Lithium Batteries with Nearly Maximum Metal Storage - ACS Nano

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Lithium Batteries with Nearly Maximum Metal Storage Abdul-Rahman O. Raji, Rodrigo Villegas Salvatierra, Nam Dong Kim, Xiujun Fan, Yilun Li, Gladys A. L. Silva, Junwei Sha, and James M. Tour ACS Nano, Just Accepted Manuscript • Publication Date (Web): 16 May 2017 Downloaded from http://pubs.acs.org on May 17, 2017

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Lithium Batteries with Nearly Maximum Metal Storage Abdul-Rahman O. Raji,†,¶ Rodrigo Villegas Salvatierra,†,¶ Nam Dong Kim,† Xiujun Fan,‡ Yilun Li,† Gladys A. L. Silva,† Junwei Sha† and James M. Tour†,‡,§,* †

Department of Chemistry, ‡The NanoCarbon Center, §Department of Materials Science

and NanoEngineering, Rice University, 6100 Main Street, Houston, Texas 77005, USA ¶

These authors contributed equally to this work. *Email: [email protected]

Abstract The drive for significant advancement in battery capacity and energy density inspired a revisit to the use of Li metal anode. We report the use of a seamless graphene-carbon nanotube (GCNT) electrode to reversibly store Li metal with complete dendrite formation suppression. The GCNT-Li capacity of 3351 mAh g-1GCNT-Li approaches that of bare Li metal (3861 mAh g1

Li),

indicating the low contributing mass of GCNT, while yielding a practical areal capacity up

to 4 mAh cm-2 and cycle stability. A full battery based on GCNT-Li/sulfurized carbon (SC) is demonstrated with high energy density (752 Wh kg-1total electrodes, where total electrodes = GCNT-Li + SC + binder), high areal capacity (2 mAh cm-2), cyclability (80% retention at > 500 cycles) and is free of Li polysulfides and dendrites that would cause severe capacity fade.

Keywords: Li metal anodes, graphene, carbon nanotubes, full battery, Li dendrites

Post-Li-ion batteries, such as Li-S and Li-air batteries, require high gravimetric capacity anodes and cathodes. Ideally, during the charging of a battery, the maximum gravimetric capacity would be achieved if Li is deposited on the anode directly as pure Li metal rather than

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stored in intercalation compounds such as graphite as in Li-ion batteries (LIBs). The theoretical capacity based on lithiated graphite LiC6 is ~ 339 mAh g-1 while pure Li metal can theoretically deliver ~ 3860 mAh g-1 assuming 100% of Li usage in the discharge operation.1,2 This enormous capacity compared to commercial Li-ion anodes explain the revisiting of Li metal after more than 30 years of the first attempts to incorporate this low density metal in high energy density batteries.3,4 However, Li metal problematically forms dendrites and related unstable structures during battery operation.5,6 This results in low coulombic efficiency (CE) and cycle life and poses serious safety concerns as the dendrites can cause short circuits.5,6 Recent literature on Li metal anodes has reported different approaches to minimize or suppress the Li dendrites formation using either modified Li metal foils or electrodeposited Li metal. The reported strategies include specially designed electrolytes,7-9 protective separators to buffer dendrites,10,11 encapsulated Li metal.12,13 Although the effect of dendrite suppression is clear in this research, a comparison of performance is difficult because of the restricted testing conditions, namely areal capacity and areal current density, which sometimes are below the commercial standards for LIBs (~2.0 mAh cm-2 and < 2 mA cm-2, considering a single sided coated electrode). Another interesting strategy is the use of three-dimensional (3D) porous frameworks as host structures for Li metal. In this approach, Li metal is electrodeposited in a 3D structure where it is accommodated and distributed in the empty volume of the porous framework, which reduces local current density and minimizes Li dendrite formation.14-20 The use of scaffolds or 3D frameworks implies that the gravimetric or volumetric capacity of the Li metal anode is reduced by including the mass or volume of the framework component. An ideal framework structure for Li dendrite suppression would involve a high surface area, low density material with a homogenously conductive surface for Li deposition that would maximize the


