S-Doped Porous Graphene Microspheres with Individual Robust Red

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S-Doped Porous Graphene Microspheres with Individual Robust RedBlood-Cell-Like Microarchitecture for Capacitive Energy Storage Xinlong Ma, Xinyu Song, Guoqing Ning, Liqiang Hou, Yanfang Kan, Zhihua Xiao, Wei Li, Guixuan Ma, Jinsen Gao, and Yongfeng Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b01953 • Publication Date (Web): 01 Aug 2017 Downloaded from http://pubs.acs.org on August 8, 2017

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Industrial & Engineering Chemistry Research

S-Doped Porous Graphene Microspheres with Individual Robust Red-Blood-Cell-Like Microarchitecture for Capacitive Energy Storage Xinlong Ma, † Xinyu Song, † Guoqing Ning,* Liqiang Hou, Yanfang Kan, Zhihua Xiao, Wei Li, Guixuan Ma, Jinsen Gao, and Yongfeng Li* State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing, Changping 102249, China. KEYWORDS: S-doped porous graphene, Red-blood-cell-like, Individual microarchitecture, Robust scaffold, Capacitive behavior

ABSTRACT: Three-dimensional individual S-doped porous red-blood-cell-like graphene (SRBCG) microspheres with double concave-surface morphology duplicated from a templatedirected chemical vapor deposition process in a fluidized bed reactor, not only exhibit high porosity, good structural stability and strong anti-compression property, but also present superior capacitive energy-storage abilities with respects to symmetric supercapacitor (great compatibility at different rates) and Li ion capacitor (no capacitance loss after 3500 cycles at 2 A g-1). The well-kept integrity of electrode configuration after cycling is benefited from the intrinsic robust scaffold which acts as structural buffer for volume expansion to inhibit structure collapse. The

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unique individual microarchitecture with well-developed pore channels of SRBCG can effectively prevent the obtained graphene from aggregation or restacking, expanding the contact area between electrolyte ions and electrode. The excellent capacitive behaviors of SRBCG are guaranteed by the unique robust microarchitecture accompanied with the good structural stability. Additionally, the fluidized bed technology is conducive to the realization of the homogeneous growth and scalable production of SRBCG. 1. INTRODUCTION Graphene has been successfully employed in the energy-storage and conversion applications, such as in Li ion batteries (LIBs),1, 2 Li ion capacitors (LICs),3 electrocatalysts,4, 5 sodium ion batteries,6 electrochemical capacitors,7-9 and Li-S batteries10 due to the strong mechanical property, high specific surface area (SSA), good chemical stability and excellent conductivity. However, most synthesized graphene materials like reduced graphene oxide (RGO) nanosheets and graphene films derived from chemical vapor deposition (CVD) process using metal (nickel or copper) foils as substrates, usually present the plane-laminated structure, and then irreversible agglomeration and stacking are easily aroused by the strong π-π interaction and high inter-sheet junction contact resistance, which leads to decreasing the conductivity and inhibiting the full potential properties of graphene.11, 12 Recent studies have demonstrated that three-dimensional (3D) individual architectures of graphene accompanied with abundant interconnected porous networks in bulk phase were dedicated to preventing the restacking of graphene.13,

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The porous structures of graphene

materials make them attractive as the key components in electrochemical energy storage and conversion devices. For instance, as electrodes for lithium ion batteries and supercapacitors, the


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robust 3D porous framework of graphene not only contributes to providing the conductive networks, but also efficiently provides a large accessible surface area for electrolyte ions transport or charge storage. As electrocatalysts for fuel cells and dye-sensitized solar cells, the well-defined porosity is favorable for the electron transport, electrolyte-electrode interaction and electrolyte-reactant diffusion.15 Besides, the curved surface of graphene also contributes to reducing the agglomeration degree.16 Nevertheless, the concave-convex position and degree are mainly based on the natural curvature property of graphene. Thus, the controllability of curved positions is usually poor and the bending angle is really small. Moreover, the crucial premise of the enhancement of electrochemical performance is to guarantee that the graphene possesses the unique individual microarchitecture, large SSA, stronger mechanical stability and intrinsic robust scaffold accompanied with the satisfactory porosity.17-19 To effectively suppress the restacking phenomenon and sufficiently manifest the electrochemical energy-storage ability of graphene, the promising strategy is to combine the 3D individual robust scaffolds with the unique curved surface structure. CVD process can not only control the growth of few-layered graphene with high SSA and extraordinary conductivity, but also perfectly duplicate the morphology of templates.20, 21 Therefore, the convinced advantages of 3D individual microarchitecture accompanied with the porous networks and curvature for graphene, as well as the duplicated property of CVD approach, trigger us to fabricate the 3D double concave-surface porous graphene with individual robust microarchitectures using redblood-cell (RBC)-like MgO microspheres as templates in a controllable manner. The in-situ S incorporation into the carbon framework, which is conducive to distorting lattice structure of graphene and improving interaction with electrolyte ions, is simultaneously realized along with the growth of RBC-like graphene.


