r-GO Composite with Enhanced Pseudocapacitance as High

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SnSe/r-GO Composite with Enhanced Pseudocapacitance as High-performance Anode for Li Ion Batteries Yayi Cheng, Jianfeng Huang, Jiayin Li, Liyun Cao, Zhanwei Xu, Xiaomin Luo, Hui Qi, and Penghui Guo ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00441 • Publication Date (Web): 12 Apr 2019 Downloaded from http://pubs.acs.org on April 14, 2019

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ACS Sustainable Chemistry & Engineering

SnSe/r-GO Composite with Enhanced Pseudocapacitance as High-performance Anode for Li Ion Batteries Yayi Cheng †,‡, Jianfeng Huang † , Jiayin Li † , Liyun Cao †, Zhanwei Xu †, Xiaomin Luo †, Hui Qi †, Penghui Guo † †

School of Materials Science & Engineering, Shaanxi Key Laboratory of Green Preparation and Functionalization for Inorganic Materials, Shaanxi University of Science and Technology, Xi’an 710021, China.

‡ Xi’an

Aeronautical University, 259 West Second Ring, Xi’an 710077, China.

ABSTRATE: Ultrafine SnSe nanocrystals uniformly dispersed on the reduced graphene oxide (r-GO) surface was synthesized by a facile one-step solvothermal technique. The as formed SnSe with size less than 5 nm connect with r-GO by Sn-C and Sn-O-C bonds. This chemically bonded SnSe/r-GO composite exhibits enhanced electrochemical properties that the reversible capacity can be maintained around 1046 mAh g−1 at 200 mA g−1 and ~514 mAh g−1 at 2000 mA g−1. Further analysis finds that the superior Li-ions storage performance of SnSe/r-GO electrode is dominated by a surface-controlled pseudocapacitive behavior, which contributes 88.7 % of the total capacity at 0.5 mV s-1. Such high ratio of pseudocapacitive storage in the SnSe/r-GO electrode could be ascribed to the synergistic effect of chemically bonded ultrafine SnSe nanocrystals with conductive r-GO networks. KEYWORDS: SnSe nanocrystals, Quantum dots, (deletion) Pseudocapacitance,

Corresponding author. Tel./fax: +86 29 86168868.



E-mail addresses: [email protected] (Huang. Jianfeng)

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Reduced graphene oxide, Anode, Li-ions batteries INTRODUCTION Alloy type anodes such as Silicon and Tin-based electrode with high theoretical specific capacity have great potential to substitute the commercial graphite (~372 mAh g−1) electrode used in Lithium ions battery (LIBs).1,

2

Especially Tin-based

anodes attract extensive attention due to its nature abundance, environment-friendly and nontoxicity,3-6 such as elemental Tin (Sn), Tin oxide (SnO, SnO2), Tin sulfide (SnS, SnS2) Tin selenide (SnSe, SnSe2). Among Tin-based materials, SnSe is particularly regarded as one of the promising anode materials for the next-generation LIBs because of its high theoretical capacity (847 mAh g − 1) and good conductivity.7 Moreover, SnSe with layered structures are loosely bound by van der Waals force between the interlayer, which could provide easier charge transfer routes.8-10 However, similar to other alloyed materials, the huge volumetric change and slow electrochemical reaction kinetics of SnSe during the lithiation/delithiation process lead to unsatisfactory cycling and rate performance.8, 11-13 To solve the above issues, constructing SnSe with conductive carbon composite/hybrid materials is an effective and frequently-used way.14-16 Since the carbonaceous materials can not only provide highway for the fast electron transport, but also act as matrix to buffer the volume change during charge/discharge cycle.17, 18 These SnSe/carbon-based materials improve electrochemical reaction activity of SnSe, resulting in high reversible capacity and good cycling stability. Thus far, graphene,19 carbon nanotubes,20 conductive carbon black7,