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gravimetric capacity of the Li metal anode. In addition, a non-tortuous path for Li plating/stripping is desired for reversible operation and high rate applications. We report here a seamless graphene-carbon nanotube (GCNT)21 electrode that is capable of reversibly storing Li metal with complete suppression of dendrite formation. As a low density material (~ 0.05 mg cm-3), GCNT can store large amounts of Li metal homogeneously distributed as a thin coating over CNT bundles, therefore suppressing the dendrite formation during reversible plating and stripping operation. The low contributing mass of GCNT enables the GCNT-Li capacity (3351 mAh g-1GCNT-Li) to approach that of bare Li metal (3861 mAh g-1Li) with high areal capacity (up to 4 mAh cm-2) and reversibility. In order to show the feasibility of the anode, a full battery (FB) is further reported by matching the GCNT-Li anode with a sulfurized-carbon (SC) cathode with high sulfur content (up to 60 wt%). This affords a stable device with an operation voltage of 2.15 V, high energy density (752 Wh kg-1total electrodes, total electrodes = GCNT-Li + SC + binder), high areal capacity (2 mAh cm-2), good cyclability (80% retention at > 500 cycles) and the system is free of Li polysulfides and dendrites that would cause severe capacity fade.

Results/Discussion

Individual metallic CNTs have an electrical conductivity of 106 to 107 S m-1,22 on the order of in-plane conductivity of graphite and up to the conductivity level of copper, a high specific surface area (2600 m2 g-1),21,23 and when grown vertically from a substrate, a 3D structure suitable for non-tortuous Li plating.24 Here, we first grow graphene via chemical vapor deposition (CVD) on a Cu substrate, followed by deposition of iron nanoparticles and aluminum


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oxide and subsequent CVD growth of CNTs at 750 °C using acetylene as the carbon source (Figure 1a).21,25 This method has been previously shown to produce CNTs that are covalently and seamlessly connected to the underlying graphene (Figure 1b),21,25 providing ohmic conductance between the underlying Cu onto which the graphene is grown and the CNTs on top of the graphene. An average electronic conductivity of 1.45 × 103 S m-1 was measured from the I-V curves for the entire electrode from the bottom of the Cu current collector through the top of the GCNT (Figure S1). By adjusting the time of growth, different GCNT thicknesses from 10 to 50 µm can be obtained. The CNTs exist in bundles (Figure 1c,d), which are superlattices held together by van der Waals interactions.26 The CNT bundles are mainly vertical at their bottom but can bend at the top. In addition to an inter-tube spacing of ~ 3.4 Å, the CNT bundles have 6 Å channels.27 The graphene and CNTs are confirmed in Figure 1e,f and the radial breathing modes (RBMs) of the latter at 100 to 300 cm-1 indicate single- to few-walled CNTs (Figure 1g).28,29 The CNTs seamlessly connect to the graphene substrate and are able to store Li ions,27,30,31 however due to low areal mass density, the capacity generated by the GCNTs is negligible (< 0.3 mAh cm-2) (see Figure S2 for a detailed comparison) when compared to the GCNT-Li (up to 4 mAh cm-2), in which Li rather than GCNT is the active material.


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Figure 1. Growth schematic and structural characterization of GCNT. a) Schematic of GCNT growth. E-beam deposited 1 nm iron nanoparticles are non-continuous and they serve as the catalysts for the CNT growth while a 3 nm layer of aluminum oxide provides the support for a vertical tip-growth. b–d) Scanning electron micrograph (SEM) images of GCNT showing a CNT carpet grown vertically from a graphene-covered Cu substrate. e) Raman spectrum of graphene as-grown on Cu. The graphene is conformally connected to its native Cu substrate upon which it is grown. The G band appears at 1589 cm-1, 2D band at 2705 cm-1, IG/I2D ratio > 1, and a trace D band at ~1360 cm-1. The Raman scattering signatures signify a high quality multilayer graphene. The skewed baseline occurred because the spectrum is obtained atop Cu. f)


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Raman spectrum of CNTs grown on the Cu-graphene substrate with the G band at 1587 cm-1, 2D band at 2652 cm-1, D band at 1336 cm-1. g) Raman RBMs of the CNTs in expanded format.

