Phase transformation and sulfur vacancy modulation of 2D layered tin sulfide nanoplates as highly durable anodes for pseudocapacitive lithium storage

Journal Pre-proofs Phase transformation and sulfur vacancy modulation of 2D layered tin sulfide nanoplates as highly durable anodes for pseudocapacitive lithium storage Junjun Zhang, Dongwei Cao, Yang Wu, Xialan Cheng, Wenpei Kang, Jun Xu PII: DOI: Reference:

S1385-8947(19)33137-7 https://doi.org/10.1016/j.cej.2019.123722 CEJ 123722

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Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

24 September 2019 26 November 2019 4 December 2019

Please cite this article as: J. Zhang, D. Cao, Y. Wu, X. Cheng, W. Kang, J. Xu, Phase transformation and sulfur vacancy modulation of 2D layered tin sulfide nanoplates as highly durable anodes for pseudocapacitive lithium storage, Chemical Engineering Journal (2019), doi: https://doi.org/10.1016/j.cej.2019.123722

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Phase transformation and sulfur vacancy modulation of 2D layered tin sulfide nanoplates as highly durable anodes for pseudocapacitive lithium storage

Junjun Zhanga,c,1, Dongwei Caob,1, Yang Wua, Xialan Chenga, Wenpei Kangb,*, Jun Xua,*

aSchool

of Electronic Science & Applied Physics, Hefei University of Technology, Hefei 230009,

P.R. China. bCollege

of Science, School of Materials Science and Engineering, China University of Petroleum

(East China), Qingdao 266580, P.R. China. cSchool

of Physics and Materials Engineering, Hefei Normal University, Hefei 230601, P.R. China.

*Corresponding authors: E-mail: [email protected] (J. Xu); E-mail: [email protected] (W. Kang) 1The

authors contributed equally to this work.

1

Abstract: Despite the fulfilling progress in fabricating metal chalcogenides-based battery electrodes, most effort focuses on construction of hybrid architectures and/or foreign elemental doping for improving electrochemical performance. In this work, we report a self-template strategy to synthesize hexagonal SnS2 and orthorhombic SnS nanoplates with abundant S vacancies as advanced anode materials of lithium-ion batteries (LIBs). Phase evolution from hexagonal SnS2 to orthorhombic SnS by thermal annealing is investigated. The resultant tin sulfide nanoplates featuring abundant S vacancies and decreased bandgaps can provide more active sites, higher Li+-ion diffusion mobility and better electronic conductivity, which are beneficial for improving electrochemical reaction kinetics. Consequently, both the S-vacancy-rich SnS2 (SVR-SnS2) and the SVR-SnS nanoplates show significantly enhanced cycling performance and rate capability compared with the interlayer-expanded SnS2 (IE-SnS2) nanoplates. Remarkably, the SVR-SnS nanoplates can exhibit outstanding rate capability (510 mAh g−1 at 10 A g−1) and excellent long-term cycling performance. A reversible capacity as high as 765 mAh g−1 at a high current of 2 A g−1 can be delivered even after 1200 cycles. It is the best cycling performance of SnS-based electrodes for LIBs to date. Besides the benefits from S vacancies, the pseudocapacitive contribution is also responsible for the fast and stable lithium storage capability. The present work provides a new strategy for modulating 2D layered tin sulfide nanoplates as promising anode materials for LIB applications.

Keywords: Tin sulfide nanoplates; Phase evolution; Sulfur vacancies; Layered structure; Lithium-ion batteries

2

1. Introduction Lithium-ion batteries (LIBs) as a promising energy storage device have achieved a great commercial success in portable electronics and electric vehicles [1-6]. However, the commercialized graphite anodes cannot meet the ever-growing demand of next-generation LIBs owing to its low theoretical capacity (372 mAh g1) [7]. Thus, it is crucial and urgent to develop high-capacity anode materials to enhance energy density of batteries [8,9]. Recently, a variety of layered metal sulfides (MoS2, WS2, TiS2, VS2, SnS2) have been intensively investigated as promising candidate anode materials owing to their lamellar structure analogous to graphite but much larger interlayer spacing and higher specific capacity [10-21]. Among the various metal sulfides, layered tin sulfides (SnSx) have attracted great interest owing to their high theoretical capacity, low discharge potential and earth abundance of the constituent elements [22].There are two typical SnSx semiconductors with a single oxidation state, i.e., the hexagonal SnS2 and the orthorhombic SnS. The layered hexagonal SnS2 with an interlayer spacing of 0.589 nm has a theoretical capacity of 1231 mAh g1, while the layered orthorhombic SnS exhibits an interlayer spacing of 0.559 nm and a high theoretical capacity of 1137 mAh g1 [22]. The large interlayer spacings of SnS2 and SnS benefit for fast Li+-ion intercalation/extraction and relaxation of the alloying/dealloying volume change. However, SnSx electrodes suffer from low electronic conductivity and large volume expansion, consequently poor cycling stability [22-25]. The electrochemical lithium storage mechanism of SnSx involves the conversion reaction (the decomposition of SnSx into metallic Sn) and the alloying processes (the subsequent formation of Li4.4Sn alloy) [22-25]. The Li-alloying/dealloying mechanism is the intrinsic driving source for electrochemical activity, 3

