Polyvinyl Alcohol Assisted Fabrication of Hollow Carbon Spheres

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Polyvinyl Alcohol Assisted Fabrication of Hollow Carbon Spheres/Reduced Graphene Oxide Nanocomposites for High-Performance Lithium Ion Battery Anodes Yunqiang Zhang, Qiang Ma, Shulan Wang, Xuan Liu, and Li Li ACS Nano, Just Accepted Manuscript • Publication Date (Web): 16 Apr 2018 Downloaded from http://pubs.acs.org on April 16, 2018

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Polyvinyl Alcohol Assisted Fabrication of Hollow Carbon Spheres/Reduced Graphene Oxide Nanocomposites for High-Performance Lithium Ion Battery Anodes Yunqiang Zhang,†,# Qiang Ma,†,# Shulan Wang,*,† Xuan Liu,*,† and Li Li*,‡ † Department of Chemistry, School of Science, Northeastern University, Shenyang 110819, P. R. China. ‡ School of Metallurgy, Northeastern University, Shenyang 110819, P. R. China. # These authors contributed equally to this work. *Corresponding author. Email: [email protected] (Shulan Wang); [email protected] (Xuan Liu); [email protected], [email protected] (Li Li)

Abstract Three-dimensional

hollow

carbon

spheres/reduced

graphene

oxide

(DHCSs/RGO)

nanocomposites with high-level heteroatom doping and hierarchical pores are fabricated via a versatile method. Polyvinyl alcohol (PVA) that serves as dispersant and nucleating agent is used as the non-removal template for synthesizing melamine resin (MR) spheres with abundant heteroatoms which are subsequently composited with graphene oxide (GO). Use of PVA and implementation of freezing treatment prevent agglomeration of MR spheres within GO network. Molten KOH is used to achieve one-step carbonization/activation/reduction for synthesis of DHCS/RGO. DHCSs/RGO annealed at 700 oC shows superior discharge capacity of 1395 mA h/g at 0.1 A/g and 606 mA h/g at 5 A/g as well as excellent retentive capacity of 755 mA h/g after 600 cycles at a current density of 2 A/g. An extra CO2 activation leads to further enhancement of electrochemical performance with outstanding discharge capacity of 1709 mA h/g at 0.1 A/g and 835 mA h/g at 2 A/g after 600 cycles. This work may improve our understanding for synthesis of graphene-like nanocomposites with hollow and porous carbon architectures and fabrication of high performance functional devices. Keywords: hollow carbon spheres; reduced graphene oxide; polyvinyl alcohol; molten KOH; lithium ion battery anodes

Lithium-ion batteries (LIBs) with advantages of high energy density as well as environmental benignity currently dominated the rechargeable energy storage market since their 1 ACS Paragon Plus Environment

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commercialization1 and the corresponding market demand is expected to increase steadily in the next few decades, considering widespread popularization and large scale applications of portable electronic devices and energy products. Graphite has been widely used as anode materials in commercial LIBs owing to its great physical and chemical stability, high electronic conductivity, and low-cost for financial advantages. However, the low theoretical capacity (372 mA h/g) and poor rate capability of graphite significantly limits its commercial use in LIBs for high-power/energy devices, leaving high demand for development of alternative anode materials with highly reversible capacity and large current discharge capacity.2,3 Many carbonaceous anode materials (carbon nanotubes (CNTs),4 reduced graphene oxide (RGO),5 carbon nanofibers,6 ordered porous carbon,7 hollow carbon spheres (HCSs),8 and their micro-/nano-composites9) show excellent electrochemical performances due to their satisfactory cycling stability and electrochemical kinetics. Among these candidates, heteroatom-doped hollow carbon spheres (H-HCSs) are highly attractive because lithium is stored within these materials by both adsorption and Faradaic reactions. Lithium storage by adsorption is similar to that of electrical double layer capacitors which is of high power density while the Faradaic reaction can provide high energy density. Many efforts have been devoted to design and optimize the structure and properties of H-HCSs for LIBs and to highlight pivotal factors such as porous structure, cavity dimension, and heteroatomic content, leaving immense space for improvement in electrochemical performances.10,11 As the anode materials of LIBs, H-HCSs and their composites possess proper pore structure which can shorten the diffusion pathway of Li-ions and buffer the volume change arising from charge and discharge, facilitating transportation of Li ions and alleviating capacity-fading of the materials.10,12 Plentiful heteroatom-doped active sites and defects in the materials promote the reversible capacity of the materials.13,14 Most H-HCSs fabrication flows are complicated with high expenses and high facility requirements but low yield. In some cases, further removal of template materials or 2 ACS Paragon Plus Environment

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purification process is needed, which strongly limits scaling up and commercialization.11,15 The pore size of H-HCSs duplicated from templates is sometimes not suitable for achieving superior capacity while in most cases a hierarchical porous structure is required combining advantages from pores of different length scales.16 Currently, it is still challenging to develop a feasible, simple, economical and effective method to realize functionalization of H-HCSs in the molecular level as high performance LIB anode materials. Reduced graphene oxide (RGO), a two-dimensional nanoflake of sp2-bonded carbon atoms, has attracted considerable attentions in many research fields due to its advantages of high specific surface area and great mechanical/electrical properties.17-19 As the anode for LIBs, RGO exhibits excellent buffering capability for Li-ions owing to formation of Li2C6 and high reversible capacity at low current densities.20 However, it suffers from poor cyclic stability and rapid capacity fading with increase in the current density due to restack and aggregation of RGO nanosheets, which limits insertion/extraction of Li-ions and blocks the charge/discharge processes of the anode.21,22 Assembling RGO into a three-dimensional (3D) porous nanostructure can efficiently separate the RGO sheets and avoid wrapping of the layers,23 leading to rapid transmission of Li-ions and full utilization of active components with improved rate capability and cycling stability. Integration of porous spheres, which can buffer the volume change during lithium insertion/extraction and promote penetration of electrolytes into the structure of active materials, into RGO to form an interconnected carbon network can profit from both components.24 However, controlling a homogeneous distribution of the spheres within RGO is still not effectively solved for similar architectures reported before,25 which strongly retards their energy related applications. In addition, appropriate activation of graphene based composite materials can adjust their porosity for significant enhancement in the electrochemical performances.26,27 Therefore, synthesis of efficiently activated homogeneous graphene based composites with abundant hetero-atoms is highly desired for assembly of high performance energy storage devices. 3 ACS Paragon Plus Environment

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Herein, we explore a species of all-carbon composites integrated with N and O-doped hollow carbon spheres (DHCSs) and RGO by a simple and scalable freezing-assisted strategy, in which the DHCSs were coated and bridged by thin and unstack RGO nanoflakes to form a homogeneous 3D porous network architecture. Polyvinyl alcohol (PVA) is used as the templating dispersant and nucleating agent to form monodisperse melamine resin (MR) spheres with uniform sizes and heterogeneous inner/outer compositions, and more importantly, to efficiently prevent aggregation of DHCSs dispersed within RGO network in the aqueous environment combined with carefully controlled freezing treatment. The hetero-atoms are introduced from the resin itself. Molten KOH is used to accelerate breaking of the chemical bonds from carbonaceous precursors and complete reduction of graphene oxide (GO) for formation of mesoporosity-dominated hierarchical porous structures, which is favorable for rapid diffusion of Li-ions, resulting in an obviously increased rate performance of anode materials. By carefully optimizing the processing procedures, DHCSs/RGO annealed at 700 o

C shows a high initial discharge capacity of 1247 mA h/g at 2 A/g in 1 M LiPF6 in ethylene

carbonate/dimethyl carbonate (EC/DMC) and maintains 755 mA h/g after 600 cycles, which is rarely reached by peer carbonous materials. The extra CO2 activation further improved its electrochemical performance including the discharge capacity of 1380 mA h/g at 2 A/g and 835 mA h/g after 600 cycles. The advantages of current work includes: (1) Using PVA as the template nucleation and dispersant agent to achieve uniform carbon spheres with tunable cavity dimensions while PVA can be transferred to carbon by calcinations with no extra templation removal steps required; (2) Offering homogeneous three-dimensional structures with MR spheres anchored on the RGO framework that can effectively prevent agglomeration of nanosheets, which takes the best advantages of stacked graphene-like structures for good cycling stability and rate capability; (3) Selecting melamine resin which has abundant heteroatoms (N and O) within the molecular structure as the carbon source to guarantee uniform and straightforward hetero-atom doping to RGO during carbonation. Effective 4 ACS Paragon Plus Environment

