CHEMSUSCHEM FULL PAPERS DOI: 10.1002/cssc.201300461

Coaxial Carbon/Metal Oxide/Aligned Carbon Nanotube Arrays as High-Performance Anodes for Lithium Ion Batteries Fengliu Lou,[a] Haitao Zhou,[a] Trung Dung Tran,[b] Marthe Emelie Melandsø Buan,[a] Fride Vullum-Bruer,[c] Magnus Rønning,[a] John Charles Walmsley,[b, d] and De Chen*[a] Coaxial carbon/metal oxide/aligned carbon nanotube (ACNT) arrays over stainless-steel foil are reported as high-performance binder-free anodes for lithium ion batteries. The coaxial arrays were prepared by growth of ACNTs over stainless-steel foil followed by coating with metal oxide and carbon. The carbon/ manganese oxide/ACNT arrays can deliver an initial capacity of 738 mAh g1 with 99.9 % capacity retention up to 100 cycles and a capacity of 374 mAh g1 at a high current density of 6000 mA g1. The external carbon layer was recognized as a key component for high performance, and the mechanism of performance enhancement was investigated by electrochemi-

cal impedance spectroscopy, electron microscopy, and X-ray diffraction analysis. The layer increases rate capability by enhancing electrical conductivity and maintaining a low masstransfer resistance and also improves cyclic stability by avoiding aggregation of metal-oxide particles and stabilizing the solid electrolyte interface. The resultant principle of rational electrode design was applied to an iron oxide-based system, and similar improvements were found. These coaxial nanotube arrays present a promising strategy for the rational design of high-performance binder-free anodes for lithium ion batteries.

Introduction Lithium ion batteries have attracted widespread interest since they were commercialized in 1991 due to their higher specific energy relative to nickel–metal and lead–acid batteries. However, their specific power remains too low to power hybrid and full electric vehicles. The specific energy needs to be improved further to meet the requirements for future applications.[1–6] 3D nanomaterials could be a new approach to enhance the specific power by tuning the pore structure for fast mass transfer of the electrolyte and reducing electron and lithium ion dif[a] Dr. F. Lou, Dr. H. Zhou, M. E. Melandsø Buan, Prof. M. Rønning, Prof. D. Chen Department of Chemical Engineering Norwegian University of Science and Technology Sem Sælands vei 4, 7491 Trondheim (Norway) Fax: (+ 47)73595047 E-mail: [email protected] Homepage: http://www.nt.ntnu.no/users/chen/index.htm [b] Dr. T. D. Tran, Prof. J. C. Walmsley Department of Physics Norwegian University of Science and Technology Høgskoleringen 5, 7491 Trondheim (Norway) [c] Prof. F. Vullum-Bruer Department of Materials Science and Engineering Norwegian University of Science and Technology Sem Sælands vei 12, 7491 Trondheim (Norway) [d] Prof. J. C. Walmsley SINTEF Materials and Chemistry Høgskoleringen 5, 7465 Trondheim (Norway) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201300461. Part of a Special Issue on “The Chemistry of Energy Conversion and Storage“. To view the complete issue, visit: http://onlinelibrary.wiley.com/doi/10.1002/cssc.v7.5/issuetoc.

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fusion distances and improve the specific energy by fully utilizing the electrochemically active materials and minimizing overpotentials.[7, 8] Porous 3D carbon materials have gained increasing attention as electrodes or support materials for electrodes, such as nanotube arrays,[9–11] nanotube films,[12, 13] nanosheet arrays,[14, 15] nanosheet films,[16, 17] and porous materials.[18, 19] In our previous work, aligned carbon nanotubes COVER (ACNTs) were grown on Al foil to generate a 3D current collector with high electrical conductivity, regular channels, and large surface area. Another thin film of conducting polymer or metal oxide was coated onto the nanotubes as an extra electrochemically active phase to store charge for supercapacitors or lithium ion battery cathodes.[20, 21] ACNTs on Al foil serve as a highly conductive and large-surface-area current collector with regular channels to improve electronic conductivity, reduce the thickness of active materials, and facilitate electrolyte transport. The thin active film coated on ACNTs has a large surface area, hence increasing the accessible area of electrolyte to the active materials and decreasing the charge-diffusion distance in the active materials. These features resulted in a significant improvement of specific power of the electrodes for energy storage. The conducting polymer or metal oxide thin film was employed as an extra ChemSusChem 2014, 7, 1335 – 1346

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electrochemically active phase to store the energy that makes at low potential. Coaxial MnO2/CNT arrays have also been prethe whole material electrochemically active and enables the pared by deposition of MnO2 and carbon inside porous alumihigh specific energy for the electrodes. The advantages of cona templates as anodes for lithium ion batteries.[24] However, axial array electrodes have been confirmed by good electrocyclic stability is poor, which indicates the ACNT–metal oxide chemical performance.[20, 21] Herein, we explored the coaxial core–shell arrays do not provide sufficient stability for highperformance lithium ion battery anodes. multilayer-array structures as anodes for lithium ion batteries Herein, we report on the synthesis and characterization of aiming to achieve high specific capacity and improve cyclic coaxial carbon/metal oxide/ACNT arrays over stainless-steel foil stability and rate capability. as binder-free anodes for lithium ion batteries. Well-aligned Transition-metal oxides have been investigated intensively CNTs were grown directly over stainless-steel foil through as anode materials for lithium ion batteries since the pioneera one-step floating catalyst method, followed by metal oxide ing work of Tarascon et al.[22] due to their higher specific cacoating by spontaneous deposition, and carbon coating by pacity relative to graphite.[22–26] The mechanism of lithium ion chemical vapor deposition (Scheme 1). The coaxial carbon/ storage in metal oxides differs from classical lithium insertion– manganese oxide/ACNTs on stainless-steel foil were investigatextraction or lithium alloying processes. It involves the formaed as anodes for lithium ion batteries. The new electrode has tion and decomposition of Li2O, accompanying the reduction and oxidation of metal oxides.[22] Only nanosized metal oxides have high performance due to the fact that the extraction of lithium from bulk Li2O is thermodynamically unfavorable.[22, 27] Therefore, the preparation of stable nanosized metal oxides is a challenge for their high electrochemical performance. Various kinds of metal oxides, such as MnO,[28–30] Mn3O4,[31, 32] Mn2O3,[32] MnO2,[24, 33–35] Fe3O4,[36] [37–40] [41, 42] [22] [22] Fe2O3, SnO2, CoO, and Cu2O, have been investigated as anode materials for lithium ion batteries. Manganese oxide and iron oxide were selected Scheme 1. Fabrication of C/MOy/ACNT arrays over stainless-steel foil. as active materials herein due to their high theoretical capacity (1233 and 1005 mAh g1 for MnO2 and unique features, such as a 3D sandwich structure, high porosiFe2O3, respectively), wide availability, low cost, and environty with regular channels, a thin layer of active materials, and mental friendliness. However, metal oxide anode materials high conductivity, which provide high specific capacity and suffer from poor lithium ion storage performance due to their rate capability, as well as good cyclic stability. The influence of low electrical conductivity, strain caused by pulverization, disthe external carbon layer on the increased rate capability and solution into the electrolyte, or aggregation. One strategy to cyclic stability is systematically investigated. The principles for enhance electrical conductivity and accommodate strain is to rational electrode design are proposed, which is further concoat the metal oxide thin film onto highly conductive carbon firmed by the coaxial carbon/iron oxide/ACNT arrays as anodes nanomaterials. In previous work, metal oxides were deposited for lithium ion batteries. onto carbon nanotubes (CNTs),[26, 33] carbon nanofibers,[43, 44] carbon nanosprings,[45] graphene,[28, 46, 47] carbon nanohorns,[35] and mesoporous carbon,[31] etc. as anodes for lithium ion batteries. Some promising results, in terms of specific capacity, Results and Discussion have been obtained,[48] but the cyclic stabilities and rate capaACNTs on stainless-steel foils bilities reported are relatively poor.[33, 44] The coating of another carbon layer over the surface of metal oxides could prevent Copper foil has been widely used as an anode current collector dissolution and aggregation. Some metal oxides coated with for lithium ion batteries due to its high stability at low potencarbon have previously been investigated as anodes for lithitials. However, it is difficult to grow ACNTs directly over copper um ion batteries.[29, 49, 50] It was found that the cyclic stability foil because of its high surface-tension energy.[54] Stainless steel [26] was improved after carbon coating. However, the detailed is chemically and thermally stable relative to copper,[14] so it mechanisms behind performance improvement are rarely inwas selected as a conductive substrate for the growth of vestigated. ACNTs herein. ACNTs have been synthesized on stainless steel Metal oxides have been coated over ACNTs by different previously.[55–57] However, either a buffer layer[55, 57] or surface methods (e.g., electrochemical deposition or spontaneous depmodification[56] was necessary. A one-step floating catalyst [51–53] [20] osition) for supercapacitors method was employed for the growth of ACNTs herein, which and lithium ion batteries. simplified the process. ACNTs had lengths reaching up to That binder-free electrode shows high rate capability and long about 50 mm after a 30 min floating catalyst process (see the life cycle due to its unique structure. However, in previous Supporting Information, Figure S1). The yield of ACNTs was in studies ACNTs were grown either on rare transition metal Ta the range of 0.5–0.8 mg cm2 ; no disordered carbon was obfoil, Si wafers or Al foil. None of these are suitable as anodes for lithium ion batteries owing to either high cost or instability served, indicating that high-quality ACNTs were synthesized by  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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www.chemsuschem.org sults indicated that the ACNT–MnOx core–shell arrays were successfully synthesized on stainless-steel foil. The microstructure and morphology of C/MnOy/ACNTs obtained by additional carbon coating of MnOx/ACNTs are presented in Figure 1 e and f, respectively. The MnOx thin film transformed to discrete nanoparticles coated on the surface of the CNTs due to the high-temperature treatment during carbon coating. MnOy particles coated the CNTs uniformly with particle sizes in the range of 10–20 nm (Figure 1 e). MnOy and the outer carbon layer constitute about 72 and 5 wt %, respectively, in C/MnOy/ ACNTs. To investigate the distribution of MnOx over CNTs before the final carbon coating, electron energy loss spectroscopy (EELS) was carried out on a single MnOx coated CNT (Figure 2 b). Mn was detected over the surface of the tube, whereas C could be found mainly in the center of the tube, which demonstrated the CNT–MnOx core–shell structure. EELS analysis was also carried out to investigate the external carbon coating over MnOy after the final stage of processing. Elemental mapping (Fig-

Figure 1. a) TEM and b) SEM images of ACNTs; c, e) TEM and d, f) STEM images of MnOx/ACNTs (c and d), and C/MnOy/ACNTs (e and f).

the one-step floating catalyst method (Figure 1 a). The CNTs obtained also showed good alignment with similar length and orientation (see the Supporting Information, Figure S1 and Figure 1 b). The presence of mesoporous and regular channels formed between the CNTs could be observed. The mesoporous channels could improve mass transfer of the electrolyte, thereby improving the rate capability of the electrodes. Physiochemical properties of MnOx/ACNTs and C/MnOy/ ACNTs Figure 1 c and d present the TEM and STEM images of MnOx coated ACNTs, respectively, obtained through the spontaneous reduction of potassium permanganate by carbon; MnOx refers to the manganese oxide dried at 100 8C, whereas MnOy refers to the manganese oxide treated at 400 8C. The loading of MnOx was about 2 mg cm2, which resulted in a MnOx content in the range of 75–80 wt % of the total mass of the electrode. A uniform and thin MnOx film with a thickness of several nanometers was coated over the entire surface of CNTs. The MnOx layer was porous and had a high surface roughness. The re 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 2. a) STEM image of MnOx/ACNTs and b) the corresponding EELS elemental mapping: Mn in blue and C in red; c) STEM image of C/MnOy/ACNTs and d) the corresponding EELS elemental mapping: Mn in red and C in green; note that the superimposition of Mn (red) and C (green) results in yellow.

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Figure 3. a) XRD patterns and b) XPS high-energy resolution spectra of Mn 2p of the obtained materials.

ure 2 d) was acquired from the area marked by a green rectangle in the STEM image of a single CNT (Figure 2 c). The presence of a carbon layer (in green) coated over the MnOy nanoparticles was clearly observed after chemical vapor deposition in an acetylene atmosphere. Hence, the carbon/MnOy/carbon sandwich structure was clearly demonstrated. The crystallographic phases were studied by XRD analysis (Figure 3 a). Uncoated CNTs show a (002) plane at 2q = 26.28, indicating that ACNTs are highly graphitized. Another three weak and broad peaks are observed at 2q = 12.4, 37.1, and 65.88 after MnOx coating, indicating that a K-birnessite-type manganese oxide (JCPDS 42-1317) is present. This is consistent with our previous report and the literature.[20, 58] K ions will convert into K2O during initial lithiation and will not be reduced to metallic K due to the high KO bond energy.[59] Weak and broad diffraction peaks suggest low crystallinity or small size of MnOx. The XRD pattern of C/MnOy/ACNTs shows the presence of MnO (JCPDS 002-1158). A small peak at 2q = 32.38 in addition to the presence of what might look like a shoulder for two of the MnO peaks at 36.2 and 59.68, might also indicate that there is some Mn3O4 (JCPDS 01-070-9110) in the sample. This composition is the result of MnOx reduction by CNTs or acetylene during carbon coating at high temperature, leading to the decrease in the average oxidation state of Mn. Thus the theoretical capacity of manganese oxide would be reduced

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www.chemsuschem.org after carbon coating based on the reaction mechanism between manganese oxide and lithium. X-ray photoelectron spectroscopy (XPS) was employed to investigate the surface chemical state. The Mn 2p peak was examined at high-energy resolution (Figure 3 b). The two peaks located at 642.1 and 653.8 eV for MnOx/ACNTs correspond to Mn 2p3/2 and Mn 2p1/2, respectively, with a spin-energy separation of 11.7 eV. These values are consistent with those reported for MnO2.[33] Fitted XPS spectra of Mn 2p3/2 (see the Supporting Information, Figure S2 a) can be referenced to literature values of 640.8 eV for Mn2 + , 642.0 eV for Mn3 + , and 642.6 eV for Mn4 + , with a ratio of about 1:3:16.[60, 61] Based on XPS results, the average oxidation state of Mn in MnOx/ACNTs is 3.75 + . The Mn 2p3/2 and Mn 2p1/2 peaks located at 641.4 and 653.4 eV for C/MnOy/ACNTs in Figure 3 b are characteristic of a mixture of MnO and Mn3O4, which might indicate that the weak diffraction lines in the X-ray diffractogram indeed is caused by Mn3O4.[60] The fitted XPS spectra of Mn 2p3/2 (see the Supporting Information, Figure S2 b) can be referenced to literature values of 640.8 eV for Mn2 + and 642.1 eV for Mn3 + , with a ratio of about 1:1. Accordingly, the average oxidation state of Mn in this sample is 2.5 + . XPS results demonstrate that MnOx was reduced partially during carbon coating at high temperature, which could result in the reduction of lithium ion storage capacity of manganese oxide. Thermogravimetric analysis was carried out to investigate the effect of carbon coating on thermal properties (Figure 4). The weight loss of MnOx/ACNTs occurred in several steps,

Figure 4. Thermogravimetric analysis curves of the obtained materials.

which began with the loss of adsorbed and crystalline water from 100 to 300 8C (see the Supporting Information, Figure S3) and was followed by the reduction of MnOx by CNTs. For C/ MnOy/ACNTs only a small weight loss was observed when the temperature reached 650 8C, which indicated that water has been removed during the carbon coating process because of the collapse of the MnOx layered structure. That was confirmed by mass spectroscopy results (see the Supporting Information, Figure S3), in which water was not detected after carbon coating. The elimination of water would reduce unwanted side reactions in the battery. These side reactions could cause instabilities in the solid electrolyte interphase (SEI) and the dissoluChemSusChem 2014, 7, 1335 – 1346

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CHEMSUSCHEM FULL PAPERS tion of manganese oxide.[62] Therefore, the cyclic stability of assembled batteries might be improved after carbon coating.

Electrochemical performance A comparative study of electrochemical performance between MnOx/ACNTs and C/MnOy/ACNTs was conducted. Performances were evaluated and compared in terms of specific capacity, Coulombic efficiency, cyclic stability, and rate capability.

High specific capacity and initial Coulombic efficiency The voltage profiles of MnOx/ACNTs and C/MnOy/ACNTs charged and discharged at a current density of 60 mA g1 (0.05 C) between 0.01 and 3.0 V versus Li/Li + for the first two cycles are presented in Figure 5 a and b, respectively. In the first lithiation process of MnOx/ACNTs, the potential decreases quickly to 1.0 V versus Li/Li + due to the reduction of high-oxidation state Mn to MnII. This is followed by a sudden drop to a plateau at 0.45 V versus Li/Li + and then decreases continuously to 0.01 V versus Li/Li + , which can be attributed to the reduction of MnII to metallic Mn. In the first delithiation process, two plateaus are observed at 1.1 and 1.9 V versus Li/Li + , which can be ascribed to the oxidation of Mn from metallic Mn to MnII and from MnII to even higher oxidation states, respective-

Figure 5. Voltage profiles of a) MnOx/ACNTs and b) C/MnOy/ACNTs in the first two cycles; voltage cycled at a current density of 60 mA g1 (0.05 C) between 0.01 and 3.0 V versus Li/Li + ; capacity is calculated based on the total weight of both carbon and manganese oxide.

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www.chemsuschem.org ly.[34] The first lithiation and delithiation capacities are 1557 and 1044 mAh g1, respectively, based on the total weight of both CNTs and MnOx, with an initial Coulombic efficiency of 67 %. The capacities are higher relative to MnOx coated CNT networks (see the Supporting Information, Table S1), which demonstrates the advantages of aligned structures. The bare ACNT anode can only deliver a capacity of 228 mAh g1 (see the Supporting Information, Figure S4 a). Subtracting the possible contribution from ACNTs in MnOx/ACNTs (77 wt % MnOx), an initial capacity of 1292 mAh g1 can be attributed to the MnOx phase. That value is even higher than the theoretical capacity of MnO2, which is about 1233 mAh g1. This is likely due to the charge used for SEI formation or the introduction of defects on ACNTs. According to Chen et al.,[63] many defects are generated on the walls of CNTs during the reduction of potassium permanganate. The lithium ion storage capacity of CNTs increases significantly after the introduction of surface defects.[64] As a result, the capacity contribution from ACNTs in MnOx/ ACNTs is expected to be higher than the capacity of pure ACNTs. Notably, initial Coulombic efficiency is higher than reported literature values,[24, 27, 33, 34] which are generally about 50 % (see the Supporting Information, Table S1). The initial irreversibility can be attributed to either the partially irreversible manganese oxide conversion reaction with lithium or the formation of SEI.[34] In this regard, the high initial Coulombic efficiency of MnOx/ACNTs can be ascribed to the MnOx thin film coated over the highly conductive CNTs, which makes the conversion reaction between MnOx and lithium highly reversible. A plateau at 0.35 V versus Li/Li + is seen in the first lithiation process of C/MnOy/ACNTs, which is lower than that of MnOx/ ACNTs. The lower lithiation plateau indicates a large overpotential, which might be due to the increased crystallinity of manganese oxide during carbon coating.[32, 34] However, the difference disappears in the following lithiation process. The initial lithiation and delithiation capacities of C/MnOy/ACNTs are 993 and 738 mAh g1, respectively, with a Coulombic efficiency of 74 %. Clearly, the capacity decreases after carbon coating, which can be attributed to the decrease of the average oxidation state of Mn, as measured by XRD and XPS. Initial Coulombic efficiency increases from 67 % to 74 % after carbon coating. As discussed above, the morphology, oxidation state, and water content of MnOx/ACNTs were changed during carbon coating. Therefore, the reason for the increase in initial Coulombic efficiency is not obvious. To isolate the effect of carbon coating, a reference sample of MnOy/ACNTs was prepared at similar conditions to the carbon coating process, but in hydrogen rather than acetylene. The properties of MnOy/ ACNTs, such as morphology and oxidation sate (see the Supporting Information, Figure S5) were similar to those of C/ MnOy/ACNTs, except for the outer carbon layer. A comparative study of these two samples makes it possible to elucidate the effects of carbon coating on lithium ion storage performance. The initial lithiation and delithiation capacities of MnOy/ ACNTs are 990 and 751 mAh g1, respectively (see the Supporting Information, Figure S6), resulting in a Coulombic efficiency of 75 %, which is similar to that of C/MnOy/ACNTs. The almost identical Coulombic efficiency regardless of carbon coating ChemSusChem 2014, 7, 1335 – 1346

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suggests that the carbon layer did not directly contribute to the improvement in Coulombic efficiency. According to XRD and XPS analyses, the oxidation states of Mn in MnOx/ACNTs and C/MnOy/ACNTs or MnOy/ACNTs are close to those of MnO2 and MnO, respectively. MnO has higher reaction reversibility with lithium than MnO2.[32] Therefore, the improved initial Coulombic efficiency of C/MnOy/ACNTs can be ascribed to improved reaction reversibility, which might be due to fewer phase transformations in the conversion reaction. The high specific capacity can be attributed to the manganese oxide thin film as well as the high conductivity of ACNTs, which enables access to all the electrochemically active material. The high initial Coulombic efficiency can be ascribed to the manganese oxide thin film, which facilitates high reversibility of the conversion reaction between manganese oxide and lithium. The improvement in initial Coulombic efficiency after carbon coating can be attributed to the decrease of the Mn oxidation state, which improves reaction reversibility. Improved stability by carbon coating and its mechanism Cyclic stability of MnOx/ACNTs, C/MnOy/ACNTs, and MnOy/ ACNTs was evaluated at a current density of 60 mA g1 (0.05 C) for the first two cycles and 600 mA g1 (0.5 C) for the following cycles between 0.01 and 3.0 V versus Li/Li + , as presented in Figure 6 a and b and Figure S6 b in the Supporting Information, respectively. The obvious capacity loss from the third cycle is ascribed to the increase of current densities from 60 (0.05 C) to 600 mA g1 (0.5 C). MnOx/ACNTs suffer from capacity fading after about 60 cycles. A capacity of 540 mAh g1 is achieved after 100 cycles, which corresponds to 51.8 % of the initial capacity. C/MnOy/ACNTs exhibit a gradual capacity increase up to about 60 cycles. With the purpose of elucidating influences of the external carbon layer on anode performance, a reference electrode, namely MnOy/ACNTs with properties similar to those of C/ MnOy/ACNTs, but without an external carbon layer, was investigated at the same conditions. The initial capacity of MnOy/ ACNTs is similar to that of C/MnOy/ACNTs, but suffers from continuous capacity fading (see the Supporting Information, Figure S6 b). It leads to a low capacity of 352 mAh g1 after 100 cycles, which is 46.9 % of the initial capacity. Therefore, the improved cyclic stability cannot simply be ascribed to the relatively low initial capacity of C/MnOy/ACNT. This gradual capacity-increase phenomenon has been observed for other MnOx based anode materials.[30, 34, 65, 66] The mechanism behind this phenomenon can be ascribed to the improvement in conversion-reaction reversibility, possibly due to reconstruction of MnOx during cycles,[34, 65] which has been confirmed by the reduction of charge-transfer resistance with cycling (Table 1). There is no obvious fading in capacity during the last 40 cycles and a high capacity of 737 mAh g1 is achieved after 100 cycles. It indicates a significantly improved cyclic stability resulting from the external carbon layer. This capacity is twice that of the theoretical capacity of graphite, which is used as an anode for commercial lithium ion batteries. Moreover, the cyclic stability of C/MnOy/ACNTs is better than what has been  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 6. Cyclic stability of a) MnOx/ACNTs and b) C/MnOy/ACNTs at a current density of 60 mA g1 (0.05 C) for the first two cycles and 600 mA g1 (0.5 C) for the following cycles between 0.01 and 3.0 V versus Li/Li + ; capacity is calculated based on the total weight of both carbon and manganese oxide.

Table 1. Impedance parameters fitted from equivalent circuit. Electrode

MnOx/ACNTs

C/MnOy/ACNTs

Cycles

Rs [W]

RSEI [W]

Rct [W]

5 50 100 5 50 100

9.9 10.8 15.6 3.5 3.3 3.5

23.4 27.3 30.2 20.6 21.9 22.0

28.7 29.6 47.2 11.8 5.8 7.6

achieved in published data on manganese oxide/CNT network structures (see the Supporting Information, Table S1). The average Coulombic efficiency of 100 cycles of C/MnOy/ ACNTs (98.1 %) is higher than that of MnOx/ACNTs (97.5 %) and MnOy/ACNTs (97.4 %), indicating less side reactions taking place after carbon coating. The presence of the carbon layer improves Coulombic efficiency by blocking liquid electrolyte penetration to the MnOy surface, which can partially suppress the formation of a new SEI film.[34] The lithium ion storage mechanism of manganese oxide can be simplified to the following reactions, assuming complete ChemSusChem 2014, 7, 1335 – 1346

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conversion of metal oxides:[22, 24, 27] MnOz þ 2z Liþ þ 2z e $ Mn þ z Li2 O

ð1Þ

2z Li $ 2z Liþ þ 2z e

ð2Þ

MnOz þ 2z Li $ Mn þ z Li2 O

ð3Þ

This process involves the reversible formation and decomposition of Li2O, accompanied by the reduction and oxidation of metal oxides. The reduction of manganese oxide by metallic lithium is thermodynamically feasible. However, the reverse reaction is only thermodynamically favorable for nanosized materials.[27] Accordingly, the preparation of stable nanosized manganese oxide is of critical importance for the synthesis of high-performance manganese oxide based anode materials. The microstructures and crystallographic phases of cycled materials were investigated by STEM, TEM (Figure 7), and XRD analyses (see the Supporting Information, Figure S7). The cycled electrodes were rinsed with diethyl carbonate to remove LiPF6 left on the samples. Aggregation is significant for MnOx/ACNTs and the mesoporous channels between CNTs are totally blocked by MnOx and bared CNTs are observed as well in Figure 7 a and c. According to XRD results, cycled materials contain Mn3O4, Mn2O3, and MnO2. The peak intensity of cycled MnOx/ACNTs is much stronger than that of C/MnOy/ACNTs, indicating that aggregation resulted in larger particle sizes. The lack of peaks in the C/MnOy/ACNT diffractogram might indicate amorphization of the material or reduction of particle size during cycling. The increase of particle size makes the extraction of lithium from Li2O difficult, which results in the fading of lithium ion storage capacity upon extended cycling. Moreover, the blocking of channels between CNTs increases the masstransfer resistance of the electrolyte, which results in the decrease of rate capability. However, C/MnOy/ACNTs are very

Figure 7. a, b) STEM and c, d) TEM images of MnOx/ACNTs (a and c) andC/ MnOy/ACNTs (b and d) after 100 cycles at 60 mA g1 (0.05 C) for the first two cycles and 600 mA g1 (0.5 C) for the following cycles between 0.01 and 3 V versus Li/Li + .

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stable and the MnOy layer and channel structure, as well as the external carbon layer are well preserved after extended cycling (Figure 7 b and d; also see the Supporting Information, Figure S8). Therefore, the aggregation of manganese oxide is prevented effectively by the external carbon layer. This could enable the reversible reduction and oxidation of manganese oxide without sintering, providing the high cyclic stability of C/ MnOy/ACNTs. In addition, it could maintain the low mass-transfer resistance, which enables the high rate capability. It is obvious that a significant aggregation occurred for MnOx/ACNTs during cycling. Solid-state reactions at the interface between two nanoparticles are typically involved in the aggregation. The volume of the MnOx film is expected to be enlarged significantly during the lithiation process due to its high lithium storage capacity. It seems that the volume of the film was enlarged to such an extent that an interface formed between the adjacent MnOx films. A slow solid-state reaction caused aggregation. After 100 cycles, large particles were formed from several pieces of the original MnOx thin film (Figure 7 c). Coating by a layer of a stable material, such as carbon, is a powerful strategy to suppress the formation of the interface between manganese oxide and thus prevent aggregation. In this regard, it is crucial to form a uniform carbon layer to cover the whole surface of the manganese oxide film. The chemical vapor-deposition carbon coating process used herein is clearly a suitable method to achieve this objective. Electrochemical impedance spectroscopy (EIS) is a powerful technique to determine the kinetic parameters of the electrochemical process on the electrode. EIS measurements were carried out by using a three-electrode cell with MnOx/ACNTs or C/MnOy/ACNTs, lithium foils, and lithium O-ring as working counter and reference electrodes, respectively. The fresh cells were cycled five times to form a stable SEI film and percolate electrolyte into the electrodes. The Nyquist plots obtained at 0.45 V versus Li/Li + during the lithiation process for MnOx/ ACNTs and C/MnOy/ACNTs after 5, 50, and 100 cycles are presented in Figure 8. The voltage of 0.45 V versus Li/Li + was selected because it is the lithiation-plateau potential of MnOx, at which charge-transfer resistance is minimal.[67] The Nyquist plots show two partially overlapped and suppressed semicircles at high and medium frequencies and an inclined line at low frequencies. The high-frequency semicircle reflects the resistance of lithium ion migration through the SEI film and the corresponding capacitance.[68] The medium-frequency semicircle reflects charge-transfer resistance and interfacial capacitance.[68] The inclined line at low frequencies is responsible for the solid-state lithium ion diffusion in the active materials.[68] Impedance data were analyzed by fitting an equivalent circuit (Figure 8 a, inset). It consists of an ohmic resistance (Rs), which represents the total resistance of the electrolyte, separator, electrode, and electrical contacts; a SEI-film resistance (RSEI), which is related to the lithium ion migration resistance through the SEI film; a charge-transfer resistance (Rct); a SEI capacitance (QSEI) and double-layer capacitance (Qdl), along with a diffusion component (QD). Constant-phase elements (CPE) instead of pure capacitances were used to fit SEI and doublelayer capacitances due to the observation of the suppressed ChemSusChem 2014, 7, 1335 – 1346

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www.chemsuschem.org extent of reaction. All EIS data were acquired at the same temperature and extent of reaction, so Rct was only affected by the specific surface area and activation energy. As can be found in Figure 7 a, aggregation is considerable for MnOx/ACNTs, which significantly reduces specific surface area, resulting in the increase of Rct. The decrease of Rct for C/MnOy/ACNTs with cycling might be attributed to the reduction of activation energy due to the formation of defects and deformation.[34] Therefore, the improved cyclic stability of manganese oxide after carbon coating can be ascribed to the following reasons: first, the presence of a carbon layer is effective in preventing the aggregation of manganese oxide, which enables the reversible conversion reaction between manganese oxide and lithium and provides the high capacity; second, the SEI formed on the outer carbon layer might be more stable and less resistant to charge transfer relative to the SEI formed on manganese oxide.

High rate capability In addition to high specific capacity and good cyclic stability, manganese oxide also exhibits good rate capability after carbon coating (Figure 9 a and b). C/MnOy/ACNTs show better

Figure 8. Electrochemical impedance spectra of a) MnOx/ACNTs and b) C/ MnOy/ACNTs measured at 0.45 V versus Li/Li + during lithiation after various cycles; the equivalent circuit used for curve fitting is shown in the inset in (a).

semicircles. The low-frequency region cannot be modelled well by a finite Warburg element, so a CPE was employed instead. Table 1 presents the equivalent circuit parameters for both MnOx/ACNTs and C/MnOy/ACNTs obtained from the simulation of experimental data after different cycles. Rs decreases after carbon coating, which can be attributed to the increase of electrical conductivity of the electrode because both electrolyte and separator are the same for the two systems. RSEI increases with the increase of cycle number for MnOx/ACNTs, whereas it remains almost constant for C/MnOy/ACNTs. It might be that the SEI forming on the carbon layer is more stable and exhibits lower resistance relative to the SEI forming directly on manganese oxide. However, the elimination of water during carbon coating will reduce side reactions, which most likely will also affect the composition of the SEI film. The Rct value of MnOx/ACNTs increases from 28.7 to 47.2 W after 100 cycles, whereas it decreases from 11.8 to 7.6 W for C/ MnOy/ACNTs. Rct can be expressed as: Rct ¼ R T=ðF A j0 Þ

ð4Þ

where R, T, F, A, and j0 are the gas constant, absolute temperature, Faraday constant, specific surface area, and exchange-current density, respectively; j0 is determined by the activation energy of the conversion reaction, the temperature, and the  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 9. a) Rate capability of obtained materials; b) voltage profiles of C/ MnOy/ACNTs at various current densities; capacity is calculated based on the total weight of both carbon and manganese oxide.

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CHEMSUSCHEM FULL PAPERS rate performance relative to MnOx/ACNTs, especially at high current densities. Notably, the capacity retention of C/MnOy/ ACNTs is about 50 % at a high current density of 6000 mA g1 (5 C), corresponding to a capacity of 374 mAh g1. This value is even higher than the theoretical capacity of graphite. However, the capacity retention of MnOx/ACNTs is only about 10 % at the same current density. The rate capability of C/MnOy/ACNTs is higher relative to previously reported values on manganese oxide/CNT coaxial structures (see the Supporting Information, Table S1). The good rate capability obtained for C/MnOy/ACNTs can be ascribed to its unique sandwich structure. ACNTs increase the electrical conductivity of the obtained electrodes and form regular mesoporous channels, which decrease mass-transfer resistance. The manganese oxide thin film decreases electron and lithium ion diffusion distances significantly. The outer carbon layer further increases electrical conductivity of the obtained electrode and prevents aggregation, which maintains the mesoporous channels between the CNTs and nanosized manganese oxide. Moreover, the stabilized thin manganese oxide film on CNTs effectively releases the strain due to its elastic properties, thus significantly improving mechanic stabilities.[20] As a result, the coaxial carbon/metal oxide/ACNT arrays on metal foils could be a powerful strategy for the rational design

www.chemsuschem.org of electrodes to achieve high specific energy and specific power as well as good cyclic stability. The strategy will be further benchmarked by the C/FeOy/ACNT system in the next section.

C/FeOy/ACNTs as anodes for lithium ion batteries Iron oxide was coated over ACNTs by spontaneous reduction of K2FeO4 followed by annealing in argon. The morphology, elemental distribution, and crystallographic phases were characterized by electron microscopy, energy dispersive X-ray spectroscopy (EDS), and XRD analyses, respectively (see the Supporting Information, Figure S9–11). The lithium ion storage capacity of FeOx/ACNTs and C/FeOy/ ACNTs was evaluated by charging and discharging at a current density of 50 mA g1 (0.05 C) between 0.01 and 3 V versus Li/ Li + (Figure 10 a and b). The initial lithiation and delithiation capacities of FeOx/ACNTs were 1301 and 788 mAh g1, respectively, with a Coulombic efficiency of 60.5 %. The initial lithiation and delithiation capacities decreased to 1141 and 746 mAh g1, respectively, after carbon coating, whereas Coulombic efficiency increased to 65.4 %. Hence, similar effects of carbon coating were obtained for the iron oxide based system as for the manganese oxide system.

Figure 10. Voltage profiles of a) FeOx/ACNTs and b) C/FeOy/ACNTs in the first two cycles; voltage cycled at a current density of 50 mA g1 (0.05 C) between 0.01 and 3.0 V versus Li/Li + ; cyclic stability of c) FeOx/ACNTs and d) C/FeOy/ACNTs at a current density of 50 mA g1 (0.05 C) for the first two cycles and 500 mA g1 (0.5 C) for the following cycles between 0.01 and 3.0 V versus Li/Li + ; capacity is calculated based on the total weight of both carbon and iron oxide.

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CHEMSUSCHEM FULL PAPERS The cyclic stability of FeOx/ACNTs and C/FeOy/ACNTs was investigated at a current density of 50 mA g1 (0.05 C) for the first two cycles and 500 mA g1 (0.5 C) for the following cycles between 0.01 and 3.0 V versus Li/Li + (Figure 10 c and d). FeOx/ ACNTs suffered from serious capacity fading after only 20 cycles and a capacity of 23 mAh g1 was left after 100 cycles. Cyclic stability improved significantly after carbon coating. In addition, the decrease in the initial capacity of FeOx caused by external carbon coating was much smaller than that of MnOx (Figure 10 and 6). The carbon coating induced reduction of Fe oxides seemed to be less significant relative to that of Mn oxides. In addition, the particle size of Fe oxides is much larger than that of Mn oxides (see the Supporting Information, Figure S9 d and f), which could result in a lower potential of structure reconstruction during the cycle. Consequently, no obvious increase in capacity of C/FeOy/ACNTs was observed with cycle number, which was different from C/MnOy/ACNTs. After a mild capacity fading at the beginning, capacity stabilized at about 550 mAh g1. C/FeOy/ACNTs showed good rate capability as well (see the Supporting Information, Figure S12). They could deliver a capacity of 470 mAh g1 at a high current density of 2000 mA g1 (2 C). Therefore, the electrochemical performance of iron oxide improved significantly after carbon coating. The results clearly confirm our strategy of the rational electrode design.

Conclusions Coaxial carbon/metal oxide/ACNT arrays were prepared by the growth of ACNTs over stainless-steel foil and metal oxide coating through spontaneous deposition followed by carbon coating through chemical vapor deposition. The materials obtained showed high performance as binder-free anodes for lithium ion batteries, in terms of specific capacity, cyclic stability, and rate capability. The high electrochemical performance of coaxial carbon/metal oxide/ACNT arrays could be ascribed to this unique sandwich structure. ACNTs over stainless-steel foil serve as a highly conductive and large-surface-area 3D current collector with regular mesoporous channels between the CNTs, which enables the high rate capability of the obtained materials. The thin metal oxide film makes the entire material electrochemically accessible and active, which enables the high specific capacity of the obtained materials. The chemical vapordeposition process provided a uniform thin layer of carbon at the external surface of the metal oxide layer, which effectively prevented aggregation of the metal oxide film and provided a more stable SEI film, which ensured good cyclic stability. Carbon/metal oxide/ACNT arrays as anodes for lithium ion batteries deliver a stable capacity of 738 mAh g1 without any decrease in capacity after 100 cycles and a capacity of 374 mAh g1 at a high current density of 6000 mA g1. This synthesis approach provided a promising route for the production of high-performance binder-free anodes for lithium ion batteries.  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemsuschem.org Experimental Section Material synthesis Stainless-steel foil (Alfa Aesar, Type 304, 25 mm) was cut into circular discs with a diameter of 16 mm. ACNTs were synthesized directly on the stainless-steel foil by using a one-step floating catalyst method.[20] A solution (0.05 m) of ferrocene (Sigma–Aldrich, 98 %) in ethanol (Sigma–Aldrich, 99.8 %) was injected into a quartz reactor (inner diameter: 40 mm) at 0.1 mL min1 with a motorized syringe pump after the furnace reached 675 8C. H2 (10 %)/Ar (Yara Praxair, 99.999 %) was employed as a carrier gas with a total flow rate of 200 sccm. Finally, the furnace was cooled to room temperature under an argon atmosphere. The weight of ACNTs was calculated by subtracting the weight of stainless-steel foil from that of the ACNT/stainless-steel foil sample. MnOx was applied to the ACNTs through spontaneous reduction of potassium permanganate (Sigma–Aldrich, 99.0 %) by carbon. The obtained ACNTs over stainless-steel foil were immersed into an aqueous solution of KMnO4 (20 mL, 0.1 m) for 1 h at room temperature and another 12 h at 70 8C followed by washing with distilled water before drying at 100 8C under vacuum for 24 h to give MnOx/ACNTs. The weight of MnOx was found by measuring the weight difference before and after MnOx coating. FeOx was applied to ACNTs by the redox reaction with potassium ferrate. The obtained ACNTs were soaked in a solution of K2FeO4 (20 mL, 25 mm, Sigma–Aldrich, 97.0 %) in aq. KOH (9 m, Sigma–Aldrich, > 85.0 %) for 12 h at room temperature. The electrodes were then washed with distilled water before drying at 100 8C under vacuum for 24 h. Finally, the obtained material was annealed under an argon atmosphere at 400 8C for 2 h to give FeOx/ACNTs. The weight of FeOx was found by measuring the weight difference before and after FeOx coating. Carbon coating was performed in a quartz reactor using chemical vapor deposition in an acetylene atmosphere. MnOx/ACNTs or FeOx/ACNTs was inserted into the quartz reactor and heated to 400 8C at 10 8C min1 in an argon atmosphere. The gas stream was then switched to a flow of acetylene (10 %) in argon for 5 min. Finally, the sample was cooled to room temperature under an argon atmosphere to give C/MnOy/ACNTs or C/FeOy/ACNTs. A reference sample of MnOy/ACNTs was prepared by annealing MnOx/ ACNTs at 400 8C under H2 (10 %) in argon for 5 min. The weight of MnOy was calculated by subtracting the weight of ACNTs from that of MnOy/ACNTs. The MnOy weight was supposed to be the same in C/MnOy/ACNTs and MnOy/ACNTs due to the similar oxidation state of Mn. Therefore, the weight of the outer carbon layer was calculated by subtracting the weight of MnOy/ACNTs from C/MnOy/ ACNTs.

General characterization Morphology was observed by SEM analysis (Zeiss Ultra, 55 Limited Edition) at an acceleration voltage of 10 kV with an in-lens detector. The detailed microstructure was observed by STEM analysis (Hitachi, S-5500) at an acceleration voltage of 30 kV and TEM analysis (JEOL JEM 2010F) at an acceleration voltage of 200 kV. EDS and EELS analyses were performed on a Hitachi S-5500 instrument at an acceleration voltage of 30 kV and a JEOL JEM 2010F instrument at an acceleration voltage of 200 kV, respectively. A Bruker AXS D8 FOCUS X-ray diffractometer with CuKa radiation (l = 1.541837 ) was employed for the investigation of phase composition of the obtained electrode materials. XPS analyses were carried out on a Kratos Axis Ultra DLD spectrometer with a monochromatic AlKa radiation (hn = 1486.58 eV). High-resolution spectra were collected ChemSusChem 2014, 7, 1335 – 1346

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CHEMSUSCHEM FULL PAPERS at a pass energy of 20 eV. Thermogravimetric differential thermal analysis experiments were carried out on a STA 449 C analyzer (Netzsch, Germany) at a heating rate of 10 8C min1 between 30 and 650 8C under an argon flow (80 sccm); a mass spectrometer was connected to the STA 449 C analyzer to investigate the composition of the exhaust gases.

Electrochemical characterization Coin cells 2016 were assembled by using the prepared coaxial nanotube arrays over stainless-steel foil (Celgard 2400), LiPF6 (1 m)/ ethylene carbonate:diethyl carbonate (v/v = 3:7), and lithium foil as the working electrode, separator, electrolyte, and counter electrode, respectively, in an argon filled glove box with both H2O and O2 less than 0.1 ppm. The cells were charged and discharged between 0.01 and 3.0 V versus Li/Li + using an eight-channel battery analyzer (MTI corporation, USA). All the prepared coin cells were stabilized overnight and then discharged and charged at 60 and 50 mA g1 (0.05 C) for MnOx and FeOx based electrodes in the first two cycles, respectively. All the reported specific capacities herein are based on the total weight of the metal oxides and carbon. Impedance measurements were carried out on a potentiostat (Princeton VersaSTAT, USA) in Swagelok three-electrode cells employing active material, lithium foil, and lithium O-ring as working, counter, and reference electrodes, respectively. An applied alternative voltage signal of 10 mV was imposed to the bias voltage and the frequency range was from 100 KHz to 0.1 Hz. Impedance data were analyzed by using the ZSimpWin program.

Acknowledgements We thank NTNU Nanolab for the facility support. The supports from VISTA, a basic research program funded by Statoil, conducted in close collaboration with the Norwegian Academy of Science and Letters, and Zhengzhou Research Institute of Chalco are gratefully acknowledged. The support from the FREECATS project financed by the European Community’s Seventh Framework Programme (FP-7), grant no. 280658 is also acknowledged. Keywords: carbon · electrochemistry · electron microscopy · nanotubes · x-ray diffraction [1] A. Zhamu, G. Chen, C. Liu, D. Neff, Q. Fang, Z. Yu, W. Xiong, Y. Wang, X. Wang, B. Z. Jang, Energy Environ. Sci. 2012, 5, 5701 – 5707. [2] M. M. Thackeray, C. Wolverton, E. D. Isaacs, Energy Environ. Sci. 2012, 5, 7854 – 7863. [3] J. R. Szczech, S. Jin, Energy Environ. Sci. 2011, 4, 56 – 72. [4] B. J. Landi, M. J. Ganter, C. D. Cress, R. A. DiLeo, R. P. Raffaelle, Energy Environ. Sci. 2009, 2, 638 – 654. [5] L. Ji, Z. Lin, M. Alcoutlabi, X. Zhang, Energy Environ. Sci. 2011, 4, 2682 – 2699. [6] V. Etacheri, R. Marom, R. Elazari, G. Salitra, D. Aurbach, Energy Environ. Sci. 2011, 4, 3243 – 3262. [7] K. Evanoff, J. Khan, A. A. Balandin, A. Magasinski, W. J. Ready, T. F. Fuller, G. Yushin, Adv. Mater. 2012, 24, 533 – 537. [8] D.-W. Wang, H.-T. Fang, F. Li, Z.-G. Chen, Q.-S. Zhong, G. Q. Lu, H.-M. Cheng, Adv. Funct. Mater. 2008, 18, 3787 – 3793. [9] W. Lu, A. Goering, L. Qu, L. Dai, Phys. Chem. Chem. Phys. 2012, 14, 12099 – 12104. [10] C. K. Chan, H. Peng, G. Liu, K. McIlwrath, X. F. Zhang, R. A. Huggins, Y. Cui, Nat. Nanotechnol. 2008, 3, 31 – 35. [11] P. L. Taberna, S. Mitra, P. Poizot, P. Simon, J. M. Tarascon, Nat. Mater. 2006, 5, 567 – 573.

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aligned carbon nanotube arrays as high-performance anodes for lithium ion batteries.

Coaxial carbon/metal oxide/aligned carbon nanotube (ACNT) arrays over stainless-steel foil are reported as high-performance binder-free anodes for lit...
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