Encapsulating Metallic Lithium into Carbon Nanocages Which

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Encapsulating Metallic Lithium into Carbon Nanocages Enables Low Volume Effect and Dendrite-Free Lithium Metal Anode Hailin Fan, Qingyuan Dong, Chunhui Gao, Bo Hong, Zhian Zhang, Kai Zhang, and Yanqing Lai ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b09321 • Publication Date (Web): 05 Aug 2019 Downloaded from pubs.acs.org on August 5, 2019

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ACS Applied Materials & Interfaces

Encapsulating Metallic Lithium into Carbon Nanocages Enables Low Volume Effect and Dendrite-Free Lithium Metal Anode

Hailin Fan, Qingyuan Dong, Chunhui Gao, Bo Hong*, Zhian Zhang, Kai Zhang, Yanqing Lai* School of Metallurgy and Environment, Central South University, Changsha, Hunan 410083, China

ABSTRACT: Metallic lithium, with its high capacity and low redox potential, shows significant development potential for high-energy-density lithium batteries. Unfortunately, huge volumetric changes, uncontrollable Li dendrites, and interfacial parasitic reactions limit its commercial application. Herein, we demonstrate a rational strategy of encapsulating metallic lithium into the interior spaces of hollow carbon nanocages for dendrite-free lithium metal anodes. We find that the polyvinylidene difluoride binder (PVDF) modified thin-layer carbon walls on the carbon nanocages can guide the lithium deposition into the interior spaces of these hollow carbon nanocages and simultaneously reduce the interfacial parasitic reaction between deposited Li metal and electrolyte. In addition, due to the high specific surface area and huge interior spaces of the carbon nanocages, the local current density can be reduced and the large volume changes are mitigated. Specifically, this electrode exhibits negligible volume changes at 1.0 mAh/cm2 and a 14.9% volume change at 3.0 mAh/cm2. The Cu foil electrode exhibits 87.9% and 234.3% volume changes at the corresponding deposition capacities. Consequently, carbon-nanocage-modified electrode exhibits an outstanding Coulombic efficiency of 99.7% for nearly 150 cycles at a current density of 1.0 mA/cm2, while Cu foil electrode exhibits less than a 70.0% Coulombic efficiency after only 43 cycles. When paired with a sulfur cathode, the carbon-nanocage-modified electrode exhibits better cycling and rate performances than the pristine Cu foil electrode. KEYWORDS: hollow carbon nanocages, induced deposition, interfacial parasitic reaction, minimum volume change, dendrite-free Li metal anode, uniform growth

1. INTRODUCTION Owing to its high theoretical capacity, highly negative potential, and good conductivity, Li metal is the most attractive material for Li rechargeable batteries.1-3 However, its unsatisfactory cycle life, low Coulombic efficiency, safety issues caused by huge volume changes, uncontrollable dendrite growth, and interfacial parasitic reactions

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limit its industrial applications.4-6 To address these problems, four main pathways have been proposed: forming a stable solid electrolyte interphase (SEI), applying surface modifications using coatings or interlayers, using gel or solid electrolytes, and employing 3D lithium anodes. Stable and uniform SEIs can be obtained by using optimized solvents,7,8 salts,9-11 and electrolyte additives,12-16 which inhibit the Li dendrite formation at the low current densities, but produce ideal results at the improved current densities. Surface modifications using coatings17-19 or interlayers2022

on the Li/Cu foil can reduce the activity of the lithium metal and hinder the decomposition of electrolytes,