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heteroatom doping within the microstructure of carbonous materials provides plentiful active sites for surface redox reactions, and increase interactions between carbonous materials and Li-ions. This is beneficial to Li adsorption and insertion/extraction, as well as improvement in the electric conductivity and electron transfer rate; (4) Implementing controlled freezing as well as freeze-drying to treat MR/GO that is coated with KOH at the molecular level, to effectively avoid aggregation and to fabricate a homogeneous carbon network; (5) Using molten KOH with excellent solubility, flow mobility, de-oxygenation and etching capability to achieve carbonation, activation and reduction of GO with a one-step protocol; (6) Designing hierarchical porous and hollow interconnected framework with appropriate pore sizes and surface areas, which can shorten the ion/electron diffusion distance, promote the corresponding transportation rate, as well as fully buffer penetration of lithium ions for stable electrochemical performances. At the most but not the least, the current material can be potentially explored for broad applications in different energy conversion, catalysis and energy storage fields with appropriate optimization of processing conditions. Results and Discussion The interconnected porous DHCSs/RGO nanocomposites are synthesized by the route described in Figure 1A. Firstly, lichee-shaped MR spheres (Figure S1A-C) are prepared through poly-condensation of melamine and formaldehyde oligomes with a precisely controlled amount of PVA that serves as the nucleating agent for uniform growth and distribution of oligomer molecules and the dispersant for blocking adhesion and aggregation of polymer microspheres. PVA, which is water-soluble without aggregation in water, has been extensively studied for its biological and medical applications.28 In this stage, the hydroxyls groups in PVA favor adsorption, nucleation and growth of oligomers on the surface of PVA clews. Interestingly, oligomers entangled in PVA are difficult to further polymerize in a large degree. As a result, nanospheres with uniform sizes and complicated inner/outer compositional chemistry are constructed. Herein, the dimension, cavity and morphology of 5 ACS Paragon Plus Environment

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MR are tunable by adjusting the amount of PVA addition and the reaction time. Exfoliated GO nanoflakes and MR spheres were then suspended into the KOH solution for molecular dispersion of KOH into the composites. During this step, PVA addition and controlled freezing are two vital factors to distribute MR uniformly and homogeneously within the GO nanoflakes. Without addition of PVA (Figure S2A and B), as-prepared MR do not show regular shapes but has random dimensions, with heavy aggregation observed when integrated with GO nanoflakes. Furthermore, no hollow structures are observed after carbonation and annealing (Figure S2C and D). All of the control results confirm the key role of PVA in formation of porous MR carbon spheres and control of their dimensional homogeneity as well as a uniform distribution within the GO nanoflakes. Similarly, carefully controlled heating at 60 oC for water evaporation also leads to formation of heavy carbon clusters (Figure S3). Aggregation of the MR spheres is avoided by freezing treatment while the crumpled thin GO nanoflakes are covered on their surface to form homogeneous composites (Figure 1B and D). The as-prepared MR spheres with the lichee-shape have a uniform dimension of around 750 nm. Hollow spheres are formed with shrinkage in the size to 450 nm after annealing at 700 oC with molten KOH, as shown in Figure 1C and E. The selected area electron diffraction (SAED) pattern (inset in Figure 1E) indicates the amorphous feature of DHCSs/RGO-700. Morphology and microstructure of DHCSs/RGO composites sintered at 600 and 800 oC are also shown in Figure S4A, B, D, E. It can be noticed that the cavity size of the DHCSs/RGO composites increases with an increasing carbonation temperature. Samples without KOH addition during synthesis (NACs-700, Figure S4C and F) shows similar shapes to DHCSs/RGO-700, indicating KOH activation brings no destruction to the hollow structure during high temperature calcinations. In Figure 1F-H, energy dispersive X-ray spectroscopy (EDS) of DHCSs/RGO-700 was presented to show the elemental distribution of C, N and O as chromatic images. All elements are evenly distributed through samples without hot spots. High-resolution TEM images of DHCSs/RGO-700 (Figure 1I) show that the three6 ACS Paragon Plus Environment

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dimensional interconnected network of DHCSs/RGO-700 is consisted of micropores and mesopores formed by activation of KOH and thermal decomposition of organic components. This hierarchical porous structure provides feasible and favorable conducting channels for transportation of ions/electrons and buffers volume changes of the active materials during lithium insertion/desertion, leading to significant improvement in the cycling and stability performances of the electrodes.9,29 Powdered X-ray diffraction (XRD) patterns which are used to investigate compositions of crystalline phases are presented in Figure 2A. The peaks centered at about 2θ=42.9o and 25.7o are correlated with (100) and (002) planes of graphite.2,26 Compared with NACs-700, peaks of DHCSs/RGO are widened and weakened due to degradation in the graphitization degree that was partially destroyed by KOH, as consistent with a previous report.30 The characteristic peak of GO at around 9.9o (Figure S5A) disappears in DHCSs/RGO and NACs-700, confirming GO has been successfully transformed into RGO after high temperature annealing. Raman spectra of the samples are performed to investigate the status of graphitic carbon and are shown in Figure 2B. The characteristic peaks located at approximately 1352 and 1626 cm– 1

can be assigned to the D-band, which corresponds to the defect/disordered sp2-hybridized

carbon atoms, and G-band, which is related to the E2g graphitic mode of carbon materials, respectively.31,32 The intensity ratio (IG/ID) between the G-band and the D-band can be used as the indicator to the ordering degree of the carbonaceous materials. Apparently, MR/GO nanocomposites without annealing (IG/ID=0.81) are more disordered than NACs-700 (IG/ID=1.15) since high temperature annealing leads to an enhanced graphitization degree of MR. Note that all DHCSs/RGO composites show a decreased graphitization degree with low IG/ID values (0.8–0.9) compared with NACs-700, indicating formation of more disordered structure resulting from KOH addition. KOH herein serves functionality of both carbon activation as well as GO reduction. Transformation of GO to RGO can be accomplished by several reduction methods, including high temperature calcination for thermal exfoliation, 7 ACS Paragon Plus Environment

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electrochemical reduction, chemical functionalization, etc.33 Herein, we attribute reduction of GO to RGO into two combined effects which includes the contribution from thermal reduction with high temperature annealing and alkaline deoxygenation. It has been reported that GO can be reduced in the alkaline solution34 while KOH serves as the molten media with excellent chemical solubility/reactivity for accelerating the breaking of chemical bonds from carbonaceous precursors and deep GO deoxygenation in the current case. The discontinued graphene layers and the abundant defects in DHCSs/RGO composites are not only beneficial to insertion/extraction of Li-ions but also to diffusion of Li-ions, both of which contribute to the high cycling performance and stability. The surface organic groups of samples were checked by Fourier transform infrared spectrometer (FTIR) and the spectra were shown in Figure 2C. The MR/GO nanocomposites show similar characteristic peaks to GO (Figure S5B). The peak at 3370 cm–1 is attributed to the stretching vibration of O–H groups, while those at 1201, 1003, and 876 cm–1 belong to the C–O/C–O–C groups. In addition, the bands at the wavenumber of 1493 and 1172 cm–1 correspond to the absorption peaks of methylene and the sharp peak at 1370 cm–1 is related to the O–H deforming vibration band. The bands detected at 1564 and 817 cm–1 refer to the 1,3,5-s-triazine ring.35,36 Most FTIR feature peaks of MR/GO composites disappear after high temperature treatment, and the two remaining broad peaks located at 1571 and 1214 cm–1 correspond to the stretching vibrations of C=N/C=C and C-N/C–O.37,38 In addition to carbon, binding energy peaks at 394–406 eV and 526.7–539.7 eV which are related to heteroatoms N and O can be clearly observed in the X-ray photoelectron spectroscopy (XPS) survey (Figure 2D). No additional peaks are observed. The relative peak intensity of the heteroatoms decreases with an increasing temperature, which indicates a decrease in the corresponding content percentage. Detailed elemental analysis data is shown in Table 1. The high percentages of heteroatoms within the carbonaceous materials are beneficial for enhancement of their electrochemical reactivity during lithium insertion/extraction.39,40 Detailed 8 ACS Paragon Plus Environment

