Cheng, Linna Dai, Jianguang Guo, Zhen Liang, Lijie Ci* SDU & Rice Joint Center for Carbon Nanomaterials, Key Laboratory for Liquid-Solid Structural Evolution & Processing of Materials (Ministry of Education), School of Materials Science and Engineering, Shandong University, Jinan, 250061, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 May 2019 Received in revised form 5 August 2019 Accepted 20 August 2019 Available online 10 September 2019
Graphitic material has captured tremendous attentions as anode material for potassium ion batteries (PIBs). Nevertheless, the large radius of potassium-ions results in sluggish potassiation kinetics and huge volume expansion, leading to unsatisfying performance. Herein, a fabrication facile, cost-effective and high carbon yield nitrogen/oxygen co-doped amorphous carbon (NOC) with pitch and urea as precursors is reported. Pre-oxidation process is employed, maintaining the amorphous structure of pitch derived carbon against its soft carbon nature. The NOC electrode delivers reversible capacities of 347 (300th cycle) and 167 mAh g1 (1000th cycle) at 100 and 2000 mA g1, respectively. Rearrangement of graphene layers in short range benefits the structure stability against volume change. Kinetics analyses prove that surface-induced capacitive process dominates in K-ion storage mechanism, which contributes to the remarkable electrochemical performance. Pouch full cells were assembled, delivering a capacity of 316 mAh g1 at 100 mA g1. In view of the cost-effectiveness and electrochemical performance, this work offers a strategy for the fabrication of low-cost and high-performance PIB anode materials. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Potassium ion batteries (PIBs) Nitrogen-doped carbon Pre-oxidized pitch Surface-dominated Diffusion coefficient
1. Introduction With the ever-growing demand for efficient and rechargeable lithium ion batteries (LIBs), the consumption of lithium is rapidly increasing, which raises concerns about the sustainability of limited and unevenly distributed lithium resources [1e4]. As a result, substitutes for lithium are highly sought after [5e9]. As group IA elements, sodium and potassium share similar chemical properties to lithium [10,11]. Therefore, sodium and potassium are likely the most ideal alternatives to lithium [12e14]. Compared to lithium (0.0017 wt%), potassium is abundant in the Earth's crust (2.09 wt%), approximating to sodium (2.36 wt%) [3,15]. Furthermore, close to the redox potential of lithium (3.04 V), potassium has a lower redox potential compared to sodium (2.93 V vs. 2.71 V), it thus offers a higher energy density than the latter [16e18]. In addition, the high potential present in carbon material in the potassiation process (0.2 V vs. 0.05 V in sodiation process) suggests a higher level of safety [19,20]. What's more, compared to sodium, smaller solvated ions formed by potassium ions (K-ions), along with lower desolvation energy, benefit the
* Corresponding author. E-mail address: [email protected] (L. Ci). https://doi.org/10.1016/j.carbon.2019.08.059 0008-6223/© 2019 Elsevier Ltd. All rights reserved.
diffusion kinetics dramatically [21,22]. Furthermore, compared to high-cost copper collectors, low-cost aluminum collectors are readily available. That is due to the inexistence of alloy formation between potassium and aluminum even at low potentials [22]. Despite numerous merits, the pursuit of potassium ion batteries (PIBs) is a challenging one [10]. For LIBs, graphite is the most widely utilized and commercialized anode material, exhibiting a decent reversible capacity of ~372 mAh g1 through the formation of LiC6 [15,19,23,24]. Similarly, potassium forms KC8 with graphite via intercalation, exhibiting a maximum capacity of 279 mAh g1 [10,15,23,25]. However, the larger radius of potassium ion compared to lithium ion (1.38 Å vs. 0.76 Å) results in severe capacity fading of graphitic electrode, which is caused by the enormous volume expansion upon full potassiation (~61%) [26e28]. Furthermore, the larger radius of K-ions leads to unsatisfying rate capability on account of the sluggish diffusion kinetics of intercalation [10]. To overcome these difficulties, various modified graphite materials have been designed [10]. An et al. chose expanded graphite as anode, delivering capacities of 263 and 170 mAh g1 at 10 and 200 mA g1 [17]. Li et al. proposed a unique structure of sandwichlike FeCl3@C, with a better performance of 269 mAh g1 at 50 mA g1 [20]. However, the reversible capacity and rate capability
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of modified graphite are still unsatisfying, compelling researchers to look for more promising materials [28]. Recent studies have diverted attention to non-graphitic carbon [21]. Yamamoto et al. introduced a cellulose-derived carbon material delivering a capacity of 290 mAh g1 [29]. Chen et al. put forward a carbonizationetching strategy for preparation of sulfur/oxygen dual-doped porous carbon. The large BET surface area and enlarged interlayer space boost the K-ion storage [16]. Likewise, Li et al. reported a facile strategy for nitrogen doped porous carbon. The material exhibits a superior cycling performance along with a decent rate capability, but its low carbon yield of less than 5% results in the overwhelming difficulty of practical application [30]. Elements doped amorphous carbon appears to be an optimal alternative for PIBs, but there are still many aspects to be explored [21,29]. As a cost-effective product, pitch is extensively used as the carbon source due to its high carbon yield [31e33]. However, pitch tends to be graphitized at high temperature due to its inherent nature, which, to a degree, undermines its advantages as anode material precursor for PIBs [34,35]. Fortunately, researchers have reported that oxygen-containing functional groups can be introduced via pre-oxidation, the formed cross-links can prevent melting and rearrangement [35e37]. Nitrogen doping could effectively enhance the electrochemical performance of carbon materials through the introduction of nitrogen-containing functional groups, which can enrich the active sites in carbon and boost K-ion diffusion [4,25,38]. Inspired by abovementioned works, herein, a brand new strategy for nitrogen-doped amorphous carbon is introduced. Low-cost pitch is adopted as the carbon source, with a pre-oxidation process introduced in advance. Cost-effective urea is employed as a nitrogen-doping precursor. The nitrogen/ oxygen co-doped amorphous carbon (NOC) displays high capacity and remarkable cycling stability and rate capability. The charge/ discharge process of NOC exhibits a sloping profile. Intrigued by this, kinetics analyses were carried out to verify the domination of surface-induced capacitive process. In addition, galvanostatic intermittent titration technique (GITT) was employed, and it showed that the K-ions present faster diffusion in NOC. After the cycling process, short-range ordered structure was observed in NOC via high-resolution transmission electron microscopy (HRTEM). Ex-situ Raman spectroscopy measurement were performed, further proving the emergence of the rearrangement in short range. The unique structure can accommodate the volume change caused by the intercalation of K-ions. Pouch full cells were then assembled, and subsequently delivered satisfying performance. This work shed some light on the exploration of large-scale fabrication of PIBs’ anode, offering a universal scheme of characterizations and analyses for amorphous carbon-based anode in alkali-ion based battery system and beyond. 2. Experimental section 2.1. Synthesis of PAC and NOC Pre-oxidized pitch (POP) was obtained via pre-oxidation treatment at 250 C for 120 min in the air atmosphere, with a heating rate of 10 C min1. Pitch derived amorphous carbon (PAC) was prepared by means of calcination of POP powder in a nitrogen atmosphere. With a heating rate of 10 C min1, the POP was first heat-retained at 450 C for 120 min, and then carbonized at 600, 800, and 1000 C for 180 min, respectively. Subsequently, the asprepared black products were ball milled for 60 min in a planetary ball mill machine, the final products were labeled as PAC-600, PAC-800, and PAC-1000. Nitrogen/oxygen co-doped amorphous carbon (NOC) was prepared by ball milling mixture process of POP and urea (weight ratio 4:1) for 30 min in advance, and then
following the same procedure of PAC-600. 2.2. Synthesis of KPB 1.62 g K3C6H5O7$H2O and 0.63 g FeCl2 were dissolved into 50 mL of deionized water. 50 ml K4Fe(CN)6 solution (0.1 M in deionized water) was dropwise added in under stirring. After that, the mixture was sequentially stirred for 2 h. After an aging process of 10 h, centrifugation was conducted and the precipitates were washed and centrifuged for 3 times with deionized water and ethanol respectively. Subsequently, the collected precipitates were dried in a vacuum oven at 80 C for 12 h. 2.3. Characterization The observation of morphology and microstructure of the obtained specimens were conducted with a Hitachi SU-70 fieldemission scanning electron microscopy (FESEM) and a JEOL JEM2100 transmission electron microscopy (TEM). The structure of NOC and PAC-x was characterized via X-ray diffraction measurement (XRD) with a Rigaku Miniflex 600 X-ray diffractometer, with Cu Ka radiation (l ¼ 1.5406 Å) at 40 kV and Raman spectra on a Renishaw InVia Reflex. X-ray photoelectron spectroscopy (XPS) was performed with an ESCALAB 250 spectrometer (Al Ka X-ray source, excitation energy of 1486.71 eV) to analyze the element contents. The specific surface areas (SSA) and pore width distributions were determined with an ASAP 2020 analyzer using the BrunauerEmmett-Teller (BET) method. The specimens were first degassed at 150 C for 2 h, and the nitrogen adsorption/desoption isotherms were acquired at 77 k. A Mettler-Toledo TGA2 Thermo Analyzer was used to perform the thermogravimetric analyses of pitch precursor (air atmosphere, 35 Ce600 C, 20 mL min1) and POP (nitrogen atmosphere, 35 Ce1000 C, 20 mL min1). 