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In this work, S-doped RBC-like porous graphene (labeled as SRBCG) building blocks covered with staggered-mesh structure derived from the fluidized bed technology, present excellent structural integrity, strong anti-compression property and remarkable electrochemical energystorage ability. The symmetric supercapacitor and LIC fabricated by SRBCG present excellent rate capabilities and long-term cycling stabilities. 2. EXPERIMENTAL SECTION 2.1. Materials. Commercial ammonia solution (Sinopharm Chemical Reagent Beijing Co. Ltd., China), magnesium sulfate heptahydrate and sodium dodecyl benzene sulfonate (Tianjin Fuchen Chemical Reagents Factory, China). All reagents were analytical grade andused as received without further purification. 2.2. Synthesis of SRBCG. First, RBC-like basic magnesium carbonate microspheres were prepared by the surfactant-consumed self-assembly process reported in the previous work,22 and were calcined at 700 °C for 1 h, yielding in porous MgO microspheres. Then, the graphene microspheres were synthesized by a CVD approach using methane as carbon source and the asproduced MgO microspheres as templates. In a typical CVD synthesis, MgO microspheres were fed into a fluidized bed reactor from the top hopper after the reaction temperature reached to 900 °C in an argon flow at atmospheric pressure. Then methane was bubbled into the reactor for 10 min. The black materials were taken out and purified by excessive amount of hydrochloric acid after the reactor was cooled to room temperature. Then the residues were filtrated by deionized water and dried in the vacuum oven overnight to obtain the final graphene. 2.3. Characterizations. The morphologies of the as-prepared materials were characterized by scanning electron microscopy (SEM, Quanta 200F, FEI, Holand) and transmission electron


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microscopy (TEM, F20, FEI) equipped with energy dispersive spectroscopy (EDS). The X-ray photoelectron spectroscopy (XPS) spectrum was performed with a photoelectron (PHI Quantera Scanning X-ray Microprobe). X-ray diffraction (XRD) measurements were conducted on a Bruker D8 Advance diffractometer. Brunauer-Emmett-Teller SSA and pore size distribution (PSD) were measured by N2 adsorption-desorption using the Micromeritics ASAP 2020. Raman spectrum was obtained with He-Ne laser excitation at 633 nm using a Horiba Jobin Yvon LabRAM HR800 Raman spectrophotometer. 2.4. Electrochemical measurements of supercapacitors. The slurry containing 80 wt% SRBCG, 10 wt% carbon black and 10 wt% polytetrafluoroethylene was coated onto a foam nickel plate (1 cm2) to prepare the working electrodes, following by a 110 °C oven drying overnight. The mass loading of the active material for the electrode was in the range of 1.5-5 mg. The platinum plate and Hg/HgO electrode (saturated calomel electrode) were used as the counter and the reference electrodes, respectively. Electrochemical tests of the three-electrode system were carried out in 6 M KOH aqueous electrolyte solution at room temperature. The symmetric two-electrode supercapacitors were fabricated with aqueous electrolyte (6M KOH solution) and organic electrolyte (Tetraethylammonium Tetrafluoroborate), respectively. The mass loading of the active materials for each electrode is about 2.4 mg. Cyclic voltammetry (CV) and galvanostatic charge/discharge measurements were measured by the CHI660D electrochemical workstation. The total capacitance (Ctotal) for the two-electrode symmetric supercapacitor is calculated by the equation Ctotal=I×∆t/(m×∆V), and the specific capacitance (Csp) of the single electrode is calculated from Csp=4×Ctotal, where I is the discharge current (A), ∆t is the discharge time (s), ∆V is the discharge voltage (V) excluding the potential drop potential window (V) and m is the total mass of the active material for both electrodes (g).