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quantitative analysis of the surface elements is resolved through deconvolution of the C1s, N1s, and O1s XPS profiles (Figure 2E–G and Figure S6) and listed in Table S1. Taking DHCSs/RGO-700 as an example, the high-resolution C1s spectrum contains four component peaks centered at 284.6, 285.1, 286.3, and 289.1 eV, corresponding to C=C (33.30% (in total area)), C–C (39.40%), C–O/C–N (15.90%), and C=O (11.40%), respectively.41 The observed peaks of C–O, C–N, and C=O are indicative of successful embedding of N and O heteroatoms into carbon matrix, which can contribute to the reversible capacity by Li-ion adsorption and surface redox reactions.14,42 In the fitted N1s spectrum, the binding energy peaks located at 398.5 eV and 400.1 eV are assigned to pyridinic N (N-6, 32.64%) and pyrrolic N (N-5, 37.99%), respectively, which are the predominant N-containing groups in DHCSs/RGO-700. The N-5 and N-6 species provide abundant electrochemically active sites for enhancing the capacitive performances of the DHCSs/RGO-700 for the LIBs.39 The peak of quaternary N (N-Q, 19.58%) at 401.0 eV is beneficial to improvement in conductivity of nanocomposites.14 In addition, the peak at 402.4 eV is related to N-oxides (N-X, 9.79%). The O 1s spectrum can be deconvoluted into the four peaks at 530.5 (25.6%), 531.4 (23.2%), 532.4 (28.7%), and 533.8 eV (22.5%) that are ascribed to the C–N=O, C=O, C–OH, and C–O–C groups, respectively.14,43 The Faradaic reaction between oxygen-containing groups and Li-ions contributes to enhancement in storage capacity.44 Figure 2H presents the nitrogen adsorption/desorption isotherm of the as-made all-carbon composites. Detailed porous properties are shown in Table 2. Note that the specific surface area and pore structure of the samples are obviously influenced by the carbonization temperatures. Compared with that of DHCSs/RGO-600, the hysteresis loops in the nitrogen adsorption/desorption isotherm of DHCSs/RGO-700 and DHCSs/RGO-800 are considerably enlarged at high relative pressures (P/P0 > 0.4) due to the significantly expanded mesopores.45 These mesopores are mainly derived from further evaporation and decomposition of unstable components in MR spheres/GO nanocomposites during pyrolysis and activation processes. In 9 ACS Paragon Plus Environment

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addition, the difference in the shape of nitrogen adsorption/desorption isotherms between DHCSs/RGO-700 and NACs-700 is mainly attributed to KOH activation that can etch the margin of micorpores and transform them into mesopores. Figure S7 shows the pore size distribution curves of all samples, in which mesopores with dimensions of 2.6–13 nm take the primary percentage in the hierarchical pores of DHCs/RGO. On the other hand, more micropores are observed in NACs-700, which is consistent with the surface area information shown in Table 2. With increase in the carbonization temperature from 600 to 700 oC, both SBET (52.31 and 417.31 m2/g for the samples annealed at 600 and 700 oC) and total pore volume (Vtotal) (0.09 and 0.71 cm3/g for the samples annealed at 600 and 700 oC) increase dramatically. A further increased annealing temperature leads to destruction of pore and hollow structures with coarsening of materials. Both SBET (331.98 m2/g) and Vtotal (0.44 cm3/g) of DHCSs/RGO-800 are lowered compared with their peers annealed at 700 oC. In addition, the significantly increased surface area of DHCSs/RGO-700 compared with RGO itself indicates restrain of restacking of RGO nanoflakes from combination of DHCSs and RGO. Note that the surface area of NACs-700 without KOH treatment is higher than DHCSs/RGO700, which is due to the high transformation percentage from mesopores to micropores (Smicro/SBET = 17.88%). A hierarchical pore structure with an appropriate pore size distribution is one of the key factors to influence performances of Li-ion storage during electrochemical processes. Micropores can serve as reservoirs for Li-ion storage while mesopores are important for providing transmission channels to Li ions with a lower transport resistance. The optimized hierarchical pore combination is required for electrode fabrication for high performance lithium ion batteries and this is the reason why we design herein to investigate the influence of processing conditions on the pore structures as well as the electrochemical performances. The corresponding electrical conductivities were measured through a fourprobe method. The average conductivities of DHCSs/RGO-600, DHCSs/RGO-700, DHCSs/RGO-800, NACs-700, and RGO are around 10.1, 9.4, 9.2, 11.5, and 8.1 S/m, 10 ACS Paragon Plus Environment

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respectively. The packing densities of DHCSs/RGO-600, DHCSs/RGO-700, DHCSs/RGO800, NACs-700, and RGO electrodes are about 0.41, 0.23, 0.29, 0.25, and 0.38 g/cm3, respectively. The apparent reduction packing density for DHCSs/RGO-700 should be due to its obvious increase of Vtotal. The electrochemical performance of different DHCSs/RGO comoposites and control samples are analyzed and shown in Figure 3. The first five cyclic voltammogram (CV) curves of DHCSs/RGO-700 measured at a scan rate of 0.1 mV/s is presented in Figure 3A. The first cycle, which shows a different shape in the CV curve from other cycles, has a strong discharge peak located at 0.64 V that is related to the irreversible reactions on electrode surfaces and formation of the solid electrolyte interface (SEI) layer.2 In addition, an appreciable voltage hysteresis can be observed in the charging band that indicates removal of inserted Li-ions within the region of 0.01–3V. The peak at 0.16 V is attributed to extraction of Li-ions from graphite layers of DHCSs/RGO-700 composites, while the peak at 1.0–1.5 V refers to lithium desertion from the cavities and pores.46,47 The weak peak detected in the potential range of 2–3 V corresponds to heteroatoms on the surface of DHCSs/RGO-700.46,48 These different Li-ion storage sites in DHCSs/RGO-700 electrode guarantee a high specific capacity for lithium insertion/extraction, while the fact that all CV curves overlap with each other after the first cycle indicating good stability and reversibility of the as-prepared electrode materials. In Figure 3B, the initial discharge and charge capacity of the DHCSs/RGO-700 electrode are measured to be as high as 2506 and 1458 mA h/g at 0.1 A/g, respectively, resulting in a lower first-cyclic coulombic efficiency of 58.18%. The low first-cyclic coulombic efficiency can be attributed to SEI film formation of DHCSs/RGO-700 with high surface area to trap Liions and/or irreversible embedded lithium in special positions near the vicinity of residual H atoms.8,49 The capacity becomes basically stable and reversible from the second cycle, which is in a good match to the observation of CV curves that the reduction peaks are present in the 11 ACS Paragon Plus Environment

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first cycle but disappeared in subsequent cycles. The DHCSs/RGO-700 electrode shows a high discharge capacity of approximately 1400 mA h/g at 0.1 A/g in the 3–10 cycles, which are 3.76 times the value of the theoretical specific capacity of commercial graphite (372 mA h/g). The large improvement in capacitance is due to various Li-ion storage mechanisms existing in this carbonaceous composite other than the graphite intercalation principle.8 During the discharge process, Li-ions are adsorbed onto the surface of both crumpled RGO nanosheets and DHCSs, intercalated into the defects (mesopores, vacancies, and interconnected RGO edges), and bound with heteroatoms at a voltage over 0.5 V.50,51 The specific capacity of the voltage range below 0.5 V is related to intercalation of Li-ions into the graphite carbon layers of DHCSs/RGO-700 with a weak potential plateau, which is similar to traditional graphite materials.5 The rate capability and cycling stability of the DHCSs/RGO anodes at the current densities of 0.1–5 A/g is presented in Figure 3C. DHCSs/RGO-700 shows a superior rate capability and cycle stability among all samples. The discharge capacity of DHCSs/RGO-700 is 1395 mA h/g at the current density of 0.1 A/g and decreases to 787 mA h/g at the current density of 2 A/g, and 606 mA h/g at the large current density of 5 A/g, which are far higher than values from other reported peer carbon materials (carbon nanofibers,31 porous carbon microspheres,52 CNT/GN47) tested at the same current density. More importantly, when the current density recovers to 0.1 A/g after cycles at different current densities, the discharge capacity can still be turned back to 1383 mA h/g for DHCSs/RGO-700, implying an excellent cycle stability and reversibility of the electrode. To highlight the outstanding electrochemical performances of the DHCSs/RGO nanocomposites, the specific capacity values and stability of other similar peer carbon materials in literature are listed in Table S2. The cycling performances of the DHCSs/RGO nanocomposites were further examined through 300 galvanostatic discharge-charge (GDC) cycles between 0.01 and 3.0 V at 0.1 A/g, as shown in Figure 3D and S8. The capacity loss in the initial few cycles is due to irreversible 12 ACS Paragon Plus Environment

