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Scalable synthesis of sub-100 nm hollow carbon nanospheres for energy storage applications

Hongyu Zhao1,2,§, Fan Zhang2,§, Shumeng Zhang1, Shengnan He1, Fei Shen2, Xiaogang Han2 (), Yadong Yin3, and Chuanbo Gao1 () Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-017-1800-3 http://www.thenanoresearch.com on Aug. 14, 2017 © Tsinghua University Press 2017

Just Accepted

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Nano Research DOI 10.1007/s12274-017-1800-3

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Scalable Synthesis of Sub-100 nm Hollow Carbon Nanospheres for Energy Storage Applications

Hongyu Zhao, Fan Zhang, Shumeng Zhang,

Shengnan He, Fei Shen, Xiaogang Han*, Yadong

Yin, Chuanbo Gao*.

Xi’an Jiaotong University, China

University of California, Riverside, United States.

Sub-100 nm hollow carbon nanospheres (as small as ~ 32.5 nm) have been synthesized in high yield by employing reverse micelles as nanoreactors, which show excellent capacity and cycling stability when used as an anode material for lithium/sodium-ion batteries.

Provide the authors’ webside if possible.

Chuanbo Gao, http://gaochuanbo.gr.xjtu.edu.cn

Xiaogang Han, http://xiaogang.han.gr.xjtu.edu.cn

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Scalable Synthesis of Sub-100 nm Hollow Carbon Nanospheres for Energy Storage Applications

Hongyu Zhao1,2,†, Fan Zhang2,†, Shumeng Zhang1, Shengnan He1, Fei Shen2, Xiaogang Han2 (), Yadong Yin3, and Chuanbo Gao1 ()

1 Center for Materials Chemistry, Frontier Institute of Science and Technology, Xi’an Jiaotong University, Xi’an, Shaanxi 710054, China 2 Center of Nanomaterials for Renewable Energy, Key Lab of Smart Grid of Shaanxi Province, State Key Lab of Electrical Insulation and Power Equipment, Xi’an Jiaotong University, Xi’an, Shaanxi 710054, China 3 Department of Chemistry, University of California, Riverside, CA92521, United States † These authors contributed equally to this work

Received: day month year Revised: day month year Accepted: day month year (automatically inserted by the publisher)

© Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2014

KEYWORDS hollow carbon nanospheres, reverse micelles, nanoreactors, templating synthesis, energy storage

ABSTRACT Sub-100 nm hollow carbon nanospheres of a thin thickness are highly desirable anode materials for energy storage applications, which, however, remain a great challenge to synthesize by a conventional strategy. In this work, we demonstrate that hollow carbon nanospheres of unprecedentedly small sizes down to ~ 32.5 nm and thickness of ~ 3.9 nm can be produced on a large scale by a templating process in a unique reverse micelle system. Reverse micelles enable a spatially confined Stöber process that affords uniform silica nanospheres with significantly reduced sizes compared with that from a conventional Stöber process, and a subsequent well-controlled sol-gel coating process of a resorcinol-formaldehyde resin on these silica nanospheres as a precursor of the hollow carbon nanospheres. Due to the short diffusion length arising from the hollow structure, as well as the small size and microporosity, these hollow carbon nanospheres show excellent capacity and cycling stability when used as an anode material for lithium/sodium-ion batteries.

Nano Research DOI (automatically inserted by the publisher) Research Article

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1. Introduction

Carbon nanomaterials have received considerable attention due to their high surface area, electrical conductivity, chemical stability, low cost, and thus potential applications in energy storage [1-5], catalysis [6-8], electronics [9], and biomedicine [10, 11]. Over the past decades, considerable progress has been made in the synthesis and applications of various types of carbon nanomaterials, such as carbon nanotubes (or nanofibers) [12-15], graphene [16-19], porous carbon materials [20-25], and carbon nanospheres [23-26]. Recently, hollow carbon nanospheres prove to be highly promising, which demonstrate to be an attractive anode material in lithium-ion batteries (LIBs) [27-33]. The unique hollow structure may boost lithium-ion transport in the carbon shells by offering a large surface area and a short diffusion distance, and on the other hand, it favors effective accommodation of the strain caused by the volume change during lithium ion insertion/extraction due to the remarkable elasticity of the carbon thin shells. From the perspective of surface diffusion and volumetric energy/power density, hollow carbon nanospheres of a small size and a thin thickness are highly desirable for fabricating an anode for the LIBs. However, due to the limitations of the available synthesis strategies, especially the availability of ultrasmall templates in a templated synthesis, the size of the hollow carbon nanospheres are usually larger than 100 nm [27-32], leaving a lot of room in developing alternative strategies that can afford hollow carbon nanospheres with significantly decreased size for even enhanced activity and stability in energy storage applications.