In an activation step, the GCNT is treated by a pre-lithiation method (see Methods). This step is important for a high 1st cycle CE and will be discussed later. The GCNT-Li is produced by plating Li onto the highly porous and high surface area GCNT by galvanostatic deposition. The morphology of the CNT bundles induces formation of Li metal on the CNT surfaces as a film or non-dendritic coating (Figure 2a). The amount of deposited Li metal is regulated by time control. The potential for Li deposition was measured at ~ -50 mV vs Li/Li+ (Figure 2b), due to overpotential as verified later from the current density dependence. Reversible Li plating and stripping from the GCNT are observed (Figure 2b). These are confirmed by a change in color of the GCNT from black to silver, indicating formation of Li metal (Figure 2c), and back to black upon Li stripping. SEM images of the GCNT (Figure 2d) and GCNT-Li (Figure 2e-g) show that the Li is not deposited atop the GCNT as a separate film, but is rather plated homogenously throughout the volume of the pillared CNT structure (Figure 2f), suggesting Li formation inside and outside the CNT bundles. The surface of the GCNT is homogeneously conductive based on its seamless connection with the basal graphene plane. Therefore, the CNT bundles are accessible to promote Li plating. The base-view SEM image (Figure 2g) also indicates the presence of a deposited Li film, which underscores the significance of the micrometer-sized pores in Li-ion diffusion through the GCNT. No discernible exfoliation of the CNT bundles in the delithiated GCNT (Figure 2h,i) is observed. In Figure 2j,k, the transmission electron micrograph (TEM) images of the lithiated CNTs show deposition in the form of nanoparticles on the surface of the CNTs.


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The SEM images of the GCNT-Li presented in Figure 2d–i show no evidence of formation of dendritic, mossy, and related structures that have hindered application of Li metal anodes. The dendrite-free appearance even holds at 4 mAh cm-2. In contrast, deposition over flat substrates (graphene-covered copper foil, CuG) as shown in Figure 2l, using the same electrolyte, produces irregular deposits of Li (Figure 2m,n). Mossy structures are observed in less than

10

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A

previous

report

using

the

same

electrolyte

(4

M

lithium

bis(fluorosulfonyl)imide (LiFSI) in 1,2-dimethoxyethane (DME), see Methods)32 also has demonstrated dendrite suppression on flat Cu foil, although the tests were performed under low areal capacity (0.5 mAh cm2) conditions. Ether-based solvents have long been known for improved Li metal battery performance.33 The shape of the Li particles in this same work also demonstrate a change in morphology in which higher current densities (> 1 mA cm-2) result in the formation of dendrite-shaped Li particles,32 as is observed under our conditions (at 2 mA cm2

) when testing pure CuG substrate. The 3D, high surface area of the GCNT enables the dendrite

suppression effect at high current densities and high capacity, in which the Li metal is infused inside the CNT bundles structure. The high surface area of GCNT contributes to reduce the local current density of Li deposition; the Li deposition occurs on a much larger area compared to a flat Cu foil. The low local current density also implies a low rate of Li-ion reduction to Li metal over the GCNT without exceeding the diffusion limit of Li-ions from the electrolyte into the pores. Therefore, the GCNT structure enables an extension of the Sand’s time for dendrite-free Li plating.34 In addition, the seamless connection of the bottom of the vertically oriented CNTs with the graphene grown on Cu would remove interfacial resistance, thus improving the overall electrode electronic conductivity (Figure S1) and facilitating current distribution throughout the electrode to promote a homogenous Li deposition in the GCNT carpet at high current density


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with low overpotential. This demonstrates that in addition to the electrolyte, the GCNT is necessary to obtain the observed results.

