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Block Copolymer Directed Ordered Mesostructured TiNb2O7 Multimetallic Oxide Constructed of Nanocrystals as High Power LiIon Battery Anodes Changshin Jo,† Youngsik Kim,‡ Jongkook Hwang,† Jongmin Shim,† Jinyoung Chun,† and Jinwoo Lee*,† †
Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Korea School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798, Korea
S Supporting Information *
ABSTRACT: In order to achieve high-power and -energy anodes operating above 1.0 V (vs Li/Li+), titanium-based materials have been investigated for a long time. However, theoretically low lithium charge capacities of titanium-anodes have required new types of high-capacity anode materials. As a candidate, TiNb2O7 has attracted much attention due to the high theoretical capacity of 387.6 mA h g−1. However, the high formation temperature of the TiNb2O7 phase resulted in large-sized TiNb2O7 crystals, thus resulting in poor rate capability. Herein, ordered mesoporous TiNb2O7 (denoted as m-TNO) was synthesized by block copolymer assisted self-assembly, and the resulting binary metal oxide was applied as an anode in a lithium ion battery. The nanocrystals (∼15 nm) developed inside the conﬁned pore walls and large pores (∼40 nm) of m-TNO resulted in a short diﬀusion length for lithium ions/electrons and fast penetration of electrolyte. As a stable anode, the m-TNO electrode exhibited a high capacity of 289 mA h g−1 (at 0.1 C) and an excellent rate performance of 162 mA h g−1 at 20 C and 116 mA h g−1 at 50 C (= 19.35 A g−1) within a potential range of 1.0−3.0 V (vs Li/Li+), which clearly surpasses other Ti-and Nb-based anode materials (TiO2, Li4Ti5O12, Nb2O5, etc.) and previously reported TiNb2O7 materials. The m-TNO and carbon coated m-TNO electrodes also demonstrated stable cycle performances of 48 and 81% retention during 2,000 cycles at 10 C rate, respectively.
the introduction of nano- or composite structures, phase control, surface modiﬁcation, and doping.6−14 However, although the stability and rate capability have been successfully enhanced, the biggest drawbacks to the use of anodes with a Ti4+/Ti3+ redox couple remain the theoretically low lithium charge capacity (<200 mA h g−1) and loss of energy density from the relatively high Li+ insertion voltage (>1.5 V vs Li/Li+). Consequently, both a high capacity and low redox potential (above 1 V) are critical requirements for their potential use as anodes in the LIBs of high power systems, including electric vehicles (EVs) and energy storage systems (ESSs).
Figure 1. Schematic representations of the (a) block copolymer assisted one-pot synthesis and (b) synthetic mechanism for preparing ordered mesoporous TiNb2O7 (m-TNO). The crystal structure of TNO, viewed along the b-axis, is also shown.
availability of the silica template. It is therefore highly preferable to synthesize mesoporous materials for batteries via softtemplate approaches. However, there are only a few studies that demonstrate the successful synthesis of ordered mesoporous multimetal oxides under high temperature heat-treatment conditions (≥700 °C) due to the thermally unstable nature of pluronic BCP.25−27 Therefore, we applied the controlled sol−gel reaction and BCP-assisted synthetic method to develop the ordered mesoporous structured TNO with high crystallinity. Here, nanosized TNO crystals, developed in conﬁned nanopore walls under 900 °C, form a highly ordered and interconnected mesoporous structure. The resulting unique features, such as well-connected nanosized TNO crystals (15 nm), high surface area (74 m2 g−1), and large pores (∼40 nm), result in favorable electron and lithium ion diﬀusion processes at the interface and bulk phase, and electrolyte penetration.24,28,29 As a result, the m-TNO electrode exhibits a long cycle life and high reversible capacity of 289 mA h g−1 at a 0.1 C rate (1 C = 387 mA g−1) and 162 mA h g−1 at 20 C; and even at 50 C is still 116 mA h g−1. These Li-storage capacities and rate performances of an m-TNO electrode are at the highest level among the intercalation-based anode materials. This enhanced electrochemical performance resulting from the ordered mesoporous structure points to the possibility of the successful application of TiNb2O7 anodes in a variety of LIB ﬁelds.
