Solid-Solution Anion-Enhanced Electrochemical Performances of

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Solid-Solution Anion Enhanced Electrochemical Performances of Metal Sulfides/Selenides for Sodium Ion Capacitors: The Case of FeS2-xSex Yaqiong Long, Jing Yang, Xin Gao, Xuena Xu, Weiliu Fan, Jian Yang, Shifeng Hou, and Yitai Qian ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00931 • Publication Date (Web): 15 Mar 2018 Downloaded from http://pubs.acs.org on March 15, 2018

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ACS Applied Materials & Interfaces

Solid-Solution Anion Enhanced Electrochemical Performances of Metal Sulfides/Selenides for Sodium Ion Capacitors: The Case of FeS2-xSex Yaqiong Long,a Jing Yang,a Xin Gao,a Xuena Xu,a Weiliu Fan,a Jian Yang,a,* Shifeng Hou,b Yitai Qiana a

Key Laboratory of Colloid and Interface Chemistry, Ministry of Education, School of

Chemistry and Chemical Engineering, Shandong University, Jinan, 250100, Peoples Republic of China b

National Engineering Research Center for Colloidal Materials, Jinan, 250100, Peoples

Republic of China KEYWORDS: Sulfides, Solid solution, Sodium ion batteries, Sodium ion capacitors, DFT calculations

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ABSTRACT: Transition metal sulfides/selenides are explored as advanced electrode materials for non-aqueous

sodium-ion capacitors, using FeS2-xSex as an example. Solid

solution of S/Se in FeS2-xSex allows it to combine the high capacity of FeS2 and the good diffusion kinetics of FeSe2 together, thereby exhibiting excellent cycle stability (~220 mAh g-1 after 6000 cycles at 2 A g-1) and superior rate capability (~210 mAh g-1 at 40 A g-1) within 0.8-3.0 V. These results are much better than those of FeS2 and FeSe2, confirming the advantages of S/Se solid solution as supported by EIS spectra, DFT calculations and electron conductivity. As FeS2-xSex is paired with activated carbon as Na-ion capacitors, this device is also better than sodium-ion batteries of FeS2-xSex//Na3V2(PO4)3 and sodium-ion capacitors of metal oxides//AC, particularly at high rates. These results open a new door for the applications of sulfides/selenides in another device of electrochemical energy storage.

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INTRODUCTION Electrochemical energy storage devices, particularly lithium-ion batteries, have widely applied in portable electronics, electric vehicles (EVs), stationary power station, and so on.1-4 However, the uneven distribution and limited resources of lithium in earth can not meet the demands of these applications. So, sodium-ion batteries (NIBs) are developed as an alternative to address this issue.5-6 But the larger ionic radius and heavier mass of Na ions than Li ions, make the electrode materials suffered more from sluggish kinetics and huge volume change, bringing great challenges to the society. Thus, electrode materials that mitigate or overcome these shortcomings, become highly desirable. Recently, transition metal sulfides/selenides (TMSs) come into sights as anode materials in NIBs,7-16 because of their high specific capacity, enhanced conductivity, moderate bonding of M-S, and versatile crystal structures. All these features make them intriguing as electrode materials in NIBs. One of the most impressive TMSs is FeS2 microspheres,9 which could present an extraordinary cycling stability (90 % of capacity after 20000 cycles) and unprecedented rate capability (~ 170 mAh g-1 at 20 A g-1) over 0.8-3.0 V. The similar results on good cyclability and high rate capability are also observed for other sulfides/selenides, such as FeSe2 microspheres,10 N-doped carbon/CoS nanotubes,11 urchin-like CoSe2 nanostructure,12 CuS/rGO nanosheets,13 SnS0.5Se0.5 powders,14 MoSe2/N,P-co-doped rGO,15 VS2 nanosheets,16 etc.. However, these TMSs always experience multiple redox reactions upon cycling, resulting in severely overlapped voltage plateaus, or slope lines in galvanostatic discharge/charge profiles.7-16 These results would greatly affect the practical applications of TMSs, because they are neither single-voltage plateau electrodes nor quasi single-voltage plateau electrodes. What’s worse, many of these redox reactions locate between 1.0-2.0 V, which is too high as an anode material and too low as a cathode material. So, how to 3