Herein, for the first time, we report a robust and scalable templating synthesis route [34, 35] to

uniform sub-100 nm hollow carbon nanospheres with a thin thickness by employing reverse micelles as discrete nanoreactors (Scheme 1) [36-40]. The roles of the reverse micelles are two-fold. First, by extending the conventional Stöber process [41] to reverse micelles, synthesis of uniform silica

nanospheres with tunable small sizes (~ 20–50 nm) becomes possible due to the spatial confinement of the reaction, which paves a way to the templated synthesis of hollow carbon nanospheres. Second, the reverse micelles as the nanoreactors make the sol-gel chemistry of a resorcinol-formaldehyde (RF) resin highly controllable, which allows subsequent coating of the silica nanospheres with RF of a controllable thickness as a carbon precursor [39, 42]. As a result, hollow carbon nanospheres with unprecedentedly small sizes down to ~ 32.5 nm and thickness of ~ 3.9 nm were eventually obtained from this unique reverse micelle system after further silica-protected carbonization of RF and etching of silica. In addition, the abundant reverse micelles as nanoreactors enable high concentrations of the products from a small volume of the reaction system, and thus large-scale production of the hollow carbon nanospheres for practical energy storage applications. When these hollow carbon nanospheres are employed as an anode material in LIBs, it demonstrates an initial capacity of 687 mA·h·g–1 and steady long-term cycling over 1000 cycles with 99.66% of coulombic efficiency (CE). Comparable results are found in sodium-ion batteries (SIBs), which exhibits an initial capacity of 222 mA·h·g–1 with 99.53% of CE over 200 cycles. These electrochemical evaluations well illustrate the superior activity and cycling stability of the sub-100 nm hollow carbon nanospheres in energy storage applications, thanks to their unprecedentedly small size and unique hollow nanostructure.

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Scheme 1 A cartoon illustrating the synthesis route to the hollow carbon nanospheres.

2. Experimental

2.1 Materials

Resorcinol, formaldehyde (HCHO, 37 wt%), 3-aminopropyltriethxoysilane (APS), diethylamine (DEA), polyoxyethylene(10) cetyl ether (Brij C10), and tetraethyl orthosilicate (TEOS) were purchased from Sigma-Aldrich. Cyclohexane was purchased from Alfa Aesar. Ammonium hydroxide (NH3·H2O, 25%) was purchased from Tianjin Kemiou Chemical Reagent. Isopropanol was purchased from Tianjin Zhiyuan Chemical Reagent. All chemicals were used as received.

2.2 Synthesis of hollow carbon nanospheres

In a typical synthesis, 2.125 g of Brij C10 was dissolved in 7.5 mL of cyclohexane at 50 °C. Then, 0.2 mL of deionized water was added dropwise under stirring. After a transparent solution was formed, 0.57 mL of NH3·H2O (25 %) and 2 min later, 0.2 mL of TEOS were added in sequence. The Stöber process was proceed for 2 h to yield silica nanospheres. Then, 50 μL of APS was added into the above reaction system and stirred overnight at 50 °C to obtain amino-functionalized silica nanospheres. Subsequently, 0.8 mL of NH3·H2O (25%) and 1 mL of resorcinol-formaldehyde (RF) precursor (prepared by mixing 0.25 g of resorcinol and 0.35 mL of formaldehyde (37%) in 3 mL of ethanol) were added to this solution and stirred for 24 h (Fig. S1 in the Electronic Supplementary Material (ESM)). During this process, the reaction solution gradually turned dark-brown, suggesting the formation of a RF resin. Finally, another layer of silica was coated these SiO2@RF core/shell nanospheres by injecting 2 mL of TEOS into the reaction solution and stirring for 12 h. The solid

product was collected by centrifugation, washed with isopropanol and water, and dried in air. The solid was then annealed at 600 °C (or 1000 °C) for 3 h in N2 for carbonization of the RF layer, and etched with NaOH (2 M) at 50 °C for 24 h to remove silica. The hollow carbon nanospheres (~ 32.5 nm) were collected by centrifugation, washed with water and dried in air.

2.3 Size tuning of the hollow carbon nanospheres

The hollow carbon nanospheres with varied sizes were synthesized following a similar procedure by tuning the amount of the TEOS (x) in the modified Stöber process, the amount of the RF precursor (y) in the RF coating, and the amount of the TEOS (z) in the coating of the silica layer. Synthesis conditions of hollow carbon nanospheres of ~ 43.0 nm: x = 1 mL, y = 4 mL, z = 8 mL; hollow carbon nanospheres of ~ 63.5 nm: x = 3 mL, y = 5 mL, z = 10 mL. After addition of TEOS (z), 0.5 mL of DEA was added for compensation of the catalyst for the sol-gel process.

