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Robust Lithium Metal Anodes Realized by Lithiophilic 3D Porous Current Collectors for Constructing High-Energy Lithium−Sulfur Batteries Fei Pei,† Ang Fu,† Weibin Ye,‡ Jian Peng,† Xiaoliang Fang,*,† Ming-Sheng Wang,‡ and Nanfeng Zheng*,† †

Pen-Tung Sah Institute of Micro-Nano Science and Technology, State Key Laboratory for Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials, National & Local Joint Engineering Research Center for Preparation Technology of Nanomaterials, and College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, Fujian 361005, China ‡ Department of Materials Science and Engineering, College of Materials, Xiamen University, Xiamen, Fujian 361005, China S Supporting Information *

ABSTRACT: Lithium−sulfur (Li−S) batteries are attractive candidates for next-generation rechargeable batteries. With the steady development of sulfur cathodes, the recent revival of research on dendrite-free Li metal anodes offers opportunities to improve the stabilities and safety of Li−S batteries. However, the low capacities and low Li utilizations of current Li anodes hinder the improvement of the energy densities of Li−S batteries. Here, we present a facile approach to fabricate lithiophilic three-dimensional porous current collectors by modifying commercial metal foams with yolk−shell structured N-doped porous carbon nanosheets. Benefiting from the structure-based rational design, this current collector is able to generate dendrite-free Li anodes with improved Coulombic efficiencies and life spans, enabling carbon/sulfur cathodes to exhibit significantly enhanced stabilities (e.g., 78.1% of capacity retention after 1400 cycles). More importantly, we successfully constructed a high-areal-capacity Li−S full cell (9.84 mAh cm−2) with 82% Li utilization. This work provides a promising route toward high-energy-density Li−S batteries. KEYWORDS: lithium metal anodes, current collectors, carbon nanosheets, Li−S batteries, high sulfur loading

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practical application of Li−S batteries is severely restricted by both cathodes and anodes.3 Significant progress in the sulfur hosts, electrolytes, and functional separators has allowed increasing the sulfur utilization while reducing the shuttling of polysulfide intermediates, thereby greatly improving the capacity and cycling stability of sulfur cathodes.1−4 However,

o meet the ever-growing demands for portable electronics and electric vehicles, high-energy rechargeable batteries beyond lithium-ion batteries are being intensively investigated. As a representative class of lithium metal batteries, lithium−sulfur (Li−S) batteries have received a great deal of attention owing to their overwhelming energy density advantages.1,2 Although the coupling of sulfur (theoretical capacity of 1675 mAh g −1 ) and lithium (theoretical capacity of 3860 mAh g−1) provides Li−S batteries with a theoretical energy density as high as 2600 Wh kg−1, the © XXXX American Chemical Society

Received: May 15, 2019 Accepted: July 9, 2019 Published: July 9, 2019 A

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Figure 1. (a) Schematic illustration for the fabrication of the dendrite-free Li anode by designing NPCN-wrapped 3D metal foam as the current collector (inset: NPCN ink screen printed onto the polyethylene terephthalate substrate). (b) SEM and (c) TEM images of NPCN. (d) N2 adsorption−desorption isotherm and pore-size distribution of NPCN. (e) XPS spectrum of NPCN (inset: N 1s spectrum, and peaks i−iii correspond to pyridinic N, pyrrolic N, and quaternary N, respectively). (f) Dip-coating process for the fabrication of Cu@NPCN. (g, h) SEM images of Cu@NPCN (inset: optical image of Cu@NPCN).

the growth of Li dendrites, thus the easy formation of “dead Li” and an unstable solid electrolyte interphase (SEI) layer, has been hindering Li anodes from matching the development of sulfur cathodes.5 These long-standing issues of Li anodes often result in Li metal batteries with low Coulombic efficiencies (CEs), short cycling life spans, and safety concerns (e.g., internal short circuit). Most current sulfur cathodes are therefore evaluated by employing materials with Li in excess (e.g., Li foils) to reduce the negative influence of Li dendrites.4,6 The development of durable and safe Li anodes has become one of the most pressing issues for Li−S batteries. Recently, tremendous efforts have been devoted to understanding Li plating/stripping electrochemistry and to developing stable anodes for Li metal batteries.7 Several strategies including the optimization of electrolytes,8−11 the fabrication of artificial SEI layers,12−14 the introduction of protection layers,15,16 and the design of current collectors or electrode structures17−19 have been developed to suppress Li dendrite growth and to improve the CE of Li anodes. Since Li nucleation and growth highly depend on the plating matrix, manipulating current collectors has been developed as a straightforward route to regulate Li plating/stripping.17 Desirable current collectors should reduce the local current density while providing mechanical support to accommodate the huge volume change of the anode caused by the “hostless” Li plating/stripping. Fabricating three-dimensional (3D) current collectors is emerging as an effective strategy for

designing dendrite-free Li anodes.20−25 However, the Li/3D current collector composite anodes still face some problems, such as low areal capacities, heavy weights, and complex manufacturing procedures. Moreover, it is worth noting that the Li areal capacity utilizations in many Li metal batteries are typically lower than 3 mAh cm−2.23,26 In the case of Li−S batteries, the dendrite-free Li anodes are usually paired with low-sulfur-loading cathodes, leading to low Li utilizations and low energy densities.6,27−29 There is no doubt that employing high-sulfur-loading cathodes and reducing the dosage of Li are highly desirable for the development of Li−S batteries. Until now, dendrite-free Li anodes have not yet been applied in the construction of high-areal-capacity Li−S batteries (>8 mAh cm−2) with low limited Li. Consequently, designing robust Li/ 3D current collector composite anodes to meet the requirements of high-sulfur-loading Li−S batteries is attractive yet still challenging. Herein, we report a facile and effective route to fabricate robust Li anodes by rationally designing 3D current collectors with optimized surface and skeleton characteristics. Metal foams wrapped with yolk−shell structured N-doped porous carbon nanosheets (designated as M@NPCN, M = Cu or Ni) were used as 3D current collectors with the ability to accommodate high amounts of Li and guide uniform Li nucleation/growth (Figure 1a). Taking advantage of this convenient and controllable fabrication method, the fabricated M@NPCN current collectors not only can improve the B

