Nitrogen-Doped Hollow Carbon Nanospheres for High-Performance

Apr 7, 2017 - The improved electrochemical performance can be ascribed to (1) the Li+ adsorption dominated energy storage mechanism prevents the volum...
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Nitrogen-Doped Hollow Carbon Nanospheres for High-Performance Li-Ion Batteries Yufen Yang,† Song Jin,‡ Zhen Zhang,† Zhenzhen Du,‡ Huarong Liu,*,† Jia Yang,† Hangxun Xu,*,† and Hengxing Ji*,‡ †

CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, and ‡CAS Key Laboratory of Materials for Energy Conversion, iChEM, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China S Supporting Information *

ABSTRACT: N-doped carbon materials is of particular attraction for anodes of lithium-ion batteries (LIBs) because of their high surface areas, superior electrical conductivity, and excellent mechanical strength, which can store energy by adsorption/desorption of Li+ at the interfaces between the electrolyte and electrode. By directly carbonization of zeolitic imidazolate framework-8 nanospheres synthesized by an emulsion-based interfacial reaction, we obtained N-doped hollow carbon nanospheres with tunable shell thickness (20 nm to solid sphere) and different N dopant concentrations (3.9 to 21.7 at %). The optimized anode material possessed a shell thickness of 20 nm and contained 16.6 at % N dopants that were predominately pyridinic and pyrrolic. The anode delivered a specific capacity of 2053 mA h g−1 at 100 mA g−1 and 879 mA h g−1 at 5 A g−1 for 1000 cycles, implying a superior cycling stability. The improved electrochemical performance can be ascribed to (1) the Li+ adsorption dominated energy storage mechanism prevents the volume change of the electrode materials, (2) the hollow nanostructure assembled by the nanometersized primary particles prevents the agglomeration of the nanoparticles and favors for Li+ diffusion, (3) the optimized N dopant concentration and configuration facilitate the adsorption of Li+; and (4) the graphitic carbon nanostructure ensures a good electrical conductivity. KEYWORDS: carbon nanospheres, hollow structures, nitrogen-doping, porous materials, lithium-ion batteries

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INTRODUCTION Lithium-ion batteries (LIBs) with high energy and power density have become increasingly important for various applications such as smart electrical grid, electrical vehicles, and portable electronics.1−3 However, the low energy density of graphite-anode (specific capacity of 372 mA h g−1) impedes the development of the Li-technology. Recently, substantial efforts have been devoted to developing silicon or graphite-silicon composite anodes owing to the high theoretical capacity of silicon (4200 mA h g−1).4 But the large volume change of silicon during Li+ insertion/extraction causes rapid capacity decay upon cycling, which severely limits their applications in Li-ion batteries. Therefore, there has been growing interest in seeking alternative anode materials in LIBs. Emerging carbon materials such as carbon nanotubes,5 graphene,6 and porous carbons7 are very promising materials for LIBs due to their high surface areas, superior electrical conductivity, and excellent structure stability. More importantly, unlike the graphite or silicon, in which the Li+ ions are only reversibly stored by Faradaic reaction, these carbon-based electrodes with high surface areas can store additional electric energy via the adsorption/desorption of Li+ onto the surface of electrode materials. This additional Li storage process is similar to the © XXXX American Chemical Society

electrochemical double-layer capacitors (EDLCs). Therefore, this unique feature enables carbon-based materials to possess high specific capacities of >1500 mA h g−1 with long cycling life.6,8,9 There are many important factors that can influence the electrochemical properties of carbon-based anode materials such as the surface area, porous structure, and heteroatomdoping level.10 Particularly, the N-doped carbons are very attractive anode materials for LIBs as the N dopant with proper configurations can provide a large number of anchoring sites for Li+ adsorption due to the stronger electronegativity of N than C atoms.11,12 Doping of nitrogen atoms incorporated into graphitic lattice can strengthen the interaction between the Ndoped anchoring sites and the Li+ because of the improved local density of states which are favored for Li+ adsorption. Therefore, carbon materials with high N doping and optimized configurations are highly desirable for fabricating LIBs with high energy and power density. As a new class of crystalline materials, metal−organic frameworks (MOFs) with variable chemical compositions Received: November 18, 2016 Accepted: April 7, 2017 Published: April 7, 2017 A