21-23

and carbon fibers24

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have been widely used as carbon matrix to build SnSe/C materials, which shows enhanced electrochemical performance in comparison with bare SnSe. Nevertheless, these SnSe/C composite listed above are usually synthesized by high energy ball milling method, which is difficult to realize the accurate control of SnSe morphology and interface structure with carbon. The as-prepared SnSe/C with messy stacked structure gives rise to congested Li+ and electrons transfer channels,25 thus resulting in diffusion-controlled electrochemical storage mechanism with sluggish charge transport rate. Recently, researchers find that the pseudocapacitive storage behavior will take place on the electrodes surface during the Li-ion insertion/extraction process when the materials structure is controlled at ultrafine nanoscale (< 10 nm).26-28 The pseudocapacitive behavior has been considered as one type of Li-ion storage mechanism with fast surface redox reactions to offer excellent rate performance and high reversible capacity.29 Xiaochuan Ren et al. reported SnSe nanoplates vertically grown on nitrogen-doped carbon (SnSe/NC) with high pseudocapacitive Na-ion storage and enhanced rate capability.25 Thereby, constructing nanostructed electrode could be potentially applied to control pseudocapacitive storage mechanism for achieving electrochemical performance enhancement of SnSe. Herein, we report chemically bonded SnSe nanocrystals anchored on reduced graphene oxide (SnSe/r-GO) for high performance anode in LIBs, which are prepared by a simple solvothermal method. The SnSe/r-GO electrode shows surface pseudocapacitive dominated mechanism, which is supposed to be the main reason for



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the outstanding electrochemical performance of SnSe/r-GO with high reversible capacity and fast charge rate. It delivers a high reversible capacity of 1046 mAh g-1 at current density of 200 mA g-1 and even ~514 mAh g-1 maintained at 2000 mA g-1. We believe that our work could provide a novel structure design strategy for obtaining the surface

pseudocapacitive

storage

mechanism

with

superior

electrochemical

performance. EXPERIMENTAL SECTION Materials synthesis. In a typical process to prepare SnSe/r-GO composite, 60 mg graphene oxide (GO) was dispersed in 60 mL ethylene glycol by ultrasonication for 120 min to form homogeneous mixture. Then 0.3418 g SnCl2··2H2O was added in above mixture with stirring about 30 min to form solution A. After that, 0.1185 g of Se powder was dissolved in 5 mL hydrazine hydrate (N2H4 · H2O, 50 %) under constant stirring for 1 h to form solution B. The precursor was prepared by solution B was added dropwise into the solution A with vigorous stirring at 500 r min-1. Finally, the precursor was transferred into a 100 mL Teflon-lined steel autoclave and heated at 180 °C for 12 h. After cooling down to room temperature, the precipitates were collected through centrifugation and washed repeatedly with D.I. water and absolute ethyl alcohol following by freeze-drying to obtain the SnSe/r-GO composite. In addition, pure SnSe was synthesized under the same process with the absence of GO. And also the as-prepared SnSe with GO (raw materials) were mixed as a comparison. Materials characterization. The phase and crystal structures of the as-prepared samples were recorded by X-ray diffraction (XRD, Rigaku D/max-Rb) using Cu Kα


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radiation at λ = 1.5406 Å. The microstructures were determined by field emission scanning electron microscopy (FE-SEM, S-4800) and transmission electron microscopy (TEM, Tecnai G2F20S-TWIN system). The SnSe loading mass was characterized by TG curves at heating rate of 10°C min-1 in air from 50 to 800 °C. Raman spectra were carried out to characterize the graphitization degree of GO and SnSe/r-GO composite. X-ray photoelectron spectroscopy (XPS, Axis Ultra spectrometer with Mg Kα X-ray source) was done in order to detect the surface chemical composition of the samples. Electrochemical test. The electrochemical properties were tested by assembled CR2032-type half cells in an Ar-filled glove box (Mbraun, Germany, O2 and H2O contents < 0.5 ppm). The Li foil was used as both counter and reference electrode. The working electrode was prepared by mixing active materials (70 wt%) with super P (20 wt%), and sodium carboxymethyl cellulose (CMC, 10 wt%) in deionized water to form a slurry, which was painted on the Cu foil and dried at 80 °C for 24 h in a vacuum oven. Then the foil was punched into circular working electrode of 1.58 cm in diameter and the active materials loading on each electrode was about 1 mg cm-1. The electrolyte used in cells was 1 M LiPF6 dissolved in a mixture of ethylene carbonate (EC), ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC) (1:1:1 in volume), and the separator was microporous polypropylene film. Galvanostatic charge/discharge measurements were conducted on a Neware battery testing system under various current density with a cut-off voltage range between 0.001 V and 3.0 V (vs. Li/Li+). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy



(EIS) were performed using a CHI 600D electrochemistry workstation at room temperature. RESULTS AND DISCUSSION

Figure. 1 (a) XRD patterns and (b) TG curves of SnSe and SnSe/r-GO composite. The TG measurement was performed under air from 50 to 800 °C at 10 °C min-1. Figure. 1a shows the XRD patterns of SnSe and SnSe/r-GO composite synthesized under the same condition. All diffraction peaks of both pure SnSe and SnSe/r-GO composite are well indexed to the orthorhombic structure of SnSe (JCPDS No. 48-1224) without detecting other impurities. This indicates that the product phase is not affected by the addition of GO in the process of preparation SnSe/r-GO. The average grain size of SnSe/r-GO and bare SnSe was calculated to be about 3.1 nm and 14.6 nm based on (111), (400), (311) peaks by Scherrer’s formula. TG analysis is performed to determine the r-GO content in the SnSe/r-GO composite, which is conducted under air from 50 to 800 °C at a ramp of 10 °C min-1. As shown in Figure. 1b, the TG curves can be understood by divided into four steps. The first step from 50 to 100 ° C, slight weight loss of both SnSe and SnSe/r-GO originates from the evaporation of the surface moisture.8, 30 From 100 to 400 °C, the weight of pure SnSe


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increase due to the oxidation reaction from SnSe to SnO2 and SeO2.31-33 In this process, the weight of SnSe/r-GO is almost unchanged, attributing to the offset between weight loss from the removal of oxygen-containing functional groups on r-GO and weight increase from the oxidation of SnSe.14 Thereafter, solid SeO2 begins to volatilize till 600 °C, resulting in the weight decrease. For the last step between 600 and 700 ° C, the abrupt weight loss of SnSe/r-GO corresponds to the combustion reaction of the r-GO. Combined with the total weight loss of pure SnSe (10.1 %) and SnSe/r-GO (26.82 %), the estimated content of r-GO in the SnSe/r-GO composite is 18.6 %.

Figure. 2 SEM images of SnSe (a, b) and SnSe/r-GO composite (c, d). Morphology of as-prepared SnSe and SnSe/r-GO composite are characterized by scanning electron microscopy (SEM), as shown in Figure. 2. SEM images of pure SnSe shown in Figure. 2a and 2b present uniform nanoparticles structure with the size less than 20 nm. In Figure. 2c, the low-magnification SEM image of SnSe/r-GO composite exhibits homogeneous nanosheets without agglomeration, which is



supposed to be the r-GO nanosheets. This also indicates that the GO is dispersed well in the precursor. Further observation the single nanosheet in Figure. 2d, it can be seen that many ultrafine SnSe nanoparticles are anchored on the r-GO nanosheet surface. The particle size of SnSe on the r-GO surface is obviously much smaller in comparison with the pure SnSe nanoparticles (Figure. 2b), demonstrating that the addition of GO could suppress the growth of SnSe nanocrystals.

Figure. 3 TEM image, HRTEM image and SAED pattern of SnSe (a, b, c) and SnSe/r-GO composite (d, e, f); (g) is the STEM image of SnSe/r-GO composite with the corresponding Sn, Se and C elemental mapping images. Transmission electron microscopy (TEM) was carried out to further study the fine microstructure of SnSe and SnSe/r-GO composite. As shown in Figure. 3 (a, b),


