Understanding Shuttling Effect in Sodium Ion Batteries for the Solution

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C: Energy Conversion and Storage; Energy and Charge Transport

Understanding Shuttling Effect in Sodium Ion Batteries for the Solution of Capacity Fading: FeS as an Example 2

Shihan Qi, Liwei Mi, Keming Song, Kaiwei Yang, Jianmin Ma, Xiangming Feng, Jianmin Zhang, and Weihua Chen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11069 • Publication Date (Web): 10 Jan 2019 Downloaded from http://pubs.acs.org on January 10, 2019

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The Journal of Physical Chemistry

Understanding Shuttling Effect in Sodium ion batteries for the Solution of Capacity Fading: FeS2 as an example Shihan Qi,1,2Liwei Mi3, Keming Song1, Kaiwei Yang,1 Jianmin Ma2, Xiangming Feng,1 Jianmin Zhang*1 and Weihua Chen*,1 1

College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, China

2 School 3

of Physics and Electronics, Hunan University, Changsha 410082, PR China

Center for Advanced Materials Research, Zhongyuan University of Technology, Zhengzhou 450007, China

*E-mail: [email protected]; [email protected]

ABSTRACT: Pyrite (FeS2) has been regarded as one of the most promising electrode materials for sodium ion batteries owning to high theoretical capacity and low cost. However, the short cycle life of FeS2 electrode in sodium storage hampers its further development. Although some researchers focused on the mechanism of capacity fading in Na/FeS2 battery, such as irreversible crystal transform, terrible volume change and so on, whether and how the shuttling effect of FeS2 electrode exists in half-cell or full-cell systems still remains so far. In this work, the shuttling effect on sodium ion batteries with FeS2 as electrode is investigated systematically in both half-cell and full-cell systems and it is confirmed the shuttle of Fe element and polysulfides, simultaneously. These shuttling effects were influenced significantly by some factors. In detail, excessive voltage will cause oxidation for shuttled Fe and polysulfides, side reactions often take place at low voltage; carbonate electrolyte can react with polysulfides; stable solid electrolyte interface could prevent the loss of elements. According to these influence factors, as an example, a designed nano FeS2@C composite was synthesized to limit the shuttling effects. As a result, in a

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large operating voltage range of 0.01-3 V, it shows high capacity (about 600 mAh g-1) and good cyclic performance with ether-based electrolyte.


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Introduction Sodium ion batterries (SIBs) as cost-effective energy storage technology is an attractive subject of research today.1,2 Development of superior SIBs needs high-performance electrode materials, accompanied therefore, metal sulfides have attracted increasing research attention as a promising anode electrode materials in recent several years owning high theoretical capacity and the modifiable possibility of composition and structure indicating promotion opportunities.3-5 Among various metal sulfides, such as SnS2,6,7 SnS,8 MoS2,9,10 TiS211 and so on,12 pyrite (FeS2)13,14,15 is one of the most highlighted candidate materials for its low price and abundant natural resources. Nevertheless, the short cycle life is the main problem which hampers the commercial prospect of this material.16,17 To overcome this challenge, some researchers devoted their efforts on the electrochemical reaction mechanism in Na/FeS2 battery.18,19 According to Kitajou’s work,20 the electrochemical reactions of sodium storage can be simply divided into two steps, the intercalation step and conversion step, as described by Eqs 1-2. FeS2 + xNa+ + xe- → NaxFeS2 NaxFeS2 + (4-x)Na+ + (4-x)e- → Fe + 2Na2S

[1] (below 0.8 V)17

[2]

Firstly, it should be pointed out that, the intermediate, NaxFeS2, can not return to pristine FeS2 after initial cycle,15,17,19 which will lead to low capacity of subsequent cycles. And then, although the above 4-electron reactions can bring high theoretical specific capacity, it also causes problems of morphology pulverization and low reversibility.21 These factors are considered as the main defects which lead to sodium storage capacity fading. However, whether other factor can also shorten the cycle life of pyrite electrode is debatable. Because one of the full discharged products of FeS2 electrode is Na2S, which is also the reaction product of ambient temperature Na-S battery,22 some researchers believe that the shuttling of sodium polysulfides also exists in Na/FeS2 cell.4,18 However, to the best of our knowledge, there has been no direct evidence to prove this view so far. Benefited from the new studies of FeS2 electrode and the development of ambient temperature Na-S battery, some new perspectives to



investigate this issue have emerged, such as the various species of sodium polysulfides, the influences of different kinds of electrolyte and so on.23 To investigate the shuttling effect for FeS2 electrode and its impact on electrochemical performance of SIBs can renew the recognition of electrochemical behavior of FeS2 electrode and help researchers to develop some more effective method to enhance its performance, or even for other metal sulfides materials. Half-cell system is a kind of suitable system to investigated electrochemical behavior of other electrodes, on one hand, for the stable potential of Na metal anode during electrochemical reaction so that it can be regarded as a reference electrode. However, on the other hand, for its lower melting point (about 97 °C) and more serious dendrite growth problem than Li metal,4 Na metal anode is regarded as an extremely unsafe electrode. Based on security consideration, half cell with Na metal electrode can not be large-scale commercialized. Therefore, with the development of research for SIBs, increasingly importance has been attached to full cell system. With above perspectives, both half-cell and full-cell systems for FeS2 electrode should be studied to understanding the shuttling effect. In this work, the shuttling effect of FeS2 in both half-cell and full-cell systems were studied in multiple perspectives. Above all, the direct evidences to prove the existence of sodium polysulfides in both Na/FeS2 half cell and FeS2/Na3V2(PO4)3 full cell (FeS2/NVP) were reported. Similar with Na-S cell, the sodium polysulfides can also shuttle to the side of counter electrode. And then, what was so astounding was that Fe element can also shuttle to the other side of separator. To the best of our knowledge, this phenomenon has not been reported in sodium battery or sodium-ion battery so far. Furthermore, the factors which make influences on above shuttling effects, including different working voltages, various kinds of electrolytes and interfacial phenomena of FeS2, were also investigated. What is more, a nano-sized FeS2@C composite was synthesized and tested in the optimized conditions, including controlled operating voltage range and ether-based electrolyte. As a result, good cyclic and rate performances were obtained. Based on these above investigations, a novel comprehensive review of pyrite’s electrochemical mechanism and capacity


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fading has been built.