but it causes problems of large volume change and drastic pulverization, leading to rapid capacity fading and performance deterioration. To relax volume expansion and improve electronic conduction of the SnSx electrodes, several strategies have been developed during the past decade. Extensive effort has been devoted to (i) modification with conductive materials such as graphene [25-32], carbon nanotubes [33-35], carbon fibers [36], carbon nanospheres [37,38], amorphous carbon coating [39], carbon paper [23,40], conductive polymers [41,42]; (ii) morphology engineering to buffer volume change and reduce ion transport pathway [43-46]; (iii) construction of heterostructures or hybrid electrodes [47-51]. Recently, foreign elemental doping and defect modulation have been developed as a new strategy to improve electrochemical performance of SnSx electrodes [52-54]. For example, Lu et al. reported the synthesis of Mo-doped SnS2 nanosheets grown on carbon cloth for lithium storage, exhibiting a high reversible discharge capacity of 1950.8 mAh g1 after 200 cycles at 1 A g1 [52]. Pang and coworkers designed and synthesized Co-doped SnS2 ultrathin nanosheet assembled nanocages, delivering an excellent discharge capacity of 809 mAh g−1 at a current density of 100 mA g−1 with a 91% retention after 60 cycles [53]. Despite these advances, the cycling stability of the SnSx-based electrodes is still unsatisfied (typically below 300 cycles) for practical LIB applications. Herein, we report a facile solvothermal synthesis of interlayer-expanded SnS2 (IE-SnS2) nanoplates. After a simple post thermal treatment, the IE-SnS2 nanoplates can be converted to the S-vacancy-rich (SVR) hexagonal SnS2 and orthorhombic SnS nanoplates, respectively, by tuning the heating temperature. Abundant S vacancies are achieved during the phase conversion process, which is beneficial to provide improved electronic conductivity, higher 4

Li-ion diffusion mobility, reduced interfacial charge transfer resistance, and more active sites for electrochemical performance. Consequently, significantly improved lithium storage performance has achieved for the SVR-SnSx nanoplates.

2. Experimental 2.1 Synthesis of the IE-SnS2 nanoplates 1 mmol of stannic chloride pentahydrate (SnCl4·5H2O) and 3 mmol of sulfur powders were added to a Teflon-lined autoclave containing 30 mL of octylamine as the solvent. After stirring for 5 min, the autoclave was sealed and maintained in an oven at 120 C for 12 h. A yellow product was then collected by centrifugation, washed with absolute ethanol and deionized water several times, and finally dried in vacuum. 2.2 Synthesis of the SVR-SnS2 nanoplates The freshly-prepared IE-SnS2 sample was heated in a chemical vapour deposition (CVD) furnace at a heating rate of 10 C min1 to 300 C and then maintained for 30 min in a nitrogen flow (20 sccm). 2.3 Synthesis of the SVR-SnS nanoplates The freshly-prepared IE-SnS2 sample was heated in the CVD furnace at a heating rate of 10 C min1 to 600 C and then maintained for 60 min in a nitrogen flow (20 sccm). 2.4 Materials characterization Scanning electron microscopy (SEM) images were recorded on a field-emission scanning electron microscope (TESCAN MIRA3, or Philips XL30 FEG). Transmission electron microscopy (TEM) images and selective area electron diffraction (SAED) patterns were 5

