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Fabrication of SnS2/Mn2SnS4/Carbon Heterostructures for SodiumIon Batteries with High Initial Coulombic Efficiency and Cycling Stability Xing Ou, Liang Cao, Xinghui Liang, Fenghua Zheng, Hong-sheng Zheng, Xianfeng Yang, Jeng-Han Wang, Chenghao Yang, and Meilin Liu ACS Nano, Just Accepted Manuscript • Publication Date (Web): 20 Feb 2019 Downloaded from http://pubs.acs.org on February 20, 2019
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Fabrication of SnS2/Mn2SnS4/Carbon Heterostructures for Sodium-Ion Batteries with High Initial Coulombic Efficiency and Cycling Stability
Xing Ou,a,§ Liang Cao,a,§ Xinghui Liang,a Fenghua Zheng,a Hong-Sheng Zheng,b Xianfeng Yang,c Jeng-Han Wang,b Chenghao Yang,a,* Meilin Liua,d
a
Guangzhou Key Laboratory for Surface Chemistry of Energy Materials, New Energy Research
Institute, School of Environment and Energy, South China University of Technology, Guangzhou 510006, P. R. China b
Department of Chemistry, National Taiwan Normal University, Taipei, 11677, Taiwan
c
Analytical and Testing Center, South China University of Technology, Guangzhou, 510641 P. R.
China d
School of Materials Science & Engineering, Georgia Institute of Technology, Atlanta,
GA30332-0245, USA
Corresponding author. Tel.: +86-803-39381203; E-mail:
[email protected] (C. Yang).
§ These
authors contributed equally.
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Abstract SnS2 has been extensive studied as an anode material for sodium storage owing to its high theoretical specific capacity, whereas, the unsatisfied initial coulombic efficiency (ICE) caused by the partial irreversible conversion reaction during the charge/discharge process is one of the critical issues that hamper its practical applications. Hence, heterostructured SnS2/Mn2SnS4/carbon nanoboxes (SMS/C NBs) have been developed by a facial wet-chemical method and utilized as the anode material of SIBs. SMS/C NBs can deliver an initial capacity of 841.2 mAh g-1 with high ICE of 90.8%, excellent rate capability (752.3, 604.7, 570.1, 546.9, 519.7 and 488.7 mAh g-1 at the current rate of 0.1, 0.5, 1.0, 2.0, 5.0 and 10.0 A g-1, respectively) and long cycling stability (522.5 mAh g-1 at 5.0 A g-1 after 500 cycles). The existence of SnS2/Mn2SnS4 heterojunctions can effective stabilize the reaction products Sn and Na2S, greatly prevent the coarsening of nanosized Sn0 and enhance reversible conversion-alloying reaction, which play a key role in improvingthe ICE and extending the cycling performance. Moreover, the heterostructured SMS coupled with interacted carbon network provide efficient channels for electrons and Na+ diffusion, resulting an excellent rate performance. Key words: sodium-ion batteries, SnS2, initial coulombic efficiency, conversion reaction, in-situ XRD, in-situ TEM
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Owing to the abundant resource and harmless characteristics, sodium ion batteries (SIBs) have recently been regarded as the most promising alternative to the lithium ion batteries (LIBs), especially in the application of large-scale renewable energy storage system.1 Although presenting the similar electrochemical reaction mechanism to LIBs, the sluggish reaction kinetic and unsatisfactory electrochemical performance induced by the larger radius Na+ ion (Na+ vs Li+ =0.102 vs 0.076 nm) intercalation in the electrode materials severely hinder the wide-scale practical usage of SIBs.2, 3 In order to match with relatively mature cathode system, the exploration of suitable anode materials for SIBs with higher energy and better cyclicity is still a great challenge. For instance, the commercial graphite of LIBs anode exhibits reversible capacity of only 35 mAh g-1 for sodium storage.4 Metal oxides/sulfides have been intensive investigated and regarded as potential anodes for high performance SIBs, attributing to their high theoretical capacity induced by conversion reaction.5, 6 Generally, the sodium accommodation reaction mechanism of these conversion-type anode materials (MxAy, M stands for the metal, A for the anion, such as O, S, Se) can be presented as: MxAy + (y n) Na+ ↔ x M + y NanA, where n represents the oxidation of A.7-9 Moreover, some metal elements in IV or V groups (Sn, Sb, P, etc.) can further undergo the alloying reaction with Na+ after the conversion reaction, providing extra sodium storage capacity: M + zNa+ ↔ NazM.10-12 As a typical member, SnS2 has attracted extensive interests because of its high theoretical specific capacity of 1136 mAh g-1 (SnS2 + Na+ ↔ Sn0 + Na2S ↔ Na15Sn4) and rapid Na+-ions migration capability.13,14 While,
