Dual Functionalities of Carbon Nanotube Films for Dendrite-Free and

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Dual-Functionalities of Carbon Nanotube Films for DendriteFree and High Energy-High Power Lithium-Sulfur Batteries Keyu Xie, Kai Yuan, Kun Zhang, Chao Shen, Weibang Lv, Xing-Rui Liu, Jian-Gan Wang, and Bingqing Wei ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14039 • Publication Date (Web): 13 Jan 2017 Downloaded from http://pubs.acs.org on January 14, 2017

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ACS Applied Materials & Interfaces

Dual-Functionalities of Carbon Nanotube Films for Dendrite-Free

and

High

Energy-High

Power

Lithium-Sulfur Batteries Keyu Xie,*, a Kai Yuan,a Kun Zhang,a Chao Shen,*, a Weibang Lv,b Xingrui Liu,a Jian-Gan Wanga and Bingqing Wei a, c

a

State Key Laboratory of Solidification Processing, Center for Nano Energy Materials, and

School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, China. b

Advanced Materials Division, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese

Academy of Sciences, Suzhou 215123, China c

Department of Mechanical Engineering, University of Delaware, Newark, DE19716, USA.

Email: [email protected]; [email protected]

KEYWORDS: CNT film, polysulfide, lithium dendrite, lithium-sulfur battery, new cell configuration

ACS Paragon Plus Environment

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ABSTRACT: As a promising Li-metal battery, Li-S battery owns an ultra-high theoretical energy density of 2600 Wh kg-1. However, most of the previous work has been mainly focused on tackling the “polysulfide shuttle” originated from the S cathode, while the dendrite problem coming from the Li-metal anode has often been overlooked. Herein, to solve the issues arising from both the cathode and anode simultaneously, we propose a novel cell configuration for the first time by inserting CNT films on both sides of the separator in Li-S batteries, in which the cathode-side CNT film works as a shield to suppress the “polysulfide shuttle” and the anode-side CNT film acts as a powerful shield to prevent the Li dendrite growth. In the new cell configuration, the S/rGO cathode with a high S loading of about 4.0 mg cm-2 displays a high specific capacity (1336 mAh g-1 at 0.2 C), excellent rate ability (1070, 833, 656, and 444 mAh g1

at 0.5 C, 1 C, 2 C, and 5 C, respectively), and sustainable cycling stability for 150 cycles with

high Coulombic efficiency (>99%) at 1 C, while the Li metal anode displays an ultra-smooth surface. We believe this work will aid in developing other metal-based (e.g. Na, K, Zn, and Al) batteries.


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INTRODUCTION The next-generation batteries require a higher energy storage density than the current Li-ion batteries (LIB) for the future electrical grid, electric vehicles, and portable electronic applications.1-3 Li metal, with a high theoretical specific capacity (3860 mAh g-1), low redox potential (-3.040 V vs. the standard hydrogen electrode), and low gravimetric density (0.59 g cm3

), holds promise as the “Holy Grail” of high-energy Li-metal batteries, such as Li-O2 and Li-S

batteries.4-6 During the past 40 years, considerable efforts have been dedicated to conquering the biggest

challenge:

the

uncontrollable

growth

of dendritic

Li,

which

hinders

the

commercialization of the rechargeable Li-metal batteries. The growth of dendritic Li during the deposition/stripping cycles leads to repeatedly break and rebuild the solid electrolyte interface (SEI) film, as well as continuous consumption of Li and electrolyte, which dramatically decreases the Coulombic efficiency. More severe is that the Li dendrites will pierce through the separator, launching internal short-circuiting, and even catastrophic safety risks.7 That is why the battery industry abandons rechargeable Li-metal batteries when graphite arises as anode material for LIBs. Various approaches have been employed to achieve the inhibition of Li dendrites to revive the rechargeable Li-metal batteries. Adding additives (e.g. fluoroethylene carbonate,8 hydrogen fluoride,9 lithium polysulfide/lithium nitrate,10 and trace amount water11) into liquid electrolytes is proved to be an efficient interface engineering strategy to obtain a stable SEI film on Li surface, and thus restrain the growth of dendritic Li. Recently, a novel self-healing electrostatic shield concept, which can alter the formation of Li dendrites, has been proposed by Xu and Zhang by adding a trace amount cesium or rubidium ions into the liquid electrolyte.12,13 Meanwhile, coating Li metal with an artificial protection layer, such as a Al2O3 layer,14,15 a


