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A Dealloying Synthetic Strategy for Nanoporous Bismuth-Antimony Anodes for Sodium Ion Batteries Hui Gao, Jiazheng Niu, Chi Zhang, Zhangquan Peng, and Zhonghua Zhang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b00643 • Publication Date (Web): 02 Apr 2018 Downloaded from http://pubs.acs.org on April 3, 2018

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ACS Nano

A Dealloying Synthetic Strategy for Nanoporous Bismuth-Antimony Anodes for Sodium Ion Batteries

Hui Gao,1 Jiazheng Niu,1 Chi Zhang,2 Zhangquan Peng,2,3,* Zhonghua Zhang1,2,* 1

Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials (Ministry of

Education), School of Materials Science and Engineering, Shandong University, Jingshi Road 17923, Jinan, 250061, P.R. China 2

School of Applied Physics and Materials, Wuyi University, 22 Dongcheng Village, Jiangmen

529020, P.R. China 3

State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry,

Chinese Academy of Sciences, Changchun, Jilin 130022, P.R. China *Corresponding author. Email: [email protected] (Z. Zhang) and [email protected] (Z. Peng)

1

ACS Paragon Plus Environment

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ABSTRACT: Metal-based anodes have recently aroused much attention in sodium ion batteries (SIBs) owing to their high theoretical capacities and low sodiation potentials. However, their progresses are prevented by the inferior cycling performance caused by severe volumetric change and pulverization during the (de)sodiation process. To address this issue, herein an alloying strategy was proposed and nanoporous bismuth (Bi)-antimony (Sb) alloys were fabricated by dealloying of ternary Mg-based precursors. As an anode for SIBs, the nanoporous Bi2Sb6 alloy exhibits an ultralong cycling performance (10000 cycles) at 1 A/g corresponding to a capacity decay of merely 0.0072% per cycle, due to the porous structure, alloying effect and proper Bi/Sb atomic ratio. More importantly, a (de)sodiation mechanism ((Bi,Sb) ↔ Na(Bi,Sb) ↔ Na3(Bi,Sb)) is identified for the discharge/charge processes of Bi-Sb alloys by using operando X-ray diffraction and density functional theory calculations.

KEYWORDS: sodium ion batteries • bismuth-antimony anodes • dealloying • cycling performance • operando X-ray diffraction

2


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Over the past decades, sustainable energies including solar, wind, geothermal power, and so on, have aroused extensive attention due to severe environmental problems caused by ever-growing consumption of fossil fuels.1-2 However, the energies generated from sustainable power sources are usually not constant and reliable.3 Thus, to smoothly integrate power into the grid, developing grid-scale energy storage systems (ESSs) technologies based on batteries is essential and urgent.4-5 With the significant merits of high energy density and long lifespan, lithium ion batteries (LIBs) are definitely strong contenders.6-8 Nevertheless, the high cost and non-uniform distribution of lithium resource greatly limit the application of LIBs in EESs, in which the cost is the most important issue.9-10 On the contrary, sodium ion batteries (SIBs) have become a promising alternative to LIBs owing to the natural abundance and thus much low cost of Na.11-13 Hence, enormous efforts have been devoted to electrode materials of SIBs. Compared with the rapid progress in cathode,14-23 the development of anode is slow. Despite some carbon-based materials, polyanionic compounds and metal sulfides have exhibited moderate specific capacities, long life-span as well as good rate performance in SIBs,24-30 it is still an urgent need to explore high-performance anodes with high gravimetric/volumetric capacities and superior cycling performance.31-33 Notably, comprehending the sodiation/desodiation

mechanisms,

especially

the

relationship

between

electrochemical performances and microstructures of electrode materials is crucial for efficiently exploring appropriate anodes.34 With the advantage of effectively real-time detection on the dynamic electrochemical process, operando (in situ) spectroscopic 3


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techniques such as X-ray Diffraction (XRD) are powerful methods to probe reaction mechanisms in SIBs.35-41 Recently, metal-based materials including metals (Pb, Sn, Bi Sb,) and metalloids (Si, P, Ge, As), have exhibited promising potentials to be employed in SIBs, in terms of their high theoretical capacities (300 – 2000 mAh/g) and low sodiation potentials (less than 1.0 V (vs. Na+/Na)).42-44 Even so, the severe volumetric expansion associated with alloying process causes aggregation and pulverization of active materials, resulting in the serious capacity fading upon cycling.32 To address these issues, one feasible approach is to design nanoarchitectures with hollow/porous structures to alleviate the volume variations and facilitate the transportation of electrons and ions.45-47 The other strategy is to engineer bimetallic alloys, enhancing the stability of the material via synergistically serving as the buffer substrates. Moreover, among the reported bimetallic anode materials, bismuth-antimony (Bi-Sb) alloys have proved promising owing to three merits: (1) The BiSb alloys combine the high theoretical capacity (660 mAh/g) of Sb and relative small volumetric expansion (250%, smaller than Sn (423%), P (440%) and Sb (390%)) of Bi.31-32, 36, 45-46, 48-49 (2) Considering the same main group of Bi and Sb, their physiochemical properties are similar, ensuring flat potential plateaus.50 (3) Because Bi can mix with Sb at any molar ratio, the compositional design of Bi-Sb alloys is flexible. Zhao et al. reported BiSb-C anode via high-energy mechanical milling and achieved the initial desodiation capacity of 375 mAh/g. However, the cycling performance is inferior with the capacity retention of merely 78% after 50 cycles. Furthermore, to the best of our 4


