Nanosheet Anodes for Advanced Sodium-Ion Batteries

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Scalable Fabrication of Core-Shell Sb@Co(OH)2 Nanosheet Anode for Advanced Sodium Ion Batteries via Magnetron Sputtering Ying Zhang, Hui Gao, Jiazheng Niu, Wensheng Ma, Yujun Shi, Meijia Song, Zhangquan Peng, and Zhonghua Zhang ACS Nano, Just Accepted Manuscript • Publication Date (Web): 30 Oct 2018 Downloaded from http://pubs.acs.org on October 30, 2018

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Scalable Fabrication of Core-Shell Sb@Co(OH)2 Nanosheet Anode for Advanced Sodium Ion Batteries via Magnetron Sputtering Ying Zhang,1,§Hui Gao,1,§Jiazheng Niu,1 Wensheng Ma,1 Yujun Shi,1 Meijia Song,1 Zhangquan Peng,2 Zhonghua Zhang1,* 1Key

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 2State

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

Chinese Academy of Sciences, Changchun, Jilin 130022, P.R. China §The authors equally contributed to this work. *Corresponding author. Email: [email protected] (Z. Zhang)

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ABSTRACT: Antimony (Sb) has captured extensive attention as a promising anode for sodium ion batteries (SIBs) due to its high theoretical capacity and moderate sodiated potential, but is held back for practical applications owing to its pulverization induced by dramatic volumetric variations during (de)sodiation process. Herein, we report a core-shell Sb@Co(OH)2 nanosheet anode fabricated via magnetron sputtering Sb onto the mass-productive Co(OH)2 substrate anchored on stainless steel mesh, which is scalable and suitable for flow-line production. In SIBs, the Sb@Co(OH)2 anode displays the superior rate performance (383.5 mAh/g at 30 A/g), high discharge capacity and excellent stability. Compared with the sputtered Sb film electrode, the improved performance of the core-shell Sb@Co(OH)2 nanosheet anode can be attributed to the open framework of the Co(OH)2 substrate, not only accelerating the ion & electron transfer, but also serving as the buffer for alleviating the volumetric variation and the supporting scaffold for prohibiting the aggregation. More importantly, the (de)sodiation mechanism of the Sb@Co(OH)2 anode was explored by operando (insitu) X-ray diffraction and the similar alloying-dealloying processes (Sb ↔ NaxSb ↔ Na3Sb) for the 1st, 13th, 30th cycles illustrate the excellent stability of the electrode. KEYWORDS: sodium ion batteries, antimony anode, magnetron sputtering, operando (in-situ) X-ray diffraction, sodiation/desodiation mechanism

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Sodium ion batteries (SIBs) have triggered immense scientific and engineering interests as the alternatives to lithium ion batteries (LIBs) in view of the naturally abundant and economic property of Na over Li.1-5 Despite the cathodes for SIBs have exhibited moderate performance based on the design line analogous with that of stateof-the-art LIBs,1, 6-11 the exploration for the high performance anodes has been hindered by the lack of proper electrode materials.2, 12, 13 For example, the graphite used widely in LIBs demonstrates inferior capacity (less than 100 mAh/g) in SIBs, since Na with the larger radius (1.02 Å) can’t easily intercalate into the graphite like Li.14,

15

Furthermore, crystalline Si and Ge with high theoretical capacities for LIBs show irreversible sodium storage without special treatment.16 Hence, the major issue for SIBs is to develop the appropriate anodes with high gravimetric/volumetric capacities and long lifespan via the rational structural & compositional modulations. Additionally, the operando (in-situ) X-ray diffraction (XRD) is employed to real-timely probe the phase evolution of the electrode during the cycles to clarify the (de)sodiation mechanism and provides the sight for designing high performance electrodes.17-19 However, to the best of our knowledge, the operando XRD researches in previous study only revealed the mechanism during initial several cycles, while the operando results after tens of cycles are critical to understand the stability or failure of the electrode in depth. Antimony (Sb) has been advocated as the promising candidate for high-energy density SIBs owing to its high theoretical capacity (660 mAh/g) and moderate sodiation potential (about 0.5 - 0.6 V (vs. Na+/Na)).20, 21 However, the adverse factors of Sb anode in SIBs, including the pulverization and aggregation induced by the large volumetric 3


