Solution Assembled Single-Walled Carbon Nanotube Foams

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Solution Assembled Single Walled Carbon Nanotube Foams; Superior Performance in Supercapacitors, Lithium Ion, and Lithium Air Batteries Rachel Carter, Landon Oakes, Adam Cohn, Jeffrey Holzgrafe, Holly F Zarick, Shahana Chatterjee, Rizia Bardhan, and Cary L. Pint J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp5054502 • Publication Date (Web): 13 Aug 2014 Downloaded from http://pubs.acs.org on August 14, 2014

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The Journal of Physical Chemistry

Solution Assembled Single Walled Carbon Nanotube Foams; Superior Performance in Supercapacitors, Lithium Ion, and Lithium Air Batteries Rachel Carter1, Landon Oakes1, 2, Adam Cohn1, Jeffrey Holzgrafe1, Holly Zarick3, Shahana Chatterjee1, Rizia Bardhan2, 3, and Cary L. Pint1, 2, * 1

Department of Mechanical Engineering, Vanderbilt University, Nashville, TN 37235

2

Interdisciplinary Materials Science Program, Vanderbilt University, Nashville, TN 37235

3

Department of Chemical and Biomolecular Engineering, Vanderbilt University, Nashville, TN

37235

ACS Paragon Plus Environment

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ABSTRACT We demonstrate a surfactant-free, solution processing route to form three-dimensional freestanding foams of pristine single-walled carbon nanotubes (SWCNTs) and explore the diverse electrochemical energy storage applications of these materials.

This route utilizes

SWCNT dispersions in organic n-methylpyrrolidone solvents, and subsequent electrophoretic assembly onto a metal foam sacrificial template which can be dissolved to yield surfactant-free, binder-free freestanding SWCNT foams. We further provide a comparison between surfactantfree foams and conventional surfactant-based solvent processing routes, and assess performance of these foams in supercapacitors, lithium-ion batteries, and lithium-air batteries. For pristine SWCNT foams, we measure up to 83 F/g specific capacitance in supercapacitors, specific capacity up to 2210 mAh/g for lithium-ion batteries with up to 50% energy efficiency, and specific discharge capacity up to 8275 mAh/g in lithium-air batteries. For lithium-air batteries, this corresponds to a total energy density of 21.2 kW/kg and 3.3 kW/kg for the active mass and total battery device respectively, approaching the 12.7 kW/kg target energy density of gasoline. In comparison, SWCNT foams prepared with surfactant exhibit poorer gravimetric behavior in all devices, and compromised Faradaic storage that leads to depreciated amounts of usable, stored energy. This work demonstrates the broad promise of SWCNTs as lightweight and highly efficient

energy storage materials, but

also

emphasizes

the importance of clean

nanomanufacturing routes which are critical to achieve optimized performance with nanostructures. KEYWORDS Electrophoretic deposition, lithium ion batteries, lithium air batteries, lithium oxygen batteries, supercapacitors, single walled carbon nanotubes, foams, surfactants, NMP, nanomanufacturing


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INTRODUCTION Single-walled carbon nanotubes (SWCNTs) possess extraordinary promise as nextgeneration building blocks for the advanced design of materials with applications across energy, electronics, sensing, and composite materials.1

The electrical, thermal, and mechanical

properties that exist at the length scale of individual SWCNTs have driven interest for nextgeneration applications.2-3

However, central to materials innovation is the construction of

hierarchical three-dimensional (3-D) assemblies from these one-dimensional building blocks without compromising the inherent physical and surface characteristics that make SWCNTs appealing.4-8 One route to achieve such materials is to synthesize SWCNTs in aligned structures that can be directly implemented in diverse applications from energy storage and conversion to optoelectronic devices.9-15

Such approaches maintain pristine SWCNT character in usable

architectures, but face challenges regarding the scalability and cost involved for practical implementation.

This makes another route involving liquid phase processing of SWCNTs

attractive, as routes to solubilize SWCNTs, or more generally CNTs, into solvents is straightforwardly achieved using surfactants.16-19 However, assembling these materials makes surfactant residue removal challenging or impossible, with the surfactant compromising desired properties of the material.20 This has motivated surfactant-free routes utilizing superacids or organic polar solvents, which can natively disperse both graphene and SWCNTs.21-29 Surfactantfree superacid dispersions of SWCNTs22-23 have been demonstrated as an enabling route toward the assembly of multifunctional fibers with extraordinary mechanical and conductivity properties retained due to the pristine character of the SWCNTs.30 On the other hand, n-methylpyrrolidone (NMP) is a polar solvent that can directly disperse graphene or SWCNTs,24-25, 27-28 but routes to directly process functional materials from such polar solvent dispersions have yet to be realized.


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Current advances in electrophoretic deposition (EPD), a route for electric field-assisted solution assembly, require the use of surfactant in order to achieve deposition and assembly of SWCNTs and graphene.31-35 In this work, we demonstrate EPD directly from SWCNTs dispersed in polar NMP solvents, without the necessity of surfactant-based dispersions. As SWCNTs and more generally CNTs appeal to many applications, they are well-suited for electrochemical energy storage devices due to high surface area, good conductivity, and native electrochemical stability.36 This has enabled their implementation in supercapacitors,37-57 pseudocapacitors,58-61 lithium-ion batteries,62-86 and lithium-air batteries.87-95 For supercapacitor electrodes, the high surface-area of SWCNTs is advantageous to enable high specific capacitances either in as-grown structures or posthumously assembled SWCNT electrodes. Previous studies have emphasized the impact of processing and impurities on the gravimetric non-Faradaic specific capacitance,51 with most studies reporting values below 100 F/g and widely varying based on processing approaches.37-38, 41-42, 46, 51-52, 54 As-grown SWCNT materials in vertical arrays have demonstrated the best specific capacitance performance (160 F/g), with good rate capabilities.55-56 This highlights the importance of pristine SWCNTs to reduce the chemical and gravimetric effects of surfactants and other impurities from processing. In addition to non-Faradaic storage systems, both SWCNTs67, 81, 83, 85-86 and multi-walled CNTs68, 73, 76, 80, 82 have also been widely applied toward the development of advanced battery electrodes. For lithium-ion batteries, these low-dimensional nanostructures enable Li storage significantly exceeding the specific capacity of bulk graphitic carbons (372 mAh/g). Reversible capacities have been measured up to 1400 mAh/g for CNTs grown on conductive substrates,73 but range between 300-1250 mAh/g, depending on processing techniques. For SWCNTs, the first-cycle charge capacity is often measured to exhibit >2500 mAh/g with a significant charging


