Galvanic Replacement Synthesis of Highly Uniform Sb Nanotubes

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Galvanic Replacement Synthesis of Highly Uniform Sb Nanotubes: Reaction Mechanism and Enhanced Sodium Storage Performance Yan Liu, Bin Zhou, Sheng Liu, Qingshan Ma, and Wen-Hua Zhang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.9b01660 • Publication Date (Web): 07 May 2019 Downloaded from http://pubs.acs.org on May 7, 2019

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Galvanic Replacement Synthesis of Highly Uniform Sb Nanotubes: Reaction Mechanism and Enhanced Sodium Storage Performance

Yan Liu, Bin Zhou*, Sheng Liu, Qingshan Ma, and Wen-Hua Zhang*

Sichuan Research Center of New Materials, Institute of Chemical Materials, China Academy of Engineering Physics, Chengdu 610200 (China) *Corresponding author: [email protected]; [email protected]

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ABSTRACT: One-dimensional (1D) nanotubes are very useful for achieving excellent performance in lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs) due to the tubular structures can effectively alleviate the structural strain and shorten the ion diffusion length during repeated cycling. In this work,

we report

a Cu 2 Sb-mediated growth strategy to controllably fabricate highly uniform Sb nanotubes (NTs), as well as Cu@Cu 2 Sb and Cu 2 Sb@Sb composite NTs, via a facile galvanic

replacement

reaction

using

Cu

nanowires

(NWs)

as

sacrificial

templates. Benefiting from their structural merits, the Sb NTs manifest excellent sodium storage performance with superior rate performance (286 mAh g-1 at 10 A g-1) and extraordinary cycling stability (342 mAh g-1 after 6000 cycles at 1 A g-1). Furthermore, a full cell with Sb NTs as anode and Na 3 (VOPO 4 ) 2 F as cathode exhibits a high energy density (252 Wh kg-1) and high output voltage (2.7 V), revealing their significant application promise in the next-generation SIBs.

KEYWORDS :

antimony, nanotubes, galvanic replacement reaction, anode

materials, sodium-ion batteries, Cu 2 Sb

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Sodium-ion batteries (SIBs) have attracted a great deal of attention as a promising substitute for lithium-ion batteries (LIBs) for large-scale energy storage applications due to their low cost, natural abundance, and relatively low electrochemical redox potential.1-3 However, the significantly larger ionic radius of Na+ than Li+ (0.102 nm vs. 0.076 nm) leads to slower Na+ insertion and extraction, larger volumetric changes, and a less stable solid electrolyte interphase (SEI), which means that anode materials that are promising for LIBs are inadaptable for SIBs.4-8 Therefore, appropriate anode materials that can effectively host large Na+ ions are needed to produce high-performance SIBs. To this end, a great deal of effort has been devoted to exploring anode materials with high rate capability and long cycling stability; examples include alloy-type materials (e.g., Sn, P, Sb, and Bi),9-19 conversion-type metal oxides and metal sulfides,20-24 intercalation compounds,25-30 and carbonaceous materials.31, 32 Among these candidates, Sb shows great promise owing to its high theoretical capacity (660 mAh g-1 for Na 3 Sb), good electronic conductivity, and appropriate operating voltage (0.5-0.8 V vs. Na/Na+).33, 34 However, Sb-based anodes suffer from severe pulverization caused by drastic volume expansion of Sb (approximately 290%) during sodiation/desodiation processes, resulting in fast capacity fading.35 Forming a one-dimensional (1D) tubular structure is an effective way to alleviate the volume expansion problems of alloy-type anodes (such as Si and Ge) in LIBs.36-40 Theoretically, this strategy should also be valid for Sb-based anodes in SIBs. Recently, a variety of Sb-based tubular composites (such as double-walled Sb@TiO 2-x NTs,41 Sb@C coaxial NTs5 and peapod-like Sb@N-C NTs42) were tested 3 ACS Paragon Plus Environment

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in SIBs and showed an improved rate capacity and long-term cycling performance. However, compared to purely metallic Sb, the introduction of carbon or TiO 2-x will lower the specific capacity of the composite anodes. Therefore, purely metallic Sb NTs would be a better choice for sodium storage.