use TMSs in NIBs becomes a dilemma. But if we treat the features of TMSs in another way, for example, in asymmetric sodium ion capacitors (NICs), these shortcomings might be compromised. Asymmetric NICs usually have one electrode worked via anion separation and accumulation at the electrolyte/electrode interface and the other electrode worked via Na-ion intercalation.17-20 Because the electrodes do not need to show a voltage plateau upon cycling in NICs, the dilemmas on the number and potential of voltage plateaus of TMSs are greatly mitigated. Moreover, NICs have a number of advantages over batteries, like high power density, long cycling life and fast discharge/charge ability.21-23 These features would allow them to fill the power and energy gap between traditional capacitors and batteries. Thus, the application of TMSs in NICs is quite promising. Herein, FeS2-xSex is selected as a model of TMSs to demonstrate this proof of concept, due to its typical characteristics of sulphides/selenides. The solid solution of S/Se in FeS2-xSex enables it to combine high specific capacity of FeS2 and good redox kinetics of FeSe2 together, thereby exhibiting ultralong cycle life (~220 mAh g-1 after 6000 cycles at 2 A g-1) and superior rate capability (~210 mAh g-1 at 40 A g-1). These performances are much better than those of FeS2 and FeSe2, confirming the advantages of S/Se solid solution in FeS2-xSex as supported by EIS spectra and DFT calculations. Although the similar strategy was tested in a few cases,14,24 these works focused on the electrochemical performances of TMSs and ignored the identification on the advantages of S/Se solid solution, as compared to MS or MSe alone. Actually, this identification is important, because it might offer a general strategy for TMSs to improve electrochemical performances. Then, FeS2-xSex is coupled with activated carbon to construct NICs. As expected, this device shows superior performances to NIBs also with FeS2-xSex as the anode material, both in rate capability and cycling stability. The cycle life goes up to 1000 cycles without severe loss in energy density. 4


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Such a long life is not reported for full cells using sulfides/selenides as anode materials. Moreover, the performances of FeS2-xSex//activated carbon could be promoted, by optimizing size, shape and structure of FeS2-xSex and carbon materials. EXPERIMENTAL SECTION Materials Synthesis. In a typical protocol, FeSO4·7H2O (1 mmol) and urea (5 mmol) were dissolved in a mixed solvent by 15 mL of dimethylformamide (DMF) and 20 mL of ethylene glycol (EG),9,25 giving a brown solution. Then, sublimed sulfur and selenium powders with their total molar number at 6.5 mmol, were added into the above solution with stirring. The resultant suspension was transferred to a Teflon-lined stainless steel autoclave that was then sealed and kept at 190 °C for 18 h. After that, the product was collected by centrifugation and rinsed by deionized water and absolute alcohol for several times. Finally, the product was dried in an oven at 60 °C overnight and used for structure characterization and electrochemical measurements. Instruments. X-ray powder diffraction (XRD) patterns were obtained on an advanced X-ray diffractometer (Bruker D8, Germany) with Cu kα line (λ= 0.1548 Å) as radiation source. SEM images were achieved from a field-emission scanning electron microscope (SUPRA 55, Germany), where the powders were stuck to a conductive tape to reduce the surface charge accumulation and coated by gold via sputtering. TEM and HRTEM images were taken from a transmission electron microscope (JEOL JEM 1011, Japan) and a field-emission transmission electron microscope (JEOL 2100F, Japan). The powders were first ultrasonically treated in absolute ethanol for several minutes. Then, the suspension was carefully casted on a carbon-supported copper grid. After the quick evaporation of ethanol, the powders deposited on the copper grid were ready for the observation with transmission electron microscopes. Surface composition was identified by X-ray photoelectron spectrometer (Thermo 5



Scientific ESCALAB 250, USA). All the signals were calibrated with the binding energies of C 1s at 284.6 eV. DC conductivity was tested by four-point probe (SZ-82, Suzhou, China) method. Electrochemical Measurements. Electrochemical properties of FeS2-xSex were measured in a setup of CR2032 coin cells. In these cells, the working electrode was fabricated by 70 wt% of FeS2-xSex, 20 wt% of acetylene black, and 10 wt% of poly(vinylidene fluoride) (pVdF, DodoChem). These components were hand-milled in several droplets of N-methyl-2pyrrolidone (NMP, Aladdin, >99.5%), resulting in a black slurry. Then, the slurry was casted on a clean copper foil and treated by a blade. The coated foil was dried at 60 °C in vacuum overnight. After that, the foil was roll-pressed and punched into discs with a diameter of 12 mm as working electrodes. Before the assembly into coin cells, these discs were weighted to calculate the mass loading of FeS2-xSex (1.0-1.3 mg cm-2). These discs as the working electrode assembled with a hand-made Na foil as the reference and counter electrode, and glass microfibers (Whatman GF/F) as the separator in an argon-filled glovebox (Mikrouna, Super 1220/750/900), which was then wetted with 1.0 M sodium trifluomethanesulfonate (NaSO3CF3) in diglyme (DGM) as the electrolyte. Other electrolytes like ethylene carbonate/ diethyl carbonate (EC/DEC, 1:1 in volume ratio) and propylene carbonate (PC), were also tested. Cyclic voltammograms (CV) were acquired from an electrochemical work-station (CHI 760D, Chenhua Instruments Co., China) at room temperature over the voltage range of 0.8-3.0 V. Galvanostatic discharge and charge tests were carried out within the same voltage range on battery cyclers (Land CT2001A, China). Electrochemical impedance spectra (EIS) were conducted on an electrochemical workstation (AUTOLAB PGSTAT302N, Switzerland) in the frequency range of 100 kHz to 0.01 Hz. The electrode was cycled and then stabilized at 1.7 V for several hours before this measurement. To demonstrate the promising potential, FeS2-xSex was paired with activated carbon for asymmetric sodium hybrid capacitors. Here, the counter cathode was fabricated by 80 wt% of 6