recorded from a transmission electron microscope (JEOL JEM-2100F) operated at 200 kV. X-ray diffraction (XRD) patterns were obtained from a Rigaku D/Max 2500 v diffractometer with a Cu K radiation. Thermogravimetric analyses (TGA) were performed with a Mettler-Toledo TGA/DSC1 system with a heating rate of 10 C min1 in nitrogen atmosphere. X-ray photoelectron spectroscopy (XPS) measurement was carried out by using a VG ESCALAB 220i-XL UHV system with an Al Ka X-ray source (1486.6 eV). Raman spectra were obtained from a Lab RAM HR spectroscope with a 532 nm excitation. Electron paramagnetic resonance (EPR) spectra were obtained from a Bruker EMXplus-10/12 spectrometer at room temperature. UV-vis spectra were recorded on an UV-vis spectrophotometer (Cary-500 spectrometer, Varian Ltd.). 2.5 Electrochemical measurements Electrochemical measurements were carried out by assembling CR 2032 type coin cells in an Ar-filled glove box. Each cell was fabricated by a working electrode, a Li metal foil as the counter electrode, and a separator film (Celgard 2400). A solution of LiPF6 (1.0 M) dissolved in ethylene carbonate and diethyl carbonate with volume ratio of 1:1 and 5% fluoroethylene carbonate was used as the electrolyte. The working electrodes were typically prepared by coating slurries of the SnSx samples mixed with carbon black and sodium carboxymethyl cellulose binder at a weight ratio of 3: 1: 1 on copper foils. The coated foil was dried at 80 C and punched into discs of 12 mm in diameter. The discs were then dried at 110 C for 4 h in vacuum before transferred into the Ar-filled glove box. The active material loading is in the range of 1.2-1.5 mg cm−2. Galvanostatic cycling tests of the assembled cells were carried out on a Neware system (5V10mA or 5V50mA) at room temperature. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were 6

performed using an electrochemical workstation (Gamry 30115).

3. Results and discussion The strategy to synthesize 2D layered tin sulfide nanoplates is illustrated in Scheme 1. Sn-octylamine and S-octylamine complexes as the precursors were prepared by dissolving SnCl4 and S powders in octylamine. Under the solvothermal condition, they reacted with each other to form octylamine-intercalated SnS2 nanoplates with an expanded interlayer spacing. The freshly-prepared interlayer-expanded SnS2 (IE-SnS2) nanoplates were then annealed in nitrogen atmosphere at controlled temperatures to prepare S-vacancy-rich (SVR) tin sulfides. The thermal dissociation of the IE-SnS2 involves two stages depending on S depletion: (i) formation of S-vacancy-rich SnS2 (i.e., SnS2x) with low S depletion (at lower annealing temperature of 300 C), named as SVR-SnS2; (ii) phase transformation to orthorhombic SnS with high S depletion (at higher annealing temperature of 600 C), named as SVR-SnS. SEM images of the freshly-prepared solvothermal sample are shown Fig. S1a and S1b, which reveal the sample consists of 2D nanoplates with sizes of 10-15 m. A TEM image in Fig. S1c further confirms the nanoplates with a hexagonal shape. Polycrystalline nature is demonstrated by an SAED pattern in Fig. S1d, which reveals (100) and (110) diffraction rings of hexagonal SnS2. HRTEM images of the freshly-prepared solvothermal SnS2 sample are presented in Fig. S1e and S1f. The fringe lattice of 0.32 nm can be well indexed to the interplanar spacing of (100) plane of hexagonal SnS2. The fringe lattice of 0.92 nm corresponding to the expanded (001) interlayer spacing is observed. The enlarged interlayer 7

spacing is probably attributed to octylamine intercalation. After annealing the IE-SnS2 at 300 C, the sample can well reserve the nanoplate morphology (Fig. 1a and 1b). An HAADF image of the nanoplate is presented in Fig. 1c, and corresponding Sn and S EDX mappings are displayed in Fig. 1d and 1e. It is revealed that the two elements are homogeneously distributed throughout the nanoplate. However, a typical SAED pattern of the nanoplates shows a set of diffraction spots (Fig. 1f), indicating its improved crystallinity. Fig. 1g presents an HRTEM image of the annealed SnS2 nanoplate. The fringe lattice of 0.59 nm matches well with the interplanar spacing of (001) plane of hexagonal SnS2. Such an observation reveals the interlayer spacing is restored to the normal value after annealing treatment to remove the intercalated octylamine. When the IE-SnS2 was annealed at 600 C, the sample is conformally converted to SnS nanoplates as shown in Fig. 1h. The SAED pattern in Fig. 1i can be indexed to orthorhombic SnS with a zone axis of [001]. In the HRTEM image (Fig. 1j), the fringe lattice of 0.28 nm and 0.56 nm match well with the interplanar spacing of (002) and (004) planes of orthorhombic SnS. Fig. 2a shows XRD patterns of the samples before and after post annealing treatment. Curve (i) is a diffraction pattern measured from the freshly-prepared solvothermal sample. All the peaks matched well with the calculated diffraction peaks of an expanded hexagonal SnS2 cell along c-axis. There are two peaks at 2 = 9.32 (d = 0.948 nm) and 2= 18.69 (d = 0.474 nm), which can be indexed respectively to the (001) and (002) planes of the interlayer-expanded hexagonal SnS2. This shift of the (001) diffraction peak to lower 2 degree reveals the interlayer spacing of SnS2 is significantly enlarged [55,56]. This scenario is in good agreement with the HRTEM observation in Fig. S1f. Curve (ii) is an XRD pattern 8