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Figure 2. (a) Optical image of the Janus-structured Cu@NPCN and SEM image of the Janus-structured Cu@NPCN after Li plating. (b) SEM image of the Cu surface with Li dendrites. (c) Morphology characterization of Cu@NPCN during the Li plating/stripping process at 1 mA cm−2. (d) Li plating/stripping states I−VIII marked in the galvanostatic discharge/charge voltage curve. (e) Schematic illustration of the configuration of the in situ TEM cell. (f) In situ TEM images of NPCN during Li plating/stripping.

prefabricated carbon nanomaterials was adopted in this work (Figure 1a). Compared with the chemical vapor deposition growth of carbon on a metal surface reported previously, this postmodification strategy has two important advantages, namely, designable carbon building blocks and easy processing.30,31 To maximize these two advantages, several structural characteristics were integrated into our prefabricated carbon building blocks (i.e., yolk−shell structured NPCN): (1) a highly porous structure to increase the surface area and to reduce the local current density; (2) heteroatom doping to improve the lithiophilicity and optimize the Li deposition behavior; (3) a 2D structure to form a continuous compact thin film and to increase the contact area between the metal surface and the carbon layer; and (4) available hollow inner cavities to optimize the Li deposition behavior. A yolk−shell structured NPCN consisting of a graphene core and an N-doped hollow porous carbon shell was synthesized according to our previous report.32 The typical scanning electron microscopy (SEM) and transmission electron microscope (TEM) images show that the surface of NPCN is highly porous (Figure 1b and c). The lateral size and thickness of NPCN are about several micrometers and 35 nm, respectively (Figure S1). The N2 adsorption and desorption measurement reveals that the specific surface area of NPCN is 2237 m2 g−1, with a narrow pore-size distribution centered at 3.2 nm (Figure 1d). As measured by X-ray photoelectron spectroscopy (XPS), NPCN contained 86.9 at. % C, 8.5 at. % O, and 4.6 at. % N (Figure 1e). Three characteristic peaks fitted in the N 1s spectrum (peaks i−iii in the inset of Figure 1e) can be assigned to pyridinic N (398.5 eV), pyrrolic N (400.9 eV), and quaternary N (403.5 eV) species, respectively. These O and N species are helpful to improve the wettability

stabilities of Li anodes but also have good potential for largescale preparation. When used as anodes, the Li/M@NPCN composites can significantly prolong the life span of highsulfur-loading Li−S batteries.

RESULTS AND DISCUSSION Design and Fabrication of M@NPCN. Since highsurface-area current collectors can reduce the local current density and increase the Li plating sites, replacing the traditional two-dimensional (2D) planar Cu foil with 3D porous Cu current collectors has a positive effect on suppressing Li dendrite growth.17 Unfortunately, the utilization of 3D porous structures does not increase the surface areas of Cu current collectors significantly. More importantly, Cu is not lithiophilic and shows a high overpotential for Li deposition.19 Alternatively, 3D porous carbon matrixes are more efficient to reduce the nucleation overpotential and induce homogeneous Li nucleation, although they suffer from low conductivity, mechanical strength, and Li intercalation.22,23 To integrate the advantages of metal and carbon, modifying 3D porous metal frameworks with a carbon layer (i.e., 3D M@C current collectors) is an obvious choice providing that the following requirements can be met: (1) the manufacturing process should be convenient and capable of large-scale fabrication; (2) the structural parameters of the carbon layer should be tunable in order to maximize the stabilities and areal capacities of the Li anodes qualified for high-sulfur-loading cathodes; (3) the weights of the current collectors should be at least comparable with those of commercial metal foils. Inspired by the fabrication of battery electrodes in which the slurry of active materials is coated on current collectors to form stable electrodes, modifying a commercial metal foam with C

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Figure 3. (a) CEs of Cu foil, Cu foam, and Cu@NPCN with an areal capacity of 1 mAh cm−2 at a current density of 1 mA cm−2. (b) Corresponding voltage profile of Cu@NPCN during the Li plating/stripping process. (c) CEs of Cu foil, Cu foam, Cu@G, Cu@LSA-NPCN, Cu@LNC-NPCN, Cu@CS-NPCN, and Cu@NPCN with an areal capacity of 1 mAh cm−2 at a current density of 3 mA cm−2. (d) Voltage profiles of Cu@NPCN with an areal capacity of 1 mAh cm−2 at different current densities. (e) CE of the Cu@NPCN with an areal capacity of 6 mAh cm−2 at a current density of 1 mA cm−2.