DOI: 10.1021/acsami.6b14840 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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before drying at 80 °C in a vacuum oven. By carbonization of the ZIF8 nanospheres with different shell thicknesses, we are able to obtain nitrogen-doped carbon nanospheres with shell thicknesses of ∼20 and ∼50 nm, and solid spheres, respectively. They are named as the thinshell NHCNSs (T-NHCNSs), medium-shell NHCNSs (M-NHCNSs) and solid NCNSs (S-NCNSs).The hollow thin-shell ZIF-8 nanospheres directly pyrolysis at 700, 800, and 900 °C, which are named as the NHCNSs-700, NHCNSs-800 and NHCNSs-900, respectively. General Characterizations. SEM images were obtained using a JSM-2100F (JEOL Ltd.) at 5.0 kV accelerating voltage. TEM images were conducted at 200 kV accelerating voltage (Hitachi Model H7560). BET was performed on a Micromeritics Tristar3020. Raman spectra were obtained with a 532 nm laser using an inVia Raman Microscope (Renishaw). XRD was conducted with a D/max-TTR III diffractometer (Cu Kα, λ = 1.54178 Å). XPS analysis was collected on a Thermo ESCALab MKII (Mg Kα radiation 1253.6 eV). FT-IR specta were collected on a Bruker Vector 22 Fourier transform infrared spectrometer. Electrochemical Tests. The electrochemical performance of the N-doped carbon nanospheres anodes were measured with 2032-type coin cells assembled in an Ar-filled glovebox. The electrolyte was composed of 1 M LiPF6 in ethylene carbonate (EC) and diethyl carbonate (DEC) (v:v = 1:1). A polypropylene membrane from Celgard Inc. was used as the separator. The working electrode was prepared by bush coating a slurry, which contains NHCNSs, acetylene black, and polyvinyl difluoride (PVDF) in a weight ratio of 50:15:35 onto a copper foil with a spreader followed by drying in vacuum oven overnight. The “bush coating” is an electrode fabrication process which is widely used both in industry and academic research. The diameter of anodes were 12 mm. The areal density of the NHCNSs in the electrode was 0.3 mg cm−2. Battery tests of the as-assembled cells were performed using an LAND-CT2001A electrochemical workstation at different rates in the voltage range 0.01−3 V. The capacities and rates were calculated on the basis of the mass of active materials. The CV was performed on a PARSTAT4000 electrochemical workstation. The scan rate was 0.1 mV s−1 with the scanned potential window between 0.01 and 3 V. The specific energy values E (W h kg−1) of the NHCNSs were obtained by numerically integrating the t−V graph area. According to the equation