collected from the sample after 300 C annealing. All the diffraction peaks can be well indexed to hexagonal SnS2 (JCPDF: 23-0677). The (001) diffraction peak appears at 2= 15.03, indicating the interlayer spacing is recovered to 0.59 nm. The increased peak intensity also supports the enhanced crystallinity relative to the IE-SnS2. Besides removal of the intercalated octylamine, some S vacancies can be generated at 300 C through the reaction of SnS2 ↔ SnS2x + x S (g). Curve (iii) is an XRD pattern of the sample after 600 C annealing. All diffraction peaks match well with those of orthorhombic SnS (JCPDF: 75-0925). Phase transformation from hexagonal SnS2 to orthorhombic SnS occurs at 600 C by the reaction of SnS2 ↔ SnS2x + x S (g) ↔ SnS + S (g). The by-product S vapour can be effectively evaporated. Raman analysis of the three samples was performed as shown in Fig. 2b. Curve (i) of the IE-SnS2 has a response peak at 311 cm1, which is attributed to A1g vibration mode of Sn(IV)-S in layered SnS2 [23,49].Curve (ii) is a Raman spectrum of the sample after 300 C annealing. Besides the strong resonance peak at 311 cm1, a minor peak at 90 cm1 corresponding to Ag vibration mode of Sn(II)-S in SnS is observed [57,58]. It indicates the formation of trace amount of Sn2+ ions owing to generation of S vacancies in the hexagonal SnS2. Curve (iii) is recorded from the sample after 600 C annealing. The band intensity of the peak at 311 cm1 is significantly decreased, while three new resonance peaks at 90, 157 and 225 cm1 appear. These three peaks are attributed respectively to the Ag, B3g and Ag modes of Sn(II)-S bonding in orthorhombic SnS [57,58]. Thus, this observation suggests existence of residual Sn4+ ions in the orthorhombic SnS. The samples obtained by annealing the IE-SnS2 at 400 and 500 C have also been investigated (Fig. S2). Both are a mixture of 9

hexagonal SnS2x and orthorhombic SnS, and the hexagonal phase gradually disappears as the annealing temperature increases. Chemical state and composition of the IE-SnS2, the SVR-SnS2 and the SVR-SnS samples were further investigated by XPS analysis. Fig. 2c shows Sn 3d core-level spectra of the three samples. For the IE-SnS2 sample (curve i), the spectrum has two peaks at 486.9 eV for Sn 3d5/2 and 495.3 eV for Sn 3d3/2, indicating formation of Sn4+ [23,33]. The spectrum of the SVR-SnS2 sample (curve ii) has a couple of prominent peaks at 495.3 eV for Sn 3d3/2 and 486.9 eV for Sn 3d5/2 corresponding to Sn4+, as well as a couple of tiny peaks at lower binding energy (494.2 eV for Sn 3d3/2 and 485.7 eV for Sn 3d5/2) corresponding to Sn2+ [59]. The atomic ratio of Sn2+/Sn4+ is estimated to be 8: 92. It further supports the formation of SVR-SnS2. When some S vacancies are generated in SnS2, the electrons participating in the bonding will be transferred to the neighboring Sn4+ cations, thus resulting in formation of some Sn2+ cation sites [60,61]. In curve (iii), the spectrum of the SVR-SnS sample also contains both Sn4+ bands and Sn2+ bands. The atomic ratio of Sn2+/Sn4+ is evaluated to be 62: 38, revealing the formation of SnS but existence of some residual SnS2x. Fig. 2d displays the S 2p core-level spectra of the three samples. Both the spectra of the IE-SnS2 (curve i) and the SVR-SnS2 (curve ii) present a couple of peaks at 162.9 eV for S 2p1/2 and 161.7eV for S 2p3/2, indicating sulfur in the two samples exists in S2 ions [23,33]. The S 2p XPS spectrum of the SVR-SnS (curve iii) reveals that sulfur has a chemical valance of −2, contaminated with a small amount of elemental S. From the XPS data, the atomic ratio of S/Sn is estimated to be 1.9 for the IE-SnS2. This value decreases to be 1.6 for the SVR-SnS2 (i.e., SnS2x,