and lithiophility of carbon materials.31,33 The highly porous 2D structure, large surface area, and high heteroatom content make NPCN a candidate material to modify 3D metal current collectors. However, the development of a facile and effective method for fabricating 3D M@C current collectors is a prerequisite. Fortunately, NPCN has a favorable dispersibility in water and can be further made into a stable ink-like dispersion by using N-lauryl acrylate as a stabilizer. This “NPCN ink” is paintable on various substrates including plastics, metals, and papers (Figure 1a). By using this “NPCN ink”, we successfully prepared large-size 3D porous M@C current collectors. As shown in Figure 1f, a motor-driven pulley was used to control the dip-coating deposition of NPCN on the Cu foam. A Cu@NPCN foam of 20 cm × 30 cm in size was easily fabricated by the ink-coating process. Simply by controlling the concentration of the “NPCN ink” and the deposition cycles, the mass of NPCN in Cu@NPCN was controlled to be as low as 0.3 mg cm−2, and the practical surface area of Cu@NPCN was 0.67 m2 per 1.0 cm−2Cu foam. The SEM images (Figure 1g and h) and the corresponding elemental mapping images (Figure S1) revealed that the surface of the Cu foam was completely wrapped with NPCN. Since N-lauryl acrylate is also a polymer binder with high affinity for both carbon materials and Cu, the NPCN layer showed an excellent adhesion to the Cu surface and did not peel off even after an ultrasonic bath treatment, enabling Cu@ NPCN with a high electrical conductivity (ca. 2.4 × 105 S m−1). Moreover, the wetting properties of Li droplets indicate that the lithiophobic Cu foam became lithiophilic after

decorating with NPCN (Figure S2), which is beneficial to induce a uniform Li nucleation with small nucleation overpotential.24,34 Li Deposition Behavior on M@NPCN. The Li deposition behavior was investigated using the CR2032 coin cells made of Li foil (i.e., the reference and counter electrode) and Cu@ NPCN (i.e., the working electrode). The electrolyte was 1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in a mixture of 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) (1:1, w/w) containing 2 wt % LiNO3. For comparison, we first fabricated a Janus-structured Cu@NPCN current collector with half Cu surface and half NPCN surface by controlling the dip-coating process (Figure 2a). As expected, after plating 1 mAh cm−2 of Li on the Janus Cu@NPCN current collector, the Cu side was covered with numerous and randomly oriented Li dendrites of tens of micrometers in length, while no obvious Li dendrites were observed on the NPCN side (Figures 2a,b and S3). This result confirms that NPCN is able to suppress the formation of Li dendrites. To further investigate the Li plating/stripping behaviors on M@ NPCN, Li was plated and striped at a constant current of 1 mA cm−2. During the plating/stripping process, seven typical stages labeled in the potential versus time curve (marks I−VII) were measured by ex situ SEM (Figure 2c and d). From stages I to II, Cu@NPCN was initially inserted with Li+. When the voltage was below 0 V, the nucleated Li started to grow on Cu@NPCN, and the Li nucleation overpotential for Cu@ NPCN was about 12 mV.19 At stage II, the plated Li was homogeneously coated on the entire skeleton of Cu@NPCN, D

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Figure 4. Voltage profiles of Li plating/stripping of the Li|Li/Cu foil, Li|Li/Cu foam, and Li|Li/Cu@NPCN symmetric cells with an areal capacity of 1 mAh cm−2 at current densities of 0.5 mA cm−2 (a) and of 1 mA cm−2 (b). (c) Rate capabilities of the Li|Li/Cu foil, Li|Li/Cu foam and Li|Li/Cu@NPCN symmetric cells measured at current densities of 0.5, 1, 3, 5, 10, and 20 mA cm−2.

Li nucleation was clearly observed on the surface of NPCN. With the increasing deposition of Li, NPCN was decorated with the dense Li nanoparticles, while no dendritic Li filament was observed, demonstrating that NPCN can offer plenty of lithiophilic sites for Li nucleation and growth. When Li was stripped from the Li/NPCN composite, the plated Li nanoparticles became gradually smaller and eventually disappeared, thereby revealing high Li plating/stripping reversibility on NPCN. Interestingly, it was observed that Li was preferentially filled in the inner cavity of NPCN during the early stages of the Li plating process. Once the inner cavity was fully filled, Li was observed to grow on the outer surface of NPCN (Figure S5). This phenomenon reveals that the porous structure of NPCN facilitates the access of Li+ and the inner surface of NPCN is also available for Li deposition. The control experiment with the core−shell structured NPCN further demonstrated that the inner cavity of the yolk−shell structured NPCN helps to improve the electrochemical performance of the Li/Cu@NPCN anodes (vide inf ra). Electrochemical Performance of Li/M@NPCN. Considering that CE is a direct parameter reflecting the efficiency and sustainability of Li anodes, the continuous Li plating/stripping on Cu@NPCN was carried out in the above-mentioned twoelectrode configuration with different charge/discharge current densities and Li areal plating capacities. As shown in Figures 3a and S6, with an NPCN loading of 0.3 mg cm−2, Cu@NPCN delivered a high and stable average CE around 99.0% for 400 cycles at a current density of 1 mA cm−2 and Li areal plating capacity of 1 mAh cm−2. In comparison, both the Cu foil and the Cu foam exhibited rapid CE decays. When using 97% CE as a dividing line to define the stable Li plating/stripping, 54