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trapping of Li-ions and re-formation of the thin SEI layer.53 After 300 cycles, the DHCSs/RGO electrodes show high capacitance retention characteristics. Particularly for DHCSs/RGO-700, the specific capacity value (1360 mA h/g) in the 300th cycle is about 97.07% the value of the third discharge capacity (1401 mA h/g) at 0.1 A/g. In addition, the high reversible capacity of DHCSs/RGO-700 was maintained at 867 and 755 mA h/g even with an increase in the current density to 1 and 2 A/g, respectively, after 600 cycles (Figure 3E and S9), which further confirms the stable inherent microstructure of DHCSs/RGO-700 during the electrochemical reactions. The coulombic efficiency of DHCSs/RGO-700 rises to approximately 99–100% after the first few cycles. The excellent electrochemical performances indicate as-prepared DHCSs/RGO-700 can serve as the potential electrode active material for fabrication of LIBs with high capacitive reversibility, great rate capability and a long cycle life. The transport kinetics of electrons and Li-ions within the framework of active materials was analyzed by electrochemical impedance spectra (EIS) tests, and the Nyquist plots of the DHCSs/RGO nanocomposites and control samples after 3 charge/discharge cycles are presented in Figure 3F. The sloping line in the low-frequency segment is related to the Li-ion diffusion behavior within active materials.54 In the high-frequency region, the equivalent series resistance (Re) originates from the electrolyte, separator, and electrode can be calculated according to the x axis intercept,55 and the results are listed in Table S3. The smallest Re value of the DHCSs/RGO-700 electrode compared to the other electrodes can be ascribed to its larger SBET and introduction of thin RGO nanoflakes and heteroatoms for improvement in electrical conductivity of the active materials. Additionally, the width of the semicircle of the high-frequency region corresponds to charge transfer resistance (Rct) at the electrode and electrolyte interface.18 The Rct value of DHCSs/RGO-700 is calculated to be 25.11 Ω, which is lower than that of DHCSs/RGO-600 (80.54 Ω), DHCSs/RGO-800 (42.21 Ω), NACs-700 (51.35 Ω), and RGO (63.16 Ω), indicating DHCSs/RGO-700 has an excellent electrical 13 ACS Paragon Plus Environment

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conductivity and a fast charge transfer process for Li-ion insertion and extraction. Notably, the Nyquist plot of DHCSs/RGO-700 after 200 cycles (Figure S10) remains almost unchanged, which is consistent with GCD measurement as the evidences for long cycle-life electrochemical performances of the as-prepared active materials. The high coulombic efficiency and stable EIS results from DHCSs/RGO-700 confirms high stability of the 3D porous structure of the samples as well as the SEI layer during the GCD process. The faradaic contribution from the Li-ion insertion process, surface charge-transfer process (surface

pseudo-capacitance)

and

non-faradaic

contribution

from

the

Li-ion

adsorption/desorption processes (double layer capacitance) are the three components of the total stored charge.56 To explain the charge storage mechanism of the electrode materials, these capacitive effects can be characterized according to the CV curves at various scan rates (Figure 4A) using the following Equation (1):57 I=avb

(1)

where the measured current I follows a power-law relationship with the scan rate v. The value of b, which can be decided through the curve of log (I) vs log (v), is the indicator of the kinetics of charge storage and can reveal the corresponding storage mechanism of lithium ion battery. Studying the b value can provide some constructive clues for the design of high performance energy storage device. The current is either dominated by the diffusioncontrolled process (b=0.5) or entirely controlled by surface capacitance (b=1).56 Figure 4B shows the b-voltage plots of the DHCSs/RGO-700 electrode in LIBs. The fitting variance (R2) of b value at different voltages is close to 1. The inset in Figure 4B is the current response plotted against scan rates of DHCSs/RGO-700 at 0.1, 0.75, 1.5, and 2 V, respectively. At the voltage of 0.1 V and 0.3 V, the b-value is 0.57 and 0.66, indicating the current comes mainly from Li-ion intercalation into graphene layers. A series of b-values larger than 0.73 in the voltage range of 0.45–2.5 V demonstrate current is primarily capacitive for the as-prepared DHCSs/RGO-700 electrode in this region. 14 ACS Paragon Plus Environment

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To further quantify the contribution from the surface capacitance and semi-infinite linear diffusion of Li-ions, an important method was proposed by Dunn et al.58 as described in Equation (2): I(V)= k1v + k2v1/2

(2)

where I(V) is the total current response at a given voltage V, k1v and k2v1/2 the current contributions from the surface capacitive effects and the diffusion-controlled intercalation process, respectively. Fractions of the current from two capacitive contributions can be quantified by calculating k1 and k2. To acquire the k1, k2 value, the Equation (2) was rearranged into I(V)/v1/2= k1v1/2 + k2 and the straight lines of I(V)/ν1/2 versus ν1/2 at different voltages were plotted. The k1 and k2 value at each fixed voltage can be obtained from the slope and intercept of a straight line, respectively. Figure 4C shows the typical current separates into the capacitive current (purple region) and the diffusion-controlled current (green region) at a sweep rate of 0.5 mV/s. A surface capacitance-controlled capacity accounting for ~46.2% of the total charge storage is obtained. The contribution from the surface capacitive increases to 52.6%, 57.4%, and 66.8% as the scan rate rises to 1, 2, and 5 mV/s, respectively (Figure 4D), suggesting the important role of capacitive charge storage in the total capacity of the electrode, especially at high sweep rates. The high capacitive contribution is due to the high surface area, N/O doping, a short ion diffusion length and rapid electron transfer, which are the key factors leading to high-rate capability of the DHCSs/RGO-700 electrode. The large current discharge capacity of DHCSs/RGO-700 electrode is also confirmed by its smaller Re and Rct. The evaluation for the influence of mechanical vibration on energy storage system is crucial for the design of high performance and long-life battery with minimizing the impact from variation induced resonance.59 Considering the reliability, durability and safety of LIBs, the impact of vibration to such systems should be deep-dived, including the natural frequency, the damping level and mode shapes of cells when loaded with mechanical vibration. To 15 ACS Paragon Plus Environment

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identify and assess the shapes of mode produced within the designed-structure, frequency response functions must be documented from different locations with the response matrix produced. Mode shapes cannot be concluded based on onefold frequency response function/test

site.59

A

model

experiment

system

(sixteen

cells

packed

in

the

polymethylmethacrylate mold and an accelerometer stick in the position 1) was designed specially to measure natural frequencies and mode shapes of cells, as shown in Figure 4E. MATLAB and IDEAS was used to obtain the frequency response functions and mode shapes for the LIBs, respectively. Figure 4F shows 16 frequency response plots for cells which are correlated with affected sites defined in Figure 4E. In Figure 4F, the two peaks in the magnitude plot demonstrate that the cell has two natural frequencies of ~29.2 and 55.6 Hz, under which the LIBs assembled from the current active materials should not be kept for long durations. Otherwise, structural deformation and damage that can lead to the failure of cells will occur. Bending and torsional modes are present in the current assembled cell, as shown in Figure S11A and Figure S11B, which correspond to the first and second natural frequency, respectively. To further introduce micro-pores into DHCSs/RGO nanocomposites, the extra CO2 activation step was adopted to evaluate the electrochemical performance changes of the current architecture. The C atoms inside the carbon can react with CO2 following with the release of CO, during which the micropores were introduced with the increased SBET (Figure S12) of nanocomposites. Figure 5A shows the rate capabilities and coulombic efficiencies of DHCSs/RGO-700-CO2 electrode at different current densities. The discharge capacities are 1709, 1490, 1362, 1160, 1050, 889, 801, and 716 mAh/g at 0.1, 0.2, 0.3, 0.5, 1, 2, 3, and 5 A/g, respectively. Apparently, the discharge capacity of DHCSs/RGO-700-CO2 is significantly enhanced compared with DHCSs/RGO-700 at the same current density due to its elevated SBET of 823.3 m2/g. The as-prepared sample also showed outstanding rate capability without decaying and the discharge capacity can be returned to 1692 mAh/g when the rate 16 ACS Paragon Plus Environment