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variations (about 350%) during the (de)sodiation process, severely impede its practical applications.22, 23 To address the problem, the major efforts are mainly focused on the nanoarchitecture design with hollow/porous framework and the introduction of the carbonate substrate as the conductive network as well as the supporting scaffold, which accelerate the ion & charge transfer, promote the permeation of the electrolyte and alleviate the volumetric change.24, 25 However, these strategies are hard to be applied in industrial manufacture due to the reduction of the volumetric specific capacities, complicated fabrication process and thus the high cost of electrode. Moreover, the traditional electrode synthesis by coating slurry is relatively complex and decreases the gravimetric/volumetric capacities owing to the addition of binder and conductive agent. In contrast, the magnetron sputtering method has been regarded as a facile way to obtain the self-supporting electrode with high gravimetric/volumetric capacity by fabricating the nanoscale compact film without adding any additional material under the industrialized production.26-28 Simultaneously, the thin film electrodes produced by magnetron sputtering are usually flexible, exhibiting the promising potentials for the wearable devices.29, 30 However, the cycling stability of the Sb film electrode fabricated by magnetron sputtering in previous work is inferior compared with traditional electrodes, which can be mainly ascribed to the loose electrical contact between the active

material

and

the

substrate

induced

by

the

large

volumetric

expansion/contraction.31 Hence, it is of significance to modulate the surface of substrate to tighten the connection between the film and the substrate for elevating the electrochemical performance.26 Specially, the substrate with open framework can 4


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efficiently disperse the stress in the active material as well as facilitate the permeation of the electrolyte.32 For instance, Liu et al. reported the significant electrochemical improvement of 3D C/Sn/Ni coated on the TMV1cys substrate compared with 2D Sn thin film on the substrate without modulation.26 Nevertheless, it is still a great challenge to realize the mass-produced surface adjustment of the substrate. Co(OH)2, as a traditional supercapacitor material, was found to be used as anode in LIBs.33-35 Other than conventional substrates (carbon, graphene) for Sb in SIB, a sheet-like structure of Co(OH)2 can enhance ion & electron transfer and contact between active materials and the electrolyte via forming a layered assembly and the Co(OH)2 with superior intercalative pseudo-capacitor behaviors may be beneficial to the rate performance of the whole anode.5, 33, 36, 37 In this work, the core-shell Sb@Co(OH)2 nanosheet anode was fabricated via coating Sb onto the mass-produced Co(OH)2 substrate based on the magnetron sputtering method. Notably, the whole fabrication process is scalable and productive, which is appropriate for the —flow-line production. As an anode for SIBs, it delivers the high discharge capacity (up to 972.6 mAh/g), excellent stability over 200 cycles at 0.2 A/g as well as the superior rate performance (retain 383.5 mAh/g at 30 A/g). Compared with the sputtered Sb film electrode, the improved performance of core-shell Sb@Co(OH)2 nanosheet anode stems from the open framework of the Co(OH)2 substrate, not only promoting the ion & electron transportations, but also serving as the buffer for mitigating the volumetric variation and the supporting network for minimizing the aggregation. Furthermore, the (de)sodiation mechanism of core-shell 5


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Sb@Co(OH)2 nanosheet anode has been detected utilizing the operando XRD and we find the similar two-step (de)sodiation process (Sb ↔ NaxSb ↔ Na3Sb) for the 1st, 13th, 30th cycles, revealing the excellent stability of the electrode.

RESULTS AND DISCUSSION As schematically illustrated in Figure 1a, the liquid Ga film (Step 2) can be painted by special brush (or spray coating in practical industrial production) on a large scale under low operation temperature (approximately 35 °C, higher than the melting point of Ga (29.8 °C)). The annealing temperature for the formation of CoGa3 film (Step 3) is merely 180 °C, significantly reducing the energy consumption. Coupling with the magnetron sputtering method (Step 1 & 5) and large-scale dealloying strategy (Step 4), the fabrication procedure of core-shell Sb@Co(OH)2 nanosheet anode is efficient, productive and environmentally friendly, which is suitable for the flow-line production, while the detailed description is elucidated in experimental section. Moreover, the macrographs of the samples during each step are exhibited in Figure 1b-d. Notably, the bright silvery white Ga film (Step 2, Figure 1b) became dark after the annealing process (Step 3, Figure 1c), illustrating the reaction between the substrate Co and liquid Ga to form CoGa3. Subsequently, the film was blackened after the dealloying process (Step 4, Figure 1c), demonstrating the formation of Co(OH)2 at the expense of CoGa3. After sputtering Sb (Step 5), the as-prepared film is flexible and able to be distorted at any angle (Figure 1d), exhibiting the promising potentials for electrodes in wearable devices. The XRD results in Figure S1 and 2a reveal the pure CoGa3 phase (JCPDS # 656