Figure 2. Morphology of the GCNT-Li. a) Schematic of GCNT-Li formation. b) Voltage vs time of lithiation and delithiation processes of GCNT-Li. c) Photograph of GCNT, lithiated, delithiated and re-lithiated (scale bar = 1 cm). d) SEM image of GCNT-Li (0.7 mAh cm-2 at 2 8 ACS Paragon Plus Environment

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mA cm-2) by top-view after 250 cycles. SEM images of GCNT-Li (4 mAh cm-2 at 2 mA cm-2) after 10 cycles by e) side-view, f) expanded top-view and g) expanded side-view. SEM images of delithiated GCNT-Li (0.7 mAh cm-2 at 2 mA cm-2) after 250 cycles by h) top-view and i) expanded top-view. j) TEM image of a CNT from GCNT-Li and k) its higher magnification. l) Schematic of Li deposited on graphene grown on Cu. m) SEM image of Li deposited directly on graphene grown on Cu foil (0.7 mAh cm-2 at 2 mA cm-2) after 10 cycles, with no GCNT, showing the mossy and dendritic Li deposition and n) its higher magnification.

Figure 3 shows the electrochemical behaviour of the GCNT-Li anodes vs CuG-Li. Figure 3a shows representative curves of the Li plating (discharge) and stripping (charge) from the 6th cycle, expressed in terms of gravimetric capacity. The measured capacity at 2 mAh cm-2 is 3120 mAh g-1GCNT-Li, having as reference the masses of both GCNT and plated Li metal, which represent ~ 80% of the capacity of the pure Li metal. As discussed previously, the GCNT has minimal contribution to the anode capacity at the same voltage range (Figure S2). The discharge and charge curves are characterized by remarkably flat voltages at –50 mV and 50 mV, respectively (Figure 3a), characteristic of Li metal plating and stripping.32,35 The charge/discharge curves are also similar in CuG-Li (Figure S3) at the first cycles. It is evident that the plated Li in the GCNT is metallic in contrast with Li-intercalated graphite (LiC6) where Li exists as an ion.35,36 The Li metal in GCNT-Li is further confirmed by X-ray diffraction (XRD) (Figure S4). Next, we compared the polarization evolution between charge and discharge over cycling. The polarization remains similar for GCNT-Li as it is observed over 200 h of continuous cycling (300 cycles) (partial cycling time shown in Figure 3b and Figure S5). In comparison, CuG-Li shows oscillating CE and increased polarization over time (partial cycling


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time shown in Figure 3c). This could be caused by the irregular Li particles formed on the bare CuG substrate (Figure 2l–n). Figure 3d shows the cycling stability and CE. Although the discharge (plating) capacity is fixed by the time-controlled galvanostatic deposition, the charge (stripping) capacity is regulated by voltage cut-off at 1 V, therefore the ratio between these capacities (CE) indicates the degree of reversibility of Li metal after deposition. The first cycle CE is 96.2%, indicating a good reversibility after the first Li plating in the GCNT structure. After 225 cycles, there is no charge capacity fading and the CE is 99.6% (Figure 3d), indicating that during cycling, the Li could be continuously infused in the GCNT structure. Conversely, we found that plating Li metal onto a compact, pure single-walled CNTs electrodes (not GCNT) produced a separate layer of Li metal atop the electrode (Figure S6,7), indicating that an open structure and non-tortuous path enabled by the GCNT is necessary for the efficient Li incorporation and dendrite suppression.


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Figure 3. Electrochemical characteristics of the GCNT anode. a) Charge/discharge profile of GCNT-Li. Gravimetric capacity is based on the mass of GCNT-Li (see Methods). b) Voltage profile of GCNT over time (Inset: expanded charge/discharge cycle). c) Voltage profile of CuGLi (no GCNT) over time (Inset: expanded charge/discharge cycle). d) Cycle performance and CE of GCNT-Li. The current density is 2 mA cm-2 (5.2 A g-1GCNT-Li). The reference dotted black line is set to 99.9%.


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The GCNT-Li structure also enables a stable cycling operation at different areal densities (0.4 to 4 mAh cm-2) (Figure 4a,b). The voltage also shows areal capacity dependence. A small voltage gap of 100 mV between the Li plating and stripping curves is observed for 0.7 mAh cm-2 increasing to 200 mV at 4 mAh cm-2 due to the thicker plated Li (Figure 4a). By adjusting the amount of Li in the GCNT-Li, it is possible to optimize the specific capacity to reach maximum metal storage per carbon mass. The GCNT-Li at 4 mAh cm-2 has a capacity of 3351 mAh g1