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plateau between 1-2 V vs. Li/Li+, but the charging characteristics beyond the first charge vary widely based on the material processing.79 In addition to Li-ion batteries, SWCNTs and CNTs have been widely assessed as cathodes in lithium-oxygen, or lithium-air battery systems, which have promise as energy carriers to replace gasoline.96-97 In such batteries, CNTs are of interest due to their lightweight, conductive properties that enable high specific storage of reversible lithium peroxide surface species. So far, one report by Wang et al. for multi-walled CNTs has demonstrated specific capacities up to ~ 8500 mAh/g at slow charging currents (0.1 mA/cm2),88 though most CNT materials exhibit capacities typically less than 4000 mAh/g.89-90, 93-95 SWCNT materials have been demonstrated to exhibit capacities of 2540 mAh/g when coupled with carbon nanofibers,87 even though SWCNT materials have not been strongly assessed for their performance as Li-air cathodes. As the oxygen evolution reactions (OER) and oxygen reduction reactions (ORR) occur on the CNT surface, different processing techniques and CNT surface characteristics have led to wide variation in capacities between ~ 700 – 4500 mAh/g. In this spirit, our work explicitly demonstrates the importance of pristine assembly routes for freestanding SWCNT binder-free electrodes in electrochemical energy storage. Forming 3-D SWCNT foams directly from surfactant-free organic solvents leads to device performance in supercapacitors, lithium-ion batteries, and lithium-air batteries competitive or better than performance previously reported for SWCNT materials. However, assembly of SWCNT foams mediated by TOAB surfactant suspensions leads to degradation of both the gravimetric and Faradaic chemical storage behavior in varying degrees for these devices in the order of lithiumair cathodes > lithium-ion anodes > supercapacitors. This work motivates the development of scalable solution-based routes to produce pristine, three-dimensional assembled carbon


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nanostructures to exploit their exceptional properties as catalytic sites or stable electrodes in a wide range of devices. EXPERIMENTAL DETAILS Freestanding SWCNT Electrode Fabrication and Characterization: HiPCO SWCNTs (Unidym, purified) were deposited on a Ni foam template (positive terminal) with the use of electrophoretic deposition (configuration shown in supporting information). Well-dispersed solutions of SWCNTs of two types were created. The first consisted of 1mg/2mL of as received SWCNTS and THF and 10:1 mass ratio of TOAB (Sigma Aldrich) to SWCNTs. The second solution contained 3mg/4mL of SWCNTs and NMP (Sigma Aldrich). Both solutions were treated with sonication for 30 min prior to deposition. The solution containing THF and TOAB was centrifuged (9000 rpm for 10min) twice to remove excess surfactant prior to deposition, and redispersed using sonication. To perform electrophoretic deposition, a Ni foam (MTI) was immersed in a beaker filled with SWCNTs and connected to the negative terminal of the DC power source. A counter electrode is connected to the positive terminal in a parallel plate set up with a 0.5 cm separation distance, and a 30 V potential was applied across the plates. The SWCNT coated Ni foam is then removed from the negative terminal and dried vertically in the hood for 12 hours. The dried Ni foam coated in SWCNTs is etched in HCl (37% Sigma Aldrich) for 48 hours. The freestanding SWCNT foam material is then transferred to a nanopure water bath for 1 hour and then further transferred to a new water bath for another hour. This material is dried under vacuum for 30 minutes leaving the freestanding SWCNT foam. Analysis of SWCNT foam materials was carried out using Scanning Electron Microscopy (Zeiss) (SEM) and Thermogravimetric Analysis (TGA). The SWCNT foams were also analyzed with a Renishaw inVia Raman microscope using 785 laser excitations.


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Supercapacitor Device Fabrication Electrochemical Testing: Electrodes made from foam pieces of equivalent mass are separated with a polymer separator (Celgard) and coated with ionic liquid electrolyte

(EMIBF4

98%

Sigma

Aldrich).

These

supercapacitor

electrodes

were

electrochemically tested using a Metrohm Autolab multichannel testing unit. The tests included cyclic voltammetry (CV) with various scan rates (25-100mV/s) from -3 to 3V, electrochemical impedance spectroscopy (EIS), and galvanostatic charge discharge measurements at various charging currents (0.1 to 20 A/g). Lithium-ion Battery Device Fabrication and Electrochemical Testing: Half-cell devices were assembled in an Ar glove box using stainless steel coin cells (MTI). The freestanding foam is assembled as anode material with 2500 Celgard seperator saturated with 1 M LiPF6 in 1g/1mL of ethylene carbonate (EC) and diethyl carbonate (DEC) (Sigma Aldrich) separating the anode material from pure lithium foil (Sigma Aldrich). The device is crimped in a 2032 coin cell and tested utilizing a Metrohm autolab multichannel testing system. Cyclic voltammetry was performed on the devices between 0-3.7 V at various scan rates (0.5-100 mV/s) and Galvanostatic charge-discharge measurements were carried out for 7 constant currents ranging from 186 mA/g to 14.88 A/g. Cycling studies were performed at 744 mA/g, and samples were tested after 300 cycles at the 7 current densities initially utilized to assess post-cycling performance. Lithium-air Battery Device Fabrication and Electrochemical Testing: Mesh coin cells containing 0.1 M LiClO4 in TEGDME were assembled in an Ar glove box using free-standing SWCNT foam as the cathode material separated from Li foil with a Celgard 2500 membrane. These batteries were sealed in an MTI test cell under 1 atmosphere of a mixture of ultra-high purity 20% oxygen and 80% Argon gas at room temperature. The devices were tested under