Results and Discussion

Figure 1. (a) Cyclic voltammetry (CV) curves of Na/FeS2 cell with a scan rate of 0.2 mV/s; Cyclic performance for (b) Na/FeS2 cell and (c) FeS2/NVP cell at a rate of 0.5C.

To investigate the shuttling effect’s impact on cyclic life in multi-dimensions, FeS2 micro-spheres as electrode material were tested in both half-cell system (Na/FeS2

cell)

and

full-cell

system

(FeS2/NVP

cell),

respectively.

The

characterizations of FeS2 micro-spheres are shown in Figure S2. In brief, these FeS2 particles are spherical with the size about 5 μm and consist of many nano-fragments. Besides, in a typical crystal structure, a S ion is embedded into a Fe face-centered cubic sublattice. Each S atom binds with three Fe atoms. According to XRD pattern, the crystal phase of as-prepared FeS2 particles is relatively pure. There exist a couple of points, particle sizes and test conditions, to be noted. Firstly, mirco-sized FeS2 suffers more terrible morphology pulverization and lower reversibility than nano-sized samples. Secondly, a large operating voltage range of 0.01-3 V and relative low rate of 0.5C can make the mentioned 4-electrons take place completely. The the aim of choices about particle sizes and test conditions is to fully expose the shortages of FeS2 electrode. As a result, FeS2 electrode does not possess a good cyclic performance. In detail, as can be seen in Figure 1a, in half cell, there exists an irreversible sharp peak in the 1st discharge at about 0.9 V. This peak corresponds to the intercalation of Na+ into FeS2 crystal cell to form NaxFeS2 (Eqs. 1, intercalation step).16 In this reaction step, no more than 2 electron transferred.17 This reaction step will not take place in subsequent cycles. The reductive peak located at about 0.35 V is related to the formation of Na2S and Fe0 (Eqs. 2, conversion step). Unfortunately, the



areas of this peak decreased continuously with cycle number increasing, which means capacity decay. Besides, the oxidized peak at about 1.4 V corresponds to the counter reaction for the conversion step. Furthermore, the small redox peaks located at 2.05 and 2.5 V is probably corresponding to the side reaction of polysulfides.24 As displayed in Figure 1b, after initial 20 cycles, the capacities and coulombic efficiencies became stable. An ultra-low capacity of 80 mAh g-1 remained after 100 cycles. As mentioned above, because of the low melting point and dendrites problem, using Na metal electrode directly has been regarded as a dangerous strategy. Therefore, for large-scale commercialization, full-cell system, which do not utilize Na metal electrode, has attracted much researchers’ attention.25,26 As shown in Figure 1c, similar with half cell, this full cell with FeS2 electrode suffered a rapid capacity decay. In the initial 10 cycles, more than 70% capacity degraded. Only a little capacity of less than 100 mAh g-1 remained. The morphology and cyclic performance of NVP electrode are shown in Figure S3. Obviously, the NVP electrode went through a relative stable cycling test, which suggests that the bad cyclic performance of full cell is not caused by this positive electrode. To explain the disappeared discharged plateau, in situ X-ray diffraction (XRD) was carried out. As shown in Figure 2a, at the bottom of 1D view, there existed no obvious peaks around 31°. With the discharging going on, a new peak, which belonged to the (221) crystal face of NaFeS2 (JCPDS:34-0935), emerged around 31°. Meanwhile, the peaks belonging to pristine FeS2, which were at 28.5° and 32.9°, became weak gradually. When the discharging plateau at about 1.1 V of the initial cycle ends, the peak around 31° became strongest. The above phenomenon can prove the correctness of Eqs 1. Significantly, this new peak around 31° no longer disappeared after its emergence. It suggests that FeS2 could not totally return to pristine state after cycling. In another word, a part of NaxFeS2 always reserve in FeS2. As can be seen in the 2D view as shown in Figure 2b, the periodic strengthening and weakening of this peak were caused by the periodic electrochemical reaction during galvanostatic tests. In detail, when the voltage reached to 0.01 V or 2.8 V, this peak


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weakened. When the voltage reached to around 1 V, the signal of this peak became strongest. As every coin has two sides, residue of NaxFeS2 will cause two contrary influences. On the positive side, the conductivity of NaxFeS2 is higher than pure FeS2, which leads to a higher reversibility. On the negative side, the incomplete electrochemical performance can decrease the capacity for FeS2 electrode.

Figure 2. In situ X-ray diffraction (XRD) analysis of the FeS2 electrode during initial 3 cycles in a 2-thera range of 28-34°: (a) 1D and (b) 2D in situ XRD patterns. (c) Corresponding charge-discharge profiles.