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SnSe presents slightly glomerate nanoparticles having sizes of approximately 10~20 nm. The selected area electron diffraction (SAED) pattern of SnSe (Figure. 3c) with several polycrystalline rings could be ascribed to the SnSe (201), (400), (102) and (212) diffraction planes. In Figure. 3d, the SnSe nanocrystals are uniformly deposited on graphene nanosheets, which is in accordance with the SEM images (Figure. 2c and 2d). Further observation from the lattice resolved HRTEM (Figure. 3e), the average particle size of SnSe nanocrystals in SnSe/r-GO composite is extracted about 3~5 nm and some interplanar spacing of SnSe lattice planes are observed. The SAED pattern of SnSe/r-GO shown in Figure. 3f indicates the high purity of SnSe in the composite. Meanwhile, the EDX elemental mapping in Figure. 3g further suggests a uniform distribution of the Sn, Se and C components, demonstrating the homogeneous coating of SnSe on graphene nanosheets. This tight contact between SnSe and r-GO is beneficial to the charge transfer and restrains the particle aggregation upon cycling.34

Figure. 4 (a) Raman spectra of the graphene oxide (raw materials) and SnSe/r-GO composite. (b) Enlarged G band of the graphene oxide and SnSe/r-GO composite. The Raman spectra of GO (raw materials) and SnSe/r-GO composite were



shown in Figure. 4 to mainly depict the carbon structure. In Figure. 4a, a typical characteristic of SnSe/r-GO with an obvious peak from 160 to 260 cm−1 is assigned to the Eg and A1g mode of SnSe.35 In addition, another two broad high-wavenumber peaks are also observed around 1350 and 1580 cm−1, corresponding to D and G band respectively.36 Generally, the D band arises from the local defects and disorders in the graphitic networks whereas G band originates from sp2-hybridized graphitic carbon structure.37 The intensity ratio of D and G band, abbreviated as ID/IG, is usually used to evaluate the concentration of carbon defects. According to the fitting results, the ID/IG value of the SnSe/r-GO composite (1.67) is higher than that of GO (1.38), indicating that the presence of SnSe nanocrystals decreases the sp2 carbon domains on the GO.38, 39 It should note that the G band of SnSe/r-GO shift to low-wavenumber in comparison to the GO. As shown in Figure. 4b, the G band of GO locates at 1591 cm-1. While after the GO surface is coated by SnSe nanocrystals, its G band shift by 9 cm-1 towards the left. This indicates that there are strong charge transportation interaction between r-GO and SnSe based on the related literature reports.40, 41


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Figure. 5 (a) Wide-survey XPS spectrum of SnSe/r-GO; (b) Se 3d spectra of SnSe and SnSe/r-GO composite; (c) C 1s and (d) O 1s XPS spectra of GO and SnSe/r-GO composite. X-ray photoelectron spectroscopy (XPS) was further performed to analyze electronic state and composition of the SnSe/r-GO composite. The survey spectrum of SnSe/r-GO (Fig. 5a) contains Sn, Se, C and O elements, in which O element results from the residual oxygen-containing functional groups on r-GO surface. In Fig. 5b, the high-resolution Se 3d spectra of both SnSe and SnSe/r-GO present two peaks corresponding to Se 3d3/2 and Se 3d5/2, respectively. Whereas the peaks of SnSe/r-GO composite shift to a higher binding energy compared with bare SnSe, indicating the presence of oxidation state of Sn, Se and the strengthened electron interaction between SnSe and r-GO,16 in agreement with the Raman spectrum of SnSe/r-GO. The



C 1s and O 1s spectra of pristine GO and SnSe/r-GO composite were shown in Fig. 5c and 5d. In comparison, there are two more peaks for SnSe/r-GO composite appearing at 283.6 and 286.8 eV that can be assigned to the Sn-C and Sn-O-C bonding in C 1s spectra.41-43 Further observation by the O 1s spectra in Fig. 5d, the SnSe/r-GO could be deconvoluted into two peaks corresponding to Sn-O-C (531.01 eV) and C-O-C (531.8 eV) bonds, which also confirms the existence of Sn-O-C bonds. Thereby, we conclude that the interaction and intimate contact between SnSe nanocrystals and r-GO are through the Sn-C and Sn-O-C bonds. These chemical bonds could not only act as a linkage for charge transfer but aslo sustain stable structure of SnSe/r-GO composite, further affecting the electrochemical reaction activity and behavior of electrode material during its lithiation/delithiation process.