revealing a uniform Li nucleation. With more Li plating, the macropores of Cu@NPCN become much narrower. After the Li areal plating capacity reached 5 mAh cm−2, the top macropores of Cu@NPCN were fully filled with Li, and the resulting Li/Cu@NPCN composite presented a very smooth Li surface without obvious dendrites, as illuminated in stage III. Impressively, this smooth Li surface can be maintained even at a high Li plating areal capacity of 10 mAh cm−2 (stage IV) (Figure S4). When the stripping process was carried out, all the plated Li could be stripped reversibly from the Li/Cu@ NPCN composite electrode. Stages IV−VII showed that Li was gradually stripped from Li/Cu@NPCN accompanied by the gradual reappearing of the macroporous structures of Cu@ NPCN. After the stripping process was finished, Cu@NPCN recovered its original structure and morphology. In addition, no obvious Li dendrites or “dead Li” were observed on the surface of Cu@NPCN after 50 plating/stripping cycles with 1 mAh cm−2 of Li areal plating capacity and 1 mA cm−2 of current density (stage VIII). Since NPCN plays a critical role in the suppression of Li dendrite formation, the in situ TEM technique was utilized to directly observe the Li plating/stripping process. As shown in Figure 2e, an electrochemical dry cell was fabricated in a dualprobe biasing TEM holder. The Li metal-decorated Cu probe and the NPCN-decorated Cu probe were used as the two electrodes of the dry cell. After being exposed to air for a few seconds, LixO grown on the surface of the Li metal was employed to serve as a solid electrolyte and to separate Li metal and NPCN.19 Figure 2f shows the typical morphologies of NPCN captured during the Li plating/stripping process. When an appropriate voltage bias (3.0 V) was applied, uniform E

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Figure 5. (a) Cycling performance of the Li/Cu foil|C/S, Li/Cu foam|C/S, and Li/Cu@NPCN|C/S full cells with 2.5 mg cm−2 of sulfur at 1 C. (b) Cycling performance and (c) charge/discharge curves of the Li/Cu@NPCN|C/S full cell with 2.5 mg cm−2 of sulfur at 2 C. (d) Charge/discharge curves and (e) rate capabilities of the Li/Cu@NPCN|C/S full cell with 4 mg cm−2 of sulfur. (f) Charge/discharge curves and (g) cycling performance of the Li/Cu@NPCN|C/S full cell with 4 mg cm−2 of sulfur at 1 C.

and 126 cycles were achieved for Cu foil (77.2% CE after 66 cycles) and Cu foam (78.9% CE after 150 cycles), respectively, implying that the repeated growth of Li dendrites on the pristine Cu surface was negative for the service life of Li anodes. The charge/discharge profiles (Figures 3b and S7) of Cu@NPCN revealed a nucleation overpotential of 18 mV, lower than those of Cu foil (37 mV) and Cu foam (24 mV). Generally, increasing the electrode surface area can reduce the local current density, which is beneficial to suppressing Li dendrite growth.17 With an NPCN loading of 0.2 mg cm−2, Cu@NPCN provided stable CE values (ca. 99%) for only 215 cycles (Figure S8). However, the high surface area also increased the area of the SEI layer, leading to high Li consumption. When the NPCN loading increased to 0.4 mg cm−2, the CE of Cu@NPCN was stable for 270 cycles (Figure S8). Therefore, the postmodification strategy used herein for fabricating 3D M@C current collectors can optimize the balance between the electrode surface area and Li consumption in SEI formation. Since the charge/discharge current density can affect the CE, a high current density of 3 mA cm−2 was used on Cu@ NPCN. With a Li areal plating capacity of 1 mAh cm−2, the CE of Cu@NPCN stabilized around 98.2% for 250 cycles, whereas the CEs of the Cu foil and Cu foam quickly decreased to 78.9% (after 45 cycles) and 79.0% (after 95 cycles), respectively (Figure 3c). In order to reveal the relationship between the structural parameters of NPCN and the electrochemical performance of Cu@NPCN, we further fabricated graphenewrapped Cu foam (Cu@G), low-surface-area NPCN-wrapped Cu foam (Cu@LSA-NPCN), low-nitrogen-content NPCNwrapped Cu foam (Cu@LNC-NPCN), and core−shell structured NPCN-wrapped Cu foam (Cu@CS-NPCN) as

the control current collectors (for details see the experimental section in the Supporting Information). With the same mass loading of 0.3 mg cm−2 of carbon materials, the stabilities of these current collectors increased in the order Cu@G (79.4% CE after 137 cycles) < Cu@LSA-NPCN (83.6% CE after 170 cycles) < Cu@CS-NPCN (82.3% CE after 199 cycles) < Cu@ LNC-NPCN (83.4% CE after 220 cycles). Compared with these current collectors, the superior stability of Cu@NPCN indicates that the combination of high surface area, high N content, and inner cavity of NPCN are crucial to improve the electrochemical performance of Li/Cu@NPCN anodes, confirming the key points mentioned above for the design of Cu@NPCN collectors. Figure 3d shows the charge/discharge profiles of Cu@NPCN at different current densities. The masstransfer potentials (i.e., the absolute value of the discharge voltage platform) were 23, 31, and 45 mV for 3, 5, and 10 mA cm−2, respectively, indicating that 3D Cu@NPCN can provide a low mass-transfer resistance and fast kinetics of Li+ migration at high current densities.19,34 When the Li areal plating capacity was increased to 6 mAh cm−2, Cu@NPCN exhibited a high CE of ∼99.4% after 100 cycles at 1 mA cm−2 (Figure 3e). Even at an ultrahigh Li areal capacity of 50 mAh cm−2, Cu@ NPCN still retained a CE of 99.2% after 1000 h of Li plating/ stripping at 1 mA cm−2 (Figure S9). To further evaluate the electrochemical performance of Cu@NPCN, the Li/Cu@NPCN, Li/Cu foam, and Li/Cu foil electrodes with a Li areal capacity of 3 mAh cm−2 were paired with Li foils to construct the Li|Li/current collector cells. Figure 4a shows the voltage profiles of the Li|Li/Cu@NPCN, Li|Li/Cu foam, and Li|Li/Cu foil cells charged/discharged at a current density of 0.5 mA cm−2 and areal capacity of 1 mAh cm−2. Li|Li/Cu@NPCN exhibited an outstanding cycling F