The crystal phase transform of FeS2 in full cell during galvanostatic test was also characterized by in situ XRD. As shown in Figure S4, the peak centered about 30.7° also emerges, which means the existence of intermediates, NaxFeS2. The transmission electron microscopy (TEM) image to prove the existence of NaxFeS2 was shown in Figure S5. Besides, this peak appears and disappears periodically during charge and discharge. In detail, when FeS2 transformed into Fe and Na2S (or sodium polysulfides) in the high voltage region, the peak disappeared. This result suggests that both in half cell and full cell, FeS2 electrodes went through the same electrochemical reaction pathways. According to the similar decay model and same reaction pathways, it is perfectly possible that the main reason of capacity decay for full cell is the capacity degeneration for FeS2 electrode.



Figure 3. Characterizations for shuttling effects in Na/FeS2 cell. (a) Schematic illustration of shuttling effects during discharging process: the dissolving in electrolyte and adsorption on carbon interlayer of Fe ion and polysulfides. (b) Fe and S elemental contents on cycled carbon interlayers. (c) Raman spectra of S element on cycled carbon interlayers: I, discharged state; II, charged state. High-resolution XPS patterns for cycled carbon interlayer: S 2p at (d) discharged state and (e) charged state; Fe 2p at (f) discharged state and (g) charged state. All the tested carbon interlayers were went through 5 cycles’ galvanostatic charging-discharging.

Based on the well-investigated Li/FeS2 cell,27 many researchers believe that the shuttling effect of polysulfides also exist in Na/FeS2 battery.4,18 However, there has been no direct evidence so far. To verify this hypothesis, a special battery with a carbon interlayer to trap the dissolved polysulfides in electrolyte was designed, as schematically illustrated in Figure 3a. Here, the carbon interlayer was placed between the separator and Na Foil without directly contacting FeS2 electrode. After 5 cycles of galvanostatic tests, X-Ray photoelectron spectroscopy (XPS), Raman spectra and energy dispersive X-ray spectroscopy (EDS) were carried out to characterize the species and existence of polysulfides which were absorbed on carbon interlayer. The contents of S and Fe elements at different state are displayed in Figure 3b. To remove the interference caused by the different masses of each FeS2 electrode, the contents are divided by the corresponding mass of tested FeS2 electrode. First of all, S element can be detected, which means S element shuttled through separator to the Na foil side. It should be noticed that the contents of S at discharging state is twice as at charging state. What is so astounding is that the signal of Fe element could also be detected on the carbon interlayer. This astonishing outcome suggests that the shuttling effect of Fe elements also exists in Na/FeS2 battery. To our best of our knowledge, there exists no work which has reported this phenomenon in Na/FeS2 battery so far. The discovering


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of Fe element losing reported here provides a novel and comprehensive angel to understand the mechanism of FeS2 electrode’s capacity fading. Besides, the ratios of remained Fe atoms to remained S atoms in cycled FeS2 electrodes were also characterized and displayed in Figure S6d. In short, with the galvanostatic tests going on, the content of S element remarkably decreased. It means that polysulfides are easier to dissolve and loss. Electron paramagnetic resonance (EPR) can also prove that polysulfides exist in this system (Figure S7). To confirm the species and valences of S and Fe elements, Raman spectra and XPS measurements were carried out and the results are shown in Figure 3c-3g. To begin with, as can be seen from the Raman spectra in Figure 3c, there exists a small peak at about 340 cm-1 on profile I, which corresponds to C-S bonds.28 The most probable reason of the emergence of C-S bonds is that polysulfides can react with carbonate-based electrolyte.24 Besides, the profile I and II both possess the peak locating at 480 cm-1. According to several published works,29 this peak is caused by the stretching of S-S bonds in Na2Sx≥4. In another word, this is one of the direct evidence to prove the shuttling effect of polysulfide in Na/FeS2 battery. In addition, the peaks locating at about 720 cm-1, 900 cm-1 and 935 cm-1 are assigned as the symmetric ring deformation mode of EC, ring breathing mode of EC and totally symmetric mode of ClO4-, respectively.30 The XPS patterns of S-based materials absorbed on carbon interlayer at dicharging and charging states are shown in Figure 3d and 3e, respectively. The broad peak located at 168.5 eV can be distinguished as species with S6+. This high-valence S-based material was may formed because of the oxidation of transferred polysulfides when exposed to high-frequency X-ray of XPS. At both discharging and charging state, the peak centered about 163 eV can be corresponded to polysulfides. Besides, the content of polysulfides at discharging state is higher than charging state. It is because in a discharging process, polysulfides will gather at the side of Na electrode. The details will be discussed in the next paragraph. This result is corresponding to Raman spectra and EDS, which prove the shuttling of polysulfides again. The signals of low-valence S, locating at about 160 eV, are formed from the