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Li3PO4 layer,16 a graphite layer,17 a boron nitride/graphene layer,18 and a carbon nanospheres layer,19 has also been proposed to block the dendrite penetration, therefore forming a stable SEI film. Also, the use of the elaborately designed three-dimensional (3D) current collector,20,21 solid nanoparticle-polymer electrolyte,22 LiNO3-contained ionic liquid electrolyte23, modified separator,24 3D polymer nanofiber interlayer,4 and 3D Li-B alloys,5 does retard the Li dendrite growth to some extent. Although all the methods mentioned above have contributed to addressing the dendrite growth issue, new approaches to fundamentally restrain Li dendrite formation is still in desperate need. As one of the most promising Li-metal batteries, Li-S battery, with elemental S as a cathode (the theoretical specific capacity is 1675 mAh g-1) and Li-metal as an anode, owns an ultra-high theoretical energy density of 2600 Wh kg-1.25-27 However, the small electrochemical utilization of S, the dissolution and diffusion of polysulfide intermediates, as well as dendritic Li growth together, prevent Li-S battery from reality applications.28-30 Furthermore, for practical application, gassing in Li-S battery should also be paid attention. At present, an excess of research has been focused on tackling the first two problems originating from the S cathode by mixing the sulfur with various carbon, conductive polymers, metal oxides, and metal hydroxides, or using different binders, porous current collectors, interlayers, reformative separators, and new cell configurations.31-38 However, little attention has been paid in Li-S batteries to deal with the Li dendrite growth problem, which is widely existing in Li-metal batteries. Suo et al.39 reported that a new electrolyte with ultrahigh concentration Li salt could suppress the dendritic Li formation in Li-S batteries. Huang and co-workers17 developed a hybrid anode by in situ forming lithiated graphite as an artificial SEI film to eliminate the side reaction at the anode, and thus protect it. Most recently, Zhang et al. demonstrated the use of 3D Li7B6 nanofiber or graphene


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framework to obtain the dendrite-free Li-base anode.5,40 Manthiram41 et al. revealed a synergetic effect of a sandwich-like cathode design and an electrolyte engineering via adding metal ions (e.g., Cu, Ag, and Au) into liquid electrolytes to simultaneously minimize the adverse effect of the dissolution and diffusion of polysulfides and the growth of Li dendrites. As has been pointed out, the core to achieving the actual application of Li-S batteries in a wide variety of fields is how to tackle all of these issues, arising from both the S cathode and Li-metal anode, at the same time. A CNT film was first integrated into the cell configuration as a powerful shield to confine the diffusion of dissolved polysulfides between the cathode and anode (so-called “shuttle effect”).42 By inserting it between the cathode and separator, the migrating polysulfides can be trapped and reutilized through the conductive and porous CNT film. As a result, a significant enhancement of S utilization, capacity retention, Coulombic efficiency, and rate performance can be achieved.43 However, the other critical issue of dendritic Li, coming from the Li-metal anode, has been neglected as usual. With similar nanostructures, a 3D polyacrylonitrile (PAN) polymer microfiber has been placed on the top of the current collector to hamper the dendritic Li formation in Li-S batteries.4 This work reveals that the PAN microfiber, with excellent chemical and electrochemical stability, is helpful to guide and confine Li along the fiber scaffold, and hence, effectively avoid the growth of dendritic Li. Meanwhile, a compacted graphite film is connected with Li to work not only as a physical barrier but also as an electrochemical surfacereaction controller.17 Notice that when the graphite film and Li are separated, the graphite film functioned as a simple physical barrier. Once it connected with Li, the lithiated graphite was endowed with an exceptional ability to minimize the surface reaction on Li. Along this direction, we notice that, compared to that of PAN microfiber, a CNT film has much smaller nano space


