Nitrogen-Doped Multiwall Carbon Nanotubes for Lithium Storage with

Mar 27, 2012 - Furthermore, when integrated with 3 nm nickel oxide nanoparticles for a further capacity boost, nitrogen doping enables unprecedented c...
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Letter pubs.acs.org/NanoLett

Nitrogen-Doped Multiwall Carbon Nanotubes for Lithium Storage with Extremely High Capacity Weon Ho Shin,† Hyung Mo Jeong,‡ Byung Gon Kim,† Jeung Ku Kang,*,†,‡,§ and Jang Wook Choi*,†,§ †

Graduate School of EEWS (WCU), ‡Department of Materials Science and Engineering, and §KAIST Institute Nano Century, Korea Advanced Institute of Science and Technology, 373-1 Guseong Dong, Yuseong Gu, Daejon 305-701, Korea S Supporting Information *

ABSTRACT: The increasing demands on high performance energy storage systems have raised a new class of devices, socalled lithium ion capacitors (LICs). As its name says, LIC is an intermediate system between lithium ion batteries and supercapacitors, designed for taking advantages of both types of energy storage systems. Herein, as a quest to improve the Li storage capability compared to that of other existing carbon nanomaterials, we have developed extrinsically defective multiwall carbon nanotubes by nitrogen-doping. Nitrogen-doped carbon nanotubes contain wall defects through which lithium ions can diffuse so as to occupy a large portion of the interwall space as storage regions. Furthermore, when integrated with 3 nm nickel oxide nanoparticles for a further capacity boost, nitrogen doping enables unprecedented cell performance by engaging anomalous electrochemical phenomena such as nanoparticles division into even smaller ones, their agglomeration-free diffusion between nitrogen-doped sites as well as capacity rise with cycles. The final cells exhibit a capacity as high as 3500 mAh/g, a cycle life of greater than 10 000 times, and a discharge rate capability of 1.5 min while retaining a capacity of 350 mAh/g. KEYWORDS: Carbon nanotubes, nitrogen-doping, lithium ion capacitor, metal oxide, high capacity, long lifetime

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therefore afford to run over a large number of cycles, which could be even comparable to the cycle lives of supercapacitors. Having noticed these advantageous properties of LICs as well as the possibility of utilizing the knowledge accumulated during the development of rechargeable batteries and supercapacitors, LICs have made an incredible progress. A variety of carbon nanomaterials including activated carbon,8 carbon nanotubes (CNTs),9 and graphene10,11 have been studied as electrode materials. Also, diverse lithium sources and doping materials on the anode sides have been developed.12,13 In this study, we demonstrate an LI storage with unprecedented electrochemical performance by using Ndoped CNTs (NCNTs). Behind the excellent electrochemical properties of NCNT-based LICs, N-doping plays the following pivotal roles: (1) The N-doping process generates extrinsic defects in the walls, through which Li ions can diffuse into interwall space, thus allowing for the use of unexplored interwall space for Li storage. (2) N-doping increases capacity further as a result of more favorable binding of N-doped sites with Li ions. (3) Moreover, for a further capacity boost, metal oxide nanoparticles (NPs) as small as 3 nm were grown on the NCNT surfaces. N-doping is critical for stable operations of the NP-integrated electrodes because it assists the good dispersion of the NPs over cycling. The integrated electrodes exhibit (i) gravimetric capacities of ∼3500 mAh/g, (ii) cycle lives of larger