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reduction reaction of polysulfides by Na foil. As shown in Figure 3f and 3g, there exist 4 peaks of Fe 2p, respectively. In detail of Figure 3f, the 3 peaks centered about 709.4 eV, 715.5 eV and 720.0 eV can be corresponded to Fe2+. Interestingly, the peak located at 706.1 eV indicates the existence of Fe0. Carbon interlayer adsorbed these 2 materials. As displayed in Figure 3g, compared with discharged state, the peak area ratio of Fe2+ 2p 3/2 and Fe0 2p 3/2 is a little lower. The ratios are 70:30 at discharging state and 57:43 at charging state, respectively. It should be noticed that no signal of Fe3+ ion can be detected. This outcome means that after get out of the cells, cycled carbon interlayers were well preserved without oxidation. (Fe and Fe2+ can react with O2 to form Fe3+.) Thus, the Fe2+ ion had already existed in this system before the galvanostatic test ended. More often, Fe element shuttled to another side of cells with the state of Fe2+ ion. Based on above results, the details of electrochemical process for shuttling effects can be drawn. Firstly, some S2- ions were oxidated excessively, to form soluble long chain polysulfides (S42- and S62-). Then, with the number of cycles increasing, the ratios of remained Fe atoms to remained S atoms in cycled FeS2 electrodes decreased. It means that during charging process, after Fe0 was oxidized into Fe2+, there existed not enough S2- to react with it to form NaxFeS2. Thus, some Fe2+ dissolved into electrolyte. In a discharging process, because of polysulfides are negative ions, they will gather on the surface of anode electrode, Na metal electrode. In another word, polysulfides ions crossed through separator from FeS2 side to Na metal side. Besides, in a charging process, polysulfides ions would move from Na metal side to FeS2 side. Similarly, because Fe2+ ion is a positive ion, in a charging process, Fe2+ ions would gather on the surface of low potential electrode. The above 2 kinds of ions always move in opposite directions. Just as pointed, compared with Fe2+/Fe and S/S2- redox couple, Na+/Na redox couple has the lowest standard electrode potential (φFe2 + -0.44 V, φNa +

Na=

Fe=

-2.71 V, standard hydrogen electrode potential) . Thus, Na metal

will reduce soluble long chain polysulfides to insoluble short chain polysulfides and even S2-. Besides, Fe2+ ions will be reduced to Fe0 particles. Therefore, low valence S


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and materials can be detected in Na/FeS2 cell. This phenomenon will intensify the loss of active material and lead to rapid capacity fading. The above electrochemical processes are illustrated in Figure 3a.

Figure 4. Characterizations for shuttling effects in FeS2/NVP cell. (a) Schematic illustration of shuttling effects during discharging process: the dissolving in electrolyte and adsorption on carbon interlayer of Fe ion and polysulfides. (b) Fe and S elemental contents at different charging-discharging states on cycled carbon interlayers. (c) Linear sweeps of Fe ion-rich electrolyte and polysulfides-rich electrolyte, besides, the CV curve of Na/FeS2 cell at 1st cycle is also shown by red line. High-resolution XPS patterns for cycled carbon interlayer: S 2p at (d) discharged state and (e) charged state. All the tested carbon interlayers were went through 5 cycles’ galvanostatic charging-discharging.

After understanding the impact of Na metal electrode on cyclic performance, there is an interesting question. If the low standard electrode potential material is replaced to a higher one, will the cyclic performance be enhanced? To answer this question, herein, the NVP electrode was utilized as the positive electrode to assemble a full cell with FeS2 electrode. To investigate the shuttling effect in FeS2/NVP cell, similarly, as displayed in Figure 4a, a carbon interlayer was also placed between the separator and NVP electrode. Some characterizations were carried out on the cycled carbon interlayer. According to EDS as displayed in Figure 4b, the amounts of shuttled Fe and S elements are very small. And even, in charging process, only S signal emerged. To ensure the electrochemical steady window of polysulfides and Fe2+ in full-cell system, linear sweeps for Fe ion rich-electrolyte and polysulfide rich-electrolyte were carried out. As shown in Figure 4c, in the region below 1.2 V,



there exist strong reducing signals for both 2 above electrolytes. These reducing signals are caused by the decomposition of EC/PC electrolyte. EC and PC can be reduced to polycarbonates, sodium carbonate and so on. These materials will deposit on electrode’s surface to form SEI membrane.31 Besides, Fe2+ and polysulfides can also be reduced in this region. It should be noticed that at about 1.3 V, there exist weak oxidating peaks for both 2 electrolytes. It means that Fe0 was oxidated to Fe2+ and S2- was oxidated to S22- or other long chain polysulfides in this voltage region (the counter reaction of Eqs. 2). (To explain the electrochemical behavior with voltage variation of 2 kinds of ions more clearly, the CV curve of Na/FeS2 cell at 1st cycle is also be shown in this graph. The oxidation peaks on CV curve at about 1.4 V corrensponds to this oxidating reaction) The electrochemical reaction of NaxFeS2 turn to FeS2 (the counter reaction of Eqs. 1) occurs at about 2.5 V. However, the valences of S and Fe elements do not change. Thus there are no signals at this potential. When the voltage rise to 3.3 V, polysulfide can be oxidated to SO32- or even SO42- and Fe2+ will be turned to Fe3+.32 As well known, the operating voltage plateau of NVP electrode is at about 3.4 V.33 Thus, during both charging and discharging processes, NVP can oxidate shuttled Fe2+ and polysulfides. As mentioned earlier, Fe3+ ion can react with polysulfides, Fe3+ ion will shuttle back to FeS2’s side, where is polysulfides-rich region. As a result, the amount of Fe element on surface of carbon interlayer is low. In conclusion, unfortunately, high potential electrode can also consume active material and weaken cyclic performance. The unstable CEs and rapid capacity fading can prove this standpoint. Besides, owning the low decomposition voltage of about 3.3 V (Figure S8), NaSO3CF3 in DGM electrolyte cannot be applied to full-cell system. Similarly, only S signal can be detected by XPS measurement. Compared with Na/FeS2 cell, there exist some differences in FeS2/NVP cell’s XPS of S 2p. As shown in Figure 4d-4e, the content of high valence sulphur species was higher and compositions were complex. It suggests that polysulfides were oxidated to form high valence sulphur species, which is corresponding to the result of linear sweep. In another word, the shuttled polysulfides were consumed by NVP electrode, which will


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lead to the low CEs and capacity fading. However, almost every cathodic material for sodium ion battery possesses the operating voltage above 3.3 V. As a result, the capacity fading caused by oxidation of shuttled Fe2+ and polysulfides can hardly be avoided.