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Figure. 6 Cycling performance at 200 mA g-1 (a) and rate capacity (b) of the SnSe and SnSe/r-GO electrode; (c) Charge-discharge curves of SnSe/r-GO electrode tested at 200 mA g-1 for different cycles; (d) Differential capacity plots of SnSe/r-GO electrode for 1st and 10th cycles; (e) The electrochemical impedance spectroscopy (EIS) plots of SnSe and SnSe/r-GO electrodes after 100 cycles; (f) the corresponding equivalent circuit model for the EIS and (g) a summary of fitting results of resistance value for SnSe and SnSe/r-GO. The electrochemical performance of both SnSe and SnSe/r-GO as LIBs anode



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was evaluated by CR2032-type half cells shown in Figure. 6. In Figure. 6a, SnSe and SnSe/r-GO electrode exhibit an initial discharge capacity of 1302 and 1049 mAh g-1 at the current density of 200 mA g-1, corresponding to the Coulombic efficiency of 81.6 and 78 %, respectively. The initial capacity loss could be assigned to the decomposition of electrolyte, formation of solid-electrolyte interface (SEI) films and electrolyte intercalation into the high surface-to-volume ratio r-GO.44,

45

In the

following cycles, the reversible capacity of SnSe electrode decays severely and just 230 mAh g-1 is maintained till 150th cycle. Compared with SnSe, the capacity of SnSe/r-GO electrode gradually increases to 1080 mAh g-1 from the 2nd to 165th cycles, which may result from the material activation upon lithiation/delithiation process induced by structural changes, as reported in Sb2Se3 anode.44 Then the SnSe/r-GO electrode undergoes slight degradation of capacity owing to the little pulverization of the SnSe nanocrystals during the cycling and 1002 mAh g−1 can be maintained till 200 cycles. In addition, the cycling performance of SnSe and Graphene oxide mixture (SnSe/GO mixture) at 200 mA g-1 was also conducted shown in Figure. S1. It is found that the capacity of SnSe/GO mixture decrease gradually from 502 to 375 mAh g-1 rather than the upward trend in the SnSe/r-GO composite. The rate performance of SnSe and SnSe/r-GO composite were also investigated at different current densities and demonstrated in Figure. 6b. It is found that the SnSe/r-GO can deliver much higher reversible capacity than bare SnSe (815 vs. 582, 767 vs. 470, 670 vs. 335, 596 vs. 200, 514 vs. 88 mAh g-1) for all current densities studied from 100 to 2000 mA g-1. When the current density is finally reduced back to 500 mA g-1, the


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reversible capacity of SnSe/r-GO can quickly recover to 645 mAh g-1, very close to the prior 670 mAh g-1. Even the SnSe/r-GO electrode is further tested at a higher current density (2000 mA g-1) shown in Figure. S2, it can still demonstrates a reversible capacity of 512 mAh g-1 after 100 cycles. Therefore, concluding from the above results indicates that the SnSe/r-GO electrode exhibits the outstanding cycling and rate performance in contrast to bare SnSe. Figure. 6c and Figure. S3 compare the galvanostatic charge/discharge (GCD) profiles of SnSe/r-GO and SnSe electrode at different cycles. The two electrodes show almost similar GCD profiles at 1st and 2nd cycles, presenting two obvious discharge voltage plateaus at ~1.25 V/0.3 V and charge plateaus at ~0.5 V/1.5 V. For bare SnSe electrode, the charge/discharge plateaus disappear during high voltage range after 10 cycles, while the GCD profiles of SnSe/r-GO almost coincide from 1st to 150th cycle. To further analyze the charge/discharge plateaus in detail, we measured the differential capacity plots (DCP) of SnSe and SnSe/r-GO electrodes at the 1st and 10th cycles. As illustrated in Figure. 6d and Figure. S4, the SnSe shows the similar cathodic/anodic peaks with SnSe/r-GO electrodes in the 1st cycle. During the cathodic sweep, a sharp peak around 1.2 V can be ascribed to the intercalation of Li+ into the SnSe interlayer in the form of LixSnSe,25 accompanied with the formation of solid electrolyte interface (SEI) film. The weak redox peak at ~0.87 V corresponds to the conversion reaction from SnSe to Sn and Li2Se. Additionally, the peaks below 0.6 V are assigned to alloying reaction between Sn and Li+ forming series LixSn. The anodic peaks between 0.005~0.6 V in the charging process is attributed to the gradual