GCNT-Li,

which is close (~87%) to the theoretical capacity of Li (3861 mAh g-1Li). A correct

comparison of Li metal with other anode materials would take into account their lithiated forms. Thus, the GCNT-Li (3351 mAh g-1GCNT-Li) has 1.8 times higher gravimetric capacity than Li15Si4 (1857 mAh g-1Li15Si4) and 9.9 times higher than LiC6 (339 mAh g-1LiC6), and it is significantly higher than other lithiated anode materials (Figure 4c). The low density of GCNT-Li (~ 0.05 g cm-3, with an average thickness of 50 µm) implies a large proportion of unoccupied space (porosity 97.8%, see Methods/Experimental) within the electrode that can be used to store high amounts of Li metal as a thin coating over the GCNT bundle (Figure 4c). In practical aspects, the specific capacity of electrodes must also include the current collectors. A detailed comparison is provided in Table S1, in which a high gravimetric capacity (~ 750 mAh g-1) is also obtained when including the Cu current collector mass, significantly higher when compared to graphite anode materials. The prospects of even higher gravimetric capacities of the GCNT-Li, including the current collectors, involve replacing the dense Cu metal foils by conductive carbon films, as recently demonstrated.31 In Figure 4d, the GCNT is shown to plate and strip Li at a current density as high as 10 mA cm-2 (25 A g-1 GCNT-Li), producing a capacity of ~ 0.7 mAh cm-2 (2100 mAh g-1GCNT-Li), which is independent of the current density. The flatness of the curves is still maintained up to 4 mA cm-2 (10 A g-1 GCNT-Li). However, during the GCNT-Li cycling at 10 mA


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cm-2, a significant polarization is observed from the Li plating/stripping curves with loss of the characteristic flatness at lower current densities. The high current capability supersedes values reported for LIB electrodes.37,38 The excellent electrical conductivity of the GCNT monolith facilitates electron transport without the need for conductive additives. The vertical carpet nature of the CNTs would enhance Li-ion diffusion through non-tortuous Li plating and stripping with flexible CNT movements.39 By controlling the time of GCNT growth (Figure S8), it is possible to tune the thickness of the GCNT carpets in order to optimize the volume occupied by the resulting GCNT-Li structure after Li plating. Figure 4e shows that the volumetric capacity of the electrode could reach up to 1600 mAh cm-3 at an areal capacity of 4 mAh cm-2 based on the thickness of the GCNT-Li (~25 µm). Upon Li plating and associated solid electrolyte interphase (SEI) formation, the thickness increased from ~17 µm to ~25 µm (Figure S8). Due to the low thickness of the GCNT, it is expected that the Li will be able to penetrate through the depth of the GCNT, thus utilizing the available volume very efficiently. The CEs of the first cycle and the first few cycles are important factors in predicting the performance of an electrode in a FB. Using the pre-lithiation method (see Methods), a sample cycled to 4 mAh cm-2 shows a cumulative capacity loss of 8.7% during the first 5 cycles (which can be seen in Figure S9a). The pre-lithiation involves a spontaneous reduction of the GCNT electrode by the Li foil with partial lithiation of the GCNT (forming Li+GCNT - not GCNT-Li) and formation of the SEI layer. Both processes can lead to an increase of the first cycle CE compared to the GCNT without pre-lithiation treatment (Figure S9b). Furthermore, we observed that the areal capacity influenced the first cycle capacity loss. The average CE of the first cycle can achieve > 90% at 4 mAh cm-2 and > 80% at the 1 mAh cm-2 anode (Figure S9b). Regarding the pre-lithiation process, the Li ions in the lithiated GCNT can drive the subsequent Li metal


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plating, which could explain the conformal production of Li metal films over the volume of the CNT bundles, as observed in our SEM and TEM images (Figure 2). In single-walled CNTs, it is known that Li ions are located preferentially in the interstitial channels between the individual CNTs, which expands the CNT bundle structure during Li+ intake.40 Therefore, the process to deposit Li metal could start from these expanded bundles or from the initial Li ions previously stored in the bundle. This is evident in the SEM images in which the Li metal is preferentially deposited over the CNT bundles, keeping its shape throughout the cycling. No Li metal is observed between the bundles or outside, as separate films. This observation indicates a selfseeded mechanism of dendrite–free Li metal deposition using as starting material the lithiated GCNT. A similar reported structure was observed using metals as seeds for Li plating.41 This effect demonstrates a clear difference between our work and other reports that use a similar idea of having 3D scaffolds to minimize Li dendrites.12,13,15,17