2.4 Electrochemical measurements

The electrochemical measurements were conducted with 2025-type coin cells. The working electrode consisted of the active material (hollow carbon nanospheres), Super P, and a polymer binder (polyvinylidene difluoride, PVDF, Aldrich) in a weight ratio of 80:10:10, and a separator (polypropylene membrane, 25 μm thick). A pure lithium (or sodium) metals served as both the counter and the reference electrode. The electrolyte was composed of 1.0 M LiPF6 (or NaClO4) in a 1 : 1 (V/V) mixture of ethylene carbonate and diethyl carbonate. Approximately 75 μL of the electrolyte was used in the fabrication of each cell. Cell assembly was carried out in an Ar-filled glove box with both moisture and oxygen concentration

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below 1.0 ppm. Galvanostatic charge-discharge cycling was performed between potential window of 0.01-2.0 V versus Li/Li+ (or Na/Na+) with a Land CT2001 tester.

2.5 Characterizations

Transmission electron microscopy (TEM) was performed on a Hitachi HT-7700 microscope operating at 100 kV. High angle annular dark field scanning transmission electron microscopy (HAADF-STEM) was performed on an FEI Tecnai G2 F20 FEG-TEM microscope operating at 200 kV. X-ray diffraction (XRD) patterns were recorded on a Rigaku SmartLab Powder X-ray diffractometer equipped with Cu Kα radiation (λ = 1.5418 Å) and a D/teX Ultra detector, scanning from 10 to 90° (2θ) at a rate of 5° min–1. Nitrogen physisorption was measured at 77 K on a Quantachrome Quadrasorb SI-3. The samples were degassed at 120 °C and ~ 104 Pa for 3 h prior to the test. Raman spectroscopy was measured on a LabRAM HR800 confocal Raman spectrophotometer equipped with a 633 nm He–Ne laser. Fourier transform infrared spectroscopy (FTIR) was measured on a Nicolet 6700 FTIR spectrometer.

3. Results and Discussion

The synthesis of sub-100 nm hollow carbon nanospheres relies on the well-established sol-gel chemistry of silica and RF that makes it possible to fabricate core/shell nanostructures in a well-controlled layer-by-layer manner as a precursor [39, 42], and on the spatially confined reaction system in reverse micelles that enables a

significant reduction and precise control of the size of the nanospheres (Scheme 1) [36-40]. Typically, sub-100 nm silica nanospheres were first synthesized as the templates for the hollow carbon nanospheres. Conventional Stöber method is usually conducted in bulk water/alcohol solutions, with a great success in producing silica nanospheres of usually large sizes, typically from ~ 100 nm to a few micrometers, via a cluster agglomeration mechanism [41]. Although silica nanospheres of a significantly reduced size have been reported by employing amino acids as the catalyst in place of ammonia [43], mass production of the silica nanospheres remains a grand challenge. In this work, the Stöber process is conducted in discrete reverse micelles, and thus the agglomeration of silica clusters has been spatially confined, favorable for a significant reduction in the size of the silica nanospheres. In a typical synthesis, reverse micelles were first formed by dissolving Brij C10 in cyclohexane. Then, organosilanes were added, including TEOS and APS. The Stöber process was initiated by introducing ammonia into the reverse micelle system, which catalyzes the hydrolysis and condensation of the organosilanes. As a result, amino group functionalized silica nanospheres of 21.8 ± 0.6 nm have been obtained from a typical synthesis, showing a much decreased size compared with those from a conventional Stöber process, and a narrow size distribution as indicated by TEM (Fig. 1(a)). These silica nanospheres are appropriate templates for the synthesis of hollow carbon nanospheres of a small size.

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Figure 1 Synthesis of sub-100 nm hollow carbon nanospheres. (a) TEM image of the Stöber SiO2 nanospheres synthesized in reverse micelles with modification of amino groups. Inset: Size distribution of the silica nanospheres; unit: nm. The average size was denoted as “mean ± standard deviation” hereafter. (b) HAADF-STEM image of the SiO2@RF core/shell nanospheres. Inset: A high-magnification image. (c, d) TEM images of the hollow carbon nanospheres by carbonization of the SiO2@RF@SiO2 nanospheres at 600 and 1000 °C, respectively, and etching of silica. Inset of (c): Size distribution of the hollow carbon nanospheres, unit: nm. (e) Plot of the thickness of the carbon nanoshells (carbonized at 600 °C) as a function of the amount of the resorcinol in a typical synthesis. The error bars represent the standard deviation for each point.