whereas it will lead to longer ion transport paths and increased polarization. Gel or solid electrolytes with high shear moduli are widely used as mechanical barriers to prevent the growth of lithium dendrites,23-25 but it is difficult to inhibit the growth of mossy lithium beneath the physical barriers.26 Meanwhile, low ionic conductivity as well as poor interface contact at the room temperature limit the practical applications as well.27 Even worse, the above three methods cannot solve the severe volume expansion and contraction issue that occurs during lithium deposition and dissolution.28,29 Three-dimensional (3D) lithium metal anodes with enhanced specific surface areas can effectively lower the current density and limit volume changes during the cycles. This is beneficial for extending the Sand's time and achieving dendrite-free lithium deposition. Based on this strategy, 3D porous current collectors, such as porous nano-copper,30-33 Ni foam,34-37 TiC/C nanowire arrays,38 graphene frameworks,39-42 graphite microtubes,43 hollow carbon granules,44,45 and CNT sponges,46 are widely considered as advanced 3D hosts for uniform lithium deposition. However, a high specific surface area will lead to severe interfacial parasitic reactions between the lithium metal and electrolyte, which usually results in a low Coulombic efficiency. For instance, a 3D Cu skeleton fabricated using self-assembly and dehydration reduction only exhibited a ~97% Coulombic efficiency after 50 cycles.30 Similarly, Zhang et al. exploited N-doped graphene as a porous current collector, rendering a 98% Coulombic efficiency after around 200 cycles.42 In addition, considering the large-scale synthesis and practical applicability, most 3D porous current collectors still suffer from issues with cost and the manufacturing process used for lithium ion batteries. Therefore, developing an effective and scalable approach to simultaneously minimize the volume changes, inhibit dendrite growth, and reduce the occurrence of parasitic reactions is urgently needed. In this study, we utilize commercialized hollow carbon nanocages as coatable nanoparticles to guide dendrite-free Li deposition. This strategy does not change the existing preparation method of the negative electrodes in lithium ion batteries. In general, hollow carbon nanocages, with their high specific surface areas and huge interior spaces,


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can lower the local current density and reduce the volume changes. In addition, PVDF modified thin-layer carbon walls on the carbon nanocages can be employed as an artificial solid electrolyte interphase layer, which will promote uniform lithium ionic flux and nucleation/deposition of Li ions into the interior spaces of the designed 3D host, resulting in reduced occurrence of parasitic reactions.45,47 More importantly, this dispersed SEI layer helps to prevent stress concentration during the repeated Li deposition/dissolution.48 Consequently, the proposed strategy can not only create uniform Li nucleation and stable Li deposition, even at a high current density of 10.0 mA/cm2, but can also limit the parasitic reactions and volume changes, resulting in an outstanding Coulombic efficiency and improved lifespan simultaneously (Figure 1(a)). On the contrary, the pristine Cu foil electrode, with its nonuniform and rough surface, leads to heterogeneous Li nucleation followed by dendrite growth and SEI rupture, resulting in the accelerated accumulation of SEIs and the continuous formation of dead Li during the repeated cycles (Figure 1(b)). As a result, Cu foil produces a low Coulombic efficiency and limited cycle life.

Figure 1. Schematic representation of Li deposition/dissolution processes on the (a) carbon nanocagemodified and (b) Cu foil electrodes.

2. EXPERIMENTAL PROCEDURES 2.1 Preparation of carbon-nanocage-modified and sulfur electrodes. Carbon nanocage powders (Shanghai Shangmu Technology Co, Ltd.) and polyvinylidene difluoride binder (PVDF) with a mass ratio of 4:1 were mixed in an N-methyl-2-pyrrolidone (NMP) solution for 30 min, after which they were coated onto a Cu foil and