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Galvanostatic charge discharge at three constant current discharge rates of 1 A/g, 0.5 A/g, and 0.1 A/g using a Metrohm Autolab multichannel testing system. RESULTS AND DISCUSSION Central to the fabrication of pristine 3-D SWCNT foam materials is the ability to assemble SWCNTs directly from surfactant-free solutions. To achieve this, we utilize NMP polar solvent suspensions of SWCNTs and EPD assembly, with the experimental setup shown in supporting information. This gives us the ability to form pristine coatings of SWCNTs on the surface of metal foams, where the metal foam is then dissolved in HCl to yield a freestanding foam material of SWCNTs (Figure 1a-c). After rinsing and drying this material, a structure remains composed only of SWCNTs that retains the porous microstructure of the metal foam. This material exhibits mechanical properties reflecting SWCNT components that are adjoined through van der Waals interactions, and can be flexed through small angles and maintain minor mechanical stresses, but do not exhibit the structural integrity of bulk materials – in a fashion similar to previous SWCNT freestanding films and fibers.8,30 Unlike CVD processes, where selfassembly during growth dictates the microstructure of CNT or SWCNT materials, even in tunable post-growth processing,98 this approach enables careful tuning of the microstructure based on the choice of metal foam in the framework of a scalable, low-power (e.g. up to 120 V, low currents) solution processing route.

Furthermore, as opposed to conventional solvent

processing techniques utilizing surfactants, the use of NMP maintains the pristine character of the SWCNT.

Characteristic SEM images of 3-D foam structures showing the bundle

microstructure and the microscale foam features after removal of the sacrificial metal template are shown in Figure 1 e-f.


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In order to assess the benefit of pristine SWCNT foams for applications, we also performed EPD assembly from tetrahydrofuran (THF) solvents containing SWCNTs suspended with tetraoctylammoniumbromide (TOAB) surfactant.

This was performed in an identical

manner as that shown in Figure 1 on metal foams to produce equivalent freestanding materials, except based on solutions where the SWCNTs are surfactant suspended. SEM comparison of NMP and surfactant-suspended SWCNTs at the same magnification (Figs. 2a, 2b) indicates notably different surface properties of the deposited materials. The pristine (NMP) foam (Fig. 2a) exhibits mat-like bundles of SWCNTs that appear free of excess contamination and consistent with the nature of bundles that are dispersed in NMP polar solvents. In the case of the surfactant-suspended SWCNTs, the resulting material evidently contains a surface residue of TOAB surfactant on the exterior of SWCNT bundles (Figure 2b). As we emphasize in this work, for applications that require the surface of the SWCNT as an active component to enable device performance, this insulating surfactant coating will significantly inhibit the performance of these materials. In order to quantify the different material characteristics evident in Figure 2a-b, thermogravimetric analysis (TGA) was carried out in air on both the pristine and surfactantcoated foams (Figure 2c) at 20 oC/min. It is known that as-grown HiPCO SWCNTs start to oxidize and decompose in air at temperatures above 400 °C with the most prevalent mass loss at temperatures above 600 °C.99-100 This is generally consistent with what is measured for the SWCNT foams prepared from NMP solvents, with a slight (~10%) mass loss at low temperatures that we associate to residue from chemical processing of the materials. However, for foams prepared from surfactant-suspended SWCNTs, the derivative weight loss analysis of the TGA measurements exhibits multiple peaks associated with thermal decomposition. We associate the derivative peaks at 500 °C and below primarily to weight loss due to removal of


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excess surfactant impurities, emphasizing the surfactant to be a significant portion of the total foam material weight that is not removed in the washing and HCl treatment required to dissolve the sacrificial foam and isolate the freestanding material. Notably, as virtually all of the material mass burns away below temperatures of 800 °C, this supports the notion that our SWCNT materials remain free of excess metal impurities. This emphasizes the significantly enhanced pristine character of SWCNT foams using EPD from NMP polar solvents without the use of surfactant in comparison to conventional EPD routes where surfactant is required.

In this

manner, to the best of our knowledge, our work is the first to show that SWCNTs, or generally low-dimensional carbon nanostructured materials, can be EPD coated into structures without the use of surfactants to aid dispersion and enable deposition. Finally, as the SWCNTs that we utilize in this study are HiPCO produced material, we show Raman spectra of a typical foam material with 785 nm laser excitations in Figure 2d. Evident from this spectra is the presence of radial breathing modes (200-500 cm-1), D band (~ 1293 cm-1), G band (~ 1590 cm-1), and 2D mode (2578 cm-1) signatures – which are identical between the SWCNTs processed in NMP and THF-TOAB surfactant solutions. Overall, SWCNTs are excellent candidates for broad applications in electrochemical devices. Their native high surface area, conductivity, electrochemical stability, and low mass density make them excellent for applications in energy storage systems. Our aim in this study is two-fold, specifically to (i) emphasize EPD from NMP-SWCNT suspensions as a viable route to assemble high performance, broadly applicable energy storage device electrodes, and (ii) demonstrate the importance of clean, surfactant-free processing routes on the performance of these electrode materials in different electrochemical platforms. To begin, we first demonstrate the utility of these SWCNT foam materials as binder-free electrodes in electrochemical


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supercapacitors (Figure 3). Supercapacitors operate on the premise of non-Faradaic charge storage through ions, in our case supplied using an ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4), that assemble on the surface of a nanostructured conductive material.

To analyze the electrochemical stability of these devices, we performed cyclic

voltammetry (CV) analysis between voltages of -3V to 3V. For foams prepared using surfactantfree NMP dispersions, relatively smooth CV curves are obtained (50 mV/sec) that emphasize good non-Faradaic storage character, whereas foams prepared using TOAB surfactants exhibit evident bumps in CV scans (~ 0.8/-0.8 V) that correspond to reduction/oxidation peaks originating from the surfactant residue. Additionally, at the same scan rate, the surfactant-free foam material exhibits a greater specific average capacitance evidenced by the greater width between positive and negative CV scans. Based upon assessment of the electrochemical window to be near 2.5-2.7 V from CV scans, we performed Galvanostatic charge-discharge cycling of these devices to assess the charge storage at different charging rates (Figs. 3b-d). Figure 3b shows three consecutive charge-discharge cycles from foam materials prepared with both the NMP-SWCNT surfactant-free dispersions and the TOAB-SWCNT surfactant dispersions, showing evidently greater charge storage capability based on equivalent charge-discharge cycles at 1 A/g currents. In both of these cases, the charge-discharge curves exhibit triangular behavior that represents good non-Faradaic energy storage, and this is maintained uniformly over cycling as shown. To assess the rate capability and energy-power performance of these devices, we performed Ragone analysis by utilizing charge currents ranging from 0.1 A/g to 20 A/g. We find at low charging currents of 0.1 A/g, we observe specific capacitances of ~ 83 F/g for the surfactant-free foam materials, and ~ 46 F/g for the electrodes prepared with TOAB surfactant. At faster charging rates, we find the pristine SWCNT foams to exhibit better rate capability than