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knowledge, the intensive study on sodiation/desodiation mechanisms of BiSb alloys via in situ or ex situ methods has not been reported.50-52 Herein, nanoporous (np) BixSby (x, y= 2, 6; 4, 4; 6, 2) alloys have been designed and synthesized via a facile one-step dealloying process. As an anode in SIBs, the np-Bi-Sb alloys show greatly improved cycling performance, as benchmarked with the monometallic electrode (Bi or Sb). Specially, the np-Bi2Sb6 alloy delivers unrivalled specific capacity (high up to 562.1 mAh/g at 200 mA/g) and ultralong cycling performance (a capacity loss of only 0.0072% per cycle at a high current density of 1 A/g over 10000 cycles). The excellent performance is attributed to the nanoporous structure and the alloying effect, which promote the permeation of electrolyte, accelerate the transportation of electrons and ions as well as synergistically mitigate the volumetric expansion/contraction. Notably, we propose a sodiation/desodiation mechanism ((Bi,Sb) ↔ Na(Bi,Sb) ↔ Na3(Bi,Sb)) through the operando XRD technique combined with density functional theory (DFT) calculations.

RESULTS AND DISCUSSION The as-spun Mg92Bi2Sb6, Mg92Bi4Sb4 and Mg92Bi6Sb2 precursor foils exhibit similar diffraction patterns and are comprised of Mg phase (JCPDS # 35-0821) and Mg3(Bi,Sb)2 intermetallic phase (the peak positions of Mg3(Bi,Sb)2 locate between those of Mg3Bi2 (JCPDS # 65-8732) and Mg3Sb2 (JCPDS # 65-3458)), Figure S2. After dealloying, the XRD patterns (Figure1a) of the three samples are indexed to the 5


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single phase of BiSb alloy, while the minor shifts of peaks for each sample are attributed to the compositional difference between Bi and Sb. The scanning electron microscopy (SEM) images (Figure 1b-d) elucidate that all three np-Bi-Sb samples exhibit typical three-dimensional bicontinuous ligament-channel structures with various sizes of ligaments. The sizes of ligaments are 15 - 25, 25 - 35 and 45 - 70 nm for np-Bi2Sb6, np-Bi4Sb4 and np-Bi6Sb2 respectively. Additionally, the EDX results (Figure S3) illustrate that the actual atomic ratios of np-Bi2Sb6, np-Bi4Sb4, np-Bi6Sb2 are close to their nominal compositions, suggesting that the compositions of np-Bi-Sb alloys can be facilely adjusted through changing the Bi/Sb ratios in the Mg-Bi-Sb precursors. Transmission electron microscopy (TEM) was used to further characterize the microstructure of np-Bi-Sb alloys (Figure 1e-g and Figure S4-S5). The nanoporous feature can be clearly observed in np-Bi4Sb4 and the sizes of ligaments are 25 ± 5 nm (Figure 1e and Figure S4b), in good agreement with the SEM results. The high-resolution TEM (HRTEM) image (Figure 1f) reveals that the crystal sizes (~ 25 nm) are approximately close to the ligaments sizes. Meanwhile, lattice fringes can be well distinguished, and well match with the crystal plane of BiSb (012) (Figure 1f). The corresponding selected-area electron diffraction (SAED) pattern (Figure 1g) exhibits diffraction rings ascribed to reflections of the BiSb phase, confirming the nanocrystalline nature of the selected region (~ 500 nm in diameter). Figure S5 demonstrates the TEM, HRTEM images and SAED patterns of the np-Bi2Sb6 and np-Bi6Sb2 samples. The similar nanoporous structures can be observed with various 6


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sizes (22 ± 8 nm for np-Bi2Sb6; 60 ± 10 nm for np-Bi6Sb2) of ligaments, in good coincidence with the SEM results. Figure 1h schematically illustrates the typical three-dimensional bicontinuous ligament-channel structure of the np-Bi-Sb alloys. The formation of such structure is ascribed to the dealloying process. During the dealloying, the active element (Mg) in the Mg-Bi-Sb precursors was selectively etched away, while the inert elements (Bi and Sb) were retained and re-organized to form the nanoporous structure with nanoscale ligaments.53-54 Notably, the sizes of ligaments become smaller with the increase of Sb content in np-Bi-Sb alloys, which can be ascribed to two aspects as follows. (1) The diffusion coefficients of Bi and Sb are different in aqueous solutions. Schoenleber et al. reported that the diffusion coefficient (D = 12.2 × 10-6 cm2/s) of Bi is nearly two times that (D = 7.0 × 10-6 cm2/s) of Sb in 0.10 M tartaric acid.55 In this work, the concentration of the tartaric acid is 2 wt.% (equal to 0.13 M), close to that in the work by Schoenleber et al., indicating that Bi can diffuse quickly in comparison with Sb in the 2 wt.% tartaric acid. During the dealloying process, the size of ligaments is significantly influenced by surface diffusion of more noble atoms, and increases with increasing diffusion coefficients of them.56-57 Moreover, previous reports have demonstrated the ligament sizes (100 ± 20 nm) of nanoporous Bi (dealloyed from Mg92Bi8 in 2 wt.% tartaric acid) are larger than that ( ~ 10 nm) of mesoporous Sb-based nanocomposite (dealloyed from Mg90Sb10 in 2 wt.% tartaric acid).58-59 The above-mentioned results elucidate that the Sb tends to form thinner ligaments compared with Bi by the dealloying strategy due to the lower diffusion coefficient. (2) The metallic radius (1.61 Å) of Sb is smaller than that (1.82 7