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activated carbon (AC, YP-50F), 10 wt% of acetylene black and 10 wt% of carboxyl methyl cellulose (CMC) on an aluminum foil. The mass ratio of activated carbon to FeS2-xSex was approximately 4:1. 1.0 M NaClO4 in tetraglyme was used as the electrolyte, due to its good stability at a high voltage. The anode was electrochemically activated for several cycles before it was assembled in capacitors. The specific capacity of FeS2-xSex//AC was computed by the total mass of FeS2-xSex and AC. A typical NIB, based on FeS2-xSex//Na3V2(PO4)3/C (FeS2-xSex//NVP), was also assembled to illustrate the advantage of NIC. Na3V2(PO4)3/C was synthesized by a sol-gel synthesis process followed by high-temperature calcination.26 Then, the cathode was fabricated by a similar procedure to that of activated carbon used in NICs, except the recipe as 80 wt% of Na3V2(PO4)3/C, 10 wt% of acetylene black and 10 wt% of pVdF. The mass ratio of Na3V2(PO4)3/C to FeS2-xSex was ~3:1. Similar to the case of FeS2-xSex//AC, the specific capacity for FeS2-xSex//NVP was calculated by the total mass of FeS2-xSex and Na3V2(PO4)3/C. Computational Details. Density functional theory (DFT) calculations are performed by the Vienna ab initio Simulation Package (VASP) with project-augmented wave approach (PAW).27,28 The exchange-correlation function was described by generalized gradient approximation (GGA) with Perdew and Wang (PW91) method.29,30 The cutoff energy of plane-wave was set to 360 eV. A k-point Monkhorst-Pack grid of 3×3×7 was used for Brillouin zone. The structure relaxation and energy calculation were achieved, until the Hellmann-Feynman force on each atom and total energy were less than 0.02 eV/Å and 10-5 eV respectively. Then, a double-sized Monkhorst-Pack grid (6×6×14) was computed for electronic structure. To speed up the convergence, Gaussian smearing with a width of 0.1 eV was employed. The supercell of Na2FeS2 and Na2FeS2-xSex are constructed by using Li2FeS2 structure.9

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RESULTS AND DISCUSSION

Figure 1. (a) XRD pattern of FeS1.6Se0.4. (b) The relationship between lattice constant (a=b=c) and x in FeS2-xSex (x=0-0.6). (c,d) SEM images, (e) HRTEM image and (f) elemental mapping images of FeS1.6Se0.4. FeS2-xSex was synthesized by a simple solvothermal reaction of FeSO4, S and Se. The molar ratio of Se/S in FeS2-xSex could be easily tailored by controlling Se/S in the reaction. Because FeS2-xSex at different ratios of Se/S exhibits the same structure, one of them, FeS1.6Se0.4, is selected as a representative to illustrate their commons. As shown in Figure 1a, all the diffraction peaks of FeS1.6Se0.4 are indexed as pyrite phase (JCPDS Card, No. 42-1340), the same one as that of FeS2. But the peaks slightly shift to the low angles (Figure S1). This shift becomes pronounced, as the content of Se in FeS2-xSex increases (Figure 1b). It indicates that Se has been successfully incorporated 8