subsequently dried at 60°C in a vacuum oven. The mass loading of the carbon nanocages on the Cu foil was only about 0.1 mg/cm2. To prepare the sulfur electrode, S, PVDF, and carbon black with a mass ratio of 8:1:1 were mixed in an NMP solution for 30 min to form a slurry. The slurry was subsequently coated onto Al foil and dried at 60°C in a vacuum oven. The areal mass loading of sulfur on the Al foil was about 1.6 mg/cm2. 2.2 Characterizations and measurements. The microscopic morphologies and corresponding structures of the carbon nanocage powders were characterized by scanning electron microscopy (SEM, FEI Nova NanoSEM 230), transmission electron microscopy (TEM, JEM-2100F), Raman spectroscopy (Renishaw, Invia), and X-ray photoelectron spectroscopy (XPS, Thermo Escalab 250Xi). The Brunauer-Emmett-Teller (BET) surface area of the carbon nanocage powder was obtained by a volumetric sorption analyzer (ASAP 2460). The morphologies of the Cu foil and carbon-nanocage-modified electrodes before and after deposition were characterized by SEM and TEM. The cycling performances were acquired using CR2032 coin-type cells on the battery system (LAND CT2001A). The working electrode was Cu foil or carbon-nanocage-modified Cu foil, while the counter electrode is Li sheet. 1.0 M LiTFSI in DME/DOL (volume ratio 1:1) with the additive of LiNO3 (1 wt.%) was utilized as the electrolyte in each cell. The nucleation overpotential test was performed on the Cu foil electrode and carbon-nanocage-modified electrode at 0.01, 0.02, 0.05, 0.1, and 0.2 mA/cm2, respectively. As for the Coulombic efficiency, Li deposition was finished when a targeted area capacity (0.5, 1.0, and 3.0 mAh/cm2) was arrived; and a cutoff potential of 1.0 V was employed for Li dissolution process. For the Li-S full cells, metallic lithium was first deposited on the Cu foil electrode and carbon-nanocage-modified electrode at 0.1 mA/cm2 for 30 h. Subsequently, Li-S full cells were assembled and evaluated within the voltage range of 1.7–2.8 V. Electrochemical impedance spectroscopy was performed on a Solartron 1470E cell test system with frequencies from 10 mHz to 100 kHz. All of these electrochemical tests were implemented at room temperature.

3. RESULTS AND DISCUSSION Figure 2(a) and (b) show the SEM and TEM images of the carbon nanocage powders. Carbon nanocage particles with thin carbon walls of ~4.7 nm and tempered sizes of 200–400 nm create a hollow structure, which can maintain a high specific surface area even after the electrode preparation.44,45,49 Meanwhile, huge interior spaces of the carbon nanocage particles are conducive to the storage of metallic Li.44,50 Owing to the nanometer sizes and huge interior spaces, the carbon nanocage powders exhibits a high specific surface area of 104.7 m2/g (Figure S1). To


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assess the regularity of the carbon nanocage powders, Raman spectroscopy is used, and the Raman spectrum is shown in Figure S2. The carbon nanocage powders exhibits two main characteristic peaks in the Raman spectra, originating from the D band at ~1350.1 cm-1 and G band at ~1590.5 cm-1, indicating that there are moderate defects in the carbon nanocages because of the moderate intensity ratio of the D and G bands (ID/IG>0.8).51 The defects in the carbon nanocages are further clarified by XPS analysis (Figure S3). The carbon nanocages are mainly composed of carbon and oxygen. The C mainly originates from C=C, C-C, C-O, and C=O, corresponding to peaks at 284.8, 285.4, 286.7 and 288.3 eV,52 respectively, while the O is attributed to C-O and C=O, with peaks at 532.6 and 533.3 eV, respectively. C-O and C=O, as polar surface functional groups, will enhance the electrolyte uptake capabilities, facilitate homogeneous Li ionic flux,53 and preferentially induce Li nucleation.35,54-56

Figure 2. (a) SEM image and (b) TEM image of carbon nanocage powders. (c) Deposition curves at 0.01 mA/cm2. (d) Li nucleation overpotentials at different current densities. To verify that the preferentially controlled nucleation of metallic Li occurred in the carbon nanocages, we employ Cu foil and carbon-nanocage-modified Cu foil as the working electrodes, and then perform deposition tests at



various current densities (Figure 2(c, d) and Figure S4). Cu foil electrode yields a Li nucleation overpotential of 18.0 mV at 0.01 mA/cm2, which is 2.1 times larger than that of the carbon-nanocage-modified electrode (8.4 mV) (Figure 2(c, d)). When the current density is increased to 0.02 mA/cm2, the nucleation overpotential on the Cu foil electrode increases rapidly to 28.8 mV, whereas the carbon-nanocage-modified electrode retains a low overpotential of Li nucleation (11.8 mV) (Figure S4(a) and Figure 2(d)). With the continued increase of the current density from 0.05 to 0.2 mA/cm2, the nucleation overpotentials on the Cu foil electrode are still much larger than those on the carbonnanocage-modified electrode (Figure S4(b-d) and Figure 2(d)). These results indicate that Li metal is more easily deposited on the carbon-nanocage-modified electrode.57-59 The low overpotential will lead to fast ion diffusion at the interface and promote the growth of Li metal along the diameter instead of the length, resulting in dendrite-free Li deposition.60