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Figure 4. Li storage and rate capabilities of the GCNT anode. a) Li storage capacities of the GCNT from 0.4 to 4 mAh cm-2. b) Cycling stability of different areal capacity anodes (1 to 4 mAh cm-2) expressed as areal capacity vs cycles. c) Comparison of the gravimetric capacity of GCNT-Li (with different areal capacities) with other anode materials with respect to the mass of the anode at the fully lithiated state. The areal capacities of the GCNT-Li are from 0.4 to 4 mAh cm-2, represented by GCNT-Li-0.4 to GCNT-Li-4. d) Charge-discharge profiles measured at different current densities expressed in current density per area and per mass of electrode (GCNT-Li). The plating and stripping voltages are below and above 0 V, respectively, due to the overpotential, which increases with current density. e) Voltage vs volumetric capacity. 15 ACS Paragon Plus Environment

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We also demonstrated that a FB can be obtained by matching our dendrite-free GCNT-Li anode with a SC cathode with S content of ~ 60 wt% (Figure S10). The S content in the cathode is reduced to 48 wt% with the addition of binder and carbon additives (see Methods). Cathodes based on SC have advantages over elemental sulfur (S8) cathodes, such as high compatibility with different electrolytes and absence of Li polysulfide diffusion (Figure S10);42 the latter

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Figure 5. FB with a GCNT-Li anode and SC cathode. a) CVs of GCNT-Li and SC cathode half-cells in 4 M LiFSI/DME at 0.5 mV s-1. b) Galvanostatic charge/discharge curves of the FB at 0.1 C with areal capacity of 2 mAh cm-2. c) Photograph of a FB prototype powering a LED. d) Sequential rate performance test (0.2 to 9 C) and cycling stability of the FB. Inset: CE (%) of the rate and stability test. The reference blue dashed line is set to 99.9%. e) Self-discharge (SD) tests of the FB after 8 h and 1 week showing charge curve followed by continuous discharge curve during and after the open circuit period. Inset: Voltage vs capacity of the SD tests. f) Ragone plot of the GCNT-Li/SC FB, here considering the combined mass of the anode and cathode active materials (Li and S) and the full electrode mass (including binder, carbon additives, GCNT, excess of Li), excluding the current collector.

Figure 5a shows the cyclic voltammograms (CVs) of the GCNT-Li and the SC cathode (3rd cycle) half-cells, each with total areal capacity of ~2 mAh cm-2. The first cycle of the SC cathode half-cell has a CE of 83% and the first cycle of the GCNT-Li anode half-cell has an average CE of 85% (Figure S10), both requiring a small excess of Li from the anode in the FB. The

galvanostatic charge/discharge curve of the FB in Figure 5b shows that the discharge curve extends from 2.1 to 1.7 V. The specific capacity based on S mass is very close to that observed in the half-cells (Figure S10). A pouch FB based on GCNT-Li/SC is shown in Figure 5c. The FB can be cycled continuously at different rates (1 C = 1/discharge time, h) from 0.2 to 9 C. A cycle stability over 500 cycles is obtained at 1 C with ~80% capacity retention (Figure 5d) and CE close to 99.9% (inset Figure 5d). The self-discharge (SD) was also tested in the FB, in which a stable voltage of 2.15 V can be achieved even after 1 week (Figure 5e) of SD. A capacity retention of 94 and 81% is measured after 8 h and 1 week of SD, respectively (inset, Figure 5e). Finally, the Ragone plot is calculated and presented in the Figure 5f for a range of energy and 17 ACS Paragon Plus Environment