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the surfactant prepared foams (supporting information). We attribute this improved performance to the gravimetric and chemical properties of residual surfactant in ion storage. The improved rate capability with the pristine SWCNT foams reflects the notion that the best rate capabilities thus far have been measured on CVD prepared SWCNT foams, and most SWCNT foams prepared from liquid processing routes have exhibited poor rate capability. To further analyze the energy-power performance of freestanding SWCNT electrodes, we calculated the energy density based on directly integrating the area under the discharge curve using the relation: ௧

‫ܫ = ܧ‬௖ න ܸ(‫ݐ݀)ݐ‬ ଴

where Ic is the charging current and V(t) is the discharge profile as a function of time. In the case where V(t) is a linear function, the energy density approximation of E = ½CV2 holds true. This relation was explicitly utilized, as the assessment of energy density in these devices using the common 1/2CV2 relationship can significantly overestimate the total stored energy in a supercapacitor device by over a factor of 2X compared to the true stored energy (supporting information). To find the power density, we calculate P = E/∆t, where ∆t is the total discharge duration. Ragone plots for these devices are shown in Figure 3d for both the surfactant-free and surfactant-prepared foam materials. This demonstrates energy densities of ~ 25 Wh/Kg and power densities approaching ~ 10 kW/Kg. Whereas the measured power density is lower in comparison to ultra-thin coatings of nanostructures,101-103 these foam materials are macroscopically thick (500-800 µm), binder-free, and still capable of maintaining ion-accessible surface area for non-Faradaic storage.

Furthermore, the energy-power performance of the


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SWCNT foams prepared from NMP surfactant-free solutions demonstrate over 2X improvement in energy and power performance compared to equivalent foams prepared with surfactants. In order to better understand the role of surfactant residue presence to influence the charge storage properties, we performed electrochemical impedance spectroscopy (EIS), with Nyquist plots shown in Figure 3e. For the pristine SWCNT foams, we observe a semicircle at high frequencies transitioning to a mid-frequency spike that remains linear at lower frequencies where double layer formation occurs. Separating these features is a knee frequency located at 13.6 Hz. In contrast to this behavior that is evident of a typical supercapacitor, EIS analysis of TOAB surfactant prepared foams exhibits the presence of multiple high to low frequency semicircles. This indicates that unlike the pristine SWCNT foams that exhibit one distinct time constant for capacitive energy storage, the electrochemical interface between the surfactantprepared SWCNT foam and the electrolyte is inhomogeneous due to the presence of surfactant, leading to multiple time constants associated with ion absorption on the SWCNT surface. In order to quantitatively assess this behavior, we utilized equivalent circuits to fit the EIS data in Figure 3e, and confirmed the fits with Bode magnitude and phase plots (supporting information). We utilized a modified Randles’ circuit as an equivalent circuit model, with an additional transmission line representation for the surfactant-prepared foam to represent the multiple time constants observed in the Nyquist plot.

From these fits, we can quantitatively assess the

equivalent series resistance (ESR), which is the total combined electrode and solution resistance components, and the average charge-transfer resistance (Rct), which are plotted in Figure 3F. Consistently for both the ESR and Rct, where the latter is measured across the primary circuit in the transmission line model, we observe that the pristine foam exhibits a ~ 3-4X smaller value for both of these quantities. This means that the surfactant plays the role of both (i) causing the


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electrode resistance to decrease due to its electrically insulating nature, compromising the ESR, and (ii) causing ion absorption on the surfactant-coated SWCNT surface to be significantly more torturous compared to the pristine foams. We present the role of residual surfactant, supported by results from device analysis in Figure 3, in a scheme shown in Figure 4a. Overall, we generally find that residual surfactant leads to two primary effects in SWCNT foam supercapacitors, (1) poorer gravimetric performance due to the residual mass of the surfactant in the material – a mechanism universal in all specific measurements discussed in this work, and (2) compromised non-Faradaic ion storage behavior on SWCNT surface sites where residual surfactant resides. The latter point is reflected in the higher power loss (ESR) and Rct value in the equivalent circuit analysis for the surfactantprepared SWCNT foams. To compare how the range of the values measured in this work relate to those reported previously in the literature for SWCNTs, we plot the specific capacitance measured at 0.1 A/g to the maximum specific capacitances measured for SWCNTs with a variety of electrolytes, processing techniques, and material morphologies (Figure 4b). Evident from Figure 4b is that most values measured for SWCNTs remain below 90 F/g and comparable to our maximum specific capacitance of 83 F/g measured for the pristine SWCNT material. As most SWCNT materials are processed with binders or surfactant-processing, the results for our surfactant-prepared samples are consistent with the average results measured using such liquid processing techniques. The highest numbers reported thus far have been centered on materials where direct CVD growth was used to obtain the materials or care was taken in processing to limit impurities.37, 55 This emphasizes the importance of clean, functional materials to exploit the non-Faradaic ion storage properties of SWCNT materials and emphasizes our SWCNT foams as being competitive with some of the best SWNCT supercapacitors developed thus far.