SnS2
suffers
from
huge
volume
expansion/contraction
during
continuous
(de)conversion-(de)alloying reactions cycling, which inevitably results in their structure collapse/pulverization as well as specific capability degradation.15, 16 Incorporation of nanostructured 3
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SnS2 into carbonaceous materials have been reported as effective improvements to mitigate the volume variation and boost its electronic conductivity, and the fabricated SnS2-based anode materials demonstrate enhanced cycling performance.17, 18 Ideally, in the reversible conversion reaction, the Sn0 metal and Na2S will be fully converted back to SnS2 during the desodiation process.19,20 However, the driving force derived from the internal-stress of recrystallization often induces the coarsening of Sn nanograins in the mixture of Sn0/Na2S, which will greatly reduce the conversion reaction interface between Na2S and metallic Sn0 and restricts the feasible Na+ diffusion, resulting in an incomplete conversion reaction of Sn0/Na2S back to the SnS2 during the desodiation process.21-25 The irreversible conversion reaction of Sn0/Na2S will cause undesirable initial coulombic efficiency (ICE) with large capacity loss, and it has detrimental influence on energy density of SIBs.26, 27 It is noted that, compared with the enormous effect on the improved cycle stability of SnS2-based anodes,28, 29 fewer attentions have been devoted to addressing the challenges of irreversible conversion reaction and low ICE problems. Herein, we report the rational design and fabrication of nanobox-like SnS2/Mn2SnS4/carbon (SMS/S) heterostuctures as SIBs anode. Figure 1a presents the synthetic strategy of the SMS/S nanoboxes (NBs). First, uniform MnSn(OH)6 nanoboxes were synthesized through a simple alkaline assisted co-precipitation process. Then, MnSn(OH)6 nanoboxes were tightly wrapped by PDA carbon using an in-situ polymerization method along with ultra-sonication. Finally, the SMS/C sulfide nanoboxes were obtained by carbonization and sulfidation of MnSn(OH)6/PDA precursors under a reductive atmosphere. The heterojunctions formed between SnS2 and Mn2SnS4 can effectively adapt the volume expansion upon extensive sodiation/desodiation process, prevent the agglomerate and coarsening of Sn nanoparticle, and improve the conversion-alloying reaction 4
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reversibility. Accordingly, the SMS/C heterostructure displays a high ICE, excellent rate performance and outstanding cycling stability. RESULTS AND DISCUSSION
Figure 1. (a) Schematic illustration of the formation of SMS/C composite. (b,c) SEM, (d) TEM, (e,f) HRTEM, (g) EDX elemental mapping images, XPS spectra of (h) Sn3d, (i) Mn2p and (j) C 1s for the SMS/C composite. 5
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Morphology of as-prepared MnSn(OH)6 precursors and SMS/C NBs is characterized by scanning electron microscope (SEM). As displayed in Figure S1, MnSn(OH)6 precursors display a regular nanoboxe-like morphology and high monodispersivity, with an average size of about 100 nm. After carbonization and sulfidation treatments, the fabricated SMS/C composite shows a nanoboxes-like morphology (Figure 1b and c), and SMS is homogenously coated by a thin carbon shell. A typical transmission electron microscope (TEM) image of SMS/C reveals that the intimate contact between the adjacent nanoboxes (about 80 nm) and the interacted carbon network (Figure 1d and 1e), which is consisted with the SEM observation. Moreover, the ultrathin PDA derived carbon layer (~8 nm) is homogenously coated on the surface of SMS NBs (Figure 1e), indicating the successful fabrication of integral and compact carbon shell. As expected, the PDA derived carbon coating can effectively confine the growth and agglomeration of SMS nanograins during the annealing process, and serves as a electronic conductive bridge to connection the contiguous SMS/C NBs.30 The corresponding SAED pattern of SMS/C NBs presented in the inset of Figure 1e indicates the coexistence of Mn2SnS4 and SnS2. High