nergy storage systems (ESSs) represented by rechargeable batteries and supercapacitors are receiving increasing attention, as their applications are successively expanding from small-scale mobile electronics to large-scale transportations and utility grids.1−4 Moreover, the large-scale applications are expected to play important roles in addressing urgent energy and environmental issues.1,2 As widely accepted, rechargeable batteries and supercapacitors have complementary characteristics such that strong points of the former usually become weak points of the latter, or vice versa. The complementary characteristics originate from different storage mechanisms of both types of ESSs. Rechargeable batteries undergo redox reactions mediated by carrier ions in the bulk electrode materials, whereas supercapacitors function based on adsorption/desorption on the electrode surfaces (electric double layer capacitors) or redox reactions on the near-surfaces of electrodes (pseudocapacitors). Thus, rechargeable batteries usually hold higher energy densities, but lower power densities, compared to those of supercapacitors. As an attempt to take only advantages of both types of ESSs, recently, lithium ion capacitors (LICs)5−7 have been designed and demonstrated. In LICs, unlike supercapacitors, Li sources exist in the anodes so that their energy densities are much larger than those of supercapacitors. Similarly, unlike rechargeable batteries, the electrodes of LICs function based on adsorption/desorption and thus facilitate fast kinetics for discharge/charge, which enables higher power densities than those of rechargeable batteries. Furthermore, LICs do not undergo the significant volume expansion of electrodes and can © 2012 American Chemical Society

Received: January 9, 2012 Revised: March 2, 2012 Published: March 27, 2012 2283

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Figure 1. Li storage in the interwall space of NCNTs. TEM images of an individual NCNT at the same spot (a) before and (b) after the full lithiation. The scale bars are 20 nm. (Middle inset) Enlarged TEM images with lattice distances. (b, right inset) An FT pattern of the selected areas. (b, bottom inset) A low-magnification TEM image of lithiated NCNT. The scale bar is 50 nm. (c) The gravimetric capacities and cycling performance of as-grown NCNTs and commercial multiwall CNTs. The current densities of both cells were 80 mA/g. (d) A schematic illustration showing that lithium ions insert into the interwall space of NCNTs through wall defects originating from defected nitrogen configurations.

interwall spacing increase in the HRTEM images. Third, for the same spot in NCNT, the TEM images show an increase in the wall thickness from 15.7 to 17.8 nm, once again as a result of the increased interwall spacing. Lastly and most significantly, the capacity of the NCNT electrode increases continuously with cycling (Figure 1c). Initially, at a C/4 rate (1C: charge or discharge takes 1 h), its capacity starts at 340 mAh/g, but reaches 1920 mAh/g by the 200th cycle. The final capacity is converted to 2300 F/g when evaluated as capacitors, which is the largest among ever reported. This capacity increase can be rationalized by the fact that over successive cycling Li ions diffuse into the interwall space through interwall defects that are always generated during N-doping processes (Figure 1d). Moreover, it is not unlikely that N-doped sites increase the capacity substantially compared to that of the N-free counterpart because the binding of Li ions onto the N-doped sites is energetically more favorable. More specifically, the larger Li accommodation is attributed to the N-doped sites that hold higher electronegativity and thus electrostatically attract larger number of Li ions. Although N-contents in our NCNTs appears small (2.4 atom %), it has been suggested that the Ndoping indeed modify the electronic properties of neighboring carbon atoms,15−17 which is also likely to contribute to the increase of the capacity. Although CNTs with no N-doping but with bamboo structures would be the most ideal control sample, commercial CNTs without bamboo structures were used as a control because CNTs with bamboo structures are almost impossible to synthesize. This is associated with the fact that bamboo structures originate from lattice distortion during growth, which is usually accompanied with doping with foreign elements. We also performed Raman spectroscopy to investigate crystallinity of NCNTs before and after electrochemical processes (Supporting Information Figure S11). The IG/ID value of the as-grown NCNTs is 1.14, and this value remains almost unchanged after electrochemical lithiation/ delithiation, indicating that the lithium insertion/extraction