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Whereas the high surface area of SWCNTs makes them excellent candidates for supercapacitors, SWCNTs have also demonstrated promise for improved Faradaic lithium metal storage in comparison to bulk carbon materials. In this manner, we employed these foams as Liion battery anodes in half-cell configurations involving Li foil cathodes, celgard 2630 separators, 1M LiPF6 in EC-DMC electrolyte, and pressed into coin cells for testing. To assess their lithium storage capability, we first performed Galvanostatic charge-discharge measurements with charging currents ranging from 0.186 A/g to 14.9A/g (0.5 to 40C for conventional graphite) over a range of 0 – 3.7 V vs. Li/Li+ (Figure 5a, 5b). Comparison of charge-discharge curves at three different rates of 0.186 A/g (C/2), 1.49 A/g (4C), and 7.44 A/g (20C) indicates very clear chargedischarge characteristics between the foams prepared with and without surfactant. The most notable difference in these curves is the presence of a significant low-voltage plateau between 1 – 2 V vs. Li/Li+ that is evident in the NMP (pristine) prepared foam, whereas the surfactantprepared foam indicates storage mostly occurring at voltages above 2 V. The difference in this storage behavior directly translates to a key differentiating feature of these two materials in terms of their overall energy efficiency. The lower voltage plateau for the pristine SWCNT foam leads to a total energy efficiency of 46%, calculated by integrating the charge and discharge peaks and comparing the ratio. Comparably, the energy efficiency for the surfactant-prepared foam is only 24% due to the higher voltage storage and the same or lower voltage during discharge. Previous studies of SWCNT materials have emphasized this low-voltage plateau only present for the first charge-discharge cycle,79 and our work emphasizes the critical concept that this plateau is the native intercalation characteristic of a pristine SWCNT material, and is compromised by the presence of surfactant or impurities at the SWCNT-electrolyte interface. Whereas most studies on SWCNTs and graphene for Li-ion battery anodes only emphasize capacity as a metric of


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performance, our work emphasizes that processing can significantly impact the energy efficiency – one of the most critical parameters for realization of such materials for battery applications. This trend is better elucidated in cyclic voltammetry (CV) curves carried out at a voltage scan rate of 0.5 mV/second (Figure 5c). For the NMP-prepared surfactant-free foam material, a large peak centered around ~ 1.6 V vs. Li/Li+ on the positive voltage scan, and two peaks located at ~ 1.3 V and 0.65 V vs. Li/Li+ on the negative voltage scan correlate to the reversible redox couple associated with Li storage on SWCNTs. However, in the case of the surfactant-prepared foam material, these peaks are present, but are visibly small. This further reinforces the notion discussed in reference to Galvanostatic testing that the pristine SWCNT material can natively store charge with significantly higher energy efficiency than the surfactant-prepared SWCNT foam. This suggests that it requires more energy, on average, to decompose the LiPF6 electrolyte for metal storage on a SWCNT surface with surfactants, and that Li is extracted upon discharge at a lower voltage in comparison to pristine SWCNT surfaces. To further emphasize these trends at different charge-discharge rates from 0.5 – 40 C (graphite), we plot the capacity as a function of the charge rate (in A/g) for both the pristine and surfactant-prepared SWCNT foams from 0 – 3.7 V (vs. Li/Li+). To emphasize the difference in usable energy that is stored with good energy efficiency between these two samples, we also plot the specific capacity that is associated with charging to 2 V. This emphasizes that the NMP prepared foam material leads to both (i) an increased total capacity of the device in comparison to TOAB-prepared SWCNT foams (1300 mAh/g versus 690 mAh/g), and (ii) a significantly greater percentage of the total capacity associated with charge-transfer reactions occurring below 2 V where this charge could be practically exploited in an energy-efficient full-cell lithium-ion battery. Notably, for pristine SWCNT foams, the < 2V capacity at a standard rate of 2C for graphite is still greater than the


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maximum theoretical capacity of graphite, emphasizing the promise of this material for a true energy efficient, advanced storage material. In order to assess the cycling behavior and the impact of cycling on the SWCNT foam storage behavior, we cycled these devices through over 250 Galvanostatic charge-discharge cycles at a rate of 0.74 A/g (2C) (Figure 6A). During this cycling process, we observe that the specific capacities of both devices improve, stabilize, and then start to decrease before further stabilizing. Cycling for 500 cycles at 0.74 A/g yields a 500th cycle capacity for the pristine foam comparable to the capacity upon device assembly (supporting information), which represents ~ 2 months of continuous charging/discharging. Throughout this cycling process, however, the Coulombic efficiency remains at 100% (Figure 6A) emphasizing that all charge stored in the device is reversed through reversible redox storage. After 250 continuous Galvanostatic chargedischarge cycles, we observe the capacity at 0.186 A/g (C/2 for graphite) to be 2210 mAh/g (pristine) and 1840 mAh/g (surfactant-prepared). This emphasizes that continuous cycling of the devices leads to a greater improved capacity of the surfactant-prepared SWCNT foam relative to the pristine foam. To better understand the mechanism of this effect, we performed CV analysis (0.5 mV/second) of the devices after 250 Galvanostatic charge-discharge cycles at 2C rates (Figs. 6c, 6d). For the NMP-prepared SWCNT foam, we observe the CV analysis to retain the large intercalation peak at ~ 1.6 V, but we notice the improved storage at higher voltages and a slightly broadened peak close to 0 V in the negative scan, emphasizing slightly lower energy efficiency. However, the overall charge intercalation peak at 1.6 V is virtually identical to the initial analysis, indicating no degradation to the electrolyte decomposition process. On the other hand, for the surfactant-prepared SWCNT foams, we observe that cycling the device acts to amplify and better distinguish the charge and discharge peaks at 1.6 and 0.65 V vs. Li/Li+,


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respectively. This collective behavior compared between these two devices tends to suggest that (i) the SWCNT foams initially prepared with surfactant indicate significantly different chargedischarge energetics that can be associated with the surfactant bridging the electrode-electrolyte interface, and (ii) successive cycling of the surfactant-prepared SWCNT foams over 250 cycles leads to electrochemical performance indicative of a more pristine SWCNT-electrolyte interface. This suggests that cycling the SWCNT foams leads to charge transfer redox reactions that act to chemically degrade the surfactant and improve the device performance. Whereas such sidereactions are not beneficial for practical Li-ion battery design, it emphasizes that SWCNTs are promising alternatives for high capacity battery electrodes, and that to fully exploit their promise, one must carefully consider the impact of material processing on the energy storage metrics sought in a commercial cell. A schematic demonstrating the discussed impact of surfactants on the lithium metal storage capability of SWCNTs is illustrated in Figure 7a. Here, illustrative energy diagrams are constructed to visualize the observations made in this study for Faradaic charge-storage reactions occurring between the LiPF6 electrolyte and the surface of the pristine and surfactant-prepared SWCNT foams. Based on our observations in this study, we generally find that the presence of surfactant leads to higher average energies for electrolyte decomposition and metal storage, and lower average energies for the replenishing of electrolyte with Li metal. By cycling the material, we observe this to improve for the surfactant-prepared foam, as charge-transfer reactions remove surfactant from the SWCNT surface while the pristine SWCNT material shows nearly identical or slightly degraded performance. In order to assess how these SWCNT foam materials in our work compare with other SWCNT and also multi-walled CNT electrodes, we included the capacities measured in our devices both before and after 250 cycles in Figure 7b in comparison


