Insight on Failure Mechanism of Sn Electrodes for Sodium-Ion

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Insight on Failure Mechanism of Sn Electrodes for SodiumIon Batteries: Evidence of Pore Formations during Sodiation while Crack Formation during De-sodiation Tao Li, Umair Gulzar, Xue Bai, Marco Lenocini, Mirko Prato, Katerina E. Aifantis, Claudio Capiglia, and Remo Proietti Zaccaria ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01934 • Publication Date (Web): 08 Jan 2019 Downloaded from http://pubs.acs.org on January 14, 2019

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ACS Applied Energy Materials

Insight on Failure Mechanism of Sn Electrodes for Sodium-Ion Batteries: Evidence of Pore Formations during Sodiation while Crack Formation during De-sodiation Tao Li,†‡ Umair Gulzar,*†‡ Xue Bai,†§ Marco Lenocini,† Mirko Prato,† Katerina E. Aifantis,¶ Claudio Capiglia,*#¥ and Remo Proietti Zaccaria∥† †Istituto

Italiano di Tecnologia, via Morego 30, Genova 16163, Italy

‡DIBRIS, §Key

University of Genova, via Opera Pia 13, Genova 16145, Italy

Laboratory for Liquid−Solid Structural Evolution & Processing of Materials (Ministry of

Education), Shandong University, Jinan 250061, China ¶

Mechanical and Aerospace Engineering, University of Florida, Gainesville, FL 32611, USA

#Recruit

R&D Co., Ltd., Recruit Ginza 8 Bldg. 8-4-17, Ginza Chuo-Ku, Tokyo 104-8001, Japan

¥Nagoya

Institute of Technology, Gokiso-cho, Showa-ku, Nagoya, Aichi, 466-8555, Japan

∥Cixi

Institute of Biomedical Engineering, Ningbo Institute of Materials Technology and

Engineering, Chinese Academy of Sciences, Ningbo 315201, China *Corresponding authors Umair Gulzar [email protected]; [email protected] Claudio Capiglia [email protected], [email protected]

KEYWORDS: sodium-ion battery, Sn anode, failure mechanism, porous structure, mechanistic study, impedance spectroscopy, diffusion coefficient

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ABSTRACT: The development of Sn based anode materials for sodium ion batteries is mainly hindered by the limited understanding of sodiation/de-sodiation mechanisms inside the active material, which typically results in electrode damage. Herein, we report a post-mortem ex-situ scanning electron microscopic analysis of Sn thin film motivated by the intention to elucidate these structural mechanisms. Our results reveal for the first time that the surface of Sn electrode film becomes highly porous during sodiation with no presence of obvious crack, a surprising result when compared to previous reports performed on Sn particles. Even more surprisingly, sequential ex-situ SEM observations demonstrate that once the de-sodiation starts and reaches the second desodiation plateau (0.28V), obvious cracks in the Sn film are instead observed along with porous islands of active material. These islands appeared as aggregated particles which further split into smaller islands when the de-sodiation potential reaches its maximum value (2.0V). Finally, for the first time the experimental value of sodium diffusion coefficient inside Sn was measured (3.9 × 10−14 cm2 s−1) using electrochemical impedance