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between the CNTs, which is supposed to be a stronger shield for Li dendrite formation when it is put directly on the Li anode. Moreover, the addition of conductive CNT with considerable BET surface area can lower local current density. Then small local current density can make lithium deposit more uniform44, 45. To verify this hypothesis, we first introduced a CNT film on the top of a Cu foil current collector to investigate the cycling stability of Li metal anode (Figure 1b). And as expected, the CNT film worked as a solid shield to embed the Li into the CNT scaffold. Furthermore, as a proof-of-concept platform, a novel cell configuration with CNT films on both sides of the separator has been developed for dendrite-free Li-S batteries.

EXPERIMENTAL SECTION Synthesis of CNT Film: The continuous CNT film was made using a modified floating catalyst chemical vapor deposition method similar to our previous work.46,47 In this process, a feedstock consisting of about 96.5 wt% ethanol (carbon source), 1.9 wt% ferrocene (catalyst precursor) and 1.6 wt% thiophene (promoter) was injected into a hot furnace along with carrier gas (H2 and Ar). The injection rates of the feedstock and carrier gas were about 0.15 ml/min and 600 ml/min, respectively. Upon entering the furnace at a temperature of around 1150 °C, these compounds broke down and reacted rapidly to form CNTs, which then interacted to form a continuous stocking-like aerogel. This CNT aerogel was continuously blown out at the exit of the furnace and collected layer by layer onto a rotating spindle. After spraying ethanol, CNT film was densified and then peeled off from the spindle. The thickness of the CNT film can be well controlled by the film collection time. The CNT film was punched into circular discs for the cell assembling and electrochemical testing.


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Materials Characterizations: The morphologies of the samples were observed under a field-emission scanning electron microscopy (FESEM, FEI Tecnai G2 F30). Raman spectra were collected by a 488 nm laser under ambient conditions with a Renishaw inVia instrument at room temperature. The resistances were tested by four-point probe meter (SZT-2A). Fabrication of free-standing S/rGO Electrode: First, GO was prepared by oxidation of natural graphite powder via the modified Hummers’ method.48 Second, rGO foam was synthesized using a hydrothermal method and followed by freeze-drying. Third, the rGO foam and S (99.98%, Aldrich) were put in the tube furnace and heated at 155 °C for 15 h under nitrogen atmosphere. After that, the free-standing S/rGO foam was then pressed and punched into circular discs. Details in the morphologies and structures of GO, rGO foam and S/rGO foam can be found in Supporting Information (Figure S1, S2, S3 and S4). Electrochemical Measurements: CR2016 coin cells with the CNT/Cu foil or the bare Cu foil as the working electrode and a Li metal as the counter/reference electrode were assembled in the argon-filled glove box and then tested on an 88-channel battery tester (Arbin Instruments, BT2000, USA) for repeated Li deposition/stripping testing. The Li deposition capacity is set at 1.0 mAh cm-2 and the cut-off potential for the stripping process is configured to be 1.0 V. The electrolyte was 1 M lithium bis(trifluoromethane sulfonyl)imide (LiTFSI) and 0.1 M LiNO3 in a mixed solvent of 1,3dioxolane (DOL) and 1,2-dimethoxyethane (DME) with a volume ratio of 1:1. For the symmetrical cell tests, the balanced Li-CNT/Li-CNT or Li/Li coin cells were assembled in the argon-filled glove box with two identical electrodes. The amount of plated Li is 1.0 mAh cm-2,


