In-Situ High Level Nitrogen Doping into Carbon Nanospheres

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In-Situ High Level Nitrogen Doping into Carbon Nanospheres Boosting Capacitive Charge Storage as Both Anode and Cathode for A High-Energy 4.5 V Full Carbon Lithium Ion Capacitor Fei Sun, Xiaoyan Liu, Hao Bin Wu, Lijie Wang, Jihui Gao, Hexing Li, and Yunfeng Lu Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b00134 • Publication Date (Web): 30 Apr 2018 Downloaded from http://pubs.acs.org on April 30, 2018

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In-Situ High Level Nitrogen Doping into Carbon Nanospheres Boosting Capacitive Charge Storage as Both Anode and Cathode for A High-Energy 4.5 V Full Carbon Lithium Ion Capacitor Fei Sun†,‡, Xiaoyan Liu‡,§, Hao Bin Wu‡, Lijie Wang†, Jihui Gao*,†, Hexing Li§ and Yunfeng Lu*,‡ †

School of Energy Science and Engineering, Harbin Institute of Technology, Harbin, 150001,

China ‡

Department of Chemical and Biomolecular Engineering, University of California Los

Angeles, CA 90095, USA §

Department of Chemistry, Shanghai Normal University, Shanghai, 150001, China

*Corresponding author. E-mail: [email protected] (J. Gao); [email protected] (Y. Lu). ABSTRACT：：To circumvent the imbalances of electrochemical kinetic and capacity between Li+ storage anode and capacitive cathode for lithium ion capacitors (LICs), we herein, demonstrate an efficient solution by boosting the capacitive charge storage contributions of carbon electrode to construct a high performance LIC. Such a strategy is achieved by in-situ and highly doping nitrogen atoms into carbon nanospheres (ANCS), which increases the carbon defects/active sites, inducing more rapidly capacitive charge-storage contributions for both Li+ storage anode and PF6- storage cathode. High level N doping induced capacitive enhancement is successfully evidenced by constructing a symmetric supercapacitor using commercial organic electrolyte. Coupling a pre-lithiated ANCS anode with a fresh ANCS cathode enables a full-carbon LIC with a high operation voltage of 4.5 V and high energy/power densities thereof. The assembled LIC device delivers high energy densities of 206.7 Wh kg-1 and 115.4 Wh kg-1 at power densities of 225 W kg-1 and 22.5 kW kg-1, respectively, as well as an unprecedented high-power cycling stability with only 0.0013% capacitance decay per cycle within 10000 cycles at a high power-output of 9 kW kg-1. KEYWORDS: Lithium ion capacitor; in-situ; nitrogen doping; capacitive mechanism

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Rapid growing market demands of electronic devices and hybrid electric vehicles urgently await the developments of advanced energy storage system.1-3 Among them, lithium-ion batteries (LIBs) and supercapacitors (SCs) are recognized as two typical and efficient systems which show complementary charge-storage features and have been widely applied in laptop computers, mobile phones, electric vehicles and elevators.2,4,5 However, due to the different charge-storage mechanism, these two electrochemical energy storage systems have their own shortcomings such as low power densities (less than 1000 W kg-1) and short cycling life (less than 1000 cycles) for LIBs and low energy densities (less than 10 Wh kg-1) for SCs.5-8 For the aim to bridge the gap between LIBs and SCs, lithium ion capacitors (LICs), which integrate a lithium storage anode and a capacitor-type cathode, have been proposed in early 2000s and extensively explored in recent years.2,5,6,9-12 Based on the working mechanism of a LIC that cations (Li+) insertion/extraction into a LIB anode and anions (PF6-) adsorption/desorption into a capacitor cathode, various hybrid LIC systems

such

as

SiC//CAC,5

AC//B-Si/SiO2/C,13

TiC//PHPNC,6

3D

graphene//Fe3O4-graphene,14 SnO2–C//porous carbon,15 AC//hard carbon,16 AC//soft carbon,17 AC//LTO,18 SWNT//V2O5,19 porous graphene //Li4Ti5O12/C,20,21 as well as AC//graphite,22 have been successfully explored, which mainly focus on optimizing the combination of anode and cathode. However, these hybrid systems using dissimilar cathode and anode make it difficult to achieve the synergetic improvements in energy density (1/CLIC=1/Canode+1/Ccathode) 23,24