Figure 3. SEM images of (a–e) Cu foil electrode and (f–j) carbon-nanocage-modified electrode: (a, f) The morphologies of pristine Cu foil and modified electrodes; (b, g) Li deposition of 0.1 mAh/cm2, (c, h) 0.5 mAh/cm2, (d, i) 1.0 mAh/cm2, and (e, j) 5.0 mAh/cm2. Inset in the upper right corner shows the reduced SEM images of the carbon-nanocage-modified electrode. To study the deposition behavior of Li metal on these electrodes, we conduct a continuous plating test at 1.0 mA/cm2, as shown in Figure 3. Abundant defects are found on the surface of the Cu foil (Figure 3(a)), which will lead to nonuniform Li protrusion growth (Figure 3(b)), followed by the localized Li accumulation, and resulting in the formation of Li dendrites (Figure 3(c)).30 When the Li deposition capacity is increased to 1.0 mAh/cm2, the Cu foil surface exhibits unlimited growth of fibrous Li dendrites (Figure 3(d)).61 When 5.0 mAh/cm2 of metallic lithium is electroplated on this surface, loose and porous dendrite coatings appears (Figure 3(e)). This will cause


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uncontrollable interfacial parasitic reactions and huge safety risks.45 In sharp contrast, the carbon-nanocage-modified electrode exhibits a distinct three-dimensional structure (Figure 3(f)), which will lower the effective local current density of the electrode during the cycling.30,35,62 There is almost no change in the morphology of the carbonnanocage-modified electrode with a low area capacity of 0.1 mAh/cm2, indicating that uniform Li nucleation occurs on this electrode (Figure 3(g)). With the increased capacity of 0.5 or 1.0 mAh/cm2, no significant changes occur, indicating that there is uniform deposition of Li ions into the interior spaces of the 3D host (Figure 3(h, i)). When the capacity grows to 5.0 mAh/cm2, only compact, nodule-like and dendrite-free Li deposition coatings covers the surface of this electrode (Figure 3(j) and inset of Figure 3(j)).

Figure 4. SEM images of (a–d) Cu foil and (e–h) carbon-nanocage-modified electrodes at cross-sections: (a, e) The cross-sectional morphologies of pristine Cu foil and modified electrodes. Li deposition of (b, f) 1.0 mAh/cm2, (c, g) 3.0 mAh/cm2, and (d, h) 5.0 mAh/cm2. TEM images of carbon-nanocage-modified electrode (i) before and after Li deposition of (j) 1.0 mAh/cm2, (k) 3.0 mAh/cm2, and (l) 5.0 mAh/cm2.