hold great potential for use as templates and/or precursors to synthesize heteroatom-doped carbon materials for LIBs.13,14 The pioneering work by Chen’s group8 has shown that direct pyrolysis of zeolitic imidazolate framework-8 (ZIF-8) at 800 °C yielded graphene nanoparticles with a N content of 17.72 wt %. As-prepared carbon materials could deliver a reversible capacity of 785 mA h g−1 after 1000 cycles at 5 A g−1 owing to the high level of N-doping. In addition, the nanostructured graphene analogues also play a vital role in improving the electrochemistry performance because of the nanometer size effects. However, the nanosized particles with a high surface energy tend to aggregate and also show a pronounced electrochemical agglomeration during charge/discharge cycling,15 which reduces the effective surface area for Li+ adsorption and increases diffusion length of the Li+, leading to a reduced gravimetric capacity and deteriorated rate capability. The formation of hollow nanostructure has been proved to be a very effective strategy to avoid the electrochemical agglomeration of the primary particles and maintain the porous structure.16−19 Here, we developed a facile template-free approach to produce N-doped hollow carbon nanospheres (NHCNSs) by directly carbonization of hollow ZIF-8 nanospheres synthesized by an emulsion-based interfacial reaction. The emulsion-based method is simple and scalable for the fabrication of hollow ZIF8 nanospheres.20 Thus, we can conveniently obtain different NHCNSs with different shell thicknesses by varying the synthesis conditions used in the preparation of hollow ZIF-8 nanospheres. In this way, we can investigate the effect of hollow nanostructure on the electrochemical performance of NHCNSs for LIBs. As a result, the as-prepared NHCNSs with a high surface area of 1083 cm2 g−1 and high N content of 16.6 at % can deliver a revisible specific capacity of 2053 mA h g−1 at a current density of 100 mA g−1, retaining about 97% of the second discharge capacity when tested as the anode for LIBs. Notably, even at a high current density of 5 A g−1, a reversible capacity as high as 879 mA h g−1 could be maintained after 1000 cycles, implying a superior cycling stability. Moreover, the specific energy of NHCNSs-based anode reaches ∼500 Wh kg−1 with a specific power of ∼50 W kg−1. Remarkably, the electrode can also deliver a specific energy of ∼290 Wh kg−1 with a high specific power of ∼1650 W kg−1. An in-depth study indicates that the thin outer porous shell in NHCNSs provides a shorter diffusion length than the solid spheres for Li+, which is favorable for the charge/discharge process. The improved specific capacity can be ascribed to the high content of pyridinic and pyrrolic N doping along with a high degree of graphitization.

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t2

E=

∫t1

IV dt

where I is the current density, V (V) is the scanned potential window, t1 and t2 represent the start time and end time in the discharge process, respectively. The specific power values P (W kg−1) were calculated according to

P = E /Δt where Δt represents the discharge time.

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RESULTS AND DISCUSSION The NHCNSs with the optimized structure for LIBs can be obtained by direct pyrolysis of ZIF-8 hollow nanospheres with a thin outer shell at 800 °C (Figures S1 and S2). The average diameter of the NHCNSs is ∼150 nm and the shell thickness is ∼20 nm (Figure 1a−f). Meanwhile, the outer shell is consisted of the primary nanoparticles with a diameter of ∼10 nm. This structure is the same as the ZIF-8 hollow nanospheres (Figure S2), indicating a good thermal stability of the hollow structure. Furthermore, the nanoscale cavities and nanopores of ZIF-8 are also preserved after carbonization as confirmed by the high surface area of 1038 m2 g−1 and the average pore size of 1.1 nm (Figure S3a, b). The high surface area ensures a much larger electroactive surface for Li+ absorption to form the EDL, and the nanometer-sized pores are favored for Li+ diffusion. The Raman spectrum (Figure 1g) shows dominant D, G, and 2D

EXPERIMENTAL SECTION

Preparation of the ZIF-8 Precursors. The preparation of ZIF-8 nanospheres were synthesized by an emulsion-based interfacial reaction. Details of the synthesis procedure can be found in our previous work.20 By finely tuning the reaction times, we obtained three ZIF-8 nanospheres with shell thicknesses of ∼20 nm (4 h) and ∼50 nm (24 h), and solid ZIF-8 spheres (48 h). Preparation of the Nitrogen-Doped Carbon Nanospheres. The as-prepared ZIF-8 precursors was positioned at the center of a tube furnace, and the temperature increased to 800 °C at atmospheric pressure under a mixed flow of Ar (300 sccm). The ZIF-8 was annealed for 6 h at 800 °C followed by cooling to room temperature at a rate of 10 °C min−1 under a flow of pure Ar (300 sccm). The sample was subsequently placed in 35% HCl aqueous solution to remove the Zn component. At last, the sample was rinsed in DI water and ethanol B