Recently, titanium−niobium oxides have been introduced as alternative anode material due to their high lithium charge capacity and broad lithium charge potential (1.7 to 1.0 V).15−20 Speciﬁcally, a TiNb2O7 anode, reported by Goodenough’s group in 2011,15,16 has exhibited a promising anode performance in which, due to the multiple redox couples (Ti4+/Ti3+, Nb5+/Nb4+, and Nb4+/Nb3+), ﬁve Li ions can be inserted into one formula unit of TiNb2O7. The multiple redox couples correspond to a high theoretical capacity of 387.6 mA h g−1, which clearly surpasses that of other titanium- and niobiumbased anodes. Following the ﬁrst report on TiNb2O7, several groups have reported the fabrication of nanostructured TiNb2O7 materials.15−18 Although the use of nanostructures has led to some improvement in the electrochemical performance of TiNb2O7, there is undoubtedly still a possibility for great enhancement considering that the average size of TiNb2O7 particles in previous studies was in the order of 100−200 nm. Because TiNb2O7 crystals are formed above 900 °C, primary particles inevitably grow to quite a large size, thus resulting in a collapse of the nanostructure and a poor rate capability. In other words, in order to achieve a high rate capability and capacity of TiNb2O7 anodes, it is crucial to fabricate nanostructures composed of small-sized nanocrystals that provide an interconnected pore structure. We have therefore endeavored to synthesize eﬀective nanostructured electrode materials that remain constructed of small nanocrystals, even after high temperature heat-treatment. Speciﬁcally, we report the synthesis of an ordered mesoporous TiNb2O7 (denoted as m-TNO) through a block copolymer (BCP)-assisted, one-pot assembly method along with its successful application as a high capacity and high rate anode in LIBs. Although mesoporous structured multimetallic oxides for LIB application have been previously reported,21−24 most such synthesis has been limited to the tedious hard-template method in which pore structures are only limited by the
RESULTS AND DISCUSSION
Synthesis of m-TNO. As illustrated in Figure 1a, all precursors, including hydrolyzed titanium isopropoxide and niobium ethoxide, and the structure-directing agent poly(ethylene oxide)-b-poly(styrene) (PEO-b-PS) prepared by atomic transfer radical polymerization (ATRP),30 are added into an organic solvent. A small amount of tetraethyl orthosilicate (TEOS) is also added to improve the thermal and mechanical stability during heat treatment. During the B
evaporation-induced self-assembly (EISA) process, the mixture constructs an ordered mesoporous structure.31−33 Finally, the m-TNO is synthesized after the heat-treatment followed by a silica etching process. The central points to establish ordered m-TNO constructed of nanocrystals are controlled sol−gel chemistry of multicomponent metal alkoxides and selective incorporation of those inorganic species within microdomains of BCP. The formation mechanism of m-TNO is illustrated in Figure 1b. Generally, transition metal alkoxide precursors are reactive toward hydrolysis and condensation, which often results in the formation of highly aggregated, macrophase-separated particles. This becomes a severe issue, particularly in the multicomponent metal alkoxide mixtures, due to their diﬀerent growth kinetics and interactions. Thus, here we employed a highly acidic condition using concentrated (35−37%) HCl. To begin with, the controlled amount of 35% HCl ([H2O]/ [M(OR)x] > 2) is added into a mixture of Ti and Nb-alkoxide with a molar ratio of 1:2. The high acidity accelerates the hydrolysis of metal alkoxides, but it considerably inhibits inorganic condensation, balancing their reaction kinetics to mix them homogeneously in nanoscale.34,35 This step allows one to prepare small hydrophilic entities (Ti, Nb sol) that can be mixed favorably with a hydrophilic block of BCP. The average size of the resulting Ti and Nb sol was in the range of 2 to 4 nm, determined by dynamic light scattering analysis (DLS, Figure S1, Supporting Information), which is below the size of the PEO block used in this work (7.5 nm). It should be noted that the size of the sol−gel derived nanoparticles should be smaller than that of the block they mix with; otherwise, the particles are segregated from BCP domains, resulting in macroscopic separation.36 Next, the preformed hydrophilic sols consisting of positively charged Ti and Nb entities in water are transferred to the TEOS/PEO-b-PS/THF solution. Ti and Nb entities selectively interact with the PEO block via Cl− mediation under acidic conditions (S0H+X−I+ interaction; S, surfactant; X, anion; I, inorganic framework) and hydrogen bonding. During the subsequent EISA