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Å) of Bi (based on the database of Materials Studio), resulting in the smaller lattice parameters (a= 4.307 Å, c= 11.273 Å) of Sb (JCPDS # 35-0732) compared with that (a= 4.547 Å, c= 11.862 Å) of Bi (JCPDS # 44-1246). Additionally, the specific surface areas of np-Bi2Sb6, np-Bi4Sb4 and np-Bi6Sb2 were determined to be 16.26, 7.01 and 5.97 m2/g using the Brunauer-Emmett-Teller (BET) method, respectively (Figure S6). Furthermore, Figure S7 shows the XRD results of the as-spun Mg92Bi8 and Mg92Sb8 foils as well as the as-dealloyed np-Bi and np-Sb powders. The porous characteristics of np-Bi and np-Sb are exhibited by the SEM images (Figure S8). The electrochemical properties of the np-Bi-Sb alloys were evaluated by utilizing cyclic voltammograms (CVs) and galvanostatic discharge/charge tests. Figure 2a shows the CVs of the np-Bi2Sb6 electrode in the initial five cycles at a scan rate of 0.1 mV/s between 0.01 - 2.0 V (vs. Na+/Na). In the first cathodic scan, one board peak at 0.69 V (vs. Na+/Na) can be detected, followed by a strong broad peak at 0.20 V (vs. Na+/Na), which can be attributed to the formation of solid electrolyte interphase (SEI) film and the two-step alloying processes of (Bi,Sb) → Na(Bi,Sb) → Na3(Bi,Sb), respectively. In the subsequent cathodic scans, two peaks at 0.53/0.21 V (vs. Na+/Na) are observed, corresponding to the formation of Na(Bi,Sb) and Na3(Bi,Sb), separately. In the anodic scan, a strong peak at 0.84 V (vs. Na+/Na) is ascribed to the two-step dealloying processes of Na3(Bi,Sb) → Na(Bi,Sb) → (Bi,Sb). The CVs of the np-Bi4Sb4 and np-Bi6Sb2 electrodes are exhibited in Figure S9. In the 1st cathodic scan, the peaks at 1.07 V (vs. Na+/Na) for np-Bi4Sb4 and 0.78 V (vs. Na+/Na) for np-Bi6Sb2 are associated with the formation of SEI film. Meanwhile, the peaks at 0.59/0.37 V 8


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(vs. Na+/Na) (for np-Bi4Sb4) and 0.48/0.22 V (vs. Na+/Na) (for np-Bi6Sb2) are attributed to the two-step alloying processes of (Bi,Sb) → Na(Bi,Sb) → Na3(Bi,Sb) respectively, while the peaks at 0.74/0.81 V (vs. Na+/Na) and 0.75/0.82 V (vs. Na+/Na) in the anodic scan are ascribed to the reversible dealloying processes for np-Bi4Sb4 and np-Bi6Sb2, separately. In the succeeding cycles, four reduction/oxidation peaks with minor shift compared with the initial cycle for the two samples are consistent with the two-step alloying/dealloying processes. Figure 2b shows the galvanostatic discharge-charge curves of the np-Bi2Sb6 electrode in different cycles at a current density of 200 mA/g over a potential window of 0.01 - 2.0 V (vs. Na+/Na). Only one voltage plateau in the 1st discharge is observed, different from the two voltage plateaus in the 2nd discharge, which is in good agreement with the CV results. Notably, by comparing with the discharge curves in the 2nd, 10th and 100th cycles, we can find that the distinction of the two voltage plateaus becomes smaller and smaller until disappearing with increasing cycles. The initial discharge/charge capacities are 789.6/551.2 mAh/g with the initial coulombic efficiency (ICE) of 69.8%, and the irreversible capacity is due to the formation of SEI film. Furthermore, the discharge-charge curves of the 1600th and 2000th cycles are substantially overlapped, implying the excellent stability during the long cycling. In addition, Figure S10 displays the galvanostatic discharge-charge curves of the np-Bi4Sb4 and np-Bi6Sb2 electrodes in different cycles at 200 mA/g, and the initial discharge/charge capacities are 705.7/455.6 mAh/g (ICE: 64.6%, np-Bi4Sb4) and 553.3/339.5 mAh/g (ICE: 61.4%, np-Bi6Sb2). Figure 2c demonstrates the cycling 9


