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A self-cleaning Li-S battery enabled by a bifunctional redox mediator
Journal of Power Sources 361 (2017) 203e210
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
A self-cleaning Li-S battery enabled by a bifunctional redox mediator Y.X. Ren, T.S. Zhao*, M. Liu, Y.K. Zeng, H.R. Jiang Department of Mechanical and Aerospace Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong SAR, China
h i g h l i g h t s Indium iodide as a self-defense redox mediator is proposed for Li-S batteries. The deposited indium (In) layer protects the Li anode from side reactions. I =I 3 redox mediator is capable of decomposing Li2Sx (x ¼ 1, 2) side products.
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Article history: Received 6 March 2017 Received in revised form 22 June 2017 Accepted 27 June 2017
The polysulfide shuttle effect and lithium dendrite growth in lithium-sulfur (Li-S) batteries can repeatedly breach the anodic solid electrolyte interphase (SEI) over cycling. As a result, irreversible shortchain sulfide side products (Li2Sx, x ¼ 1, 2) keep depositing on the Li anode, leading to the active material loss, increasing the Liþ transport resistance, and thereby reducing the cycle life. In this work, indium iodide (InI3) is investigated as a bifunctional electrolyte additive for Li-S batteries to protect the Li anode and decompose the side products spontaneously. On the one hand, Indium (In) is electrodeposited onto the Li anode prior to Li plating during the initial charging process, forming a chemically and mechanically stable SEI to prevent the Li anode from reacting with soluble polysulfide species to form Li2Sx (x ¼ 1, 2) side products. On the other hand, by adequately overcharging the battery, the triiodide/iodide redox mediator is capable of chemically transforming side products deposited on the Li anode and separator into soluble polysulfides, which can be recycled by the cathode. It is shown that the battery with the InI3 additive exhibits a prolonged cycle life, and is capable of retrieving its capacity by a facile overcharging process. © 2017 Elsevier B.V. All rights reserved.
Keywords: Lithium-sulfur battery Redox mediator Side reactions Solid electrolyte interphase
1. Introduction With the depletion of fossil fuel, there is a large demand for high-energy rechargeable batteries, for portable electronics, electric vehicles and grid-scale stationary storage systems. Lithium or sodium-based batteries with high-energy alkaline metal anodes have thus been developed for these purposes [1e3]. Among the systems studied, Li-S batteries are especially attractive due to the striking merits including the high specific energy of sulfur cathode (2600 Wh kg1, 2800 Wh L1, respectively), reasonably fast kinetics, environmental benignity and low cost [1,2,4e8]. However, the dissolution of intermediate polysulfides in the electrolyte, the so called “shuttle effect”, leads to the active material loss, anode degradation and thus a shortened cycle life, which is a formidable
* Corresponding author. E-mail address: [email protected] (T.S. Zhao). http://dx.doi.org/10.1016/j.jpowsour.2017.06.083 0378-7753/© 2017 Elsevier B.V. All rights reserved.
challenge facing the wide deployment of Li-S batteries [9e13]. To suppress the polysulfide shuttle effect, strategies have been proposed to localize the polysulfide species in the cathodic side [14e22]. Specifically, an ion selective separator, which allows the Liþ transport but inhibits the diffusion of polysulfides, can be exploited for use [23e26]. However, in most cases, irreversible capacity loss can be still observed over cycling due to the insufficient ion selectivity. In another approach, a chemically and mechanically stable passivation layer (solid electrolyte interphase) is desirable to form onto the Li anode, which can shield the Li anode from reacting with polysulfide shuttles [27,28]. Electrolyte additives such as LiNO3 and FEC (Fluoroethylene carbonate) have been proposed to facilitate the formation of a more stabilized solid electrolyte interphase (SEI) on the Li metal anode surface [29e32]. However, a critical issue facing Li metal is the volume change over the repeated stripping/plating cycles, leading to the collapse of SEI over cycling [16,33,34]. In consequence, short-chain sulfides
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formed by the side reactions between Li metal and polysulfides can be continuously deposited onto the anode surface over cycling as shown in Figs. 1(a, b), leading to the irreversible capacity loss and considerably lowered Liþ conductivity [35,36]. Considering the detrimental effects of side product accumulation, it occurs to us that a potential strategy is to allow for a timely removal of side products, which can be possibly revealed by oxidizing redox mediators [37e39]. Specifically, over charging, reduction state redox species can be charged into the oxidation state at the cathode, which is capable of migrating out of the cathode and oxidizing the side products (solid Li2S and Li2S2 species) into soluble long-chain polysulfides, which also results in the regeneration of reduction state redox species. The redox mediator thus acts as an electron-hole transfer agent that facilitates efficient oxidation of side products. However, just like polysulfide species, usually the redox mediator can react with the Li anode, while decomposing those deposited Li2S and Li2S2 species [38]. In that case, in the corrosive electrolyte containing polysulfides and redox mediator, anode engineering approaches are critically needed to reinforce the SEI and shield the Li anode. Motivated by these issues, we investigated the indium iodide (InI3) additive as a bifunctional redox mediator for Li-S batteries, which can spontaneously fulfill the roles of side product (Li2S and Li2S2) decomposition and Li anode protection. We demonstrate that, on the one hand, a stable and uniform solid electrolyte interphase (SEI) can be enabled by a deposited metallic In layer, which adheres onto the Li anode surface and functions as a protective shield for the bulk phase of Li anode as shown in Fig. 1c. On the other hand, we propose that a timely decomposition of side products can be realized by overcharging the batteries into the voltage window of iodide/triiodide redox reaction, which chemically transforms the short-chain Li2Sx (x ¼ 1, 2) deposited outside the cathode into soluble and electrochemically active polysulfides as shown in Fig. 1d. Owing to the side product decomposition by overcharging, the Liþ transport resistance can be lowered, and the battery's capacity can be retrieved, enabling a Li-S battery with the self-cleaning capability.