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and the current density is 1.0 mA cm-2 in each cycle. For the CNT coupled Li-S batteries, the CR2016 coin cells were assembled in an argon-filled glove box with the S/rGO cathode, cathode-side CNT film, polypropylene separator, anode-side CNT film, and Li metal foil in sequence. The areal mass loading of sulfur is ~4.0 mg cm-2 and the amount of used electrolyte in each cell is ~20 µL (electrolyte volume : sulfur weight= 1 L:50 g). For the conventional Li-S batteries, the CR2016 coin cells were assembled under the same conditions but without CNT films. The Li-S coin cells were galvanostatically charged-discharged at different current densities between 1.5 and 3.0 V (vs. Li/Li+) using a CT2001A cell test instrument (LAND Electronic Co, BT2013A, China) or an 88-channel battery tester (Arbin Instruments, BT2000, USA). A Solartron electrochemical workstation (1260+1287) was employed for electrochemical impedance spectrometry tests in the frequency range of 100 kHz to 10 mHz. The anode sample of SEM characterization was prepared in the argon-filled glove box by disassembling the cell, and a DME: DOL (1:1 v/v ) solution was used to washing the remaining electrolyte quickly.

RESULTS AND DISCUSSION


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Figure 1. Schematic illustrations of Li deposition. a) On the bare Cu current collector, Li dendrites formed during the Li deposition process. b) For the CNT film modified Cu current collector, Li is uniformly trapped into the quasi-3D CNT skeleton.

Figure 2. SEM images of top-view a) and cross-section b) images of the CNT film. c) Nitrogen adsorption-desorption isotherms of CNT film and d) corresponding pore-size-distribution.

During the repeated Li deposition and stripping processes, the bare planar Cu foil is usually covered with lots of Li dendrites (Figure 1a), which may lead to the internal short circuit, and even safety risks. At the same time, the SEI film on the Li surface is continuously broken and repaired during cycling, resulting in low Coulombic efficiency and cycle stability. A CNT film with the thickness of ~6 µm (Figure 2a, b) is placed on top of the planar Cu foil (CNT/Cu) in


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order to solve these problems. The entangled CNT film, with the BET surface area of 186 m2 g-1 and the mass loading of ~0.45 mg cm-2 (Figure 2c, d), is fabricated using a modified floating catalyst chemical vapor deposition method.46,47 These multilayered and intertwined CNT films can be considered as a quasi-3D architecture. The small nano space among these CNTs and the high BET surface could effectively restrain the growth of Li in the quasi-3D CNT skeleton while offering open ion channels for electrolyte infiltration. As a consequence, uniform Li is expected to form on the top of the Cu foil and the inner space of the quasi-3D CNT skeleton (Figure 1b).

Figure 3. Cycling performances of CNT/Cu substrate and bare Cu substrate at various current densities: a) 0.25 mA cm-2, b) 0.5 mA cm-2 and c) 1.0 mA cm-2. The deposition capacity of Li plated is fixed at 1 mAh cm-2.


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CR2016 coin cells were assembled with the CNT/Cu or Cu substrates as the working electrodes and a Li foil as the counter electrode to compare the cycling stability. Herein, the Coulombic efficiency, which is an indicator of long-term cycling stability, is defined as the ratio of the amount of Li stripped back to the total amount of Li plated onto the working electrode during each cycle. Figure 3 shows the Coulombic efficiency of Li deposition on the CNT/Cu or Cu substrates at the current densities of 0.25 mA cm-2, 0.5 mA cm-2, and 1.0 mA cm-2, respectively, with a set deposition capacity of 1 mAh cm-2. The CNT/Cu substrate presents a more stable cycling performance and a longer cycle life than its Cu counterpart. When the current density is 0.25 mA cm-2 (Figure 3a), the Coulombic efficiency of the Cu substrate starts as high as 95% and then drops dramatically to ~70 % at the 50th cycle. This fast decaying Coulombic efficiency is attributed to the gradual increase of the surface area and electrolyte consuming during the Li dendrite growth. As for the CNT/Cu substrate, the Coulombic efficiency gradually increases from less than ~50% at the 1st cycle to be stable at ~95% after 20 cycles. With the current density further increased up to 1.0 mA cm-2, the more apparent decay of the Coulombic efficiency of the Cu substrate can be found. The Coulombic efficiency promptly decreases from 86.3 % to less than 15% at the 100th cycles (Figure 3c). In addition, the Coulombic efficiency of the Cu substrate predominantly and irregularly fluctuates during the long-term Li deposition and stripping cycling due to partial short-circuits.37 In stark contrast, the Coulombic efficiency of the CNT/Cu remains nearly constant at a high level of ~90% after the first several cycles. Hence, it can be concluded that, as expected, the CNT film does benefit to providing Cu substrate with a significantly improved cycling stability for Li deposition and stripping.