resolution TEM (HRTEM) image of SMS/C NBs is shown in Figure 1f, the well-defined lattice fringes with inter-planar distance of 0.316, 0.588 and 0.606 nm are associated to (101) and (126) planes of SnS2 and (110) plane of Mn2SnS4, respectively. The obvious grain boundaries between Mn2SnS4 and SnS2 confirm the formation of Mn2SnS4/SnS2 heterojunctions. EDX element mappings in Figure 1g illustrate that Sn, Mn, S, N and C are homogenously distributed within the SMS-C NBs, further implying SMS NBs are uniformly dispersed in the carbon framework. Figure S3 displays the XRD patterns of as-prepared SMS/C NBs, which can be well indexed to the mixture of hexagonal-type of SnS2 (PDF. 23-0677) and orthorhombic-type Mn2SnS4 (PDF. 73-0829) without any other impurity peaks. The diffraction patterns of SMS/C NBs is well consisted with that 6
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of pure SMS, implying carbon surface modification has little impact on SMS crystal structure.31 Thermogravimetric analysis (TGA) of SMS and SMS/C NBs is tested in air from 30 to 1000 oC, and the results are presented in Figure S7. Carbon content in SMS/C NBs is calculated to be 18.5 wt%. The surface chemical state and valence state of SMS/C NBs have been studied by X-ray photoelectron spectrometer (XPS). Figure 1h displays the XPS spectrum of Sn 3d core-level, the peaks centered at 485.7 and 494.2 eV are attributed to Sn 3d5/2 and Sn 3d3/2 of SMS/C, respectively.32 Meanwhile, the peaks at 642.8 and 654.0 eV are associated to Mn 2p3/2 and Mn 2p1/2 of SMS/C, respectively (Figure 1i),33 indicating the existence of Sn4+ and Mn2+ in SMS/C NBs. In contrast, the XPS spectra of pristine SnS2/C and MnS/C have also been investigated. Compared to SnS2/C (486.9 and 495.3 eV for Sn 3d5/2 and Sn 3d3/2) and MnS/C (641.4 and 652.7 eV for Mn 2p3/2 and Mn 2p1/2), it is noted that the characteristic peaks of Mn 2p for SMS/C NBs shift towards the lower and higher binding energy with the opposite direction, respectively, suggesting a decreased electron cloud density around Mn2SnS4. The electron cloud bias from SnS2 to Mn2SnS4 is a solid evidence of enhanced interaction and strong coupling between SnS2 and Mn2SnS4, confirming the construction of nanosized heterojunction between SnS2 and Mn2SnS4.34,
35
C1s core level XPS
spectra of SnS2/C, MnS/C and SMS/C NBs have also been studied (Figure 1j), three peaks located at 284.4, 286.1 and 288.9 eV are associated to the bond of C-C, C-O and O=C-O, respectively.36 Interestingly, the new peak broaden peak in 285.2 eV ascribed to the C-N bond has been observed in SMS/C NBs. It is noted that the C-N bond is an evidence of N-doping into C as well as the intimate and strong chemical bonding between SMS and C.29 As presented in Figure S8a for the S 2p spectrum of SMS/C composite, the peaks at 161.1 and 162.3 eV are attributed to the S2-, and the peaks at 163.6 and 164.8 eV are assigned to the presence of sulfur covalently bonded to carbon layer, 7
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with formation of C-S-C bridge.37,38 Meanwhile, for XPS spectrum of N 1s (Figure S8b), the peaks at 398.3, 399.9 and 400.7 eV are attributed to the pyridinic N, pyrrolic N and graphitic N, respectively, manifesting the doping of nitrogen element into the SMS/C composite.37,39 The intimate heterojunctions formed between SMS and C is expected to play a vital role in stabilizing SMS and reaction products, and providing high electronic conductive during the sodiation/desodiation process.30,40 Figure 2a displays the initial galvanic discharge/charge curves SnS2, SnS2/C and SMS/C NBs at 0.1 A g-1 in 0.1-3.0 V. SMS/C NBs have similar initial discharge/charge profiles with that of SnS2/C. But, compared with the SnS2 and SnS2/C sample, SMS/C NBs show much lower sodiation potentials for conversion reaction, suggesting the relative inferior working plateau with enhanced ICEs. SMS/C NBs deliver the initial discharge/charge capacities of 926.4 and 841.2 mAh g-1, respectively, closed to the theoretical capacity (965.32mAh g-1 =1100 mAh g-1×81.5wt% (SnS2) + 372 mAh g-1×18.5wt% (carbon)), with a high ICE of 90.8% (Figure S9). While, the ICE of SnS2 and SnS2/C are only calculated to be 67.9% and 85.4%, respectively. Based on Equation 3, if partial conversion reaction of Sn/Na2S mixture happened during the desodiation process and only SnS was obtained, the