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1. INTRODUCTION Sodium-ion batteries (SIBs) are being considered a viable replacement to existing lithiumion batteries (LIBs) for stationary applications.1 In search of electrode materials for SIBs, hard carbon has shown a reasonable capacity of ~300 mAh g−1 and being considered the “First Generation” anode for sodium ion systems.2 Due to limited capacity of hard carbon, alloying material, preferentially tin (Sn) have been proposed as a high capacity alternative (847 mAh g−1 at Na15Sn4) for SIBs.3 However, Sn based electrodes suffer from large volumetric expansion (~420%) which leads to severe mechanical stress causing electrode degradation.4 Moreover, an important challenge to overcome in SIBs is the sluggish reaction kinetics due to the large ionic radius of Na ion (102 pm) which impedes fast sodium transport.5 Great efforts have been devoted to solve these issues by engineering various nanostructured Sn 6,7 or Sn/carbon nanocomposites.8–17 Nevertheless, it is quite important to understand the nature of damage in Sn based electrode during the process of sodiation/de-sodiation. Many theoretical and experimental works have been conducted to understand the sodium storage mechanisms in Sn anodes to establish their electrochemical reactions as well as their viability as anodes in SIBs18–23. Recent efforts in this direction were done by Huang et al.24 who investigated the structural evolution and phase transformation of Sn nanoparticles using in-situ transmission electron microscopy (TEM). Negligible fractures were observed during multiple sodiation/de-sodiation of Sn particles below ~200 nm. Similar sodiation/de-sodiation experiments performed using in-situ synchrotron hard Xray nanotomography could not detect cracks in Sn particles with dimensions below 500 nm.25 By comparing sodium insertion with its lithium counterpart, authors concluded that sodiation process results in large volume expansion and fracture of nanoparticles, whereas de-sodiation only induces the volume shrinkage with negligible pulverization and maintains microstructural integrity. All 3



these observations are in striking contrast with the electrochemical behaviour of Sn when cycled against lithium, where de-lithiation causes severe structural damage. Moreover, phase diagrams for Li-Sn and Na-Sn reveal that the melting points of Na-Sn compounds are substantially lower than the Li-Sn analogues,26 therefore, plasticity mechanisms might be more active in the Na-Sn electrodes. A different approach was applied by Datta et al.27 where an ex-situ morphological study was performed on Sn/C composite electrodes (Crystal size: ~90 nm) after 20 cycles. Unfortunately, the results were not conclusive, and the lack of clear evidence was associated to the cell environments where results are compromised by the introduction of multiple particle sizes distributions, binding materials and conductive matrix covering the active particles. A morphological and electrochemical characterization of carbon and binder free Sn electrode, on a macroscopic level, is important to understand the actual mode of damage in Sn based electrodes. The information could indeed be helpful for designing carbon and binder free Sn electrodes as already occurred for lithium-ion batteries.28,29 Moreover, the planar geometry associated to the macroscopic level electrode, will experience a relatively simple state of stress produced by one-dimensional gradient in sodium concentration allowing the current density, used in the cycling, to be easily interpreted. In the present work, a detailed morphological characterization of Sn thin film electrodes before and after sodiation is presented in order to understand the mode and evolution of damage using ex-situ scanning electron microscopy (SEM). Contrary to lithiation/de-lithiation inside Sn particles25,30 and Si nanowires31,32 where pore formation only occurs during the process of de-lithiation, our results revealed that Sn electrode, during sodiation, undergoes a volumetric expansion accompanied by the swelling of electrode surface along with pore formations (pore size: 0.2–1.8 m). On the other hand, de-sodiation of the electrode up to 0.28 V showed severe cracks along the lateral size of the electrode resulting in 4


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~50 µm islands of active material which seems to be an aggregation of ~5 µm particles anchored to each other. Finally, electrochemical impedance spectroscopy was used to calculate the first experimental value of Na diffusion inside thin film Sn electrode. 2. EXPERIMENTAL SECTION 2.1. Materials Preparation Sn thin film of around 1 µm in thickness was deposited on a large piece of copper (Cu) foil substrate (Sigma Aldrich) by physical vapor deposition (PVD) method using Sn foil (purity of 99.998%, purchased from Sigma Aldrich Co, LLC) as the tin source. Small disks of 15 mm in diameter were cut from as-prepared large piece of Sn thin film and stored in a MBraun glove box under Ar atmosphere with a water and oxygen content less than 0.1 ppm before making a cointype cell. 2.2. Electrochemical Cycling The electrochemical performance of the as-prepared Sn thin film electrode was evaluated using R2032-type coin cells. The Cu foil substrate served as the current collector to obtain a uniform current distribution. The assembly of the sodium half-cell was conducted in the Ar-filled glove box with sodium metal employed as the counter and reference electrode, Whatman fibre glass as separator, and 1.0 M of NaClO4 in propylene carbonate (PC) as electrolyte. Galvanostatic cycling of the cells at the low current of 20 A (C/40 in rate) was carried out within a voltage range of 0.005−2.0 V (vs. Na+/Na) using Biologic BCS-805 multichannel battery test system at room temperature.