2.4. Electrochemical measurements The electrodes were fabricated by mixing active materials (obtained specimens and KPB, 80 wt%), super P (10 wt%), and polyvinylidene fluoride (PVDF, 10 wt%) as binder. The homogenously dispersed slurries were coated onto copper foil (obtained specimens) and aluminum foil (KPB) respectively, and then dried in a vacuum oven at 80 C for 10 h until the solvents had completely evaporated. For half cells, the dried anode electrodes were circular punched (diameters of 14 mm). The type CR-2032-coin cells were assembled with potassium metal as counter electrodes in a glove box (argon-filled, H2O < 1 ppm, and O2 < 1 ppm). As for pouch full cells, aluminum-plastic film (D-EL35H, DNP) was formed on a KJ MSK-120 and then top side sealed on a KJ MSK-140. The electrodes were punched into rectangles (cathode: 57 44 mm; anode: 58 45 mm) with uncovered tabs on a die-cutting machine (KJ MSK-180). Afterwards, the electrodes were severed by separators, wrapped with insulating tape, and put into the aluminum-plastic film after the uncovered tabs were ultrasonic-welded with a KJ MSK-800 W to achieve parallel connection. After the electrolyte injection process in the glove box, the pouch full cell was vacuumsealed with a KJ MSK-115A-S. As for the electrolyte, 0.8 M KPF6 solution was dissolved into ethylene carbonate/diethyl carbonate (EC/DEC 1:1 by volume) mixture as electrolyte. Glass fiber membranes (Whatman GF/D) were applied as separators. The charge/ discharge performance was tested within the voltage window of 0.01e3.0 V for half cells and 2.0e4.0 V for full cells on battery testers (LAND-CT2001A). The CV measurements were conducted within the voltage range of 0.01e3.0 V at various scan rates of 0.1e2.0 mV s1 with an electrochemical workstation (Autolab, Metrohm). As for galvanostatic intermittent titration technique
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(GITT) measurements, the method of pulse currents (100 mA g1 for 15 min) separated by relaxation intervals (30 min) was adopted. For ex-situ HRTEM and ex-situ Raman spectra, the cycled coin cells were disassembled in glove box, the electrode was washed in DEC solvent and then dried in vacuum overnight. 3. Results and discussion 3.1. Morphology and structure characterization Pitch derived carbon tends to be graphitized due to its soft carbon nature, while the pre-oxidation introduced oxygencontaining functional groups can form cross-links to prevent melting, therefore avoid the reordering phenomenon effectively [35]. To determine the optimal temperature for pre-oxidation, thermos-gravimetric analysis (TGA) was performed from 35 to 600 C (10 C min1) under air atmosphere. As shown in Fig. S1a, there is a rapid increase of 0.7% of weight gain between 200 and 250 C, which can be ascribed to the oxygen acquisition [36]. Until 250 C, the mass increase ratio amounts to be 0.9%, and the oxygen acquisition should be higher considering the escape of H2O and methylene hydrogen [35]. After that, the mass decreased quickly due to the violent reaction with oxygen, and eventually burned off [36]. Refer to this, pre-oxidized pitch (POP) was therefore obtained in the air at 250 C for 120 min. Fig. 1a illustrates the typical preparation process. Pitch was first modified by air pre-oxidation treatment, and the formed cross-links could avoid melting and rearrangement at high temperature. Specifically, with the gradually decomposition of oxygen containing functional groups during calcination, small molecules like CO and CO2 escape, leading to an even more disordered structure [39]. As for the nitrogen doping of NOC, urea was evenly mixed with POP followed by facile calcination
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treatment. Subsequently, ball-milling was conducted to achieve the acquirement of particles morphologies. For the sake of understanding the microstructure of POP derived and pitch derived carbon, high-resolution transmission electron microscopy (HRTEM) images were recorded respectively in Fig. 1b and c. Compared with the POP derived carbon (1000 C), the carbon layers of pitch derived carbon are clearer and more parallel, which indicate a higher degree of graphitization. Meanwhile, the selected area electron diffraction (SAED) further demonstrates the abovementioned graphitization difference. POP derived carbon exhibits more disordered carbon layers in random directions and more dispersing SAED rings, revealing the lower graphitization degree of POP derived carbon arisen from the pre-oxidation process. The disordered structure of POP derived carbon results in the absence of plateau capacity as the later test shows, which affects the electrochemical performance in positive impacts. Calcination temperatures have effects on the microstructure of carbonaceous materials to a degree, and influence the electrochemical performance [32]. To investigate the thermal stability of POP, TGA was conducted from 35 to 1000 C at a rate of 10 C min1 (nitrogen atmosphere). As shown in Fig. S1b, POP starts to lose weight at approximate 300 C and reaches a maximum rate at 423 C. The weight loss is almost complete at ~550 C. Different pitch derived amorphous carbon (PAC) specimens were obtained accordingly, labeled PAC-600, PAC-800, and PAC-1000, reflecting the calcination temperatures of 600, 800, and 1000 C. Based on the latter characterizations and tests, 600 C is adopted as the optimal calcination temperature for NOC. Therefore, the resultant specimen prepared is labeled as NOC-600. Figs. S2aeS2d shows the morphologies of NOC-600, PAC-600, PAC-800 and PAC-1000 investigated by field emission scanning electron microscope (FESEM), and NOC-600 (Fig. S2e) and PAC-600
Fig. 1. (a) Schematic illustration of the preparation process with and without pre-oxidation; HRTEM images with SAED insets of (b) POP derived carbon and (c) pitch derived carbon calcinated at 1000 C. (A colour version of this figure can be viewed online.)
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(Fig. S2f) were further examined by transmission electron microscopy (TEM). The specimens exhibit irregular morphologies with uniform particle sizes, which should be ascribed to the preceding ball-milling. The HRTEM image (Fig. S2g) presents the disordered microstructure of NOC-600, which results from the pre-oxidation process and the nitrogen doping. XRD and Raman spectroscopy were employed to characterize the microstructures. As depicted in Fig. 2a, the series of XRD patterns possess two distinct broad diffraction peaks, which can be respectively indexed to (002) and (100) diffraction mode, indicating the formation of amorphous carbon structure. Particularly, with the calcination temperature increasing, the (002) peak shifts from 25.32 to 25.54 and grows sharper as the intensity increases. This phenomenon suggests the decrease of interlayer distance and the increased degree of graphitization. Note that NOC-600 possesses the broadest peak width and lowest peak intensity of XRD pattern, indicating its lowest degree of graphitization, which is attributed to the successful doping of nitrogen. The grafting of nitrogen groups effectively expanded the carbon layers of NOC-600, which is calculated to be 0.356 nm according to the Bragg equation (2dsinq ¼ nl), larger than graphite's 0.335 nm. The carbon matrix with expanded carbon interlayer distance can accommodate the volume change during potassiation/depotassiation, contributing to the stability during cycling, and also accelerate the K-ions insertion/extraction kinetics [17,30]. Raman spectra (Fig. 2b) present two characteristic bands which correspond to D-band (the disorder/defect-induced mode) and G-band (the in-plane vibrational mode) at around 1336 and 1586 cm1 [26,40]. It is noticeable that, with the increase of calcination temperature, the half widths at half maximum (HWHM) of D-band and G-band of PAC-x decrease, which is consistent with the uptrend of the temperature, indicating the transformation from disordered to ordered structure [33]. The peak areas are fitted by Lorenz model, and ID/IG
is calculated from the ratio of peak areas of D-band to G-band. NOC600 possesses the highest ID/IG of 2.74, indicating a most disordered degree of microstructure along with abundant structural defects which is ascribed to the nitrogen doping [30,41]. X-ray photoelectron spectroscopy (XPS) analysis was conducted to investigate the element doping configuration in NOC. The XPS survey spectra (NOC-600 and PAC-600, Fig. 2c) present peaks at approximately 283.9, 401.2, and 531.9 eV, corresponding to C 1s, N 1s, and O 1s, respectively [25]. The relative atomic percentages of C (91.8 at%), N (1.7 at%) and O (6.4 at%) are calculated for NOC-600 while the ratios for PAC-600 are 93.7 at% (C) and 5.6 at% (O) with the absence of N. In view of the similar content of carbon and oxygen of NOC-600 and PAC-600, it's reasonable to ascribe the enhanced electrochemical performance of NOC to nitrogen doping. High resolution XPS spectra are further characterized, as drawn in Fig. 2d, N 1s is deconvoluted into tree peaks at 398.3, 399.9, and 401.5 eV, which can be indexed to pyridinic nitrogen (N-6), pyrrolic nitrogen (N-5), and graphitic nitrogen (N-Q), respectively [21,42]. It's obvious that N-5 and N-6 dominate in the nitrogen containing species of NOC-600, and researches have shown that the electrochemical active character of N-5 and N-6 is significantly beneficial for the rate capability, the doped nitrogen can create plenty of active sites and thereby enhance the surface-driven process [23]. Meanwhile, N-Q supplies decent electroconductive enhancement via electron-donor characteristic change [21,23,43]. Oxygen element of NOC is derived from the innate oxygen-containing functional groups of pitch and the pre-oxidation process. High resolution XPS spectrum of O 1s (Fig. 2d) is deconvoluted into three peaks denoted as -C¼O (O-I), C-O (O-II), and -COOH (O-III) at 531.1, 532.3, and 533.3 eV, respectively [38,42]. Despite the reduction of electroconductivity they lead to, oxygen atoms can increase defect density and disorder degree, as well as effectively improve the wettability of the NOC, and facilitate the utilization of the surface area [25,43].