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power densities. At the lowest power density, the energy density of the GCNT-Li/SC full-cell is 1423 Wh kg-1active materials (752 Wh kg-1total electrodes), where active materials = Li + S only and total electrodes = GCNT-Li + SC + carbon additives + binder. This is 3× higher energy density than seen in Li-S full-cells with respect to the mass of active materials (Li-S).44 Moreover, the data appear attractive when compared commercial LIB performances with 310 Wh kg-1active (220 Wh kg-1total

electrodes),

45

materials

where active materials = graphite + LiCoO2; total electrodes =

graphite + LiCoO2 + carbon additives + binder. But a definitive comparison with a commercial cell is difficult at this stage since commercial cells are dual-sided and stacked, designed to minimize the contribution of current collectors and packaging materials. Further data on the gravimetric energy density calculation is shown in Tables S2 and S3. In a non-optimized device, we achieved a volumetric energy density of 234 Wh/Ltotal electrodes. There is no dendritic or mossy Li in the full-cell electrodes after 500 cycles (Figure S11). These results represent a promising achievement for a Li polysulfide- and dendrite-free battery.

Conclusions In conclusion, the concept of a 3D GCNT structure for reversible and stable Li storage can be extended to other materials with the combined features of a conductive high surface area, high porosity and low mass density, yielding dendrite-free Li-based batteries. The high surface area electrode results in an effective decrease in the local current density of Li plating, promoting the homogenous deposition of Li metal as dendrite-free thin metal coating over the bundles of GCNT. In addition to this effect, the high porosity (97.8%) and low gravimetric density of the GCNT contributes minimally to the weight of the GCNT-Li electrode. This holds promise for achieving superior energy density due to the near theoretical Li storage capacity and serves as


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the basis for the demonstrated SC||GCNT-Li full-cell in a high concentration electrolyte to produce a safe, stable, and high performance battery, thus becoming a harbinger of future systems.

Methods/Experimental Graphene-carbon nanotubes (GCNT) preparation. The preparation of GCNT was similar to the previously reported methods.21,25 First, Bernalstacked multilayer graphene was grown on copper foil (25 µm) using the CVD method as reported elsewhere.46 The catalysts for CNT growth are deposited by e-beam evaporation over the graphene/Cu foil in the order graphene/Fe (1 nm)/Al2O3 (3 nm). The CNT growth was conducted under reduced pressure using a water-assisted CVD method at 750 °C. First, the catalyst is activated by using atomic hydrogen (H•) generated in situ by H2 decomposition on the surface of a hot filament (0.25 mm W wire, 10 A, 30 W) for 30 s, under 25 Torr (210 sccm H2, 2 sccm C2H2 and water vapor generated by bubbling 200 sccm of H2 through ultra-pure water). After the activation of the catalyst for 30 s, the pressure is reduced to 8.3 Torr and the growth is carried out for 15 min. The GCNTs were weighed on a Cahn Instruments C31 Microbalance with an accuracy of 0.0012%, ultimate precision of 0.1 µg at the fraction of load being 0.0001%. The average mass loading is 265 µg ± 100 µg per cm2. The porosity ϕ of the GCNT (97.8%) was calculated based on the eq ϕ = (1 - dGCNT/dgraphite) × 100% and the average density of the GCNT carpets (dGCNT = ~0.05 mg cm-3) and the density of graphite (dgraphite = 2.266 g cm-3). The weight of Cu/graphene/Fe/ AlOx after the graphene growth and metal deposition is recorded. Then the weight after the CNT growth is recorded. The weight difference is attributed to the carbon


