New Nanoconfined Galvanic Replacement Synthesis of Hollow Sb@C

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New Nanoconfined Galvanic Replacement Synthesis of Hollow Sb@C YolkShell Spheres Constituting a Stable Anode for High-Rate Li/Na-Ion Batteries Yan Yu, Jun Liu, Litao Yu, Chao Wu, Yuren Wen, Kuibo Yin, Fu-Kuo Chiang, Renzong Hu, Jiangwen Liu, Litao Sun, Lin Gu, Joachim Maier, and Min Zhu Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b00083 • Publication Date (Web): 13 Feb 2017 Downloaded from http://pubs.acs.org on February 14, 2017

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New Nanoconfined Galvanic Replacement Synthesis of Hollow Sb@C Yolk-Shell Spheres Constituting a Stable Anode for High-Rate Li/Na-Ion Batteries †

†

§

∥

⊥

∥

Jun Liu, Litao Yu, Chao Wu, § Yuren Wen, ∥ Kuibo Yin, ⊥ Fu-Kuo Chiang, ∥ ,Ϯ § † Renzong Hu, Jiangwen Liu, Litao Sun,⊥ Lin Gu,∥, Ϯ Joachim Maier,§ Yan Yu, , ,* †

⊥

∥

§

‡

†

and Min Zhu ,*

†

School of Materials Science and Engineering and Guangdong Provincial Key

Laboratory of Advanced Energy Storage Materials, South China University of Technology, Guangzhou, 510641, China ‡

Key Laboratory of Materials for Energy Conversion, Chinese Academy of Sciences,

Department of Chemistry and Materials Science, University of Science and Technology of China, Hefei, 230026,Anhui, China §

Max Planck Institute for Solid State Research, Heisenbergstr. 1, Stuttgart, 70569,

Germany ∥

Beijing National Laboratory for Condensed Matter Physics, Institute of Physics

Chinese Academy of Sciences, Beijing 100190, China ⊥

SEU-FEI Nano-Pico Center, Key Laboratory of MEMS of Ministry of Education,

Southeast University, Nanjing 210096, China Ϯ

National Institute of Clean-and-Low-Carbon Energy, Beijing 102209, China

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ACS Paragon Plus Environment

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Abstract In the current research project, we have prepared a novel Sb@C nanosphere anode with biomimetic yolk-shell structure for Li/Na-ion batteries via a nanoconfined galvanic replacement route. The yolk-shell microstructure consists of Sb hollow yolk completely protected by a well-conductive carbon thin shell. The substantial void space in the these hollow Sb@C yolk-shell particles allows for the full volume expansion of inner Sb while maintaining the framework of the Sb@C anode and developing a stable SEI film on the outside carbon shell. As for Li-ion battery anode, they displayed a large specific capacity (634 mA h g−1), high rate capability (specific capabilities of 622, 557, 496, 439 and 384 mA h g−1 at 100, 200, 500, 1000, and 2000 mA g-1, respectively) and stable cycling performance (a specific capacity of 405 mA h g−1 after long 300 cycles at 1000 mA g-1). As for Na-ion storage, these yolk-shell Sb@C particles also maintained a reversible capacity of approximate 280 mA h g−1 at 1000 mA g-1 after 200 cycles.

Keywords: rechargeable batteries, alloying reaction, carbon-encapsulation, yolk-shell structure, antimony

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Going along with the urgent requirement on rechargeable energy storage devices for electric vehicles (EVs) or hybrid EVs, Li-ion batteries now become one type of the most attractive energy storage technologies.1-8 As a similar energy storage device, Na-ion batteries re-attracted intensive research attention recently owing to the relatively low cost and infinity of Na resources.7,8 Though the progress of these rechargeable batteries development is substantial, their energy density, rate capability and cycle performance are not in sufficient to satisfy the large market demand.1,9,10 The widely-used commercial graphite carbon anode however displays inferior rate capability and serious safety problem as arising from its low Li+ diffusion coefficient and close potential for Li+ insertion and Li deposition.1 As an alternative anode material, the alloying-type Sb has a theoretical gravimetric and volumetric capacity of approximate 660 mA h g–1 and 1750 Ah L-1 (corresponding to Li3Sb/Na3Sb), respectively, and thus increase the gravimetric and volumetric capacity by about two to three times.11-13 Furthermore, the Sb anode can provide safer Li+/Na+ insertion potentials of approximate 0.5 to 0.8 V that are still small but sufficiently distant from deposition.14-16 It is widely known that the distinct shortcoming of alloying-type metallic Sb and Sn anodes is their huge volume expansion when Li+/Na+ inserts the host.15,18-20 For Sb anode, accompanied with the transformation of Sb to Li3Sb or Na3Sb, there is approximate 150% and 390% volume increase, respectively.15,20,21 Such huge volume changes cause negative cracking of the solid electrolyte interphase (SEI) layer coating on the anode material, and eventually Li/Na exhaustion due to fresh SEI formation.