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2.5 Electrochemical measurements of LIC. For the preparation of the slurry, 80 wt% SRBCG, 10 wt% LA133 binder and 10 wt% carbon black were mixed in water to obtain a slurry. Then the slurry was coated on an aluminum foil and copper foils to prepare the cathode and anode, respectively, which were dried at 110 °C for 12 h. The foils were cut into disks (13 mm in diameter) and coin-type cells were assembled in the glove box. For the half cell test, the electrolyte was composed of 1 M LiPF6 dissolved in a mixture of ethylene carbonate and dimethyl carbonate (1:1, v/v) and Li metal foils were used as reference/counter electrodes. The LIC was fabricated using SRBCG as anode and cathode, respectively. CV and AC impedance spectrum measurements were carried out on the CHI660D electrochemical workstation with frequency range of 0.1 Hz to 100 KHz. 3. RESULTS AND DISCUSSION 3.1. SRBCG synthesis and characterization. The growth of SRBCG based on CVD process is schematically illustrated in Scheme 1. SEM images of RBC-like basic magnesium carbonate microspheres prepared by our previously-reported surfactant-consumed self-assembly approach are shown in Figure 1a and b. Basic magnesium carbonate microspheres with a red-blood-cell (RBC)-like appearance were synthesized by the sodium dodecyl benzene sulfonate (SDBS)participated self-assembly process. The growth of the microspheres from a tiny SDBS micelle to a sphere of several microns is driven by the continuous generation of new hydrophobic centers because of the consumption of hydrophilic poles (SO3-). Therefore, there are some S-containing functional groups in RBC-like basic magnesium carbonate microspheres. After that, RBC-like MgO microspheres with a small amount of S-containing functional groups were obtained by the calcination treatment. Porous MgO microspheres detected by the XRD with well-kept RBC-like appearance were obtained after calcination (Figure 1c and Figure S1a). The PSD of porous MgO


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microspheres reveals that mesopores with the sizes of 2-10 nm are dominant, and the SSA and pore volume are 105 m2 g-1 and 0.29 m3 g-1, respectively (Figure S1b), which is conducive to the adsorption of methane. The growth of graphene is achieved by methane cracking on the surface and pores of as-obtained MgO microspheres. For the mechanism of the graphene growth, methane is firstly adsorbed on the surfaces and pore channels of MgO templates, and then MgO flakes with the oxygen-terminated surface contributes to catalyzing the cracking of methane.14 After that, C atoms rearrange to form graphene through the self-assembly process. Finally, the graphene will be uniformly deposited on the pore channels and exposed surfaces of MgO templates. The in-situ S incorporation into graphene framework is simultaneously realized during the growth of graphene because of the existence of a small amount of S-containing functional group in templates.22 During the preparation of graphene, S incorporation is realized by the doping reaction between methane and sulfate group in S-containing functional groups. S-doped graphene is directly obtained by this CVD process. Therefore, the undoped graphene can not be synthesized by the same procedure. However, our target is to synthesize the S-doped graphene. Although the comparison conductivity between S-doped graphene and bare graphene can not be achieved, the enhanced electrical conductivity of S-doped graphene in comparison with undoped graphene has been demonstrated by previous contributions. Yun et al. have reported that the electrical conductivity of S-doped graphene nanosheets (1743 S m-1) is two orders of magnitude higher than that of pristine graphene nanosheets (32 S m-1).23 In our recent studies, S-doped graphene, S-doped carbon nanotubes and S-doped porous carbon all exhibited higher electrical conductivity as compared to the undoped ones.24-26 After the removal of templates, the 3D RBClike graphene with the concave-surface is obtained (Scheme 1). The fluidized bed technology sufficiently guarantees the homogeneous growth and mass production of SRBCG. The robust