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was restored to be 0.1 A/g after 90 cycles at various rates, which is attributed to the increased Vtotal of 1.01 cm3/g as well as improved micropore content (Vmicro/Vtotal = 26.93%) and more evenly distributed mesoporous and macroporous structure. In Figure 5B, the Nyquist plot of DHCSs/RGO-700-CO2 after 300 cycles are nearly overlapping with the curve after 3 cycles, demonstrating its good cycle life. This is also consistent with the results of the multi-cycle GCD measurement (high reversible capacities were maintained at 835 mA h/g at 2 A/g) as shown in Figure 5C. In addition, the coulombic efficiency of DHCSs/RGO-700-CO2 (Figure 5A and C) is approximately 99–100% after initial few cycles in which SEI layer is formed, indicating its superior capacitive reversibility for the repeated insertion/desertion processes that is the very critical performance required for their practical and manufacture-scaled applications. The

excellent

electrochemical

performance

of

the

DHCSs/RGO-700

(-CO2)

nanocomposites can be attributed to its homogeneous structure and high-level heteroatom doping. DHCSs wrapped with RGO nanosheets form homogeneous 3D network architecture, in which RGO serves as bridges for electron transmission between DHCSs. Meanwhile, abundant pores in DHCSs act as reservoirs for storage of Li-ions. Reasonable SBET and more significantly, appropriate combination of the hierarchical pore structure provides a large electrode/electrolyte interface and ample numbers of active sites to adsorb Li-ions and thus improve kinetics of charge-transfer reaction. Heteroatom doping and other defects in the asobtained composite provide different Li-ion storage mechanisms other than the accepted graphite intercalation mechanism, all of which contribute to the large reversible specific capacity. The thin shell and porosity of DHCSs also significantly decreases the solid-state transport lengths for Li-ion diffusion, which is beneficial to increase the rate capability of the electrode materials. In the current case, the detailed sphere dimension is evaluated with the electron microscopy while other characterization method, such as dynamic light scattering, also can be used to trace the size change before and after electrochemical reaction. In addition, 17 ACS Paragon Plus Environment

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the 3D network architecture remits expansion/shrinkage during the discharge/charge processes and maintains the porous hollow sphere structure within the framework which leads to a long cycle life of the as-prepared electrode. Besides PVA, other polymers, such as poly(sodium 4-styrene sulfonate), polyethyleneglycol, and polyethyleneimine, can be attempted to sever as the similar role and should be considered as a future extended focus work. Conclusions In summary, DHCSs clad in RGO nanosheets introduced a homogeneous 3D interconnected carbon network architecture by using PVA as the template for dispersion and nucleation followed with uniform molten KOH activation and reduction of MR spheres/GO composites in Ar. The dimension-controllable spheres prepared from PVA-assisted polymerization of melamine and formaldehyde efficiently prevent agglomeration when integrated with graphene-like nanosheets and thus significantly enhance the cycling performance and rate capability. Obtained DHCSs/RGO-700 shows a SBET of 417.31 m2/g with a large pore volume of 0.71 cm3/g and high-level intrinsic heteroatom doping. As the anode material for LIBs, DHCSs/RGO-700 shows superior electrochemical performances including ultra-high reversible capacity, good rate capability and a long cycle life. The reversible capacity is as high as 1360 mA h/g at 0.1 A/g even after 300 cycles. The detailed kinetics of charge storage about contributions from surface capacitive and diffusion-controlled intercalation is investigated while natural frequencies and modes shapes of the cell with mechanical loading are also tested, both of which provide important clues for design of high performance batteries. The extra CO2 activation leads to the further enhancement of electrochemical performances with outstanding rate capability, cycling performance and high coulombic efficiency. The discharge capacity of DHCSs/RGO-700-CO2 maintained 1692 mA h/g when the rate was restored to be 0.1 A/g after 90 cycles at different rates. Moreover, the optimized microstructure and processing engineering of the architecture broadly extend its applications 18 ACS Paragon Plus Environment

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and functionality to various fields including lithium-sulfur battery, supercapacitor, hydrogen storage, and adsorbent, etc. Experimental Section Material. Expanded graphite powders were purchased from Alfa Aesar. KOH (≥85%), acetic acid, melamine, acetylene black, poly(vinylidene fluoride) (PVDF), formaldehyde solution (37%) were obtained from Sinopharm Chemical Reagent Co., Ltd. Polyvinyl alcohol (PVA, Mw=2 000) were supplied by Tianjin Bodi Chemical Co., Ltd. All other chemicals were commercially purchased of analytical grade from Aladdin biochemical technology Co., Ltd. Fabrication of graphene oxide (GO). GO was synthesized from expanded graphite powders by the modified Hummers method. In a typical synthesis, 2.0 g expandable graphite and K2S2O8 were added into 20.0 mL concentrated H2SO4 under ultrasonic oscillation followed with slow addition of 5.0 g KMnO4 into the mixture by continuous magnetic stirring at 80 °C for 1 h. Afterwards, 100.0 mL distilled water was slowly dropped into the reaction system. After magnetic stirring for 20 min, 30% H2O2 (30.0 mL) was added into the solution, resulting in a saffron yellow suspension. Finally, the product was collected by centrifugation and washed with 6% diluted HCl and distilled water. The obtained solid GO was dried at 60 °C for 12 h for further use. Synthesis of MR spheres. MR spheres were prepared by controlled polymerization of melamine and formaldehyde with PVA served as the dispersant and nucleating agents for templating. In the current preparation procedure, 3.0 g melamine and 6.0 mL formaldehyde solution were added into a 50.0 mL beaker and maintained at 65 oC for 25 min. 0.6 g PVA was dispersed in 100.0 mL distilled water and then mixed with 1.2 mL acetic acid in the beaker followed with magnetic stirring at 65 oC for 8 min to form the MR spheres. The asmade MR spheres were collected by centrifugation and washed with distilled water and

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absolute ethyl alcohol for multiple times. The product was then obtained by drying at 50 oC overnight. Preparation of DHCSs/RGO nanocomposites. GO powders and MR spheres with a mass ratio of 1:3 were added into distilled water (200.0 mL), followed by ultrasonic treatment for 3 h for complete dispersion of the components. 100.0 mL KOH aqueous solution was added into the mixed suspension of GO/MR spheres (mGO/MR

spheres

: mKOH=1 : 4) for intensive

mixing. The mixture was transferred for freezing for 24 h with a mixture of MR spheres/GO/KOH obtained by freeze-drying. The mixture was placed into an iron crucible for carbonation at different temperatures from 600 to 800 oC for 1 h with a heating rate of 10 o

C/min under Ar protection. The as-prepared samples were then washed with 0.2 M HCl and

distilled water for several times to remove residual KOH and other impurities. The final product was obtained by subsequent centrifugation and vacuum drying at 60 oC. The samples are denoted as DHCSs/RGO-X (X=600, 700, 800) in which X refers to their carbonation temperatures. As a comparison, MR spheres/RGO fabricated from the same route with an annealing temperature of 700 oC without PVA addition is designated as DSCSs/RGO-700. MR spheres/RGO prepared in the same route with conventional drying at 60 oC is designated as CDHCSs/RGO-700. MR spheres/GO annealed at 700 oC without KOH addition is another control cell designated as NACs-700. RGO was prepared by annealing the mixture of GO and KOH at 700 oC with a mass ratio of 1:4. DHCSs/RGO-700 was then heated in the argon gas to 700 ºC, following with the ventilation of CO2 gas flow with 50 cc/min for 15 min. The sample with CO2 activation is named as DHCSs/RGO-700-CO2. Materials Characterization. Scanning electron microscopy (SEM, Hitachi S-4800, accelerating voltage of 5.0 kV) and transmission electron microscopy (TEM, JEOL JEM-2100, accelerating voltage of 200 kV) were conducted to investigate the microstructural and morphological features of the materials. Powdered X-ray diffraction (XRD) patterns were recorded by an X-ray diffractometer (Germany, Bruker AXS D8 ADVANCE) using Cu-Kα 20 ACS Paragon Plus Environment

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radiation (λ=1.5406 Å). Raman spectroscopy was used to examine the chemical composition. Elemental analysis was carried out on an Elementar Vairo EL III (Germany). Fourier transform infrared spectrometer (FTIR; TENSOR 27) was used to study the variation of the chemical structure using KBr pellets. The electron binding energy of elements is analyzed through X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific ESCALAB 250 using monochromatic Al Kα radiation (1486.6 eV)). The samples were degassed under vacuum for 24 h following with obtaining the N2 adsorption/desorption isotherms (American Micromeritics ASAP 2020) at 77.1 K. The nonlocal density functional theory was used to check the pore size distribution of samples based on N2 adsorption data. The total pore volume was analyzed based on the adsorbed amount at a relative pressure P/P0=0.99. The micropore information was estimated by the t-plot theory, and the mesopore information was analyzed using the adsorption data based on the Barrett–Joyner–Halenda (BJH) theory. The average conductivity was characterized using a four-probe testing instrument (RTS-8, Tianjin comprehensive technology Co., Ltd.). The samples were pressed into small pieces of 1.0 cm diameter and approximately 0.1 cm thickness under the pressure of 15 MPa. Electrochemical

Measurements.