Subsequently, a thin layer of an RF resin was coated on these silica nanospheres as a precursor of the carbon nanoshells, which was experimentally achieved by introducing resorcinol and formaldehyde into the reverse micelles, with their polymerization triggered by ammonia. The resulting SiO2@RF core/shell nanostructure can be clearly revealed by HAADF-STEM imaging, which essentially reflects the atomic number (Z) contrasts (Fig. 1(b)). The thickness of the RF layer is uniform on the silica nanospheres, which is ~ 12 nm in average. The success of the RF coating can be attributed to the well-controlled sol-gel chemistry of the RF resin in the reverse micelles, and to the effective functionalization of the silica nanospheres by amino groups that provide strong interactions between the silica surface and the RF resin. A control experiment indicates that the coating of RF on the silica nanospheres fails in the absence of amino groups (Fig. S2 in the ESM). With an increase in the density of the amino groups on the silica nanospheres, an RF shell can be synthesized

on the silica nanospheres with continuously increasing thickness (Fig. S2 in the ESM). It is reasonable that the amino groups on the silica nanospheres may form strong electrostatic interactions with the phenolic groups of the resorcinol, which favors the enrichment of resorcinol on the silica surface for their effective polymerization with formaldehyde, forming a complete shell of the RF resin. The higher the density of the amino groups on the silica nanospheres, the thicker the RF shell will growth due to an increased thickness of the resorcinol layer chemisorbed on the silica surface (Fig. S2 in the ESM). The successful coating of the RF resin on the silica nanospheres paves a way to their eventual conversion into hollow carbon nanospheres.

To prevent the sintering of the SiO2@RF core/shell nanospheres, an additional layer of silica was coated on the nanospheres before the thermal carbonization treatment at 600 °C in a nitrogen atmosphere. It is known that the silica overlayer may greatly help prevent the sintering of the

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nanoparticles it encloses by avoiding their direct contact in a thermal process, and is thus favorable for retaining their colloidal property after the silica etching [44]. As expected, after further etching of silica by a diluted solution of NaOH, uniform hollow carbon nanospheres with complete shells were obtained as a stable colloid (Fig. 1(c)). The colloidal property of the hollow carbon nanospheres can be attributed not only to the sintering-free carbonization process, but also to the abundant remaining phenolic groups, as evidenced by FTIR (Fig. S3 in the ESM), which provide electrostatic repulsive forces among the hollow carbon nanospheres. For comparison, aggregates of hollow carbon nanospheres were obtained in the absence of silica protection, which is indicative of sintering of the carbon nanospheres during the thermal carbonization process (Fig. S4 in the ESM). TEM image shows that the hollow carbon nanospheres obtained in this work are highly uniform, with an average size of ~ 32.5 ± 1.9 nm and a shrunk thickness of ~ 3.9 ± 0.5 nm (Fig. 1(c)). To the best of our knowledge, these hollow carbon nanospheres are among the smallest ones that can be synthesized from a templating synthesis. When carbonation was carried out at an elevated temperature of 1000 °C, the resulting carbon nanospheres retained their hollow structure with high structural integrity due to the excellent protection by the silica layers, with fine lattice fringes in the carbon shells becoming more prominent, suggesting an enhanced graphitization degree of the hollow carbon nanospheres (Fig. 1(d)). In addition, the thickness of the RF shells can be conveniently tuned by the amount of the resorcinol in the presence of excess formaldehyde, which varies from ~ 3.9 to ~ 10.2 nm in a typical demonstration, making the synthesis of the hollow carbon nanospheres highly tunable for different applications (Fig. 1(e) and Fig. S5 in the ESM).

The structural and porous properties of the hollow carbon nanospheres (carbonized at 1000 °C) were further examined by Raman spectroscopy, XRD, and nitrogen physisorption (Fig. 2). The Raman spectrum of the hollow carbon nanospheres displays typical G and D bands, corresponding to the in-phase vibration of the graphite lattice and disordered carbon, respectively (Fig. 2(a)) [45]. The

XRD pattern of the hollow carbon nanospheres shows significant broadening of reflection peak at ~ 26.7° (2θ), which corresponds to the reflection from the graphite, and thus confirms the presence of graphitic carbon in the hollow carbon nanospheres (Fig. 2(b)) [46]. It is therefore inferred that during the thermal carbonization process at 1000 °C, the amorphous carbon becomes partially graphitized. It is worth noting that a high carbonization temperature of 1000 °C favors an enhanced graphitization degree of carbon, but a prolonged carbonization time turned out to be less effective, as indicated by Raman spectroscopy (Fig. S6 in the ESM). The graphitization of carbon enables remarkable conductivity of the hollow carbon nanospheres for the charge transfer in a lithium/sodium-ion battery. To evaluate the conductivity, the hollow carbon nanospheres were grounded with 10 wt % PVDF in N-methyl pyrrolidone (NMP) to make a slurry. The slurry was then casted onto a non-conductive substrate with a doctor blade and dried at 105 °C in air. The sheet resistance was measured to be as low as ~ 100 Ω/sq by a four-probe method. The nitrogen physisorption isotherms of the hollow carbon nanospheres is type II, which shows an abrupt nitrogen desorption at ~ 0.4–0.5 of the relative pressure, indicative of a bottle-neck structure (Fig. 2(c)). The hollow interior and the micropores in the carbon nanoshells represent the bottle and the neck, respectively, which well rationalizes the peculiar shape of the physisorption isotherms of the hollow carbon nanospheres. Based on the Brunauer-Emmett-Teller (BET) and the t-plot method, the hollow carbon nanospheres show microporous area of ~ 192 m2·g–1, with the total surface area being ~ 840 m2·g–1 (Fig. 2(d)). The emergence of the micropores in the hollow carbon nanospheres can be attributed to the evaporation of non-crosslinked species including monomers, oligomers and solvents in the polymer network, upon heating. In addition, carbonization is an elimination process that removes H, O, and some C atoms from the polymer chains, which also leads to formation of micropores in carbon [39]. The microporosity of the hollow carbon nanospheres ensures convenient accessibility of the material by the electrolyte ions in a battery application.