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performance of the np-Bi2Sb6 electrode at 200 mA/g, while the cycling performances of the np-Bi4Sb4, np-Bi6Sb2, np-Bi and np-Sb electrodes have also been shown for comparison. The discharge capacity of np-Bi2Sb6 in the 2nd cycle is 562.1 mAh/g, slightly higher than the theoretical value (559.9 mAh/g). The coulombic efficiency reaches 97% after the initial two cycles and retains stable during the following cycles. The reversible capacity after 100 cycles retains 428.6 mAh/g, still higher than the initial capacity (375.0 mAh/g) of Bi0.57Sb0.43-C electrode.50 Surprisingly, the discharge capacity is 305.2 mAh/g after 400 cycles and remains 257.5 mAh/g after 2000 cycles (Figure 2c), corresponding to a capacity loss of only 0.027% per cycle, confirming the ultralong cycling performance of np-Bi2Sb6. In contrast, the np-Bi4Sb4 electrode also exhibits the excellent cycling performance (a capacity decay of 0.037% per cycle before the 1600th cycle), but the discharge capacities are substantially lower than those of np-Bi2Sb6 during the whole cycling process. Meanwhile, the discharge capacities of the np-Sb electrode are high (~ 495 mAh/g) and stable during the initial 200 cycles but suddenly fade to vanish in tens of cycles, illustrating the inferior cycling performance. Additionally, the cycling performances of np-Bi and np-Bi6Sb2 electrodes rapidly deteriorate, performing low capacity only after dozens of cycles. Figure 2d displays the rate capability of the np-Bi-Sb electrodes at different current densities. The reversible capacities of np-Bi2Sb6 are 590.3, 542.8, 523.9, 507.3, 472.8, 403.0, and 304.2 mAh/g at current densities of 0.2, 0.5, 1, 2, 5, 10 and 15 A/g, separately. When the current density is reduced back to 0.2 A/g, the discharge capacity can recover to 604.6 mAh/g, confirming the extraordinary rate performance of 10


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np-Bi2Sb6. Specially, the np-Bi2Sb6 anode can deliver a discharge capacity of 523.9 mAh/g at 1 A/g, much higher than that (326.0 mAh/g at 1 A/g) of Bi0.57Sb0.43-C.50 The discharge capacity at 10 A/g is 403.0 mAh/g, even more than the theoretical capacity (372.1 mAh/g) of the commercial graphite in LIBs.60 In comparison, the rate capabilities of np-Bi4Sb4 and np-Bi6Sb2 are inferior, with the relatively low reversible capacities (Figure 2d). Furthermore, the corresponding discharge-charge curves of np-Bi2Sb6, np-Bi4Sb4 and np-Bi6Sb2 are displayed in Figure S11. Figure 2e shows the cycling performance of the np-Bi2Sb6 electrode at 1 A/g (1.79C). The capacity rapidly fades to 192.5 mAh/g in the initial 350 cycles, and then fluctuates up to 10000 cycles. After 10000 cycles, the np-Bi2Sb6 electrode still retains a moderate discharge capacity of ~ 150 mAh/g, corresponding to a capacity decay of only 0.0072% per cycle, exhibiting the marvelous cycling stability of the anode. It is noteworthy that the discharge capacity can remain about 150 mAh/g from 5000 to 5300 cycles when the current density rises up to 2 A/g. Meanwhile, the reversible capacity can rapidly recover after resuming the current density to 1 A/g (inset of Figure 2e). The above-mentioned results illustrate that the np-Bi2Sb6 alloy has a great potential as the anode for grid-scale ESSs, where the span life is the most important factor.9, 16 In addition, the reversible capacity of np-Bi4Sb4 is ~ 100 mAh/g after 10000 cycles, and the np-Bi6Sb2 electrode show inferior cycling performance at 1 A/g (Figure S12). Figure S13 displays the galvanostatic discharge-charge curves of the np-Bi2Sb6, np-Bi4Sb4 and np-Bi6Sb2 electrodes in different cycles at 1 A/g. Furthermore, Figure S14 exhibits the comparison of the cycling performances of the 11


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np-Bi2Sb6 and np-Bi4Sb4 electrodes based on different weight ratios (the active material: conductive acetylene black: carboxymethyl cellulose (CMC) binder) at the current density of 200 mA/g, elucidating that the system still works well with lower carbon content (15 wt.%). Figure S15 demonstrates the cycling performances of the conductive acetylene black at different current densities, illustrating the minor contribution of acetylene black to the total discharge capacity of the np-Bi2Sb6 and np-Bi4Sb4 electrodes. Ex situ TEM was performed to further explore the structural stability of the np-Bi-Sb alloys. Figure S16 shows the TEM results of the np-Bi4Sb4 electrode after 1600 cycles at 200 mA/g. The nanoporous ligament-channel structure can be sustained even after 1600 cycles (Figure S16a), proving the good structural stability. The sizes of ligaments reduce from 25 ± 5 nm to less than 20 nm due to the electrochemically induced refinement during the sodiation/desodiation processes. Moreover, well-discerned lattice fringes are observed and the lattice spacing (3.286 Å) corresponds to the BiSb (012) plane (Figure S16b). The SAED pattern is composed of polycrystalline diffraction rings which can be indexed by (Bi,Sb) phase (Figure S16c). Specially, despite the ultralong cycling performance, the outstanding stability of the np-Bi-Sb alloys needs to be activated by hundreds of cycles with the moderate capacity fading. Combined with the ex situ TEM results (Figure S16a-c), the activated process can be interpreted as the reduction of the ligament sizes as well as the formation of the amorphous structure during cycling, which could facilitate the Na ion transfer, mitigate the volumetric variation and enhance the stability of the 12