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Besides, the active material (Sb-Li/Na) could pulverize or be pushed away during charging/discharging, thus losing electrical contact from the current collector. One widely used approach for solving these problems is constructing Sb-M alloys.14,15,20 The primary beneficial role of the non-electroactive M is providing a mechanical buffer to accommodate the volume increase of Sb that otherwise would lead to disintegration. However, the introduction of non-electroactive metal M, in particular if of high molecular weight, greatly decreases its gravimetric and volumetric energy densities. A core-shell protective route has also been developed for buffering the volume change.22-26 Unfortunately, cracking of the shells still happened upon huge volume increase in these conventional core-shell structures. The recently developed yolk-shell electrodes can solve this big drawback of normal core-shell electrodes as providing sufficient void space for accommodating the volume expansion and avoiding damage SEI film.27-34 Herein, based on our recently work on Sn4P3@C yolk-shell anode,35 we have successfully developed a novel nanoconfined galvanic replacement route for a new kind of hollow yolk-shell Sb@C, i.e., carbon nanocages (shells) fully encapsulated Sb hollow yolks. The main advantage of the current hollow Sb@C yolk-shell spheres is that such microstructured electrode provides sufficient void space (the hollow core in Sb yolks and the gap between carbon shells and Sb yolks), which can perfectly accommodate the volumetric expansion during the Li-/Na-ion insertion, thus preserves the structural stability of the anode and stable SEI film. More importantly, the current nanoconfined galvanic replacement opens up a synthetic route for the production of hollow and yolk-shell particles with very

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different composition, which may also serve as nanoscale reactors in catalytic applications. The fabrication process of the hollow Sb@C yolk-shell spheres is detailed shown in Scheme 1a. Firstly, uniform hollow SnO2 nanospheres were synthesized by a facile hydrothermal route, in which the typical Ostwald ripening process results in hollow structure

(see

Experimental

Section

for

details,

Supporting

Information).

Subsequently, these uniform SnO2 hollow spheres were hydrothermally coated and annealed in H2 atmosphere, in which, SnO2 was reduced into Sn, while the carbon-rich polysaccharide derived from hydrothermal carbonization of glucose was pyrolyzed into a purely inorganic carbon shell.34 The completely hollow cores in the hollow SnO2 nanospheres provide adequate volume and give rise to a typical yolk-shell structure in the Sn@C product. In the following galvanic replacement process (just adding SbCl3 salt into the ethanol solution of yolk-shell Sn@C particles), Sn was oxidized to Sn4+ and Sb3+ simultaneously reduced to Sb, during which encapsulated solid Sn yolks were chemically transformed into hollow Sb,35,36 thus uniform hollow Sb@C yolk-shell spheres are finally achieved. As shown in Scheme 1b, during the galvanic replacement of Sn with Sb3+, the void space formation in Sb product from Sn precursor is induced by the nonequilibrium diffusion of Sn and Sb (outward diffusion of Sn and inward transport of Sb). As shown in Scheme 1c, a stable SEI layer could be well grow on the outer carbon shell, thus its continual rupturing and reforming can be ingeniously avoided. Secondly, the outer carbon layer is well electronic-conductive, allowing for superior transport kinetics and rate