, power density (high energy densities at the cost of power densities less than 3 kW kg-1)6,

and cycling stability (less than 5000 cycles.6,11,13) for the constructed hybrid devices. To this end, constructing symmetric ion capacitors using identical materials in both electrodes have been demonstrated as an effective approach to circumventing the imbalances between anode and cathode. To date, nanomaterials with intercalational pseudocapacitance such as Na3V2(PO4)325-27, carbon nanosheets28 and B-N dual-doped carbon nanofibers11 have been


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explored as identical active materials on both electrodes to construct symmetric Na ion capacitors or Li ion capacitors, which show potentials to bridge the performance gap between batteries and supercapacitors. In general, to construct a high performance LIC, circumventing the kinetics, stability and capacity discrepancies between capacitor-type cathode and battery-type anode is the key which can be resolved by exploring nanomaterials with rapidly and reversibly pseudocapacitive mechanisms to construct symmetric device configurations6. Carbonaceous materials, due to their high physicochemical compatibility, easily-tunable porosity and environmentally benign nature, offer a well-suited platform for optimizing micro- and nano-structure for both LIBs anodes and SC cathodes not only in terms of capacity/capacitance but also of rate performance and cycling stability.29-31 Moreover, doping carbon framework with heteroatoms (e.g., nitrogen,31-33 boron,11,34 sulfur,35,36 etc.) has been regarded as an effective strategy to enhance the electrical conductivity and to induce rapidly pseudocapacitive reaction mechanism of carbon eletrodes, which should be an efficient strategy to solve the kinetics imbalance between anode and cathode.6 In this regard, nitrogen-doped carbons such as N-rich hard carbon37, N-rich carbon nanosheets,28,38 N-doped porous carbon fiber39, hydrogel derived carbon N, O doped carbon40 have demonstrated excellent capacitilities as anode materials for Na ion capacitors or Li ion capacitors with their performance strongly correlated to nitrogen doping. For the N doping effects, it is generally considered that nitrogen species incorporation into carbon framework can either provide additional active sites and/or defects for pseudocapacitive Na/Li ion storage , or tailor the interlayer distance and electronic properties of carbon lattice for facilitated electron/ion transfer, all of which contribute to the improved anode perfomances.37-40 Despite progress in this aspect, there is room for advancements in both developing improved heteroatom-doped carbons and elucidating the doing effects on construted electrodes or full-cell devices.


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Based on the analyses above, we, herein, demonstrate a new type of full-carbon LIC configuration which is enabled by in-situ and highly doping nitrogen atoms into carbon nanospheres (ANCS) as both anode and cathode materials. The synthesis of ANCS is achieved by a continuous aerosol-spaying process based on the principle of co-assembly between carbon precursor (phenol and formaldehyde) and nitrogen source (melamine) with colloid silica as the pore template. Such co-assembly method allows simultaneous control over the morphology, pore structure, and degree of N doping,41-43 which could yield carbon particles that have an amorphous carbon framework, high surface area and pore volume, a uniform mesoporous structure, and a high degree of N doping (14.51at-%). These multiple structural features result in a synergism that enables the constructed ANCS electrode with outstanding electrochemical performances for use as both anode and cathode materials in lights of high-rate and long-life anodic capability as well as high-capacity cathodic properties. In particular, high-level N doping is considered to greatly increase the amorphously carbon defects (active sites), resulting in more capacitive Li+ storage contributions, which effectively upgrades the rate and cycling performances of ANCS anode to balance the capacitive ANCS cathode. Thus, a high performance full-carbon LIC is constructed, which outperforms the previously reported LICs, particularly in term of high-rate cycling stability. Schematic preparation process of ANCS is illustrated in Figure 1a and Figure S1. Specifically, we start from aqueous solutions containing the carbon precursor (a copolymer of melamine, phenol, and formaldehyde, denoted as MPF) and pore template (colloidal silica with particle size in the range of 10~15 nm). The aqueous solution is atomized using nitrogen carrier gas to continuously generated aerosol droplets which then pass through a high-temperature zone to enable the co-assembly of the MPF precursors with the silica, leading to the formation of polymer-silica nanoparticle. Subsequent carbonization and removal of the silica templates convert the particles into amorphously mesoporous nitrogen-rich carbon spheres (ANCS). In