The

electrochemical

characterization

of

the

DHCSs/RGO nanocomposites and the control samples was tested using CR2025 stainless steel coin cells with lithium wafers as the counter electrode and reference electrode. The active materials (80%) were then mixed with PVDF (10%) and acetylene black (10%) in Nmethyl-2-pyrrolidone to form the slurry. The working electrodes were prepared through coating the slurry on the surface of the copper foil current collector. The as-made working electrodes were dried at 100 oC in a vacuum oven for 10 h to remove N-methyl-2-pyrrolidone. 1 M LiPF6 in ethylene carbonate (EC) and dimethyl carbonate (DMC) (VEC:VDMC=1:1) was used as the electrolyte. The cells were assembled in an argon-filled glovebox. The cyclic voltammetry (CV) and galvanostatic discharge-charge (GDC) curves were measured using an electrochemical workstation (USA PARSTAT 273A) and a battery test system (LAND 21 ACS Paragon Plus Environment

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CT2001A) with a potential range between 0.01 and 3.0 V, respectively. Electrochemical impedance spectra (EIS) was also tested on PARSTAT 273A in the frequency range from 100 kHz to 100 mHz. Associated Content Supporting Information SEM and TEM images of samples, XRD and FTIR spectrum of GO, high-resolution XPS scans for C1s, N1s, and O1s, pore size distribution of the samples, the voltage profiles of the DHCSs/RGO composites and control samples, the Nyquist plots of the DHCSs/RGO-700, two mode shapes, XPS results of the samples, comparison of the electrochemical performance, equivalent series resistance (Re) and charge transfer resistance (Rct) of the as-made samples. The Supporting Information is available free of charge on the ACS Publications website at DOI: ….. Author Information Corresponding Authors *E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected], [email protected]. ORCID Li Li: 0000-0003-2308-916X Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant Nos.51574062). References (1)

Ai, W.; Luo, Z.; Jiang, J.; Zhu, J.; Du, Z.; Fan, Z.; Xie, L.; Zhang, H.; Huang, W.; Yu,

T. Nitrogen and Sulfur Codoped Graphene: Multifunctional Electrode Materials for HighPerformance Li-ion Batteries and Oxygen Reduction Reaction. Adv. Mater. 2014, 26, 6186– 22 ACS Paragon Plus Environment

Page 22 of 37

Page 23 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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6192. (2)

Hou, J.; Cao, C.; Idrees, F.; Ma, X. Hierarchical Porous Nitrogen-Doped Carbon

Nanosheets Derived from Silk for Ultrahigh-Capacity Battery Anodes and Supercapacitors. ACS Nano 2015, 9, 2556–2564. (3)

Xie, Z.; He, Z.; Feng, X.; Xu, W.; Cui, X.; Zhang, J.; Yan, C.; Carreon, M. A.; Liu, Z.;

Wang, Y. Hierarchical Sandwich-Like Structure of Ultrafine N-Rich Porous Carbon Nanospheres Grown on Graphene Sheets as Superior Lithium-Ion Battery Anodes. ACS Appl. Mater. Interfaces 2016, 8, 10324–10333. (4)

Landi, B. J.; Ganter, M. J.; Cress, C. D.; DiLeo, R. A.; Raffaelle, R. P. Carbon

Nanotubes for Lithium Ion Batteries. Energy Environ. Sci. 2009, 2, 638–654. (5)

Raccichini, R.; Varzi, A.; Wei, D.; Passerini, S. Critical Insight into the Relentless

Progression Toward Graphene and Graphene-Containing Materials for Lithium-Ion Battery Anodes. Adv. Mater. 2017, 29, 1603421–1603453. (6)

Tan, G.; Bao, W.; Yuan, Y.; Liu, Z.; Shahbazian-Yassar, R.; Wu, F.; Amine, K.; Wang,

J.; Lu, J. Freestanding Highly Defect Nitrogen-Enriched Carbon Nanofibers for Lithium Ion Battery Thin-Film Anodes. J. Mater. Chem. A 2017, 5, 5532–5540. (7)

Zhou, H.; Zhu, S.; Hibino, M.; Honma, I.; Ichihara, M. Lithium Storage in Ordered

Mesoporous Carbon (CMK-3) with High Reversible Specific Energy Capacity and Good Cycling Performance. Adv. Mater. 2003, 15, 2107–2111. (8)

Yang, S.; Feng, X.; Zhi, L.; Cao, Q.; Maier, J.; Mullen, K. Nanographene-Constructed

Hollow Carbon Spheres and Their Favorable Electroactivity with Respect to Lithium Storage. Adv. Mater. 2010, 22, 838–842. (9)

Luo, B.; Zhi, L. Design and Construction of Three Dimensional Graphene-Based

Composites for Lithium Ion Battery Applications. Energy Environ. Sci. 2015, 8, 456–477. (10) Yan, K.; Lu, Z.; Lee, H.-W.; Xiong, F.; Hsu, P.-C.; Li, Y.; Zhao, J.; Chu, S.; Cui, Y. Selective Deposition and Stable Encapsulation of Lithium through Heterogeneous Seeded 23 ACS Paragon Plus Environment

ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Growth. Nat. Energy 2016, 1, 16010–16017. (11) Jiang, Z.; Jiang, Z.-J.; Tian, X.; Luo, L. Nitrogen-Doped Graphene Hollow Microspheres as an Efficient Electrode Material for Lithium Ion Batteries. Electrochim. Acta 2014, 146, 455–463. (12) Han, F.-D.; Bai, Y.-J.; Liu, R.; Yao, B.; Qi, Y.-X.; Lun, N.; Zhang, J.-X. Template-Free Synthesis of Interconnected Hollow Carbon Nanospheres for High-Performance Anode Material in Lithium-Ion Batteries. Adv. Energy Mater. 2011, 1, 798–801. (13) Yang, Y.; Jin, S.; Zhang, Z.; Du, Z.; Liu, H.; Yang, J.; Xu, H.; Ji, H. Nitrogen-Doped Hollow Carbon Nanospheres for High-Performance Li-Ion Batteries. ACS Appl. Mater. Interfaces 2017, 9, 14180–14186. (14) Lu, M.; Yu, W.; Shi, J.; Liu, W.; Chen, S.; Wang, X.; Wang, H. Self-Doped Carbon Architectures with Heteroatoms Containing Nitrogen, Oxygen and Sulfur as HighPerformance Anodes for Lithium- and Sodium-Ion Batteries. Electrochim. Acta 2017, 251, 396–406. (15) Zhang, K.; Li, X.; Liang, J.; Zhu, Y.; Hu, L.; Cheng, Q.; Guo, C.; Lin, N.; Qian, Y. Nitrogen-Doped Porous Interconnected Double-Shelled Hollow Carbon Spheres with High Capacity for Lithium Ion Batteries and Sodium Ion Batteries. Electrochim. Acta 2015, 155, 174–182. (16) Wang, D. W.; Li, F.; Liu, M.; Lu, G. Q.; Cheng, H. M. 3D Aperiodic Hierarchical Porous Graphitic Carbon Material for High-Rate Electrochemical Capacitive Energy Storage. Angew. Chem. 2008, 47, 373–376. (17) Xu, C.; Xu, B.; Gu, Y.; Xiong, Z.; Sun, J.; Zhao, X. S. Graphene-Based Electrodes for Electrochemical Energy Storage. Energy Environ. Sci. 2013, 6, 1388–1414. (18) Sun, H.; Mei, L.; Liang, J.; Zhao, Z.; Lee, C.; Fei, H.; Ding, M.; Lau, J.; Li, M.; Wang, C.; Xu, X.; Hao, G.; Papandrea, B.; Shakir, I.; Dunn, B.; Huang, Y.; Duan, X. ThreeDimensional Holey-Graphene/Niobia Composite Architectures for Ultrahigh-Rate Energy 24 ACS Paragon Plus Environment