Fig. 2. Structural characterization. (a) XRD patterns; (b) Raman spectra; (c) XPS spectra; (d) High-resolution XPS spectra for N 1s and O 1s of NOC-600; (e) Nitrogen adsorptiondesorption isotherm and (f) related pore size distribution of NOC-600 and PAC-600. (A colour version of this figure can be viewed online.)
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It's widely accepted that pore structure and pore-size distribution of carbon electrode materials significantly influence their electrochemical performance [23,44]. Therefore, to get a deeper insight into NOC-600 and PAC-x, nitrogen absorption-desorption were performed. As demonstrated in Fig. 2e, NOC-600 presents a Brunauer-Emmett-Teller (BET) isotherm of Type I (IUPAC definition) with a relatively high nitrogen adsorption at low relative pressure area, indicating its typical character of microporosity [24,30]. The specific surface area (SSA) of PAC-600, PAC-800, and PAC-1000 are calculated to be 20.3, 26.0, and 28.5 m2 g1. The similar SSA values of PAC-x suggest that the employed temperature range between 600 and 1000 C has minimal impact on the SSA of the samples. In contrast, the SSA of NOC-600 is calculated to be 176.5 m2 g1, much larger than that of PAC-600 (~6 times higher). The pore volume values which are 0.03, 0.03, 0.04, and 0.19 cm3 g1 for PAC-600, PAC-800, PAC-1000, and NOC-600, respectively. The significantly increased SSA and pore volume of NOC-600 should be attributed to the additional defects and nanopores introduced by nitrogen doping. Fig. 2f and S3b exhibit the pore-size distributions of NOC-600 and PAC-600 calculated with quenched solid density functional theory (QSDFT) method [45]. Both NOC and PAC possess pores covering from micropores to macropores, proving the
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hierarchically porous architecture with the coexistent micro-mesomacro pores. It is worth noting that NOC-600 presents newly emerged pores with size ranging from 0.39 to 1.27 nm, particularly at ~1.18 nm (Fig. 2f). The abundance of micropores could provide plenty of active sites along with the enhanced affinity to K-ions, thereby not only providing storage locations to accommodate the charge, but also facilitating the transport of electrons and K-ions [10,23,46]. 3.2. Electrochemical performance The cyclic voltammetry (CV) measurement was conducted to investigate the electrochemical properties of NOC-600 and PAC600 electrodes within a voltage window ranging from 0.01 to 3.0 V (vs. K/Kþ) at 0.1 mV s1. During the first cathodic scan, an irreversible peak presents at ~1.65 V for NOC-600 (Fig. 3a), indicating the reaction between surface functional groups and K-ions [25]. In addition, both NOC-600 (Fig. 3a) and PAC-600 electrodes (Fig. 3b) exhibit peaks at ~ 0.50 V and the peaks vanish in the subsequent scans, which might be attributed to the formation of solid electrolyte interface (SEI) along with the decomposition of the electrolyte [46,47]. On account of the larger SSA, NOC-600 electrode
Fig. 3. Electrochemical performance of NOC and PAC. CV curves of (a) NOC-600 and (b) PAC-600; (c) Cycling stability of NOC and PAC at a current density of 100 mA g1; (d) Charge/ discharge voltage profiles of NOC-600; (e) Rate performance of NOC and PAC at current densities from 100 to 2000 mAh g1; (f) Charge/discharge voltage profiles of NOC-600 at various current densities and (g) cycling performance of NOC and PAC at a high current density of 1000 mA g1. (A colour version of this figure can be viewed online.)
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possesses much more prominent cathodic peak at ~ 0.50 V than PAC-600 due to the formation of extra SEI [45]. This will result in large capacity consumption and thus reducing the initial coulombic efficiency (CE), as confirmed by the subsequent testing. The sharp cathodic peak at 0.01 V and broad anodic peak at ~0.25 V are related to the intercalation and extraction of K-ions, respectively [21,43,48]. Additionally, several humps are observed at ~ 0.60 V (NOC-600), ~0.70 V (PAC-600), ~1.20 V (PAC-600), and ~1.80 V (NOC-600) due to the interaction between K-ions and heteroatoms on the surface of the materials like N and O [47]. Generally, the relatively broad profiles of CV curves (NOC-600) reveals the higher proportion of surface driven storage in K-ion storage, which could significantly improve the rate capability and cycling stability [49]. Furthermore, the CV curves of the second cycle overlapped well with the third, which indicates decent microstructure stability and a good cycling reversibility. To measure the cycling stability of the materials, the NOC and PAC-x electrodes were tested at a current density of 100 mA g1 (Fig. 3c). The initial discharge capacity of NOC-600 (2209.1 mAh g1) is higher than those of PAC-600 (610.0 mAh g1), PAC-800 (718.0 mAh g1), and PAC-1000 (618.9 mAh g1), corresponding to initial CEs of 32.8%, 43.2%, 46.9%, and 44.6%. The high initial discharge capacity along with the low initial CE can be ascribed to the K-ions consumption during the formation of SEI, which was confirmed by the abovementioned CV profile [50]. After fully activated in the first few cycles, the electrode exhibits a CE which gradually grows close to 100%. NOC-600 electrode exhibits a considerable reversible capacity of 347 mAh g1 after 300 cycles, noticeably higher than those of PAC-600 (192 mAh g1), PAC-800 (167 mAh g1), and PAC-1000 (143 mAh g1). The CE of NOC-600 electrode stabilizes after more cycles, which could be ascribed to the additional time required for the formation of stable SEI layer due to the high SSA [21]. The discharge capacity of NOC-600 electrode remains to be 93% (vs. 10th cycle)after 300 cycles, dramatically surpassing those of PAC-600 (77%), PAC-800 (70%), and PAC1000 (61%). The charge/discharge profiles of NOC-600 electrode are shown in Fig. 3d. Within the voltage window ranging from 0.01 to 3.0 V (vs. K/Kþ), the curves overlap well from 50th to 300th cycle, indicating a superior cycling stability of NOC-600 electrode. It's noteworthy that NOC-600 electrode exhibits a prominent sloping feature, which is a characteristic behavior of surface dominated Kion storage [47,51]. Surface dominated storage behavior benefits the fast K-ions transport [48,49]. In view of this, the rate performance for NOC and PAC electrodes were evaluated (Fig. 3e). NOC-600 electrode exhibits capacities of 461.2, 358.1, 300.4, and 224.0 mAh g1 at 100, 200, 500, and 1000 mA g1, exceeding PAC tremendously. And the capacity remains 162.5 mAh g1 even at a relatively high current density of 2000 mA g1. The superior rate capability of NOC-600 is in the lead among reported carbon based materials in PIBs (Fig. S4). As for PAC-x electrodes, with the increase of the calcination temperature, the rate performance becomes worse, indicating negative effect due to the higher degree of graphitization. Surprisingly, the capacities almost completely recover in the wake of the step by step reverted current density. Moreover, in the last 50 cycles at 200 mA g1, NOC-600 electrode maintained a stable cycling capacity of 315 mAh g1. Notably, for NOC-600 electrode (Fig. 3f), the proportion of plateau capacity (below ~0.3 V), which is originally small, almost vanished with the current density increased to 2000 mA g1. This phenomenon proves the domination of surface capacitive behavior in K-ion storage process particularly at high currents [21]. This will be further corroborated in the following contribution analysis of K-ion storage process (Fig. 5). Long-term cycling stability at high rate of 1000 mA g1 were evaluated (Fig. 3g). NOC-600 electrode exhibits a reversible capacity of 167
Fig. 4. (a) Schematic illustration of the K-ion storage process; (b) HRTEM images of pristine and cycled NOC-600 and (C) interlayer spacing of short range ordered structure. (A colour version of this figure can be viewed online.)