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into the lattice, thus leading to the expansion of lattice constants (Figure 1b) and the blue shift of diffraction peaks. The broad peak width and low peak intensity suggest the low crystallinity and small grain size of FeS1.6Se0.4. The grain size in the range of 12.5-14.0 nm is estimated by Scherrer equation. Such a nanoscale size not only reduces the diffusion distance of Na ions, but also brings a large number of grain boundaries that effectively enhance the charge-transfer kinetics and realize the interfacial charge storage,31,32 greatly benefiting the electrochemical performances. Finally, it should be pointed out that as-prepared FeSe2 has a different structure with that of FeS1.6Se0.4 and FeS2 (Figure S2), which makes us prefer the comparison between FeS1.6Se0.4 and FeS2. Then, SEM images were used to identify the size, shape and structure of FeS2-xSex. Again, FeS1.6Se0.4 is selected as an example for this characterization, due to the similar morphology of FeS2-xSex in different Se/S ratios (Figure S3). As shown in Figure 1c, FeS1.6S0.4 consists of spherical microparticles with an average size of 3 µm. The close check on these microspheres discloses the rough surface and numerous grains (Figure 1d), indicating highly aggregated nanoparticles. This aggregation could be attributed to the lack of the strong and effective surface protection in the synthesis. This feature is also confirmed by HRTEM image (Figure 1e). The random orientation of the fringes suggests the polycrystalline nature, well explaining why the microsphere size is much bigger than the grain size calculated from XRD pattern. The fringe spacings at 0.289 nm and 0.259 nm could be attributed to {200} and {210} planes of pyrite phase, confirming this crystal structure again. Element mapping (Figure 1f) shows that Fe, S, and Se are uniformly distributed throughout the microsphere, well consistent with the solid-solution feature of FeS1.6Se0.4.

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Figure 2. XPS spectra of FeS1.6Se0.4: (a) survey spectrum, (b) Fe 2p, (c) S 2p, (d) Se 3d. XPS spectra were measured to gain insights about the surface components, very important to the formation of a solid electrolyte interphase (SEI) film. Analogue to the cases of XRD patterns and SEM images, only the results of FeS1.6Se0.4 are discussed here, for the sake of clarity. As presented in Figure 2a, the signals of Fe, S and Se could be visualized in the survey spectrum, consistent with the formation of FeS2-xSex. The intense signals of oxygen and carbon are probably caused by surface oxidation and carbon contamination, owing to inevitable exposure of the product to air. The detailed results could be obtained from the high-resolution spectra of Fe 2p, S 2p and Se 3d (Figures 2b-2d, Table S1). The spectrum of Fe 2p could be fitted by three pairs of Fe 2p3/2 and Fe 2p1/2 with their binding energies at 707.3/720.1 eV, 711.3/725.0 eV, 713.2/ 727.5 eV (Figure 2b). They could be assigned to Fe2+ in pyrite-phase FeS2,33,34 Fe3+ in Fe-S/ Fe-O caused by surface oxidation,33 and Fe species in FeSO4 or Fe2(SO4)3 resulted from unreacted reactant or the exposure to air.33 The cases of S 2p and Se 3d are even more complicate than that of Fe 2p. As shown in Figure 2c, the intense peaks at 162.4/163.6 eV and 168.7/169.8 eV arise from lattice persulfide (S22-) from bulk-like 10


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FeS2 and sulfates (SO42-) from FeSO4/Fe2(SO4)3,34-36 which are in good agreement with those observed in the spectrum of Fe 2p. Beside them, there are several peaks over 164.5-167.0 eV, likely originated from polysulfides (Sn2-) or core-hole effects.34-36 The minor peaks at 161.2/162.5 eV could be identified as persulfide (S22-) at the surface.35 The shift between binding energies of persulfide (S22-) at the surface and in the bulk could be explained by band bending in the space charge region.35 The similar case is also obtained from Se 3d (Figure 2d), where Se species from FeS2-xSex at the surface (53.4/54.3 eV) and in the bulk (54.7/55.6 eV), or related to polysulfides or core-hole effects (56.0-57.5 eV), and selenates/selenites (58.7/ 59.3 eV) are all observed in the spectrum.37-39 The high resemblance between the cases of S and Se indicates the well mixing of them in the particles, indirectly supporting the formation of a solid solution. Electrochemical performances of FeS2-xSex as an electrode material in NIBs are optimized by voltage windows, electrolytes, additives and binders. Before the cycling at 0.5 A g-1, the electrode was activated at 50 mA g-1 for the first five cycles. First, lifting the cut-off voltage on the discharge side could avoid the complete reduction to metallic Fe (Figure S4a), thus reducing the volume change on electrode materials and enhancing the cycling stability.9 Second, replacing ester-based electrolytes (EC/DEC) by ether-based ones (diglyme) could efficiently inhibit side reactions on electrolytes (Figure S4b),9,25,40 also benefiting the capacity retention. Third, fluoroethylene carbonate (FEC) as an additive makes the cycling quickly decayed (Figure S4c). This result might be also related to the carbonate feature of FEC, deserving the detailed investigation in the future. Finally, all the tested binders, CMC, SA and pVdF, give the good stability. But pVdf is slightly better than CMC and SA (Figure S4d). On the basis of these results, FeSe2-xSx was measured within a voltage window of 0.8-3.0 V, using diglyme as the electrolyte, and pVdF as the binder.