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Figure 6. (a) Schematic illustration of the fabrication of the high-sulfur-loading Li−S full cells. (b) Charge/discharge curves and (c) cycling performance of the Li/Cu@NPCN|CNT/S and Li/Cu foil|CNT/S cells (inset: charge/discharge curve of the Li/Cu foil|CNT/S cell). (d) Comparative analysis of the Li areal capacities and Li utilizations of the Li/Cu@NPCN|CNT/S cell with those of the reported Li metal batteries (the Li utilizations and Li areal capacities were chosen from the first cycles).

sulfur loading of the carbon/sulfur cathodes used in this work are higher than many other slurry-coated carbon/sulfur cathodes,43 as most of the carbon/sulfur composites derived electrode films easily crack when the areal sulfur loading of the cathodes is higher than 3 mg cm−2.44 For comparison, four coin cells were assembled by pairing the NHCB/S cathodes (designated as C/S cathodes) with Li foil, Li/Cu foil, Li/Cu foam, and Li/Cu@NPCN, respectively. The areal capacity ratio of the Li/current collector anode to the sulfur cathode (designated as Licapacity/Scapacity) was fixed at 1.8:1 for the Li/ Cu foil|C/S, Li/Cu foam|C/S, and Li/Cu@NPCN|C/S cells. This value is lower than most of the values reported recently for Li−S full cells.27,45−48 The capacities and cycling stabilities of the Li−S full cells with 2.5 mg cm−2 of sulfur were evaluated at a current density of 1 C (1 C = 1675 mA g−1). All the cells were first activated at 0.05 C before the cycling tests, and the specific capacities were calculated based on the weight of sulfur. When excess Li was used, the Li|C/S cell exhibited a capacity of 790 mAh g−1 (90.5% capacity retention) with 99.7% CE after 300 cycles (Figure S12). When Li was used in limited amounts, the Li/Cu foil|C/S cell exhibited a capacity of 129 mAh g−1 (16.6% capacity retention) with 84.9% CE after only 92 cycles, while the Li/Cu foam|C/S cell exhibited a capacity of 280 mAh g−1 (34.2% capacity retention) with 95.0% CE after 300 cycles (Figures 5a and S13). Note that the cathode of the cycled Li/ Cu foil|C/S cell can be recovered by reassembling with fresh Li foil (Figure S14). Therefore, the poor cycling stabilities of these two cells are mainly caused by the unstable anodes, indicating that Li anodes restrict the practical use of sulfur cathodes. With the help of Cu@NPCN, the Li/Cu@NPCN| C/S cell showed high capacity (816 mAh g−1) and stable CE (ca. 99.9%) after 300 cycles (Figures 5a and S13). To visually observe the Li anodes, we disassembled the cells after the cycling tests (Figure S15). A large number of Li dendrites,

stability for 1600 h (400 cycles) with a low and stable voltage hysteresis of 20 mV, significantly superior than Li|Li/Cu foam (650 h) and Li|Li/Cu foil (400 h). When cycled at a higher current density of 1 mA cm−2, Li|Li/Cu@NPCN was run for 1200 h (600 cycles) with a voltage hysteresis of 30 mV. In contrast, the life spans of Li|Li/Cu foam and Li|Li/Cu foil decreased to 320 and 150 h, respectively (Figure 4b). The rate capabilities of the three cells were further tested by cycling from 0.5 to 20 mA cm−2 with an areal capacity of 1 mAh cm−2 (Figure 4c). Although the voltage hysteresis increased with the increase of current density, Li|Li/Cu@NPCN exhibited the lowest voltage hysteresis in comparison to Li|Li/Cu foam and Li|Li/Cu foil. The highest available current densities of Li|Li/ Cu foam, Li|Li/Cu foil, and Li|Li/Cu@NPCN were 3, 10, and 20 mA cm−2, respectively, highlighting the excellent rate capability of Li/[email protected]−25 In addition, the Cu foam used in M@NPCN can be replaced with commercial Ni foam, as metal foam is mainly responsible for providing a 3D macroporous conductive skeleton. Not surprisingly, the Li/ Ni@NPCN containing 0.3 mg cm−2 of NPCN showed a performance nearly the same as that of Li/Cu@NPCN (Figure S10). To the best of our knowledge, the electrochemical performance of Li/Cu@NPCN is among the best Li anodes reported in the literature (Tables S1 and S2).35−41 Application in Li−S Full Cells. The feasibility of M@ NPCN for Li−S full cells was explored by pairing Li/Cu@ NPCN with the carbon/sulfur cathodes. To reveal the potential of the M@NPCN anodes, the high-performance carbon/sulfur cathodes were prepared using our recently developed N-doped hollow carbon bowls (NHCB) as the sulfur hosting material.42 A NHCB/S composite with 85 wt % sulfur was synthesized by the melt-diffusion method (Figure S11). The corresponding carbon/sulfur cathodes with 68 wt % and 2.5−4 mg cm−2 of sulfur were then prepared by a traditional slurry-coating method. The sulfur content and areal G