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Figure 2 Characterizations of the hollow carbon nanospheres (carbonized at 1000 °C). (a) Raman spectra of the hollow carbon nanospheres, showing the G and D bands. (b) XRD patterns of the SiO2@C@SiO2 core/shell nanospheres and the hollow carbon nanospheres. The arrow indicates the position of the X-ray reflection from graphitic carbon. (c, d) The nitrogen physisorption isotherm and the t-plot of the hollow carbon nanospheres.

The size of the hollow carbon nanoshells can be readily controlled by tuning the size of the templates, i.e., the Stöber silica nanospheres (Fig. 3). In a conventional Stöber process conducted in bulk water/alcohol solution, the size of the silica nanospheres can be broadly tuned from ~ 100 nm to a few micrometers by adjusting the reaction conditions [41]. Similarly in our reverse micelle system, it is found that the size of the silica nanospheres can vary in a range of ~ 20–50 nm by tuning the concentration of TEOS (Fig. 3(a–c)), which well compensates the size window of the silica nanospheres from a conventional Stöber process. It is reasonable that with increasing concentration of TEOS, silicate clusters of high concentrations are formed in the individual reverse micelles, which leads to silica nanospheres of a

large size after agglomeration of these clusters. It is worth noting that when the concentration of TEOS becomes too high, aggregations of the silica nanospheres emerge, albeit an increased size of the nanospheres (Fig. S7 in the ESM). After the templating synthesis against these silica nanospheres, the resulting hollow carbon nanospheres show well-tunable sizes in a range of ~ 30–70 nm, respectively (Fig. 3(d–f)), which follows the same trend of the size of the silica nanospheres as a function of the amount of the TEOS in a typical synthesis (Fig. 3g). The high tunability of the unprecedentedly small size of the hollow carbon nanospheres paves a way to great applicability of these materials in different scenarios of the applications.

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Figure 3. Size tuning of the sub-100 nm hollow carbon nanospheres. (a–c) TEM images of the Stöber SiO2 nanospheres with average sizes of 21.8 ± 0.6 (mean ± standard deviation), 32.2 ± 2.0, and 49.8 ± 2.0 nm, respectively, obtained from reverse micelles. (d–f) TEM images of the hollow carbon nanospheres (carbonized at 600 °C) with sizes of 32.5 ± 1.9, 43.0 ± 1.5, and 63.5 ± 2.9 nm, respectively, obtained by templating against the Stöber SiO2 nanospheres. Inset: Size distributions, unit: nm. (g) Plot of the size of the Stöber SiO2 spheres and the hollow carbon nanospheres against the amount of TEOS in a typical synthesis. The error bars represent the standard deviation for each point.

As an additional merit of the reverse micelle synthesis system, large-scale production of the hollow carbon nanospheres can be conveniently achieved in the lab (Fig. 4). A large population of the reverse micelles are present in the synthesis system, which represent discrete nanoreactors for the sol-gel production of silica nanospheres and the subsequent coating processes, making it possible for large-scale production of the product in a limited volume of the reaction system. In our demonstration, ~ 5 g of the hollow carbon nanospheres have been successfully obtained from one pot of the reaction with a volume of ~ 1.3 L, which show a similar size and structure as those obtained from the typical synthesis, as confirmed

by the TEM imaging (Fig. 4(a)). We believe the scale of the production could be even higher from a much larger volume of the vessel with excellent reproducibility. The scalable synthesis of the hollow carbon nanospheres are highly desirable for many practical applications.

In addition, the sub-100 nm hollow carbon nanospheres show remarkable chemical stability, making them particularly suitable for fabricating an electrode which is in intimate contact with an electrolyte. The high chemical stability can be demonstrated by the morphology and structural integrity of the hollow carbon nanospheres that remain intact after the treatment in NaOH (2 M) or

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HCl (1 M) for 48 h at 50 °C (Fig. S8 in the ESM). In addition, the hollow carbon nanospheres show excellent thermal and hydrothermal stability. As a demonstration, the hollow carbon nanospheres remained intact after they were calcined in air at

300 °C for 12 h (Fig. S9 in the ESM) or refluxed in boiling water (100 °C) for 12 h (Fig. S10 in the ESM), which promises high applicability of this material in different aerobic applications.