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capability. More importantly, the inner void in Sb yolk and gaps between the Sb yolk and carbon shell allows electroactive Sb to swell upon alloying while preserves the electrode integrity. Low-magnification SEM image (Figure 1a) of these SnO2 precursor particles clearly shows that they have regular spherical morphology and uniform size. These monodisperse SnO2 spheres have an average diameter of about 250 nm (Figure 1b). TEM analyses were also carried out to directly reveal the hollow feature of these SnO2 nanospheres (Figure 1c). As clearly shown in this image, the core and the shell of these SnO2 particles present a distinct non-uniform contrast, indicating a hollow interior. The enlarged image, as an inset in Figure 1c is a typical hollow sphere, displaying that the thin shell consist of dense small nanoparticles. For nanoconfined galvanic replacement synthesis of the Sb@C product, the yolk-shell Sn@C precursor was easily achieved by reduction of the SnO2 hollow spheres. As clearly shown in Figure 2a, the Sn@C precursor particles derived from SnO2 hollow sphere template are still non-aggregated. Moreover, high-magnification SEM (Figure 2b) and low-magnification TEM (Figure 2c) images both exhibit that all these solid Sn yolks were fully encapsulated in carbon nanocages. Such high quality yolk-shell spheres are suitable precursors for the nanoconfined galvanic replacement. Owing to the different Θ redox potentials ( ESb 3+ Θ ( ESn 4+

Sn

Sb

= 0.241 V vs standard hydrogen electrode, SHE), Sn

= 0.0129 V vs SHE) Sn@C can be converted into Sb@C products by

exposing them to a SbCl3 solution. As shown in both SEM (Figure 2d,e, and Figure S1 in Supporting Information) and TEM (Figure 2f,g) images, it looks as if the Sb@C

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products generally inherited the size and the topology of the yolk-shell nanostructure of Sn@C. More precisely, however, the inner Sb yolks in Sb@C were converted into porous hollow spheres, much different from the Sn yolks in Sn@C. The formation mechanism of hollow Sb yolks during the nanoconfined galvanic replacement is due to interdiffusion of Sb and Sn (obviously the diffusion rate of Sn is larger than that of Sb, generating voids and eventually resulting in a typical hollow cavity).36,37 HRTEM images (Figure 2h,i) display clearly a 0.23 nm lattice spacing, corresponding to the (104) plane distance of rhombohedral Sb. Moreover, the element mapping of hollow Sb@C yolk-shell nanospheres (Figure 3) further confirms that hollow Sb yolks fully-encapsulated with thin carbon nanocages have been indeed achieved through the current spatially confined galvanic replacement. The XRD result displayed in Figure 4a further confirmed the constituents of these template, precursor and product materials. The XRD pattern (red curve) of the hollow template can be fully ascribed to the tetragonal SnO2 (JCPDS No. 41-1445), while the blue and green ones are well consistent with the pure Sn (tetragonal phase, JCPDS No. 89-2958) and pure Sb (rhombohedral phase, JCPDS No. 85-1323), respectively. Such a completely galvanic replacement can be further verified by time-dependent ex-situ XRD measurements (Figure S2, Supporting Information). These XRD patterns clearly show the gradual increase of Sb content at the expense of Sn consumption during the galvanic replacement. The weight content of the carbon in the final Sb@C nanocomposites was measured by thermogravimetric analysis (TGA, Figure 4b). This TGA curve firstly displays a weight loss of approximate 4.0 % from room

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temperature to 200 °C, which can be credited to the loss of water in the sample. The next weight gain of ~5.8% between 200 -360 °C corresponds to Sb oxidation. While the subsequent weight loss of about 7.0% is owing to the comprehensive effect of Sb oxidation and carbon oxidation with the products of Sb2O3 and CO2, respectively. As the added weight of oxygen is about 16.4% for pure Sb, the TGA result reveals the carbon content in the hollow Sb@C yolk-shell spheres be about 17.6% [16.4% + (7.0% ‒ 5.8%)]. Firstly, cyclic voltammetry (CV) and galvanostatic charging/discharging cycle tests were adopted to verify the anticipated superior electrochemical performance of these hollow Sb@C yolk-shell particles for Li-ion storage. As shown in Figure 5a, The first CV curve demonstrates a sharp cathodic peak located at 0.78 V, which could be ascribed to the lithiation of Sb forming Li3Sb. The anodic peak centered at 1.09 V is corresponding to the delithiation of Li3Sb, suggesting good reversibility of these alloying reactions. In the next three cycles, they are basically overlapped, indicating the high stability of hollow Sb@C yolk-shell particles. The electrochemical stability of these hollow Sb@C yolk-shell nanospheres was firstly tested by cycling at a current density of 50 mA g-1 (0.01–2.6 V). The first discharge and charge processes of Sb@C anode possess a large capacity of 1230 and 680 mA h g–1 (Figure 5b), respectively. Such initial large capacity loss (with a low coulombic efficiency of 55.3%), common phenomena for non-intercalation anode materials,11,12,20 could be mainly ascribed to some irreversible processes, such as formation of SEI layer, organic electrolyte decomposition, and interfacial Li storage (BET measurement