theoretical ICE is calculated to be 81%, while the complete reversible conversion reaction of Sn/Na2S mixture to SnS2 corresponds to a high theoretical ICE of 100% (Figure 2b). It is noted that, the six cells with SnS2-based anode or SnS2/C-based anode exhibits an average ICE of 67.9% and 85.4% (Figure S10 and S11), respectively, which is further lower than the theoretical ICE, indicating the existence of incomplete and irreversible conversion reaction. While, the six different batteries with SMS/C NBs anode exhibit a high average ICE value of 90.8%, which is greatly higher than the previous reported SnS2-based anode materials as presented in Figure 2c and Table S1.16, 20, 40-47 8
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Figure 2. (a) discharge/charge curves and (b) ICEs summary of six cells for SnS2, SnS2/C, SMS and SMS/C composite. (c) The ICE comparison of typical Sn-based materials in SIBs and SMS/C composite in this work. (d) CV curves at initial three cycles and (e) differential charge capacity curves versus voltage at the various cycles, (f) reversible capacities versus cycle number (separated into potential ranges of 0.1-1.0, 1.0-2.0, and 2.0-3.0 V) for the SMS/C composite in 0.1-3 V at rate of 5 A g-1. (g) Rate capability and (h) cycling performance at 5.0 A g-1 for SnS2/C and SMS/C composite. Rate performances of SnS2, SnS2/C and SMS/C NBs have been tested, the results are shown in Figure 2g. SMS/C NBs deliver a high specific capacity of 752.3, 636.5, 604.7, 570.1, 546.9, 519.7 and 488.7 mAh g-1 at the current rate of 0.1, 0.2, 0.5, 1.0, 2.0, 5.0 and 10.0 A g-1, respectively. When the current rate is set beck to 0.1 A g-1, the specific capacity of SMS/C NBs quickly recovers to 9
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746.3 mAh g-1, demonstrating an exceptional rate performance. In comparison, SnS2/C exhibits exhibit a specific capacity of 647.5, 540.6, 498.8, 469.3, 431.2, 379.4 and 361.1 mAh g-1 at 0.1, 0.2, 0.5, 1.0, 2.0, 5.0 and 10.0 A g-1, respectively. While, that of SnS2 is only 632.3, 602.7, 536.6, 452.1, 403.5, 347.2 and 326.4 mAh g-1 at 0.1, 0.2, 0.5, 1.0, 2.0, 5.0 and 10.0 A g-1, respectively (Figure S16). Cycling performance of SMS/C NBs has also been tested, and it can deliver a high reversible capacity of 522.5 mAh g-1 at 5.0 A g-1 after 500 cycles (Figure 2h), with a high capacity retention of 91.3%. For SnS2/C, a slight capacity increase has been observed in initial 200 cycles, which is ascribed to the electrode activation and continuous SEI fracture and reformation.40 It keeps stable until 400 cycles, and then quickly degrades to 223 mAh g-1 after 500 cycles. While, the specific capacity of SnS2 fast drops to almost 0 at 5.0 A g-1 after 300 cycles (Figure S16d). Additionally, the reaction kinetics of SnS2, SnS2/C and SMS/C NBs have been investigated by EIS measurement as displayed in Figure S19, and all the samples are fitted by equivalent equations.48-50
It is observed
that the charge transfer resistance (Rct) of SMS/C NBs before cycling is 12.33 Ω, which is much lower than those of pure SnS2 and SnS2/C as listed in Table S2. Meanwhile, the Na+ ion diffiusion coefficiency (DNa+) of SMS/C is 6.01×10-15 cm2 s-1), it is almost twice that of SnS2/C composite (2.87×10-15 cm2 s-1). The superior DNa+ further demonstrates a fast Na+ ions diffusion mobility of SMS/C NBs. To understand the reaction mechanisms of SMS/C NBs, the initial three cyclic voltammograms (CV) curves have been investigated within a voltage window of 0.1-3.0 V at 0.1 mV s-1, and the results are illustrated in Figure 2d. During the first cathodic scan, two peaks at 1.70 and 1.14 V are ascribed to Na+ intercalation into the SMS NBs,51 while, the peaks at 0.80 and 0.62 V are attributed to the conversion reaction accompanied by the formation of SEI film as the consequence of the 10