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Figure 4. Galvanostatic cycling voltage profiles of CNT/Cu substrate and Cu substrate at a) the 1st cycle, b) the 4th cycle, c) the 50th cycle, and d) the 100th cycle with Li metal as the reference electrode at a current density of 1.0 mA cm-2. e) The calculated voltage hysteresis, which is defined as the voltage difference between the middle of the charge and discharge curves. It is noted that the first Coulombic efficiency of the CNT/Cu is only ~ 50 % at those three current densities mentioned above. This phenomenon can be clarified by the voltage profiles of both the CNT/Cu and Cu substrates at a specified cycle (Figure 4a-d). Compared to the voltage profile of the Cu substrate, there is a voltage plateau between 1.3 V and ~0 V during the first Li deposition process on the CNT/Cu substrate, which is due to the SEI film formation on the highsurface-area carbon materials (Figure 4a).49 Meanwhile, the voltage hysteresis of the CNT/Cu cell, which is related to the polarization of the cell at the stage of cycling, is slightly higher than that of the Cu cell. Fortunately, after the first three cycles, the high voltage plateau of the CNT/Cu cell disappears, indicating that a stable SEI film formed on the CNT film (Figure 4b).


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This result reveals that the CNT film not only works as a physical barrier preventing dendrite Li formation but also offers a stable SEI film to protect the Li metal anode. And thus, the Coulombic efficiency of the CNT/Cu cell is accordingly improved close to that of the Cu substrate. In the consequential cycles, the Coulombic efficiency of the CNT/Cu cell remains ~90% of the 50th and 100th cycle, while the Coulombic efficiency of the Cu cell is only about half of the CNT/Cu cell (Figure 4c, d). Accordingly, after the first several cycles, the voltage hysteresis of the CNT/Cu cell is smaller (~40-50 mV) than that of its counterpart (~60 mV, Figure 4e). It is also comparable to or lower than other modified Cu substrates, such as by PAN (~80-100 mV),4 BN (~65 mV),18 and carbon nanosphere (~50 mV)50. The constant voltage hysteresis of the CNT/Cu cell indicates that the CNT film coupled Cu substrate has a steadily internal resistance during cycling. We ascribe this bonus to the excellent stability of the SEI film introduced by the CNT film, and the uniform Li deposition, leading to the smaller surface area compared to the dendritic Li formed on the bare Cu substrate during cycling. By contrast, the voltage hysteresis of the bare Cu substrate abruptly drops after 90 cycles, indicating the occurrence of a dendriteinduced partial short circuit (Figure 4e).20 Figure 5 shows the surface morphologies of the repeated Li deposition and stripping onto the CNT/Cu and Cu substrates after 100 cycles at a current density of 1.0 mA cm-2. Without the CNT protection, a plenty of apical and fibre-like Li dendrites and mossy Li, with the length of more than 5 µm and diameter of less than 1 µm, has appeared and protruded after cycles (Figure 5a, b, c). While Li deposition onto the CNT film coupled Cu substrate displays an utterly different morphology. From the top-view images of the CNT/Cu and Cu substrates (Figure 5a, d), it can be seen that the surface of the CNT/Cu substrate is remarkably smooth, and no dendrite protruding upward is found to avoid the potential safety risks effectively. The cross-sectional


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images further confirmed that the Li tends to fulfill the spaces between CNTs, and thus, the uniform Li is trapped inside the quasi-3D CNT skeleton (Figure 5e, f).