10

x=0.4), indicating a large amount of S vacancies is produced. This value becomes 1.3 (larger than 1.0) for the SVR-SnS, which also support the existence of residual SnS2x. Phase evolution and thermal stability of the various tin sulfide nanoplates were investigated by TG analysis performed in nitrogen atmosphere (Fig. 3a). The IE-SnS2 (curve i) shows three temperature ranges of mass loss, corresponding to evaporation of intercalated octylamine (100-250 C), generation of S vacancies to form SnS2x (250-400 C), and phase transformation from hexagonal SnS2x to orthorhombic SnS (550-700 C). For the SVR-SnS2 obtained after 300 C annealing (curve ii), only two mass loss ranges of 250-400 C and 550-700 C are observed, indicating the contaminated octylamine is completely removed. Owing to the generation of S vacancies, the SVR-SnS2 has a weight loss of 15.2% (curve ii), which is slight smaller than that (17.5%) of the pristine SnS2 according to phase conversion from SnS2 to SnS in nitrogen. The SVR-SnS sample obtained after 600 C annealing exhibits outstanding thermal stability and no obvious mass loss is observed (curve iii). On the other hand, N 1s core-level XPS spectra in Fig. S3 also confirm the intercalated octylamine disappears in the two annealed samples. Besides the stoichiometric ratio variation among the various tin sulfides, EPR analysis is further performed to verify the existence of rich S vacancies in the two annealed tin sulfide samples, as such defects in semiconductors are usually paramagnetic. Fig. 3b displays the EPR spectra of the three samples. The EPR signal at g = 2.003 for the tin sulfides can be assigned to the S vacancies [62,63]. It is obvious that both the annealed samples (SVR-SnS2 and SVR-SnS) exhibit a much higher signal than the solvothermal IE-SnS2, and the SVR-SnS sample has the strongest signal. Thus, it indicates that the annealed nanoplates possess a significantly increased density of S vacancies relative 11

to the IE-SnS2 nanoplates. The higher concentration of S vacancies in the SVR-SnS nanoplates can provide higher Li+-ion diffusion mobility as well as more active sites [47,49], consequently resulting in more excellent surface reaction kinetics for electrochemical lithium storage. The annealing treatment of the nanoplates has induced great changes in sample colour and their optical properties. As shown in Fig. 4a, it is obvious that the colour varies from orange for the IE-SnS2, to the brown for the SVR-SnS2, and further to black for the SVR-SnS. UV-vis spectra of the three samples are presented in Fig. 4b. The IE-SnS2 has an absorption edge of 470 nm, while the two annealed samples show significantly expanded absorption edge. The bandgap of the samples was evaluated by plotting the (ahv)2–hv relationship (where a = absorbance, h = Planck’s constant, and v = frequency) as shown in Fig. 4c. The IE-SnS2 has a bandgap of 2.48 eV, which is in good agreement with the reported value (~2.4 eV) of bulk SnS2 [63]. The bandgap of the SVR-SnS2 decreases to be 2.20 eV, which is attributed to S vacancy-induced varied electronic structure [60,61,64,65]. The bandgap of the SVR-SnS is estimated to be 1.33 eV that matches well with the values (1.3-1.5 eV) of bulk SnS [66]. The bandgap variation of the three samples suggests improved electronic conductivity of the SVR-SnS2 and the SVR-SnS, which is important for their application as anode materials of LIBs. I-V curves of the three samples deposited on SiO2 substrates were measured and presented in Fig. S4. It reveals the SVR-SnS film has the highest electronic conductivity. Electrochemical performance of the various tin sulfide nanoplates as anode materials of LIBs was investigated. Rate capability of the three samples is evaluated through galvanostatic 12

discharge/charge at different current densities varied from 0.1 to 10 A g−1, as shown in Fig. 5a. The average discharge capacity of the IE-SnS2 electrode is 1031 mAh g1 at 0.1 A g1, and decreases to 86 mAh g1 at 10 A g1. It shows a low capacity retention of only 8.3% with a current increase of 100-folds. The rate performance can be significantly improved for the two annealed samples. The specific capacity of the SVR-SnS2 electrode is 1209 mAh g1 at 0.1 A g1, and keeps 383 mAh g1 at 10 A g1, thereby yielding a capacity retention of 31.7%. Rate capability is further enhanced for the SVR-SnS electrode. It delivers average discharge capacities of 1220, 1074, 967, 877, 771, 638 and 510 mAh g1 when cycling at current densities of 0.1, 0.2, 0.5, 1, 2, 5 and 10 A g1, respectively. Furthermore, the capacity of the SVR-SnS electrode can be recovered to 1086 mAh g1 as the current density returns to 0.1 A g1. Obviously, the SVR-SnS electrode exhibits significantly improved discharge capacities at high current densities. The capacity retention increases from 8.3% for the IE-SnS2, to 31.7% for the SVR-SnS2, and further to 41.6% for the SVR-SnS when current density varies from 0.1 to 10 A g1. Fig. 5b shows the cycling performance of the three electrodes at a current density of 1 A g1. The IE-SnS2 electrode shows a rapid capacity fading and holds a specific capacity of 238 mAh g1 after 400 cycles. However, the capacity after 400 cycles can increase to 772 mAh g1 for the SVR-SnS2 electrode and 980 mAh g1 for the SVR-SnS electrode, demonstrating significantly improved cycling stability of the annealed samples. Even cycled at a high current density of 2 A g1, the SVR-SnS electrode can yield a reversible discharge capacity of 765 mAh g1 after 1200 cycles (Fig. 5c), exhibiting excellent long-term stability. The SVR-SnS electrode performs among the best cycling performance of the tin sulfide-based electrodes (Table S1). Specially, the cycling stability of SnS-based 13