To date, very few successful preparations of Sb NTs have been reported. Qian et al. and Wang et al. synthesized Sb NTs with nonuniform morphologies or irregularly shaped segments through a hydrothermal/solvothermal reduction method.43-46 Li et al. fabricated single-crystalline Sb NT arrays in an anodic alumina (AAO) membrane using a pulsed electrodeposition technique.47 However, these Sb NTs have not been tested in LIBs or SIBs, so their electrochemical performances in energy storage are unknown.

Herein, we report a strategy for the synthesis of uniform Sb NTs through a facile galvanic replacement route using Cu NWs as sacrificial templates and further explore their potential as anodes for SIBs. Through electron paramagnetic resonance (EPR), UV-vis, transmission electron microscopy (TEM), high-resolution TEM (HRTEM) and ex situ X-ray diffraction (XRD) analyses, we identified Cu 2 Sb as an intermediate in the replacement reaction; thus, a Cu 2 Sb-mediated mechanism was proposed for the growth of the Sb-based NTs, which is distinct from previous reports. Based on these findings, Cu@Cu 2 Sb and Cu 2 Sb@Sb NTs were prepared by precisely controlling the reaction time. When evaluated as anode materials for SIBs, all of these NTs exhibited superior long-term cycling stability, and the Sb NTs demonstrated the highest specific 4 ACS Paragon Plus Environment

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capacity and excellent rate capability. The enhanced electrochemical performance of Sb NTs can be ascribed to the combination of a 1D tubular structure and a thin wall, which can not only accommodate structural strain during repeated cycling but also shorten the Na+ diffusion length.

RESULTS AND DISCUSSION

Figure 1. a) Schematic illustration of the synthesis of Sb NTs. b) TEM image of Cu NWs. c) XRD pattern, d,e) TEM images, f,g) SEM images, and h) HRTEM image of Sb NTs. i) Elemental mapping and EDS spectrum of Sb NTs.

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The synthesis of Sb NTs is briefly illustrated in Figure 1a. Highly uniform Cu NWs with an average diameter of 42 nm (Figure 1b) were first synthesized by a hydrothermal method and then used as structural templates and reducing agents for the growth of Sb NTs. The Cu NWs were mixed with a 0.01 M SbCl 3 DMSO solution and transformed into tubular structures after a 24 h reaction at room temperature. The crystalline structure of the tubular samples was examined by powder XRD, and all of the diffraction peaks could be well indexed to rhombohedral Sb (JSPDS NO. 85-1324), implying the full conversion of Cu NWs into Sb NTs (Figure 1c). TEM, scanning electron microscopy (SEM), and HRTEM were then employed to analyse the morphology and microstructure of the Sb NTs. A narrow size distribution with an average diameter of 49.7 nm and an average wall thickness of 9.7 nm was realized for the Sb NTs (Figure S1). The hollow interior of the Sb NTs was clearly confirmed by the noticeable contrast between the centre and the edge of the samples in the TEM images (Figures 1d-e and S2) and disclosed by the open ends in the SEM images (Figure 1g). The HRTEM image (Figures 1h and S3) shows clear lattice fringes with an estimated d-spacing of 0.31 nm, corresponding to the (012) plane of Sb. Detailed elemental mapping and energy dispersive X-ray spectroscopy (EDS) (Figure 1i) revealed the complete conversion of Cu NWs to Sb NTs. In addition, the obtained Sb NTs showed good stability under ambient conditions, as evidenced by the unchanged XRD patterns, Raman spectra and X-ray photoelectron spectroscopy (XPS) spectra of the sample stored in atmospheric conditions for 3 weeks (Figure S4). Taken together, these results confirm the formation of uniform crystalline Sb NTs. 6 ACS Paragon Plus Environment

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In previous reports, two different reaction mechanisms were proposed for the galvanic replacement reaction between bulk Cu and Sb3+,48, 49 as shown below:

2Sb3+ + 3Cu = 2Sb + 3Cu2+

(1)

Sb3+ + 5Cu = Cu 2 Sb + 3Cu+

(2)