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Figure 3. (a, b) Discharge/charge curves of FeS1.6Se0.4 at 50 mA g-1 and 1A g-1.(c) Cycling performances and (d) rate performances of FeS1.6Se0.4, FeS2, and FeSe2. (e) Long-term cycling of FeS1.6Se0.4 at 2 A g-1. Figure 3a shows the galvanostatic discharge/charge profiles of FeS1.6Se0.4 at a current density of 50 mA g-1 during the first five cycles. The first discharge profile exhibits a long and flat plateau at ~ 1.35 V (Vs. Na+/Na), indicating a typically twophase process. The new phase was identified as a monoclinic phase of NaFeS2 (Figure S5).9 In the first charge process, the voltage ascends from 0.8 V to 2.4 V without pronounced plateaus, implying a different mechanism from the discharge process. The final product (NaxFeS2) does not recover the pyrite-phase structure, but keeps the monoclinic-phase structure.9,41 The discharge/charge capacity at the first cycle is 502/424 mAh g-1, resulting in an initial coulombic efficiency of 84.4 %. The discharge profile at the second cycle moves upward with its mean voltage at ~ 1.8 V, due to 12


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structure rearrangement and varied Na-intercalation mechanism. As to the charge profile at the second cycle, it almost repeats that at the first cycle. It is noticed that the discharge /charge profiles still slowly change upon cycling. As shown in Figure 3b, the discharge voltage at a fixed capacity gradually decreases, giving rise to a flat plateau at ~1.6 V and a sloped plateau at ~ 1.96 V. For the charge profiles, three voltage plateaus at ~ 1.5 V, ~1.75 V and ~ 2.0 V become prominent upon cycling. This evolution was associated with the phase transition from monoclinic phase NaxFeS2 to trigonal phase NaxFeS2 (layered structure).9 The similar changes in redox reactions with cycling are also supported by CV curves (Figure S6), where the cathodic peak at 2.0 V decreases and that at 1.6 V increases upon cycling. Meanwhile, the anodic peaks at ~1.4 V, ~ 2.1 V and ~2.2 V enhance. These results are well consistent with discharge /charge profiles. Because the similar evolution was also observed in FeS2 and FeSe2 (Figure S7), the relationship between it and Se-doping in FeS2 could be excluded. Figure 3c shows the cycling performances of FeS1.6Se0.4 at 1 A g-1. At the beginning, FeS1.6Se0.4 was treated at 50 mA g-1 for the first ten cycles. Then, the current density increases to 1 A g-1 for the left cycling. After 1000 cycles, the reversible capacity could be remained at 275 mAh g-1, corresponding to a capacity retention of 75.5 % relative to the capacity at the 11th cycle. This data is much higher than those of FeS2 (~ 195 mAh g-1) and FeSe2 (~ 219 mAh g-1). The solid-solution feature of FeS1.6Se0.4 enables it to combine the high capacity of FeS2 and good stability of FeSe2, realizing the high performances. The structure stability is checked by SEM images of the electrode after 1000 cycles. As shown in Figure S8, the compact microspheres become loose upon cycling, confirming the severe damage caused by giant strain/stress. In spite of this, these pulverized particles still closely connect to each other, thus remaining the electrochemical activity. Figure 3d shows the rate performances of FeS1.6Se0.4, FeS2, and FeSe2. As the current density increases from 0.1 to 0.2, 0.5, 1, 2, 5, 10, 20 or 30 A 13



g-1, the capacity of FeS1.6Se0.4 decreases from 378 to 370, 335, 320, 295, 264, 247, 228 or 225 mAh g-1. Even at 40 A g-1, the capacity is still 211 mAh g-1, 57.0 % at 0.2 A g-1. This data is much better than those of FeS2 and FeSe2, both of which only shows ~90 mAh g-1 at 40 A g-1. If the current density returns to 0.2 A g-1 again, the capacity comes back to 360 mAh g-1, suggesting the good durability to high rates. To evaluate the rate performances of FeE2 (E=S, Se) at different voltage windows, capacity retention, rather than specific capacity, is used. As shown in Figure S9,9,25,42-44 the result of FeS2-xSex is still better than many works on FeE2 (E=S, Se). Finally, the superior performance of FeS1.6Se0.4 at a high rate is confirmed by the long-term cycling (Figure 3e). After 6000 cycles at 2 A g-1, FeS1.6Se0.4 shows a capacity of 220 mAh g-1, giving a capacity degradation about 0.0044% per cycle. The superior performances of FeS2-xSex, particularly at high rates, deserve in-depth investigation. DC electronic conductivity of FeS2-xSex confirmed that Se doping indeed effectively improves the conductivity (Figure S10), as compared to FeS2. The same conclusion is also obtained from EIS spectra of the electrodes at the open circuit voltage (Figure 4a). EIS of FeS2, FeSe2, and FeS1.6Se0.4 could be fitted by a modified equivalent circuit, as illustrated in Figure S11. Rs presents the resistance of cell components and electrolyte. Rct indicates the charge-transfer resistance at the interfaces of electrolyte/surface film/electrode. Constant phase elements (CPEs), which arise from the non-homogeneous nature of the composite, fit the depressed semicircle well. As listed in Table S2, the Rct value of FeS1.6Se0.4 is 35 Ω, much less than 141 Ω in FeS2 and 70 Ω in FeSe2. This result could be assigned to enhanced electronic conductivity caused by Se doping and the small grain size in FeS1.6Se0.4. Se doping improves the electronic conductivity of FeS2, making FeS1.6Se0.4 better than FeS2. Small gain size provides abundant crystal boundaries for improved charge transfer,31,32 making FeS1.6Se0.4 better than FeSe2. More important, this enhancement is kept for cycled electrodes (Figure 4b), where Rct of cycled electrode based on FeS1.6Se0.4 at 7 Ω is still less than those 14