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decomposition of electrolyte.52, 53 Additionally, a broad peak occurred at 0.35 V is supposed to the alloying reaction between Sn and Na+.54 In the corresponding anodic scan, the distinct peak at 0.65 V corresponds to the dealloying process, The obvious peak at 1.23 V followed by a small peak at 1.52 V in the CV curve is assigned to the reconversion reaction between metallic Sn/Mn and Na2S.55-57 Besides, the small peak at 2.1 V is assigned to the release of Na+ for deintercalation.40 In the subsequent scans, the CV curves is well-overlapped with each other, demonstrating a high reversibility and cycling stability of SMS/C NBs. The differential charge capacity plots (DCPs) of SMS/C NBs at 5.0 A g-1 have also been studied, and the results are exhibited in Figure 2e. Combined with the CVs testing results, DCPs of SMS/C NBs can be clearly divided into three sections at three different potential regions, involving the broad peaks of dealloying reaction within 0.1-1.0 V, sharp peaks within 1.0-2.0 V for reverse conversion reaction, and obvious peaks for Na+ deintercalation within 2.0-3.0 V. Meanwhile its reversible capacity in each cycle is calculated according to their corresponding potential ranges as presented in Figure 2f. It is observed that a new obvious peak emerges after 100 cycles in 2.0-3.0 V, contributing to the increased capacities induced by the catalytic activity of irreversible consumption of the electrolyte. Especially, the peaks in 0.1-1.0 and 1.0-2.0 V ranges exhibit highly stable performance during the following 400 cycles, regarding to the integral intensity as well as potential positions. Furthermore, the reversible capacities of 130 and 400 mAh g-1 can be achieved in 0.1-1.0 and 1.0-2.0 V, respectively, which illustrate the formed metallic Sn by dealloying conversion is transferred to Sn2S3, suggesting the excellent cyclability for the highly reversible alloying and conversion reactions. In comparison, the DCPs of SnS2/C in 1.0-2.0 V displays continuous severe decay as the increasing of cycle numbers (Figure S18), demonstrating that the partial irreversible conversion reaction results 11
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in the dramatic capacity degradation. To further clearly elucidate the Na+-storage mechanisms, in-situ XRD analysis is performed against with the initial discharger/charge curves at the current rate of 0.1 A g-1 in 0.1-3.0 V,58 as presented in Figure 3. As indicated in the figures, the different phase compositions from in-situ XRD results are color-marked to better understand each (de)sodiation reaction state. Combined with the CV investigations, a 6-stage reaction scheme for SMS/C electrode is proposed. In the stage I (OCV-1.1 V), the rapid decrease in potential associated with the peaks of (110) and (400) planes for Mn2SnS4 and peak of (001) plane for SnS2 gradually shift to lower 2θ, it is attributed to the lattice expansion of Mn2SnS4 and SnS2 caused by Na+ intercalation.59 In the stage II (1.1-0.6 V), the peak of (001) plane for SnS2 disappears rapidly accompanied with the appearance of a new peak at 12.5°. This new peak is recognized to be Sn2S3, a product of Na+ partial concurrent conversion reaction taking place. Then, the peaks of Mn2SnS4 and Sn2S3 gradually weaken in intensity until complete disappear, followed by the appearance of new peaks located at 23.8° and 34.3°. These new peaks are indexed to Sn (PDF. 86-2266) and MnSn2 (PDF. 65-2701), respectively, the products of further occurrence of conversion reaction. In stage III (0.6-0.1 V), when the battery is discharge to 0.1 V, new peaks are observed at 21.4° and 37.2°, which can be indexed to Na15Sn4 (PDF. 31-1327) and Na9Sn4 (PDF. 30-1250), respectively. The observation of Na15Sn4 and Na9Sn4 at fully sodiated state implies the multi-steps alloying reaction between metallic Sn and Na+ under a sequential phase transformation. In stage IV (0.1-1.0 V), when desodiated back to 1.0 V, the peaks of Na15Sn4 and Na9Sn4 disappear, indicating dealloying reaction of the formed Na-Sn alloys to metallic Sn. In stage V (1.0-2.0 V), the peaks of NaxMnS and NaxSn2S3 are detected, demonstrating the proceeding of reverse conversion reaction among Sn, MnSn2 and Na2S. As fully desodiated back to 3.0 V, stage VI 12
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(2.0-3.0 V), the peaks of Sn2S3 (PDF: 75-2183) and MnS (PDF: 72-1534) are detected, suggesting the formation of Sn2S3 and MnS. Additionally, the evolution of heterojunctions during the initial process of Na+ ion uptake and release also can be confirmed by ex-situ HRTEM (Figure S21) and XPS (Figure S22) measurements at different sodiation/desodiation states. Therefore, the Na+-storage mechanisms of SMS/C electrode can be illustrated as follows equations: Stage I (Intercalation, OCV-1.1 V) Mn2SnS4 + xNa+ + xe- → NaxMn2SnS4 (0