dealloying reaction from LixSn to Sn, while the weak peak centered at ~1.15 V relates to the conversion of Sn to SnSe, along with the formation of LixSnSe.25 Besides, both the anodic peak about 1.78 V for SnSe electrode and extra peak at 2.2 V for SnSe/r-GO are correlated with the Li+ extraction from the host and re-formation of SnSe. At 10th cycle, the cathodic/anodic peaks in DCP profiles for SnSe/r-GO electrode nearly overlap with the 1st cycle. In contrast, the cathodic/anodic peaks of bare SnSe have disappeared at high voltage, demonstrating irreversible conversion reaction between Sn and SnSe. Consequently, the DCP profiles indicate that the SnSe/r-GO electrode shows higher conversion reaction reversibility. The electrochemical impedance spectra (EIS) of SnSe and SnSe/r-GO electrodes were carried out to gain further insight into the cause of better electrochemical performance for SnSe/r-GO composite and bare SnSe anode as reference. In Figure. 6e, both Nyquist plots consist of one depressed semicircle in medium frequency region and an inclined line in low frequency, the former corresponds to the charge transfer resistance (Rct) at the electrode/electrolyte interface and the latter is related to the process of Li+ diffusion in the bulk electrode (Warburg impedance).46-48 This could be well understood by the equivalent circuit model shown in Figure. 6f and the fitting parameters are presented in Figure. 6g. The fitting results demonstrate that SnSe/r-GO electrode shows much smaller Rct value (14.5 Ω) than that of bare SnSe (62.2 Ω), indicating the faster interface kinetics of SnSe/r-GO. Moreover, the more-vertical line at low frequency for SnSe/r-GO indicates a strengthened capacitive-like behavior in the electrode during the storage of Li+.29 The SnSe very


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differs from SnSe/r-GO that the angle of the oblique line is close to 45°, which is a phenomenon of diffusion- controlled reaction process in the electrode.

Figure. 7 (a) Cyclic voltammograms (CV) curves of SnSe/r-GO electrode at various scan rates tested after 20 cycles; (b) Plots of ln (anodic peak current) against ln (square root of scan rate) for SnSe/r-GO calculated from the corresponding CV curves; Cyclic voltammogram with the capacitive contribution (shaded region) to the cells in the SnSe/r-GO electrode at (c) 0.1 mV s-1 and (d) 0.5 mV s-1. To further explore the electrochemical reaction kinetics characteristic and charge storage behavior of SnSe/r-GO electrode, the CV measurement were investigated at various scan rates ranging from 0.1 to 0.5 mV s-1. As shown in Figure. 7a and Figure. S5, the SnSe/r-GO exhibits higher peak currents and more reduction/oxidation peaks than bare SnSe (Figure. S5) at the same scan rate, indicating that the SnSe/r-GO



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yields much higher Li-ion storage capacity.42 This charge storage behavior and reaction kinetics are analyzed by correlating the current (i) with sweep rates (ν) according to Equation (1) as follows:49, 50 i = aν𝑏