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electrode.61-62 Furthermore, as shown in Figure 2c, the cycling performance of np-Bi electrode rapidly deteriorates, which can be attributed to the higher electrical resistivity and larger ligament size of np-Bi in comparison with np-Sb (Figure S8).63 Moreover, the ligament sizes of np-Bi-Sb alloys increase with increasing Bi content as illustrated in Figure 1b-e, Figure S4b and Figure S5a,d, Notably, the capacity decrease of the np-Bi6Sb2 is serious due to the significant increase of ligament sizes compared with np-Sb induced by the addition of Bi, while the capacity decay of np-Bi2Sb6 and np-Bi4Sb4 is moderate owing to the slight increase of ligament sizes. Thus, we conclude that the initial rapid capacity decay of np-Bi-Sb alloys stems from the increase of the ligament sizes induced by the addition of Bi and the activated process of (Bi,Sb). Table S1 summarizes the discharge capacities and corresponding capacity retention ratios of np-Bi2Sb6, np-Bi4Sb4 and np-Bi6Sb2 at the current density of 200 mA/g. As mentioned above, the Bi/Sb atomic ratios in the np-Bi-Sb electrodes have significant effects on their electrochemical properties towards Na storage, including CV profiles, discharge/charge curves, specific capacity, cycling stability and rate capability. Among three BiSb electrodes, the np-Bi2Sb6 anode exhibits the best Na storage performance, even as benchmarked with the np-Bi and np-Sb electrodes. Table S2 summarizes the comparison of the present results with the literature data. The np-Bi2Sb6 anode exhibits superior cycling performance as benchmarked with that of previously reported Bi-based and Sb-based anodes for SIBs. The outstanding performance of np-Bi2Sb6 is attributed to three aspects as follows. (1) The nanoporous 13


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structure provides abundant channels for the permeation of electrolyte, facilitating the transportation of ions. Simultaneously, the nanopores can alleviate the volumetric variation. (2) The nanoscale ligaments form conductive network, efficiently accelerating the electron transport and reducing the ohmic resistance. (3) The alloying strategy with the proper Bi/Sb atomic ratio strengthens the stability of the material via each component synergistically serving as the buffer substrate, mitigating the volumetric expansion/contraction.32, 43, 45-48, 64-67 To shed light on the sodiation/desodiation mechanisms of the np-Bi-Sb alloys, the phase evolution of the np-Bi4Sb4 electrode was probed by the operando XRD technique during the initial discharge-charge processes between 0.01 – 2.0 V (vs. Na+/Na) at 25 mA/g (Figure 3). At the beginning of the discharge, the peaks at 28.0º, 38.9º and 40.9º can be denoted to the (Bi,Sb), while the peak at 44.0º is ascribed to the stainless steel mesh (Figure 3b). Meanwhile, the peaks at 39.0° and 41.7° are ascribed to the BeO (JCPDS # 35-0818) generated by the oxidation at the surface facing to the air of the Be window. With continuous discharging in stage 1 (2.40 – 0.50 V (vs. Na+/Na)), the peaks belonging to the (Bi,Sb) start to weaken. Simultaneously, the appearance and gradual increase of peaks (18.9°, 26.4º, 32.5º, 37.8° and 42.1°) indexed to the Na(Bi,Sb) are observed, elucidating that Na reacts with (Bi,Sb) to form Na(Bi,Sb). The minor shifts of the peaks for (Bi,Sb) at the very early stage of the discharge could be attributed to the intercalation of Na into the (Bi,Sb) lattice. In the subsequent discharge (0.50 - 0.01 V (vs. Na+/Na), stage 2), the peaks at 18.9º, 19.3°, 20.6°, 21.5°, 25.3°, 26.4º, 33.6°, 34.1°, 38.6° and 39.8º assigned to the Na3(Bi,Sb) 14


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appear, accompanying the gradual decrease and vanishing of the Na(Bi,Sb). Simultaneously, the peaks at 23.6° and 30.3° denoted as the NaBeF3 (JCPDS # 11-0569) start to appear and increase due to the reaction between the electrolyte and Be at low applied potentials. At the end of stage 2, except the peaks from the Be window and substrate, only Na3(Bi,Sb) can be detected, indicating the complete transformation from Na(Bi,Sb) to Na3(Bi,Sb) via the reaction with Na. In the following charge process, a two-step dealloying reactions occur (stage 3 & 4), inverse to the discharge process. The peaks assigned to Na3(Bi,Sb) begin to diminish and ultimately disappear, accompanied by the reappearance and enhancement of Na(Bi,Sb) peaks associated with the desodiation of Na3(Bi,Sb). Afterwards, the peaks of (Bi,Sb) are regained and become strong at the expense of Na(Bi,Sb), illustrating the desodiation of Na(Bi,Sb) to form (Bi,Sb). With respect to the second discharge, the similar two-step alloying processes of (Bi,Sb) → Na(Bi,Sb) →Na3(Bi,Sb) are observed, further confirming the (Bi,Sb) reacts with Na to form Na3(Bi,Sb) via the intermediate product of Na(Bi,Sb). In addition, the contour plot for the operando XRD results of the np-Bi4Sb4 electrode is shown in Figure 3a, vividly revealing the sodiation/desodiation procedure of np-Bi4Sb4 during the initial cycle. With respect to the np-Bi2Sb6 electrode, the sodiation/desodiation mechanism was also probed by operando XRD (Figure S17). The np-Bi2Sb6 electrode was subjected to two successive discharge/charge cycles and the real-time phase evolution was monitored by the operando XRD. Similar to the np-Bi4Sb4 electrode, the discharge process of np-Bi2Sb6 is correlated with the two-step sodiation ((Bi,Sb) → 15


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Na(Bi,Sb) → Na3(Bi,Sb)), and the inverse desodiation reactions (Na3(Bi,Sb) → Na(Bi,Sb) → (Bi,Sb)) occur in the charge process (Figure S17). The above-mentioned results elucidate that the BiSb alloys with various compositions exhibit the analogical reaction mechanism involving the two-step alloying/dealloying processes. As schematically demonstrated in Figure 4a, the (Bi,Sb) phase participates in the reaction as the integrated part and forms the final product of Na3(Bi,Sb)

via

the

intermediate

phase

of

Na(Bi,Sb).