electrodes for LIBs is usually investigated for less than 300 cycles in the reported references (Table S1). Our work demonstrates an excellent cycling stability and high rate capability of SnS electrodes for LIBs. It is believed that the rich S-vacancies may contribute to the superior electrochemical performance in the following three aspects: (i) reducing bandgap (Fig. 4) and improving electronic conductivity (Fig. S4) of the electrode material by changing the electronic states; (ii) generating a localized built-in electric field to promote the difussiom of Li-ions by Coulomb force [67,68]; (iii) serving as active sites for electrochemical reaction and achieving fast reaction kinetics [47,49,69]. A capacity fading of the SVR-SnS electrode in the first 280 cycles is observed (Fig. 5c). There are two possible reasons: (i) the low reversibility of the conversion reaction; (ii) the pulverization of the SVR-SnS nanoplates (Fig. S5) owing to the large volume change of alloying/de-alloying reaction. The capacity of the SVR-SnS electrode gradually recovers after the 280 cycles and then increases to 765 mAh g−1 at the 1200th cycle at 2 A g1. The pulverization of SVR-SnS nanoplates into smaller nanoparticles may adversely become a positive factor for the capacity increase. Smaller particles not only increase the contact area between SnS and the electrolyte, but also shorten Li-ion diffusion path. Consequently, more Li-ions can be stored on newly created surfaces with increased interfacial charge storage, resulting in capacity increase. This phenomenon of first decay and then recovery was also observed in previous reports of SnS-based anodes [28,70]. Electrochemical impedance spectroscopy (EIS) measurement was performed to investigate reaction kinetics of the anodes. Nyquist plots of the three electrodes before cycle are presented in Fig. 5d. The Nyquist plots are modeled with a simplified Randles equivalent circuit shown in Fig. 5e. The semicircle at the high frequency corresponds to the charge 14

transfer reaction at the electrode-electrolyte interface [25,35]. While the three electrodes have a similar internal ohmic resistance (Re) of about 3 , the SVR-SnS electrode exhibits the smallest charge transfer resistance (Rct) of 62  compared to the IE-SnS2 and the SVR-SnS2. The abundant S vacancies in the SVR-SnS nanoplate surface can provide an effective way for lithium ion intercalation and serve as nucleation sites promoting phase transitions, thereby enhancing the interfacial charge transfer. Nyquist plots of the SVR-SnS electrode after various cycles are further investigated and shown in Fig. 5f. Rct value decreases from 62  before cycle, to 21  after the 1st cycle, to 18  after the 50th cycle, and further to 15  after the 500th cycle. This suggests an increased electrical conductivity as well as enhanced electrochemical reaction kinetics of the SVR-SnS electrode along with the discharge/charge cycling. The lithium storage mechanisms of both SnS2 and SnS involve a conversion together with an alloying/dealloying reaction described by the following equations [22], (i) Conversion process For SnS2: SnS2 + 4 Li+ + 4 e ↔ Sn + 2Li2S

(1)

For SnS: SnS + 2 Li+ + 2 e ↔ Sn + 2Li2S

(2)

(ii) Alloying/dealloying process Sn + x Li+ + x e ↔ 4LixSn (0  x  4.4)

(3)

Cyclic voltammetry (CV) test was carried out to investigate the lithium storage mechanism and the possible reasons for the improved reversible capacity of the SVR-SnS2 and the SVR-SnS electrodes. Fig. 6a depicts the CV curves of the SVR-SnS2 electrode at a scan rate of 0.1 mV s1. During the first cathodic process, three cathodic peaks centred at 15