Figure 5. Surface morphologies of FeS2 electrode after 5 cycles in different electrolytes: (a) electrolyte I: 1 M NaClO4 in propylene carbonate and ethylene carbonate (EC/PC) with 5% fluoroethylene carbonate (FEC); (b) electrolyte II: 1 M NaClO4 in diglyme (DGM); (c) electrolyte III: 1 M NaSO3CF3 in DGM. (d) FT-IR spectra of FeS2 electrode after 5 cycles in above 3 electrolytes. (e) Charge-discharge profiles at 5th cycle in different electrolytes: the electrolytes (I-III), respectively. (f) The cyclic performances of FeS2 at 0.5 C in electrolytes (I-III), respectively.

There is no doubt that electrolytes and solid-liquid interface (SEI) can make a huge influence on shuttling effect of FeS2 electrode. To find out these influences, the electrochemical performances in 3 different electrolytes were characterized. Above all, SEM images of cycled FeS2 electrode after 5 cycles in 3 kinds of electrolytes were shown in Figure 5a-c, respectively. Obviously, the surface of carbonate-based electrolyte, which consists of many nano-particles, is very rough with no membrane. In contrast, although there exist some wrinkles, stable SEI membranes formed on the surfaces of two samples tested in ether-based electrolytes, which is consist with previous work.34,35 The Fourier transform infrared spectrometry (FT-IR) patterns (Figure 5d) show that the main compositions of SEI film are ROCO2Na, Na2CO3, RCOONa and RONa.36 The 3 samples from different electrolytes possess the similar patterns. In detail, the strongest peaks locating at about 1600 cm-1 and the moderate peaks



centered at about 1400 cm-1 are related to carbonyl groups (C=O) in the ROCONa. Besides, the weak peaks centering at about 2900 cm-1 are also assigned to the C-H bond in organic parts of ROCO2Na, RCOONa or RONa. Carboxylate (RCOONa) also exist in SEI films because of the peaks at about 1500 cm-1. However, there is no evidence to prove that carboxylic acid also exists. The C-O bond and the C-O-C bond also exist in SEI films for the peaks locating at about 1100 cm-1 and 800 cm-1, respectively. Some organic structure of carbonate-based electrolyte and ether-based electrolyte did not be reduction totally. In addition, the difference caused by the addition of fluoroethylene carbonate (FEC), is whether the polycarbonates exist in SEI. As can be seen in the enlarged partial view of FT-IR spectra, the peak locating at 1800 cm-1, which can be related to polycarbonates, only emerge on the pattern of the FEC-added electrolyte, 1 M NaClO4 in EC/PC (v:v=1:1, 5% FEC as the additive).37 The elemental contents (atomic percentage) of these 3 samples are shown in Figure S9d, which corresponds to the results of FT-IR. The contents of different elements in SEI film can also prove the result of FT-IR. However, based on SEM image of cycled FeS2, polycarbonates do not benefit for forming stable SEI membrane. The SEI membranes formed by ether-based electrolytes are more stable, which may prevent shuttling effect. Then, the charge-discharge profiles of micro FeS2 at 5th cycle in 3 different electrolytes are displayed in Figure 5e. Obviously, ether-based electrolytes have the higher capacity and lower polarization. Especially, the electrochemical performance of 1M NaSO3CF3 in DGM is the best. These results are consistent with previous works shown in Table S1. Two main reasons make these outcomes. The first one is that the smaller solvation of DGM molecules leads to the electrochemical reaction energy barrier, which can decrease the voltage polarization.17 The second one is that different from carbonate-based electrolyte (as the EC/PC electrolyte), ether-based electrolyte (as the DGM electrolyte) do not react with polysulfide.24 As a result, the reversibility of electrochemical reaction increase, which leads to the larger capacity and higher cyclic performance. Just as expected, as shown in Figure 5f, the DGM electrolyte possesses better cyclic performance than EC/PC electrolyte. However,


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although FeS2 shows a little higher capacity in NaSO3CF3/DGM, the shortage of narrow operating voltage window for this electrolyte cannot be ignored. The NaClO4/DGM electrolyte is suitable for assembling full cell based on FeS2 electrode. Furthermore, it is possible that NaSO3CF3/DGM electrolyte has good application prospects for sodium ion capacitor.38

Figure 6. (a) EDS of cycled carbon interlayers, with inset of SEM image for nano FeS2@C. (b) CV profiles of the first 3 cycles at 0.2 mV s-1. (c) Charging-discharing profiles and (d) cyclic performance at 0.5 C of nano FeS2@C electrode. All the electrochemical performances of nano FeS2@C electrode were obtained by Na/FeS2 cell system. The used electrolyte was 1M NaClO4 in DGM.