DOI: 10.1021/acsami.6b14840 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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photoelectron spectrum (XPS, Figure 1h) shows intensive peaks of C, N, and O, and the calculated C/O atomic ratio is 12.9, further indicating a deep carbonization level. The N content of the NHCNSs can be calculated from the XPS results and measured by the elemental analysis, which show a high content of 16.6 at % and 16.15 wt %, respectively (Table S2). This value is much larger than other commonly used N-doping methods such as the NH3 annealing of porous carbons at elevated temperatures.23−25 A high content of N-doping in porous carbons is crucial for applications in energy storage and electrocatalysis.26,27 The electrochemical performance of the NHCNSs was evaluated in a half-cell with a 1 M LiPF6 in EC/DMC electrolyte and a lithium foil as the counter/reference electrode. The potential of the NHCNSs electrode increases/decreases monotonically without obvious potential plateau when charge/ discharge at various current densities, which is a typical behavior of ion adsorption (forming electrical double-layer) dominated energy storage mechanism.28 This observed behavior agrees with the cyclic voltammetry (CV) profiles (Figure S4a) which show a large frame, except for one broad peak at around 0.55 V in the first cycle due to the formation of the solid electrolyte interface (SEI) film.12 In addition, the CV curves almost overlap after the second cycle, indicating an excellent reversibility of Li+ storage for the NHCNSs. As a result, the NHCNSs could deliver a reversible specific capacity of 1945 mA h g−1 when charged/discharged at a current density of 0.1 A g−1 (Figure 2a, b), which is 5 times of the theoretical value of graphite. This value is also close to the state-of-the-art Si-based anodes (∼1200 mA h g−1 to ∼2800 mA h g−1).4 The specific capacity drops to 60% (1167 mA h g−1) when the current density is increased 32 times (3.2 A h g−1), and is maintained at 879 mA h g−1 when charge/discharge at 5 A g−1 for 1000 cycles with an average Coulombic efficiency of 99.4% (Figure 2d), which is considerably higher than most of the N-

Figure 1. (a) SEM and (b, c) TEM images of the NHCNSs. (d−f) TEM image and corresponding EDX mapping of C and N elements acquired at the same position, showing a uniform distribution of the N-dopants. (g) Raman spectrum of the NHCNSs. (h) XPS survey spectrum of the NHCNSs, showing a high N content of 16.6 at % and C/O ratio of 12.9. NHCNSs were obtained by direct pyrolysis of ZIF8 hollow nanospheres with a thin outer shell at 800 °C.

bands that locate at 1348, 1585, and 2700 cm−1, respectively.21 The X-ray diffraction (XRD, Figure S3c) pattern shows a broad peak at approximately 25.5°, suggesting a good graphitic level of the NHCNSs as shown in Figure 1c.22 The X-ray

Figure 2. (a) Galvanostatic charge/discharge profiles of the NHCNSs under different current densities. (b) Rate capability of NHCNSs. (c) Comparison of the specific capacity using NHCNSs with other high-performance LIBs. Detailed information can be found in Table S3. (d) Longterm cycling performance of the NHCNSs anode measured at 5 A g−1 for 1000 cycles. Herein, the NHCNSs were obtained by direct carbonization of ZIF-8 hollow nanospheres with a thin outer shell at 800 °C. C

DOI: 10.1021/acsami.6b14840 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 3. (a−c) TEM images of NHCNSs with different shell thicknesses. (d−f) Corresponding galvanostatic charge/discharge profiles of NHCNSs measured at 100 mA g−1. (g) Cycling performance of the NHCNSs with different shell thicknesses at 100 mA g−1 for 100 cycles. T-NHCNSs, MNHCNSs, and S-NCNSs were fabricated by carbonization of ZIF-8 hollow nanospheres with different shell thicknesses at 800 °C.