than 10 000 cycles without capacity loss, and (iii) power capabilities such that 350 mAh/g is retained even at a 1.5 min discharge time. Furthermore, the electrodes exhibit some anomalous phenomena such as the fact that capacity increases with cycling, an observation that corresponds to gradual Li ion diffusion into unexplored interwall space. Also, we have observed subdivision of these small NPs into even smaller features as well as their dynamic diffusion between N-doped sites of multiwall CNTs without agglomeration. Overall, these N-doping effects explain the outstanding power and cycling performance. While the extraordinary results presented herein indicate that our electrodes could also function as Li battery anodes, the electrochemical properties could be applied for LIC electrodes. Regardless of the device type, the N-doping effects on the improved Li diffusion and storage will be mainly discussed. The NCNTs were prepared by a plasma enhanced chemical vapor deposition (PECVD) process. More detailed procedures are described in the Supporting Information. A transmission electron microscopy (TEM) image shows (Figure 1a) that individual NCNTs have bamboo structures with outer diameters of 40−60 nm. After electrochemical lithiation, the original shape of the NCNT appears unmodified, yet solidelectrolyte interphase (SEI) layers are formed (Figure 1b) on the NCNT surfaces. For direct comparison, we tracked the same individual NCNT (Figure 1a,b) before and after the lithiation by ex situ TEM. See the Supporting Information for the details of this analysis.14 Several analyses indicate clearly that the interwall space is indeed used for Li storage. First, highresolution (HR) TEM images, taken at the same spot, show the increased interwall spacing from 3.4 to 3.7 Å following a full lithiation (middle insets in Figure 1a,b). Second, the Fourier transformed (FT) patterns from white boxed regions in Figure 1a,b display spots assigned to the interwall (002) planes (right insets in Figure 1a,b). After full lithiation, the spot-to-spot distances in the FT patterns decrease in keeping with the 2284

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Figure 2. Electrochemical characterization of the NiO-NCNT. (a) An HRTEM image of the NiO3 nm-NCNT magnified from the red circle of the left inset. (Left inset) A scanning TEM image of the NiO3 nm-NCNT. (Right inset) A FT pattern of (a). The corresponding lattice orientations are denoted. (b) Voltage profiles of the NiO3 nm-NCNT at 200 mA/g at different cycles during galvanostatic measurements. (c) The cycling performance of the NiO3 nm-NCNT, the NiO10 nm-NCNT, NCNT, and NiO alone. The current densities of those samples were 200, 200, 80, 200 mA/g, respectively. (d) The power capability tests for the NiO3 nm-NCNT, the NiO10 nm-NCNT, and the NiO−CNT. The unit of current density denoted for each period is A/g. (e) A capacity retention test for a large number of cycles. Intermittent break points are denoted with periods. It is noteworthy that the data in (d) and (e) were obtained sequentially from the same sample. Mass loadings for the samples whose data are shown here are ∼1 mg/ cm2.

NCNT surfaces (Figure 2a left inset). Moreover, the N-doping allows for a fine-tuning of NP sizes so that conspicuously differently sized NPs can be grown on the NCNTs without agglomeration, even in the sub-10 nm regime, actually 3 and 10 nm in this investigation. High-resolution TEM images of the 3 and 10 nm examples and particle size distributions for each NP case are presented in Figure 2a, Supporting Information Figure S1a, and Supporting Information Figure S2, respectively. By contrast, NiO NPs grown on bare CNTs (NiO-CNT) by the same procedures turn out (Supporting Information Figure S3) to be agglomerated or too sparsely distributed. A FT diffraction pattern (Figure 2a right inset) as well as lattice distances in HRTEM (Figure 2a) and X-ray diffraction (XRD) data (Supporting Information Figure S4) verify that the assynthesized NPs are single-crystalline and pure NiO. In order to observe the effect of the dispersion of extremely small NPs on the electrochemical properties of LIC electrodes, coin-type half-cells were characterized by galvanostatic measurements. In agreement with previous reports,26 when they are cycled in the potential range of 0.001−3 V (vs Li/Li+), the charging and discharging curves of NiO-NCNT reveal (Figure 2b) plateaus around 1.0 V, indicative of the conversion reaction: NiO + 2Li ⇄ Li2O + Ni. NiO-NCNTs exhibit far superior capacity retentions to that of a control case (Figure 2C, Supporting Information Figure S5) of NiO alone. In the case of the NiO alone, the capacity drops rapidly with cycling as a result of NP agglomeration and electrical contact loss from carbon conducting agents. As in the NCNT case, the NiONCNT exhibits an uncommon trend of capacity rise with cycling (Figure 2c). The capacities of NiO3 nm-NCNT and