(1)

where a and b are fitting parameters. Herein, b-value is of great significance, which can be calculated by the slope of the plot of ln i vs. ln v. When b is 0.5, the charge transfer is prevailed by the diffusion-controlled process; while b is close to 1.0, the system is surface-controlled process.46, 51-53 Figure. 7c and Figure. S6 show the ln i vs. ln v plots of SnSe/r-GO and SnSe electrodes at oxidation and reduction process based on the CV curves. From these plots, the b1 (reduction peaks) and b2 (oxidation peaks) of SnSe/r-GO and SnSe are calculated to be 0.69/0.96 and 0.72/0.57, respectively. This indicates that the b-value of two electrodes are similar in discharge process (0.69 vs. 0.72), implying the diffusion and surface-controlled behavior coexistence during lithiation process. However, in the charge process, the b-value of SnSe/r-GO is nearly close to 1.0, possessing obviously surface-controlled capacitive effects rather than the diffusion-controlled Li-ions storage behavior in bare SnSe electrode (b2=0.57). The quantitative storage contributions can be evaluated by separating the current response (i) at a fixed voltage derived from the following Equation:54-58 i(V) = k1ν + k2ν1/2

(2)

In which, k1v and k2v1/2 correspond to the current contributions from the surface-capacitive effects and diffusion-controlled redox process, respectively. Whereas k1 and k2 are constant values for the same electrochemical reaction,


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determining by rearranging Equation (2) at a certain voltage. Then the capacitive and diffusion-controlled capacity can be quantitative calculated by separating the current response i into k1v and k2v1/2. As illustrated in Figure. 7c and 7d, the capacitive capacity contribution to the total charge for SnSe/r-GO electrode are 75.6 % and 88.7 % at a scan rate of 0.1 mV s-1 and 0.5 mV s-1, respectively. While the proportion of capacitive contribution for SnSe electrode is only 51.4 % at 0.5 mV s-1 shown in Figure. S7. This demonstrates that the SnSe/r-GO delivers much higher specific capacity with enhanced capacitive contribution at the same scan rates. The capacitive area with distinct peaks further indicates the pseudocapacitive behavior characteristic of SnSe/r-GO electrode. Owing to the surface-controlled pseudocapacitive storage mechanism could provide ultrafast redox reaction of the surface atoms, rather than sluggish ion insertion/extraction kinetics during the diffusion-controlled process. Therefore, the SnSe/r-GO electrode with surface pseudocapacitive Li+ storage mechanism shows superior electrochemical performance.



Figure. 8 Ex situ TEM, HRTEM images and corresponding SAED pattern of SnSe/r-GO (a, b, c) and bare SnSe electrode (d, e, f) after 100 cycles under fully charged. The white box in (b) is the local magnification area of the lattice fringes. Ex situ TEM was performed to elucidate the structure change and further interpret the pseudocapacitive effects of SnSe/r-GO composite with superior performance. As shown in Figure. 8a and 8b, SnSe with size about 3~5 nm seems to be a film that still anchors on the r-GO surface, maintaining good structural stability and close electrical contact of electrode. Further measurement from HRTEM image (Figure. 8b) and SAED patterns (Figure. 8c), the lattice spacing are well indexed into the SnSe (210), (311) and (810) planes, indicating highly reversible conversion reaction of SnSe/r-GO electrode that Sn and Li2Se could be converted into SnSe in charging process. On contrast, bare SnSe electrode has lost its original nanoparticle structure and presents pulverized and aggregated morphology after 100 cycles, as shown in Figure. 8d. The corresponding lattice fringes (Figure. 8e) and SAED pattern (Figure. 8f) of SnSe electrode are ascribed to Sn (PDF No. 65-2631) and Li2Se (PDF No. 23-0072), and no SnSe are detected. This manifests that bare SnSe electrode displays poor structural stability and conversion reaction reversibility, which are consistent with the ex situ SEM images (Figure. S8) and DQ/DV curves.


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Figure. 9 Schematic diagram of the specific charge transfer mechanism for the SnSe/r-GO composite. Based on the above results, we conclude that the enhanced electrochemical performance of SnSe/r-GO is attributed to the surface pseudocapacitive prevailed storage mechanism. This is induced by the synergistic effect of chemically bonded ultrafine SnSe nanocrystals with high conductive r-GO networks. As shown in Figure. 9, the structural advantages can be summarized as: (i) the ultrafine SnSe nanocrystals (less than 5 nm) possess great electrolyte wetting area and short Li+ transfer routes, facilitating surface redox reactions; (ii) the chemical bonds (Sn-C and Sn-O-C) promote fast charge transfer between SnSe nanocrystals with r-GO interface and improve structural stability of SnSe/r-GO electrode; (iii) r-GO with conductive networks not only provides highway for the rapid charge transport paths but also tolerate the volume change of SnSe during charging/discharging process. These merits contribute to high surface pseudocapacitive storage capacity (88.7 %) of SnSe/r-GO composite, yielding the superior cycling and rate performance. CONCLUSIONS