process, the mixture is self-organized into a highly ordered nanostructure (as-made TNO, Figure 2a).37−40 The strongly segregating PEO-b-PS is advantageous to form ordered mesostructures, achieving a sharp interface between the PEO/precursors and the PS block. The ordered mesoporous structures of the TNO samples were veriﬁed by microscopy. The transmission electron microscope (TEM) image of as-made TNO (Figure 2a) shows a highly ordered hexagonal arrangement of the mesopores. This structure is well preserved after heat-treatment at 900 °C, as conﬁrmed by TEM (denoted as m-TNO-SiO2; Figure 2b). The electron energy loss spectroscopy (EELS) images of m-TNO-SiO2 shows that all metal species are uniformly distributed inside the walls (Figure 2c to f). This distribution indicates that the mesoporous wall is composed of TNO crystals and SiO2 nanoparticles, which is supported by HR-TEM imaging (Figure S2, Supporting Information). Before TEM analysis, the m-TNO-SiO2 sample was sectioned at a thickness of 100 nm using a SIMS powertome XL. Therefore, all metal species can be viewed overlapped in mapping images because the nanometer sized TNO crystals and amorphous SiO2 are well dispersed inside the nanowall. The result of scanning electron microscopy (SEM) also reveals a clear image of a hexagonal 2-D mesoporous structure (Figure 2g; more images can be found in Figure S3, Supporting
Figure 2. Electron microscopy images. (a,b) TEM images of (a) asmade TNO and (b) m-TNO-SiO2. (c,d,e,f) EELS mapping images of the (d) Nb, (e) Ti, and (f) Si. (g,h) SEM images of (g) m-TNO-SiO2 and (h) m-TNO after silica etching.
Information). Structural collapse and formation of aggregated particles were not observed, which indicates that the TNO nanocrystals were developed inside the conﬁned space as directed by PEO-b-PS self-assembly. The resulting pore size and wall thickness of the m-TNO-SiO2 was determined to be 35 and 10 nm, respectively, from both TEM and SEM images. Considering that SiO2 is not an active material for LIBs, the SiO2 present (∼15 wt%) should ideally be removed prior to use as an anode. Although a less ordered 2-D hexagonal array is obtained after SiO2 etching (Figures 2h and S4, Supporting Information), a few nanometer-sized TNO nanoparticles in a porous matrix and interconnected structures are well preserved. This process would be beneﬁcial because it increases the surface area and creates a few nanometer sized pores generated from the removal of the amorphous SiO2 part, resulting in a high electrode/electrolyte interface area and easy electrolyte penetration (more images in Figure S4, Supporting Information). Figure S5 (Supporting Information) shows the energydispersive X-ray spectroscopy (EDX) spectra before and after silica removal. The atomic ratio of Nb:Ti:Si was 2:1:1 in mTNO-SiO2. After SiO2 etching, the EDX spectrum shows negligible Si content as conﬁrmed in Figure S5b (Supporting Information). The negligible Si residue indicates that most of the Si species stay in oxide form in the nanowall and that the silica was completely removed during etching process. Figure 3a shows the nitrogen physisorption graphs of mTNO-SiO2 and m-TNO. Both samples exhibit typical type-IV adsorption and desorption curves with a sharp capillary condensation step at 0.9−0.95 P/P0, indicating the formation of uniform and large pores. After silica removal, the m-TNO has a Brunauer−Emmett−Teller (BET) surface area of 74 m2 g−1 and pore volume of 0.37 cm3 g−1, an increase over those of m-TNO-SiO2 (surface area, 37 m2 g−1; pore volume, 0.20 cm3 g−1). The pore size of m-TNO-SiO2 was 33 nm as estimated by the Barrett−Joyner−Halenda (BJH) method, which is coC
Figure 3. (a) N2 physisorption isotherms and (b) BJH pore size distribution curves of m-TNO-SiO2 and m-TNO. (c) SAXS patterns of as-made TNO (black line), m-TNO-SiO2 (red line), and m-TNO (blue line). (d) Powder XRD patterns of m-TNO and nano-TNO (JCPDS#: 77-1374).
Figure 4. (a) Galvanostatic charge−discharge curves (initial 3 cycles) of m-TNO and bulk-TNO electrodes at 0.1 C rate conditions (= 38.7 mA g−1). (b) Cycle and rate performance plots and (c) capacity retention plots of m-TNO and bulk-TNO electrodes obtained at various current densities, 0.1 to 50 C-rate. (d) Capacity retention of various TNO electrodes and other titanium- and niobium-based electrodes as a function of rate (electrospunTiNb2O7,18 Ti2Nb10O29,19 mesoporous Li4Ti5O12-carbon,12 mesoporous TiO2-graphene,47 carbon-coated Li4Ti5O12,11 Cr-doped Li4Ti5O12,48 mesoporous TiO2(B),49 and TiO2 nanodisk45).
incident with the results of microscopy (Figure 3b). The pore size distribution of m-TNO presents a 40 nm peak with a few nanosized pores, the result of eliminating the SiO2 particles.