processes into the inner wall of NCNT do not affect the crystallinity of NCNTs. In the present situation, once the capacity becomes saturated after ∼200 cycles, the NCNT electrode exhibits a 5−10 times larger specific capacity than those of other reported CNT or graphene-based analogues9−11 (200−400 mAh/g). This enhanced binding with ions by Ndoping has been reported for CNTs and graphene, as well as being used in energy storage devices.18,19 As shown in Supporting Information Figure S5, unlike graphite NCNTs do not show “staging” in their potential profiles. This must be due to different lithiation mechanisms between both materials. For the case of graphite, Li undergoes interlayer diffusion from the edge, whereas NCNTs in our study have Li diffusion through interwall defects in the perpendicular direction to the walls. Although a good number of studies covered N-doped CNTs as Li storage media,20,21 the observed abnormal phenomena in this study have never been reported. The different observations rather indicate that the observed phenomena are sensitive to local microscopic environments of N-doped sites and thus CNT growth conditions. To provide an additional capacity boost, nickel oxide (NiO) NPs were attached onto the NCNTs’ walls by first growing Ni NPs by chemical reduction. These metal NPs on the NCNTs were fully oxidized by thermal treatment under ambient conditions. Employing these processes, NCNTs with 3 nm NiO NPs grown on the surfaces (Figure 2a) were prepared. It is recalled from previous studies15,22−25 that N-doping alters the local electronic structure of CNTs and thus nucleates NP growth preferentially at N-doped sites. Taking advantage of this observation, we have grown NiO NPs highly dispersed on the 2285

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Figure 3. TEM characterization after the first lithiation. The TEM images for the same spot of the NiO3 nm-NCNT (a) before and (b) after full lithiation down to 0.001 V vs Li/Li+. Scale bars of both insets are 50 nm. (c) An EDS mapping corresponding to Ni elements (blue dots). (d) A TEM image of the fully lithiated NiO3 nm-NCNT after the electron-beam irradiation. (Inset) A FT diffraction pattern obtained from a dark spot in (c) indicates that the agglomerated NPs during the e-beam irradiation are Ni.

the current density increases 100 times (0.2 to 20 A/g), the NiO3 nm-NCNT still retains 45% of its original capacity (Figure 2d). When the rate returns to the initial 0.2 A/g after 75 cycles, the electrode shows higher capacity than that of the first cycle and the capacity increases further thereafter. We also tested CoO3 nm-NCNTs and observed a similar excellent power capability (Supporting Information Figure S7), thus verifying that the N-doping and small NP effects are versatile for various metal oxide materials. We have expanded the cycling test beyond those in Figure 2c to verify the robust interaction between NiO and NCNTs (Figure 2e). During this test, both normal and extremely high current rates were employed to ensure the robust cell operations over more than 10 000 cycles, that is, 1 order of magnitude greater than those of usual cycling tests. Also, for a more rigorous test, we used the same sample as the one in Figure 2d. Subsequently, after the rate capability test from low to high rates summarized in Figure 2d, the sample was cycled at extremely high rates from 360 to 600C over 10,000 cycles. It should be noted that at 600C each of charge or discharge process takes only 6 s. During such long cycles which also include intermittent breaks for several days to months, the initial capacity is retained without any loss. When the current density is reduced back to 1.2 A/g (1.6C), the original capacity is recovered and persists for more than 200 cycles. Overall, the outstanding cycling data are commensurate with exceptionally robust electrochemical processes, which are ascribed once again