The main points of debate are whether (i) the Cu is oxidized to Cu+ or Cu2+ and (ii) the main reaction product is Sb or Cu 2 Sb. To answer the first question, UV-vis absorption spectra were collected, and EPR measurements were performed. As shown in Figure 2a, the UV-vis absorption spectra of CuCl-DMSO and CuCl 2 -DMSO solutions were quite different from each other. The absorption spectra of the supernatant collected after the replacement reaction were in good agreement with that of the CuCl-DMSO solution, indicating that the Cu is oxidized to Cu+ during the replacement reaction. Additionally, the EPR responses of the above three solutions were acquired (Figure 2b). The broad EPR signal (blue line) centred at 3250 G could be assigned to Cu2+ and originated from the unpaired electron in Cu2+ (the electron configuration of Cu2+ is [Ar] 3d9).50 In contrast, no signature was found in the range of 2250-4250 G for the CuCl-DMSO solution (black line) or the supernatant (red line), providing further evidence that Cu+ is the only oxidation product of the replacement reaction, thus, reaction (1) did not occur here.

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Figure 2. a) UV-vis spectra and b) EPR spectra of 0.01 M CuCl-DMSO (black), 0.01 M CuCl 2 -DMSO (blue) and the supernatant collected after the galvanic replacement reaction (red). c) Ex situ XRD patterns of aliquots obtained at different reaction times with a Cu/Sb ratio of 1/2. d) XRD patterns of the samples synthesized with different Cu/Sb ratios.

To gain further insight into the reaction process, aliquots were extracted at different times during the replacement reaction and analysed by ex situ XRD, TEM and HRTEM. The XRD results in Figure 2c showed that Cu 2 Sb was the only product of the replacement reaction at the initial stage. When the reaction proceeded for 15 8 ACS Paragon Plus Environment

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min, the Cu NWs were completely transformed into Cu 2 Sb. At the same time, the typical signal of Sb emerged, implying that a part of Cu 2 Sb was converted to Sb. At 30 min, only Sb was observed in the XRD patterns, suggesting the full conversion of Cu 2 Sb to Sb. These results strongly demonstrate that Cu 2 Sb was a stable intermediate during the replacement reaction between Cu and SbCl 3 . These aliquots were then analysed by TEM to capture the morphological evolution. As shown in Figure S5, some voids appeared inside the NW after 5 min of reaction. When the reaction proceeded for 10 min, the interior of the NW became hollow, resulting in a tubular structure. The formation of tubular structures is due to the faster outward diffusion of Cu species than the inward diffusion of Sb, namely, the nanoscale Kirkendall effect, which is commonly observed in the galvanic replacement reaction.38, 51, 52 As the reaction further proceeded, the diameters of the NTs increased slightly, and the walls of the NTs became thinner. The morphology of the NTs no longer changed until the reaction was prolonged to 30 min. HRTEM was performed to analyse the microstructure of these samples. As shown in Figures S6 and S7, the sample obtained at 10 min consisted of Cu and Cu 2 Sb (denoted Cu@Cu 2 Sb NTs), and the sample obtained at 15 min consisted of Cu 2 Sb and Sb (denoted Cu 2 Sb@Sb NTs). These results are in good accordance with the XRD patterns (Figure 2c). Additionally, the molar ratio of Sb/Cu also had a great effect on the replacement reaction. As shown in Figure 2d, excess Cu NWs (Sb/Cu≤0.5) in the reaction led to the formation of Cu 2 Sb, while excess SbCl 3 (Sb/Cu≥1) resulted in the formation of Sb. When the molar ratio of Sb/Cu was 0.6-0.9, a composite of Cu 2 Sb and Sb could be observed. 9 ACS Paragon Plus Environment

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According to the above results, the replacement reaction between Cu NWs and the SbCl 3 DMSO solution in the present experiment could be expressed as follows:

Sb3+ + 5Cu = Cu 2 Sb + 3Cu+

(3a)

3Cu 2 Sb + 2Sb3+ = 5Sb + 6Cu+

(3b)

The conversion of Cu NWs to Sb NTs involves a two-step reaction process [Equation (3a–b)], where Cu 2 Sb acts as an intermediate. The plausibility of Equation 3b was confirmed by the successful conversion of a pre-synthesized Cu@Cu 2 Sb sample to Sb by the addition of excess SbCl 3 -DMSO solution (Figure S8).