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our design, melamine with a high nitrogen content (~66.7 wt-%) is adopted as the co-monomers for polymerizing with phenol and to form the MPF which potentially enables the formation of ANCS particles with a high degree of in-situ N-doping. Besides, colloidal silica with uniform particle size distribution plays the role in pore creating, leading to the formation of ANCS particles with uniform pores. Figure 1b and inset show the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of as-obtained ANCS particles, which exhibit a spherical morphology with diameters less than 1 micrometer. Such poly-dispersed particles could enable dense packing in the electrodes where the packing voids provide effective electrolyte transfer channels. The TEM image of a typical ANCS particle shown in Figure 1b inset further affirms the spherical morphology with an amorphous carbon framework and with substantial pores distributing. To demonstrate the amorphous nature of ANCS, Figure 1c and inset further present the high-resolution TEM (HRTEM) image and corresponding fast Fourier transformation (FFT) image of ANCS, confirming the amorphous carbon structure without graphite lattice fringes.43 To better understand the composition and structure of ANCS, high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) image and the corresponding EDS mapping images are obtained, illustrated in Figure 1d from which C, N and O elements are coexisted in the ANCS particle. These elements are overlapped and homogeneously distributed within the ANCS particle.


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Figure 1 (a) Schematic illustration of ANCS formation process; (b) SEM image of ANCS particles. Inset is a TEM image of an ANCS particle; (c) HRTEM image of ANCS. Inset is the corresponding FFT image. (d) HAADF-STEM image and corresponding C, N and O element mapping of ANCS.


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Figure 2a shows the N2 adsorption/ desorption isotherm of the ANCS particles, which exhibits a typical type IV isotherm with a hysteresis loop within the relative pressure range of 0.6~1, confirming a highly mesoporous structure.41 These mesopores are uniformly distributed with diameter distribution centered at ~12 nm (Figure 2a inset), in good agreement with the particle size (10~15 nm) of employed silica templates. According to the N2 isotherm, ANCS possesses both high BET surface area and large pore volume are up to 1560 m2 g-1 and 2.56 cm3 g-1, respectively, which could provide sufficient surface and space for charge storage. To understand the effect of N-doping on the carbon framework, CS control sample from phenolic-formaldehyde precursor (without melamine) is prepared using the same method (see the Experimental section). The as-synthesized CS particles have a similar surface area and pore volume (Table S1, see the supporting information). Moreover, the control sample without silica template addition was also prepared to demonstrate the advantage of mesoporous structure, the N2 isotherm of resulting sample (denoted as ANCS_TF, TF represents template free) is illustrated in Figure S2 (see the supporting information) which yields a much lower BET surface area and pore volume of 96 m2 g-1 and 0.05 cm3 g-1, respectively. X-ray diffraction (XRD) patterns of ANCS and CS are shown in Figure 2b, from which both ANCS and CS display broad peaks of (002) diffraction, suggesting the amorphous nature of both materials. However, further comparing the XRD patterns between ANCS and CS gives us a recognition that ANCS shows relatively wide (002) and (001) peaks with a full width at half maximum (FWHM) higher than that of N-free carbon CS (the FWHM of ANCS (002) peak is ca. 10o while the FWHM of CS (002) peak is 6o, shown in Figure 2b inset figure). Based on the Scherrer equations (Equation S1~S3, see the Supplementary information), interlayer distance (d002), lateral size (La) and stacking height (Lc), which characterize the degrees of disorder and graphitization for carbon structure, can be obtained.39 These calculated parameters are also shown in the inset table of Figure 2b, from


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which N doping reduce the values of La and Lc, indicating the subdued development of continuous sp2 C planar and stacked structure due to the implant of nitrogen atoms. The decreased La and Lc sizes in ANCS are expected to provide a large number of active sites to improve the surface charge storage. A schematic illustration of the carbon layer models and related parameters before and after N doping can be visually described by Figure 2c. Consistently, the Raman spectra of ANCS and CS (Figure 2d) clearly indicate that ANCS has a lower G-to-D band ratio, demonstrating the more disordered carbon structure of ANCS particles compared with that of CS particles because of N doping.43 Based on the HRTEM, XRD and Raman results, we could concluded that N-doping results in a more amorphous carbon structure containing increased defects and active sites which are expected to increase capacitive charge storage capabilities for both Li+ storage anode and PF6- adsorbed cathode. To further explore the chemical environment of the dopants, X-ray photoelectron spectroscopy (XPS) is conducted to investigate the N species in ANCS.