NiO10 nm-NCNT increase in a continuous fashion up to 3500 and 3000 mAh/g, respectively, values that are much larger than that covered (∼1000 mAh/g) by the original conversion reaction.26 The value of 3500 mAh/g can be converted to 4200 F/g when evaluated as capacitors. This value is substantially higher than those of any LICs reported previously.8−13 Notably, the capacities illustrated in Figure 2c were measured over a 6 month period, hence supporting the robust character of the electrode during electrochemical operation. The exceptional capacities and their retentions are not only associated with the N-doping that facilitates Li diffusion into the interwall space but also with preserving the interaction with the NiO NPs during repeated cycles. A detailed mechanism of the NP interaction with N-doped sites during dynamic electrochemical processes will be presented at the end of this discussion. It should be added that all the data presented here are based on the mass (∼1 mg/cm2) of both NiO and NCNT and also that no conducting agent was used for any of the NCNT-based electrodes. The N-doping also allows the NiO-NCNTs to operate well under extreme rate conditions. Both the 3 and 10 nm cases exhibit far superior power capabilities compared to that (Figure 2d) of a NiO-CNT control. In the case of the NiO3 nm-NCNTs, even at 15 A/g, which corresponds to relatively fast charging and discharging times of 1.5 min, a specific capacity as high as 350 mAh/g is achieved. Upon increasing the current rate, the NiO3 nm-NCNT shows extraordinary performance; even when 2286

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Figure 4. XPS characterization of the NiO3 nm-NCNT and NCNT. (a) N 1s data for the NiO3 nm-NCNT taken at different stages of electrochemical and other processes. (b) A graphical illustration of Ni diffusion between N-doped sites during lithiation. (c) N 1s data for NCNT taken at the same stages. (d) Ni 2p data for the NiO3 nm-NCNT taken at different stages of electrochemical and other processes. The Ni0 position was calibrated by Ni foil.

cycling. These phenomena confirm the N-doping effects that maintain the original electrode structure of the well-dispersed NPs in a very robust manner, even during dynamic electrochemical cycling. X-ray photoelectron spectroscopy (XPS) measurements at various electrochemical points provide critical clues about the extraordinary performance (Figure 4a). As-grown NCNTs exhibit mainly two N-configurations: graphite-like (N-Q, 400.0 eV) and pyridine-like (N-6, 398.8 eV). As verified by previous theoretical and experimental studies,15 initially Ni NPs are grown preferentially on N-6 sites because of the higher binding energies compared with N-Q sites. This difference is reflected in the decreased N-6 peak in Figure 4a upon NiO NP nucleation. Notably, once the samples are lithiated, the peak at 398.8 eV becomes significantly intensified, and the N-Q peak becomes relatively weaker (Figure 4a). These peak intensity changes can be interpreted by the fact that, during lithiation processes, Ni atoms undergo diffusion and some of the final Ni elements, the conversion reaction products, get bound onto NQ sites (schematic illustration is shown in Figure 4b). This NQ site binding that corresponds to an sp2 to sp3 shift is reflected in the intensified peak at 398.8 eV. In other words, in the highly kinetic lithiation processes where lithium ions and active materials undergo diffusion in the presence of electric fields, the Ni elements might be able to leave the originally stable N-6 sites and reach metastable binding sites (N-Q sites).15 Similar metal atom diffusions between defects on graphene have been observed upon electron-beam irradiation.28,29 It is noteworthy that significant portions of N-Q still retain their sp 2 configuration, an observation that might be primarily caused by remaining N-Q sites on interwalls of NCNTs that are not accessible to Ni atoms. Furthermore, we carried out XPS analysis for the heat-treated sample whose TEM image is shown in Supporting Information Figure S9. We observed that