Figure 5. Morphology characterization of Li deposited onto CNT/Cu substrate and Cu substrate after 100 cycles. a) and b) the top-view and cross-sectional SEM images of Li metal anode. c) Magnified view of the formed Li dendrites. d) and e) the top-view and cross-sectional SEM images of Li entrapped with the CNT film. f) Magnified view of the Li trapped within the CNT film.

To further evaluate the cycling stability of the Li electrode covered with the CNT film, a symmetric Li-CNT/Li-CNT cell was assembled with two identical CNT film coupled Li metal sheets as the electrodes (Figure 6a). For comparison, a symmetric Li/Li cell was also constructed. The galvanostatic cycling experiments of the two types of the symmetric cells were conducted at a current density of 1.0 mA cm-2, with the amount of plated Li of 1.0 mAh cm-2 (the charge and discharge time add up to 2 h for each cycle). During the first 60 h, the Li-CNT/Li-CNT cell exhibits lower polarization in comparison with that of the Li/Li cell (Figure 6b), indicating that the introduction of the highly-conductive CNT films in the cell configuration is a benefit to


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reducing the voltage polarization and resulting in superior charge transfer kinetics. After cycling for 60 h, random voltage oscillations are observed in the Li/Li cell (inset of Figure 6b). These sudden and substantial rises and drops in voltage are typical of short-circuiting behavior, which is attributed to the continuously growing Li dendrites.20,50-53 In contrast, Li deposition and stripping of the CNT film coupled Li substrate show exceptional cycling stability with slight voltage oscillation, indicating more consistent Li behavior due to the existence of the CNT films. This result agrees well with the outcomes of the electrochemical and morphology characterizations discussed above.

Figure 6. Electrochemical characterization of the Li deposition/stripping in symmetrical Li/Li cell and Li-CNT/Li-CNT cell during galvanostatic cycling. a) Schematics of the two symmetrical cell configurations. b) Voltage-Time curves of Li deposition/stripping in symmetrical Li/Li cell (top) and Li-CNT/Li-CNT cell (bottom). Inset: Magnified view of the Voltage-Time curve of the symmetrical Li/Li cell. The amount of plated Li is 1.0 mAh cm-2, and the current density is 1.0 mA cm-2 in each cycle.


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Figure 7. a) Schematic illustration of the novel Li-S cell configuration. The CNT films are put on both sides of the separator. The cathode-side CNT film works as a shield for polysulfides, and the anode-side CNT film works as a solid shield to confine Li dendrite growth. b) Cycling performances and c) rate capabilities of the novel configuration (with CNT) and the conventional configuration (without CNT). d) Long-term cycle performances of Li-S with the novel configuration (with CNT) at the current density of 1C.

Based on the primary results, we have conceived a new cell design to put CNT films on both sides of the separator in a Li-S battery to simultaneously tackle the polysulfide diffusion and


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dendrite formation problems, arising from the S-based cathode and the Li-metal anode, respectively (Figure 7a). In this cell configuration, each CNT film plays its desired role, with the cathode-side CNT film being a shield to prevent polysulfide diffusion and the anode-side CNT film working as a reliable shield to control the Li dendrite growth. Cycling performance and rate capability of the novel Li-S cell configuration (with CNT) and the conventional Li-S cell configuration (without CNT) are compared in Figure 7b and c. Here, a free-standing S/rGO composite with an S loading of ~4.0 mg cm-2 is used as the cathode (Figure S1 and S4, Supporting Information, see Experimental Section for details in the synthesis of the compounds). The specific capacity of S/rGO in the CNT coupled cell configuration is as high as 1366 mAh g-1 while that of S/rGO in the conventional cell configuration is only 1196 mAh g-1 at a low current density of 0.2 C (Figure 7c). When the current density increased to 0.5C, the specific capacity of S/rGO in the CNT coupled cell remains 981 mAh g-1 after 80 cycles, which is much higher than that of S/rGO in the conventional cell (495 mAh g-1, Figure 7b). Moreover, long-term cycle performances of Li-S in the CNT coupled cell configuration at the high current density of 1C were also evaluated (Figure 7d). After the first cycle, the initial specific capacity of S/rGO is 808 mAh g-1. A reversible discharge capacity of 590 mAh g-1 can still be retained with a stable CE (>99%) after 150 cycles indicating its excellent stability and long-term cyclability even at a high charge/discharge current density. In addtion, in order to evaluate the specific capacity of S/rGO itself, correctly, a reference coin cell with bare CNT film as the working electrode and a Li metal as the counter electrode has been also assembled with the same electrolyte and evaluated at the current density of 0.2C between 1.5 and 3.0 V (vs. Li/Li+) (Figure S5, Supporting Information). It can be seen that the first discharge capacity of CNT film is ~170 mAh g-1, and the first charge capacity is only