mAh g1 after 1000 cycles, far beyond those of PAC-600 (94 mAh g1), PAC-800 (69 mAh g1), and PAC-1000 (54 mAh g1). A high capacity retention of 70.0% (NOC-600) can also be achieved after 1000 cycles, as opposed to 48.0% for PAC-600, 36.3% for PAC-800, and 27.7% for PAC-1000. The better performance is attributed to the domination of surface-induced capacitive process in the NOC600 electrode. However, there is a continuous capacity loss during the first 200 cycles, after which the capacity tends to stabilize. This can be ascribed to a certain degree of collapse of the twisted graphitic structure during the intercalation of K-ions, indicating the co-existence of surface-induced capacitive process and diffusioncontrolled intercalation process [19]. 3.3. Ex-situ analyses of K-ion storage process In view of the remarkable electrochemical performance above, ex-situ characterizations were adopted to explore the K-ion storage process. Fig. 4a illustrates the K-ion storage process. As for pristine NOC-600 electrode, a typically amorphous structure is observed in pristine state via HRTEM (Fig. 4b). After the cycling process, rearrangement in short range clearly emerges, possessing an interlayer spacing calculated to be 0.40 nm on average (Fig. 4c). It is noteworthy that the interlayer spacing of short range ordered stacks is larger than that of graphite (0.335 nm), this benefits the accommodation of large-radius K-ions and enhances the K-ion transport [17]. The co-existence of short range ordered stacks and amorphous structure can evenly distribute the stress, thus relieving the volume expansion during cycling and resulting in superior cycling stability [24,25]. Ex-situ Raman spectroscopy was conducted to further
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Fig. 5. Analysis of K-ion storage and diffusion kinetics. CV curves at various scan rates of (a) NOC-600 and (b) PAC-600; (C) b values determined from linear fitting; (d) Contributions of surface-induced capacitive process in NOC-600 and PAC-x at various scan rates; (e) Contribution of the surface-induced capacitive process in NOC-600 at 0.5 mV s1; (f) GITT profiles of NOC-600 and (g) diffusion coefficients of NOC-600 and PAC-600. (A colour version of this figure can be viewed online.)
investigate the K-ion storage process. As shown in Fig. S5, both Raman spectra (collected on pristine and cycled electrodes, respectively) present two characteristic bands corresponding to Dband and G-band. Notably, after fitted by Lorenz model, the ID/IG ratio of cycled electrode (2.24) decreases distinctively compared to the pristine electrode (2.62). This suggests that the degree of order is improved to some extent, which is attributed to the potassiation/ depotassiation process [50]. The Raman results are consistent with the results of the abovementioned HRTEM images. 3.4. Analysis of K-ion storage and diffusion kinetics To further investigate the mechanism of K-ion storage, CV measurements were conducted at various scan rates ranging from 0.1 to 2.0 mV s1. As shown in Fig. 5a, b, Figs. S6a and S6b, broad peaks of NOC-600 are maintained at various scan rates, while the peaks of PAC-x are steeper and exhibit the characteristics of polarization with the increase of calcination temperature [47]. A power-law equation is introduced to describe the relationship
between peak current (i) and scan rate (v) [40,51,52]
i ¼ avb
(1)
logðiÞ ¼ blogðvÞ þ logðaÞ
(2)
Where a is adjustable constant, i the peak current of each CV curve, and v the corresponding scan rate. The value of b could determine the contribution of surface-induced capacitive process and diffusion-controlled intercalation process during potassiation/ depotassiation process [21]. It indicates an typical diffusioncontrolled process when b ¼ 0.5 whereas an ideal surfaceinduced capacitive process when b ¼ 1 [47]. The b values are calculated from the linear fitting data that are acquired from the CV curves of various scan rates. Preferable linear relationships are achieved (Fig. 5c). NOC-600 electrode possesses the highest b value (~0.91), suggesting a surface dominated K-ion storage behavior attributed to the nitrogen doping. As predicted, the b values of PACx electrodes decrease from 0.85 to 0.80 owing to the increase of
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graphitization degree. The contribution of surface process could be separated from the mixed storage mechanisms by following Equation (3) [10,53]
pffiffiffi i ¼ k1 v þ k2 v
(3)
Where i represents the current response at a fixed potential, k1 and k2 the constants. The separated contributions of surface-induced process and diffusion-controlled process can be represented by pffiffiffi k1 v and k2 v. Fig. 5d shows the contributions of surface-induced process at scan rates of 0.1, 0.2, 0.5, 1.0, and 2.0 mV s1. Following the increase of scan rates, the capacitive contributions of all the specimens increases, which benefits the rate capabilities as well as the cycling stabilities at high rates. When it comes to the same scan rate, the growth of capacitive contribution complies well with the increased disorder degree. The typical separation of capacitive contribution for CV profiles at a scan rate of 0.5 mV s1 is presented in Fig. 5e, with red areas of capacitive behavior. NOC-600 possesses the highest capacitive contribution, the surface-dominated K-ion storage behavior should be attributed to the hierarchically porous architecture and more surface defects introduced by nitrogen doping [30]. Galvanostatic intermittent titration technique (GITT) measurements were employed to further investigate the diffusion kinetics of K-ion storage. The diffusion coefficient (Dk) of K-ions during potassiation/depotassiation can be calculated with the following Equation (4) [19,54]
Dk ¼
4mB V m DE s 2 pM b S Dt DEt
(4)
Where mB represents the active material mass loaded on the electrode; Mb the molar mass; Vm the molar volume of active material; S the electrode area in geometry; and Dt the time of periodic discharge/charge. Descriptions of DEs and DEt are shown in Fig. S7 in detail. The diffusion coefficients of K-ions in NOC-600 and PAC-600 electrodes were measured via GITT with a periodic repetition of pulse currents of 100 mA g1 for 15 min separated by relaxation intervals of 30 min (Figs. 5f and S8). The calculated coefficients are plotted as functions of evolving potassiation/depotassiation depths in Fig. 5h. As shown, due to the well-developed hierarchical porous architecture and surface defects derived from nitrogen doping, NOC-600 electrode exhibits distinctly higher coefficient than PAC-600 in general, suggesting its faster K-ion diffusion kinetics. It is worth noting that with the evolution of potassiation process, the fluctuant coefficient diminishes
moderately. This is because the ease of access to the surface sites leads to the prior occupation of K-ions. After that, K-ions diffuse inside, and have to overcome the resistant gradient from the K-ions anchored previously [10,47]. As for the depotassiation process, the tendency is almost the opposite, further proving the irreversibility of K-ion storage. 3.5. Electrochemical performance of pouch full cells Encouraged by the above tested electrochemical performance of NOC-600, pouch full cells were further assembled with NOC-600 electrode as anode and potassium Prussian blue (KPB) as cathode. KPB is regarded as a promising cathode material for PIBs due to its low cost and facile synthesis [55]. KPB was synthesized via a reported precipitation route [47]. XRD pattern and cycling performance with initial charge/discharge voltage profile of KPB are posted in Fig. S9. Considering the high K-ion consumption during the initial cycle of NOC-600, the anode is pre-potassiated with a method of direct contact with potassium foil in electrolyte to compensate for the initial capacity loss. To achieve the optimal effect, electrodes of different contact time are employed to assemble half cells. The comparison of ICEs (Fig. S10) indicates that 7.5 min is the optimal pre-potassiation time. The pouch full cells are designed in anode-limited mode with a capacity ratio of 1:1.1 (anode: cathode). At a current density of 100 mA g1, the full cell presents a discharge capacity of 316 mAh g1 within a voltage window ranging from 2.0 to 4.0 V (Fig. 6a). It's worth noting that in contrast with the plateaus of KPB half cell, the full cell exhibits distinct semi-plateaus, it should be attributed to the sloping profiles of NOC-600. The assembled pouch full cell can charge a HUAWEI honor V10 via an E50D-based 5 V voltage booster circuit (Fig. 6b). The circuit diagram is available in Fig. S11. The pouch full cell demonstrates the application of the synthesized anode material in PIBs. 4. Conclusions In summary, a facile fabrication process of nitrogen/oxygen codoped amorphous carbon material with cost-effective and high carbon yield is reported. Low-cost urea and pitch are employed as precursors along with easy pre-oxidation in advance, which maintain the amorphous structure against the soft carbon nature of pitch derived carbon. The NOC electrode delivers remarkable electrochemical performance due to its hierarchical porous architecture and nitrogen doping. The rearrangement of graphene layers in short range is observed after cycling, which can accommodate
Fig. 6. (a) Cycling performance with initial charge/discharge voltage profile of NOC-600/KPB full cell at 200 mA g1 and (b) photograph of a HUAWEI Honor V10 charged by an assembled NOC-600/KPB pouch full cell. (A colour version of this figure can be viewed online.)
Q. Sun et al. / Carbon 155 (2019) 601e610
the volume change during K-ion storage. Kinetics analysis proves that surface-induced capacitive process dominates in K-ion storage mechanism, thus accomplishing the high reversible capacity, superior cycling stability and remarkable rate capability of NOC. Quantitative calculation indicates that the diffusion coefficient of Kions in NOC electrode surpasses that in PAC electrode, proving that nitrogen doping effectively benefits diffusion kinetics. This work serves as an exploration of large-scale fabrication of amorphous carbon anode for PIBs.
[19] [20]
[21]
[22] [23]
Acknowledgment [24]
This work was supported by Startup Funding of Distinguished Professorship of “1000 Talents Program” (31370086963030), the Fundamental Research Funds of Shandong University (2016JC005, 2017JC042 and 2017JC010), Shandong Provincial Science and Technology Major Project (2016GGX104001, 2017CXGC1010, and 2018JMRH0211), and the Natural Science Foundation of Shandong Province (ZR2017MEM002).
[25]
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Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.carbon.2019.08.059.