Notably,

the

sodiation/desodiation processes are based on the synergetic effects of two elements in BiSb alloys, different from the independent reaction of each component (as illustrated in the dashed rectangle in Figure 4a) in previous reports.50-52, 65-77 To the best of our knowledge, only one literature described the possible sodiated products (Na3Sb, Na3Bi) of Bi-Sb based upon Na-Sb and Na-Bi phase diagrams.50 The reaction mechanisms of Bi-Sb alloys in SIBs have not yet been probed using in situ or ex situ techniques in the literature. Notably, the synchronous sodiation process of Bi and Sb can be ascribed to the infinite miscibility and the similar physiochemical properties between Bi and Sb. Simultaneously, the open framework facilitates the Na ion transfer, further guaranteeing the uniform sodiation process of Bi-Sb alloys. In order to rationalize this mechanism, the operando XRD results were further analyzed with the DFT calculations. The first step was to design & calculate the optimized structures of (Bi,Sb), Na(Bi,Sb) and Na3(Bi,Sb). Figure S18 schematically shows the calculation processes for atomic structures of (Bi,Sb) (left panel), Na(Bi,Sb) (middle panel) and Na3(Bi,Sb) (right panel). For (Bi,Sb), the crystal structure was 16


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obtained from the PDF card (R-3m (166), JCPDS # 35-0517), while the lattice parameters were calculated based on the fitting curve by utilizing the software Jade 6.5 (Figure S19). Considering the intermediate product during the sodiation process of Sb was uncertain (NaxSb), the crystal structure of Na(Bi,Sb) referred to that of NaBi (P4/mmm (123), JCPDS # 04-0701).78 In terms of the similar crystal structures of Na3Bi (P63/mmc (194), JCPDS # 65-3525) and Na3Sb (P63/mmc (194), JCPDS # 65-3523) as well as the infinite miscibility between Bi and Sb, we assumed the crystal structure of Na3(Bi,Sb) as the “P63/mmc” type.79 Then the initial lattice parameters of Na(Bi,Sb) and Na3(Bi,Sb) were set based upon those of the NaBi (a=b= 0.3470 nm, c= 0.4810 nm) and Na3Bi (a=b= 0.5459 nm, c= 0.9675 nm), respectively. Simultaneously, the initial atomic positions for (Bi,Sb), Na(Bi,Sb) and Na3(Bi,Sb) were set on the basis of the positions in Bi, NaBi and Na3Bi, separately.79 Afterwards, the geometry optimizations for the lattice parameters and atomic positions of three phases were conducted by the CASTEP module in Materials Studio (version 7.0, Accelrys Inc) and the settings of the related parameters were described detailedly in experimental section. After the calculation, the optimized lattice structures and parameters were shown at the bottom of Figure S18 and in Figure 4e-g. The second step was to compare the calculated XRD patterns with the operando XRD results. Based on the optimized structures, the simulations for the XRD patterns of (Bi,Sb), Na(Bi,Sb) and Na3(Bi,Sb) were carried out by utilizing the Reflex module, Figure 4b-g. The calculated peaks (line A) of the (Bi,Sb) phase are well coincident with the experimental results, proving the accuracy of the simulation (Figure 4b). Moreover, 17


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the positional distinctions between the strongest peaks (at 32.43° and 33.66°) obtained by the calculation and those (at 32.46°, line B and 33.59º, line C) probed by the experiment for Na(Bi,Sb) and Na3(Bi,Sb) are less than 0.1°, while the differences of other peaks between the calculated and experimental results are less than 0.35º (Figure 4b-d). The above-mentioned results confirm convincingly the accuracy for the definition of intermediate product as Na(Bi,Sb) as well as the final product as Na3(Bi,Sb) during the sodiation process of np-Bi-Sb alloys, and well rationalize the reaction mechanism. Notably, the intermediate products of Sb are amorphous in most previous reports.78, 80-82 For example, Li et al. reported the transformation from Sb film to crystalline hexagonal Na3Sb via amorphous NaxSb by utilizing the in situ TEM analysis and indexed SADs.80 Baggetto et al. suggested the formation of an amorphous phase composed of atomic environments similar to those found in NaSb during the sodiation process of Sb thin film by using the

121

Sb Mossbauer

spectroscopies.81 In contrast, Tian et al. recently reported reversible crystalline phase evolution (Sb ↔ NaSb ↔ Na3Sb) during cycling of the two-dimensional (2D) antimonene by using the synchrotron XRD and ex situ SAED.83 By comparing the above-mentioned reports, the c lattice parameter (3.8 Å) of antimonene are larger than that of the bulk Sb or Sb film reported before and thus we speculate that the crystallization degree of the intermediate products of Sb-based materials is influenced by the interlayer spacing along the c-axis between Sb atoms. In our work, the interlayer spacing along the c-axis (Figure S18) between Sb atoms increases to 3.89 Å 18


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(close to the value (3.8 Å reported by Tian et al.)), promoting the crystallization degree of intermediate products of Bi-Sb alloys. Moreover, the crystallization degree of ternary alloy compound (here, Na-Bi-Sb) is influenced by each component. Specially, the crystallization degree of the sodiated products (NaBi and Na3Bi) of Bi is high in previous reports.37, 59 Thus, the crystallization degree of the intermediate products of Bi-Sb alloys may be determined by the large interlayer spacing along the c-axis of Sb atoms as well as the high crystallization degree of the sodiated product of Bi.