In this work, a one-step solvothermal technique was used for synthesis of SnSe nanocrystals with reduced graphene oxide (SnSe/r-GO) composite as anode materials for Li-ions batteries. The as-prepared SnSe/r-GO composite with 3~5 nm SnSe well coated on the r-GO nanosheets by Sn-C and Sn-O-C bonds exhibit superior electrochemical performance. It delivers a reversible capacity of 1046 mAh g-1 till 150 cycles at the current density of 200 mA g-1, even at 2000 mA g-1, the capacity of around 514 mAh g-1 can be sustained. This superior property of SnSe/r-GO composite mainly derives from its highly predominated surface pseudocapacitive storage mechanism (88.7% of the total capacity), which provides ultrafast redox reaction without breaking the original structure of SnSe. By combining this electrochemical reaction mechanism with electrode microstructure, we conclude that both the chemically bonded ultrafine SnSe nanocrystals and conductive r-GO networks provide many ultrafast Li+ and electrons transfer paths for promoting surface pseudocapacitive Li-ions storage, thus resulting in excellent electrochemical properties of SnSe/r-GO composite. ASSOCIATED CONTENT Supporting Information Cycling performance of SnSe/GO mixture; Cycling performance of SnSe electrode at 2000 mA g-1; Charge-discharge curves, differential capacity plots and cyclic voltammograms (CV) curves of SnSe electrode; Plots of ln (anodic peak current) against ln (square root of scan rate) for SnSe calculated from the corresponding CV curves; CV with the capacitive contribution (shaded region) to the


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SnSe electrode at 0.5 mV s-1; Ex situ SEM of SnSe/r-GO and bare SnSe electrode after 100 cycles under fully charged. ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (No. 51472152), National Natural Science Foundation of China (No. 51702198), Innovation Team Assistance Foundation of Shaanxi Province (No. 2013KCT-06), Scientific Special Foundation of Shaanxi Province Office of Education (No. 15JK1074), State Key Laboratory of Solidification Processing in NWPU (No. SKLSP201636), China Postdoctoral Science Foundation (No. 2016M592897XB) and Graduate Innovation Fund of Shaanxi University of Science and Technology. REFERENCES (1) Chen, R.; Li, S.; Liu, J.; Li, Y.; Ma, F.; Liang, J.; Chen, X.; Miao, Z.; Han, J.; Wang, T.; Li, Q. Hierarchical Cu doped SnSe nanoclusters as high-performance anode for sodium-ion batteries. Electrochim. Acta 2018, 282, 973-980. (2) Hou, S.; Chen, T.; Wu, Y.; Chen, H.; Lin, X.; Liew, W.-K.; Chang, C.; Huang, J. Influence of Glucose Derivatives on Ball-Milled Si for Negative Electrodes with High Area Capacity in Lithium-Ion Batteries. ACS Sustainable Chem. Eng. 2019. DOI: 10.1021/acssuschemeng.8b04039. (3) Tang Q.; Su H., Cui Y.; Baker A. P.; Liu Y.; Lu J.; Song X.; Zhang H.; Wu J.; Yu H.; Qu D. Ternary tin-based chalcogenide nanoplates as a promising anode material for lithium-ion batteries. J. Power Sources 2018, 379, 182-190. (4) Tang Q.; Cui Y.; Wu J.; Qu D.; Baker A. P.; Ma Y.; Song X.; Liu Y. Ternary tin



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Synopsis: The chemically bonded SnSe/rGO composite with highly predominated surface pseudocapacitive storage mechanism shows superior electrochemical performance.


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r-GO Composite with Enhanced Pseudocapacitance as High

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