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exhibiting a much high surface area of these Sb@C spheres, Figure S3 in Supporting Information). These hollow Sb@C yolk-shell particles possess a large specific capacity of 525 mA h g-1 after 100 cycles (98% Coulombic efficiency). Charging/discharging tests of these hollow Sb@C yolk-shell particles at a serious of rates (firstly increased from 50 to 2000 mA g-1 and again decreased to 50 mA g-1) clearly show their superior rate capability (Figure 5c,d). Even, at a high rate of 2000 mA g-1, the anode of Sb@C could still have a large specific capacity of about 385 mA h g−1 (Figure 5c). They delivered a specific capacities of 623, 558, 496, 439, 385 and 590.5 mA h g-1 at 100, 200, 500, 1000, 2000 and returned back to 50 mA g-1, respectively, and these specific capacities remained as high steady values during the rate testing (Figure 5d). Such rate capability behaviors of the current hollow Sb@C yolk-shell nanospheres is superior to most of recently reported Sb-based electrodes for Li-ion storage (Table S1, Supporting Information). The long cycling of Sb@C at 1000 mA g-1 is demonstrated in Figure 5f, which shows a stable cyclability over the whole period. Specially, a large value of 405 mA h g-1 is still remained after 300 cycles, indicating the superior electrochemical stability. Such structure stability of these Sb@C yolk-shell particles is directly supported by SEM measurement (Figure S4 in Supporting Information), which demonstrates the yolk-shell spherical shape and integrated framework are well maintained after 100 cycles. Specially, the characteristically cracked Sb@C sphere shows that the gap between the yolk and shell was filled due to volume expansion of inner Sb (Figure S4b in Supporting Information). This SEM result indicates that the void in each Sb@C particle can

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provide substantial space for the huge volume expansion of Sb lithiation. For understanding the diffusion process of Li-ion and electrons in Sb@C anode, CV curves of Sb@C anode were measured with a series of scan rates (from 0.1 mV s-1 to 2 mV s-1). The anodic peak currents Ip (Amperes) at different scan rates as shown in Figure 5e were adopted to calculate the Li-ion diffusion coefficient D (cm2 s-1) of these hollow Sb@C yolk-shell particles according to the following Randles Sevcik equation:38,39 Ip=2.69 ×105ACD1/2n3/2v1/2

(1)

where A stands for the anode area (cm2), C for the shuttle concentration (mol cm-3), n for the involved electron numbers in the redox action, and v for the scan rate (V s-1). With these data, it can be got a fitting straight-line (the inset of Figure 5e) with v1/2 as x-axis and Ip as y-axis. The calculated slope of this line is about 0.054, and the measured Li-ion diffusion coefficient of the current Sb@C yolk-shell spheres thus is approximate 1.2×10-9 cm2 s-1. Such a comparatively high Li-ion diffusion coefficient is naturally beneficial for the electrode performance. As Sb is also a well known alloying-based material for Na-ion storage,40,41 the Na-ion storage performances of these hollow Sb@C yolk-shell spheres were investigated in detail, too. The CV curves of Sb@C for Na-ion batteries are displayed in Figure S5 (see Supporting Information for details). The first cathodic peaks centered at 0.75 and 0.27 V could be attributed to the sodiation of Sb (forming Na3Sb) and the formation of SEI layer, respectively, while the corresponding anodic peak occurred at 0.85 V indicates the desodiation reactions of Na3Sb. In the subsequent