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Figure 4. (a) Nyquist plots for the electrodes made from FeS2, FeSe2, or FeS1.6Se0.4 at open circuit voltage or (b) at 1.70 V after 50 cycles at a current density of 1 A g-1. (c) CV curves of FeS1.6Se0.4 at various scan rates. (d) Plots of log (scan rate) versus log (peak current). (e)

Crystal structure of layered-structure Na2FeS2-xSex. (f) Total and partial density of states of Na2FeS2-xSex (up) and Na2FeS2 (down). based on FeS2 at 16.8 Ω and on FeSe2 at 10 Ω. Compared to the cases of open-circuit voltage, all the Rct data decrease a lot, due to effectively electrochemical activation upon cycling, FeS1.6Se0.4 still holds the smallest Rct in these electrodes. Moreover, Na-diffusion coefficient of FeS1.6Se0.4 calculated from the diffusion drift, is larger than those of FeS2 and FeSe2 (Table S2). The fast diffusion kinetics could be also confirmed by CV curves of FeS1.6Se0.4 at different scan rates. As displayed in Figure 4c, these CV curves exhibit similar contours. The relation of peak current (ip) with scan rate (v), ip = avb, is closely corre15



lated to reaction kinetics. As ip is logarithmically plotted against v (Figure 4d), the slope of the fitting line represents b, an important parameter. b = 0.5 implies an ideally diffusion-controlled process, whereas b = 1.0 indicates a surface-controlled process. In most cases, b is between 0.5-1.0, suggesting a mixed processes.45-47 Here, the cathodic peak (R1) of FeS1.6Se0.4 shows slightly larger b values than those of FeS2 (Figure S12), indicating that the discharge process is less controlled by diffusion. The similar conclusion is also drawn from the side of the anodic peaks (O1, O2 and O3). The results confirm again that Se in FeS1.6Se0.4 improves the diffusion kinetic, as compared to FeS2. The enhancement of Se doping on electrochemical kinetics is also tried to be understood by DFT calculation. Because FeS2-xSex is more close to FeS2 in terms of chemical components, crystal structure, and electrochemical properties, DFT calculation on FeS2-xSex and FeS2 is conducted to disclose the underlying effect. The previous works reported that FeS2 gradually transform to triganol-phase NaxFeS2 upon cycling.9,25 Thus, DFT calculation was conducted based on this structure, where NaxFeS2 is composed of FeS4 tetrahedrons by sharing the edges. Meanwhile, half of Na atoms are intercalated in two layers built by FeS4 tetrahedrons. The other half competes with Fe for 2d sites in the structure. As sulfur atoms in FeS4 tetrahedron are partially replaced by Se, the layer spacing is likely to increase, due to the big size of Se than S. Such an expanded layer structure definitely promotes the charge transportation upon cycling. Meanwhile, Se doping also modifies the electronic structure, as evidenced by density of states (DOS) calculations on Na2FeS2 and Na2FeS2-xSex. On the basis of the structure in Figure 4e, DOS of Na2FeS2 is highly localized near the Fermi level (Figure 4f), which is mostly made up of Fe 3d states. As to Na2FeS2-xSex, the Fe 3d states are delocalized near the Fermi level, indicating the enhanced mobility of charge carriers. This result would benefit the carrier transportation and enhance the conductivity. Despite Se could effectively enhance the charge-transfer kinetics as state above, the high content of Se would reduce the 16


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specific capacity like the case of FeSe2. It is necessary to optimize the amount of Se for the balance between charge-transfer kinetics and specific capacity. FeS2-xSex in different Se/S ratios is prepared (Figure S13, Table S3) and examined under the same conditions. As pointed by Figure S14, FeS1.6S0.4 is the best of all the FeS2-xSex (x=0.2, 0.4, 0.6), in view of reversible capacity and rate capability.