The XPS overall

spectrum of ANCS and CS shown in Figure 2e, which evidence an obvious N1s signal in ANCS and the absence of N species in CS. XPS element content results indicate that ANCS possesses a high N content up to 14.51% which is much higher than that most reported N-doped carbons.31,32,44 Figure 2f shows the high-resolution N1s spectrum of ANCS which can be deconvoluted into three peaks, representing the structures of pyridinic-N (N-6) at 398.5 ± 0.2 eV, pyrrolic or pyridonic-N (N-5) at 399.8 ± 0.2 eV and quaternary N (N-Q) 401.5 ± 0.2 eV.44 Among these N species, N-6 and N-5 structures with unpaired electrons may induce surface polarization of the carbon frameworks, considered to induce extra pseudocapacitive capacity/capacitance. The N-Q structure serves as a component of the planar graphitic carbon, which could enhance electron transport throughout the carbon plane.43 All of these N species are beneficial to enhancing both anodic and cathodic performances.


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Figure 2 (a) N2 isotherm and the corresponding NLDFT pore size distribution of ANCS; (b) XRD patterns of ANCS and CS. Inset figure is the broadened XRD patterns. Inset table shows the calculated crystalline parameters based on Scherrer equations; (c) Schematic illustration of the carbon layer models and parameters before and after N doping. (d) Raman spectra of ANCS and CS; (e) Overall spectra of ANCS and CS; (f) High-resolution N1s spectrum of ANCS. Inset is the N-doped carbon surface models. To evaluate the electrochemical performances of ANCS as both anode and cathode for LICs, half-cell configurations were constructed and tested. The electrochemical results are shown in Figure 3 and Figure S3 to S10 (see the Supplementary information). The cyclic voltammograms (CV) curves of ANCS anode for the initial three cycles between 0.02 and 3.0 V (vs Li/Li+) at a scan rate of 0.5 mV s-1 are presented in Figure S3, which demonstrates typical carbonaceous anode features with a broaden reduction peak at the initial cycle suggesting solid electrolyte interface (SEI) formation, and the CV curves almost overlapped in the subsequent cycles, indicating a stable reversibility.33 Figure 3a displays the typical CV curve of ANCS anode at 0.5 mV s-1 as well as the calculated surface capacitive capacity


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contribution (shadowed area) based on a current separation method. 11, 45 For comparison, the CV curves at 0.5 mV s-1 of CS anode is also tested, shown in Figure S4 (see the supporting information). It can be seen that about 52% of the total capacity of the ANCS anode is contributed by capacitive mechanism at 0.5 mV s-1, which is higher than 38%, clearly demonstrating larger pseudocapacitance contribution of ANCS anode due to N doping. The high capacitive contribution to the total capacity may lead to the good rate capability. Figure 3b presents the charge/discharge profiles of ANCS anode for the initial three cycles at a current density of 0.1 A g-1. During the first cycle, a large irreversible capacity of 2032 mAh g-1 and a reversible capacity of 1091 mAh g-1 are obtained, resulting in the initial Coulombic efficiency (CE) of 53.7%. The irreversible capacity is owing to the formation of SEI layer and/or the irreversible lithium insertion into special positions in the carbon material. To further investigate the early CE during cycling, ANCS anode was cycled at a current density of 100 mA g-1 within 100 cycles, as shown in Figure S5 (see the supporting information). It can be observed that the CE gradually increases during the first 10 cycles, after which the charge/discharge process becomes stable and reversible with the CE of approximately 99.5%. Moreover, the gradual increase in capacity with cycling could be attributed to the activating process of the porous anode which has also been observed in carbon-based anode materials in previous literature.46 Figure 3c presents the rate performance of ANCS anode at various current densities as well as the comparison with N-free CS anode. It can be seen that ANCS anode exhibits higher capacities than CS anode at the same current densities, directly evidencing the promotion effects of N doping on the anodic properties for carbon materials. Moreover, the ANCS_TF sample obtained by removing silica template addition into precursor solution is also tested as anode material. The rate performance is shown in Figure S6, from which ANCS_TF sample exhibit much lower capacities than ANCS anode at corresponding current densities,