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electrode, knowing that the CNTs (~ 50 µm thick) contribute far more extensively to the weight than does the graphene (~ 1 nm thick). Electrochemical plating/stripping of Li into/from GCNT. The electrochemical reaction was performed in 2032 coin-type cells using GCNT substrates and Li foil as both counter and reference electrodes. The GCNT substrates are circular with total area of ~ 2 cm2. The electrolyte used was 4 M lithium bis(fluorosulfonyl)imide (LiFSI) (Oakwood Inc.) in 1,2-dimethoxyethane (DME). The LiFSI salt is vacuum dried (< 20 Torr) at 100 °C for 24 h and DME was distilled over Na strips. All the experiments are conducted inside a glove box with oxygen levels below 5 ppm. The separator was Celgard membranes K2045. Previous to the coin cell assembly, the GCNT substrate is pre-lithiated by putting one drop of electrolyte on the surface of GCNT, pressing a Li coin gently against the GCNT and leaving it with the Li coin on top for 3 h. Adding excessive amounts of the electrolyte solution during the pretreatment was found to yield ineffective pre-lithiation due to poor contact between the GCNT and the Li. After the pre-lithiation, the GCNT is assembled in a coin cell using the same Li chip used in the prelithiation. The current density for the electrochemical measurements (insertion/extraction and cycling) ranges from 1 to 10 mA cm-2, all performed at room temperature. For the Li plating (discharging process), a time-controlled process with a constant current regime was applied with no cut-off voltage limit. The stripping process (charge process) was set to a constant current regime with a cut-off voltage of 1 V (vs Li+/Li). A control experiment was carried out using a copper foil upon which graphene is grown by CVD. The gravimetric capacity was determined based on the combined mass of the GCNT, which ranges from 0.1 to 0.4 mg cm-2, and the Li metal, which was calculated from the amount of charge applied during the electrodeposition or plating (for instance, 4 mAh cm-2 is equivalent to 1.04 mgLi cm-2). 20 ACS Paragon Plus Environment

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Cathode preparation and FB SC cathodes were prepared by the decomposition of polyacrylonitrile (PAN) (Sigma-Aldrich, Mw 150k) in the presence of excess elemental sulfur. In a typical preparation, PAN, S and graphene nanoribbons (GNR) (EMD-Merck) in the mass ratio of 55:11:1 were ground together using a mortar and pestle. The GNRs improved the conductivity of the final material. The resulting powder is heated from room temperature to 450 °C at a rate of 5 °C min-1 in an argon atmosphere (1 atm). After 6 h, the SC powder was removed and used without purification. The SC powder has approximately 60 wt% S. The SC cathodes were prepared by mixing the SC powder with carbon black (Black Pearls 2000, Cabot Corp.) and polyvinylidene fluoride (PVDF, Sigma Aldrich) in a mass proportion of 8:1:1, resulting in a total S content in the electrode of 48 wt%. Typical mass loading was 4–5 mg in 1 cm2 electrodes. Steel foils were used as current collectors. Electrolytes in the half-cell tests were either 4 M LiFSI/DME or 1 M LiPF6 in EC:DEC (1:1) in the voltage range of 1 to 3 V. The FB was assembled by previously producing GCNT-Li and then combining with a SC cathode using the 4 M LiFSI/DME electrolyte and Celgard K2045 as separator. The electrodes were ~1 cm2. The areal capacity of the GCNT-Li is set to match the 30% irreversible capacity loss of the first cycle of the SC cathode. Materials characterization Coin cells were disassembled inside a glove box to investigate the morphology of the GCNT electrodes after Li insertion/extraction. SEM images of the GCNT electrodes were obtained with an FE-SEM (JEOL-6500F) at an accelerating voltage of 20 kV. HRTEM images (JEOL FEG2100F) were obtained after preparing the samples by sonicating the GCNT substrate in acetonitrile and dropping the dispersion over TEM grids. Electrical conductivity was measured with a 4155C Agilent semiconductor parameter analyzer. Measurements were conducted 21 ACS Paragon Plus Environment

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between the substrate (copper foil) and evaporated areas (30 µm thick Li, contact area 62500 µm2) over the top of the CNT.

Acknowledgements R. V. S. thanks the National Council for Scientific and Technological Development (CNPq), Ministry of Science, Technology and Innovation, Brazil. This work was supported by the AFOSR MURI program FA9550-12-1-0035 and AFOSR FA9550-14-1-0111. The electrode separator material Celgard K2045 was kindly donated by Celgard, LLC.

Supporting Information: including additional electrochemical and X-ray diffraction characterization, SEM images and energy density calculations, can be found via the Internet at http://pubs.acs.org.

Competing financial interests: Rice University has several patent filings on GCNT electrodes and their use in energy devices. That intellectual property has been licensed to Tubz, LLC, of which J.M.T. is a shareholder, though not an officer, director, employee or paid consultant. Almost all of the results in this manuscript were obtained before licensing the intellectual property to Tubz. Potential conflicts are managed by the Rice University Office of Sponsored Programs and Research Compliance (SPARC, http://sparc.rice.edu/coi).

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