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Carbon journal homepage: www.elsevier.com/locate/carbon
Nitrogen-doped carbon derived from pre-oxidized pitch for surface dominated potassium-ion storage Qing Sun, Deping Li, Jun Cheng, Linna Dai, Jianguang Guo, Zhen Liang, Lijie Ci* SDU & Rice Joint Center for Carbon Nanomaterials, Key Laboratory for Liquid-Solid Structural Evolution & Processing of Materials (Ministry of Education), School of Materials Science and Engineering, Shandong University, Jinan, 250061, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 May 2019 Received in revised form 5 August 2019 Accepted 20 August 2019 Available online 10 September 2019
Graphitic material has captured tremendous attentions as anode material for potassium ion batteries (PIBs). Nevertheless, the large radius of potassium-ions results in sluggish potassiation kinetics and huge volume expansion, leading to unsatisfying performance. Herein, a fabrication facile, cost-effective and high carbon yield nitrogen/oxygen co-doped amorphous carbon (NOC) with pitch and urea as precursors is reported. Pre-oxidation process is employed, maintaining the amorphous structure of pitch derived carbon against its soft carbon nature. The NOC electrode delivers reversible capacities of 347 (300th cycle) and 167 mAh g1 (1000th cycle) at 100 and 2000 mA g1, respectively. Rearrangement of graphene layers in short range benefits the structure stability against volume change. Kinetics analyses prove that surface-induced capacitive process dominates in K-ion storage mechanism, which contributes to the remarkable electrochemical performance. Pouch full cells were assembled, delivering a capacity of 316 mAh g1 at 100 mA g1. In view of the cost-effectiveness and electrochemical performance, this work offers a strategy for the fabrication of low-cost and high-performance PIB anode materials. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Potassium ion batteries (PIBs) Nitrogen-doped carbon Pre-oxidized pitch Surface-dominated Diffusion coefficient
1. Introduction With the ever-growing demand for efficient and rechargeable lithium ion batteries (LIBs), the consumption of lithium is rapidly increasing, which raises concerns about the sustainability of limited and unevenly distributed lithium resources [1e4]. As a result, substitutes for lithium are highly sought after [5e9]. As group IA elements, sodium and potassium share similar chemical properties to lithium [10,11]. Therefore, sodium and potassium are likely the most ideal alternatives to lithium [12e14]. Compared to lithium (0.0017 wt%), potassium is abundant in the Earth's crust (2.09 wt%), approximating to sodium (2.36 wt%) [3,15]. Furthermore, close to the redox potential of lithium (3.04 V), potassium has a lower redox potential compared to sodium (2.93 V vs. 2.71 V), it thus offers a higher energy density than the latter [16e18]. In addition, the high potential present in carbon material in the potassiation process (0.2 V vs. 0.05 V in sodiation process) suggests a higher level of safety [19,20]. What's more, compared to sodium, smaller solvated ions formed by potassium ions (K-ions), along with lower desolvation energy, benefit the
* Corresponding author. E-mail address: [email protected] (L. Ci). https://doi.org/10.1016/j.carbon.2019.08.059 0008-6223/© 2019 Elsevier Ltd. All rights reserved.
diffusion kinetics dramatically [21,22]. Furthermore, compared to high-cost copper collectors, low-cost aluminum collectors are readily available. That is due to the inexistence of alloy formation between potassium and aluminum even at low potentials [22]. Despite numerous merits, the pursuit of potassium ion batteries (PIBs) is a challenging one [10]. For LIBs, graphite is the most widely utilized and commercialized anode material, exhibiting a decent reversible capacity of ~372 mAh g1 through the formation of LiC6 [15,19,23,24]. Similarly, potassium forms KC8 with graphite via intercalation, exhibiting a maximum capacity of 279 mAh g1 [10,15,23,25]. However, the larger radius of potassium ion compared to lithium ion (1.38 Å vs. 0.76 Å) results in severe capacity fading of graphitic electrode, which is caused by the enormous volume expansion upon full potassiation (~61%) [26e28]. Furthermore, the larger radius of K-ions leads to unsatisfying rate capability on account of the sluggish diffusion kinetics of intercalation [10]. To overcome these difficulties, various modified graphite materials have been designed [10]. An et al. chose expanded graphite as anode, delivering capacities of 263 and 170 mAh g1 at 10 and 200 mA g1 [17]. Li et al. proposed a unique structure of sandwichlike FeCl3@C, with a better performance of 269 mAh g1 at 50 mA g1 [20]. However, the reversible capacity and rate capability
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of modified graphite are still unsatisfying, compelling researchers to look for more promising materials [28]. Recent studies have diverted attention to non-graphitic carbon [21]. Yamamoto et al. introduced a cellulose-derived carbon material delivering a capacity of 290 mAh g1 [29]. Chen et al. put forward a carbonizationetching strategy for preparation of sulfur/oxygen dual-doped porous carbon. The large BET surface area and enlarged interlayer space boost the K-ion storage [16]. Likewise, Li et al. reported a facile strategy for nitrogen doped porous carbon. The material exhibits a superior cycling performance along with a decent rate capability, but its low carbon yield of less than 5% results in the overwhelming difficulty of practical application [30]. Elements doped amorphous carbon appears to be an optimal alternative for PIBs, but there are still many aspects to be explored [21,29]. As a cost-effective product, pitch is extensively used as the carbon source due to its high carbon yield [31e33]. However, pitch tends to be graphitized at high temperature due to its inherent nature, which, to a degree, undermines its advantages as anode material precursor for PIBs [34,35]. Fortunately, researchers have reported that oxygen-containing functional groups can be introduced via pre-oxidation, the formed cross-links can prevent melting and rearrangement [35e37]. Nitrogen doping could effectively enhance the electrochemical performance of carbon materials through the introduction of nitrogen-containing functional groups, which can enrich the active sites in carbon and boost K-ion diffusion [4,25,38]. Inspired by abovementioned works, herein, a brand new strategy for nitrogen-doped amorphous carbon is introduced. Low-cost pitch is adopted as the carbon source, with a pre-oxidation process introduced in advance. Cost-effective urea is employed as a nitrogen-doping precursor. The nitrogen/ oxygen co-doped amorphous carbon (NOC) displays high capacity and remarkable cycling stability and rate capability. The charge/ discharge process of NOC exhibits a sloping profile. Intrigued by this, kinetics analyses were carried out to verify the domination of surface-induced capacitive process. In addition, galvanostatic intermittent titration technique (GITT) was employed, and it showed that the K-ions present faster diffusion in NOC. After the cycling process, short-range ordered structure was observed in NOC via high-resolution transmission electron microscopy (HRTEM). Ex-situ Raman spectroscopy measurement were performed, further proving the emergence of the rearrangement in short range. The unique structure can accommodate the volume change caused by the intercalation of K-ions. Pouch full cells were then assembled, and subsequently delivered satisfying performance. This work shed some light on the exploration of large-scale fabrication of PIBs’ anode, offering a universal scheme of characterizations and analyses for amorphous carbon-based anode in alkali-ion based battery system and beyond. 2. Experimental section 2.1. Synthesis of PAC and NOC Pre-oxidized pitch (POP) was obtained via pre-oxidation treatment at 250 C for 120 min in the air atmosphere, with a heating rate of 10 C min1. Pitch derived amorphous carbon (PAC) was prepared by means of calcination of POP powder in a nitrogen atmosphere. With a heating rate of 10 C min1, the POP was first heat-retained at 450 C for 120 min, and then carbonized at 600, 800, and 1000 C for 180 min, respectively. Subsequently, the asprepared black products were ball milled for 60 min in a planetary ball mill machine, the final products were labeled as PAC-600, PAC-800, and PAC-1000. Nitrogen/oxygen co-doped amorphous carbon (NOC) was prepared by ball milling mixture process of POP and urea (weight ratio 4:1) for 30 min in advance, and then
following the same procedure of PAC-600. 2.2. Synthesis of KPB 1.62 g K3C6H5O7$H2O and 0.63 g FeCl2 were dissolved into 50 mL of deionized water. 50 ml K4Fe(CN)6 solution (0.1 M in deionized water) was dropwise added in under stirring. After that, the mixture was sequentially stirred for 2 h. After an aging process of 10 h, centrifugation was conducted and the precipitates were washed and centrifuged for 3 times with deionized water and ethanol respectively. Subsequently, the collected precipitates were dried in a vacuum oven at 80 C for 12 h. 2.3. Characterization The observation of morphology and microstructure of the obtained specimens were conducted with a Hitachi SU-70 fieldemission scanning electron microscopy (FESEM) and a JEOL JEM2100 transmission electron microscopy (TEM). The structure of NOC and PAC-x was characterized via X-ray diffraction measurement (XRD) with a Rigaku Miniflex 600 X-ray diffractometer, with Cu Ka radiation (l ¼ 1.5406 Å) at 40 kV and Raman spectra on a Renishaw InVia Reflex. X-ray photoelectron spectroscopy (XPS) was performed with an ESCALAB 250 spectrometer (Al Ka X-ray source, excitation energy of 1486.71 eV) to analyze the element contents. The specific surface areas (SSA) and pore width distributions were determined with an ASAP 2020 analyzer using the BrunauerEmmett-Teller (BET) method. The specimens were first degassed at 150 C for 2 h, and the nitrogen adsorption/desoption isotherms were acquired at 77 k. A Mettler-Toledo TGA2 Thermo Analyzer was used to perform the thermogravimetric analyses of pitch precursor (air atmosphere, 35 Ce600 C, 20 mL min1) and POP (nitrogen atmosphere, 35 Ce1000 C, 20 mL min1). 