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demonstrating the essential role of mesoporous structure in Li+ transfer and storage. To further demonstrate the N doping effects, electrochemical impedance spectroscopies for ANCS and CS anode are obtained, shown in Figure 3d. As can be seen, the difference between ANCS and CS electrodes should be mainly due to the charge transfer resistance effects, which are reflected by the semicircles at high frequency. ANCS electrode exhibits smaller charge transfer resistance than that of CS electrode, suggesting fast and highly efficient faradaic redox reactions at the electrode surface. Considering the similar pore structure and the same mass loading for ANCS and CS electrodes, such optimized electrode kinetics of ANCS electrode should be derived from N doping. Particularly, ANCS anode also display an outstanding high-rate cycling stability shown in Figure 3e (the initial 100 cycling data is shown in Figure S7, supporting information). There is no capacity decay for ANCS anode after 2000 cycles at a high current density of 2 A g-1, demonstrating its promising candidate as durable anode for LICs.


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Figure 3. Electrochemical performances of half-cell configurations. (a) Typical CV curve of ANCS anode at a scan rate of 0.5 mV s-1 within the potential range of 0.02-3.0 V vs. Li/Li+; (b) Charge-discharge curves of ANCS anode for 1st, 2nd, and 3rd cycles at a current density of 0.1 A g-1; (c) Rate performances of ANCS and CS anodes; (d) Nyquist plots of ANCS and CS electrodes; (e) Cycling stability of ANCS anode at 2 A g-1; (f) Charge-discharge curves of ANCS cathode at different current densities within the potential range of 2.0-4.5 V vs. Li/Li+; (g) Cycling stability of ANCS cathode at a current density of 2 A g-1.

The cathode performances of ANCS is next investigated by setting the potential range as 2.0~4.5 V (vs Li/Li+). The CV curves of ANCS cathode at various scan rates (Figure S8, see the supporting information) clearly exhibit quasirectangular shapes, indicating a typical


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capacitive behavior. The deviation part from the quasirectangular CV shape should be ascribed to the pseudocapacitance contribution due to the interaction between electrolyte and N-containing carbon surface. Furthermore, ANCS cathode also behaves a good rate capability with rectangular CV shape preservation even at a high scan rate of 50 mV s -1 (Figure S9, see the supporting information). The comparison of CV curves for ANCS and CS cathode at a scan rate of 50 mV s-1 (Figure S5, see the Supplementary information) suggests a larger area of the CV curve for doped ANCS cathode than that the non-doped CS cathode, demonstrating improved capacitance due to N doping. Figure 3f shows the linear charge-discharge curves of ANCS cathode at different current densities from 0.3 to 10 A g-1 from which ANCS delivers high specific capacities of 113 mAh g-1 (162.7 F g-1) at a low current density of 0.3 A g-1 and still maintains 67 mAh g-1 (96.5 F g-1) at a high current density of 10 A g-1. In addition, ANCS cathode also exhibits an excellent cycling stability with 88.3 % capacity retention after 9000 cycles (Figure 3g, only 0.0013% capacity decay per cycle) at a current density of 2 A g-1 (the initial 200 cycling data is shown in Figure S10, supporting information). The large specific capacities and high cycling stability of ANCS cathode are superior to many reported LIC cathodes,17,18,20-22 demonstrating its promising cathode candidate for the construction of high-performance LICs. To further demonstrate the enhanced capacitive charge storage properties due to high level N doping, symmetric EDLC devices were constructed using commercial organic electrolyte (1M tetraethylammonium tetrafluoroborate in acetonitrile). Figure 4 present the electrochemical performances of ANCS based EDLC as well as the comparative CS based EDLC. Figure 4a shows the CV curves of ANCS-EDLC which can be stably operated at 3.0 V without obvious polarization even at a high scan rate of 200 mV s-1. Figure 4b compares the CV curve of ANCS-EDLC and CS-EDLC at 100 mV s-1, clearly indicating the both higher capacitance and better rate property of ANCS-EDLC relative to CS-EDLC. Figure 4c presents the


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galvanostatic charge-discharge curves of ANCS-EDLC. Linear voltage profiles indicating ideal capacitive charge storage mechanism, in good agreement with the rectangular CV shapes. The specific capacitances are 180 F g-1 and 113 F g-1 at 0.5 A g-1 and 10 A g-1, respectively. Figure 4d compare the galvanostatic charge-discharge curves of ANCS-EDLC and CS-EDLC at 2 Ag-1.