These structural characterizations are further supported by the small-angle X-ray scattering (SAXS) patterns of the TNO samples, as shown in Figure 3c. Several scattering peaks of asD
second cycle, the m-TNO electrode demonstrates stable lithium insertion and extraction performance; the bulk-TNO electrode exhibits smaller Li+ insertion/extraction values of 246 and 228 mA h g−1, respectively. Although a TNO phase in the m-TNO begins to form around 700−750 °C as shown in Figure S8a (Supporting Information), the m-TNOs still exhibit lower reversible capacities and Coulombic eﬃciencies as the synthesis temperature is reduced (Figure S8b, Supporting Information), which indicates that the high crystallinity of TNO is important and that the BCP self-assembly method is a powerful tool for producing TNO nanomaterials with high crystallinity. The high-rate performance of the m-TNO electrode is illustrated in Figure 4b. Under 0.1 to 50 C rates, the electrode shows stable Li+ insertion/extraction capacities (charge− discharge proﬁles are in Figure S9, Supporting Information). The reversible capacity of the m-TNO electrode was reduced from 289 to 215 and 190 mA h g−1 at 5 and 10 C rates, respectively. The capacity value at 10 C is clearly superior to the theoretical capacities of Li4Ti5O12 and anatase TiO2 (175 and 168 mA h g−1). Moreover, at a 20 C rate, the reversible capacity of 162 mA h g−1 is much higher than the best value obtained in previous studies (∼63 mA h g−1 at 20 C).18 Even at a 50 C rate (= 19.35 A g−1), the m-TNO electrode exhibits a reversible capacity of 116 mA h g−1. Furthermore, the m-TNO electrode also exhibits stable cycle performance at diﬀerent charging rates. When the current density is set at a 2 C rate, the reversible capacity is well maintained without capacity loss (244 mA h g−1). In comparison, all capacity values of the m-TNO electrode surpass those of the bulk-TNO electrode. This diﬀerence in the rate capability of the electrodes is quite evident in Figure 4c. About 65 and 40% of the maximum capacity of mTNO electrodes are at 10 and 50 C, whereas there are just 41 and 6% retention in the bulk-TNO cell under similar conditions. In Figure S10 (Supporting Information), the electrochemical performance of a sol−gel derived nano-TNO electrode shows a high reversible capacity of 263 mA h g−1 at 0.1 C but suﬀers severe capacity decay under high rate conditions, with 6% retention at a 50 C rate. Moreover, the mTNO electrode exhibits higher speciﬁc capacities and outstanding rate performance under diﬀerent current densities, compared to those of the state of the art TiO2, Li4Ti5O12, and TNO electrodes (Figure 4d). The excellent rate performance of the m-TNO electrode is favorable for use in high-power battery systems. For use as an anode, the long-term stability of any electrode material is so important that a cycle test was conducted to check the reversibility of the m-TNO electrode. This test was conducted after 10 cycles of charge−discharge to ensure stabilization. As shown in Figure 5a, the capacity of the mTNO/Li cell is somehow increased, reaching a maximum capacity of 208 mA h g−1 after just a few cycles at a 10 C rate. The m-TNO/Li cell was able to insert/extract Li ions corresponding to 100 mA h g−1 or 48% retention of its maximum capacity, after 2000 cycles. After the cycle test, the nanoparticles in m-TNO were somehow aggregated and broken oﬀ from the m-TNO particles (TEM images, Figure S11, Supporting Information). However, it is worthwhile to note that even though m-TNO suﬀered structural deformation upon cycling, the cycled electrode still exhibited interconnected porous networks as well as ordered mesostructures of initial mTNO to some extent. We additionally conducted ex-situ XRD analysis on the m-TNO electrode after the 2000th delithiation