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to the highest capacities reported in other studies.62-86

Prior to cycling, our work shows

comparable storage capacities to the best reported CNT battery anode materials at chargedischarge currents (0.186 A/g) that are ~ 4X higher than other studies. After cycling for 250 Galvanostatic cycles at 0.74 A/g, we observe storage capacities greater than the best measured capacities on SWCNT materials thus far. Notably, as these measurements are carried out at charging/discharging currents of 0.186 A/g, compared to other studies that report values at < 100 mA/g currents, we emphasize the storage behavior at comparable rates for this material should be enhanced, even though such low rates are not practical for true operation of these devices. Additionally, where many of these reports involve capacities that exist at high voltages and poor energy efficiencies, we emphasize the important role of clean material processing on achieving materials that exhibit large capacities and the majority of that capacity usable in a full-cell device with good energy efficiency. Whereas SWCNTs have great promise for lithium-ion batteries, significant effort has recently been focused toward the development of lithium-air batteries, which have the potential to exhibit energy densities superior to lithium-ion batteries and capable of replacing fossil fuels.96-97 To realize such devices, cathodes that are electrically conductive, lightweight, and porous must be developed. Whereas previous reports have primarily focused on multi-walled CNTs as electrodes for lithium-air batteries,88-95 we demonstrate here that SWCNT foams can be viable high performance materials for lithium air batteries. In order to assemble lithium-air batteries, we sandwiched freestanding SWCNT foams with lithium foil in a mesh coin cell using 1 M LiClO4-TEGDME electrolyte solutions. We then pressurize the coin cell holder with 20% O2/Ar mixture, and carry out measurements on devices.

Shown in Figure 8a, 8b are

characteristic discharge curves from Li-air batteries prepared from identically prepared SWCNT


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foams utilizing both NMP SWCNT suspensions (Fig 8a), and SWCNT-TOAB surfactant suspensions (Figure 8b). Discharge voltages characteristic of lithium peroxide formation are evident in the Li-air battery prepared using pristine SWCNT foams, with a discharge plateau that extends to a total discharge capacity of 8275 mAh/g at a rate of 0.1 A/g, measured in the reversible regime above 2 V. At higher rates of 0.5 A/g and 1 A/g (rates comparable to supercapacitors and lithium-ion batteries), the voltage and width of the discharge plateau drops, but still exhibits 4695 mAh/g and 2420 mAh/g total capacity, respectively. This discharge capacity of the SWCNT foams exceeds that previously measured for SWCNT materials,87 and is consistent with the best results measured for MWCNT foams materials that remain supported by Ni foam substrates (~ 8500 mAh/g).88 On the other hand, for the TOAB surfactant-prepared SWCNT foam materials, the discharge characteristics of the devices were poor, exhibiting a maximum of ~ 250 mAh/g at discharge rates of 0.1 A/g. At higher rates (0.5 A/g and above), the measured capacity was virtually zero, and this behavior was consistently measured across many test devices, with none of them showing any notable capacity. This significant difference in device performance emphasizes that the presence of surfactant on the SWCNT surface completely inhibits the nucleation of reversible charge storage products in the voltage range of 22.7 V vs. Li/Li+ that are typically present in discharge processes with good lithium-air battery cathode materials.97 This leads us to conclude the difference between surfactant prepared and pristine foam materials (illustrated in Figure 8c), where surfactant plays an inhibitory effect on the nucleation process of reversible lithium-peroxide charge storage species on the SWCNT surface. Unlike lithium-ion batteries, where the role of the SWCNTs is to generate chargetransfer sites to form monovalent lithium, or supercapacitors where the role of the SWCNTs is to produce a conductive surface to form a double-layer, lithium-air batteries require a cathode


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material to support nucleation sites for chemical species that are catalytically formed on the SWCNT surface during discharge. This means that the presence of surfactant, unique from supercapacitors and lithium-ion batteries, can completely inhibit the Faradaic storage process in lithium air batteries and only enable the formation of irreversible products based on the reaction between the electrolyte and surfactant at low rates. On the other hand, the pristine surface characteristics of the SWCNTs enables a discharge capacity that can be maintained at over 2000 mAh/g at high rates of up to 1 A/g, which is a rate commonly employed in supercapacitor electrodes where conceptually similar surface ion absorption processes dictate the device performance. Whereas the prospect of these materials and the role of surfactants are highlighted by discharge capacity, we have emphasized the rechargeable character of these devices in Galvanostatic curves shown in the supporting information. We generally observe OER voltages between 3.7 – 4.1 V (vs. Li), which is consistent with other studies involving pristine carbon nanomaterials.88,91 Recent efforts have shown such disparity between OER and ORR can be reduced using electrocatalysts,104 which could improve the total energy efficiency of these devices. Comparing these results to other results for SWCNTs and MWCNTs reported in the literature, our capacities at 0.1 A/g rates are comparable to the best reported values for these materials (8275 mAh/g) and significantly improved compared to most CNT-based electrodes (Figure 8d).88-95 It should be noted that whereas is has become common to define charging currents in A/cm2, we have utilized charging currents normalized to the active mass of the material, consistent with our supercapacitor and lithium-ion battery measurements. This is due to the notion that practical industry applications of such batteries require performance that scales to active material mass (or volume), and not surface area. Nonetheless, we have included performance of both approaches to analyzing storage capability in Figure 8d. Previous studies