The as-obtained Sb, Cu 2 Sb@Sb and Cu@Cu 2 Sb NTs were evaluated as anode materials for SIBs. Figure 3a demonstrates the 1st, 2nd, 5th, 10th, 25th and 50th cyclic voltammetry (CV) curves of Sb NTs obtained at a scan rate of 0.1 mV s-1. In the first cathodic scan, two broad bands at approximately 1.2 V and 0.2 V are observed. The former is probably associated with the formation of SEI film and the reduction of Sb 2 O 3 in which the Sb 2 O 3 comes from the surface oxidation of Sb NTs (Figure S4),6, 53

and the latter can be ascribed to the conversion of crystalline Sb to Na x Sb alloy

compounds.54 In the subsequent cathodic scans, three apparent reduction peaks are observed at 0.69, 0.56, and 0.42 V, which could be attributed to a sequence of sodiation reactions proposed by Grey et al..55 In the reverse anodic scan, three oxidation peaks at 0.75, 0.82 and 0.92 V are observed, corresponding to the multi-step sodium extraction reaction.5, 55 Notably, all CV curves (even for the 50th cycle) nearly overlap except the first cycle, demonstrating the highly reversible nature of the Sb NT 10 ACS Paragon Plus Environment

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electrode. To gain further insight into the sodiation/desodiaion processes of the Sb NT electrode, in situ Raman studies were performed. As shown in Figure S9, the Sb NT electrode undergoes a reversible reaction between Sb and Na 3 Sb during cycling, and the crystalline Sb transformed into amorphous phase during the first CV cycle. The formation of amorphous Sb during cycling was in accordance with the previous in situ XRD studies conducted by Monconduit et al.56

Figure 3. a) CV curves of Sb NT electrode recorded at a scan rate of 0.1 mV s-1. b) Charge/discharge curves of the Sb NT electrode during the 1st, 2nd, 5th,10th, 25th, and 50th cycles at a current density of 0.1 A g-1. c) Rate capability and d,e) cycling performance of the electrodes based on Sb, Cu 2 Sb@Sb and Cu@Cu 2 Sb NTs. 11 ACS Paragon Plus Environment

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Figure 3b displays the galvanostatic discharge/charge profiles of Sb NT electrodes at a current density of 0.1 A g-1 with a cut-off window of 0.01-2.00 V. Obvious plateaus (in the voltage range of 0.5-0.8 V) are observed in the profiles, which is in good agreement with the CV curves. Interestingly, there are two distinct plateaus below 0.5 V in the discharge curves, and the length of the higher plateau increases with the cycle number, while the length of lower plateau decreases with the cycle number (Figure S10). To verify whether this phenomenon is peculiar to Sb NTs, we also investigated the sodium storage performance of the commercial bulk Sb, and we found that the discharge curves of the bulk Sb exhibit a similar phenomenon to that of Sb NTs (Figure S11). Moreover, this phenomenon has also been observed in previous studies using Sb nanoparticles as anodes for SIBs.10, 57 These results imply that this is an intrinsic phenomenon in metallic Sb based SIBs although there is no explicit explanation on this yet. The initial discharge and charge capacities are 779 and 553 mAh g-1, respectively, implying an initial coulombic efficiency (CE) of 71%. The initial capacity loss could be mainly ascribed to the formation of a SEI layer.54 Figure 3c shows the rate performance of Sb, Cu 2 Sb@Sb and Cu@Cu 2 Sb NT electrodes at varying current densities from 0.1 to 10 A g-1. Among them, Sb NTs exhibited higher reversible capacities than the Cu 2 Sb@Sb and Cu@Cu 2 Sb electrodes under all current densities. The much lower capacities of Cu 2 Sb@Sb and Cu@Cu 2 Sb may be ascribed to their low Sb contents (Table S1). Impressively, the Sb NTs could maintain a reversible sodium storage capacity of 286 mAh g-1 with a mass loading of 1 mg cm2, even at a high rate of 10 A g-1. However, when further increasing the mass loading to 12 ACS Paragon Plus Environment