polyvinyl difluoride binders after multiple cycling process as shown in the SEM and TEM images (Figure S4c−e). In addition to the above merits of the NHCNSs, the hollow nanostructure plays a key role in the enhanced electrochemical performance. By carbonization of the ZIF-8 nanospheres with different shell thicknesses, we are able to obtain NHCNSs with shell thicknesses of 20 and 50 nm, and solid spheres, respectively. They are named as the thin-shell NHCNSs (TNHCNSs, Figure 3a), medium-shell NHCNSs (M-NHCNSs, Figure 3b) and solid NCNSs (S-NCNSs, Figure 3c). The charge/discharge profiles of these three different carbon nanospheres are similar and show a Li+ adsorption dominated storage mechanism (Figure 3d−f, Figure S5).The reversible specific capacities of the second cycle are 2117, 1675, and 1482 mA h g−1 for the T-NHCNSs, M-NHCNSs, and S-NHCNSs (at a current density of 0.1 A g−1), respectively. The TNHCNSs show a very stable cycling performance that outputs a specific capacity of 2053 mA h g−1 at the 80th cycle with a charge/discharge current density of 0.1 A g−1. Meanwhile, the specific capacities of the M-NHCNSs and S-NCNSs increase to 1856 and 1843 mA h g−1, respectively. The low initial values and large increment of the specific capacities of M-NHCNSs and S-NHCNSs have been typically observed in anodes consisted of nonconductive metal oxides.38−40 This phenomenon was attributed to the activation process during which the nonconductive or solid electrochemical active material hampers the migration of electrons (or the diffusion of Li+). Characterization results show that the T-NHCNSs, M-NHCNSs and SNCNSs have similar structures regarding their size (Figure 3a− c), carbonization level, surface area, pore size, and N-dopant content (Figure S7 and Table S2). Therefore, the kinetic limit observed in M-NHCNSs and S-NCNSs with thicker shells should account for the observed phenomenon. The shell

doped carbon-based anode materials (Figure 2c and Table S3).8,9,29−37 The increase of the specific capacities of NHCNSs during cycling can be attributed to the activation process of the electrochemical active material. The high surface area and abundant nanopores in NHCNSs enable sufficient active sites for Li+ adsorption, yet the electrode/electrolyte interface area is dependent on the wettability of electrolyte. Charge/discharge cycling improves the wettability of electrolyte thus increases the electrode/electrolyte interface, leading to the increase of the specific capacity. The charge/discharge profiles of the NHCNSs of the 1000th cycle (Figure S4b) exhibited the same characteristics compared to the first few cycles. The linear time-dependent change in potential without obvious potential plateau is a typical behavior of ion adsorption dominated energy storage mechanism (formation of electrical double layer), indicating an excellent reversibility of Li+ storage for the NHCNSs. For commercial LIBs, the high specific power is typically achieved at the expense of reduced specific energy, and vice versa. For example, a commercial Li-ion cell for electric vehicles can deliver a specific energy of 220 W h kg−1 with a specific power of 50 W kg−1. However, the specific energy drops to less than 10 W h kg−1 when the cell works at a specific power of 1000 W kg−1. The NHCNSs anode can deliver a remarkable high specific energy of ∼290 W h kg−1 at a specific power of ∼1650 W kg−1 with respect to the total mass of the electrode (including the active materials, acetylene black and polyvinyl difluoride). Alternatively, the cell could work in a high energy mode at a specific energy of ∼500 W h kg−1 with a specific power of ∼50 W kg−1. Therefore, NHCNSs stand out as a novel anode material for LIBs with high power and energy density. The hollow spherical structure of the NHCNSs can be well preserved and connected together by the surrounding D

DOI: 10.1021/acsami.6b14840 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 4. (a) FTIR spectra and (b) XPS N 1s spectra of NHCNSs carbonized at different temperatures. (c) Correlation between the contents of N doping and different N configurations with pyrolsysis temperatures. (d) Cycling performance of the NHCNSs prepared at different temperatures at a charge/discharge current density of 100 mA g−1.