2. Experimental 2.1. Material preparation The Li2S8 solution was prepared by dissolving a desired amount of stoichiometric S and Li2S in 1,3-dioxolane (DOL)/1,2dimethoxyethane (DME) solution (1:1 in volume) with the addition of 1 M LiTFSI and LiNO3 additive (1 wt%). For the typical preparation of 250 mM Li2S8 solution, 280 mg of S and 57.5 mg of Li2S were added to 5 mL of DOL/DME (1:1) based electrolyte. The obtained suspension was stirred at 80 C overnight to yield redbrown Li2S8 solution. The battery's specific capacity was calculated based on the mass of S2 8 in Li2S8 in consistence with the previous literature [22,24]. 2.2. Cell assembly and test The electrochemical performance was tested in the Swageloktype lithium-sulfur battery fixture as shown in Fig. S1, which were assembled in an argon-filled glove box with oxygen and H2O maintained below 0.1 ppm. All tests were performed at the temperature of 23 þ 2 C. A piece of Li foil (16 mm diameter) was placed onto the bottom Cu cell body and a piece of Celgard 2500 separator (18 diameter) was placed onto the Li foil, followed by the addition of 50 mL 50 mM InI3 electrolyte (theoretical capacity of In3þ /In is 0.2 mAh) to saturate the separator. Subsequently, a piece of hydrophilic carbon cloth (12 mm, 1.13 cm2) was employed as the cathode and 25 mL 250 mM Li2S8 catholyte was uniformly dropped onto the carbon cloth cathode, with a theoretical areal capacity of 2.37 mAh cm2 (S8/Li2S). The galvanostatic discharge and charge tests were conducted on a battery cycling system (Neware, CT3008W) at room temperature (298 K). The electrochemical impedance spectroscopy (EIS) measurements were conducted on a potentiostat (Princeton Applied Research, PARSTAT M2273) via the two-electrode setup, where the lithium (Li) metal anode performs as both the reference and counter electrode and the cathode performs as the working and
Fig. 1. (a, b) A schematic diagram of side product (Li2S and Li2S2) accumulation. Over cycling, soluble polysulfides can be transformed to insulating short-chain sulfides and deposited onto the Li anode. (c, d) A schematic diagram illustrates the merits of the bifunctional InI3 electrolyte additive: (c) the electrodeposited In layer mitigates side reactions between Li metal and polysulfides, reducing the accumulated side products; (d) by overcharging, I can be charged into I 3 , which can chemically decompose the side products into soluble polysulfides.