2.4. Electrochemical measurements The electrodes were fabricated by mixing active materials (obtained specimens and KPB, 80 wt%), super P (10 wt%), and polyvinylidene fluoride (PVDF, 10 wt%) as binder. The homogenously dispersed slurries were coated onto copper foil (obtained specimens) and aluminum foil (KPB) respectively, and then dried in a vacuum oven at 80 C for 10 h until the solvents had completely evaporated. For half cells, the dried anode electrodes were circular punched (diameters of 14 mm). The type CR-2032-coin cells were assembled with potassium metal as counter electrodes in a glove box (argon-filled, H2O < 1 ppm, and O2 < 1 ppm). As for pouch full cells, aluminum-plastic film (D-EL35H, DNP) was formed on a KJ MSK-120 and then top side sealed on a KJ MSK-140. The electrodes were punched into rectangles (cathode: 57 44 mm; anode: 58 45 mm) with uncovered tabs on a die-cutting machine (KJ MSK-180). Afterwards, the electrodes were severed by separators, wrapped with insulating tape, and put into the aluminum-plastic film after the uncovered tabs were ultrasonic-welded with a KJ MSK-800 W to achieve parallel connection. After the electrolyte injection process in the glove box, the pouch full cell was vacuumsealed with a KJ MSK-115A-S. As for the electrolyte, 0.8 M KPF6 solution was dissolved into ethylene carbonate/diethyl carbonate (EC/DEC 1:1 by volume) mixture as electrolyte. Glass fiber membranes (Whatman GF/D) were applied as separators. The charge/ discharge performance was tested within the voltage window of 0.01e3.0 V for half cells and 2.0e4.0 V for full cells on battery testers (LAND-CT2001A). The CV measurements were conducted within the voltage range of 0.01e3.0 V at various scan rates of 0.1e2.0 mV s1 with an electrochemical workstation (Autolab, Metrohm). As for galvanostatic intermittent titration technique
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(GITT) measurements, the method of pulse currents (100 mA g1 for 15 min) separated by relaxation intervals (30 min) was adopted. For ex-situ HRTEM and ex-situ Raman spectra, the cycled coin cells were disassembled in glove box, the electrode was washed in DEC solvent and then dried in vacuum overnight. 3. Results and discussion 3.1. Morphology and structure characterization Pitch derived carbon tends to be graphitized due to its soft carbon nature, while the pre-oxidation introduced oxygencontaining functional groups can form cross-links to prevent melting, therefore avoid the reordering phenomenon effectively [35]. To determine the optimal temperature for pre-oxidation, thermos-gravimetric analysis (TGA) was performed from 35 to 600 C (10 C min1) under air atmosphere. As shown in Fig. S1a, there is a rapid increase of 0.7% of weight gain between 200 and 250 C, which can be ascribed to the oxygen acquisition [36]. Until 250 C, the mass increase ratio amounts to be 0.9%, and the oxygen acquisition should be higher considering the escape of H2O and methylene hydrogen [35]. After that, the mass decreased quickly due to the violent reaction with oxygen, and eventually burned off [36]. Refer to this, pre-oxidized pitch (POP) was therefore obtained in the air at 250 C for 120 min. Fig. 1a illustrates the typical preparation process. Pitch was first modified by air pre-oxidation treatment, and the formed cross-links could avoid melting and rearrangement at high temperature. Specifically, with the gradually decomposition of oxygen containing functional groups during calcination, small molecules like CO and CO2 escape, leading to an even more disordered structure [39]. As for the nitrogen doping of NOC, urea was evenly mixed with POP followed by facile calcination
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treatment. Subsequently, ball-milling was conducted to achieve the acquirement of particles morphologies. For the sake of understanding the microstructure of POP derived and pitch derived carbon, high-resolution transmission electron microscopy (HRTEM) images were recorded respectively in Fig. 1b and c. Compared with the POP derived carbon (1000 C), the carbon layers of pitch derived carbon are clearer and more parallel, which indicate a higher degree of graphitization. Meanwhile, the selected area electron diffraction (SAED) further demonstrates the abovementioned graphitization difference. POP derived carbon exhibits more disordered carbon layers in random directions and more dispersing SAED rings, revealing the lower graphitization degree of POP derived carbon arisen from the pre-oxidation process. The disordered structure of POP derived carbon results in the absence of plateau capacity as the later test shows, which affects the electrochemical performance in positive impacts. Calcination temperatures have effects on the microstructure of carbonaceous materials to a degree, and influence the electrochemical performance [32]. To investigate the thermal stability of POP, TGA was conducted from 35 to 1000 C at a rate of 10 C min1 (nitrogen atmosphere). As shown in Fig. S1b, POP starts to lose weight at approximate 300 C and reaches a maximum rate at 423 C. The weight loss is almost complete at ~550 C. Different pitch derived amorphous carbon (PAC) specimens were obtained accordingly, labeled PAC-600, PAC-800, and PAC-1000, reflecting the calcination temperatures of 600, 800, and 1000 C. Based on the latter characterizations and tests, 600 C is adopted as the optimal calcination temperature for NOC. Therefore, the resultant specimen prepared is labeled as NOC-600. Figs. S2aeS2d shows the morphologies of NOC-600, PAC-600, PAC-800 and PAC-1000 investigated by field emission scanning electron microscope (FESEM), and NOC-600 (Fig. S2e) and PAC-600
Fig. 1. (a) Schematic illustration of the preparation process with and without pre-oxidation; HRTEM images with SAED insets of (b) POP derived carbon and (c) pitch derived carbon calcinated at 1000 C. (A colour version of this figure can be viewed online.)
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(Fig. S2f) were further examined by transmission electron microscopy (TEM). The specimens exhibit irregular morphologies with uniform particle sizes, which should be ascribed to the preceding ball-milling. The HRTEM image (Fig. S2g) presents the disordered microstructure of NOC-600, which results from the pre-oxidation process and the nitrogen doping. XRD and Raman spectroscopy were employed to characterize the microstructures. As depicted in Fig. 2a, the series of XRD patterns possess two distinct broad diffraction peaks, which can be respectively indexed to (002) and (100) diffraction mode, indicating the formation of amorphous carbon structure. Particularly, with the calcination temperature increasing, the (002) peak shifts from 25.32 to 25.54 and grows sharper as the intensity increases. This phenomenon suggests the decrease of interlayer distance and the increased degree of graphitization. Note that NOC-600 possesses the broadest peak width and lowest peak intensity of XRD pattern, indicating its lowest degree of graphitization, which is attributed to the successful doping of nitrogen. The grafting of nitrogen groups effectively expanded the carbon layers of NOC-600, which is calculated to be 0.356 nm according to the Bragg equation (2dsinq ¼ nl), larger than graphite's 0.335 nm. The carbon matrix with expanded carbon interlayer distance can accommodate the volume change during potassiation/depotassiation, contributing to the stability during cycling, and also accelerate the K-ions insertion/extraction kinetics [17,30]. Raman spectra (Fig. 2b) present two characteristic bands which correspond to D-band (the disorder/defect-induced mode) and G-band (the in-plane vibrational mode) at around 1336 and 1586 cm1 [26,40]. It is noticeable that, with the increase of calcination temperature, the half widths at half maximum (HWHM) of D-band and G-band of PAC-x decrease, which is consistent with the uptrend of the temperature, indicating the transformation from disordered to ordered structure [33]. The peak areas are fitted by Lorenz model, and ID/IG
is calculated from the ratio of peak areas of D-band to G-band. NOC600 possesses the highest ID/IG of 2.74, indicating a most disordered degree of microstructure along with abundant structural defects which is ascribed to the nitrogen doping [30,41]. X-ray photoelectron spectroscopy (XPS) analysis was conducted to investigate the element doping configuration in NOC. The XPS survey spectra (NOC-600 and PAC-600, Fig. 2c) present peaks at approximately 283.9, 401.2, and 531.9 eV, corresponding to C 1s, N 1s, and O 1s, respectively [25]. The relative atomic percentages of C (91.8 at%), N (1.7 at%) and O (6.4 at%) are calculated for NOC-600 while the ratios for PAC-600 are 93.7 at% (C) and 5.6 at% (O) with the absence of N. In view of the similar content of carbon and oxygen of NOC-600 and PAC-600, it's reasonable to ascribe the enhanced electrochemical performance of NOC to nitrogen doping. High resolution XPS spectra are further characterized, as drawn in Fig. 2d, N 1s is deconvoluted into tree peaks at 398.3, 399.9, and 401.5 eV, which can be indexed to pyridinic nitrogen (N-6), pyrrolic nitrogen (N-5), and graphitic nitrogen (N-Q), respectively [21,42]. It's obvious that N-5 and N-6 dominate in the nitrogen containing species of NOC-600, and researches have shown that the electrochemical active character of N-5 and N-6 is significantly beneficial for the rate capability, the doped nitrogen can create plenty of active sites and thereby enhance the surface-driven process [23]. Meanwhile, N-Q supplies decent electroconductive enhancement via electron-donor characteristic change [21,23,43]. Oxygen element of NOC is derived from the innate oxygen-containing functional groups of pitch and the pre-oxidation process. High resolution XPS spectrum of O 1s (Fig. 2d) is deconvoluted into three peaks denoted as -C¼O (O-I), C-O (O-II), and -COOH (O-III) at 531.1, 532.3, and 533.3 eV, respectively [38,42]. Despite the reduction of electroconductivity they lead to, oxygen atoms can increase defect density and disorder degree, as well as effectively improve the wettability of the NOC, and facilitate the utilization of the surface area [25,43].