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have emphasized capacities of up to ~ 8500 mAh/g for MWCNTs supported by Ni foam materials at discharge rates of 0.1 mA/cm2,88 with previous SWCNT materials demonstrating less than 3100 mAh/g at comparable current density. To further emphasize the comparison between surfactant-prepared and pristine SWCNT foams illustrated in Figure 8c, we performed TEM imaging of the pristine and surfactant-prepared foams after the first discharge at 0.1 A/g discharge rates. Whereas we observe the presence of Li2O2 discharge products in the pristine SWCNT foams (Figure 8e, 8f), we do not observe clustered discharge products in the surfactantprepared foams due to both the virtually negligible capacity and the low ORR potentials vs. Li that indicates parasitic reactions with the surfactant. Our work here underlines the critical importance of SWCNT material processing and impurities on enabling practical, high rate and high capacity lithium-air discharge characteristics and further emphasizes the notion that SWCNT foams are a high performance and lightweight electrode material for lithium-oxygen or lithium-air batteries. Finally, as our efforts have demonstrated a common material that can be applied across broad electrochemical energy storage device platforms, we analyzed the best devices studied in this work (prepared with pristine SWCNT foams) in order to assess their relative true powerenergy performance. In order to compare our results with both other literature reported values, where it is conventional to only report active mass, and numbers relevant to industrial applications, we report (a) the power and energy performance normalized to the active electrode mass, and (b) the power and energy normalized to the total device mass, which includes the electrode, electrolyte, and active lithium metal in the case of batteries – both are shown in Figure 9. The latter calculation is relevant for porous electrode materials where independently assessing the volumetric or specific performance without consideration of electrolyte can lead to numbers


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that are challenging to interpret and compare. In all cases, energy densities are calculated based on direct integration of the discharge curves for each device (using previous equation 1) indicative of the true stored energy measured in the device. Comparing the active material energy-power performance, our analysis emphasizes that the SWCNT foam materials exhibit excellent performance scaling across all three device platforms. Specifically in lithium-ion batteries, the SWCNT materials exhibit improved power capability compared to supercapacitors formed with the same material. This high power performance is significant since (i) these pristine SWCNT foam electrodes exhibit energy efficiencies that are ~ 50%, which is, to the best of our knowledge, the best value reported for functional SWCNT materials, and (ii) a majority of the energy is stored at voltages < 2 V, which is usable in a practical Li-ion cell coupled with common cathode materials. In this notion, this energy and power performance will improve under the notion that this anode is coupled with a cathode that does not limit the device power capability, and that the total voltage of the cell will increase, thus leading to a greater total energy stored with the same capacity. Furthermore, utilizing these devices for lithium-air battery cathodes yields power densities comparable to low-rate supercapacitors (2-3 kW/kg), but with energy density up to 21.2 kWh/kg. Compared to active mass, these metrics for battery electrodes are among the best reported thus far, and emphasize promise for these materials compared to other candidates studied in the literature. This idea is not significantly altered when considering the total device mass (Figure 9b). In this case, we performed measurements of the total amount of electrolyte mass contained in the device, and found that this made up ~ 25% of the electrode mass. Furthermore, based on the molar mass of Li (6.94 g/mol) and a total assumed charge of 96500 C/mol (one charge per Li ion), we calculated the total active metal mass in addition to the electrolyte and electrode mass. This mass therefore represents the total weight of an optimized


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battery system that has practical significance for comparison to commercial products, which are assessed for the specific performance of the whole packaged device. Most notably, after this analysis we find that Li-air batteries prepared with SWCNT exhibit energy densities up to 3.3 kWh/kg, which is approaching the energy density of gasoline at 12.7 kWh/kg. This result plays a significant role because it demonstrates that these devices, coupled with lightweight porous SWCNT foams, have energy storage capability based on total device mass within 3-4X of that achieved with gasoline. As the basic premise of lithium-air batteries is energy performance capability to replace fossil fuels, this work gives promise to this vision using SWCNT foams and through direct total mass measurements that we propose should be a benchmark for Li-air battery assessment and improvement. Overall, these results collectively suggest that freestanding SWCNT foams are a universal platform for high performance electrochemical energy storage, and that the route to achieve such high performance is to harness the utility of clean, liquid processing techniques that minimize impurities which can inhibit usable, efficient charge storage behavior. In the broadest sense, beyond electrochemical energy storage, our work emphasizes a message transferrable to areas of catalysis, sensing, energy conversion, or electronics, which emphasizes the foundation for innovation with such materials is the development and understanding of clean solution processing routes to manipulate and assemble SWCNTs into macroscopic, functional structures. CONCLUSION We demonstrate a route to use electrophoretic assembly to produce freestanding SWCNT foam materials, and compare the impact of processing these materials from pristine, NMPSWCNT suspensions and TOAB-SWCNT surfactant suspensions. Our results indicate that the


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residual TOAB surfactant in the foam due to processing compromises the gravimetric and Faradaic storage characteristics of electrochemical energy storage devices in the order of lithiumair batteries > lithium-ion batteries > supercapacitors, where the greater value indicates more compromised storage performance. In the extreme case of lithium-air batteries, we find the presence of surfactant to completely inhibit the formation of reversible charge storage products upon discharge, with the surfactant-prepared foam to only exhibit ~ 3% of the total capacity of the pristine foam. Whereas these results emphasize the importance of clean solution processing of SWCNT macroscopic materials, our results also give significant promise to the utilization of NMP-assembled SWCNT materials in electrochemical energy storage.

For SWCNT foam

supercapacitors, we demonstrate specific capacitances up to 83 F/g, with true energy densities near ~ 25 Wh/Kg.

For SWCNT foam lithium-ion battery anodes, we observe reversible

capacities of 2210 mAh/g, with energy efficiencies up ~ 50%, half-cell energy (power) density over 1 kWh/kg (10 kW/kg) and demonstrated cycling behavior greater than 500 cycles. For SWCNT foam lithium-air battery cathodes, we observe high specific capacities of 8275 mAh/g at discharge rates of 0.1 A/g, yielding half-cell energy density of 21.2 kWh/kg (3.3 kWh/kg for total cell including electrode, electrolyte, active metal), with power densities comparable to lithium-ion cells. This performance is approaching the energy capability of fossil fuels with 12.7 kWh/kg, which is the best and most promising result measured thus far to our knowledge. We emphasize such excellent performance to be achievable through the development and utility of clean, surfactant or impurity-free assembly processes which preserve the native SWCNT surface properties, and is transferrable to diverse applications such as catalysis, sensing, electronics, and energy conversion, among others.