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ACS Nano “dead Li”, and cracks were observed for both the Li foil and Li/ Cu anodes. On the contrary, Li/Cu@NPCN remains a smooth surface without obvious Li dendrites. More impressively, the long-term cycling test at a high rate of 2 C (ca. 8.4 mA cm−2) indicated that the Li/Cu@NPCN|C/S cell delivered a capacity of 690 mAh g−1 with 78.1% capacity retention after 1400 cycles (Figures 5b,c and S16). During the cycling test, the average CE was ca. 99.7% and the capacity decay rate was as low as 0.016% per cycle. In addition, we demonstrated that Li/Cu@NPCN can also operate with C/S cathodes having higher sulfur loadings. As shown in Figure 5d and e, the Li/Cu@NPCN|C/ S cell with 4 mg cm−2 of sulfur showed outstanding rate capabilities and cycling stabilities. When cycled at 0.2 (1.3), 0.5 (3.4), 1 (6.7), 1.5 (10.1), 2 (13.4), 2.5 (16.8), and 3 C (20.1 mA cm−2), the capacities of the Li/Cu@NPCN|C/S cell were 1031 (4.12), 893 (3.57), 826 (3.30), 789 (3.30), 763 (3.16), 745 (2.98), and 723 mAh g−1 (2.89 mAh cm−2), respectively. After 500 cycles at 1 C, a high capacity of 743 mAh g−1 (ca. 3 mAh cm−2) was still retained with 86.4% capacity retention and 99.2% CE (Figure 5f and g). Moreover, when the Licapacity/ Scapacity was further decreased from 1.8:1 to 1.5:1, the Li/Cu@ NPCN|C/S cell with 4 mg cm−2 of sulfur maintained high capacity and good stability. A discharge capacity of 621 mAh g−1 with 72.3% capacity retention was obtained after 500 cycles at 1 C (Figure S17). The overall performance of Li/Cu@ NPCN|C/S cells is more superior than most of the Li−S batteries reported so far, especially considering their sulfur content, areal sulfur loading, and Licapacity/Scapacity (Table S3).6,28,45−48 Although considerable progress has been made in suppressing Li dendrite growth, the active material loading of cathodes and Li areal capacity utilization of anodes in most Li metal batteries are still far behind the practical energy density requirements.26,43 In order to highlight the advantages of the Li/M@NPCN anode, a high-sulfur-loading Li−S cell was further constructed with a low Licapacity/Scapacity ratio of 1:1. As shown in Figure 6a, a carbon/sulfur cathode with 70 wt % and 10 mg cm−2 of sulfur (theoretical areal capacity of 16.75 mAh cm−2) was fabricated using commercial carbon nanotubes (CNTs) as the sulfur hosting material (Figure S18). To suppress the polysulfide shuttling of the CNT/S cathode, we used our recently reported polysulfide barrier-modified Celgard separator for constructing the Li−S full cell.49 It should be pointed out that the polysulfide barrier layer (0.075 mg cm2) used in this work caused negligible effects on the overall sulfur content of the batteries (Figure S19). Since the practical areal capacity of the CNT/S cathode was ca. 12 mAh cm−2 when a Li foil was used as the anode (Figure S18), the predeposited Li of Li/Cu@NPCN was controlled to be 12 mAh cm−2. At this ratio, the full cell showed a high energy density only when the Li/Cu@NPCN anode consistently provided high Li areal capacity utilization during the charge/ discharge processes, which is very difficult for current Li anodes.26 Excitingly, the charge/discharge testing of the full cell revealed high energy density and good cycling stability (Figure 6b and c). During the activation at a current density of 0.84 mA cm−2, the initial areal capacity of the Li/Cu@NPCN| CNT/S cell was as high as 11.86 mAh cm−2, corresponding to an ultrahigh Li utilization of 98.8%. After 45 cycles at 1.68 mA cm−2, an areal capacity of 9.84 mAh cm−2 was still maintained with 98.3% CE and 82% Li utilization. In contrast, the capacity of the Li/Cu foil|CNT/S cell rapidly decreased from 10.0 to 4.2 mAh cm−2 and failed after only 16 cycles (Figure 6c). As

shown in Figure 6d and Table S4, the areal capacity and Li utilization of the Li/Cu@NPCN|CNT/S cell are significantly higher than those shown by state-of-the-art Li metal batteries.7,26,29 As is well known, commercial Cu foams suffer from high weight (usually >30 mg cm−2), higher than those of commercial Cu foil current collectors, leading to a decrease of the battery energy density. To address this issue, we developed a facile method to prepare lightweight M@NPCN current collectors by etching commercial Ni foams (18.4 mg cm−2) in a ferric chloride solution for 1 h. The low-weight Ni@ NPCN (9.6 mg cm−2) was successfully obtained after the subsequent dip-coating deposition of NPCN (Figure S20). The obtained Ni@NPCN is slightly lighter than a commercial 12 μm thick Cu foil (ca. 10.0 mg cm−2). As a current collector for the Li anode, the lightweight Ni@NPCN dramatically enhanced the performance of the Li−S full cells. A Li/Ni@ NPCN|C/S cell with 4 mg cm−2 of sulfur delivered a high capacity of 702 mAh g−1 with 81.3% capacity retention and 99.6% CE after 500 cycles at 1 C (Figure S21). Using the same strategy, the weight of Ni@NPCN can be further reduced to 7.3 mg cm−2, which is even comparable to that of the 8 μm thick Cu foil (ca. 7.14 mg cm−2) (Figure S22). Moreover, since M@NPCN determines the geometric sizes of the Li/M@ NPCN anode, the high Li areal capacity is helpful to improve the volumetric energy density of the Li/M@NPCN anode. For instance, the volumetric energy density of Li/Ni@NPCN with 12 mAh cm−2 of Li is about 1665 mAh cm−3, corresponding to 80.8% of the theoretical value of Li (Figure S23). Therefore, the development of a Li/M@NPCN anode with high Li areal capacity, high Li utilization, and lightweight M@NPCN current collector is a promising route to high-energy Li−S batteries.