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three cycles, these CV curves are nearly overlapped, indicating the good stability of hollow Sb@C yolk-shell nanospheres. Figure 6a exhibits the capacity-voltage curves of Sb@C at different rates (firstly increased from 50 to 2000 mA g-1 and again decreased to 50 mA g-1). At low and high current densities (Figure 6b,c), these hollow Sb@C yolk-shell hollow nanospheres display a relatively stable cycling process. A similar low Coulombic efficiency of the first cycle (60.3% for Na-ion vs. 55.3% for Li-ion) is still observed, mainly ascribed to the SEI layer formation and electrolyte decomposition. Even though, such Sb@C anode delivers large specific capacities of 600, 548, 463, 408, 329, 279 and 463.4 mA h g-1 at 50, 100, 200, 500, 1000, 2000 and back returned to 50 mA g-1, respectively. Figure 6d exhibites the long-cycle performance of these uniform Sb@C yolk-shell spheres, demonstrating superior long-term cyclability, with about 280 mA h g-1 capacity at a high rate of 1000 mA g-1. The little lower specific capacity when compared to a Li-ion cell (280 mA h g-1 vs. 405 mA h g-1) may be related to the slower diffusion rate and a smaller theoretical capacity due to the large radius of Na+. For verifying this conclusion, the kinetic analysis of the Sb@C anode was also carried out with CV testing of Na//Sb@C at different sweep rates (Figure 6e). The obtained Na-ion diffusion coefficient according to the relationship between the peak currents and sweep rate (Figure 6f) is little smaller than that of Li-ion (3.0×10-10 cm2 s-1 for Na-ion vs. 1.2×10-9 cm2 s-1 for Li-ion). Even though the Na-ion storage performance of these hollow Sb@C yolk-shell nanospheres is not as good as that of Li-ion storage, it still outperforms most of Sb-based anodes for Na-ion batteries (Table S2, Supporting Information).

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For directly confirming the main advantages of the current hollow yolk-shell structure of Sb@C anode, we used in-situ TEM technique, which can powerfully observe the structural changes during alloying/dealloying processes, and hence gain insight into the volume change process.31-33,42 The in-situ electrochemical experimental setup contains Sb@C anode, Li counter electrode and solid electrolyte Li2O. Appling a constant potential of 3 V (respect to Li counter electrode) on the hollow Sb@C yolk-shell nanosphere initiates the lithiation process. As clearly shown in Figure 7a and b, as the current hollow yolk-shell construction can provide substantial internal space for accommodating the volume increase, the typical Sb@C sphere underwent little swelling, with the outer diameter having increased from 224 nm to 245 nm (9.4% diameter expansion and 30.8% for volume expansion). To further explore the microstructure change of the inner Sb hollow yolks during lithiation and delithiation, we aimed at a closer view of the microstructure evolution of a bigger hollow Sb@C yolk-shell sphere (Figure 7c-e, and Movie S1,2, Supporting Information). As clearly shown in these TEM images (Figure 7c,d, and Movie S1, Supporting Information), the tiny Sb nanoparticles assembled in the inner hollow yolk were coarsened and the gap between the yolk and shell was fully filled after lithiation. Despite the obvious size expansion of Sb nanoparticle units, there were no crackings or fractures happened in the whole Sb@C yolk-shell sphere during the alloying process. Such electrode integrity during lithiation process is much beneficial for the electrical contact. During the delithiation process, the inner Sb hollow yolk shrank a little connected with the formation of a gap as a result of the Li+ extraction. Notice

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that the size of inner Sb nanoparticle units was gradually decreased, greatly similar to the pristine stage (Figure 7c,e, Movie S2, Supporting Information). Both SEM and in-situ TEM measurements indicate that the framework of the Sb@C particles could keep integrated upon lithiation, and that the anode made of the current hollow and yolk-shell structures undergo only marginal microstructural damage upon cycling. In summary, we have established an innovative and effective nanoconfined galvanic replacement approach to encapsulate electroactive Sb in carbon nanocages. Such materials show attractive Li+/Na+ storage performances. The internal void space in these hollow yolk-shell spheres allows for the full expansion of Sb nanoparticle units, thus preserving the structural integrity of Sb@C and stable SEI film. The assembled Li-ion batteries using Sb@C as anode exhibited a long cycling life and an admirable rate capability (reversible capabilities of 623, 558, 496, 439 and 385 mA h g-1 at 100, 200, 500, 1000, 2000 mA g-1, respectively). A large specific capacity of 405 mA h g-1 and a high Coulombic efficiency of 99.1% after 300 cycles at 1000 mA g-1 were achieved. Such hollow Sb@C yolk-shell spheres also showed a stable cycling performance and superior capability for Na-ion storage. The current nanoconfined galvanic replacement approach may also be effective for constructing yolk-shell catalytic particles with large surface area, in which the inert shells can kinetically protect the encapsulated catalytic yolks from aggregation and deactivation, while reactants and products can still diffuse in and out through the outer shells to reach the inner catalytic yolks.