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ACKNOWLEDGEMENTS We thank Andrew Westover and Keith Share for discussions related to this work. This work was supported in part by NSF grant CMMI 1334269 (L.O.), an ORAU Powe Junior Faculty Award, and by an NSF REU DMR 1005023 (J.H.). SEM materials analysis was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. Supporting Information Available: (i) Description of experimental EPD setup used to make SWCNT foams, (ii) rate capability analysis of SWCNT supercapacitors, (iii) assessment of the true versus the approximated energy-power performance for SWCNT supercapacitors, (iv) equivalent circuit analysis for SWCNT foam supercapacitors including model description, (v) cycle lifetime to 500 cycles for the SWCNT foam Li-ion battery, (iv) full charge-discharge profiles for SWCNT electrodes used in Li-air devices. This material is available free of charge via the internet at http://pubs.acs.org.


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FIGURES

Figure 1. (A) SWCNT-NMP surfactant-free suspensions utilized to make freestanding foams, (B) Ni foam coated with SWCNTs from NMP solutions using EPD, (C) picture of a freestanding SWCNT foam following dissolution of the sacrificial Ni foam substrate, and (D) picture of a freestanding SWCNT foam following metal foam removal and drying. (E-F) SEM images at different magnifications showing the nanostructured and microstructured features of SWCNT foams that remain in-tact in the freestanding material. (E) SB = 1 µm, (F) SB = 20 µm.


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Figure 2. (A-B) SEM images at similar magnifications showing the different surface topology of the SWCNTs in foams prepared with NMP surfactant-free solutions (A) and TOAB-THF surfactant suspensions (B). (C) Thermogravimetric analysis of SWCNT foams prepared using NMP (pristine) solutions and TOAB (surfactant) solutions. Derivative weight loss curves are shown with dotted lines. (D) Raman spectra of SWCNT foam materials showing features consistent with HiPCO SWCNTs. SB = 200 µm in both images.


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Figure 3. SWCNT foam supercapacitor device analysis. (A) Cyclic voltammetry at 50 A/g for pristine (NMP) and surfactant (TOAB) prepared SWCNT foams, (B) Galvanostatic chargedischarge measurements at 1 A/g for pristine and surfactant prepared foams, (C) Rate capability plot showing specific capacitance as a function of charging current for pristine and surfactant prepared SWCNT foams, (D) Ragone energy-power plot based on integrated energy densities from Galvanostatic charge-discharge curves at different rates. (E) Nyquist plot showing the real (Z) and imaginary (Z’’) device resistance as a function of frequency. (F) Key fitting parameters from equivalent circuit fits (supporting information) derived from electrochemical impedance spectroscopic analysis showing the comparative equivalent series resistance (ESR) and charge transfer resistance (Rct) for pristine and surfactant-prepared SWCNT foams.


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Figure 4. (A) Schematic illustrating the effect of surfactant on the double layer storage behavior of SWCNT foams based on electrochemical measurements, (B) Comparative analysis of supercapacitor results for pristine and surfactant-prepared SWCNT foams based on previous literature assessments of SWCNT supercapacitors.


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Figure 5. SWCNT foam lithium-ion battery anode performance analysis. (A-B) Galvanostatic charge-discharge measurements of NMP prepared (pristine) (A) SWCNT foams and TOAB surfactant-prepared SWCNT foams (B) at charging rates of 0.186, 1.49, and 7.44 A/g. (C) Cyclic voltammetry curves taken at 0.5 A/g for surfactant-prepared and pristine SWCNT foams emphasizing the effect of surfactant on the lithium storage behavior. (D) Rate capability curves


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showing the specific capacity as a function of the Galvanostatic charging current for pristine and surfactant-prepared SWCNT foams, including the charge capacity measured below 2 V that is associated with the charge storage plateau clearly visible in curves shown in (A-B).

Figure 6. Cycling performance of SWCNT foams in lithium-ion batteries. (A) Specific capacity measured over 250 Galvanostatic charge-discharge cycles at 0.744 A/g for the pristine and surfactant-prepared foam anodes (cycling up to 500 cycles shown in supplementary information). (B) Rate capability plots taken after 250 Galvanostatic charge-discharge cycles showing the performance of surfactant prepared and pristine SWCNT foam anodes post-cycling. (C-D)


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Cyclic voltammetry curves at 0.5 A/g showing the effect of cycling on the lithium storage properties of (C) pristine SWCNT foams, and (D) surfactant-prepared SWCNT foams.

Figure 7. (A) Schematic illustration of the effect of surfactant on the intercalation and deintercalation behavior of SWCNT foam anode materials, with generalized energy diagrams for these processes that represent the measurements from this work. (B) Comparative analysis of results for SWCNT foams compared to other literature-reported SWCNT and MWCNT materials used as lithium-ion battery anodes.


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Figure 8. SWCNT foam lithium-air battery performance analysis. (A-B) Discharge curves at different rates from 0.1 A/g to 1 A/g for pristine, NMP prepared SWCNT foams (A) and surfactant-prepared SWCNT foams (B) measured down to 2 V vs. Li/Li+.

(C) Schematic

illustration demonstrating the nucleation of reversible charge storage products on pristine SWCNT foams compared to a low capacity of irreversible products on surfactant-prepared SWCNT foams. (D) Comparative analysis between first discharge capacities measured in this work versus other SWCNT and MWCNT materials studied for Li-air battery cathodes. (E-F) HR-TEM characterization of pristine SWCNT foams post-discharge showing the formation of reversible products on SWCNT bundles. (E) High magnification image of a nucleated Li2O2 cluster on a SWCNT bundle, and (F) lower magnification image showing several nucleation sites on each SWCNT bundle. SB = 20 nm in (E) and (F).


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Figure 9.

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Ragone plots showing the relative energy-power performance of pristine NMP

prepared SWCNT foams in templates of supercapacitors, lithium-ion batteries, and lithium-air batteries for (a) SWCNT active material mass only, and (b) total device mass including SWCNT, electrolyte, and active lithium metal. Energy and power are based on integrated discharge characteristics of these devices.


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Solution Assembled Single-Walled Carbon Nanotube Foams

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