thickness of hollow nanostructures could affect the mean Li+ diffusion time, τLi, which can be defined by the equation τed = L2 /2D

presence of nitrile N. The N contents are determined to be 16.6 and 3.9 at. % for NHCNSs-800 and NHCNSs-900, respectively. The N content and composition of the NHCNSs prepared at different temperatures are summarized in Figure 4c and Table S2. These results indicate that higher carbonization temperature significantly decreases the N content, which should be avoided for preparing N-doped carbon materials for LIBs. Although the NHCNSs-700 has the highest N content of 21.7 at %, the low carbonization level results a high content of electrochemically inactive nitrile N (Figure 4a, c).8 The sum of the electrochemically active pyridinic and pyrrolic N42,43 in the NHCNSs-700 (11.9 at %) is slightly lower than that in theNHCNSs-800 (12.2 at %). But the NHCNSs-800 could deliver a much higher specific capacity of 2080 mA h g−1 than the NHCNSs-700 (1633 mA h g−1) after 80 cycles at a charge/ discharge current density of 100 mA g−1. This is due to the fact that the electrochemically active pyridinic and pyrrolic N requires additional sp2-hybridized carbon to generate delocalized π electrons with high density of states (DOS) to effectively adsorb Li+. This effect is similar to the recent experimental finding that the DOS dominated quantum capacitance of the nanometer-sized carbon is a key factor to improve EDL capacitance.44,45 The NHCNSs carbonized at different temperatures store energy by the similar Li+ adsorption mechanism (Figure S9), thus an adequate carbonization level and high content of the pyridinic and pyrrolic N are crucial to the electrochemical performance. This experimental finding would benefit us to the selection of the optimal N-doping level and Ncontaining species for the N-doped carbon materials for applications in LIBs. The NHCNSs derived from hollow ZIF-8 nanospheres show fascinating electrochemical properties with high capacity and remarkable reversibility when being used as anode materials in LIBs. This improved electrochemical performance can be ascribed to (1) the Li+ adsorption dominated energy storage mechanism prevents the volume change of the electrode materials; (2) the hollow nanostructure assembled by the nanometer-sized primary particles prevents the agglomeration

(1)

where the D is the diffusion coefficient, and L is the diffusion length (shell thickness). The T-NHCNSs has a shell thickness of ∼20 nm which is 26% of the diameter of the S-NCNSs (∼75 nm). Considering the same D for both T-NHCNSs and SNCNSs, the diffusion time for Li+ in S-NCNSs is 3.5 times longer than that of the T-NHCNSs. The electrochemical impedance spectroscopy (EIS) measurements (Figure S6) presented a depressed semicircle and a smaller interfacial charge-transfer resistance of the T-NHCNSs electrode, indicating excellent electron conduction of the T-NHCNSs and high Li+ ion migration speed. Therefore, the outer carbon shell can control the diffusion of Li+ in LIBs. The balance between the carbonization level and the N content is also crucial to the electrochemical performance of the NHCNSs.8 We thus prepared NHCNSs by carbonization of hollow ZIF-8 nanospheres at the temperatures of 700, 800, and 900 °C, which are named as the NHCNSs-700, NHCNSs-800, and NHCNSs-900, respectively. The morphology and porous structure of NHCNSs prepared at different temperatures are almost identical (Figure S8). The NHCNSs-700 contains abundant oxygen-containing functional groups which are visible in the FTIR spectrum (Figure 4a). Particularly, there is an intensive band centered at 2195 cm−1, which can be associated with the nitrile groups (−CN).8 These bands are not visible in the FITR spectra of the NHCNSs-800 and NHCNSs-900, indicating the inadequate carbonization of the NHCNSs-700. The N 1s peak in the XPS spectrum of the NHCNSs-700 (Figure 4b) shows an intensive band in the binding energy range of 396−406 eV, which can be deconvoluted by peaks centered at ∼398.3 (pyridinic N), ∼399.5 (nitrile N), ∼400 (pyrrolic N), ∼400.8 (graphitic N), and ∼403.0 eV (Noxide).41 The N content of the NHCNSs-700 calculated from XPS is 21.7 at %. The XPS spectra of the NHCNSs-800 and NHCNSs-900 also display intensive N 1s peaks without the E