To further study the difference in the Li deposition behavior on these electrodes, the cross-sectional morphologies before and after Li deposition of 1.0, 3.0, and 5.0 mAh/cm2 are shown in Figure 4. Cu foil exhibits a uniform thickness of ~9.9 μm (Figure 4(a)). After deposition for 1.0 mAh/cm2, an Li layer with a thickness of 8.7 μm appears on the surface, which is much thicker than the theoretical value of 4.9 μm,63 indicating that loose lithium deposition occurs. This originates from the growth of dendrites, as confirmed by the surface morphology shown in Figure 3(d). If the area capacity grows to 3.0 and 5.0 mAh/cm2 (Figure 4(c, d)), loose lithium deposition induced by fibrous growth of Li dendrites further aggregates, resulting in a significant increase in the Li coating thickness (23.2 μm for 3.0 mAh/cm2 and 43.5 μm for 5.0 mAh/cm2). The loose lithium deposition leads to huge Cu foil electrode volume changes of 87.9% for deposition capacities of 1.0 mAh/cm2, 234.3% for 3.0 mAh/cm2, and 439.4% for 5.0 mAh/cm2. By comparing the Li deposition behavior on the Cu foil electrode, the carbon-nanocage-modified electrode exhibits a significantly different deposition behavior. As shown in Figure 4(e), the carbon nanocage layer coated on the Cu foil has a thickness of 29.1 μm. After Li deposition of 1.0 mAh/cm2, the carbon-nanocagemodified electrode exhibits a negligible thickness change (29.1 μm) (Figure 4(f)), indicating that there is complete Li deposition in the interior spaces of this 3D host. Even after Li deposition of 3.0 mAh/cm2, this electrode only exhibits a small thickness change from 29.1 to 34.9 μm, corresponding to a volume change of 14.9%. (Figure 4(g)). If an additional 2.0 mAh/cm2 of metallic Li is further deposited on this electrode, lithium metal on the surface further accumulates, resulting in the appearance of an Li layer with a thickness of 11.6 μm (Figure 4(h)). This means that the carbon-nanocage-modified electrode can stably store more than 5.0 mAh/cm2 of Li metal, corresponding to an ultrahigh specific capacity of 3584.2 mAh/g, which is comparable to the homologous materials reported in recent literature, as summarized in Table S1. These results indicate that the carbon-nanocage-modified electrode exhibits dense lithium deposition and minimum volume changes, even at a high deposition capacity. Furthermore, TEM images further clarify that the lithium ions deposits into the interior spaces of the carbon nanocages (Figure 4(i-l)). This interesting result can be attributed to the special structure of the carbon-nanocage-modified electrode. Carbon nanocage grains are dispersed by a PVDF insulator layer, and consequently, the conductive thin-layer carbon walls on the carbon nanocages will provide nucleation sites for the lithium metal. Moreover, the interior spaces of the carbon nanocages are free of mechanical stress, while PVDF/C interface usually exhibits high mechanical stress.45,47 These reasons lead to preferential Li deposition into the carbon nanocages instead of outside the electrode or on the Cu foil. Thence, lithium metal mainly deposits on the carbon walls at a low area capacity (Figure 4(i, j)). Upon


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increasing the Li deposition capacity to 3.0 mAh/cm2, Li metal in the interior spaces of carbon nanocages further accumulates, as shown in Figure 4(k). As a consequence, if the area capacity is further increased to 5.0 mAh/cm2, the interior spaces of the carbon nanocages are almost completely filled with metallic Li (Figure 4(l)).

Figure 5. TEM images and elemental maps of the carbon nanocages (a, b) before and (c, d) after Li deposition of 1.0 mAh/cm2 at 1.0 mA/cm2. Additionally, to further confirm uniform lithium deposition into the carbon nanocages, elemental analyses are carried out, as shown in Figure 5. Before Li deposition, the carbon walls on the carbon nanocages are very thin and uniform (Figure 5(a) and S5(a)), exhibiting simple distributions of carbon and oxygen elements with no signals from nitrogen, fluorine, or sulfur (Figure 5(b)), which is consistent with the XPS result shown in Figure S3. However, after Li deposition at 1.0 mA/cm2, these carbon walls exhibits an increased thickness from 4.7 to 21.4 nm (Figure 5(c)), corresponding to a uniform deposition of Li metal with a thin SEI (2 ~ 4 nm) on the carbon walls (Figure S5(b)). Furthermore, nitrogen, fluorine, and sulfur elements introduced by the reaction between metallic Li and the electrolyte are found and uniformly distributed on the carbon nanocages (Figure 5(d)). More importantly, these