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~8mAh g-1. This may be attributed to the formation of SEI film during the first cycle as mentioned before.44 After several cycles, the capacity of CNT film between 1.5 and 3.0 V (vs. Li/Li+) gradually decreased to ~6 mAh g-1, indicating the contribution of the CNT film at the cathode-side to the total specific capacity of S/rGO cathode can be ignored. Form the typical cycling voltage profiles of the conventional Li-S cell configuration and the novel Li-S cell configuration (Figure S6, Supporting Information), it can be also seen that, beside the two separate reduction plateaus at the potential of ~2.3 and ~2.0 V originated from the reduction of S,54 there is not other discharge plateau. This result further confirm that the contribution of specific capacity from CNT film can be ignored. Rate capability of the two Li-S batteries were also evaluated. As shown in Figure 7c , the reversible capacity of the S/rGO electrode in the CNT coupled cell slightly reduces to about 1366, 1070, 833, 656, and 444 mAh g-1 at different current densities of 0.2C, 0.5C, 1C, 2C, and 5C, respectively. While they are about 1196, 891, 732, 556, and 195 mAh g-1 at the same current rates, respectively in the conventional cell, suggesting that a higher specific capacity and a higher discharge rate can be realized at the same time for the CNT coupled cell.55 According to our and others work,56-59 the improved specific capacities of the S/rGO electrode in the CNT coupled cell is attributed to the cathode-side CNT film shield, which can suppress the polysulfide shuttling and thus result in the enhancement of the sulfur utilization. This can be further verified by the enhanced specific capacity of S/rGO cathode with only cathode-side CNT film (Figure S7, Supporting Information). Meanwhile, the excellent rate ability is mainly benefited from the integration of the highly-conductive CNT film, which can also be considered as the top current collector of both S/rGO cathode and Li anode. The conclusion is in agreement with the results of the electrochemical impedance spectroscopy (Figure S8, Supporting Information) and the


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hysteresis voltage discussed before. Compared with the previous works, which only on focused on tackling the “polysulfide shuttle” originated from the S cathode by using various interlayers or modifying separators (Table S1, Supporting Information),60-65 our CNT coupled cell, with much higher sulfur loading (~4 mg cm-2), displays a high specific capacity of 1336 mAh g-1 at low current density of 0.2 C, competitive rate ability (1070, 833, 656, and 444 mAh g-1 at 0.5 C, 1 C, 2 C, and 5 C, respectively), as well as stable cycle performance for 150 cycles with high Coulombic efficiency (>99%) at 1 C. Furthermore, with the anode-side CNT film, the dendrite problem coming from the Li-metal anode in Li-S battery can be eliminated from the root as well. This result can be further verified from the following SEM images.


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Figure 8. Morphology characterization of Li metal anode with/without CNT film. a), b) and c) The top view SEM images of the Li metal anode without CNT film. d), e) and f) The top view FESEM images of the CNT film covered Li metal anode. g) The cross-sectional SEM image of the CNT film covered Li metal anode. (top: CNT film; middle: Li metal anode; bottom: Cu foil, respectively). h) and i) The magnified view of the CNT film. j), k) and l) The magnified view of the Li metal anode.