DOI: 10.1021/acsami.6b14840 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces of the nanoparticles and favors for Li+ diffusion; (3) the optimized N-dopant concentration and configuration facilitate the adsorption of Li+; (4) the graphitic carbon nanostructure ensures a good electrical conductivity.

(4) Lee, J. K.; Oh, C.; Kim, N.; Hwang, J. Y.; Sun, Y. K. Rational Design of Silicon-Based Composites for High-Energy Storage Devices. J. Mater. Chem. A 2016, 4, 5366−5384. (5) 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. (6) Zhang, F.; Zhang, T. F.; Yang, X.; Zhang, L.; Leng, K.; Huang, Y.; Chen, Y. S. A High-Performance Supercapacitor-Battery Hybrid Energy Storage Device Based on Graphene-Enhanced Electrode Materials with Ultrahigh Energy Density. Energy Environ. Sci. 2013, 6, 1623−1632. (7) Mao, Y.; Duan, H.; Xu, B.; Zhang, L.; Hu, Y. S.; Zhao, C. C.; Wang, Z. X.; Chen, L. Q.; Yang, Y. S. Lithium Storage in NitrogenRich Mesoporous Carbon Materials. Energy Environ. Sci. 2012, 5, 7950−7955. (8) Zheng, F. C.; Yang, Y.; Chen, Q. W. High Lithium Anodic Performance of Highly Nitrogen-Doped Porous Carbon Prepared from a Metal-Organic Framework. Nat. Commun. 2014, 5, 5261. (9) Qie, L.; Chen, W. M.; Wang, Z. H.; Shao, Q. G.; Li, X.; Yuan, L. X.; Hu, X. L.; Zhang, W. X.; Huang, Y. H. Nitrogen-Doped Porous Carbon Nanofiber Webs as Anodes for Lithium Ion Batteries with a Superhigh Capacity and Rate Capability. Adv. Mater. 2012, 24, 2047− 2050. (10) Paraknowitsch, J. P.; Thomas, A. Doping Carbons Beyond Nitrogen: An Overview of Advanced Heteroatom Doped Carbons with Boron, Sulphur and Phosphorus for Energy Applications. Energy Environ. Sci. 2013, 6, 2839−2855. (11) Wang, H.; Maiyalagan, T.; Wang, X. Review on Recent Progress in Nitrogen-Doped Graphene: Synthesis, Characterization, and Its Potential Applications. ACS Catal. 2012, 2, 781−794. (12) Reddy, A. L. M.; Srivastava, A.; Gowda, S. R.; Gullapalli, H.; Dubey, M.; Ajayan, P. M. Synthesis of Nitrogen-Doped Graphene Films for Lithium Battery Application. ACS Nano 2010, 4, 6337−6342. (13) Liu, B.; Shioyama, H.; Akita, T.; Xu, Q. Metal-Organic Framework as a Template for Porous Carbon Synthesis. J. Am. Chem. Soc. 2008, 130, 5390−5391. (14) Jiang, H. L.; Liu, B.; Lan, Y. Q.; Kuratani, K.; Akita, T.; Shioyama, H.; Zong, F.; Xu, Q. From Metal-Organic Framework to Nanoporous Carbon: Toward a Very High Surface Area and Hydrogen Uptake. J. Am. Chem. Soc. 2011, 133, 11854−11857. (15) Guo, Y. G.; Hu, J. S.; Wan, L. J. Nanostructured Materials for Electrochemical Energy Conversion and Storage Devices. Adv. Mater. 2008, 20, 2878−2887. (16) Xu, F.; Tang, Z. W.; Huang, S. Q.; Chen, L. Y.; Liang, Y. R.; Mai, W. C.; Zhong, H.; Fu, R. W.; Wu, D. C. Facile Synthesis of Ultrahigh-Surface-Area Hollow Carbon Nanospheres for Enhanced Adsorption and Energy Storage. Nat. Commun. 2015, 6, 7221. (17) Jayaprakash, N.; Shen, J.; Moganty, S. S.; Corona, A.; Archer, L. A. Porous Hollow Carbon@Sulfur Composites for High-Power Lithium-Sulfur Batteries. Angew. Chem., Int. Ed. 2011, 50, 5904−5908. (18) Liu, R.; Mahurin, S. M.; Li, C.; Unocic, R. R.; Idrobo, J. C.; Gao, H.; Pennycook, S. J.; Dai, S. Dopamine as a Carbon Source: The Controlled Synthesis of Hollow Carbon Spheres and Yolk-Structured Carbon Nanocomposites. Angew. Chem., Int. Ed. 2011, 50, 6799−6802. (19) Lai, X. Y.; Halpert, J. E.; Wang, D. Recent Advances in Micro-/ Nano-structured Hollow Spheres for Energy Applications: From Simple to Complex Systems. Energy Environ. Sci. 2012, 5, 5604−5618. (20) Yang, Y. F.; Wang, F. W.; Yang, Q. H.; Hu, Y. L.; Chen, Y. Z.; Liu, H. R.; Zhang, G. Q.; Lu, J. L.; Jiang, H. L.; Xu, H. X. Hollow Metal-Organic Framework Nanospheres via Emulsion-Based Interfacial Synthesis and Their Application in Size-Selective Catalysis. ACS Appl. Mater. Interfaces 2014, 6, 18163−18171. (21) Ferrari, A. C.; Basko, D. M. Raman Spectroscopy as a Versatile Tool for Studying the Properties of Graphene. Nat. Nanotechnol. 2013, 8, 235−246. (22) Li, Z. Q.; Lu, C. J.; Xia, Z. P.; Zhou, Y.; Luo, Z. X-ray Diffraction Patterns of Graphite and Turbostratic Carbon. Carbon 2007, 45, 1686−1695.