elements are concentrated on the carbon wall, as illustrated in Figure S6, which is consistent with the result of Figure 4(j). In addition, XPS spectrum further confirm that SEI constituents on the carbon-nanocage-modified electrode are composed of Li2CO3, LiF, LiNxOy, ROCOOLi, and, ROLi, etc (Figure S7).47,64 As compared to SEI constituents on the Cu foil electrode, the carbon nanocage-modified electrode exhibits higher contents of LiF and LiNxOy, indicating more stable and robust SEI film.16,65 To demonstrate the advantages of carbon-nanocage-modified electrode, cycle performances of the Cu foil electrode and carbon-nanocage-modified electrode are exhibited in Figure 6. At a current density of 1.0 mA/cm2 with the low area capacity of 0.5 mAh/cm2, the carbon-nanocage-modified electrode exhibits 98.9% Coulombic efficiency over 250 cycles, while the Cu foil electrode quickly drops to below 70.0% in Coulombic efficiency after 85 cycles (Figure 6(a)). If the area capacity increases from 0.5 to 1.0 mAh/cm2, the carbon-nanocage-modified electrode exhibits an outstanding Coulombic efficiency of 99.7% after around 150 cycles, indicating that there is negligible reaction between metallic Li and electrolyte. For Cu foil electrode, the Coulombic efficiency decreases below 70.0% after 43 cycles (Figure 6(b)). This leads to a significant interfacial reaction induced by the Li dendrites. The carbon-nanocage-modified electrode retains a high Coulombic efficiency for 100 cycles at 5.0 mA/cm2. However, Cu foil electrode oscillates after only 15 cycles (Figure 6(c)). Moreover, even at the high current density of 10.0 mA/cm2, the carbon-nanocage-modified electrode operates stably for 60 cycles (Figure S8(a)). Currently, the area capacities of commercial electrodes in lithium-ion batteries exceed 3.0 mAh/cm2.64 Consequently, we perform further cycling performance tests at 3.0 mAh/cm2, as displayed in Figure S8(b). The carbon-nanocage-modified electrode exhibits a stable lifetime for 60 cycles, two times longer than the cycling life on the Cu foil electrode. In addition, the carbon-nanocage-modified electrode exhibits a reduced charge transfer resistance before and after 50 cycles at 5.0 mA/cm2 (Figure S9). Furthermore, many electrolyte reduction products are nonuniformly distributed on the surface of the Cu foil (Figure S10(a)), which leads to continuous growth, rupture, and accumulation of an SEI and to the formation and accumulation of dead lithium. As for the carbon-nanocage-modified electrode, only trace amounts of dead lithium or mossy lithium are found, as shown in Figure S10(b). This is mainly due to the huge structural advantages of the designed 3D host, which lower the local current density, adjust the volume change, inhibit parasitic reactions, and even prevent stress concentration during the repeated cycles. To further clarify the cycling stability, 5.0 mAh/cm2 of Li is first deposited on the different electrodes to manufacture a simulated symmetric battery. The voltage-time profiles of these batteries at 3.0 mA/cm2 with the


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deposition capacity of 3.0 mAh/cm2 are shown in Figure S11. The voltage polarization on the carbon-nanocagemodified electrode retains ~34.8 mV for over 100 h, while the voltage hysteresis on the Cu foil electrode reaches 60.1–92.2 V during the cycles. Moreover, the Cu foil electrode is short circuited after only 20 h. Such outstanding performances originate from the structural advantages of the carbon-nanocage-modified electrode, including the high specific surface area and huge interior spaces, which are beneficial for lowering the actual current density, relieving the volumetric change, and even preventing interfacial parasitic reactions during the cycling. Furthermore, thin-layer carbon walls as the dispersed SEI layer can ease the stress concentration during the repeated cycles.45,47,48

Figure 6. Current densities and deposition capacities during cycle testing of the Cu foil and carbon-nanocagemodified electrodes: (a) At 1.0 mA/cm2 with an area capacity of 0.5 mAh/cm2; (b) At 1.0 mA/cm2 with an area capacity of 1.0 mAh/cm2; (c) At 5.0 mA/cm2 with an area capacity of 1.0 mAh/cm2.