1.20, 0.91 and 0.17 V are observed. The peaks at 1.20 V and 0.91 V could be attributed to the intercalation of Li+ ions into SnS2 layers (SnS2 + x Li+ + x e ↔ LixSnS2) as well as the conversion of LixSnS2 into metallic Sn (LixSnS2 + (4-x) Li+ + (4-x) e ↔ Sn + 2Li2S) [35,52]. The cathodic peak at 0.17 V can be assigned to the Li-Sn alloying reaction (Equation 3) and the formation of solid electrolyte interface (SEI) layer [35]. In the subsequent cycles, the small shoulder at 0.91 V disappears, while the two prominent peaks shift to higher potentials (from 1.20 V to 1.29-1.33 V and from 0.17 V to 0.20-0.27 V). During the anodic process, three corresponding anodic peaks at 0.53, 1.24 and 1.89 V are observed. They can be assigned to the dealloying of LixSn, the formation of LixSnS2 and the extraction of Li+ ions from the layered LixSnS2, respectively. Fig. 6b shows CV curves of the SVR-SnS electrode for the initial five cycles. In the first cathodic process, the curve shows a broad band at potential of 0.8-1.4 V and a sharp peak at 0.19 V, which can be ascribed respectively to the decomposition of SnS into metallic Sn and Li2S (Equation 2) as well as the subsequent generation of LixSn alloy (Equation 3). Correspondingly, there are also three oxidation peaks at 0.50, 1.26, and 1.87 V in the anodic sweeps. Compared with the SVR-SnS2 electrode, the initial five CV profiles of the SVR-SnS electrode nearly overlap together, which indicates its more excellent electrochemical reversibility. Fig. 6c and 6d display the galvanostatic charge/discharge profiles of the two electrodes in the cutoff voltage 0.01−3.0 V at a current density of 1 A g1. As shown in Fig. 6c, the SVR-SnS2 electrode delivers the first discharge and charge capacities of 1062 and 1568 mAh g−1, respectively, yielding an initial Coulombic efficiency of 67.7%. Remarkably, the SVR-SnS electrode exhibits a much larger initial Coulombic efficiency of 73.4% with a 16

reversible capacity of 1022 mAh g−1. It is believed that the enhanced initial Coulombic efficiency can be ascribed to the increased conductivity of the SVR-SnS, which minimizes the undesirable reactions with electrolyte. The gradual decrease in capacity is observed for the two electrodes. After 100 cycles, however, the SVR-SnS electrode maintains a discharge capacity of 807 mAh g−1, which is higher than that (782 mAh g−1) of the SVR-SnS2 electrode. In order to understand merits of the SVR-SnS electrode for a better reversibility and electrochemical performance, the reaction kinetics of the SVR-SnS electrode is investigated by CV analysis tested at different sweep rates. Fig. 7a presents the CV curves at various sweep rates ranging from 0.2 to 1.0 mV s1. All the curves show a similar profile with a couple of prominent cathodic and anodic peaks. The current (i) in CV curves obeys the power-law equation below [38,71], i = avb

(4)

where v is sweep rate, a and b are adjustable parameters. The relationship between i and v can be described by the following Equation 5, log(i) = blog(v) + log(a)

(5)

The b value as determined by the slope of the log(i)-log(v) plot can be used to identify the capacitive or diffusion-controlled mechanisms in LIBs. b = 0.5 means a diffusion-controlled behaviour, while b = 1.0 represents a capacitive behaviour [38,71]. Fig. 7b presents the log(i)-log(v) plots of the SVR-SnS electrode at the cathodic and anodic peaks. Thus, b values of the cathodic and anodic peaks are evaluated to be 0.81 and 0.89, respectively. Thus, it indicates a mixed diffusion and capacitive mechanism with a

17

favoured pseudocapacitive effect of the SVR-SnS electrode. The capacitive contribution ratio at a fixed sweep rate can be calculated by the following Equation 6 [38,71], i(V) = k1v + k2v0.5

(6)

where k1 and k2 are adjustable parameters. The quantities, k1v and k2v0.5 in Equation 6 represent the fraction of the surface capacitive behavior and the diffusion controlled behaviour, respectively. Fig. 7c shows the whole pseudo-capacitive contribution area at a sweep rate of 0.6 mV s1. The capacitive contribution is estimated to be 73.8% of the total charge. The contribution ratios at different sweep rates are presented in Fig. 7d. With the increasing sweep rate, the capacitive contribution increases gradually. The pseudo-capacitive contributions at sweep rates of 0.2, 0.4, 0.6, 0.8 and 1.0 mV s1 are 63.9%, 68.8%, 73.8%, 77.4% and 81.0%, respectively. The pseudo-capacitive storage becomes more prominent at higher sweep rates, which supports the fast Li+ intercalation/extraction for outstanding reversibility and rate capability of the SVR-SnS electrode [38,71].