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2.3. Characterization Powder X-ray diffraction (XRD) was carried out utilizing a Rigaku Smart Lab 9 kW diffractometer equipped with Cu Kα X-ray source operated at 40 kV and 150 mA. X-ray photoelectron spectroscopy (XPS) analysis has been carried out with a Kratos Axis UltraDLD spectrometer (Kratos Analytical Ltd., UK) using a monochromatic Al Kα source (hν = 1486.6 eV) operated at 20 mA and 15 kV. The analyses have been carried out on 300 × 700 μm area. Scanning electron microscopy (SEM) images were obtained using a JEOL JEM 6490LA (Jeol, Tokyo, Japan) equipped with a cold FEG, operating at 15 kV acceleration voltage. Both XRD and SEM analysis of the tin film electrode before and after cycling were conducted. In particular, the after cycling analysis was conducted on coin cells completely discharged/charged which were disassembled inside Argon filled glove box with H2O and O2 level below 0.1 ppm. The Sn film electrodes were washed with PC and then dried at 50 °C for 3 h. The resulting dried electrodes were carefully kept in a vial before performing XRD and SEM studies. 3. RESULT AND DISCUSSION Sn thin films were prepared by PVD method on a battery grade Cu foil substrate and cut into circular disks with surface area of 1.76 cm2. Surface morphologies of the Cu substrate and asprepared Sn thin film are shown in Figure 1a, b, respectively. In particular, Figure 1b shows a dense and sinusoidal surface composed of coarse particles with diameter in the range 3–4 m while Figure 1c shows a cross-sectional image of Sn thin film (1 µm thickness) deposited on top of 25 m thick Cu substrate.

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Figure 1. SEM images describing the surface morphology of a) copper substrate, b) Sn thin film located on top of the Cu substrate and (c) the cross section of the as-prepared Sn thin film by PVD method on Cu foil substrate. The thickness of Sn film is around 1 m, as shown in (c). (d) XRD pattern of the Sn thin film electrode before cycling.

It is well-known that the crystal structure and the crystallographic orientation of the deposited material depends on experimental conditions and the crystal orientation of the substrate.33 Therefore, XRD analysis was performed to understand the crystallographic structure of the asprepared Sn film. The XRD patterns displayed in Figure 1d reveal that the main diffraction peaks well match with the reference Sn phase (JCPDS No. 04-0673). As expected, the Sn film shows a preferred crystalline orientation along [200] direction while peaks corresponding to the other planes have limited amplitudes, which can be associated to the growth mechanism under our experimental conditions. Moreover, no peaks of SnO2 or SnO were identified during the XRD analysis which may suggests the amorphous nature of SnO2 or SnO. Therefore, XPS analysis was carried out to confirm the presence of Sn based oxide layer. Data collected over the typical range for Sn 3d peaks are reported in Figure S1, together with the outcome of the fitting procedure. The 7



best fit has been obtained with three sets of spin-orbit split doublets, representative of the different oxidation states of Sn at the surface of the electrode. The splitting between the two components of each doublet has been set to 8.4 eV34 while the intensity ratio between the two components is set to 3:2 due to spin-orbit coupling. The position of each Sn 3d doublet will be conveniently identified by the position of the 3d5/2 component, i.e. the component within the doublet centred at lower binding energy and showing higher intensity. Here, the “green” doublet (Sn3d5/2 at (484.7±0.2) eV) is representative of metallic Sn of the electrode, the “cyan” doublet (Sn3d5/2 at (486.0±0.2) eV) is representative of SnO species and the “blue” doublet (Sn3d5/2 at (487.1±0.2) eV) is representative of SnO2 species, in nice agreement with previously reported results.35 The presence of the metallic Sn related peaks is an indication that the thickness of the oxide layer is quite thin. Indeed, by using the formula from Powell et al.36 we calculated an inelastic electron mean free path λ of approx. 1.8 nm in SnO2 at the kinetic energy of the electrons photoemitted from metallic Sn. Since the probing depth of XPS is in the order of 3λ, the XPS data suggest that the thickness of the Sn oxide layer is well below 6 nm. Electrochemical characterization was performed by cycling as-prepared electrodes against freshly cut sodium reference electrode in a coin cell configuration. The employed electrolyte was formed by 200 L of 1.0 M NaClO4 in PC with a mass loading for 1 m thin layer of tin equal to 0.9 mg/cm2.