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scaffold covered with staggered-mesh structure for the graphene is conducive to inhibiting the stacking. The thin core and thick periphery of SRBCG is also observed by the TEM image (Figure 1d). The remarkable corrugations coupled with mesopores are clearly observed in SRBCG sheet (Figure 1e). The SRBCG consisting of 2-4 layers is revealed by high-resolution TEM (Figure 1f). The EDS elemental mapping confirms that C, O and S elements are homogeneously distributed in SRBCG (Figure S2). By adjusting the reaction time (10, 12 and 15 min), S contents in SRBCG are in the range of 0.96-1.2 atom%, indicating that the operation parameter has little effect on the S content in SRBCG and the S doping level has a close relationship with the amount of S-containing functional groups in the templates. Because the templates are derived from the surfactant-directed self-assembly process, the amount of Scontaining functional groups in templates is constant, resulting in the almost constant S concentration in SRBCG. Considering from the aspect of S doping, S doping level is almost kept constant under different reaction time. However, the graphene layer will increase with the increase of reaction time. In order to obtain the few-layered graphene with high quality, the reaction time of 10 min was employed in this study.


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Scheme 1 Schematic illustration of the growth of SRBCG.

Figure 1. (a,b) SEM images of the RBC-like basic magnesium carbonate microspheres. (c) RBC-like porous MgO microspheres. (d-f) TEM images of SRBCG. The PSD of SRBCG is concentrated at 2-6 nm (Figure 2a). The SSA and the total pore volume of the SRBCG are 1431 m2 g-1 and 1.96 cm3 g-1, respectively. The occurrence of capillary condensation is revealed by the obvious adsorption hysteresis of N2 adsorption/desorption isotherm for SRBCG (the inset of Figure 2a). SRBCG with the doping level of 0.96 atom% is detected by the XPS (Figure 2b). The high-resolution S2p is split into three peaks composed of 2p3/2 (S1, 164.1 eV) and 2p1/2 (S2, 165.2 eV) of thiophene-S and different oxidized S (S3, 168.9 eV) (Figure 2c).27-30 The presence of the C-S peak (C3, 286.2 eV) further implicates that S atoms are covalently bonded with the carbon (Figure 2d). Besides, C1s also exists in other forms of C=C bond (C1, 284.7 eV), C-C (C2, 285.5 eV), C-O bond (C4, 287.0 eV) C=O (C5, 288.3 eV), O-C=O (C6, 289.8 eV) and π-π interactions (C7, 290.8 eV).31


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Figure 2. The PSD curve (a) and XPS survey spectrum (b) of SRBCG. S2p (c) and C1s (d) spectra of SRBCG. The inset in a is N2 adsorption/desorption isotherm. 3.2. Capacitive behaviors of SRBCG in supercapacitor. The SRBCG electrode was fabricated in order to evaluate its electrochemical property in supercapacitor. Using carbon black as the conductive additive is ascribed to the following two factors. Firstly, as shown in Figure S3, the carbon black presents a spherical morphology with diameter of 20-50 nm, much smaller than SRBCG. Thus the carbon black can be filled into the gap between adjacent SRBCG microspheres, enhancing the overall electrical conductivity of the electrode. Secondly, the ratio between graphene and carbon black employed in this work is convenient for parallel comparison with literatures. Electrodes with 1, 1.5, 2, 2.8 and 5 mg (SRBCG-1 to SRBCG-5) were prepared to investigate their influences on capacitive behaviors. In CV curves at 50 mV s-1 (threeelectrode in KOH electrolyte), rectangle-shaped profiles are observed (Figure S4a), which is the