CONCLUSIONS In summary, a facile postmodification strategy was developed to fabricate 3D porous M@C current collectors. With a large surface area, high nitrogen doping, and available hollow inner cavity, NPCN was proved to be an ideal building block for preparing the lithiophilic M@C current collectors. Since NPCN can guide uniform Li nucleation and growth, Li metal deposition on M@NPCN was effectively carried out while avoiding the growth of Li dendrites at a high Li deposition capacity of 10 mAh cm−2. The Li/M@NPCN composite anode showed a high CE and a low hysteresis after cycling for 1600 h in a Li/Cu@NPCN|Li cell. When coupling the Li/M@NPCN anode with the C/S cathode at a low Licapacity/Scapacity of 1.8:1, the Li−S full cell showed a high cycling stability over 1400 cycles. By partially etching metal foam supports, the weight of M@NPCN current collectors can even compete with those of commercial metal foils. More importantly, a high-sulfur-loading Li−S full cell assembled using the Li/M@NPCN anode exhibited both high areal capacity (9.84 mAh cm−2) and high Li utilization (82%), revealing the energy density advantages of the Li/M@NPCN anode. We hope this work can open opportunities to generate durable and safe Li anodes for practically viable Li−S batteries. EXPERIMENTAL SECTION Preparation of M@NPCN. NPCN was synthesized according to a protocol previously reported by our group.32 The metal foams used in this work were obtained by pressing commercial Cu or Ni foams (HGP Technology Co., Ltd.) at 0.2 MPa for 0.5 min and H

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ACS Nano subsequently washed with ethanol three times before further use. The “NPCN ink” was prepared by adding 280 mg of NPCN and 12 mL of 1 wt % N-lauryl acrylate (Chengdu Yindile Power Supply Technology) aqueous solution to 210 mL of a water/ethanol mixture (v/v = 1:1). After ultrasonication for 10 min, the metal foam was immersed into the as-obtained “NPCN ink” and then coated with an NPCN layer by a dip-coating process. The as-obtained M@NPCN was dried at 60 °C for 6 h, and the loading of the NPCN layer on the metal foam was controlled to be 0.3 mg cm−2. Li/M@NPCN was prepared by predeposition of Li onto the M@NPCN. Characterization. SEM and TEM analyses were carried out on Zeiss SIGMA and TECNAI F-30 high-resolution TEM microscopes, respectively. For the ex situ SEM tests, all samples were prepared in an Ar-filled glovebox. Coin cells were carefully disassembled, and the Li/ M@NPCN electrodes were soaked and washed with DOL (3−5 times) and then dried in a glovebox. The treated Li/M@NPCN electrodes were then sealed in atmosphere-protected bottles and transferred immediately to the vacuum chamber of the SEM device. N2 adsorption−desorption isotherms were performed on a TriStar II 3020 system. The XPS spectra were recorded on a PHI QUANTUM 2000. The electrical conductivity was measured by using the fourprobe method. Electrochemical Measurements. Two-electrode configuration coin cells were assembled in an Ar-filled glovebox to investigate the Li deposition behavior and CE. M@NPCN was used as the work electrode, while Li foil (China Energy Lithium Co., Ltd.) was used as the reference/counter electrode. The diameters of the Li foil and Cu@NPCN are both 12 mm. LiTFSI of 1 M in a mixed solvent of DOL/DME (v/v = 1:1) with 2.0 wt % LiNO3 additive was employed as the electrolyte. The dosage of the electrolyte in each cell was 80 μL. To remove surface contaminants and form a stable SEI layer on the current collectors, the coin cells were first cycled at 0.05 mA cm−2 with a voltage ranging from 0 to 1.0 V for 5 cycles. Fixed amounts of Li were then plated onto M@NPCN at a constant current and stripped when charging to 1 V. To evaluate the cycling stability and voltage hysteresis, 3 mAh cm−2 of Li was predeposited onto M@ NPCN. The resulting Li/M@NPCN electrode was paired with Li foil to form a Li|Li/current collector cell. Galvanostatic cycling was conducted at different current densities. To evaluate the performance of Li−S full cells, NHCB was synthesized according to our previous work.35 NHCB was mixed with sublimed sulfur (15:85, w/w) in a sealed glass bottle, and the NHCB/S composite was obtained after heating at 155 °C for 12 h. The cathode slurry was prepared by mixing NHCB/S, Super P, and N-lauryl acrylate with a mass ratio of 8:1:1 in deionized water. To increase the areal sulfur loading of the cathode, the CNT paper (∼0.7 mg cm−2) obtained by vacuum filtration dispersion of CNT (Wuhan ATMK Super EnerG Technologies) was used as the cathode current collector. The NHCB/S cathodes were prepared by coating the slurry onto the CNT paper and then drying at 60 °C for 12 h. The areal sulfur loading of the NHCB/S cathodes was 2.5 to 4 mg cm−2. The diameters of the NHCB/S cathode and Li/Cu@NPCN anode were controlled at 12 and 14 mm, respectively. In an Ar-filled glovebox, a CR2032 coin cell with the NHCB/S cathode, the Li/Cu@NPCN anode, and a polypropylene separator was assembled by using 1 M LiTFSI in a mixed solvent of DOL/DME (v/v = 1:1) with 2.0 wt % LiNO3 additive as the electrolyte. The ratio of electrolyte to sulfur in each Li−S full cell was 10 μL mg−1. The performance of the abovementioned half cells, Li|Li/current collector cells, and Li−S full cells was measured on a LAND CT2001A cell test instrument.