Figure 4. A large-scale synthesis of the hollow carbon nanospheres (carbonized at 600 °C), which yields ~ 5 g of the final product from one pot. (a) Typical TEM image of the hollow carbon nanospheres obtained from the large-scale synthesis. (b) Digital photograph of the product in a form of powder.

Thanks to the small size, thin shell thickness, high microporosity, conductivity and chemical stability, these hollow carbon nanospheres may become excellent candidates as an anode material for LIBs or SIBs. For demonstration, a working electrode was fabricated with the hollow carbon nanospheres (~ 30 nm, carbonized at 1000 °C) for electrochemical study (Fig. 5). Figure 5a shows the charge-discharge curves for the 10th, 100th, 500th, and 1000th cycle of the electrode cycled between 0.01 and 2.0 V (versus Li/Li+) at a current density of 500 mA·g–1. It is clear that the voltage profiles overlap very well from the 100th cycle onwards, indicating excellent reversibility of the electrochemical reactions. An evident voltage plateau can be noticed in the initial discharge process which finally delivers a high capacity (687 mA·h·g–1) with a low coulombic efficiency (Fig. S11 in the ESM). The irreversible capacity loss of the hollow carbon nanospheres is mainly due to the irreversible formation of the solid electrolyte interface (SEI) layer, which is inevitable for most anode materials [47-49]. The first irreversible capacity loss is related to the large surface area of the material that can increase the

electrolyte-electrode contact. On the other hand, stable cycling performance is also highly desirable for practical applications. Figure 5b shows the cycling behavior of the hollow carbon nanospheres in LIBs at a current density of 500 mA·g–1. A capacity of 440 mA·h·g–1 is achieved in the 10th cycle. With increasing cycle numbers, the capacity fades very slowly, which shows a reversible and stable capacity of 360 mA·h·g–1 with a coulombic efficiency of 99.66% even in the 1000th cycle, confirming the excellent cycling stability of the hollow carbon nanospheres. The high stability can be attributed to hollow structure of the carbon spheres, which readily accommodates the strain caused by the volume change during the lithium ion insertion/extraction, and thus retains the excellent structural integrity during the cycling process, as evidenced by TEM imaging (Fig. S12 in the ESM). For comparison, at a current density of 500 mA·g–1, commercial graphite delivers a low capacity of 137.9 mAh·g–1 for LIB at the first cycle (Fig. S13 in the ESM). Although the capacity gradually increases upon cycling, the maximal capacity of the commercial graphite reaches 300 mAh·g–1 after 100 cycles, which is still much lower

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than that of the hollow carbon nanospheres (375 mAh·g–1 after 100 cycles). The sub-100 nm hollow carbon nanospheres show excellent rate performance, which confirms high electrochemical

kinetics of the carbon electrode as a result of the short diffusion distance and excellent electronic conductivity of the hollow carbon nanospheres (Fig. S14 in the ESM).

Figure 5 Electrochemical performance of the hollow carbon nanospheres (carbonized at 1000 °C). (a, b) Performance of the hollow carbon nanospheres in LIBs. (a) Voltage profiles at the current density of 500 mA·g–1. (b) Long-term cycling performance at the current density of 500 mA·g–1. (c, d) Performance of the hollow carbon nanospheres in SIBs. (c) Voltage profiles at the current density of 100 mA·g–1. (d) Long-term cycling performance at the current density of 100 mA·g–1.

The sub-100 nm hollow carbon nanospheres show stable cycling performance not only in LIBs, but also in SIBs. Figure 5c presents the charge-discharge curves of the hollow carbon nanospheres for the 10th, 100th, 500th, and 1000th cycle between 0.01 and 2.0 V (vs Na/Na+) at a current density of 100 mA·g–1. The hollow carbon nanospheres deliver sodiation and desodiation capacities of 208 and 197 mA·h·g–1 in the 10th cycle. Obviously, they exhibit a much lower initial coulombic efficiency (Fig. S15 in the ESM), which can be ascribed to its high surface area. The electrode was further performed at 100 mA·g–1 for 200 cycles after cycling at 20 mA·g–1 for 5 cycles (Fig. 5d), which exhibits a stable capacity around 161 mA·h·g–1 with 99.53% of the coulombic efficiency over 200 cycles, much superior to commercial graphite (Fig. S13 in the ESM). The hollow structure of the carbon nanospheres was also retained during the cycling process (Fig. S12 in

the ESM).