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characterization of double layer capacitor behavior.32 The areal capacitances (normalized to the area of the current collector) almost increase with the increase of loading mass (Figure S4b), which is attributed to the fact that more electrode materials are capable of adsorbing more electrolyte ions, increasing the capacitance. Rate capabilities of SRBCG-1 to SRBCG-5 electrodes are shown in Figure 3a. Compared to areal capacitances, specific capacitances based on the electrode mass for these five samples are not affected significantly with the increase of the loading mass when the scan rate is lower than 10 mV s-1 (Figure 3a). With the increase of the scan rate, the drop extent of capacitance for SRBCG-5 electrode is higher than that of SRBCG-1 to SRBCG-2.8 electrodes, which is attributed to the fact that the thick coating leads to the large resistance of electrolyte ions diffusion. Specific capacitances at 10 mV s-1 for all samples are in the range of 203-250 F g-1, higher than the previously-reported values for graphene-based electrode materials (e.g. 206 F g-1 at 5 mV s-1 for the mesoporous graphene nanoball,33 193 F g-1 at 10 mV s-1 for the RGO/CNT@AC hybrid,34 120 F g-1 at 10 mV s-1 for the porous graphene paper35 and around 180 F g-1 at 10 mV s-1 for the porous graphene/AC composite.36 The specific capacitance presents a good linear relationship with areal capacitance at different scan rates (Figure S5). Loading mass between 2 and 2.8 mg should be the optimized value to obtain relatively high specific and areal capacitances. CV curves of the two-electrode symmetric supercapacitor assembled with KOH electrolyte display good rectangular-like shapes, indicating an ideal capacitive property and high-power behavior (Figure 3b).37 The voltages of charge/discharge profiles of the symmetric supercapacitor are linear response to time and exhibit equilateral triangle shape, showing an excellent reversibility (Figure 3c). 87.3% of the initial capacitance (102 F g-1 at 2.5 A g-1) is preserved at 50 A g-1, exhibiting the great compatibility (Figure 3d). Compared to capacitances calculated from galvanostatic charge/discharge profiles in


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the three-electrode system, the utilizable ratios of the single electrode capacitances measured in the symmetric supercapacitor at different rates are as high as above 85 %, reflecting the satisfactory electrochemical performance of the electrode and the perfect assembly of the supercapacitor. The supercapacitor was consecutively cycled at 2.5 A g-1 for 10000 cycles (96.3% retention) and 5 A g-1 for 20000 cycles (97.0% retention), demonstrating the excellent durability (Figure 3e and Figure S6). The charge/discharge curves for different cycling stages at 2.5 and 5 A g-1 are kept well, exhibiting a stable electrochemical process (Figure 3f and Figure S7). The 3D RBC-like microarchitecture efficiently overcomes the stacking of graphene, dedicating to enhancing the accessibility between electrolyte ions and electrode. The large SSA and well-defined mesopores for SRBCG are favorable for shortening the ion diffusion length to the interior surfaces of the bulk materials, facilitating the rapid electrolyte ions transport.38-40 The unique electrode property of SRBCG is also benefited from S doping which is in capable of improving conductivity.25 The hydrophilic groups (C-SOx-C) give rise to promoting the wettability of graphene, which sufficiently permits easy access to the surface of the graphene for electrolyte ions.


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Figure 3. (a) Specific capacitances of SRBCG-1 to SRBCG-5 at different scan rates. CV curves (b) and galvanostatic charge/discharge profiles (c) of SRBCG symmetric supercapacitor. (d) Rate performances of SRBCG three-electrode system and symmetric supercapacitor. Cycling performance (e) and galvanostatic charge/discharge profiles (f) at 2.5 A g-1 of SRBCG symmetric supercapacitor. 3.3. SRBCG as the cathode for LIC. The superior rate capability and high capacitance of SRBCG in supercapacitor trigger us to assemble it as cathode in LIC that combines the merits of large Li storage capacity for LIBs and rapid charge/discharge and long-term electrochemical stability of capacitors, resulting in higher energy densities than supercapacitors and higher power


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densities than LIBs.3,

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Quasi-rectangular CV curves of the SRBCG cathode display ion

adsorption/desorption processes and a reversible redox reaction at the electrode-electrolyte interface (Figure S8). The reversible capacity of SRBCG decreases very slowly as the increase of rates, illustrating an excellent rate performance (Figure 4a). Charge/discharge profiles without obvious potential drop embody the high coulombic efficiency and small internal resistance (Figure S9). As shown in Figure 4b, the broad working potential of the SRBCG cathode is composed of the p-type storage (3-4.5 V) and the n-type storage (1.5-3 V).44 The ascendant capacity at 1 A g-1 is ascribed to the interfacial storage sites caused by reversible formation and decomposition process of an organic polymeric/gel-like film covering on the interface of electrode during the extended cycles (Figure S10), which can undertake excess Li+ through the “pseudocapacitance-type behavior”.44 Additionally, the penetrable degree of electrolyte ions into the electrode is promoted during the electroactivation process.38 After 10000 cycles at 5 A g-1, not only excellent cycling stability with 91% capacity retention is retained (Figure 4c), but also the powerful evidence based on TEM observation for SRBCG cathode after cycling has confirmed the strong ability for maintaining the structural stability (insets of Figure 4c).