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sensing electrodes. Because Li stripping and plating is regarded as reversible and exhibits a stable potential, in most of the existing LiS battery research, the Li anode can serve as the reference electrode [22,40e42]. Here, the EIS measurement using a frequency range from 100 kHz to 100 mHz with a wave amplitude of 5 mV was applied to the charged batterie at the open circuit voltage. Besides, the cyclic voltammetry (CV) of the Li-S batteries was tested at a scanning rate of 0.1 mV s1 with the carbon cloth electrode as the working electrode and Li foil as both reference and counter electrodes. 2.3. Characterization The battery components (anode and separator) after cycling were rinsed in pure DME for 10 min and then dried before SEM observation in an argon-filled glove box with oxygen and H2O maintained below 0.1 ppm. JSM-6700F field emission SEM instruments were used for micrograph observation at an acceleration voltage of 5.0 kV. The UV-Vis spectra were collected by SEC2000 UVevisible spectrophotometer (ALS Co., Ltd.) An SCE-C thin layer quartz glass cell with an optical path length of 4.5 mm was used as the holder. X-ray photoelectron spectroscopy (XPS) measurements were conducted on a Physical Electronics PHI5802 instrument using an X-rays magnesium anode (monochromatic Ka X-rays at 1253.6 eV) as the source. C 1s region was used as the reference and set at 285 eV. 3. Results and discussion The first step to validate the self-cleaning concept is to investigate the electrochemical and chemical compatibility between iodine and sulfur based redox species in the ether based electrolyte. The Li-polysulfide battery containing 25 mL 250 mM Li2S8 as the
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active material and 50 mL 50 mM InI3 as the electrolyte additive was assembled and the as-prepared battery was charged firstly to 3.4 V. A layer of protective In layer (with a theoretical areal loading of 0.072 mg cm2) can be deposited onto the Li anode following the reaction of In3þþ3e/In (0.375 V vs SHE), which occurs prior to Li plating Liþþe/Li (3.04 V vs SHE). The battery was rested for 30 min before discharge and the corresponding voltage profile for the 1st charge-discharge cycle can be found in Fig. S2. Separated discharge voltage plateaus representing the triiodide reduction (~2.8 V), sulfur dissolution (~2.3 V) and Li2S precipitation (~2.1 V) can be found, indicating that there are minor side reactions between the Li metal and corrosive polyiodide and polysulfide species, owing to the protective effects of the deposited In layer. Cyclic voltammetry (CV) was also exploited for the Li-S battery with the addition of InI3 as the electrolyte additive as shown in Fig. 2a. It is found that the sulfur part and iodine part were well separated and consistent with the reported CVs for Li-S and Li-I2 batteries respectively [43e45]. The anodic peak of I =I 3 redox reaction is at 3.0 V, which is considerably higher than the anodic peaks for Li2S oxidation (~2.4 V). It is thereby deduced that I 3 is capable of oxidizing Li2S. To confirm this issue, we dispersed the Li2S powders (commercially available from Sigma-Aldrich) into the DOL/DME electrolyte via sonication and further added excessive LiI3. Simply after 30-min rest at the room temperature, those sediment Li2S powders were found to be well dissolved, as can be confirmed in the UV-Vis spectra in Fig. 2b, two absorption peaks at around 295 and 366 nm, representing I 3 species, decreased with the existence of Li2S6 species [43]. The peak representing Li2S6 was found to emerge at around 470 nm [46]. This result suggests that LiI3 is sufficiently oxidative to transform the insoluble Li2S to soluble polysulfide species. On the other hand, as shown in Fig. 2(c and d), we show that during the initial charging process, a coarse layer of deposited In can be observed to passivate the Li surface uniformly. Also, from the
Fig. 2. (a) Cyclic voltammetry (CV) of Li-S battery with the addition of InI3; (b) UV-Vis spectra of the 5 mM LiI3 in DME solution and the mixture of 5 mM LiI3 þ 0.50 mg Li2S in DME, the inset shows the photography of Li2S suspension (0.20 mg mL1 in DME), 10 mM LiI3 in DME solution and their mixture. (c) Pristine Li anode; (d) passivated Li anode with the deposited In layer.