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5976) and Co(OH)2 phase (JCPDS # 30-0443) after the annealing process (Step 3) and dealloying (Step 4), illustrating the complete alloying process between Co and Ga as well as the transformation from CoGa3 to Co(OH)2, respectively. The XRD patterns (Figure 2a) elucidate that the core-shell Sb@Co(OH)2 nanosheet anode is composed of the Co(OH)2 phase and Sb phase (JCPDS # 35-0732), while the peak (at 28.7°) ascribed to Sb is board, illuminating its amorphous-like characteristic (low crystallinity). Specially, the amorphous state of Sb may boost its (de)sodiation performance due to the merits of amorphous material as following. (1) The short-range structural ordering contributes to Na+ insertion. (2) The amorphous material was found to accommodate lattice distortions without emerging phase transitions, which may lead to the improvement of specific capacities and electrochemical cycling stability.35,

38

The

scanning electron microscopy (SEM) images (Figure 2b-e and Figure S2c-f) display that the Co(OH)2 and core-shell Sb@Co(OH)2 nanosheet electrodes are both composed of the interlaced polygonal sheet with similar diagonals (approximately several micrometers) and distinct thickness. As depicted in Figure S3, the average thickness of core-shell Sb@Co(OH)2 nanosheet is 270 nm which is much larger than that (around 20 nm) of Co(OH)2 substrate, implying the total thickness of Sb coating is approximately 250 nm. Specially, the Sb coating (Figure 2e) consists of uniformly distributed and compactly aggregated particles with sizes of 150 ± 60 nm, while smaller particles (diameter: 30 ± 15 nm) can be observed in a zoom-in SEM image (inset of Figure 2e). Additionally, Figure S2a-b, g-j exhibits the SEM results of Co, Sb and CoGa3 anchored on the SS-mesh, illustrating the smooth surface of Co, Sb and CoGa3 7


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film. The energy dispersive X-ray (EDX) results (Figure S4) demonstrate that the mass fraction of Sb in core-shell Sb@Co(OH)2 nanosheet anode is more than 70 wt.%, which is of importance for its practical application.39 Moreover, the actual atomic ratio of Co and O is close to 1:2, corresponding to the nominal composition of Co(OH)2 substrate. Transmission electron microscopy (TEM) was employed to further probe the microstructure of core-shell Sb@Co(OH)2 nanosheet (Figure 2f-h). The bright-field TEM image (Figure 2f) verifies the polygonal shape of nanosheets with vertex angle of 120 degree, consistent with the SEM results (Figure 2d, e). One hexagonal sheet is displayed as an inset in Figure 2f. Moreover, the high-resolution TEM (HRTEM) image (Figure 2g) reveals two kinds of well-distinguished lattice fringes, corresponding to the crystal plane of (012) for Sb (color: deep yellow) as well as (100) for Co(OH)2 (color: red). The corresponding selected-area electron diffraction (SAED) pattern (Figure 2h) displays the diffraction rings attributed to reflections of the Sb shell as well as the spottype pattern ascribed to the [001]-zone axis of the Co(OH)2. Above-mentioned phenomena not only reveal the single crystalline feature of the Co(OH)2 sheet but also confirm the amorphous-like (nanocrystalline) nature of Sb coating, in good agreement with the XRD results (Figure 2a). In addition, Figure S5 displays more TEM results of the core-shell Sb@Co(OH)2 nanosheets. The electrochemical performance of the core-shell Sb@Co(OH)2 nanosheet anode was explored utilizing the cyclic voltammograms (CVs) and galvanostatic discharge/charge tests. Figure 3a displays the typical curves of initial CV scans at 0.1 mV/s. It is observed that the first cathodic scan differs from the following cathodic 8