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Figure 4. Rate performance (a), galvanostatic charge/discharge curves (b) and cycling performances at 1 A g-1 (c) of SRBCG cathode. Insets in c are TEM images of SRBCG cathode after cycling. 3.4 SRBCG as the anode for LIC. For the anode test, the SRBCG anode delivers average capacities of 855 and 472 mAh g-1 at 0.5 and 1 A g-1, respectively. It is capable of retaining high Li storage capacity (257 mAh g-1) at rates up to 3.6 A g-1 (Figure 5a). In lithiation/delithiation profiles (Figure 5b), the capacity coming from Li intercalation into graphene layers (0.01-0.5 V) is 403 mAh g-1 at 0.8 A g-1, exhibiting a much enhanced capacity as compared to graphite (372 mAh g-1), and the rest of capacity is contributed by Faradaic capacitance on the surface or on the edge sites of sheets.45, 46 The ratio of Li intercalation capacity is dominant when the current density is less than 2 A g-1, and it accounts for about 50% as the rate increases from 2.4 to 3.6 A g-1. After 200 consecutive cycles at 1 A g-1 (Figure S11), the reversible capacity increases to 510 mAh g-1 and then the anode is successfully cycled for another 4000 cycles coupled with the


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coulombic efficiency of nearly 100% at 3 A g-1 with the capacity retention of 82%, which reveals the favorable durability of the cell upon cycling (Figure 5c and Figure S12). The Nyquist plots of the SRBCG anode before and after cycling have been compared and interpreted with the help of an appropriate electric equivalent circuit (Figure S13). The charge-transfer resistance after cycling (Rct=18.9 Ω) is significantly reduced in comparison with the initial value (Rct=53.2 Ω) due to the gradual activation of electrode and better contact between electrolyte and electrode. The excellent anode performance is mainly ascribed to the abundant mesopores and highly 3D robust scaffold with hollow structure, which provides large interfacial surfaces and a large amount of Li storage sites, resulting in a shortened diffusion distance, sufficient electrolyte/electrode contact and improved kinetics. These features are beneficial for ensuring the integrity of the electrode, restraining the volume expansion during electrochemical process and facilitating the fast penetration of Li+ and diffusion of electrolyte. Moreover, the RBC-like morphology is well kept even after a calendaring at 10 MPa for the electrode, presenting a strong anti-compression property of SRBCG (Figure S14). The empty space of the hollow structure in the SRBCG can efficiently buffer the pressure and keep the structure stable, therefore resulting in an excellent cycling stability. The superior reversible capacity, rate capability and durability of SRBCG anode are in favor of providing the energy demand for the full battery.


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Figure 5. The rate capability (a), lithiation/delithiation profiles at different current densities (b) and cycling stability at 3 A g-1 (c) of SRBCG anode. 3.5 Electrochemical performance of LIC. The gradual deviation of the rectangular shape for CV curves of the LIC assembled using SRBCG as anode and cathode, reflects the combination of LIB and capacitor energy-storage behaviors (Figure 6a and Figure S15), being consistent with the approximately triangular shape of charge/discharge profiles (Figure 6c). More importantly, CV curves for the battery exhibit similar shape without visible distinction as the scan rate increases from 5 to 50 mV s-1, demonstrating that the LIC owns a great compatibility for