4. Conclusions In summary, we have explored an effective approach to synthesize hexagonal SnS2 and orthorhombic SnS nanoplates by morphology-conformal conversion from the IE-SnS2 nanoplates. The SVR-SnS2 and SVR-SnS nanoplates are prepared respectively by adjusting the annealing temperature. Conversion from the IE-SnS2 to SnS involves three steps of removing intercalated octylamine, generation of S vacancies, and phase transformation. The bandgap decreases from 2.48 eV for the IE-SnS2, to 2.20 eV for the SVR-SnS2, and further to 1.33 eV for the SVR-SnS, indicating improved 18

electronic conductivity. Increased density of S vacancies is observed in the annealed samples, thereby contributing to more active sites for enhanced electrochemical reaction kinetics. When employed as anode materials for LIBs, the SVR-SnS2 and SVR-SnS nanoplates show much better electrochemical performance than the IE-SnS2. It is observed an obvious increase in capacity retention from 8.3% for the IE-SnS2, to 31.7% for the SVR-SnS2, and further to 41.6% for the SVR-SnS when current density increases from 0.1 to 10 A g1. Specially, the SVR-SnS electrode exhibits outstanding rate performance (510 mAh g−1 at 10 A g−1) and long cycling life (765 mAh g−1 at 2 A g−1 after 1200 cycles). The exceptional performance of the SVR tin sulfide electrodes suggests that the vacancy modulation strategy will open up a new opportunity for developing layered metal chalcogenides as advanced electrodes for the next-generation LIBs.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51972092, 51802145 and 51702366), and the Fundamental Research Funds for the Central Universities (Grant No. PA2018GDQT0009).

Appendix A. Supplementary data

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Scheme 1 Schematic illustration for the synthesis of IE-SnS2, SVR-SnS2 and SVR-SnS.

29

Fig. 1 (a-g) SEM image (a), TEM image (b), HAADF image (c), EDX mappings (d,e), SAED pattern (f), HRTEM image (g) of the SVR-SnS2 nanoplates, inset of (g): profile plot of the calibration for measuring the spacings. (h-j) TEM image, SAED pattern, HRTEM image of the SVR-SnS nanoplates.

30

Fig. 2 (a) XRD patterns, (b) Raman spectra, (c,d) Sn 3d (c) and S 2p (d) core-level XPS spectra of the (i) IE-SnS2, (ii) SVR-SnS2, and (iii) SVR-SnS nanoplates.

31

Fig. 3 (a) TG curves of the (i) IE-SnS2, (ii) SVR-SnS2, (iii) SVR-SnS nanoplates performed in nitrogen atmosphere. (b) EPR spectra of the three samples. Inset of (b): an enlarged view of the curves in panel (b).

32

Fig. 4 Photographs (a), UV-Vis spectra (b), and (ahv)2-hv plots (c) of the (i) IE-SnS2, (ii) SVR-SnS2, (iii) SVR-SnS nanoplates.

33

Fig. 5 (a) Rate performance of the three electrodes. (b) Cycling performance of the three electrodes at a current density of 1 A g1. (c) Long-term stability and corresponding Coulombic efficiency of the SVR-SnS electrode at a current density of 2 A g1; the initial discharge/charge cycle was conducted at a small current density of 50 mA g1. (d) Nyquist plots of the three electrodes before cycling. (e) An equivalent circuit to fit the Nyquist plots and evaluate the Rct values. (f) Nyquist plots of the SVR-SnS electrode at various cycles.

34

Fig. 6 (a,b) CV curves of (a) the SVR-SnS2 electrode and (b) the SVR-SnS electrode at a sweep rate of 0.1 mV s1. (c,d) Representative discharge/charge curves of (c) the SVR-SnS2 electrode and (d) the SVR-SnS electrode at a current density of 1 A g1.

35

Fig. 7 (a) CV curves of the SVR-SnS electrode at various sweep rates. (b) log (peak current) vs. log (v) plots for the anodic and cathodic peaks. (c) Pseudocapacitive contribution (red region) in CV curve of the SVR-SnS electrode at a sweep rate of 0.6 mV s−1. (d) Pseudocapacitive contribution at different sweep rates.

36

Declarations of interest: none. We wish to confirm that there are no known conflicts of interest associated with this publication.

37

Graphical abstract

Tin sulfide nanoplates with abundant sulfur vacancies have been synthesized and demonstrated as lithium-ion battery anode materials with excellent long-term cycling stability and superior rate performance.

38

Highlights · S-vacancy-rich SnS2 and SnS nanoplates are synthesized. · Phase transformation from SnS2 to SnS involves generation S vacancies. · The S vacancies contribute to improved electronic conductivity. · The SVR-SnS anode exhibits excellent long-term cycling stability for LIBs.

39