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scans and the broad strong peak at 0.44 V (vs. Na+/Na) can be ascribed to the alloying of Sb with Na as well as the irreversible reduction of the electrolyte for the solid electrolyte interphase (SEI) film formation,40 while the peaks centered at 0.89/1.44 V (vs. Na+/Na) correspond to the desodiation process during the anodic scan. From the second scan onward, the CVs profiles are substantially overlapped regardless of the little shift of the peak (at 0.6 V (vs. Na+/Na)) in the second cathodic scan, elucidating the good stability of the electrodes. Based on the CV results of Co(OH)2 and Sb shown in Figure S6, we can conclude that the peaks located at 1.11/1.60 V (vs. Na+/Na) in the cathodic/anodic scans can be attributed to sodiation/desodiation reactions of Co(OH)2, while the peaks situated at 0.37/0.68 V (vs. Na+/Na) in the cathodic scan and 0.89 V (vs. Na+/Na) in the anodic scan can be ascribed to the alloying/dealloying processes of Sb coating (Figure 3a). Notably, the intensities of CV peaks for the Co(OH)2 electrode (Figure S6a) decrease rapidly, revealing the instability of Co(OH)2 during the (de)sodiation process. In contrast, the intensities of CV peaks contributed by Co(OH)2 in the core-shell Sb@Co(OH)2 nanosheet anode retain stable, implying that the framework of Co(OH)2 sheets can be efficiently protected by the Sb coating and inversely stabilize the whole electrode during cycling. Figure 3b demonstrates the rate capability of the core-shell Sb@Co(OH)2 nanosheet and Sb electrodes at stepwise current densities of 0.2, 1, 5, 10, 20, 30 A/g. Compared with the Sb electrode, the significantly improved rate capability of the coreshell Sb@Co(OH)2 nanosheet electrode can be obtained, delivering the discharge capacities of 882.6, 729.2, 645.2, 617.2, 483.3, 383.5 mAh/g at 0.2, 1, 5, 10, 20, 30 A/g, 9


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respectively. When the current density comes back to 0.2 A/g, the discharge capacity of the core-shell Sb@Co(OH)2 nanosheet anode can recover up to 749.1 mAh/g, exhibiting the much better performance in contrast with that (merely 269.4 mAh/g) of Sb. Furthermore, Figure S7 displays the corresponding discharge-charge profiles of the core-shell Sb@Co(OH)2 nanosheet and Sb electrodes at various current densities. Figure 3c exhibits the cycling performance of the core-shell Sb@Co(OH)2 nanosheet anode at 0.2 A/g, as benchmarked with the Sb electrode. The core-shell Sb@Co(OH)2 nanosheet anode delivers superior reversible capacities than the Sb electrode throughout the cycling. For the core-shell Sb@Co(OH)2 nanosheet anode, the high discharge capacity of 972.6 mAh/g can be obtained during the 2nd cycle, much higher than the theoretical capacity (660 mAh/g) of Sb,22, 41 which can be attributed to the additional capacity provided by Co(OH)2 substrate. The discharge capacities of the core-shell Sb@Co(OH)2 nanosheet anode decrease slightly during initial dozens of cycles and subsequently keep stable up to 200 cycles, with the capacity of 645.5 mAh/g at the 200th cycle, exhibiting the excellent long-term lifespan. Figure 3d demonstrates the corresponding discharge-charge curves of the core-shell Sb@Co(OH)2 nanosheet anode in different cycles at 0.2 A/g. The initial discharge/charge capacities are high up to 1460/960 mAh/g with the initial Coulombic efficiency (ICE) of 65.8%, while the irreversible capacity can be attributed to the SEI formation. The voltage profile during the first discharge exhibits only one sloping line, differing from the three voltage plateaus in the second discharge, which is in good coincidence with the CV results (Figure 3a). Notably, the short voltage plateau close to 1.2 V (vs. Na+/Na) starts to 10


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appear during the second discharge, while this plateau can’t be observed on the galvanostatic discharge−charge curves of Sb in Figure S8, implying that Na+ can react with the Co(OH)2 substrate through the Sb coating. Subsequently, the plateau gradually shortens and finally disappears after about 50 cycles, elucidating that the reaction is partly irreversible. Meanwhile, the variations of the overpotential of the discharge−charge profiles are not obvious during the 50th to 150th cycles, confirming the marvelous stability of the Sb@Co(OH)2 electrode. Even at 0.5 A/g (Figure 3e), the core-shell Sb@Co(OH)2 anode also shows the excellent durability and maintains a discharge capacity of 555.9 mAh/g after 180 cycles. Figure S9 exhibits the galvanostatic discharge-charge curves of the Sb@Co(OH)2 anode in different cycles at 0.5 A/g. To assess the capacity contribution from the Co(OH)2 substrate, Figure 3f depicts the cycling performance based on the area specific capacity of the core-shell Sb@Co(OH)2 nanosheet and Co(OH)2 electrodes at the current density of 0.0584 mA/cm2. The initial discharge/charge capacities of the Sb@Co(OH)2 nanosheet and Co(OH)2 electrodes are 0.53/0.32 mAh/cm2 and 0.19/0.09 mAh/cm2, respectively. Subsequently, the reversible capacities of Co(OH)2 drop dramatically to 0.03 mAh/cm2 after merely 50 cycles, accounting for 12.5% of that (0.24 mAh/cm2) for the core-shell Sb@Co(OH)2 anode. After 200 cycles, the reversible capacities of Co(OH)2 further decrease to 0.02 mAh/cm2, responsible for only 9.5 % of that (0.21 mAh/cm2) for Sb@Co(OH)2, elucidating the minor capacity contribution from the Co(OH)2 substrate for the whole Sb@Co(OH)2 electrode. Figure S10 demonstrates the corresponding 11