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Xiaoliang Fang: 0000-0001-6048-9926 Ming-Sheng Wang: 0000-0003-3754-2850 Nanfeng Zheng: 0000-0001-9879-4790 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by the National Key R&D Program of China (2017YFA0207302), the MOST of China (2015CB932300), the NSF of China (21731005, 21420102001, 21721001), and the Fundamental Research Funds for the Central Universities (20720160080, 20720180026). REFERENCES (1) Manthiram, A.; Fu, Y.; Chung, S. H.; Zu, C.; Su, Y. S. Rechargeable Lithium-Sulfur Batteries. Chem. Rev. 2014, 114, 11751− 11787. (2) Service, R. F. Lithium-Sulfur Batteries Poised for Leap. Science 2018, 359, 1080−1081. (3) Bruce, P. G.; Freunberger, S. A.; Hardwick, L. J.; Tarascon, J. M. Li-O2 and Li-S Batteries with High Energy Storage. Nat. Mater. 2012, 11, 19−29. (4) Pang, Q.; Liang, X.; Kwok, C. Y.; Nazar, L. F. Advances in Lithium-Sulfur Batteries Based on Multifunctional Cathodes and Electrolytes. Nat. Energy 2016, 1, 16132. (5) Lin, D. C.; Liu, Y. Y.; Cui, Y. Reviving the Lithium Metal Anode for High-Energy Batteries. Nat. Nanotechnol. 2017, 12, 194−206. (6) Chung, S. H.; Chang, C. H.; Manthiram, A. Progress on the Critical Parameters for Lithium-Sulfur Batteries to be Practically Viable. Adv. Funct. Mater. 2018, 28, 1801188. (7) Cheng, X. B.; Zhang, R.; Zhao, C. Z.; Zhang, Q. Toward Safe Lithium Metal Anode in Rechargeable Batteries: A Review. Chem. Rev. 2017, 117, 10403−10473. (8) Suo, L.; Hu, Y. S.; Li, H.; Armand, M.; Chen, L. A New Class of Solvent-in-Salt Electrolyte for High-Energy Rechargeable Metallic Lithium Batteries. Nat. Commun. 2013, 4, 1481. (9) Lu, Y. Y.; Tu, Z. Y.; Archer, L. A. Stable Lithium Electrodeposition in Liquid and Nanoporous Solid Electrolytes. Nat. Mater. 2014, 13, 961−969. (10) Li, W. Y.; Yao, H. B.; Yan, K.; Zheng, G. Y.; Liang, Z.; Chiang, Y. M.; Cui, Y. The Synergetic Effect of Lithium Polysulfide and Lithium Nitrate to Prevent Lithium Dendrite Growth. Nat. Commun. 2015, 6, 7436. (11) Xin, S.; You, Y.; Wang, S. F.; Gao, H. C.; Yin, Y. X.; Guo, Y. G. Solid-State Lithium Metal Batteries Promoted by Nanotechnology: Progress and Prospects. ACS Energy Lett. 2017, 2, 1385−1394. (12) Fu, K. K.; Gong, Y. H.; Liu, B. Y.; Zhu, Y. Z.; Xu, S. M.; Yao, Y. G.; Luo, W.; Wang, C. W.; Lacey, S. D.; Dai, J. Q.; Chen, Y. N.; Mo, Y. F.; Wachsman, E.; Hu, L. B. Toward Garnet Electrolyte-Based Li Metal Batteries: An Ultrathin, Highly Effective, Artificial Solid-state Electrolyte/Metallic Li Interface. Sci. Adv. 2017, 3, No. e1601659. (13) Li, G. X.; Huang, Q. Q.; He, X.; Gao, Y.; Wang, D. W.; Kim, S. H.; Wang, D. H. Self-Formed Hybrid Interphase Layer on Lithium Metal for High-Performance Lithium-Sulfur Batteries. ACS Nano 2018, 12, 1500−1507. (14) Tu, Z. Y.; Choudhury, S.; Zachman, M.; Wei, S. Y.; Zhang, K. H.; Kourkoutis, L. F.; Archer, L. A. Fast Ion Transport at Solid-Solid Interfaces in Hybrid Battery Anodes. Nat. Energy 2018, 3, 310−316. (15) Zheng, G. Y.; Lee, S. W.; Zheng, L.; Lee, H. W.; Yan, K.; Yang, H. B.; Wang, H. T.; Li, W. Y.; Chu, S.; Cui, Y. Interconnected Hollow

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.9b03784. Additional SEM, TEM, optical photographs, and electrochemical performance, including 23 figures and 4 tables(PDF) I

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