CONCLUSIONS In summary, nanoporous Bi-Sb alloys with different Bi/Sb ratios were successfully fabricated through a one-step dealloying of Mg-based precursors. As anodes for SIBs, the np-Bi-Sb alloys show enhanced electrochemical performance towards Na storage as benchmarked with pure Bi or Sb, especially the cycling stability. Specially, the np-Bi2Sb6 electrode delivers a reversible capacity of 257.5 mAh/g after 2000 cycles at 200 mA/g, and exhibits the ultralong cycling performance at 1 A/g corresponding to a capacity decay of merely 0.0072% per cycle over 10000 cycles. The extraordinary electrochemical performance of np-Bi2Sb6 can be ascribed to the nanoporous structure with proper Bi/Sb atomic ratios, effectively promoting the permeation of electrolyte and transportation of electrons/ions, as well as synergistically alleviating the volumetric expansion. Notably, a sodiation/desodiation mechanism of (Bi,Sb) ↔ Na(Bi,Sb) ↔ Na3(Bi,Sb) is first proposed by utilizing the 19


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operando XRD technique combined with the DFT calculations. The present results could provide useful information on design of advanced anode materials and understanding of electrode processes in SIBs.

EXPERIMENTAL SECTION Materials synthesis The np-Bi2Sb6, np-Bi4Sb4, np-Bi6Sb2, np-Bi and np-Sb were fabricated by chemical dealloying of rapidly solidified Mg-based precursors, as illustrated in Figure S1. In a typical synthesis of np-Bi2Sb6 alloy, ternary Mg92Bi2Sb6 (nominal composition, at.%) was selected as the precursor for dealloying. Pure Mg, Bi and Sb blocks (purity, 99.9 wt.%) were melted in a graphite crucible using an electric resistance furnace (at 740 °C for 30 min) under the protection of covering flux, and then cast into a mold to form a Mg92Bi2Sb6 ingot. Utilizing a single-roller melt-spinning apparatus, the ingot was further rapidly solidified into foils (at a speed of 1000 revolutions per minute (rpm)) under an argon atmosphere. Afterwards, the dealloying of as-spun foils was performed in a 2 wt.% tartaric acid solution at room temperature until no obvious bubbles emerged. Then the as-dealloyed samples were rinsed thoroughly with deionized water and ethanol, followed by drying at 60 °C for 5h in vacuum. For the fabrication of np-Bi4Sb4, np-Bi6Sb2, np-Bi and np-Sb, the experimental procedures were similar except that the precursors were Mg92Bi4Sb4, Mg92Bi6Sb2, Mg92Bi8 and Mg92Sb8, respectively.

20


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Microstructural characterization The phase constitutions of the as-prepared samples were identified using an XD-3 diffractometer (Beijing Purkinje General Instrument Co., Ltd, China) equipped with Cu Kα radiation. Scanning electron microscope (SEM, ZEISS, SIGMA 300) and transmission electron microscope (TEM, JEOL JEM-2100) were employed to probe the microstructures of the as-dealloyed powders. An energy dispersive X-ray (EDX) analyzer coupled with SEM was used to determine chemical compositions of Bi-Sb alloys. N2 adsorption and desorption isotherms were achieved at 77 K utilizing a Gold APP V-Sorb 2800 surface area and porosity analyzer. For post-cycling TEM observations, coin-type cells were performed to the required cycles and disassembled in an argon-filled glove box (Mikrouna Co., Ltd, China) with oxygen and moisture levels below 0.1 ppm. Then the electrodes were washed with a diethyl carbonate (DEC) solution to remove residual sodium salt, subsequently stripped from the substrate and dispersed in ethanol by ultrasonication before testing.

Electrochemical measurements For the np-Bi2Sb6 sample, the electrode was fabricated by a scraping-coating method. The slurry was prepared by mixing the active material (np-Bi2Sb6 powders), conductive acetylene black (Super P) and carboxymethyl cellulose (CMC) binder based on a weight ratio of 60:25:15 in deionized water. The mixture was stirred for around 12 h to form a homogenous slurry and then coated onto stainless steel mesh (SS-mesh, stainless steel 304, thickness: 0.1 mm) for operando XRD or copper foil for 21