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galvanostatic discharge-charge curves of the core-shell Sb@Co(OH)2 nanosheet and Co(OH)2 electrodes in different cycles at 0.0584 mA/cm2. The electrochemical impedance spectroscopy (EIS) was utilized to gain the indepth insight into the ion-charge transfer mechanism of the core-shell Sb@Co(OH)2 nanosheet anode. Figure 3g and Figure S11 demonstrate the Nyquist plots of the coreshell Sb@Co(OH)2 anode at open circuit voltage (OCV) as well as after the 1st, 2nd, 5th, 10th and 50th cycles. The equivalent circuits (Figure S12) were employed to model the EIS results. The diffusion coefficients (DNa) of Na+ were calculated in terms of the following equations,42, 43 1

𝑍′ = 𝑅𝑐𝑡 + 𝑅𝑒 +𝜎𝜔 ― 2 𝐷𝑁𝑎 =

𝑅2𝑇2 2𝐴 𝑛 𝐹 𝐶𝑁𝑎2𝜎2 2 4 4

(1) (2)

where R, T, n, A, F, CNa, ω and σ stand for the gas constant, the absolute temperature, the number of electrons per molecule during reaction, surface area of the electrode, the Faraday's constant, the concentration of Na+, the angular frequency and the Warburg factor corresponding to the slopes in Figure 3h, respectively. The relationships between Z' and ω-1/2 within the low frequency range at OCV, 1st and 50th cycles were plotted to calculate the Warburg factor and subsequently determine the DNa (Figure 3h).42 From the results summarized in Table S1, the Rct (charge transfer resistance) of the core-shell Sb@Co(OH)2 nanosheet anode decreases after the first cycle and further reduces after 50 cycles. More significantly, the value of DNa at OCV (8.2×10-16 cm2/s) increases 82 (6.7×10-14 cm2/s) and 146 times (1.2×10-13 cm2/s) after 1st and 50th cycles respectively, illustrating the much faster diffusion of Na+ with the cycling onward. The significant 12


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increase of the DNa can be ascribed to the morphological evolution of Sb@Co(OH)2 nanosheets upon the cycling onward.44 As illustrated in Figure S13, the surfaces of Sb@Co(OH)2 nanosheets after 100 cycles become loose compared with the original compact Sb coating, which is beneficial for the penetration of electrolytes and Na ion transfer.45 The ex-situ SEM (Figure S13) was employed to clarify the structure and morphology of the core-shell Sb@Co(OH)2 nanosheet anode after 100 cycles. The polygonal nanosheets can still be observed, suggesting the outstanding structural stability of the Co(OH)2 substrate which is significant for stabilizing the Sb shell during cycles. Based on the above-mentioned results, the significant improvement of the coreshell Sb@Co(OH)2 nanosheet anode compared with the Sb electrode stems from the following two aspects. (1) The open framework of the Co(OH)2 substrate is not only favorable to the electrolyte penetration and facilitates the ion & electron transportations,46 but also serves as the buffer for alleviating the volumetric expansion/contraction and the supporting network for prohibiting the aggregation. Simultaneously, the reaction between Na+ and Co(OH)2 substrate can contribute additional capacity (around 10%) for the whole electrode. (2) The particles in the Sb coating are in nano scale as indicated in inset of Figure 2e. Such structure can efficiently reduce the distance for the electron & ion transfer and mitigate the volumetric variation due to the size effect.32,

47-53

The “ electron & ion transfer ” process for the

Sb@Co(OH)2 anode is schematically illustrated in Figure S14. The Sb@Co(OH)2 nanosheets can serve as the path for the electron transfer, while the interlayer of 13