Fig. 2. Structural characterization. (a) XRD patterns; (b) Raman spectra; (c) XPS spectra; (d) High-resolution XPS spectra for N 1s and O 1s of NOC-600; (e) Nitrogen adsorptiondesorption isotherm and (f) related pore size distribution of NOC-600 and PAC-600. (A colour version of this figure can be viewed online.)
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It's widely accepted that pore structure and pore-size distribution of carbon electrode materials significantly influence their electrochemical performance [23,44]. Therefore, to get a deeper insight into NOC-600 and PAC-x, nitrogen absorption-desorption were performed. As demonstrated in Fig. 2e, NOC-600 presents a Brunauer-Emmett-Teller (BET) isotherm of Type I (IUPAC definition) with a relatively high nitrogen adsorption at low relative pressure area, indicating its typical character of microporosity [24,30]. The specific surface area (SSA) of PAC-600, PAC-800, and PAC-1000 are calculated to be 20.3, 26.0, and 28.5 m2 g1. The similar SSA values of PAC-x suggest that the employed temperature range between 600 and 1000 C has minimal impact on the SSA of the samples. In contrast, the SSA of NOC-600 is calculated to be 176.5 m2 g1, much larger than that of PAC-600 (~6 times higher). The pore volume values which are 0.03, 0.03, 0.04, and 0.19 cm3 g1 for PAC-600, PAC-800, PAC-1000, and NOC-600, respectively. The significantly increased SSA and pore volume of NOC-600 should be attributed to the additional defects and nanopores introduced by nitrogen doping. Fig. 2f and S3b exhibit the pore-size distributions of NOC-600 and PAC-600 calculated with quenched solid density functional theory (QSDFT) method [45]. Both NOC and PAC possess pores covering from micropores to macropores, proving the
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hierarchically porous architecture with the coexistent micro-mesomacro pores. It is worth noting that NOC-600 presents newly emerged pores with size ranging from 0.39 to 1.27 nm, particularly at ~1.18 nm (Fig. 2f). The abundance of micropores could provide plenty of active sites along with the enhanced affinity to K-ions, thereby not only providing storage locations to accommodate the charge, but also facilitating the transport of electrons and K-ions [10,23,46]. 3.2. Electrochemical performance The cyclic voltammetry (CV) measurement was conducted to investigate the electrochemical properties of NOC-600 and PAC600 electrodes within a voltage window ranging from 0.01 to 3.0 V (vs. K/Kþ) at 0.1 mV s1. During the first cathodic scan, an irreversible peak presents at ~1.65 V for NOC-600 (Fig. 3a), indicating the reaction between surface functional groups and K-ions [25]. In addition, both NOC-600 (Fig. 3a) and PAC-600 electrodes (Fig. 3b) exhibit peaks at ~ 0.50 V and the peaks vanish in the subsequent scans, which might be attributed to the formation of solid electrolyte interface (SEI) along with the decomposition of the electrolyte [46,47]. On account of the larger SSA, NOC-600 electrode
Fig. 3. Electrochemical performance of NOC and PAC. CV curves of (a) NOC-600 and (b) PAC-600; (c) Cycling stability of NOC and PAC at a current density of 100 mA g1; (d) Charge/ discharge voltage profiles of NOC-600; (e) Rate performance of NOC and PAC at current densities from 100 to 2000 mAh g1; (f) Charge/discharge voltage profiles of NOC-600 at various current densities and (g) cycling performance of NOC and PAC at a high current density of 1000 mA g1. (A colour version of this figure can be viewed online.)
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possesses much more prominent cathodic peak at ~ 0.50 V than PAC-600 due to the formation of extra SEI [45]. This will result in large capacity consumption and thus reducing the initial coulombic efficiency (CE), as confirmed by the subsequent testing. The sharp cathodic peak at 0.01 V and broad anodic peak at ~0.25 V are related to the intercalation and extraction of K-ions, respectively [21,43,48]. Additionally, several humps are observed at ~ 0.60 V (NOC-600), ~0.70 V (PAC-600), ~1.20 V (PAC-600), and ~1.80 V (NOC-600) due to the interaction between K-ions and heteroatoms on the surface of the materials like N and O [47]. Generally, the relatively broad profiles of CV curves (NOC-600) reveals the higher proportion of surface driven storage in K-ion storage, which could significantly improve the rate capability and cycling stability [49]. Furthermore, the CV curves of the second cycle overlapped well with the third, which indicates decent microstructure stability and a good cycling reversibility. To measure the cycling stability of the materials, the NOC and PAC-x electrodes were tested at a current density of 100 mA g1 (Fig. 3c). The initial discharge capacity of NOC-600 (2209.1 mAh g1) is higher than those of PAC-600 (610.0 mAh g1), PAC-800 (718.0 mAh g1), and PAC-1000 (618.9 mAh g1), corresponding to initial CEs of 32.8%, 43.2%, 46.9%, and 44.6%. The high initial discharge capacity along with the low initial CE can be ascribed to the K-ions consumption during the formation of SEI, which was confirmed by the abovementioned CV profile [50]. After fully activated in the first few cycles, the electrode exhibits a CE which gradually grows close to 100%. NOC-600 electrode exhibits a considerable reversible capacity of 347 mAh g1 after 300 cycles, noticeably higher than those of PAC-600 (192 mAh g1), PAC-800 (167 mAh g1), and PAC-1000 (143 mAh g1). The CE of NOC-600 electrode stabilizes after more cycles, which could be ascribed to the additional time required for the formation of stable SEI layer due to the high SSA [21]. The discharge capacity of NOC-600 electrode remains to be 93% (vs. 10th cycle)after 300 cycles, dramatically surpassing those of PAC-600 (77%), PAC-800 (70%), and PAC1000 (61%). The charge/discharge profiles of NOC-600 electrode are shown in Fig. 3d. Within the voltage window ranging from 0.01 to 3.0 V (vs. K/Kþ), the curves overlap well from 50th to 300th cycle, indicating a superior cycling stability of NOC-600 electrode. It's noteworthy that NOC-600 electrode exhibits a prominent sloping feature, which is a characteristic behavior of surface dominated Kion storage [47,51]. Surface dominated storage behavior benefits the fast K-ions transport [48,49]. In view of this, the rate performance for NOC and PAC electrodes were evaluated (Fig. 3e). NOC-600 electrode exhibits capacities of 461.2, 358.1, 300.4, and 224.0 mAh g1 at 100, 200, 500, and 1000 mA g1, exceeding PAC tremendously. And the capacity remains 162.5 mAh g1 even at a relatively high current density of 2000 mA g1. The superior rate capability of NOC-600 is in the lead among reported carbon based materials in PIBs (Fig. S4). As for PAC-x electrodes, with the increase of the calcination temperature, the rate performance becomes worse, indicating negative effect due to the higher degree of graphitization. Surprisingly, the capacities almost completely recover in the wake of the step by step reverted current density. Moreover, in the last 50 cycles at 200 mA g1, NOC-600 electrode maintained a stable cycling capacity of 315 mAh g1. Notably, for NOC-600 electrode (Fig. 3f), the proportion of plateau capacity (below ~0.3 V), which is originally small, almost vanished with the current density increased to 2000 mA g1. This phenomenon proves the domination of surface capacitive behavior in K-ion storage process particularly at high currents [21]. This will be further corroborated in the following contribution analysis of K-ion storage process (Fig. 5). Long-term cycling stability at high rate of 1000 mA g1 were evaluated (Fig. 3g). NOC-600 electrode exhibits a reversible capacity of 167
Fig. 4. (a) Schematic illustration of the K-ion storage process; (b) HRTEM images of pristine and cycled NOC-600 and (C) interlayer spacing of short range ordered structure. (A colour version of this figure can be viewed online.)