For the aim of solving problem of shuttling effect, as a simple solution example, carbon coating nano FeS2 (FeS2@C) (Figure S10a) with the content of FeS2 in composite is 74.9%( Figure S10b), was synthesized because nano-sized particle can avoid morphology pulverization so that the bad impact on cyclic performance of this factor can be avoided. Carbon coating can perform confinement effect.39 In addition, NaClO4 in DGM was selected as electrolyte for form stable SEI on FeS2 particles. As expected, the element detection of the cycled carbon interlayer after 5 cycles in Na/Nano FeS2 cell (Figure 6a) shows that no signals of Fe and S elements can be detected. It means shuttling effect is suppressed. The CV profiles for FeS2@C composite are shown in Figure 6b. Compared with CV of micro FeS2 tested in EC/PC electrolyte, two main differences should be noticed. The first one is that the sharp of CV for this composite is more likely to a rectangle. It means more pseudocapacitive contribution in the total capacity by the synergistic effects of carbon



coating layer and ether-based electrolyte.13,23,40,41 The second one is that there is no redox peaks above 2 V. Because polysulfides cannot dissolve into electrolyte in this system, the side reaction will not occur at about 2.4 V. The charging-discharing curves (as shown in Figure 6c), no plateau emerges above 2 V, are correspond with CV result. The curves of 2nd cycle, 5th cycle and 50th cycle are mainly overlapped, which means the good cyclic performance. In addition, the rate performance was also tested. As can be seen in Figure S10c, the nano FeS2@C delivered a high discharging capacity of 330 mAh g-1 at 5C. At last, as shown in Figure 6d, nano FeS2@C electrode showed a better cyclic performance. In detail, no obvious fading can be observed and after 200 cycles, about a capacity of 560 mAh g-1 was reserved. Besides, the coulombic efficiencies were close to 100%. Besides, FeS2@C/NVP cell was also assembled. Its cyclic performance is shown in Figure S11. These results confirm that shuttling effects is crucial for the cyclic performance batteries with FeS2 electrodes, and the up-mentioned measures are effective to prevent the shuttling effects of Fe and S elements.

Conclusions In this study, the shutting effects of polysulfides and Fe element in both Na/FeS2 cell and FeS2/NVP cell were detected successfully. The differences between the above 2 systems and the influencing factor on shutting effects were studied systematically. In detail, polysulfides and Fe2+ ion will form and cross through separator during charging-discharging process. They can be reduced by low potential electrode (Na metal) and oxidated by high potential electrode (NVP electrode). Besides, carbonate-based will aggravate shuttling effect for the unstable SEI membrane and side reaction with polysulfides. Shuttling effect has a serious negative impact on the cyclic performance for FeS2 electrode. To suppress shuttling effect and enhance cyclic performance, for example, carbon coating and choice of ether-based electrolyte strategies were utilized. As a result, cyclic performance was enhanced effectively in half cell and full cell, respectively. These results confirm that shuttling effect is crucial for the cyclic performance batteries with FeS2 electrodes, and the


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up-mentioned measures are effective to prevent the shuttling effects of Fe and S elements. The above conclusions can tentatively build a new comprehensive review of pyrite’s electrochemical mechanism, which can be expand to some other metal sulfides, and contribute to develop some more effective strategies to prolong its cyclic life in sodium ion battery. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (No. 21771164, U1804129, 21671205, U1804126) and Youth backbone teacher training program of Zhengzhou University. Supporting Information Available: Experimental section, sectional view of carbon interlayer, morphologies and charging-discharging curves of FeS2 micro-sphere electrode and NVP electrode, in situ XRD pattern of FeS2/NVP cell, TEM, ex situ XRD pattern and elemental analyzes of cycled FeS2, elemental characterization and EPR pattern of cycled carbon interlayer, linear sweep of electrolytes, charging-discharging curves in different electrolytes, XRD, TGA and rate performance of nano FeS2@C, cyclic and rate performances of FeS2@C/NVP cell.

REFERENCES 1. Shen, L.F.; Yu, Y. Greener and Cheaper. Nature Energy 2017, 2, 836-837. 2. Dunn, B.; Kamath, H.; Tarascon, J.M. Electrical Energy Storage for the Grid: A Battery of Choices. Science 2011, 334, 928-935. 3. Zhang, Y.; Zhou, Q.; Zhu, J. X.; Yan, Q. Y.; Dou, S. X.; Sun, Y. P. Nanostructured Metal Chalcogenides for Energy Storage and Electrocatalysis. Adv. Funct. Mater. 2017, 1702317. 4. Xiao, Y.; Lee, S. H.; Sun, Y. K. The Application of Metal Sulfides in Sodium Ion Batteries. Adv. Energy Mater. 2016, 1601329. 5. Duan, X.C.; Xu, J.X.; Wei, Z.X.; Ma, J.M.; Guo, S.J.; Liu, H.K.; Dou, S.X.; Atomically Thin Transition-Metal Dichalcogenides for Electrocatalysis and Energy Storage. Small Methods 2017, 1, 1700156. 6. Jiang, Y.; Wei, M.; Feng, J. K.; Ma, Y. C.; Xiong, S. L. Enhancing the Cycling Stability of Na-ion Batteries by Bonding SnS2 Ultrafine Nanocrystals on Amino-Functionalized Graphene Hybrid Nanosheets. Energy Environ. Sci. 2016, 9, 1430-1438. 7. Chen, W. H.; Song, K. M.; Mi, L. W.; Feng, X. M.; Zhang, J. M.; Cui, S. Z.; Liu, C. T. Synergistic Effect Induced Ultrafine SnO2/Graphene Nanocomposite as an Advanced Lithium/Sodium-Ion Batteries Anode. J. Mater. Chem. A 2017, 5, 10027-10038. 8. He, P. L.; Fang, Y. J.; Yu, X. Y.; Lou, X. W. D. Hierarchical Nanotubes Constructed by Carbon-Coated Ultrathin SnS Nanosheets for Fast Capacitive Sodium Storage. Angew. Chem., Int. Ed. 2017, 56, 12202-12205. 9. Cui, C.Y.; Wei, Z.X.; Xu, J.T.; Zhang, Y.Q.; Liu, S.H.; Liu H.K.; Mao M.L.; Wang, S.Y.; Ma, J.M.; Dou, S.X.; Three-Dimensional Carbon Frameworks Enabling MoS2 as Anode for Dual Ion Batteries with Superior Sodium Storage Properties. Energy Storage Mater. 2018, 15, 22-30.