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above 3 mg cm2, the specific capacity of Sb NT electrode decreased obviously at high rates, due to the thick and compact films which is unfavourable for Na+ diffusion (Figure S12 and S13). Figure 3d demonstrates the cycling performance of the Sb, Cu 2 Sb@Sb and Cu@Cu 2 Sb electrodes at a current density of 0.1 A g-1. The Sb NTs exhibited a large specific capacity of 546 mAh g-1 after 100 cycles (capacity retention = 98.7%), which was much higher than that of Cu 2 Sb@Sb (325 mAh g-1) and Cu@Cu 2 Sb (106 mAh g-1). Moreover, the Sb NTs also showed excellent long-term cycling stability when cycled at a current density of 1.0 A g-1. As shown in Figure 3e, even after 6000 cycles, the capacity of the Sb NTs could be maintained at 342 mAh g-1 (capacity retention = 74%), indicated excellent electrochemical stability. The structural stability of the Sb NTs was confirmed by post-mortem SEM and TEM characterizations, which showed that the tubular structure of Sb NTs can be maintained for over 500 cycles (Figures S14 and S15). Table S2 lists some of the best-performing Sb-based electrodes, and the electrochemical performance of the Sb NTs is among the top values reported for Sb-based SIBs.

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Figure 4. a) Schematic illustration of the Na 3 (VOPO 4 ) 2 F//Sb NTs full cell. b) Galvanostatic charge/discharge profiles, c) cycling performance, and d) Ragone plot of Na 3 (VOPO 4 ) 2 F//Sb NTs full cells, where the specific capacity, energy density and power density are calculated based on the total masses of Na 3 (VOPO 4 ) 2 F and Sb NTs. The mass ratio of cathode to anode is 5:1. The inset of (c) is the photograph of 10 light-emitting diodes lit by a Na 3 (VOPO 4 ) 2 F//Sb NTs full cell, and the inset of (d) is the average discharge voltage at different rates of the full cells.

Encouraged by the excellent sodium-storage performance of Sb NTs in half cells, we then further investigated their electrochemical performance in the full cells with Na 3 (VOPO 4 ) 2 F nanoparticls as the cathode material (Figure 4a). The Na 3 (VOPO 4 ) 2 F electrode delivers a reversible capacity of 119 mAh g-1 at 120 mA g-1 in half cells (Figure S16 and S17). Figure 4b shows the charge/discharge curves of the

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Na 3 (VOPO 4 ) 2 F//Sb NTs full cells, and an average discharge voltage of 2.7 V was obtained, which is high enough for practical application. Moreover, the full cell exhibits good cycling stability. Even after 50 cycles, it delivers a reversible capacity of 87.2 mAh g-1 (capacity retention = 94% ) at 20 mA g-1 based on the total mass of cathode and anode (Figure 4c). As shown in the Ragone plot (Figure 4d), the full cells exhibit a high energy density of 252 Wh kg-1 (based on the total mass of cathode and anode), which is among the best results reported in literatures (Table S3). Considering the battery assembly procedures are still needed to be further optimized, the electrochemical performance obtained

in the present experiment is quite prominent.

Thus a great application promise could be expected for the Sb NTs as anode materials for SIBs.

The superior electrochemical performance of the Sb NTs can be attributed to the following reasons: (a) The hollow structure of the Sb NTs can effectively alleviate the structural strain during continuous Na+ insertion and extraction, thus leading to excellent long-term cycling stability. (b) The tubular structure allows for quick ion access along the radial direction. (c) The thin wall ensures short diffusion paths for Na ions during sodiation/desodiation. (d) Sb NTs have direct 1D electronic pathways allowing for efficient charge transport. (e) The metallic nature of the Sb NTs ensures high conductivity. CONCLUSIONS In summary, highly uniform metallic Sb NTs have been successfully fabricated, via a Cu 2 Sb-mediated formation mechanism, by a facile galvanic replacement 15 ACS Paragon Plus Environment