CS-EDLC shows both less discharge time and higher IR drop than

those of ANCS-EDLC. To further demonstrate the N doping effects on the symmetric device, electrochemical impedance spectroscopies for ANCS-EDLC and CS-EDLC are tested (Figure 4e) from which ANCS-EDLC shows smaller charge transfer resistance than that of CS electrode, suggesting larger ion-accessible surface area and accelerated electrolyte ion transfer of the former due to N doping. Figure 4f further gives the Ragone plots of the ANCS-EDLC, as well as its comparison with the CS-EDLC. The ANCS-EDLC can deliver energy densities up to 58.1 Wh kg-1 and 22.4 Wh kg-1 at power densities of 0.396 kW kg-1 and 15.6 kW kg-1 respectively, obviously superior to the CS-EDLC. These results clearly demonstrates the improved capacitive performances for ANCS in terms of both increased capacitances and enhanced rate capability due to high level N doping.

Figure 4. Electrochemical performances of ANCS-EDLC and CS-EDLC in 1M TEA BF4-AN with the operation voltage of 3 V. (a) CV curves of ANCS-EDLC at different scan


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rates; (b) CV curves of ANCS-EDLC and CS-EDLC at a scan rate of 50 mV s-1; (c) Galvanostatic charge-discharge curves of ANCS-EDLC at different current densities; (d) Galvanostatic charge-discharge curves of ANCS-EDLC and CS-EDLC at 2 A g-1;(e) Nyquist plots of ANCS-EDLC and CS-EDLC; (f) Ragone plots of ANCS-EDLC and CS-EDLC.

Considering the aforementioned structure merits of ANCS as well as its comparison with N-free CS, such excellent anodic and cathodic properties of ANCS should be ascribed to the synergy of highly amorphous carbon sphere framework, large surface area and pore volume and particularly rich N species, which not only provide sufficient charge storage space and active sites for boosting the capacity/capacitance, but also offer fast channels and enhanced surface affinity for electrolyte penetration and storage. Hence, a full-carbon lithium ion capacitor (ANCS//ANCS) is constructed by coupling pre-lithiated ANCS anode and a fresh ANCS cathode. Figure 5a illustrates the charging process of the ANCS//ANCS configuration during which Li+ ions enter into ANCS structure based on multiple storage mechanisms including insertion into interlayer, adsorption onto carbon pores and N-containing active sites while PF6- ions are accumulated on the carbon pores and the N-doped carbon surface. Figure 5b and 5c show the CV and charge/discharge curves of ANCS//ANCS LIC which can be operated in a high voltage window of 0~4.5 V. The slightly distorted CV and charge/discharge curves should stem from the overlapping effects of different charge-storage mechanisms. With increasing the scan rate from 5 mV s-1 to 50 mV s-1, the CV curves of ANCS//ANCS LIC show similarly quasirectangular shapes, indicating an excellent rate performance. As shown in Figure 5c, the voltage profiles plotted are all 5th cycle for each current density (a complete set of galvanostatic charge/discharge curves within 5 cycles of the ANCS//ANCS LIC at various current densities are presented as Figure S11 to S17, supporting information). Based on the galvanostatic charge/discharge curves, the energy and power densities at various current densities can be clearly calculated (calculation details please refer to supporting information).