made TNO and m-TNO-SiO2 with a peak position ratio of 1:31/2:41/2 indicate a highly ordered 2-D hexagonal mesoporous structure with a long-range order.41,42 The dspacing value (d100) was changed from 37.2 nm (as-made TNO) to 34.9 nm (mTNO-SiO2) after heat treatment due to thermal shrinkage. The SAXS pattern of m-TNO exhibits two peaks with a 1:31/2 ratio, indicating that the m-TNO retains a 2-D hexagonal ordered mesoporous structure even after the SiO2 removal process. The powder X-ray diﬀraction (XRD) patterns shown in Figure 3d coincide with those of monoclinic TiNb2O7 (space group: C2/m, JCPDS#: 77-1374). The TNO crystals developed in conﬁned walls show broad peaks, despite being calcined at 900 °C. The average particle size is about 15 nm, as calculated from the Debye−Scherrer equation.43 Previous studies have reported that small nanocrystals facilitate fast lithium insertion/ extraction kinetics, resulting in a superior rate performance.44,45 However, TNO synthesized by the calcination of Ti and Nb sol without the addition of PEO-b-PS and TEOS shows sharp peaks, corresponding to an average crystal size of 49 nm (denoted as nano-TNO). These results indicate that both the BCP and silica source perform a key role in the development of nanosized TNO particles. Overall, the structural characterizations of the TNO samples support the successful synthesis of an ordered mesoporous structure with large pores (∼40 nm) constructed of small TNO nanocrystals (∼15 nm). This resulting structure is ideal for facilitating the transport of electrolyte and the fast diﬀusion of Li ions and electrons. Electrochemical Performance. The electrochemical performance of m-TNO as a LIB anode was investigated by a series of galvanostatic charge−discharge tests (Figure 4). Figure 4a shows the discharge (lithiation) and charge (delithiation) curves of m-TNO/Li and bulk-TNO/Li cells at potentials ranging from 1.0 to 3.0 V with a charging rate of 0.1 C (= 38.7 mA g−1). This voltage range protects the electrodes from SEI formation. Following the ﬁrst discharge, from 3.0 to 2.0 V, Li ions are inserted into m-TNO with a capacity contribution of 19 mA h g−1 (0.25 Li+); this observation being prominent only in the ﬁrst cycle. This Li+ insertion process, not found in previous studies, is likely attributed to the high surface area and small nanocrystals of its structure.44−46 In contrast, the control LIB cell with a bulk-TNO electrode (produced via solid-state reaction; Figure S6, Supporting Information) revealed a diﬀerent discharge proﬁle with a steep voltage drop from OCV to 2.0 V during the ﬁrst cycle (Figure 4a). At less than 2.0 V, the Li+ insertion curve shows sloped voltage proﬁles with a short plateau near 1.7 V. The cyclic voltammetry graphs provide further information concerning the Li+ insertion reaction of TNO electrodes (Figure S7, Supporting Information). The ﬁrst CV curve of the m-TNO electrode exhibits a weak cathodic peak at 2.5 V, coinciding with its initial discharge proﬁle (Figure S7a, Supporting Information). Some cathodic/ anodic peaks at 1.73/2.0 V, 1.56/1.73, and 1.0−1.3 V may represent the multiple redox reactions of Ti4+/Ti3+, Nb5+/Nb4+, and Nb4+/Nb3+, respectively.17,18 In comparison with the bulkTNO electrode (Figure S7b, Supporting Information), the peak intensities get smaller, and the CV curve broadens when an mTNO electrode is used. Such broad CV curves have been reported in previous nanostructured electrode studies and may originate from the increased charge contribution of surfaces and interfaces in nanomaterials.50 The ﬁrst discharge and charge capacities are 366 and 289 mA h g−1 (4.7 and 3.7 Li+ per 1 formula unit), respectively; the Coulombic eﬃciency of 86% is a typical feature of nanosized electrode materials. From the E
current densities is shown in Figure S15 (Supporting Information). During initial cycles, the reversible capacity of the cell drops signiﬁcantly. This decline may originate from Li+ trapping inside the m-TNO electrode, an imbalance of the anode/cathode loading ratio, or the sluggish kinetics of the positive electrode (Figure S15, Supporting Information). Although the reversible capacity declined from the ﬁrst cycle (190 mA h g −1), the cell performance is nonetheless gradually stabilized. During 1600 cycles under a 10 C rate (= 3.87 A g−1), the reversible capacity is reduced by only 15 mA h g−1 (0.009 mA h g−1 loss per cycle). The cycle performance of a m-TNObased full-cell could be potentially further improved by optimizing the anode/cathode mass ratio and pre-Li treatment.