This excellent performance can be attributed to the unique physicochemical properties and structures of the hollow carbon nanospheres. First, the small size of the hollow carbon nanospheres improve the volumetric energy/power density of the electrode, which partially overcomes the drawback of low volumetric energy density with conventional large hollow particles [50]. In addition, the hollow structure shortens the diffusion and migration length of the electrolyte ions, ensuring fast charge and discharge processes. Second, the high specific surface area and the porous structure allow for convenient accessibility of the electrode/electrolyte interface by the electrolyte ions. Third, the hollow carbon nanospheres possess remarkable electrical conductivity due to the high degree of the graphitization, which further facilitates the charge transfer. Finally, the high tendency of the carbon

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nanospheres to retain their hollow nanostructure facilitates their excellent cycling stability when used as an anode material.

4. Conclusions

In summary, we have demonstrated a large-scale templating synthesis of hollow carbon nanospheres with unprecedentedly small size (~ 32.5–62.5 nm) for enhanced cycling performance in LIBs or SIBs. The synthesis relies on the reverse micelles as the nanoreactors, which enables scalable production of sub-100 nm silica nanospheres as the templates by a modified Stöber process, and controlled sol-gel chemistry for sequential coating processes by silica and a RF resin (carbon precursor). The resulting hollow carbon nanospheres possess a tunable size (as small as ~ 32.5 nm) and shell thickness (~ 3.9 nm), high microporosity, electric conductivity, and chemical stability, which makes this material an excellent candidate for fabricating the anodes for the lithium/sodium-ion batteries. When used as the anode material of the LIBs, the hollow carbon nanospheres can deliver a capacity of 360 mA·h·g–1 at the current density of 500 mA·g–1 even after 1000 cycles, which is very close to the calculated specific capacitance and is much higher than that of commercial graphite. In addition, the hollow carbon nanospheres can be also used as the potential anode materials for SIBs with excellent cycling stability. We believe these sub-100 nm hollow carbon nanospheres may also find broad use in many other energy storage applications.

Acknowledgements C.G. acknowledges the support from the National Natural Science Foundation of China (21671156, 21301138), the Tang Scholar Program from the Cyrus Tang Foundation, and the start-up fund from Xi’an Jiaotong University. X.H acknowledges the programs supported by State Key Laboratory of Electrical Insulation and Power Equipment (EIPE17306) and Young Talent Support Plan of Xi’an Jiaotong University. Y.Y. acknowledges the support from U.S. Department of Energy (DE-SC0002247).

Electronic Supplementary Material: Supplementary material (Additional TEM images, FTIR spectra, Raman spectra, TGA, and electrochemical results) is available in the online version of this article at http://dx.doi.org/10.1007/s12274-***-****-* (automatically inserted by the publisher).

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Electronic Supplementary Material

Scalable Synthesis of Sub-100 nm Hollow Carbon Nanospheres for Energy Storage Applications

Hongyu Zhao1,2,†, Fan Zhang2,†, Shumeng Zhang1, Shengnan He1, Fei Shen2, Xiaogang Han2 (), Yadong Yin3, and Chuanbo Gao1 ()

1 Center for Materials Chemistry, Frontier Institute of Science and Technology, Xi’an Jiaotong University, Xi’an, Shaanxi 710054, China 2 Center of Nanomaterials for Renewable Energy, Key Lab of Smart Grid of Shaanxi Province, State Key Lab of Electrical Insulation and Power Equipment, Xi’an Jiaotong University, Xi’an, Shaanxi 710054, China 3 Department of Chemistry, University of California, Riverside, CA92521, United States † These authors contributed equally to this work

Supporting information to DOI 10.1007/s12274-****-****-* (automatically inserted by the publisher)

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Figure S1 Effect of polymerization time of RF on the morphology of the hollow carbon nanospheres. (a–c) TEM images of the hollow carbon nanospheres with RF polymerized after different lengths of time: (a) 2 h, (b) 12 h, and (c) 24 h. It is inferred that 24 h of polymerization is required for a uniform coating of RF on the silica nanospheres.

Figure S2 Effect of amino group functionalization on the coating of RF on the silica nanospheres. (a) TEM image of the product after coating of RF on the silica nanospheres without amino group functionalization. The coating of RF failed under this condition. (b–d) TEM images of the hollow carbon nanospheres by templating of amino group functionalized silica nanospheres. The volumes of APS were 20 (b), 50 (c) and 150 µL (d) for the synthesis of the respective silica nanospheres.

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Figure S3 FTIR of the hollow carbon nanospheres obtained by carbonization at 600 °C, showing the retention of phenolic groups.

Figure S4 TEM image of the hollow carbon nanospheres synthesized without silica protection before the thermal carbonization process at 600 °C. Aggregates of the hollow carbon nanospheres can be observed, instead of well dispersed ones, which can be attributed to the sintering of RF/carbon during the thermal treatment.