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Figure 2. a) Differential capacity plot obtained from galvanostatic charge/discharge profile showing distinct peaks of various phases formed during sodiation and de-sodiation. b) Galvanostatic charge/discharge profile of the asprepared Sn film (mass loading: 0.9 mg/cm2) cycled at 20 A. The black arrows in (b) indicate the electrochemical potentials at which SEM images were taken.

The sodiation of Sn is known to occur in four distinct steps corresponding to the characteristic bi-phasic reactions27,33,37,38. To observe and investigate each sodiation step, galvanostatic cycling were performed at low current density of 11.4 A/cm2 (C/40 in rate) in the voltage range of 0.005−2.0 V (vs. Na+/Na). The differential capacity plot of the Sn thin film electrode obtained from galvanostatic charge/discharge profile shown in Figure 2a clearly confirms the four distinct sodiation peaks at 0.36, 0.19, 0.08 and 0.035 V along with the de-sodiation peaks at 0.14, 0.25, 0.53 and 0.58 V. However, a close inspection shows an additional broad sodiation peak at 0.41 V (Figure S2a) which can be associated to the conversion of SnO2 layer to Sn.37 The voltage plateaus in the first discharge/charge profile (Figure 2b) are in general agreement with the density functional theory (DFT) calculations by Chevrier et al.39 However, due to the high polarization, the discharge/charge potentials of each experimental plateaus are considerably shifted compared to DFT calculated voltages. Similar results were reported in the study of sodiation/de-sodiation mechanism of Sn film electrodes prepared by electrodeposition and magnetron sputtering.7,22,37 9



Interestingly, potential dips before each bi-phasic reaction were observed during the sodiation process along with a noisy voltage profile at the end of the de-sodiation process (Figure 2b and S2b), which might be related to various complex phenomenon including pore formation, continuous electrolyte decomposition and improved diffusion in amorphous NaSnx phases. All these detrimental effects might indeed be the cause of the rapid capacity decay observed in the second cycle, therefore, suggesting the pulverization of Sn electrodes (Figure S2c). To understand the degradation mechanism of Sn electrode, ex-situ SEM analysis was performed at different stages of the first sodiation and de-sodiation process. Figure 3 shows the SEM images of Sn electrode before and after complete sodiation (0.005 V).

Figure 3. SEM images of Sn thin film a) before cycling, b) after full sodiation at 0.005 V. c) and d) are both magnified images of (b) showing the porous structure of Sn film (c) with nanoparticles on the surface (d).