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nanosheets is favorable to shuttle sodium ions back and forth. Na ions directly arrive in the whole surface of Sb coatings through fast diffusing between interlayers of nanosheets.54 Meanwhile, electrons move to the current collector along the direction parallel to the polygonal plane. Furthermore, the self-supporting electrode can avoid inadequate ion permeation and electron transfer disruption generated by the addition of binders and achieve fast ion and electron transport.55 To shed light on the (de)sodiation mechanism, the phase evolution of the coreshell Sb@Co(OH)2 nanosheet anode was probed by utilizing the operando XRD technique during the initial discharge-charge-discharge processes between 0.01 − 2.0 V (vs. Na+/Na) at 0.1 A/g (Figure 4). The XRD pattern (black) of the original core-shell Sb@Co(OH)2 nanosheet anode is displayed at the bottom for reference. At the beginning of the discharge, the broad peak centered at 28.7° can be ascribed to the Sb, while the peaks at 19.1°, 32.4° and 37.9° are well indexed to Co(OH)2. Moreover, the peaks at 38.5° and 41.2° can be attributed to the BeO (JCPDS # 35-0818) due to the oxidation of the surface facing to the air of Be window. With continuous discharge in Stage 1 (1.69 – 0.63 V (vs. Na+/Na)), the intensity of peak ascribed to Sb starts to decrease and disappear without appearance of new peak at the end of this stage, indicating the intermediate sodiated product (NaxSb) of Sb is amorphous, which is in good consistence with the previous studies.19, 21, 41, 56 Simultaneously, the peaks indexed to Co(OH)2 begin to weaken and finally vanish, illustrating the sodiated process of Co(OH)2 during the discharge process. In previous researches, the Co(OH)2 was complied with the following reversible process as anode in LIBs.34 14


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Co(OH)2 + 2Li+ + 2e- ↔ Co + 2LiOH

(3)

Regarding the similar physicochemical property between Li+ and Na+,57 we speculate that the sodiated products of Co(OH)2 in Sb@Co(OH)2 may be the low crystalline Co and amorphous NaOH, which are difficult to be detected by XRD due to the lowintensity X-ray signal of amorphous materials and the obstructive influence caused by the thick Sb shell surrounding the Co(OH)2 core. Upon further sodiation process (Stage 2, 0.63 – 0.01 V (vs. Na+/Na)), the peaks (at 18.9°, 21.1°, 33.4°, 34.2°, 38.5° and 39.9°) assigned to Na3Sb appear and increase progressively until the end of discharge, elucidating the complete transformation from Sb to Na3Sb via the amorphous intermediate NaxSb. Meanwhile, the peaks (at 24.6°, 25.3° and 29.8°) belonging to NaBeF3 (JCPDS # 11-0569) appear throughout the discharge/charge process, which is associated with the reaction between the electrolyte and Be window. When being charged back to 2 V (vs. Na+/Na) (Stage 3), the peaks attributed to Na3Sb gradually diminish and disappear during the desodiation process, while the peak ascribed to Sb is regained at the end of charge. The aforementioned results demonstrate the formation of Sb at the expense of Na3Sb and further verify that the immediate (de)sodiated product (NaxSb) between the reversible transformation of Sb and Na3Sb is amorphous. Moreover, no diffraction peak of Co(OH)2 can be probed even under the full charge state, confirming the low-crystalline (de)sodiated products of Co(OH)2. Combined with the EIS results in Figure 3g-h, the (de)sodiated procedure of Co(OH)2 can be regarded as the activation process which efficiently promotes the ion transfer via introducing a dense distribution of nanopores and inducing the size 15


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reduction as previous reports suggest.58, 59 With respect to the second discharge (Stage 4 & 5), the similar scenario, that is the two-step alloying process of Sb → NaxSb → Na3Sb, is observed, further revealing that the Sb reacts with Na to form Na3Sb via the amorphous intermediate product NaxSb. Additionally, the contour plot for the operando XRD results of the core-shell Sb@Co(OH)2 nanosheet anode is displayed in Figure 4a, vividly demonstrating the sodiation/desodiation procedure during the initial cycles. It should be noted that, to the best of our knowledge, the previous operando XRD studies are restricted to the initial cycles while the exploration for the (de)sodiation mechanism after dozens of cycles is of great importance to learn the variation of the structural & phase evolution processes between different cycles under the long-term electrochemical impact and obtain deep understanding of the stability or failure of the electrode. Herein, the (de)sodiation processes of the core-shell Sb@Co(OH)2 nanosheet anode at the 13th and 30th cycles were probed by operando XRD method (Figure 5). Similar to the phenomenon during the first cycle, the discharge procedures of the Sb@Co(OH)2 anode during the 13th and 30th cycles are associated with the two-step alloying process (Sb → NaxSb → Na3Sb), while the final products after the charge process are both Sb, implying the excellent reversible property and thus marvelous stability of the core-shell Sb@Co(OH)2 nanosheet anode. Specially, the peak of Sb before 30th cycle (Figure S15) becomes stronger and thinner than those before the 1st and 13th cycles, illustrating the increasing crystallinity of Sb with continuous cycles.