Page 24 of 37

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Storage. Science 2017, 356, 599–604. (19) Sun, M.-H.; Huang, S.-Z.; Chen, L.-H.; Li, Y.; Yang, X.-Y.; Yuan, Z.-Y.; Su, B.-L. Applications of Hierarchically Structured Porous Materials from Energy Storage and Conversion, Catalysis, Photocatalysis, Adsorption, Separation, and Sensing to Biomedicine. Chem. Soc. Rev. 2016, 45, 3479–3563. (20) Lian, P.; Zhu, X.; Liang, S.; Li, Z.; Yang, W.; Wang, H. Large Reversible Capacity of High Quality Graphene Sheets as an Anode Material for Lithium-Ion Batteries. Electrochim. Acta 2010, 55, 3909–3914. (21) Pan, D.; Wang, S.; Zhao, B.; Wu, M.; Zhang, H.; Wang, Y.; Jiao, Z. Li Storage Properties of Disordered Graphene Nanosheets. Chem. Mater 2009, 21, 3136–3142. (22) Hu, X.; Zeng, G.; Chen, J.; Lu, C.; Wen, Z. 3D Graphene Network Encapsulating SnO2 Hollow Spheres as a High-Performance Anode Material for Lithium-Ion Batteries. J. Mater. Chem. A 2017, 5, 4535–4542. (23) Guo, C. X.; Li, C. M. A Self-Assembled Hierarchical Nanostructure Comprising Carbon Spheres and Graphene Nanosheets for Enhanced Supercapacitor Performance. Energy Environ. Sci. 2011, 4, 4504–4507. (24) Zhou, G.; Zhao, Y.; Manthiram, A. Dual-Confined Flexible Sulfur Cathodes Encapsulated in Nitrogen-Doped Double-Shelled Hollow Carbon Spheres and Wrapped with Graphene for Li–S Batteries. Adv. Energy Mater. 2015, 5, 1402263–1402272. (25) Lei, Z.; Christov, N.; Zhao, X. S. Intercalation of Mesoporous Carbon Spheres between Reduced Graphene Oxide Sheets for Preparing High-Rate Supercapacitor Electrodes. Energy Environ. Sci. 2011, 4, 1866–1873. (26) Zhu, Y.; Murali, S.; Stoller, M. D.; Ganesh, K. J.; Cai, W.; Ferreira, P. J.; Pirkle, A.; Wallace, R. M.; Cychosz, K. A.; Thommes, M.; Su, D.; Stach, E. A.; Ruoff, R. S. CarbonBased Supercapacitors Produced by Activation of Graphene. Science 2011, 332, 1537–1541. (27) Zhang, L. L.; Zhao, X.; Stoller, M. D.; Zhu, Y.; Ji, H.; Murali, S.; Wu, Y.; Perales, S.; 25 ACS Paragon Plus Environment

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Clevenger, B.; Ruoff, R. S. Highly Conductive and Porous Activated Reduced Graphene Oxide Films for High-Power Supercapacitors. Nano Lett. 2012, 12, 1806–1812. (28) Pon-On, W.; Meejoo, S.; Tang, I. M. Formation of Hydroxyapatite Crystallites Using Organic Template of Polyvinyl Alcohol (PVA) and Sodium Dodecyl Sulfate (SDS). Mater. Chem. Phys. 2008, 112, 453–460. (29) Zhang, H.-J.; Wang, K.-X.; Wu, X.-Y.; Jiang, Y.-M.; Zhai, Y.-B.; Wang, C.; Wei, X.; Chen, J.-S. MoO2/Mo2C Heteronanotubes Function as High-Performance Li-Ion Battery Electrode. Adv. Funct. Mater. 2014, 24, 3399–3404. (30) Li, Y.; Li, Z.; Shen, P. K. Simultaneous Formation of Ultrahigh Surface Area and Three-Dimensional Hierarchical Porous Graphene-Like Networks for Fast and Highly Stable Supercapacitors. Adv. Mater. 2013, 25, 2474–2480. (31) Li, W.; Li, M.; Wang, M.; Zeng, L.; Yu, Y. Electrospinning with Partially Carbonization in Air: Highly Porous Carbon Nanofibers Optimized for High-Performance Flexible Lithium-Ion Batteries. Nano Energy 2015, 13, 693–701. (32) Kim, C.; Yang, K. S.; Kojima, M.; Yoshida, K.; Kim, Y. J.; Kim, Y. A.; Endo, M. Fabrication of Electrospinning-Derived Carbon Nanofiber Webs for the Anode Material of Lithium-Ion Secondary Batteries. Adv. Funct. Mater. 2006, 16, 2393–2397. (33) Dreyer, D. R.; Park, S.; Bielawski, C. W.; Ruoff, R. S. The Chemistry of Graphene Oxide. Chem. Soc. Rev. 2010, 39, 228–240. (34) Fan, X.; Peng, W.; Li, Y.; Li, X.; Wang, S.; Zhang, G.; Zhang, F. Deoxygenation of Exfoliated Graphite Oxide under Alkaline Conditions: A Green Route to Graphene Preparation. Adv. Mater. 2008, 20, 4490–4493. (35) Wu, Y.; Li, Y.; Qin, L.; Yang, F.; Wu, D. Monodispersed or Narrow-Dispersed Melamine-Formaldehyde Resin Polymer Colloidal Spheres: Preparation, Size-Control, Modification, Bioconjugation and Particle Formation Mechanism. J. Mater. Chem. B 2013, 1, 204–212. 26 ACS Paragon Plus Environment

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(36) Lai, L.; Yang, H.; Wang, L.; Teh, B. K.; Zhong, J.; Chou, H.; Chen, L.; Chen, W.; Shen, Z.; Ruoff, R. S.; Lin, J. Preparation of Supercapacitor Electrodes through Selection of Graphene Surface Functionalities. ACS Nano 2012, 6, 5941–5951. (37) Holst, J. R.; Gillan, E. G. From Triazines to Heptazines: Deciphering the Local Structure of Amorphous Nitrogen-Rich Carbon Nitride Materials. J. Am. Chem. Soc. 2008, 130, 7373–7379. (38) Yao, Y.; Zhang, Y.; Li, L.; Wang, S.; Dou, S.; Liu, X. Fabrication of Hierarchical Porous Carbon Nanoflakes for High-Performance Supercapacitors. ACS Appl. Mater. Interfaces 2017, 9, 34944–34953. (39) Zheng, F.; Yang, Y.; Chen, Q. High Lithium Anodic Performance of Highly NitrogenDoped Porous Carbon Prepared from a Metal-Organic Framework. Nat. Commun. 2014, 5, 5261–5270. (40) Long, W.; Fang, B.; Ignaszak, A.; Wu, Z.; Wang, Y. J.; Wilkinson, D. Biomass-Derived Nanostructured Carbons and Their Composites as Anode Materials for Lithium Ion Batteries. Chem. Soc. Rev. 2017, 46, 7176–7190. (41) Chen, P.; Yang, J.-J.; Li, S.-S.; Wang, Z.; Xiao, T.-Y.; Qian, Y.-H.; Yu, S.-H. Hydrothermal Synthesis of Macroscopic Nitrogen-Doped Graphene Hydrogels for Ultrafast Supercapacitor. Nano Energy 2013, 2, 249–256. (42) Wu, Z.-S.; Ren, W.; Xu, L.; Li, F.; Cheng, H.-M. Doped Graphene Sheets As Anode Materials with Superhigh Rate and Large Capacity for Lithium Ion Batteries. ACS Nano 2011, 5, 5463–5471. (43) Wang, S.; Gai, L.; Jiang, H.; Guo, Z.; Bai, N.; Zhou, J. Reduced Graphene Oxide Grafted by the Polymer of Polybromopyrroles for Nanocomposites with Superior Performance for Supercapacitors. J. Mater. Chem. A 2015, 3, 21257–21268. (44) Lee, S. W.; Yabuuchi, N.; Gallant, B. M.; Chen, S.; Kim, B. S.; Hammond, P. T.; ShaoHorn, Y. High-Power Lithium Batteries from Functionalized Carbon-Nanotube Electrodes. 27 ACS Paragon Plus Environment