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Supporting Information Available: Movie S1 and Movie S2 displaying lithiation and delithiation of Sb@C anode; experimental procedures; additional TEM and SEM images and XRD patterns of Sb@C anode; BET results of Sb@C particles; additional electrochemical performances of Sb@C and Sn@C anodes; and the electrochemical performance comparison of the current Sb@C anode with previously reported Sb-based anodes. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Authors *E-mail: [email protected] *E-mail: [email protected]

Acknowledgements We acknowledge the financial support by the Innovative Research Groups of the National Natural Science Foundation of China (No. NSFC51621001), National Key Research program of China (No. 2016YFB0100305), National Natural Science Foundation of China Projects (No. 51231003, No. 51622210, No.21373195), and the Sofia Kovalevskaja award of the Alexander von Humboldt Foundation ， the Fundamental Research Funds for the Central Universities (WK3430000004), and the Collaborative Innovation Center of Suzhou Nano Science and Technology.

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Scheme 1. Detailed formation process and advantages of hollow Sb@C yolk-shell spheres. a), Schematic evolution of hollow Sb@C yolk-shell nanospheres for Li/Na-ion batteries. b), Schematic hollowing mechanism of the inner Sb yolks based on Kirkendall effect during the nanoconfined galvanic replacement. c), Schematic structure transformation of hollow Sb@C yolk-shell nanospheres during the lithiation/sodiation process.

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Figure 1. SEM and TEM images of SnO2 template. a,b), Different-magnification SEM images of these SnO2 nanospheres showing their uniform diameter. c), Different-magnification TEM images of these SnO2 nanospheres showing their hollow feature.

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Figure 2. SEM and TEM measurements of Sn@C precursor and Sb@C product. a-c), Different-magnification SEM (a,b) and low-magnification TEM (c) images of Sn@C precursor. d-g), Different-magnification SEM (d,e) and TEM (f,g) images of the finally achieved Sb@C product. h,i), HRTEM images of Sb nanoparticles constituting of the encapsulated Sb yolks, which clearly display lattice planes of Sb, the inset of Figure 2i shows the corresponding FFT pattern. 21


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Figure 3. Elemental distribution of hollow Sb@C yolk-shell particles. a), Low-magnification SEM image (in mode of SE2) of hollow Sb@C yolk-shell nanospheres clearly showing the inner Sb hollow cores. b-d), Dark field TEM (b) and 22


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EFTEM element mapping images (c-e) of hollow Sb@C yolk-shell spheres.

Figure 4. Composition characterizations of hollow Sb@C yolk-shell particles. a), XRD patterns of SnO2 template (black), Sn@C precursor (blue), and the as-obtained obtained Sb@C product (green) clearly show that the initial used SnO2 was firstly converted into Sn, and finally Sb via the nanoconfined galvanic replacement route. b), TGA curve of yolk-shell Sb@C nanospheres under O2 atmosphere from room temperature to 700 °C. 23


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Figure 5. Superior Li-ion storage performances of hollow Sb@C yolk-shell spheres. a), The first four CV curves of Sb@C (0.1 mV s−1). b), Cycling performances of Sb@C (50 mA g-1). c), Charging/charging curves at various rates (firstly increased from 50 to 2000 mA g-1 and again decreased to 50 mA g-1). d), Rate capability at various rates. e), CV curves at different sweep rates and the corresponding relationship between peak currents and sweep rates (the inset). f), Long cycling performances of hollow Sb@C yolk-shell anode at 1000 mA g-1.

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Figure 6. Superior Na-ion storage performances of hollow Sb@C yolk-shell spheres. a), Charging/charging curves at various rates (firstly increased from 50 to 2000 mA g-1 and again to 50 mA g-1). b), Cycling performances of Sb@C (50 mA g-1). c), Rate capability at rarious rates. d), Long cycling stability of hollow Sb@C yolk-shell particles at 1000 mA g-1. e,f), CV curves at different sweep rates (e) and the corresponding relationship between peak currents and sweep rates (f).

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Figure 7. In-situ TEM observation of hollow Sb@C yolk-shell particle lithiation/delithiation. a), TEM images of the lithiation of carbon-encapsulated hollow Sb particles showing the little diameter increase of sphere (from 224 nm to 245 nm). b), the corresponding EELS spectrum of Sb@C particles before and after lithiation. c-e), Close view of the microstructure evolution of one typical Sb@C sphere for the stage of pristine (c), the first lithiation (d) and delithiation (e).

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New Nanoconfined Galvanic Replacement Synthesis of Hollow Sb@C

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