Figure 5. (a) Schematic diagram of the charge and discharge processes of FeS2-xSex//AC. (b) Galvanostatic charge/discharge curves of FeS2-xSex//AC. (c) Ragone plots of FeS2-xSex//AC, FeS2-xSex//Na3V2(PO4)3/C and other hybrid capacitors. (d) Cycling performance of FeS2-xSex//AC at 1 A g-1 and FeS2-xSex//Na3V2(PO4)3/C at 0.5 A g-1. As stated above, FeS2-xSex exhibits the electrochemical features that have been reported for many sulfides as anode materials, such as the awkward mean voltage (~ 1.5 V), the multiple voltage plateaus, but good cycling stability and rate capability. These features, particularly the first two, make them not qualified to the applications in 17



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NIBs, but the last two indicate the potential as electrode materials in NICs. To our knowledge, it is the first time to explore the applications of sulfides/selenides in NICs. To demonstrate this potential, FeS2-xSex was paired with commercial activated carbon (AC) for NICs, where the specific energy density and specific power density were calculated by the total mass of FeS2-xSex and AC. Before the test, the electrochemical performances of AC were checked in a half-cell. As illustrated in Figure S15, AC shows a large specific surface area, a nearly rectangular shape in CV curves and a good cycling stability, all of which make it capable of capacitors. Then, NICs are assembled using AC as the cathode, FeS2-xSex as the anode, and 1 M of NaClO4 in tetraglyme as the electrolyte. In such a configuration, NICs have Na+ intercalated into FeS2-xSex, and ClO4‾ absorbed on carbon surface during the charging process (Figure 5a). In the discharging process, Na+ is deintercalated from NayFeS2-xSex, accompanied by desorption of ClO4‾ from carbon surface. So, they are different from conventional batteries and supercapacitors.48,49 This is supported by the charge/discharge profiles of FeS2-xSex//AC (Figure 5b), where the profiles show neither straight lines like supercapacitors, nor voltage plateaus like batteries. The results of FeS2-xSex//AC at different current densities are summarized in the Ragone plot (Figure 5c), where it gives an energy density of 67 Wh kg-1FeSSe+AC at a power density of 172.3 W kg-1FeSSe+AC. The energy density would be reduced to 27 Wh kg-1

FeSSe+AC

at a high power density of

2543 W kg-1FeSSe+AC. These results are much better than the previous works based on transitional metal oxides//activated carbon, such as TiO2-B//AC,50 Li2CoPO4F//AC,51 TiO2-B//CNT,52 NiCo2O4 //AC,53 Na-TNT//AC,54 etc.. It should be pointed out that the performances of FeS2-xSex//AC could be further promoted, if the size, shape and structure of FeS2-xSex are specially optimized for this purpose. We also compare the results with NIBs constructed by FeS2-xSex and Na3V2(PO4)3/C, marked as FeS2-xSex// NVP, to confirm the advantages of FeS2-xSex as NICs. Na3V2(PO4)3/C was synthesized 18


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by a reported method,26 and then tested in half cells firstly. As shown in Figure S16, home-made Na3V2(PO4)3/C presents a flat voltage plateau at ~3.4/3.3 V, and a reversible capacity of 93 mAh g-1NVP/C after 100 cycles at 0.1 A g-1 NVP/C, both of which agree well with the reported data.26 After assembled with FeS2-xSex, FeS2-xSex//NVP gives an output voltage of ~ 1.4 V and a specific capacity of ~ 235 mAh g-1FeSSe at 0.4 A g-1FeSSe (Figure S17), also comparable to the previous works.5,25 Then, Figure 5c compares the Ragone plots of FeS2-xSex//NVP and FeS2-xSex//AC. It is found that the energy densities of FeS2-xSex//NVP and FeS2-xSex//AC are comparable at low power densities. However, as the power density increases, the energy density of NIBs quickly drops to ~6.8 Wh kg-1FeSSe+NVP at 767 W kg-1FeSSe+NVP, much smaller than ~55 Wh kg-1FeSSe+AC of NICs tested at a similar rate. So, it is concluded that NICs achieve a superior balance between energy density and power density. Finally, Figure 5d describes the cycling performances of FeS2-xSex//AC, where it delivers an energy density about ~34 Wh kg-1FeSSe +AC after 1000 cycles at 1 A g-1FeSSe +AC. Such a long cycle-life is not reported in NICs based on TMSs yet. In our case, FeS2-xSex//NVP only maintains an energy density of ~17 Wh kg-1FeSSe+NVP at 0.5 A g-1FeSSe+NVP after 300 cycles, confirming again the advantages of TMSs as NICs. CONCLUSIONS In summary, solid-solution FeS2-xSex microspheres are synthesized by a simple solvothermal reaction, as supported by XRD patterns, XPS spectra and EDS mapping. The solid solution of S/Se allows FeS2-xSex to combine high specific capacity of FeS2 and good redox kinetics of FeSe2 together, thereby delivering a specific capacity of 220 mAh g-1 after 6000 cycles at 2 A g-1, or 210 mAh g-1 at 40 A g-1 in a rate test. Both of them are better than FeS2 and FeSe2 alone. These improvements could be attributed to enhanced charge-transfer kinetics, which originates from increased layer spacing 19