CONCLUSIONS 16


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In summary, the core-shell Sb@Co(OH)2 nanosheet anode anchored on SS-mesh was successfully synthesized by sputtering Sb onto the mass-produced Co(OH)2 substrate, which is scalable and suitable for flow-line production. Specially, the proposed strategy efficiently takes advantages of the excellent stability and open framework of the Co(OH)2 substrate and the marvelous Na storage capacity of Sb together. As an anode in SIBs, the Sb@Co(OH)2 anode exhibits the high discharge capacity (up to 972.6 mAh/g), excellent stability over 200 cycles at 0.2 A/g as well as the superior rate performance (383.5 mAh/g at 30 A/g). As benchmarked with the Sb film electrode, the improved performance of the core-shell Sb@Co(OH)2 nanosheet anode stems from the open framework of the Co(OH)2 substrate, not only accelerating the ion & electron transportation, but also serving as the buffer for mitigating the volumetric variation. Meanwhile, the (de)sodiation mechanism of the core-shell Sb@Co(OH)2 nanosheet anode was probed by the operando XRD. The similar alloying-dealloying processes (Sb ↔ NaxSb ↔ Na3Sb) can be observed for the 1st, 13th, 30th cycles, confirming the excellent stability of the Sb@Co(OH)2 electrode. The present results elucidate the promising potentials of the core-shell Sb@Co(OH)2 nanosheet anode for high energy density SIBs, and provide more insight into the (de)sodiation mechanism for Sb-type anodes.

EXPERIMENTAL SECTION Material Preparation The Sb, Co targets and Ga were purchased from Purui Material Instrument (Co., 17


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Ltd, China) with purity of 99.99 wt.%. In the typical synthesis of Co(OH)2 substrate, the stainless steel mesh (SS-mesh, stainless steel 304, thickness: 0.1 mm) was ultrasonically cleaned in acetone for 20 min and then dried at 80 °C for 1h in vacuum before being loaded onto the sample platform in vacuum chamber. Afterwards, the chamber was pumped down to a base pressure of less than 5 × 10−4 Pa and subsequently the working pressure was fixed at 1 Pa (for all case) under the Ar flow of 28 standard cubic centimeter per minute (sccm). The Co was sputtered onto the SS-mesh to prepare Co@SS-Mesh using a direct current (DC) magnetron sputtering method at 100 W for 1 h. Then the Ga ingot (purity, 99.99 wt.%) was melted at 80 °C and obtained liquid Ga was painted onto the Co film utilizing the brush at the temperature of 35 °C. In the next moment, the sample was annealed at 180 °C for 6 h in resistance furnace to form GoGa3 film and subsequently dealloyed in 0.5 M NaOH for 9 h to obtain the Co(OH)2 substrate. Furthermore, the core-shell Sb@Co(OH)2 nanosheet anode was fabricated by sputtering Sb onto the Co(OH)2 substrate using a radio frequency (RF) magnetron sputtering apparatus at 100 W for 1.5 h, while the Sb electrode (anchored on the SSmesh) was prepared via directly coating Sb onto the SS-mesh based on the similar parameters. For ex-situ SEM tests, the electrode upon charge state after 100 cycles was disassembled from the cell and subsequently washed several times with a dimethyl carbonate (DMC) solution. After several minutes for drying, the electrode was taken out of the glove box and then probed by the SEM.

Microstructural Characterization 18


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The XRD results of the as-prepared samples were characterized by an XD-3 diffractometer (Beijing Purkinje General Instrument Co., Ltd, China) equipped with Cu Kα radiation. SEM (ZEISS, SIGMA 300) and TEM (JEOL JEM-2100) were employed to probe the microstructures of the samples. Chemical composition of the core-shell Sb@Co(OH)2 nanosheet anode was determined using an EDX analyzer.

Electrochemical Measurements The as-prepared electrodes can be directly used as the working electrode. Na foil (purity, 99.99 wt.%, Sigma-Aldrich) was used as both the counter & reference electrodes, while glass fiber (GF/D, Whatman) was employed as a separator. The electrolyte was fabricated by dissolving 1 M NaClO4 in propylene carbonate (PC) with 5 wt.% addition of fluoroethylene carbonate (FEC). The CR2032 cell was assembled in an argon-filled glove box and subsequently aged overnight before tests. Galvanostatic charge/discharge profiles were obtained by the test system (LANDCT2001A, Wuhan, China) over 0.01 - 2.0 V (vs. Na+/Na). CVs were identified by a CHI 660E potentiostat at a scan rate of 0.1 mV/s between 0.01 – 2.0 V (vs. Na+/Na). EIS measurements were carried out by a Zahner Zennium potentiostat in the full charged state (2.0 V (vs. Na+/Na)). The frequency was set from 10 mHz to 100 kHz with an excitation voltage of 5 mV. Additionally, a CR2016 coin cell equipped with one side Be window (10 mm in diameter) was utilized for operando XRD. The operando XRD patterns were obtained through the Be window of the coin cell, cycled by the test system at 0.1 A/g for the initial discharge-charge-discharge and 0.05 A/g for the 13th & 19


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30th cycles within 0.01 - 2.0 V (vs. Na+/Na).

ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website at DOI: XRD patterns and SEM images of the SS-Mesh, Co@SS-Mesh and CoGa3@SS-Mesh; Size distributions; SEM, corresponding EDX and TEM results of Sb@Co(OH)2; Some cyclic voltammograms, galvanostatic discharge−charge curves, cycling performances of Sb@Co(OH)2, Sb, Co(OH)2 electrodes for reference; Nyquist plot and corresponding equivalent electrical circuits of Sb@Co(OH)2; Ex-SEM image of Sb@Co(OH)2 after 100 cycles; XRD patterns of Sb@Co(OH)2 before 1st, 13th and 30th cycles; Fitting results for EIS data.

ACKNOWLEDGMENTS The authors gratefully acknowledge financial support by National Natural Science Foundation of China (51871133, 51671115), and the support of Department of Science and Technology of Shandong Province for Young Tip-top Talent Support Project.

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Figure 1. (a) Schematic illustration exhibiting the fabrication process for the core-shell Sb@Co(OH)2 nanosheet anode anchored on SS-mesh. Notably, the method is scalable and productive, which is suitable for the flow-line production. (b-d) The macrographs of the samples of (b) Step 1 & 2, (c) Step 3 & 4 and (d) Step 5. The macrographs in insets of (d) show the flexible, self-supported characteristic of the Sb@Co(OH)2 nanosheet anode.

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Figure 2. (a) XRD patterns of the core-shell Sb@Co(OH)2 nanosheet and Co(OH)2 electrodes anchored on SS-mesh. (b-e) SEM images of the (b,c) Co(OH)2 and (d,e) core-shell Sb@Co(OH)2 nanosheet electrodes. Inset of (e) is the SEM image at a higher magnification. (f) TEM, (g) HRTEM images and (h) SAED pattern of the core-shell Sb@Co(OH)2 nanosheet anode. Inset of (f): a hexagonal Sb@Co(OH)2 nanosheet.

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Figure 3. (a) Cyclic voltammograms of the core-shell Sb@Co(OH)2 nanosheet anode at a scan rate of 0.1 mV/s over a potential window of 0.01 - 2.0 V (vs. Na+/Na). (b) The comparison of the electrochemical performances between the core-shell Sb@Co(OH)2 nanosheet and Sb electrodes: rate capability at various current densities. (c) Cycling performances of the Sb@Co(OH)2 and Sb electrodes at 0.2 A/g. (d) Galvanostatic discharge-charge curves of the Sb@Co(OH)2 anode in different cycles at 0.2 A/g. (e) Cycling performance of the core-shell Sb@Co(OH)2 nanosheet anode at 0.5 A/g. (f) Cycling performances of the core-shell Sb@Co(OH)2 nanosheet and Co(OH)2 electrodes at 0.0584 mA/cm2. (g) Nyquist plots of the core-shell Sb@Co(OH)2 nanosheet anode at OCV as well as after the 1st, 2nd, 5th, 10th, 50th cycles. (h) Relationships between Z' and ω-1/2 in the low frequency range at OCV and after 1st & 50th cycles.

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ACS Nano

Figure 4. (a) Contour plot and (b) line plot of the operando XRD results of the coreshell Sb@Co(OH)2 nanosheet anode during the initial discharge-charge-discharge processes. The discharge (stage 1 & 2) - charge (stage 3) - discharge (stage 4 & 5) profiles of the core-shell Sb@Co(OH)2 nanosheet anode at 0.1 A/g within 0.01 – 2.0 V (vs. Na+/Na) and the XRD result of original sample are exhibited for reference.

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Figure 5. The operando XRD results of the core-shell Sb@Co(OH)2 nanosheet anode during the discharge-charge processes upon (a) the 13th cycle and (b) 30th cycle. The discharge (stage 1 & 2) - charge (stage 3) profiles of the core-shell Sb@Co(OH)2 nanosheet anode at 0.05 A/g within 0.01 – 2.0 V (vs. Na+/Na) and the XRD results of original sample are exhibited for reference.

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Table of contents entry:

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Nanosheet Anodes for Advanced Sodium-Ion Batteries

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