10. Geng, X. M.; Jiao, Y. C.; Han, Y.; Mukhopadhyay, A.; Yang, L.; Zhu, H. L. Freestanding Metallic 1T MoS2 with Dual Ion Diffusion Paths as High Rate Anode for Sodium-Ion Batteries. Adv. Funct. Mater. 2017, 27, 1702998. 11. Liu, Y. P.; Wang, H. T.; Cheng, L.; Han, N.; Zhao, F. P.; Li, P. R.; Jin, C. H.; Li, Y. G. TiS2 Nanoplates: A High-Rate and Stable Electrode Material for Sodium Ion Batteries. Nano Energy 2016, 20, 168-175. 12. Zhou, J.D.; Lin, J.H.; Huang, X.W.; Zhou, Y.; Chen, Y.; Xia, J.; Wang, H.; Xie, Y.; Yu, H.M.; Lei, J.C.; Wu, D.; Liu, F.C.; Fu, Q.D.; Zeng, Q.S.; Hsu, C.H.; Yang, C.L.; Lu, L.; Yu, T.; Shen, Z.X.; Lin, H.; Yakobson, B.I.; Liu, Q.; Suenaga, K.; Liu, G.D.; Liu, Z. A library of atomically thin metal chalcogenides. Nature 2018, 556, 355-359. 13. Kim, T. B.; Choi, J. W.; Ryu, H. S.; Cho, G. B.; Kim, K. W.; Ahn, J. H.; Cho, K. K.; Ahn, H. J. Electrochemical Properties of Sodium/Pyrite Battery at Room Temperature. J. Power. Sources 2007, 174, 1275-1278. 14. 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. 15. Chen, W. H.; Qi, S. H.; Guan, L. Q.; Liu, C. T.; Cui, S. Z.; Shen, C. Y.; Mi, L. W. Pyrite FeS2 Microspheres Anchoring on Reduced Graphene Oxide Aerogel as an Enhanced Electrode Material for Sodium-Ion Batteries. J. Mater. Chem. A 2017, 5, 5332-5341. 16. Liu, Z. M.; Lu, T. C.; Song, T.; Yu, X. Y.; Lou, X. W. D.; Paik, U. Structure-Designed Synthesis of FeS2@C Yolk-Shell Nanoboxes as a High-Performance Anode for Sodium-Ion Batteries. Energy Environ. Sci. 2017, 10, 1576-1580. 17. Hu, Z.; Zhu, Z. Q.; Cheng, F. Y.; Zhang, K.; Wang, J. B.; Chen C. C.; Chen, J. Pyrite FeS2 for High-Rate and Long-Life Rechargeable Sodium Batteries. Energy Environ. Sci. 2015, 8, 1309-1316. 18. Zhang, S. S. The Redox Mechanism of FeS2 in Non-Aqueous Electrolytes for Lithium and Sodium Batteries. J. Mater. Chem. A 2015, 3, 7689-7694 19. Shadike, Z.; Zhou, Y. N.; Ding, F.; Sang, L.; Nam, K. W.; Yang, X. Q.; Fu, Z. W. The New Electrochemical Reaction Mechanism of Na/FeS2 Cell at Ambient Temperature. J. Power. Sources 2014, 260, 72-76. 20. Kitajou, A.; Yamaguchi, J.; Hara, S.; Okada, S. Discharge/Charge Reaction Mechanism of a Pyrite-Type FeS2 Cathode for Sodium Secondary Batteries. J. Power. Sources 2014, 247, 391-395. 21. Chen, K. Y.; Zhang, W. X.; Xue, L. H.; Chen, W. L.; Xiang, X. H.; Wan, M.; Huang, Y. H. Mechanism of Capacity Fade in Sodium Storage and the Strategies of Improvement for FeS2 Anode. ACS Appl. Mater. Interfaces 2017, 9, 1536-1541. 22. Yu, X.W.; Manthiram, A. Highly Reversible Room-Temperature Sulfur/Long-Chain Sodium Polysulfide Batteries. J. Phys. Chem. Lett. 2014, 5, 1943-1947. 23. Wei, S. Y.; Xu, S. M.; Agrawral, A.; Choudhury, S.; Lu, Y. Y.; Tu. Z. Y.; Ma. L.; Archer, L. A. A Stable Room-Temperature Sodium-Sulfur Battery. Nat. Commun. 2016, 7, 11722. 24. Hu, Z.; Zhang, K.; Zhu, Z. Q.; Tao Z. L.; Chen, J. FeS2 Microspheres with an Ether-Based Electrolyte for High-Performance Rechargeable Lithium Batteries. J. Mater. Chem. A, 2015, 3, 12898-12904. 25. Slater, M. D.; Kim, D. H.; Lee, E. J.; Johnson, C. S. Sodium-Ion Batteries. Adv. Funct. Mater.