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approach. Cu@Cu 2 Sb and Cu 2 Sb@Sb NTs can also be controllably produced by precisely controlling the reaction time. When applied as anodes for SIBs, the Sb NTs delivered a high capacity of 286 mAh g-1 even at a high current density of 10 A g-1 and an ultrastable capacity retention of 342 mAh g-1 over 6000 cycles at 1 A g-1. Moreover, by coupling with a Na 3 (VOPO 4 ) 2 F cathode, the as-obtained Na-ion full battery delivered a high energy density (252 Wh kg-1) and output voltage (2.7 V), making it very attractive for practical application. The excellent sodium storage performance can be attributed to the 1D hollow structure with a thin wall that can effectively alleviate the structural strain and accelerate the ion transport. The successful synthesis of uniform Sb NTs and their promising electrochemical performance provide opportunities to achieve a broad spectrum of alloy-type metal NTs (such as Bi and Ge) for high-performance anode materials.

EXPERIMENTAL SECTION Chemicals: SbCl 3 (ultradry, 99.999%) was purchased from Alfa Aesar. CuCl 2 ·2H 2 O (99.99%), glucose (99.5%), hexadecylamine (90%), dimethyl sulfoxide (DMSO, 99.9%), NaF (99.99%), vanadyl acetylacetonate (VO(C 5 H 7 O 2 ) 2 , 98%) and Sb

powder

(99.5%)

were

purchased

from

Aladdin.

Tetrabutylammonium

tetrafluoroborate (TBABF 4 , 98%) was obtained from J&K Chemicals. Sodium carboxymethylcellulose (CMC) and carbon black were obtained from DoDoChem.

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All chemicals were used as received without further purification. The DMSO was dried for several days over freshly activated molecular sieves (type 3 Å) before use.

Synthesis of Cu NWs: Uniform Cu NWs were synthesized following a previously reported method.58 In a typical synthesis, glucose (90 mg, 0.5 mmol), CuCl 2 ·2H 2 O (52 mg, 0.3 mmol) and hexadecylamine (460 mg, 1.9 mmol) were dissolved in 20 mL of distilled water under vigorous stirring for 6 h to form a light blue suspension. Then, the above solution was transferred into a 50 mL Teflon-lined stainless steel autoclave and heated at 140°C for 6 h. After cooling to room temperature, the products were collected by centrifugation at 10000 rpm for 5 min. The obtained Cu NWs were washed with hot distilled water (60°C), ethanol and hexane several times to remove excess hexadecylamine and glucose. Afterwards, the Cu NWs were dried at 50°C under vacuum for 6 h and then transferred into an Ar-filled glovebox for further application.

Synthesis of Sb NTs: Cu NWs (64 mg, 1 mmol) were dispersed into 12.8 mL of DMSO with sonication for 1 h in a sealed vial to form a dark red suspension. SbCl 3 (456 mg, 2 mmol) and TBABF 4 (6.59 g, 20 mmol) were successively added into a bottle inside an Ar-filled glovebox, and 200 mL of DMSO was poured into the bottle under stirring. After the complete dissolution of the salts, the Cu NW suspension was added into the above solution to trigger the galvanic replacement reaction. The mixed suspension was maintained at room temperature for 48 h with gentle stirring. The obtained grey suspension was collected by centrifugation at 8000 rpm for 5 min and 17 ACS Paragon Plus Environment

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washed with ethanol three times. Finally, the product was dried at 50°C under vacuum for 6 h. The yield of the replacement reaction is 91%.

To monitor the galvanic replacement reaction between Cu NWs and SbCl 3 , 5 mL aliquots were taken from the reaction mixture after 5, 10, 15, 20, 30, and 40 min of reaction. The obtained aliquots were collected by centrifugation and washed with ethanol three times before further characterization.

To investigate the influence of the Sb/Cu molar ratio on the replacement reaction, 1 mmol of Cu NWs was mixed with 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, and 2.0 mmol of SbCl 3 , separately, in a DMSO solution, while the other reaction parameters remained the same as those for the Sb NT preparation.