and promoted electron conductivity caused by Se, as supported by EIS spectrum, DFT calculations and DC conductivity. More important, this strategy, solid solution in anions, could be applied to other sulfides/selenides for advanced performances. Encouraged by the above results, FeS2-xSex is coupled with activated carbon as Na-ion capacitors. This device presents better electrochemical performances than Na-ion batteries also made of FeS2-xSex, in terms of rate capability and cycling stability. Furthermore, these results also outperform those observed in metal oxides//activated carbons. These results demonstrate the promising potential of TMSs in NICs, thereby paving the way for sulfides/selenides in electrochemical energy storage. ASSOCIATED CONTENT Supporting Information XRD patterns, SEM images and Se/S contents of FeS2-xSex (x=0, 0.2, 0.4, 0.6, 2); Influences of voltage window, electrolytes, FEC and binders on electrochemical performance of FeS1.6Se0.4; Cyclic voltammograms or Discharge/charge curves of FeS1.6Se0.4, FeS2 and FeSe2 at different cycles; SEM images of the electrode before and after the cycling; Comparison of FeS1.6Se0.4 with the reported sulfides in terms of capacity retention; Comparison of electric conductivity of FeS2 and FeS1.6Se0.4; Equivalent circuit used for electrochemical impedance spectra; Cyclic voltammograms of FeS2 at various scan rates; Cycling performance and rate performance of FeS2-xSex (x=0.2, 0.4, 0.6); XRD pattern, N2 sorption isotherms, cyclic voltammograms, charge/discharge curves, cycling performance and rate performance of active carbon; XRD pattern, SEM image; XRD pattern, SEM images, charge/discharge curves and cycling performance of Na3V2(PO4)3/C; Charge/discharge curves of FeS2-xSex//NVP. AUTHOR INFORMATION Corresponding Author *

Jian Yang. E-mail: [email protected] 20


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Notes The authors declare no competing ﬁnancial interest.

ACKONWLEDGEMENT This work was supported by National Nature Science Foundation of China (Nos. 21471090, and 61527809), Key Research and Development Programs of Shandong Province (2017GGX40101), and Taishan Scholarship in Shandong Provinces (No. ts201511004). Ms. J. Yang and Prof. W. Fan conducted the DFT calculations related to FeS2-xSex. REFERENCES (1) Goodenough, J. B., Park, K. S. The Li-ion rechargeable battery: a perspective. J. Am. Chem. Soc. 2013, 135, 1167-1176. (2) Van Noorden, R. A better battery. Nature 2014, 507, 26. (3) Zhu, Y., Fan, X., Suo, L., Luo, C., Gao, T., Wang, C. Electrospun FeS2@carbon fiber electrode as a high energy density cathode for rechargeable lithium batteries. ACS Nano 2015, 10, 1529- 1538. (4) Douglas, A., Carter, R., Oakes, L., Share, K., Cohn, A.P., Pint, C.L. Ultrafine iron pyrite (FeS2) nanocrystals improve sodium-sulfur and lithium-sulfur conversion reactions for efficient batteries. ACS Nano 2015, 9, 11156-11165. (5) Wan, M., Zeng, R., Chen, K., Liu, G., Chen, W., Wang, L., Zhang, N., Xue, L., Zhang, W., Huang, Y. Fe7Se8 nanoparticles encapsulated by nitrogen-doped carbon with high sodium storage performance and evolving redox reactions. Energy Storage Mater. 2018, 10, 114-121.

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Table of Contents Solid-Solution Anion Enhanced Electrochemical Performances Sulfides/Selenides for Sodium Ion Capacitors: The Case of FeS2-xSex

of

Metal

Yaqiong Long,a Jing Yang,a Xin Gao,a Xuena Xu,a Weiliu Fan,a Jian Yang,a,* Shifeng Hou,b Yitai Qiana a

Key Laboratory of Colloid and Interface Chemistry, Ministry of Education, School of Chemistry and

Chemical Engineering, Shandong University, Jinan, 250100, Peoples Republic of China b

National Engineering Research Center for Colloidal Materials, Jinan, 250100, Peoples Republic of China

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Solid-Solution Anion-Enhanced Electrochemical Performances of

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