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It is quite evident that the uneven surface of Sn film (Figure 3a) consists of 3–4 m particles separated by valleys containing smaller particles of ~1 m. Once the electrode is completely sodiated up to 0.005 V, its surface becomes highly porous and inflated, suggesting severe volumetric expansion, which might be due to the formation of Na15Sn4 (Figure 3b). EDS analysis of sodiated Sn electrode show higher Na/Sn ratio compare to de-sodiated electrodes, inferring to the sodiation and de-sodiation process of Sn electrode (Figure S3). Differently from the previous report by Wang et al.25 no obvious cracks along the lateral surface of electrode were observed. However, after close inspection of the porous electrode, small cracks within the pores were identified which may be associated to inhomogeneous expansion of Sn electrode during sodiation (Figure 3c). This inhomogeneous expansion can be caused by the different crystallite size on Sn films providing different diffusion pathways. Large number of nanometer size particles were also observed on the interior surface of the pores along with additional micropores. The sizes of the small pores were in the range of 0.2–1.8 m (Figure 3d) while the nature and cause of small particles is still unknown. These results suggest that the process of sodiation causes severe expansion of the electrode making it porous without breaking or cracking the surface of the electrode. It is also interesting to note that the Sn film electrode undergoes volumetric expansion accompanied with large amount of pore formations during sodiation process. However, in case of lithiation/de-lithiation inside Sn particles 25,30 and Si nanowires 31,32 pore formation occurred only during the process of de-lithiation. Chao et al.30 attributed these pore formations inside Sn electrodes to the re-crystallization process of Sn while the observation of pore formation inside Si nanowires during de-lithiation can be interpreted as a result of the fast lithiation/de-lithiation kinetics.31 In our study, expansion of Sn electrodes and pore formations can be attributed to the

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gases produced during the electrolyte decomposition. However, further studies are required for comprehensive understanding of micropore formation during sodiation. The destruction of Sn electrode during sodiation process is not evident and, therefore, must be happening during the process of de-sodiation. To verify this hypothesis, we obtained the SEM images of electrodes at different stages of de-sodiation. During de-sodiation, Na15Sn4 with a volumetric expansion of 420% is expected to convert into Na9Sn4 with a volumetric expansion of 252%.24 However, SEM analysis performed after de-sodiation of electrodes up to 0.16V show no considerable evidence of cracking (Figure 4a and b) and suggests that volumetric contraction of 168% is not enough to produce cracks under these experimental conditions. Nevertheless, further de-sodiation of the electrode up to 0.28 V showed severe cracks along the lateral size of the electrode resulting in ~50 µm islands of active material (Figure 4c). Interestingly, each island seems to be an aggregation of ~5 µm particles one another anchored (Figure 4d). Upon complete de-sodiation process up to 2 V, the size of the islands became smaller (~5 µm) while the active material starts to peel off from the substrate surface (Figure 4e and f). We suggest that these smaller islands might be the same particles shown in Figure 4d which are just de-aggregated upon further de-sodiation making islands of the similar size. Furthermore, these smaller islands are porous and contain small particles anchored to their surface.

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Figure 4. SEM images of Sn thin film electrodes at different stages of the first de-sodiation process up to 2.0 V: a), b) at 0.16 V; c), d) at 0.28 V; e), f) at 2.0 V.

A further indication of the severe electrode destruction and islands formation is provided by the noisy voltage profile at the end of de-sodiation process shown in Figure S2c, also accompanied by a rapid decay in the capacity after the first cycle. This fast decay is due to the loss of electrical contact between active material and current collector, with particles maintaining the same size after multiple cycling (Figure S4).

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To correlate the structural evolution of Sn electrode with its electrochemical behaviour, electrochemical impedance spectroscopy (EIS) was performed at the same potentials where SEM analyses were carried out.

Figure 5. Impedance spectra (Nyquist plots) obtained a different stage of sodiation and de-sodiation. a) at OCV, b) fully sodiated at 0.005 V and de-sodiation at c) 0.16 V, d) 0.28 and 2.0 V with the high frequency region as inset.

The Faradaic impedance of an electrode material can be separated into kinetic (Zk) and diffusive impedance (Zd). Due to the conductive nature of Sn, kinetic impedance (Zk) can be represented by the charge transfer resistance, whereas diffusive impedance (Zd) describes the contribution from the concentration overpotential, hence it depends on the transport phenomena of the sodium ions.40 Moreover, Nyquist diagram of the Faradaic impedance consists of two distinct domains with semicircles in the high and low frequency regions and a straight line in the low frequency region.41 As expected, the Nyquist plot of the Sn electrodes obtained at OCV (before 14