to the N-doping effect that preserves the original small sizes of NPs and their binding with NCNTs. To elucidate the excellent specific capacity, power, and cycling performance, we observed the NiO-NCNT electrodes at various stages of cycling using HRTEM. The HRTEM images illustrated in Figure 3a,b show the same spot of NiO3 nm-NCNT before and after a full lithiation (0.001 V vs Li/ Li+). TEM images for NiO10 nm-NCNT are also shown in Supporting Information Figure S1. Apparently, in both cases there is no agglomeration of NiO NPs during the lithiation. More significantly, NiO NPs appear divided into smaller features and spread out along the NCNT. In the case of NiO10 nm-NCNTs, the smaller features are as small as 3−4 nm and so are believed to be Ni NPs, the conversion reaction products, as in previously reported cases.26 In the case of NiO3 nm-NCNTs, NiO NPs also appear subdivided and highly dispersed. The final features after full lithiation, however, are hardly visible and only SEI layers are observed, presumably because the Ni elements are too small, likely almost a few atoms size, to detect even with HRTEM (Figure 3b). Energy dispersive X-ray spectroscopy (EDS), however, indicates Ni elements are still present (blue dots in Figure 3c) over the entire NCNT. Also, as direct evidence for the Ni atoms presence on the NCNT surfaces, when we irradiated the lithiated NiO3 nm-NCNT sample with an electron-beam for a couple of minutes, 2−5 nm dark spots, indicative of Ni (an FT pattern in Figure 3d inset) started showing up because of Ni diffusion and agglomeration driven by the electron-beam energy.27 Similarly, we supplied thermal energy (heat treatment under ambient conditions) to the lithiated NiO3 nm-NCNT and observed (Supporting Information Figure S8) the formation of NiO NPs. This series of observations implies that extremely small metal oxide NPs are subdivided into even smaller features but still remain highly dispersed on the NCNTs throughout 2287

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(9) Lee, S. W.; Yabuuchi, N.; Gallant, B. M.; Chen, S.; Kim, B. S.; Hammond, P. T.; Shao-Horn, Y. Nat. Nanotechnol. 2010, 5, 531−537. (10) Jang, B. Z.; Liu, C. G.; Neff, D.; Yu, Z. N.; Wang, M. C.; Xiong, W.; Zhamu, A. Nano Lett. 2011, 11, 3785−3791. (11) Stoller, M. D.; Murali, S.; Quarles, N.; Zhu, Y.; Potts, J. R.; Zhu, X.; Ha, H.-W.; Ruoff, R. S. Phys. Chem. Chem. Phys. 2012, 14, 3388− 3391. (12) Park, M. S.; Lim, Y. G.; Kim, J. H.; Kim, Y. J.; Cho, J.; Kim, J. S. Adv. Energy Mater. 2011, 1, 1002−1006. (13) Luo, J. Y.; Xia, Y. Y. J. Power Sources 2009, 186, 224−227. (14) McDowell, M. T.; Lee, S. W.; Ryu, I.; Wu, H.; Nix, W. D.; Choi, J. W.; Cui, Y. Nano Lett. 2011, 11, 4018−4025. (15) Yang, S. H.; Shin, W. H.; Lee, J. W.; Kim, H. S.; Kang, J. K.; Kim, Y. K. Appl. Phys. Lett. 2007, 90, 013103. (16) Ruelle, B.; Felten, A.; Ghijsen, J.; Drube, W.; Johnson, R. L.; Liang, D.; Erni, R.; Van Tendeloo, G.; Dubois, P.; Hecq, M.; Bittencourt, C. J. Phys. D: Appl. Phys. 2008, 41, 045202. (17) Wei, J.; Hu, H.; Zeng, H.; Zhou, Z.; Yang, W.; Peng, P. Physica E 2008, 40, 462−466. (18) Reddy, A. L. M.; Srivastava, A.; Gowda, S. R.; Gullapalli, H.; Dubey, M.; Ajayan, P. M. ACS Nano 2010, 4, 6337−6342. (19) Jeong, H. M.; Lee, J. W.; Shin, W. H.; Choi, Y. J.; Shin, H. J.; Kang, J. K.; Choi, J. W. Nano Lett. 2011, 11, 2472−2477. (20) Bulusheva, L. G.; Okotrub, A. V.; Kurenya, A. G.; Zhang, H.; Zhang, H.; Chen, X.; Song, H. Carbon 2011, 49, 4013−4023. (21) Li, X.; Liu, J.; Zhang, Y.; Li, Y.; Liu, H.; Meng, X.; Yang, J.; Geng, D.; Wang, D.; Li, R.; Sun, X. J. Power Sources 2012, 197, 238− 245. (22) Biel, B.; Blase, X.; Triozon, F.; Roche, S. Phys. Rev. Lett. 2009, 102, 096803. (23) Huang, S. F.; Terakura, K.; Ozaki, T.; Ikeda, T.; Boero, M.; Oshima, M.; Ozaki, J.; Miyata, S. Phys. Rev. B 2009, 80, 235410. (24) Lee, D. H.; Lee, W. J.; Kim, S. O. Nano Lett. 2009, 9, 1427− 1432. (25) Jiang, K. Y.; Eitan, A.; Schadler, L. S.; Ajayan, P. M.; Siegel, R. W.; Grobert, N.; Mayne, M.; Reyes-Reyes, M.; Terrones, H.; Terrones, M. Nano Lett. 2003, 3, 275−277. (26) Poizot, P.; Laruelle, S.; Grugeon, S.; Dupont, L.; Tarascon, J. M. Nature 2000, 407, 496−499. (27) Kim, Y. T.; Uruga, T.; Mitani, T. Adv. Mater. 2006, 18, 2634− 2638. (28) Gan, Y. J.; Sun, L. T.; Banhart, F. Small 2008, 4, 587−591. (29) Cretu, O.; Krasheninnikov, A. V.; Rodriguez-Manzo, J. A.; Sun, L. T.; Nieminen, R. M.; Banhart, F. Phys. Rev. Lett. 2010, 105, 196102. (30) Dedryvere, R.; Laruelle, S.; Grugeon, S.; Poizot, P.; Gonbeau, D.; Tarascon, J. M. Chem. Mater. 2004, 16, 1056−1061. (31) Wang, Y.; Fu, Z. W.; Yue, X. L.; Qin, Q. Z. J. Electrochem. Soc. 2004, 151, E162−E167.