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CONCLUSIONS In summary, we demonstrate the facile fabrication of N-doped carbons from ZIF-8 hollow nanospheres for LIBs with high energy and power density via the Li+ adsorption mechanism. The carbon electrode prepared from N-doped hollow carbon nanospheres with a shell thickness 20 nm and N-doping content as high as 16.6 at % could deliver a reversible specific capacity of 2053 mA h g−1 at 100 mA g−1 with a long cycling life. The hollow structure with a properly tuned shell thickness effectively prevents the agglomeration of the nanoparticles and provides a short Li+ diffusion distance during cell cycling. The optimized processing temperature yields an adequate carbonization level and a high content of electrochemically active pyridinic and pyrrolic N. We anticipate that this work would benefit for the future development of the heteroatom-doped carbon materials with optimal doping levels and configurations for LIBs and other extended applications such as Li−O2 batteries and supercapacitors.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b14840. Specific surface area, pore size distribution, pore volume, XPS spectra, XRD patterns, Raman spectrum, TGA data, TEM images, and Nyquist plots of nitrogen-doped carbon nanospheres are included (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Hangxun Xu: 0000-0003-1645-9003 Hengxing Ji: 0000-0003-2851-9878 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Y. Y. and S. J. contributed equally to this work. This work is supported by the National Key Basic Research Program of China (2015CB351903), National Natural Science Foundation of China (51402282, 51373160, 21474095, 21476104, 21373197), and the Fundamental Research Funds for the Central Universities (WK3430000003). H. J. thank the 100 Talents Program of the Chinese Academy of Sciences.

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DOI: 10.1021/acsami.6b14840 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX