Silver

Self-Volatilization Approach to Mesoporous Carbon Nanotube/Silver Nanoparticle Hybrids: The Role of Silver in Boosting Li Ion Storage Hao Jiang,† Haoxuan Zhang,† Yao Fu,† Shaojun Guo,*,‡ Yanjie Hu,† Ling Zhang,† Yu Liu,*,§ Honglai Liu,§ and Chunzhong Li*,† †

Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, and §State Key Laboratory of Chemical Engineering and Department of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China ‡ Department of Materials Science and Engineering, Department of Energy and Resources Engineering, and Beijing Key Laboratory for Theory and Technology of Advanced Battery Materials, College of Engineering, Peking University, Beijing, 100871, China S Supporting Information *

ABSTRACT: One of the biggest challenging issues of carbon nanomaterials for Li ion batteries (LIBs) is that they show low initial Coulombic efficiency (CE), leading to a limited specific capacity. Herein, we demonstrate a simple template self-volatilization strategy for in situ synthesis of mesoporous carbon nanotube/Ag nanoparticle (NP) hybrids (Ag-MCNTs) to boost the LIBs’ performance. The key concept of Ag-MCNTs for enhancing LIBs is that a small trace of Ag NPs on MCNTS can greatly restrict the formation of a thicker solid electrolyte interphase film, which has been well verified by both transmission electron microscopy results and quantum density functional theory calculations, leading to the highest initial CE in all the reported carbon nanomaterials. This uncovered property of Ag NPs from Ag-MCNTs makes them exhibit a very high reversible capacity of 1637 mAh g−1 after 400 discharge/charge cycles at 100 mA g−1, approximately 5 times higher than the theoretical value of a graphite anode (372 mAh g−1), excellent rate capability, and long cycle life. KEYWORDS: Ag nanoparticle, carbon nanotube, Coulombic efficiency, solid electrolyte interphase, lithium ion battery solid electrolyte interphase (SEI) film, which is a very important index to evaluate the irreversible charge “loss” in the first cycle.14−16 This thicker SEI film during the charge− discharge cycle, formed by consuming more Li ions and electrons at the negative electrode, can result in lower Li ion concentration in the electrolyte, higher resistance, and a larger irreversible capacity, thus affecting the LIB performance. In this regard, the search for a new type of carbon nanomaterial with special control for obtaining thinner but stable SEI films becomes very important for improving the initial CE, and thus a much improved capacity and cycling stability, but is still a great challenge. Herein, we demonstrate a new self-volatilization approach to synthesize Ag nanoparticle (NP) in situ decorated mesoporous carbon nanotubes (Ag-MCNTs) by annealing Ag−V2O5/ polydopamine core/shell nanowires (NWs) under flowing argon at 700 °C. Such interesting Ag-MCNTs are highly

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echargeable lithium ion batteries (LIBs) have been of great interest during the past decade due to their high energy density and long cycle life.1−4 The efficiency of energy storage at a high charge and discharge rate is the key for their further applications in electric vehicles (EVs) and hybrid electric vehicles (HEVs). To date, well-known graphite and carbon nanomaterials show excellent cycling stability for LIBs, but are greatly limited by their low theoretical capacity (372 mAh g−1, LiC6).5−7 To further improve the specific capacity, recent reports reveal that engineering disordered carbonaceous nanomaterials with rich defects can provide a new approach to allow more Li+ intercalation (LixC6, x > 1).8−10 Another interesting approach is that the Li ion storage capacity of micro/mesoporous carbon nanotubes (CNTs) can increase from LiC6 to Li2C6 by enabling Li ion diffusion into the interior space of CNTs.11−13 Despite these interesting concepts in enhancing the capacity of LIBs, these engineered carbon nanomaterials still show limited reversible capacity. The biggest scientific challenging issue for this is that existing engineered carbon nanomaterials still show a low initial Coulombic efficiency (CE) of ∼50%, caused by the formation of a thick © XXXX American Chemical Society

Received: November 23, 2015 Accepted: December 21, 2015

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Figure 1. (a) Schematic illustration on how to use the self-volatilization approach to make Ag-MCNTs, (b) low- and (c) high-magnification TEM images, (d) N2 adsorption−desorption isotherms (the inset shows the corresponding pore size distribution curve), and (e) Raman spectrum of the as-prepared Ag-MCNTs.

Ag-CNTs. However, serious aggregation of Ag NPs occurred at more than 250 °C.17,18 To solve this problem, herein we first modified Ag NPs onto porous V2O5 NWs, followed by coating a layer of carbon precursor on their surface. Because Ag NPs were well encapsulated onto porous V2O5 NWs, the carbonization at high temperature could make Ag NPs keep their morphology. Figure 1a demonstrates the typical procedure for making Ag-MCNTs hybrids. (a) Silver−vanadium oxide (Ag− V2O5) NWs with a diameter of ∼13 nm were first synthesized via one-step hydrothermal treatment of NH4VO3, HNO3, and small amounts of AgNO3 (Figure S1). (b) The as-obtained Ag−V2O5 NWs were used as a self-sacrificing template for polymerizing a layer of polydopamine (PDA) in the presence of P123 to obtain Ag−V2O5/PDA core/shell NWs. Herein, the P123 can serve as a soft template for introducing abundant and rich mesopores onto/into MCNTs. This role of P123 has been demonstrated in previous reports on the synthesis of mesoporous carbon and mesoporous silica.19,20 (c) Annealing the Ag−V2O5/PDA core/shell NWs at high temperature (700 °C) resulted in the production of Ag-MCNTs because the V2O5 template could be volatilized near its melting point (690 °C).21,22 The disappearance of the V2O5 template after the thermal treatment was proved by X-ray diffraction (XRD) (Figure S2), showing no characteristic peaks from all vanadium oxides. Figure 1b displays a scanning electron microscopy (SEM) image of Ag-MCNTs. They have diameters of ∼10−20 nm and lengths of several micrometers. The high-magnification

particular for boosting the performance of LIBs in terms of initial CE, capacity, rate ability, and cycling stability. (a) The mesoporous wall of MCNTs allows full activation by enabling Li ion diffusion into the inner space of CNTs. (b) Nitrogen self-doping from the precursor (polydopamine) can contribute to additional capacity. (c) Most importantly, the trace amounts of Ag NPs (1.4%) can remarkably improve the first CE (87%), much higher than the corresponding MCNTs (70%), which is a brand new approach to greatly enhance the capacity, rate ability, and cycling stability of LIBs. The role of Ag NPs has also been well verified by both transmission electron microscopy results and quantum density functional theory (QDFT) calculations. As a result, the Ag-MCNTs exhibit a ultrahigh reversible capacity of 1637 mAh g−1 after 400 discharge/charge cycles at 100 mA g−1, excellent rate capability (758 mAh g−1 at 3000 mA g−1), and long cycle life (813 mAh g−1 even after another 500 discharge/charge cycles at 2000 mA g−1). The superior LIB performance of Ag-MCNTs makes them a very promising anode material for future EV and HEV applications.

RESULTS AND DISCUSSION Material Design and Structure Characterization. In order to demonstrate our concept, we first tried to directly load Ag NPs onto a carbon precursor and then carbonized the asprepared composite at high temperature to make well-defined B

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Figure 2. CVs of (a) Ag-MCNTs and (b) MCNTs for the first three cycles at a scan rate of 0.2 mV s−1; the charge−discharge curves of (c) AgMCNTs and (d) MCNTs for the first three cycles at a current density of 300 mA g−1.

Figure 3. (a) Cycling performance of Ag-MCNTs, MCNTs and CNTs, respectively, at 1000 mA g−1 for the first 400 cycles, (b) CVs of the AgMCNTs and MCNTs, respectively, before and after 400 cycles at a scan rate of 0.2 mV s−1, (c) the capacity retention of the Ag-MCNTs at various current densities, and (d) the following cycling performance of the Ag-MCNTs at 2000 mA g−1 for another 500 cycles.

Ag-MCNTs. Two characteristic carbon peaks around 1375 and 1605 cm−1 related to the disordered carbon (D-band) and graphitic carbon (G-band), respectively, are observed. The intensity ratio (ID/IG) is about 0.96, suggesting a relatively high graphitization of Ag-MCNTs. To investigate the role of Ag NPs, MCNTs were also prepared by the same procedure without the addition of AgNO3 (Figure S3). Both Ag-MCNTs and MCNTs are assembled into coin-type 2016 half-cells for evaluating their electrochemical properties. The cyclic voltammograms (CVs) of Ag-MCNTs and MCNTs are tested within the potential window of 0.01−3 V at 0.2 mV s−1 (Figure 2a,b). The big difference is that Ag-MCNTs manifest a remarkably improved first Coulomb efficiency. This interesting phenomenon was

TEM image (Figure 1c) reveals that Ag-MCNTs have a tubular nanostructure, and a few Ag NPs with a diameter of ∼2.5 nm are well incorporated into the mesporous carbon wall. The Ag content in the Ag-MCNTs is determined to be ∼1.4 wt % by inductively coupled plasma (ICP) elemental analyses. Figure 1d shows the nitrogen adsorption/desorption isotherm and the pore size distribution of Ag-MCNTs. They possess a high Brunauer−Emmett−Teller (BET) surface area of 429.3 m2 g−1 with a multimodal mesoporous size distribution (inset of Figure 1d) of small pore (10 nm) from the interior cavity of the MCNTs. Such intriguing pore structures will be favorable for increasing the diffusion of lithium ions and electrolyte to enhance the performance of LIBs. Figure 1e displays the Raman spectrum of C

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Figure 4. (a, b) High-magnification TEM images of (a) MCNTs and (b) Ag-MCNTs after the initial 3 cycles. (c) EIS of Ag-MCNT and MCNT after the first discharge to 0.01 V.

the interior space of Ag-MCNTs and MCNTs caused by the mesopores or edge defects on the walls.9,11 This is the reason that we observe the increased capacity trend for both AgMCNTs and MCNTs, which is more remarkable for AgMCNTs. The only big difference for Ag-MCNTs and MCNTs is that Ag-MCNTs show a much higher initial CE than that of MCNTs, leading to much improved capacity, rate capability, and cycling stability. Therefore, we think that the much improved CE of Ag-MCNTs is more related to Ag NPs. Here, the trace amounts of Ag NPs have dual functions, which can not only accelerate the electron transport by improving the electrical conductivity but also greatly increase the reversible capacity by improving the first CE (Figure 3). The former function was confirmed by the electrochemical impedance spectra (EIS) results (Figure S6), showing AgMCNTs have a much lower resistance (180 Ω) than the MCNTs (260 Ω). For the latter function, we believe that the Ag NPs can effectively restrict the decomposition of solvent on the surface of the electrode, i.e., forming a thinner and stable SEI film on the surface of Ag-MCNTs. To support our hypothesis, we reassembled 2016 coin-type half-cells by replacing poly(vinyl difluoride) (PVDF) binder with watersoluble sodium carboxymethylcellulose (CMC) for testing the thickness of the as-formed SEI film during the charge/discharge process. Figure 4a,b show the TEM images of the delithiated MCNTs and Ag-MCNTs after 3 charge/discharge cycles. A thinner layer of SEI film was formed on the surface of the AgMCNTs, suggesting Ag NPs can restrict the electrolyte decomposition. Furthermore, both the thinner SEI film and no structure change on MCNTs are observed even after 150 cycles, implying Ag-MCNTs are highly stable for LIBs (Figure S7). As is known, EIS is an important technique for investigating the growth kinetics and the Li ion/electron transport of SEI films.16,24,25 Figure 4c shows the EIS of the Ag-MCNT and MCNT after the first discharge to 0.01 V. Both of them are composed of two high-frequency depressed semicircles and a sloping line in the low-frequency region, in sequence related to the formation of a SEI film, the charge-transfer reaction, and a combined effect of Li+ diffusion on the electrode−electrolyte interfaces. The SEI film resistances (RSEI) of Ag-MCNTs and MCNTs were fitted by Zview software based on the equivalent circuit model (inset of Figure 4c). The fitting of impedance spectra to the proposed equivalent circuit was performed by using the code Zview, as shown in Tables S2 and S3. We find that the RSEI of Ag-MCNTs is about 25.1 Ω, much lower than

further approved by the galvanostatic charge/discharge measurement at a current density of 300 mA g−1 (Figure 2c,d). The Ag-MCNTs still exhibit a much higher first CE of 87% than the MCNTs (70%), being in good agreement with the CV results. Electrochemical Performance. Figure 3a shows the specific capacities of Ag-MCNTs, MCNTs, and commercial CNTs, respectively, for first 400 cycles at a current density of 1000 mA g−1. The Ag-MCNTs deliver a first reversible capacity of around 534 mAh g−1, which gradually reaches a maximum value of 928 mAh g−1 after 370 cycles. Under the same conditions, the MCNTs also demonstrate similar trends, in which the capacity increases from 425 mAh g−1 to 560 mAh g−1 after 260 cycles, while the commercial CNTs keep an unchanged capacity of ∼340 mAh g−1 after 400 cycles. Notably, after 400 cycles, there is no extra redox peaks in the CVs of AgMCNTs (Figure 3b), indicating that no new phase was generated for contributing an additional capacity during the cycles. The rate capability of the Ag-MCNTs was subsequently evaluated (Figure 3c). They exhibit a very high average reversible capacity of 1637 mAh g−1 at 100 mA g−1, which is approximately 5 times higher than the theoretical value of a graphite anode (372 mAh g−1). Even at a current density of 3000 mA g−1, the capacity can be maintained at 758 mAh g−1. After the deep charge/discharge for 5 cycles at 3000 mAh g−1, an average capacity of 1582 mAh g−1 can be recovered when the current density again decreases to 100 mA g−1. After another 500 cycles at 2000 mA g−1 (Figure 3d), the reversible capacity of the Ag-MCNTs still maintains a high value of 813 mAh g−1 with a CE of nearly 100%. For easy comparison, we summarized the detailed electrochemical performance of various derived-carbon anodes in the literature (Table S1).23 Although only a small trace of Ag NPs is used, it is obvious that our Ag-MCNTs are highly efficient carbon anode materials for LIBs in terms of their comprehensive electrochemical performance. Such greatly improved electrochemical performances of AgMCNTs for LIBs are mainly attributed to the mesoporous structure and N-doping of the carbon wall, as well as the decoration of Ag NPs in particular. As shown in Figure 2a,b, a couple of reversible redox peaks can be observed at 0.9 and 1.3 V for Ag-MCNTs and MCNTs during Li intercalation, whereas no such peaks appear during Li intercalation for the commercial CNTs (Figure S4). To further clarify this, their corresponding differential capacity plots are provided in Figure S5. The distinct redox peaks can be explained by the Li+ diffusion into D

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Figure 5. (a, b) Final states and the corresponding binding energy of LiF-graphite and LiF-graphite (Ag) and (c) diagrammatic sketch of the interaction potential for LiF-graphite and LiF-graphite (Ag).

that of MCNTs (∼213.2 Ω). This further implies a thinner SEI film for Ag-MCNTs, thus giving rise to an improved initial CE. To examine the strength of the SEI−electrode interaction, we use the QDFT to calculate the binding energy between the three typical components of SEI (LiF, Li2O, and Li2CO3) and the electrode. The binding energy is defined by ΔESEI‑electrode = ESEI‑electrode − EESI − Eelectrode, where ESEI‑electrode, EESI, and Eelectrode are the energy of the SEI-electrode, SEI, and electrode, respectively. The electrode is modeled by two layers of graphite with Ag doped in between. The QDFT calculation is performed in the Dmol3 module of the Material Studio software (Accelrys Inc., San Diego, CA, USA). The exchange−correlation energy is approximated by the PBE method; the core−electron interactions are treated as effective core potentials; the DND basis set is employed for numerical calculations. The COSMO model is used to calculate the solvation effect, where the dielectric constant is set as 26.2 to mimic an ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:1 in volume) mixture. Figure 5a and b show the final states and the corresponding binding energy of LiF-graphite and LiF-graphite (Ag). The binding energy and the final states for the other two components Li2O and Li2CO3 are listed in Table S4. For SEI-graphite systems, the binding energy is on the order of −10 kcal/mol, implying a chemical adsorption between SEI and graphite. In this case, it is difficult for SEI to be desorbed in the charge/discharge process, being consistent with the experimental findings. For the SEI-graphite (Ag) system, in contrast, the binding energy is positive, suggesting the repulsions between SEI and graphite (Ag). As shown in Figure 5c, the positive binding energy represents a local minimum of the interacting potential u(r), corresponding to a metastable state. According to the asymptotic limit of u(r), the global minimum should locate at r → ∞. In this case, SEI will be excluded away from the surface. As the existence of Ag leads to the repulsions between SEI and the electrode, “doping” Ag into graphite would lead to the thin adsorbed SEI. Thus, both experimental and theoretical results prove that Ag NPs from Ag-MCNTs can restrict the formation of the thinner SEI film during the charge/ discharge process and lead to very high initial CE, which can partially prevent the blocking of meso/micropores in the carbon walls and promote more Li ion and electrolyte diffusion into the inner space (Figure 6).

Figure 6. Schematic illustration of the formation of an SEI film on CNT, MCNT, and Ag-MCNT.

discharge cycles, which also has been well verified by both transmission electron microscopy results and the quantum density functional theory calculations. The resulting thinner SEI film on Ag-MCNTs is the key to greatly improve the initial CE to 87%, much higher than the corresponding MCNTs (70%) at a current density of 300 mA g−1. As a consequence, the asobtained Ag-MCNTs exhibit an ultrahigh reversible capacity of 1637 mAh g−1 after 400 discharge/charge cycles at 100 mA g−1 with excellent rate capability (758 mAh g−1 at 3000 mA g−1) and long cycle life (813 mAh g−1 even after another 500 discharge/charge cycles at 2000 mA g−1). We expect that the new strategy developed in this study may open a new way to prepare other MCNT/metal hybrid nanostructures for advanced renewable energy conversion and storage applications.

EXPERIMENTAL SECTION Synthesis of Ag−V2O5 NWs. The Ag−V2O5 nanowires were synthesized according to the established procedure with a minor modification.26 In a typical synthesis, 0.3 g of NH4VO3 and 0.5 g of triblock copolymer PEO−PPO−PEO (P123) were first dissolved in 30 mL of deionized water containing 2 mL of 1 M HNO3. The resulting mixture was further stirred for 7 h, followed by the addition of 5 mL of 0.1 M AgNO3 aqueous solution. After stirring for another 1 h, the mixture was transferred into a 50 mL Teflon-lined stainless steel autoclave, heated at 120 °C for 24 h, and then cooled to room temperature. The precipitates, Ag−V2O5 NWs, were collected by

CONCLUSION To summarize, a new self-volatilization approach has been developed for the in situ synthesis of Ag-MCNTs by annealing as-formed Ag−V2O5/polydopamine core/shell NWs at high temperature. We reveal Ag NP-supported MCNTs can greatly restrict the formation of an SEI film during the charge/ E

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ACS Nano filtration, further washed with absolute ethanol and deionized water, and finally dried by the freezing technique. As a control, pure V2O5 NWs were also synthesized by the same procedure without the addition of AgNO3. Synthesis of Ag-MCNTs. A 100 mg amount of the as-prepared Ag−V2O5 NWs was dispersed in 50 mL of deionized water containing 50 mg of 2-amino-2-hydroxymethylpropane-1,3-diol (Tris) and 100 mg of P123. Then, 100 mg of dopamine was added to the above dispersion with continuous magnetic stirring in air at room temperature for 5 h. The precipitate, i.e., polydopamine/Ag−V2O5 NWs, was collected by filtration, then washed several times with absolute ethanol and deionized water, and finally dried by the freezing technique. Finally, Ag-MCNTs were obtained by thermal treatment of polydopamine/Ag−V2O5 NWs under flowing argon at 700 °C for 4 h at a rate of 2 °C min−1. Pure MCNTs were also prepared with the same procedure using pure V2O5 NWs as core materials instead of Ag−V2O5 NWs. Characterization. The structure and morphology of as-prepared products were characterized with X-ray power diffractometer (Rigaku D/Max2550, Cu Kα radiation) at a scan rate of 1 °C min−1, scanning electron microscopy (SEM; Hitachi, S-4800), and transmission electron microscopy (TEM; JEOL, JEM-2100F) operated at 200 kV. Raman spectroscopy were performed at ambient temperature with a NEXUS 670 FT-IR Raman spectrometer. Nitrogen adsorption/ desorption curves were determined by BET measurement using a Micromeritcs ASAP 2010 surface area analyzer. Electrochemical Measurements. Electrochemical measurements were performed using coin-type 2016 cells. The working electrode was prepared by mixing the active materials, carbon black, and PVDF at a weight ratio of 80:10:10 and then pasted on pure Cu foil. The mass loading density of the active materials is 0.97 mg cm2. Pure lithium foil was used as counter electrode, and the separator was a polypropylene membrane (Celgard 2400). The electrolyte consists of a solution of 1 M LiPF6 in EC/DMC (1:1 in volume). The cells were assembled in an argon-filled glovebox. The charge and discharge measurements were carried out on a LAND-CT2001C test system at different current densities. The cyclic voltammogram experiment was performed on an Autolab PGSTAT302N electrochemical workstation at scan rates of 0.2 mV s−1.

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ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.5b07367. Figures S1−S7 and Tables S1−S3 (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected] (S. J. Guo). *E-mail: [email protected] (Y. Liu). *E-mail: [email protected] (C. Z. Li). Tel: +86-21-64250949. Fax: +86-21-64250624. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21236003, 21522602, 21322607, 21506051), the Shanghai Shuguang Scholars Program (13SG31), the Program for Professor Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning, the Research Project of Chinese Ministry of Education (113026A), Fundamental Research Funds for the Central Universities, the start-up funding from Peking University, and Young Thousand Talented Program. F

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