Page 18 of 21

Page 19 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2013, 23, 947-958. 26. Palomares, V.; Serras, P.; Villaluenga, I.; Hueso, K. B.; Carretero-Gonzalez J.; Rojo, T. Na-ion Batteries, Recent Advances and Present Challenges to Become Low Cost Energy Storage Systems. Energy Environ. Sci. 2012, 5, 5884-5901. 27. Zhang, S. S.; Tran, D. T. Mechanism and Solution for the Capacity Fading of Li/FeS2 Battery. J. Electrochem. Soc. 2016, 163(5), A792-A797. 28. Zhang, B. W.; Liu, Y. D.; Wang, Y. X.; Zhang, L.; Chen, M. Z.; Lai, W. H.; Chou, S. L.; Liu, H. K.; Dou, S. X. In Situ Grown S Nanosheets on Cu Foam: An Ultrahigh Electroactive Cathode for Room-Temperature Na-S Batteries. ACS Appl. Mater. Interfaces 2017, 9, 24446-24450. 29. Lu, X. C.; Kirby, B. W.; Xu, W.; Li, G. S.; Kim, J. Y.; Lemmon, J. P.; Sprenkle, V. L.; Yang, Z. G. Advanced Intermediate-Temperature Na-S Battery. Energy Environ. Sci. 2013, 6, 299-306. 30. Yamanaka, T.; Nakagawa, H.; Tsubouchi, S.; Domi, Y.; Doi, T.; Abe T.; Ogumi, Z. In situ Raman Spectroscopic Studies on Concentration of Electrolyte Salt in Lithium Ion Batteries by Using Ultrafine Multi-Fiber Probes. Chensuschem 2017, 10, 855-861. 31. Mogensen, R.; Brandell, D.; Younesi R.; Solubility of the Solid Electrolyte Interphase (SEI) in Sodium Ion Batteries. ACS Energy Lett. 2016, 1, 1173-1178. 32. Silván, B.; Gonzalo, E.; Djuandhi, L.; Sharma, N.; Fauth F.; Saurel, D. On the Dynamics of Transition Metal Migration and its Impact on the Performance in Layered Oxides for Sodium-Ion Batteries: NaFeO2 as a Case Study. J. Mater. Chem. A 2018, 6, 15132-15146. 33. Rui, X. H.; Sun, W. P.; Wu, C.; Yu, Y.; Yan, Q. Y. An Advanced Sodium-Ion Battery Composed of Carbon Coated Na3V2(PO4)3 in a Porous Graphene Network. Adv. Mater. 2015, 27, 6670-6676. 34. Zhang, J.; Wang, D. W.; Lv, W.; Zhang, S. W.; Liang, Q. H.; Zheng, D. Q.; Kang, F. Y.; Yang, Q. H. Achieving Superb Sodium Storage Performance on Carbon Anodes through an Ether-Derived Solid Electrolyte Interphase. Energy Environ. Sci. 2017, 10, 370-376. 35. Wang, C. C.; Wang, L. B.; Li, F. J.; Cheng, F. Y.; Chen, J. Bulk Bismuth as a High-Capacity and Ultralong Cycle-Life Anode for Sodium-Ion Batteries by Coupling with Glyme-Based Electrolytes. Adv. Mater. 2017, 29, 1702212. 36. Leskes, M.; Kim, G.; Liu, T.; Michan, A.L.; Aussenac, F.; Dorffer, P.; Paul, S.; Grey, C.P. Surface-Sensitive NMR Detection of the Solid Electrolyte Interphase Layer on Reduced Graphene Oxide. J. Phys. Chem. Lett. 2017, 8, 1078-1085. 37. Etacheri, V.; Haik, O.; Goffer, Y.; Roberts, G. A.; Stefan, I. C.; Fasching, R.; Aurbach, D. Effect of Fluoroethylene Carbonate (FEC) on the Performance and Surface Chemistry of Si-Nanowire Li-Ion Battery Anodes. Langmuir 2012, 28, 965-976. 38. Long, Y. Q.; Yang, J.; Gao, X.; Xu, X. N.; Fan, W. L.; Yang, J.; Hou, S. F.; Qian, Y. T. Solid-Solution Anion-Enhanced Electrochemical Performances of Metal Sulfides/Selenides for Sodium-Ion Capacitors: The Case of FeS2-xSex. ACS Appl. Mater. Interfaces 2018, 10, 10945-10954. 39. Chen, W. H.; Zhang, X. X.; Mi, L. W.; Liu, C. T.; Zhang, J. M.; Cui, S. Z.; Feng, X. M.; Cao, Y. L.; Shen, C. Y. High-Performance Flexible Freestanding Anode with Hierarchical 3D Carbon-Networks/Fe7S8/Graphene for Applicable Sodium-Ion Batteries. Adv. Mater. 2019, DOI: 10.1002/adma.201806664. 40. Shao, M.; Chen, Y. Y.; Zhang, T.; Li, S.; Zhang, W. N.; Zheng, B.; Wu, J. S.; Xiong, W. W.; Huo, F. W.; Lu, J. Designing MOFs-Derived FeS2@Carbon Composites for High-Rate Sodium



Ion Storage with Capacitive Contributions. ACS Appl. Mater. Interfaces 2018, 10, 33097-33104. 41. Chen, Y. Y.; Hu, X. D.; Evanko, B.; Sun, X. H.; Li, X.; Hou, T. Y.; Cai, S.; Zheng, C. M.; Hu, W. B.; Stucky, G. D. High-Rate FeS2/CNT Neural Network Nanostructure Composite Anodes for Stable, High-Capacity Sodium-Ion Batteries. Nano Energy 2018, 46, 117-127.


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Understanding Shuttling Effect in Sodium Ion Batteries for the Solution

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