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sodiation) shows a high frequency (HF) small arc followed by a large arc in the low frequency (LF) region. The HF arc (Figure 5a, inset) can be associated to the charge transfer resistance while the second arc at LF can be associated to the blocking character of non-sodiated electrode along with phase transformations at very low frequency regions (Figure 5a). Usually, the arc at LF is absent in traditional intercalation materials such as LiCoO2 or graphite due to small energetic differences between two different phases. However, large variations in physical and chemical properties of Sn and NaSnx phases cause two different time constants for sodium ion transference allowing a new arc to appear at LF region.42 Furthermore, the Nyquist plot obtained at full sodiated state (0.005V) shows a depressed semicircle and a reduced straight line (Figure 5b). It is quite interesting that the LF arc observed at OCV transforms into one straight line as the electrode potential reaches 0.005 V. This observation suggests that the high diffusive resistance associated to solid Sn is reduced significantly due to the porous structure of electrode making the straight line to appear at lower values of resistance. Moreover, the high surface area enhances the migration predisposition of the active material, resulting in a minimized polarization. Benefiting from the impedance data obtained at the LF region, the diffusion coefficient of sodium inside the electrode was calculated at 0.005 V using a model proposed by Ho et al.43 according to the following equation:

[( )( )]

1 𝑉𝑚 𝑑𝐸 𝐷𝑁𝑎 = 2 𝑧𝐹𝐴∂ 𝑑𝑥

2

where Vm is the molar volume of Sn (16.340 cm3/mol), z is the charge of sodium ions, F is the Faraday constant (96,485 C mol−1), A is the surface area of the Sn electrode (1.76 cm2) , ∂ is the slope of the plot obtained from the imaginary part of impedance (Zim) vs inverse square root of the 15



angular frequency (ω−1/2) and dE/dx is the slope obtained from galvanostatic charge/discharge capacity plot. The approximate diffusion coefficient for sodium ions inside our porous Sn electrode at 0.005V was found to be 3.9 × 10−14 cm2 s−1. This diffusion coefficient of Na ions in Sn is tempted to be the first experimental value obtained, which is much lower than the theoretical values that have been already reported in the literature44,45 and also lower than the experimental values for Li ion diffusion in Sn46,47 due to the larger radius of Na ion than Li ion. The Nyquist plot obtained during the de-sodiation process (0.16 V) clearly shows a wider depressed semi-circle than EIS at 0.005 V sodiation, suggesting the existence of multiple phases of Na15Sn4 and Na9Sn4 with higher charge transfer resistance (Figure 5c). It also indicates the possible presence of structural damage, such as cracks, which lead to the non-ideal behaviour of the electrode surface. Once the electrode potential reaches 0.28 V (Figure 5d), a small arc at HF region reappears followed by an enhanced straight line. The enhancement of the straight line indicates the blocking characteristic of de-sodiated Sn phases which appears after the turning of the film Sn electrode into small islands, therefore determining a direct contact between the copper substrate and sodium ions. Similar results are described also by the Nyquist plot at 2.0 V when the Sn film starts to peel off. 4. CONCLUSIONS We have explored the mode of damage mechanism of binder and carbon free thin film Sn electrode during sodiation/de-sodiation process. Ex-situ SEM was used to study the morphological features of the Sn electrode at different stages of sodiation and de-sodiation. Our results show considerable swelling of Sn film electrode at the end of the sodiation process, however no cracks were observed on the Sn films, a novel result with respect to previous reports.25 Cracks start instead 16


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appearing during the de-sodiation process at potential of 0.28 V where the Sn electrode forms islands of active material. Upon further de-sodiation these aggregated islands crumbled to form even small islands. Finally, all these observations were correlated with impedance spectroscopy which was also used to calculate the diffusion coefficient of Na inside Sn electrode.

ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications websites. AUTHOR INFORMATION Corresponding Authors *Umair Gulzar E-mail: [email protected]; [email protected] *Claudio Capiglia Email: [email protected], [email protected]

Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS Authors would like to acknowledge the support from the 3315 project, grant Y70001DL01, Ningbo, China. REFERNCES (1)

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Graphical Abstract

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Insight on Failure Mechanism of Sn Electrodes for Sodium-Ion

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