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The resulting Ragone plots accompanied with the representative LIC hybrid systems in previous studies are shown in Figure 5d. It can be observed that ANCS//ANCS LIC can achieve a high energy density of 206.7 Wh kg-1 at 225 W kg-1; even at a high power density of 22.5 kW kg-1, the LIC can still maintain an energy density of 115.4 Wh kg-1, demonstrating the excellent rate capability. Such high energy and power densities of ANCS//ANCS LIC are also among the first-class levels in previous reported LIC systems such as LTO//AC,20,21 MnO//AC,47,48 SnO2–C//porous carbon,15 Fe3O4/G//3DGraphene,14 AC//hard carbon,16 SWNT//V2O5,19 TiC//PHPNC,6 Nb2O5/C//AC,49 B-Si/SiO2/C//AC,13 BNC//BNC,11 and 3DaC//MnFe2O4.50 (A comprehensive summarization see Table S2, Supporting information). More remarkably, the ANCS//ANCS LIC also displays an excellent cycling stability of 86.6% capacitance retention after 10000 cycles (only 0.00134% capacitance decay per cycle) at a high current density of 4 A g-1 as well as good Coulombic efficiency of nearly 100% (Figure 5e), which represents the best high-rate cycling performance reported to date among the reported LIC configurations as per the knowledge. To further demonstrate the superior rate capability and cycling stability of our ANCS//ANCS LIC, commercial graphite is employed as the anode material to construct a LIC by coupling an ANCS cathode under the same design guidelines (denoted as Graphite//ANCS). As compared in Figure 5e, the Graphite//ANCS LIC shows a much shorter cycling life with severe capacity decay in 1000 cycles.

Figure 5f further compares the voltage drops at

various current densities for ANCS//ANCS and Graphite//ANCS. Clearly, Graphite//ANCS LIC reveals much more serious polarization that that of ANCS//ANCS LIC, especially at high current densities. The key to achieving high rate and excellent cycling performances of a lithium ion capacitor is through optimizing the microstructure of anode so that Li+ storage and release can proceed rapidly and reversibly to match the capacitive cathode. Graphite, which is an insertion-type anode, possesses relatively low theoretical capacity (372 mA h g-1), poor ion


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diffusion kinetic (DLi of 10-7~10-9 cm2 s-1) as well as mediocre cycling stability (volume change up to 150% by the formation of Li(solvent)yCn ), thus restricting the rate and long cycling capabilities of constructed graphite//ANCS LIC. By contrast, the adaption of ANCS particles greatly enhances the anodic performance in terms of fast and durable Li+ storage capabilities through inducing capacitive charge storage mechanism, thus efficiently circumventing the mismatch of kinetic and lifespan between anode and cathode, endowing the constructed ANCS//ANCS LIC with excellent rate capability and cycling stability.

Figure 5. Mechanism and electrochemical performances of ANCS//ANCS LIC (mass loading of ANCS on anode and cathode are ~1.2mg cm-2 and ~3.6 mg cm-2, respectively). (a) Schematic illustration of the work mechanism; (b) Representative CV curves within the voltage range of 0~4.5V at various scan rates; (c) Galvanostatic charge-discharge curves at


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various current densities; (d) Ragone plots in a comparison with literature; (e) Cycling performance of ANCS//ANCS at 4 A g-1. Inset is the cycling performance of Graphite//ANCS at 2 A g-1; (f) Voltage drops vs. current densities for ANCS//ANCS and Graphite//ANCS. In summary, we have developed a new type of full-carbon lithium ion capacitor which employs amorphously nitrogen-rich mesoporous carbon spheres (ANCS) as both anode and cathode materials. High level N doping induces more capacitive charge-storage contributions for both anode and cathode, achieving high rate and long life for anode while high capacitance for cathode, which efficiently circumvent the electrochemical discrepancies between Li+ storage anode and PF6- storage cathode, enabling a high-voltage and full-carbon ANCS//ANCS LIC device exhibiting among the most promising performances reported. These results also provide insights into designing high-voltage lithium ion capacitors by regulating the chemical environment of carbon nanomaterials. ASSOCIATED CONTENT Supporting Information: Experimental details, schematic illustration of the employed aerosol-assisted spraying process, structural and electrochemical characterizations of prepared samples, as well as a comparison of LIC performance with literature. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author*. Jihui Gao ([email protected]); Yunfeng Lu (luucla.ucla.edu) Notes. The authors declare no competing financial interest. ACKNOWLEDGMENTS This research was financially supported by the National Natural Science Foundation of China (Grant No. 51376054 and 51276052).


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In-Situ High Level Nitrogen Doping into Carbon Nanospheres Boosting Capacitive Charge Storage as Both Anode and Cathode for A High-Energy 4.5 V Full Carbon Lithium Ion Capacitor TOC graphic


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