N 1s and Ni 2p peaks also return to the original prelithiation ones (Figure 4a,d), suggesting that Ni elements diffuse back to the energetically more stable N-6 sites and form bigger NiO NPs (Supporting Information Figure S9). We also conducted the same XPS characterization for NiO-free NCNT samples (Figure 4c) and realized that the N 1s peaks hardly change during the lithiation and delithiation processes. This realization verifies that the N 1s peak changes of NiO3 nm-NCNT are not associated with SEI layer formation on NCNTs, but rather with the Ni diffusion process. On the other hand, the Ni 2p peaks (Figure 4d) in the XPS data confirm the conversion reaction of NiO-NCNT. It is, however, noteworthy that, unlike that of pure NiO (Supporting Information Figure S8),30 the oxidation state after lithiation is not completely 0 and is rather clearly greater than 0 (852.7 vs 853.7 eV). This observation, consistent with a previous study reporting Ni3N,31 indicates Ni binding with N-doped sites and thus supports the aforementioned Ni diffusion between N-sites. See the Supporting Information for a detailed discussion. Overall, this investigation establishes that N-doping brings multifold revolutionary effects on the LI storage of CNT-based composite electrodes. The Li storage capability in the interwall space and the dispersed binding of high capacity NPs with CNTs during long and aggressive cycling will be especially useful for future challenging energy storage applications that deal with robust and fast ionic and electric transports within high capacity materials.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental procedures/characterization, in-depth XPS analysis, additional TEM images, XRD, XPS, TGA data, and electrochemical data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: (J.W.C.) [email protected]; (J.K.K.) jeung@ kaist.ac.kr. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are pleased to acknowledge the National Research Foundation of Korea Grant funded by the Korean Government (MEST) for the financial support through the Secondary Battery Program (NRF-2010-0029031), Korea Basic Science Institute Grant (T32413), and the World Class University Program (R-31-2008-000-10055-0) for the financial support.

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REFERENCES

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