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As discussed before, the Li dendrite growth is a crucial factor in the successful development of Li-S batteries but has long been ignored. Herein, to better understand the functionality of the anode-side CNT film in the new cell configuration, we focus our investigation on the morphology evolution of the Li anode after cycling. Figure 8a, b, c show the top-view images of the Li anode without the protection of the CNT film. The surface of the Li anode is extraordinary rough and exists significant wave-like fluctuations. The magnified view further reveals that these heterogeneous microstructures compose of the overgrown Li dendrites. Figure 8c clearly shows the Li dendrites with the length of above 10 µm and diameter of less than 2 µm. In fact, this will be the root of potential safety risks. When CNT film is only covered on the top of the cathode, plenty of Li dendrites still appeared on the surface of Li anode (Figure S9, Supporting Information). In contrast, no dendrite is observed on the top of the CNT film coupled Li anode (Figure 8d, e). Moreover, as shown in Figure S10c,d, the major elements on the surfaces of CNT film at the anode-side include O, S, and F, which are attributted to the disintegration of the electrolyte. Figure 8g gives the detail information of the cross-sectional structure of the CNT film coupled Li anode. The sandwich-like structure consists of a top CNT layer, a middle Li metal layer, and a bottom Cu current collector. Notice that the Li anode below the CNT film has a very flat surface (Figure 8g, k, l), implying that the anode-side CNT film is a solid shield to suppress the growth of Li dendrites effectively. By endowing the CNT film with an upgraded shield role, the potential safety risk of Li-S cells can be minimized in addition to a higher specific capacity and a higher power capability of the cathode in the new Li-S cell configuration. Besides, additional attention should also be paid to the structure characteristics of the CNT film used in the cell, because the applications of two CNT films in the cell will increase the extra weight and volume of the whole battery, and thus decreasing the energy density of the whole


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battery. More detailed studies on the regulation of the structure characteristics of the CNT film, such as the thickness, for Li-S battery application with the new cell configuration need to be conducted in the next stage.

CONCLUSIONS In the present work, dual functionalities of CNT films, working as a shield at the cathode side to prevent polysulfide diffusion and acting as a solid shield at the anode side to suppress the growth of dendritic Li, have been successfully demonstrated. A new cell design, in which the CNT films have been integrated at the both sides of the separator, has also been proposed and realized for dendrite-free Li-S batteries. In this novel cell configuration, the cathode-side CNT film worked as a shield to suppress the polysulfide shuttle effect, and the anode-side CNT film is used to minimize the growth of Li dendrites. And hence, the critical issues arising from both the S-based cathode and Li-metal anode (e.g. the small electrochemical utilization of S, the shuttling migration of polysulfides, as well as the dendritic Li growth) can be solved at the same time in this CNT integrated cell design. In addition, this work offers a new option for the utilization of the conductive 2D/3D CNT nano and microstructures to address the uncontrollable Li dendrite growth and also, sheds light on next-generation electrochemical energy storage devices with a metal electrode. ASSOCIATED CONTENT Supporting Information FESEM images of S/rGO cathode, the corresponding elemental mapping of sulfur, carbon and oxygen; SEM, TEM, Raman results for GO; SEM, TEM, Raman results as well as photograph for rGO foam; TGA curve of the S/rGO cathode; electrochemical


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performances of pure CNT and cell with different configutations; SEM images of Li metal anode only with cathode-side, CNT film on both side of the separator after cycling. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author Email: [email protected]; [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors acknowledge the financial supports of this work by the National Natural Science Foundation of China (51674202, 51302219, and 51402236), the Natural Science Foundation of Shannxi Province (2015JM2045), the Specialized Research Fund for the Doctoral Program of Higher Education of China (No. 20136102120024), and the Fundamental Research Funds for the Central Universities (3102014JCQ01019 and 3102015BJ(II)MYZ02).

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Table of Contents


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Dual Functionalities of Carbon Nanotube Films for Dendrite-Free and

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