mAh g1 after 1000 cycles, far beyond those of PAC-600 (94 mAh g1), PAC-800 (69 mAh g1), and PAC-1000 (54 mAh g1). A high capacity retention of 70.0% (NOC-600) can also be achieved after 1000 cycles, as opposed to 48.0% for PAC-600, 36.3% for PAC-800, and 27.7% for PAC-1000. The better performance is attributed to the domination of surface-induced capacitive process in the NOC600 electrode. However, there is a continuous capacity loss during the first 200 cycles, after which the capacity tends to stabilize. This can be ascribed to a certain degree of collapse of the twisted graphitic structure during the intercalation of K-ions, indicating the co-existence of surface-induced capacitive process and diffusioncontrolled intercalation process [19]. 3.3. Ex-situ analyses of K-ion storage process In view of the remarkable electrochemical performance above, ex-situ characterizations were adopted to explore the K-ion storage process. Fig. 4a illustrates the K-ion storage process. As for pristine NOC-600 electrode, a typically amorphous structure is observed in pristine state via HRTEM (Fig. 4b). After the cycling process, rearrangement in short range clearly emerges, possessing an interlayer spacing calculated to be 0.40 nm on average (Fig. 4c). It is noteworthy that the interlayer spacing of short range ordered stacks is larger than that of graphite (0.335 nm), this benefits the accommodation of large-radius K-ions and enhances the K-ion transport [17]. The co-existence of short range ordered stacks and amorphous structure can evenly distribute the stress, thus relieving the volume expansion during cycling and resulting in superior cycling stability [24,25]. Ex-situ Raman spectroscopy was conducted to further
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Fig. 5. Analysis of K-ion storage and diffusion kinetics. CV curves at various scan rates of (a) NOC-600 and (b) PAC-600; (C) b values determined from linear fitting; (d) Contributions of surface-induced capacitive process in NOC-600 and PAC-x at various scan rates; (e) Contribution of the surface-induced capacitive process in NOC-600 at 0.5 mV s1; (f) GITT profiles of NOC-600 and (g) diffusion coefficients of NOC-600 and PAC-600. (A colour version of this figure can be viewed online.)
investigate the K-ion storage process. As shown in Fig. S5, both Raman spectra (collected on pristine and cycled electrodes, respectively) present two characteristic bands corresponding to Dband and G-band. Notably, after fitted by Lorenz model, the ID/IG ratio of cycled electrode (2.24) decreases distinctively compared to the pristine electrode (2.62). This suggests that the degree of order is improved to some extent, which is attributed to the potassiation/ depotassiation process [50]. The Raman results are consistent with the results of the abovementioned HRTEM images. 3.4. Analysis of K-ion storage and diffusion kinetics To further investigate the mechanism of K-ion storage, CV measurements were conducted at various scan rates ranging from 0.1 to 2.0 mV s1. As shown in Fig. 5a, b, Figs. S6a and S6b, broad peaks of NOC-600 are maintained at various scan rates, while the peaks of PAC-x are steeper and exhibit the characteristics of polarization with the increase of calcination temperature [47]. A power-law equation is introduced to describe the relationship
between peak current (i) and scan rate (v) [40,51,52]
i ¼ avb
(1)
logðiÞ ¼ blogðvÞ þ logðaÞ
(2)
Where a is adjustable constant, i the peak current of each CV curve, and v the corresponding scan rate. The value of b could determine the contribution of surface-induced capacitive process and diffusion-controlled intercalation process during potassiation/ depotassiation process [21]. It indicates an typical diffusioncontrolled process when b ¼ 0.5 whereas an ideal surfaceinduced capacitive process when b ¼ 1 [47]. The b values are calculated from the linear fitting data that are acquired from the CV curves of various scan rates. Preferable linear relationships are achieved (Fig. 5c). NOC-600 electrode possesses the highest b value (~0.91), suggesting a surface dominated K-ion storage behavior attributed to the nitrogen doping. As predicted, the b values of PACx electrodes decrease from 0.85 to 0.80 owing to the increase of
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graphitization degree. The contribution of surface process could be separated from the mixed storage mechanisms by following Equation (3) [10,53]
pffiffiffi i ¼ k1 v þ k2 v
(3)
Where i represents the current response at a fixed potential, k1 and k2 the constants. The separated contributions of surface-induced process and diffusion-controlled process can be represented by pffiffiffi k1 v and k2 v. Fig. 5d shows the contributions of surface-induced process at scan rates of 0.1, 0.2, 0.5, 1.0, and 2.0 mV s1. Following the increase of scan rates, the capacitive contributions of all the specimens increases, which benefits the rate capabilities as well as the cycling stabilities at high rates. When it comes to the same scan rate, the growth of capacitive contribution complies well with the increased disorder degree. The typical separation of capacitive contribution for CV profiles at a scan rate of 0.5 mV s1 is presented in Fig. 5e, with red areas of capacitive behavior. NOC-600 possesses the highest capacitive contribution, the surface-dominated K-ion storage behavior should be attributed to the hierarchically porous architecture and more surface defects introduced by nitrogen doping [30]. Galvanostatic intermittent titration technique (GITT) measurements were employed to further investigate the diffusion kinetics of K-ion storage. The diffusion coefficient (Dk) of K-ions during potassiation/depotassiation can be calculated with the following Equation (4) [19,54]
Dk ¼
4mB V m DE s 2 pM b S Dt DEt
(4)
Where mB represents the active material mass loaded on the electrode; Mb the molar mass; Vm the molar volume of active material; S the electrode area in geometry; and Dt the time of periodic discharge/charge. Descriptions of DEs and DEt are shown in Fig. S7 in detail. The diffusion coefficients of K-ions in NOC-600 and PAC-600 electrodes were measured via GITT with a periodic repetition of pulse currents of 100 mA g1 for 15 min separated by relaxation intervals of 30 min (Figs. 5f and S8). The calculated coefficients are plotted as functions of evolving potassiation/depotassiation depths in Fig. 5h. As shown, due to the well-developed hierarchical porous architecture and surface defects derived from nitrogen doping, NOC-600 electrode exhibits distinctly higher coefficient than PAC-600 in general, suggesting its faster K-ion diffusion kinetics. It is worth noting that with the evolution of potassiation process, the fluctuant coefficient diminishes
moderately. This is because the ease of access to the surface sites leads to the prior occupation of K-ions. After that, K-ions diffuse inside, and have to overcome the resistant gradient from the K-ions anchored previously [10,47]. As for the depotassiation process, the tendency is almost the opposite, further proving the irreversibility of K-ion storage. 3.5. Electrochemical performance of pouch full cells Encouraged by the above tested electrochemical performance of NOC-600, pouch full cells were further assembled with NOC-600 electrode as anode and potassium Prussian blue (KPB) as cathode. KPB is regarded as a promising cathode material for PIBs due to its low cost and facile synthesis [55]. KPB was synthesized via a reported precipitation route [47]. XRD pattern and cycling performance with initial charge/discharge voltage profile of KPB are posted in Fig. S9. Considering the high K-ion consumption during the initial cycle of NOC-600, the anode is pre-potassiated with a method of direct contact with potassium foil in electrolyte to compensate for the initial capacity loss. To achieve the optimal effect, electrodes of different contact time are employed to assemble half cells. The comparison of ICEs (Fig. S10) indicates that 7.5 min is the optimal pre-potassiation time. The pouch full cells are designed in anode-limited mode with a capacity ratio of 1:1.1 (anode: cathode). At a current density of 100 mA g1, the full cell presents a discharge capacity of 316 mAh g1 within a voltage window ranging from 2.0 to 4.0 V (Fig. 6a). It's worth noting that in contrast with the plateaus of KPB half cell, the full cell exhibits distinct semi-plateaus, it should be attributed to the sloping profiles of NOC-600. The assembled pouch full cell can charge a HUAWEI honor V10 via an E50D-based 5 V voltage booster circuit (Fig. 6b). The circuit diagram is available in Fig. S11. The pouch full cell demonstrates the application of the synthesized anode material in PIBs. 4. Conclusions In summary, a facile fabrication process of nitrogen/oxygen codoped amorphous carbon material with cost-effective and high carbon yield is reported. Low-cost urea and pitch are employed as precursors along with easy pre-oxidation in advance, which maintain the amorphous structure against the soft carbon nature of pitch derived carbon. The NOC electrode delivers remarkable electrochemical performance due to its hierarchical porous architecture and nitrogen doping. The rearrangement of graphene layers in short range is observed after cycling, which can accommodate
Fig. 6. (a) Cycling performance with initial charge/discharge voltage profile of NOC-600/KPB full cell at 200 mA g1 and (b) photograph of a HUAWEI Honor V10 charged by an assembled NOC-600/KPB pouch full cell. (A colour version of this figure can be viewed online.)
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the volume change during K-ion storage. Kinetics analysis proves that surface-induced capacitive process dominates in K-ion storage mechanism, thus accomplishing the high reversible capacity, superior cycling stability and remarkable rate capability of NOC. Quantitative calculation indicates that the diffusion coefficient of Kions in NOC electrode surpasses that in PAC electrode, proving that nitrogen doping effectively benefits diffusion kinetics. This work serves as an exploration of large-scale fabrication of amorphous carbon anode for PIBs.
[19] [20]
[21]
[22] [23]
Acknowledgment [24]
This work was supported by Startup Funding of Distinguished Professorship of “1000 Talents Program” (31370086963030), the Fundamental Research Funds of Shandong University (2016JC005, 2017JC042 and 2017JC010), Shandong Provincial Science and Technology Major Project (2016GGX104001, 2017CXGC1010, and 2018JMRH0211), and the Natural Science Foundation of Shandong Province (ZR2017MEM002).
[25]
[26]
[27] [28]
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.carbon.2019.08.059.
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