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different scan rates. The LIC affords the highest capacitance of 198 F g-1 at 0.5 A g-1, and delivers 109 F g-1 at 9 A g-1 (Figure 6b). The maximum energy density of 110 Wh kg-1 are delivered for the LIC with the power density of 713 W Kg-1 (Figure 6d). Even at a highest power density of 12828 W kg-1, this LIC still affords an energy density of 61 Wh Kg-1. The obtained values are much higher than those of most hybrid LICs based on graphene and AC, such as Graphene//TiO2,47 3D graphene//3D graphene,48 Graphene//Graphene,49 MSP-20//T-Nb2O5@C,50 G-SU//G-LTO,51 AC//LiNi0.5Mn1.5O4,52 AC//TiNb2O7,53 AC//graphene,54 AC//m-Nb2O5-C,55 AC// LiMnBO3,56 AC/Li4Ti5O12,57 and AC/TiO2-RGO.57 The capacitance at 1 A g-1 was well maintained up to 1500 cycles without significant capacitance fading, furthermore, no capacitance loss at 2 A g-1 is observed for it even after another 3500 cycles along with excellent cycling stability (Figure 7a). Charge/discharge profiles without distortion are observed during cycling, exhibiting a superior electrochemical stability (Figure 7b and c). Additionally, capacitance retentions for the LIC at different rates are higher than previously-reported durabilities for different types of LICs (Table 1). The remarkable rate capability coupled with the high power and energy densities for the LIC are also attributed to the own excellent conductive network of SRBCG, which is dedicated to the fast transport of electrons. The robust scaffold of SRBCG with large internal space is capable of alleviating the pulverization and aggregation of the electrode material, which is propitious for keeping the structural integrity of the electrode material

and

providing

buffer

function

for

the

expansion/shrinkage

during

Li

insertion/extraction, resulting in the high utilization of active materials and excellent cycling stability.


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Figure 6. (a) A schematic illustration of the assembled LIC. The rate performance (b), typical galvanostatic charge/discharge curves (c) and Ragone plots (d) of the LIC.


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Figure 7. (a) Cycling stabilities at 1 and 2 A g-1 of the LIC. Galvanostatic charge/discharge curves at 1 (b) and 2 A g-1 (c) for different cycles of the LIC.


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Table 1. Comparison of cycling stabilities for different types of LICs. Sample

Current density (A g-1)

Cycle number

Capacitance retention

Graphene hydrogels//TiO247

1

600

73%

AC//LiNi0.5Mn1.5O452

1

3000

81%

Oligomer derived carbon//Li4Ti5O1258

2

1000

81%

AC//m-Nb2O5-C55

1

1000

90%

Polyaniline//LiMnBO356

1

1000

91%

AC//LiMnBO356

1

1000

76%

Porous carbon//Li4Ti5O1259

2

2000

87%

AC//TiNb2O753

2

3000

84%

Activated Polyaniline-derived carbon//vanadium nitride-RGO60

2

1000

83%

SRBCG//SRBCG (This work)

2

3500

No capacitance decay

4. CONCLUSION The 3D individual robust scaffold of SRBCG microspheres with double concave-surface morphology derived from the controllable CVD process can effectively inhibit the stacking and guarantee mechanical strength, electrical conductivity and structural stability. Moreover, SRBCG microspheres exhibit remarkable electrochemical energy-storage abilities in terms of supercapacitor and LIC. The superior capacitive behaviors including the outstanding rate capability, electrochemical reversibility and durability are attributed to the synergistic effects of the robust matrix coupled with good electrical conductivity and a large amount of mesopores. The special structure of SRBCG is beneficial for inhibiting structure collapse and keeping the electrochemical stability and integrity of the electrode during the electrochemical process. In


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addition, the developed synthesis technology is promising to realize the mass production of SRBCG with well-controlled morphology and excellent capacitive performances. ASSOCIATED CONTENT Supporting Information XRD pattern and the PSD of RBC-like porous MgO microspheres, CV curves and rate performances of SRBCG-1 to SRBCG-5 samples, cycling stability of SRBCG symmetric supercapacitor, CV curves, galvanostatic charge/discharge curves and cycling performance of SRBCG cathode, cycling performance and galvanostatic lithiation/delithiation profiles of SRBCG anode, Nyquist plots of the SRBCG anode, SEM images of the SRBCG anode pressed at 10 MPa and CV curves of the LIC at different scan rates. These materials are available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Guoqing Ning) and [email protected] (Yongfeng Li) Author Contributions †These authors contributed equally.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 21576289), Science Foundation of China University of Petroleum, Beijing (No. 2462017YJRC003 and No.


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SYNOPSIS


25

S-Doped Porous Graphene Microspheres with Individual Robust Red

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