Galvanostatic cycle voltage profiles of Cu foil and carbon-nanocage-modified electrodes at 1.0 mA/cm2 with a deposition capacity of 1.0 mAh/cm2 are also shown in Figure S12. In the first cycle, the voltage hysteresis of the Cu foil electrode is 198.5 mV, which is about 2.3 time higher than that of carbon-nanocage-modified electrode (87.1 mV) (Figure S12(a)). After 30 cycles, the voltage hysteresis of the carbon-nanocage-modified electrode is 46.2 mV, lower than that on the Cu foil electrode (54.1 mV) (Figure S12(b)). The carbon-nanocage-modified electrode still exhibits a low voltage hysteresis of 42.6 mV after 50 cycles, while the Cu foil electrode fails, as shown in Figure S12(c). This is attributed to the excellent stability of carbon-nanocage-modified electrode, which exhibits uniform and stable Li deposition and dissolution.65 Owing to the excellent Li deposition/dissolution performance, the lithiated carbon-nanocage-modified electrode with an excessive ~12% capacity as an anode is paired with the sulfur electrode to form a Li-S battery. The ratio of the electrolyte content versus sulfur content is 20 μL/mg. The Li-S battery with the lithiated carbon-nanocagemodified electrode exhibits a higher discharge capacity and better cycling performance at 0.5C (Figure S13). In particular, it exhibits an outstanding discharge specific capacity of 746.5 mAh/g in the first cycle, 431.6 mAh/g after 100 cycles, and 315.3 mAh/g after 250 cycles. Compared with the lithiated carbon-nanocage-modified electrode, the battery with the lithiated Cu foil electrode exhibits a specific capacity of less than 300 mAh/g after only 55 cycles. This indicates that the cycling performance of the lithiated carbon-nanocage-modified electrode is much better than that of 2D lithium metal anode in the case of the limited cyclable lithium. Additionally, the Li-S battery with the lithiated carbon-nanocage-modified electrode still delivers a much better rate capability (941.7 mAh/g, 857.5 mAh/g, 699.6 mAh/g, and 576.6 mAh/g at 0.2, 0.5, 1.0, and 2.0C, respectively) than the battery with the lithiated Cu foil electrode (669.7 mAh/g, 371.1 mAh/g, 195.3 mAh/g, and 130.4 mAh/g at 0.2, 0.5, 1.0, and 2.0C, respectively), as shown in Figure S14. Moreover, the Li-S full battery with lithiated carbon-nanocage-modified electrode renders more stable charge/discharge profiles under the different charge and discharge rates (Figure S15).68

4. CONCLUSIONS In summary, we demonstrate a rational concept to encapsulate metallic lithium into hollow carbon nanocages. The hollow carbon nanocages with their thin-layer carbon walls effectively hinder parasitic reactions between metallic Li and electrolyte, yielding an excellent Coulombic efficiency and long lifespan. Furthermore, the high specific surface area and huge interior spaces of the carbon nanocages are beneficial for lowering the current density and mitigating


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volume changes, giving rise to an outstanding cycle performance at a high current density and deposition capacity. Consequently, the carbon-nanocage-modified electrode exhibited a 98.9% Coulombic efficiency for over 250 cycles and 99.7% Coulombic efficiency for nearly 150 cycles at 1.0 mA/cm2 with different area capacities of 0.5 and 1.0 mAh/cm2. These are far superior to the cycling performances obtained on the Cu foil electrode. Even at an ultrahigh current density of 10.0 mA/cm2 or practical area capacity of 3.0 mAh/cm2, this electrode still operates stably for over 60 cycles. Furthermore, the carbon-nanocage-modified electrode even exhibits a superior cycle and rate performances in a working Li-S battery.

ASSOCIATED CONTENT Supporting information available: The BET isotherm, Raman spectrogram, XPS spectrogram, SEM, TEM images of the carbon nanocages, and electrochemical characterization of the electrodes for lithium deposition/dissolution.

Conflict of interest: The authors declare no conflict of interests.

AUTHOR INFORMATION Corresponding Authors E-mail: [email protected] E-mail: [email protected]

Author Contributions B.H. and Y.Q.L. conceived and designed this work. Q.Y.D, C.H.G, Z.A.Z, and K.Z carried out parts of the electrochemical measurements. H.L.F completed the main materials characterization, and the preparation of the paper. H.L.F., B.H., and Y.Q.L. participated in the analysis and discussions of the results in this work.

ACKNOWLEDGMENTS This work was supported by the National Key R&D Program of China (2018YFB0104202).

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