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Figure S5 Control of the thickness of the hollow carbon nanospheres by tuning the amount of resorcinol in a typical synthesis. The amounts of the resorcinol were 2.27 (a), 4.5 (b) and 6.8 mmol (c), respectively.

Figure S6 Raman spectra of the hollow carbon nanospheres obtained under different carbonization conditions. After carbonization at 1000 °C for 3 h, the hollow carbon nanospheres showed increased intensity ratio of the G/D bands, relative to that of the nanospheres carbonized at 600 °C, confirming an increased graphitization degree of the carbon at an elevated temperature. When the carbonization process (1000 °C) was prolonged from 3 h to 12 h, no significant change can be observed in the G/D ratio, indicating that the graphitization of carbon cannot be effectively improved by solely relying on the prolonged carbonization time.

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Figure S7 TEM images of the sub-100 nm silica nanospheres synthesized in reverse micelles, when the volume of the TEOS was increased to 5, 6 and 10 mL in a typical experiment (see Experimental Section). Inset: Size distributions of the silica nanospheres; unit: nm. The average sizes were denoted as “mean ± standard deviation”. With increasing volume of TEOS, the size of the silica nanospheres becomes larger. However, significant aggregation of the silica nanospheres was observed, which can be attributed to incomplete hydrolysis of silicate in the presence of insufficient amount of water (628 μL in a typical synthesis), and thus increased hydrophobicity of the silica nanospheres in a polar aqueous environment. It is worth noting that the volume of water cannot be significantly increased to maintain the spherical micelles in the reverse micelle system. Therefore, it is difficult to synthesize uniform silica nanospheres of large sizes without sacrificing their dispersity by the reported method.

Figure S8 Chemical stability of the sub-100 nm hollow carbon nanospheres (carbonized at 600 °C) in acid and base. (a) TEM image of the hollow carbon nanospheres after treatment in NaOH (2 M) for 48 h at 50 °C. (b) TEM image of the hollow carbon nanospheres after treatment in HCl (1 M) for 48 h at 50 °C.

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Figure S9 Thermal stability of the sub-100 nm hollow carbon nanospheres (carbonized at 600 °C). (a) Thermal gravimetric analysis (TGA) of the hollow carbon nanospheres in air. (b, c) TEM images of the hollow carbon nanospheres after calcination in air at 300 and 400 °C, respectively, for 12 h. These results indicate that the hollow carbon nanospheres stayed stable at temperatures below ~ 400 °C in ambient air, thus are applicable in many aerobic applications at relatively high temperatures.

Figure S10 TEM image of the hollow carbon nanospheres (carbonized at 600 °C) after boiling in water (100 °C) for 12 h. The hollow nanostructure has been well retained, confirming their high hydrothermal stability.

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Figure S11 Voltage profiles for the 1st cycle of the electrode fabricated with hollow carbon nanospheres (carbonized at 1000 °C) cycled between 0.01 and 2.0 V vs Li/Li+ at a current density of 500 mA·g–1.

Figure S12 TEM images of the hollow carbon nanospheres in LIB after 1000 cycles (a) and SIB after 200 cycles (b), respectively. The hollow structure of the carbon nanospheres was successfully retained.

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Figure S13 Cycle performance of the commercial graphite for the LIB (a) and SIB (b). The capacities of commercial graphite were tested under the same conditions as Fig. 5. At a current density of 500 mA·g–1, commercial graphite delivers a low capacity of 137.9 mAh·g–1 for LIB at the first cycle (a). The capacity gradually increases upon cycling, which suggests an obvious delay of electrochemical reaction due to slow kinetics. The maximal capacity of commercial graphite reaches 300 mAh·g–1 after 100 cycles, which is still much lower than that of the hollow carbon nanospheres (375 mAh·g–1 after 100·cycles). In addition, the commercial graphite exhibits a very limited capacity for SIB, less than 15 mAh·g–1 (b), which is significantly lower than that of the hollow carbon nanospheres (166 mAh·g–1 after 100 cycles).

Figure S14 Rate performance of the hollow carbon nanospheres (carbonized at 1000 °C) in LIB. The hollow carbon nanospheres show excellent kinetics and rate performance, which deliver 470 mAh·g–1, 375 mAh·g–1, 315 mAh·g–1 and 255 mAh·g–1 at 200 mA·g–1, 500 mA·g–1, 1000 mA·g–1 and 2000 mA·g–1, respectively. It indicates great electrochemical kinetics of the carbon electrode arising from the short diffusion distance and excellent electronic conductivity of the sub-100 nm hollow carbon nanospheres.

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Figure S15 Voltage profiles for the 1st cycle of the electrode fabricated with hollow carbon nanospheres (carbonized at 1000 °C) cycled between 0.01 and 2.0 V vs Na/Na+ at a current density of 100 mA·g–1.

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