Synthesis of Na 3 (VOPO 4 ) 2 F nanoparticles: Na 3 (VOPO 4 ) 2 F nanoparticles were synthesized following a previously reported method.59 In a typical synthesis, VO(C 5 H 7 O 2 ) 2 (1.33 g, 5mmol) was dissolved in 15 mL of ethanol and 5 mL of acetone. 520 µL of H 3 PO 4 (85 wt% in water) and NaF (357.5 mg, 8.35 mmol) were successively added into the above solution. Aſter sonication at room temperature for 10 min, the obtained solution was transferred into a 50 mL Teflon-lined stainless steel autoclave and heated at 120°C for 10 h. The obtained suspension was collected by centrifugation and washed with ethanol and water three times. Finally, the product was dried at 60°C under vacuum for 6 h.

Characterization: Powder XRD was performed on a D/MAX RINT-2000 X-ray diffractometer (Rigaku). The morphologies of the samples were characterized using a 18 ACS Paragon Plus Environment

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JSM-7500F scanning electron microscope (JEOL), an HT7700 transmission electron microscope (HITACHI), and a Libra 200FE high-resolution transmission electron microscope (Zeiss). The EPR experiment was performed by a Bruker EMX plus X-band EPR spectrometer with 2.0 mW microwave power and 9.8 GHz microwave frequency. The UV-vis absorption spectra were recorded on an Evolution 201 spectrophotometer (Thermo Fisher Scientific). The contents of Sb in different samples were determined by inductively coupled plasma mass spectrometer on a VG PQ ExCell instrument (Thermo Jarrell Ash Corporation). In situ Raman spectra were recorded on a LabRAM HR800 confocal Raman microscope (Horiba Jobin Yvon).

Electrochemical measurements: The electrochemical properties of the samples were evaluated through a half-cell configuration. The working electrode was prepared by casting a slurry of active materials, carbon black and CMC with a weight ratio of 7:2:1, onto a copper foil and drying under vacuum at 60°C for 12 h. The mass loading of active material was determined to be approximately 1 mg/cm2. The cells were assembled with sodium foil as the counter electrode, glass fibre (Whatman GF/D) as a separator, and a solution of 1 M NaClO 4 in a mixture of EC and DMC (1:1 by volume) with 5 vol% fluoroethylene carbonate (FEC) as the electrolyte in an Ar-filled glove box with a water/oxygen content below 0.1 ppm. In the case of Na-ion full cell, the cathode was prepared by casting a slurry of Na 3 (VOPO 4 ) 2 F, carbon black and polytetrafluoroethylene (PTFE) with a weight ratio of 7:2.5:0.5, onto an aluminum foil and drying under vacuum at 60°C for 12 h. The anodes used in the full cell were precycled to eliminate the large capacity loss in the first cycle. The galvanostatic 19 ACS Paragon Plus Environment

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discharge-charge tests were conducted on a LAND CT3001A battery testing system. CV measurements were conducted using an Autolab 302N electrochemical workstation. Electrochemical impedance spectroscopy (EIS) was also performed on an Autolab 302N electrochemical workstation with a frequency window from 1 MHz to 0.05 Hz.

AUTHOR INFORMATION Corresponding Author *Email: [email protected]

*Email: [email protected]

ACKNOWLEDGMENT The authors thank Prof. Zhangquan Peng and Dr. Jiawei Wang for their help with the in situ Raman measurements. This work is supported by the National Natural Science Foundation of China (21805261 and 21701158), China Postdoctoral Science Foundation (2018T110995 and 2018M631101), National Postdoctoral Program for Innovative Talents (BX201700214), and Science Foundation of Institute of Chemical Materials (011100301).

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Supporting Information

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Supporting Information Available: < TEM and HRTEM images of Sb NTs; TEM images of the products obtained at different time; HRTEM images and elemental mapping of Cu@Cu 2 Sb and Cu 2 Sb@Sb NTs; XRD patterns of the samples; In operando Raman spectra; Post-mortem SEM and TEM characterization; Relative contents of Sb in Cu@Cu 2 Sb and Cu 2 Sb@Sb composite NTs; tables for comparison of electrochemical performance.> This material is available free of charge via the Internet at http://pubs.acs.org.

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