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other electrochemical measurements. The electrode was further dried at 80 °C for 12 h in vacuum and then punched into disks (12 mm in diameter) with the mass loading of 0.8 ± 0.2 mg. The np-Bi4Sb4, np-Bi6Sb2, np-Bi and np-Sb electrodes were prepared by the similar procedures. The as-prepared electrodes were utilized as the working electrodes. Na foil (purity, 99.99 wt.%, Sigma-Aldrich) was used as both the counter and reference electrodes, and glass fiber (GF/D, Whatman) was used as a separator. The 1 M NaClO4 in propylene carbonate (PC) with 5 wt.% addition of fluoroethylene carbonate (FEC) was selected as the electrolyte. The CR2032 cells were assembled in an argon-filled glove box and subsequently aged overnight before tests. Galvanostatic charge/discharge profiles were obtained by the test system (LAND-CT2001A, Wuhan, China) between 0.01 - 2.0 V (vs. Na+/Na). Cyclic voltammograms (CVs) were obtained by a CHI 660E potentiostat at a scan rate of 0.1 mV/s over a potential window of 0.01 – 2.0 V (vs. Na+/Na). For operando XRD, a CR2016 coin cell with one side beryllium (Be) window (12 mm in diameter) was employed. The operando XRD patterns were recorded based on the “time step” method with a time step of 3 seconds through the window of the coin cell, cycled by the test system at a current density of 25 mA/g between 0.01 - 2.0 V (vs. Na+/Na).

Theoretical Methodology The DFT calculations for geometry optimization were conducted by the CASTEP module in Materials Studio (version 7.0, Accelrys Inc). The generalized 22


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gradient approximation (GGA) of the Perdew–Burke–Ernzerhof (PBE) functional was used for exchange-correlation effects. Meanwhile, the ultrasoft pseudopotentials, a kinetic energy cut-off of 370 eV and a 7 × 7 × 2 Monkhorst–Pack grid were selected for all cases. The internal coordinates and lattice parameters were optimized for Na(Bi,Sb) and Na3(Bi,Sb), while only the internal coordinates were optimized for (Bi,Sb). As to the simulation for the XRD patterns, the Reflex module was employed based on the optimized structures. The step size was set as 0.01 with the range from 17.5º to 44.5º. Cu was selected as the X-ray source, while the line shift correction applied to data was determined by Bragg-Brentano correction.

ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website at DOI:

Preparation procedure; XRD patterns of foils; EDX results; more TEM results of np-Bi4Sb4; TEM, HRTEM and SAED patterns of np-Bi2Sb6 and np-Bi6Sb2; N2 adsorption-desorption isotherms; XRD and SEM of np-Bi and np-Sb; Some cyclic voltammograms, galvanostatic discharge-charge curves, cycling performances of Bi-Sb samples for reference; comparison of the performance of electrodes based on different weight ratios; cycling performances of the conductive acetylene black; ex situ TEM for the electrode after 1600 cycles; the operando XRD patterns of the np-Bi2Sb6 electrode; the designed & calculated processes; fitting curve; the capacities 23


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and retention ratios; comparison of this work with previous reports; comparison of the atomic positions before/after the Geometry Optimization.

ACKNOWLEDGMENTS The authors gratefully acknowledge financial support by National Natural Science Foundation of China (51671115), the support of Department of Science and Technology of Shandong Province for Young Tip-top Talent Support Project, and Young Tip-top Talent Support Project (the Organization Department of the Central Committee of the CPC).

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Sn and

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Figures:

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Figure 1. (a) XRD patterns of the np-Bi2Sb6, np-Bi4Sb4 and np-Bi6Sb2 samples. (b-d) SEM images of the (b) np-Bi2Sb6, (c) np-Bi4Sb4 and (d) np-Bi6Sb2 samples. (e) TEM image, (f) HRTEM image and (g) SAED pattern of the np-Bi4Sb4 sample. (h) Schematic illustration showing the ligament-channel nanoporous structure of the np-Bi-Sb alloys. Nanopore and ligament are marked by ellipse (red) and square (black), respectively.

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Figure 2. (a) Cyclic voltammograms of the np-Bi2Sb6 electrode at a scan rate of 0.1 mV/s over a potential window of 0.01 - 2.0 V (vs. Na+/Na). (b) Galvanostatic discharge-charge curves of the np-Bi2Sb6 electrode in different cycles at 200 mA/g. (c) Cycling performances of the np-Bi2Sb6, np-Bi4Sb4, np-Bi6Sb2, np-Bi and np-Sb electrodes at 200 mA/g. (d) Rate capability of the np-Bi2Sb6, np-Bi4Sb4 and np-Bi6Sb2 electrodes at various current densities. (e) Cycling performance of the np-Bi2Sb6 electrode at 1 A/g. A zoom-in image (inset) illustrates the cycling performance at 2 A/g. 39


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Figure 3. (a) Contour plot and (b) line plot of the operando XRD results of the np-Bi4Sb4 electrode during the discharge-charge-discharge processes. The discharge (stage 1 & 2) - charge (stage 3 & 4) - discharge (stage 5 & 6) profiles of the np-Bi4Sb4 electrode at 25 mA/g are also shown for reference (right part).

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Figure

4.

(a)

Schematic

illustration

exhibiting

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the

comparison

of

two

sodiation/desodiation mechanisms. (b-d) XRD patterns (left panel) and (e-g) corresponding atomic structures (middle panel) of (b,e) (Bi,Sb), (c,f) Na(Bi,Sb) and (d,g) Na3(Bi,Sb) upon the sodiation (stage 1 & 2) - desodiation (stage 3 & 4) processes (right panel). The lattice parameters are given adjacent to each atomic structure scheme. Line A, B and C are the XRD patterns selected from Figure 3b.

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