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Nat. Nanotechnol. 2010, 5, 531–537. (45) Qian, W.; Sun, F.; Xu, Y.; Qiu, L.; Liu, C.; Wang, S.; Yan, F. Human Hair-Derived Carbon Flakes for Electrochemical Supercapacitors. Energy Environ. Sci. 2014, 7, 379–386. (46) Guo, B.; Wang, X.; Fulvio, P. F.; Chi, M.; Mahurin, S. M.; Sun, X. G.; Dai, S. SoftTemplated Mesoporous Carbon-Carbon Nanotube Composites for High Performance Lithium-Ion Batteries. Adv. Mater. 2011, 23, 4661–4666. (47) Huang, S.; Wang, J.; Pan, Z.; Zhu, J.; Shen, P. K. Ultrahigh Capacity and Superior Stability of Three-Dimensional Porous Graphene Networks Containing in Situ Grown Carbon Nanotube Clusters as an Anode Material for Lithium-Ion Batteries. J. Mater. Chem. A 2017, 5, 7595–7602. (48) Su, F.; Poh, C. K.; Chen, J. S.; Xu, G.; Wang, D.; Li, Q.; Lin, J.; Lou, X. W. NitrogenContaining Microporous Carbon Nanospheres with Improved Capacitive Properties. Energy Environ. Sci. 2011, 4, 717–724. (49) Lyu, H.; Liu, J.; Qiu, S.; Cao, Y.; Hu, C.; Guo, S.; Guo, Z. Carbon Composite Spun Fibers with in Situ Formed Multicomponent Nanoparticles for a Lithium-Ion Battery Anode with Enhanced Performance. J. Mater. Chem. A 2016, 4, 9881–9889. (50) Yoo, E.; Kim, J.; Hosono, E.; Zhou, H.-s.; Kudo, T.; Honma, I. Large Reversible Li Storage of Graphene Nanosheet Families for Use in Rechargeable Lithium Ion Batteries. Nano Lett. 2008, 8, 2277–2282. (51) Wang, Z.-L.; Xu, D.; Wang, H.-G.; Wu, Z.; Zhang, X.-B. In Situ Fabrication of Porous Graphene Electrodes for High-Performance Energy Storage. ACS Nano 2013, 7, 2422–2430. (52) Zhang, L.; Dou, Y.; Guo, H.; Zhang, B.; Liu, X.; Wan, M.; Li, W.; Hu, X.; Dou, S.; Huang, Y.; Liu, H. A Facile Way to Fabricate Double-Shell Pomegranate-Like Porous Carbon Microspheres for High-Performance Li-Ion Batteries. J. Mater. Chem. A 2017, 5, 12073– 12079. (53) Sun, H.; Xin, G.; Hu, T.; Yu, M.; Shao, D.; Sun, X.; Lian, J. High-Rate Lithiation28 ACS Paragon Plus Environment

Page 28 of 37

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ACS Nano

Induced Reactivation of Mesoporous Hollow Spheres for Long-Lived Iithium-Ion Batteries. Nat. Commun. 2014, 5, 4526–4533. (54) Yang, Y.; Pang, R.; Zhou, X.; Zhang, Y.; Wu, H.; Guo, S. Composites of ChemicallyReduced Graphene Oxide Sheets and Carbon Nanospheres with Three-Dimensional Network Structure as Anode Materials for Lithium Ion Batteries. J. Mater. Chem. 2012, 22, 23194– 23200. (55) Liao, J. Y.; Higgins, D.; Lui, G.; Chabot, V.; Xiao, X.; Chen, Z. Multifunctional TiO2C/MnO2 Core-Double-Shell Nanowire Arrays as High-Performance 3D Electrodes for Lithium Ion Batteries. Nano Lett. 2013, 13, 5467–5473. (56) Liu, J.; Wang, J.; Xu, C.; Jiang, H.; Li, C.; Zhang, L.; Lin, J.; Shen, Z. X. Advanced Energy Storage Devices: Basic Principles, Analytical Methods, and Rational Materials Design. Adv. Sci. 2018, 5, 1700322–1700340. (57) Augustyn, V.; Come, J.; Lowe, M. A.; Kim, J. W.; Taberna, P.-L.; Tolbert, S. H.; Abruña, H. D.; Simon, P.; Dunn, B. High-Rate Electrochemical Energy Storage through Li+ Intercalation Pseudocapacitance. Nat. Mater. 2013, 12, 518–522. (58) Brezesinski, T.; Wang, J.; Tolbert, S. H.; Dunn, B. Ordered Mesoporous α-MoO3 with Iso-Oriented Nanocrystalline Walls for Thin-Film Pseudocapacitors. Nat. Mater. 2010, 9, 146–151. (59) Hooper, J. M.; Marco, J. Experimental Modal Analysis of Lithium-Ion Pouch Cells. J. Power Sources 2015, 285, 247–259.


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Figure 1. (A) Schematic illustration for synthesis of DHCSs/RGO composites; SEM images of (B) MR spheres/GO after mixing and (C) DHCSs/RGO-700; The corresponding TEM bright field images of (D) MR spheres/GO and (E) DHCSs/RGO-700 (SAED pattern as the inset); (F-H) EDS elemental mapping of DHCSs/RGO-700; and (I) high resolution TEM image of DHCSs/RGO-700. (The presence of micropores and mesopores are clearly observed while some mesopores are highlighted in the figure).


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Figure 2. (A) XRD spectra, (B) Raman spectra, (C) FTIR spectra, (D) XPS spectra, and (H) N2 adsorption and desorption isotherms of the samples. High-resolution XPS scans for (E) C1s, (F) N1s, and (G) O1s of the DHCSs/RGO-700.


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Figure 3. The electrochemical performances of the samples: (A) CV curves at the scan rate of 0.1 mV/s; (B) GCD curves at the current density of 0.1 A/g; (C) Rate performance at different rates; (D) Cyclic performances at the current density of 0.1 A/g; (E) Cycling performances of DHCSs/RGO-700 at the current density of 1 and 2 A/g, respectively; and (F) Nyquist plots.


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ACS Nano

Figure 4. (A) CV curves of DHCSs/RGO-700 at different scan rates; (B) b values plotted against the voltage of DHCSs/RGO-700 for cathodic scans. R2 are the fitting variance of b value at different voltages. The inset is current response plotted against scan rates of DHCSs/RGO-700 at different voltages; (C) capacitive (purple region) and the diffusioncontrolled (green region) contribution to charge storage of DHCSs/RGO-700 at 0.5 mV/s; (D) the normalized contribution ratio of capacitive (purple region) and diffusion-controlled (green region) capacities at different scan rates; (E) the setup of field test to measure natural frequencies and mode shapes of cells; and (F) frequency response for impacted locations.


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Figure 5. The electrochemical performances of DHCSs/RGO-700-CO2: (A) rate performance and coulombic efficiency at different rates; (B) Nyquist plots after 3rd and 300th GCD cycles; and (C) cyclic performances and coulombic efficiency at the current density of 2 A/g.


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ACS Nano

Table 1. Elemental analysis and XPS results of the samples. Elemental analysis (wt %)

XPS (at. %)

Samples

C

N

O

C

N

O

DHCSs/RGO-600

84.85

7.14

8.01

81.70

8.02

10.28

DHCSs/RGO-700

88.64

5.22

6.14

86.24

5.81

7.95

DHCSs/RGO-800

92.45

3.30

4.25

90.79

3.78

5.43

NACs-700

87.37

8.76

3.87

85.02

9.95

5.02


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Table 2. The pore structure information about the as-made samples. BET analysis SBET (m2/g)

Smicro (m2/g)

Vtotalc (cm3/g)

Vmicrod (cm3/g)

Vmesoe (cm3/g)

Daver (nm)

Smicro/SBET

(m /g)

DHCSs/RGO-600

52.31

4.65

40.32

0.09

0.002

0.08

7.1

8.89%

DHCSs/RGO-700

417.31

6.21

407.84

0.71

0.0014

0.69

6.9

1.49%

DHCSs/RGO-800

331.98

7.11

289.52

0.44

0.0015

0.41

5.3

2.14%

NACs-700

475.94

85.1

323.24

0.54

0.042

0.46

4.5

17.88%

RGO

174.1

1.91

162.1

0.17

0.001

0.15

6.2

Samples

a

a

Smesob 2

b

1.09% c

Smicro is the surface area of the micropores; Smeso is the surface area of the mesopores; Vtotal is the total pore volume; dVmicro is the volume of the micropores; eVmeso is the volume of the mesopores.


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ACS Nano

Table of Contents DHCSs clad in RGO nanosheets introduced a homogeneous 3D interconnected carbon network architecture by using PVA as the non-removal template for dispersion and nucleation followed with uniform molten KOH activation and reduction of MR spheres/GO composites in Ar. The uniform structure and high-level heteroatom doping of DHCSs/RGO result in an excellent electrochemical performance for LIBs anodes. Keyword: hollow carbon spheres, reduced graphene oxide, polyvinyl alcohol, molten KOH, lithium ion battery anodes Yunqiang Zhang, Qiang Ma, Shulan Wang,* Xuan Liu,* and Li Li* Polyvinyl Alcohol Assisted Fabrication of Hollow Carbon Spheres/Reduced Graphene Oxide Nanocomposites for High-Performance Lithium Ion Battery Anodes


Polyvinyl Alcohol Assisted Fabrication of Hollow Carbon Spheres

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