CONCLUSIONS In conclusion, we successfully synthesized large-pore-sized mesoporous TiNb2O7 with small-sized nanocrystals in its walls by employing a BCP-assisted, simple one-pot method. The Ti and Nb sol were conﬁned in nanospaces formed by the segregation between the PS blocks and the PEO/precursors. These precursors are converted to nanosized building blocks, which then construct highly ordered mesoporous structures. The m-TNO electrode exhibited a high reversible capacity of 289 mA h g−1 and an excellent rate performance (40% retention at a 50 C rate), both of which are attributed to the small crystallites, high surface area, and large mesopores (∼40 nm). This performance goes far beyond the observed performance of TiNb2O7 anodes and Ti- and Nb-based electrodes (see Table S1 in Supporting Information). For the fabrication of a stable and high-capacity full-cell, potential optimization processes will be the subject of future reports. On the basis of the current study, it seems highly likely that an optimized m-TNO electrode will meet the power performance requirements of various ﬁelds, including EVs and ESSs. Moreover, the synthetic method used in this work could be extended to other electrode materials that require a high synthesis temperature.
Figure 5. Cycle performance of (a) m-TNO/Li and m-TNO-C/Li cells at the 10 C rate and (b) the m-TNO/LiFePO4 cell (0.2 Cm‑TNO × 5, 1 C × 50, 2 C × 100, 5 C × 500, 10 C × 1600 cycles, 1.0 to 2.55 V range). Capacities are calculated based on m-TNO mass.
process (Figure S12, Supporting Information). The monoclinic phase of the m-TNO electrode was not changed during the cycles (at 10 C). The XRD result presents that the structural stability of the TNO phase also inﬂuences the maintenance of m-TNO structure. The cycle performance of m-TNO can be enhanced by surface modiﬁcation processes, as conﬁrmed in previous studies.51−56 Typically, a carbon layer enhances the surface stability and electrical conductivity of a material, as well as the electrical contact of particles. Therefore, a carbon layer was applied to m-TNO by coating with polydopamine and then carbonizing at 700 °C (denoted as m-TNO-C);57 the added carbon content being conﬁrmed as 10 wt % (TGA in Figure S13a, Supporting Information). As expected, the cycle stability was drastically improved by this carbon coating process. The mTNO-C electrode exhibits a higher maximum capacity of 235 mA h g−1 at 10 C (208 mA h g−1 in m-TNO electrode) and excellent cyclability with 191 mA h g−1 after 2000 cycles (81% retention, 0.022 mA h g−1 loss per cycle). Figure S14a (Supporting Information) shows the reversible charge− discharge curves for the m-TNO cell up to 2000 cycles. Corresponding dQ/dV plots showed that Li intercalation and deintercalation appeared at ∼1.6 V and ∼1.75 V, respectively (Figure S14b, Supporting Information). These data strongly supported the fact that Li+ insertion/extraction processes are well-maintained even at 10 C for 2000 cycles. The high reversible capacity and superior cycle performance of both mTNO and m-TNO-C materials are a very impressive achievement toward stable anode materials having a high Li+ insertion potential (>1.0 V vs Li/Li+). Figure 5b shows the capacity versus cycle data of a m-TNO/ LiFePO4 cell, in which the mass ratio of LiFePO4/m-TNO is ∼2. The capacity plot of the LiFePO4/Li cell at diﬀerent
S Supporting Information *
Experimental procedures and characterizations, TGA, XRD data, electrochemical data (cyclic voltammetry, galvanostatic charge−discharge curves), and ex-situ TEM and XRD. This material is available free of charge via the Internet at http:// pubs.acs.org.
The authors declare no competing ﬁnancial interest.
ACKNOWLEDGMENTS This work was supported by the Global Frontier R&D Program on Center for Multiscale Energy System under the Ministry of Education. This work was further supported by National Research Foundation of Korea (NRF2012R1A2A2A01002879) and Basic Science Research Program (NRF-2013R1A1A2074550) by the Ministry of Science, ICT and Future Planning. This work was further supported by Defense Acquisition Program Administration and Agency for Defense Development under the contract UD 110090GD, a F
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grant of the Korea Health 21 R&D Project of Ministry of Health & Welfare (A121631), and by a part of the project titled “Technology Development of Marine Industrial Biomaterials,” funded by the Ministry of Oceans and Fisheries, Korea.
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