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cross-sectional view of the Li anode deposited with the In layer (Fig. S3), the border between the In surface layer and the Li foil can be clearly seen. Over the subsequent cycles, Li-In alloy can be formed as found in the previous literature, which allows for facile Li storage and Liþ transport [34,47e49]. Moreover, a notable feature of In is that it is highly inertial to the oxidative iodine, polyiodide as well as polysulfides, thus it can be hypothesized that the Li anode can be protected from the side reactions with the In layer, which will be presented in the following section [50]. Electrochemical test was conducted to confirm the protective effect of the deposited In layer. The battery's performance over cycling was presented in Fig. 3a-c. Without the addition of InI3, the average capacity decay rate for the assembled Li-polysulfide battery is as high as 0.82% for 100 cycles. On the other hand, with the addition of InI3 as the electrolyte additive, after an initial cycle similar with the process in Fig. S2, the battery was cycled within the voltage window of 1.7e2.8 V at 0.5 C. The overall discharge capacity decay rate was lowered to 0.38% for 100 cycles, indicating that the deposited In layer, which anchors on the Li surface, can function as a stable shield for the bulk phase of Li anode against the dissolved polysulfide species. With minor side reactions occurring on the anode, the dissolved polysulfides would diffuse back into the cathode and could be utilized in the subsequent electrochemical processes. In this regard, by mitigating side reactions, the active material loss induced by polysulfide dissolution can be well decreased. Moreover, interestingly, if we assembled a Li-polyiodide battery with the addition of InI3 (50 mL 50 mM) as the active material, it is found that there is minor capacity decay. As shown in Fig. S4, though the coulombic efficiency is averaged only around ~90%, the battery's capacity can be well stabilized over cycling, implying that the side reaction actually occurs in an electrochemical approach: I 3 þ 2e 43I , with minor degradation of the Li anode [51]. Thus, it is hypothesized that the deposited In layer is capable of suppressing side chemical reactions. However, for the Li-polysulfide battery, electrodeposition of sulfides from polysulfides can keep occurring on the anode over the repeated cycles. Unlike I =I 3 redox species that are both highly soluble in the ether-based electrolyte, short-
chain sulfides are insoluble and can continuously accumulate on the Li anode, leading to the active material loss, enlarged Liþ transport resistance and thus a shortened cycle life. To decompose the insoluble short-chain sulfide side products, we carried out the proposed overcharging strategy. As shown in Fig. 3d, overcharging (the upper limit is 3.4 V also) was conceived for every 20 cycles for overall 100 cycles. With this overcharging strategy, the average capacity decay rate was lowered to 0.24% and a discharge capacity around ~850 mAh g1 was maintained over 100 cycles (Fig. 3d). Interestingly, as shown in Fig. 3(e and f), it is found that after overcharging and rest, the battery's capacity can be considerably retrieved, implying that polysulfides derived from the side products can be utilized in the subsequent cycle, also the ionic transport might be improved via removing the side products. Also, to further testify the effectiveness of the overcharging strategies. We try to elucidate the effects of resting the batteries. It is found that by resting the batteries for 60 min after 20 cycles, the battery's cyclability doesn't exhibit any substantial improvement (Fig. S5). Also, in the control cell without I =I 3 redox mediator, if the battery is overcharged, from 2.8 V to 3.4 V, the cell voltage climbs up rapidly, with minor increase of the charge capacity. Unlike the battery with the addition of InI3, the battery's capacity keeps decreasing (Fig. S6). In this regard, it can be testified that our strategy of introducing a redox mediator to timely clean (decompose) the side products in Li-S batteries shows a promising perspective for prolonging the battery's cycle life. The positive role of such an overcharging process was confirmed in the electrochemical impedance spectroscopy (EIS) measurement. A two-electrode setup is exploited, where Li metal performs as both the counter and reference electrodes. As shown in the equivalent circuit model in Fig. 4a, the high-frequency intercept is attributed to the battery's ohmic resistance and the semi-circle is attributed to the interfacial resistance (RSEI) and charge transfer resistance (Rct) [35]. The charge transfer process involves both Li stripping/plating reaction at the anode and sulfur redox reactions at the cathode. Considering that these two processes might have similar frequency response, a single Rct//CPE2 module was used to depict the charge transfer process [40,52]. Over cycling, as the side
Fig. 3. (a) Cycling performance of Li-polysulfide batteries with and without the addition of InI3 at 0.5 C; (b) voltage profiles of Li-polysulfide batteries with the addition of InI3; (c) voltage profiles of Li-polysulfide batteries without the addition of InI3. (d) Cycling performance of Li-polysulfide batteries at 0.5 C with the addition of InI3, the battery was overcharged to 3.4 V and rested for 30 min for every 20 cycles; (eef) corresponding voltage profiles at 20e22 cycles and 80e82 cycles.
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Fig. 4. (a) Equivalent circuit model and (b) the electrochemical impedance spectroscopy (EIS) results of batteries after charged at different cycle indexes.
Table 1 Summary of fitted EIS results from Fig. 4. Cycle index
2
20
21
80
81
R b ( U) RSEI (U) Rct (U) CPE1 (F) CPE2 (F) WR (S1/2) WP
4.88 2.94 6.26 3.42 106 9.78 106 8.20 0.41
8.66 11.64 9.12 2.73 105 1.19 104 13.17 0.36
5.89 2.75 5.25 4.13 106 9.16 106 16.57 0.28
9.81 12.31 8.46 1.19 105 1.08 104 13.31 0.36
6.67 3.79 7.15 5.65 105 1.29 104 7.791 0.43
products passivate the anode/separator as well as cathode/separator interphases, Liþ transport pathway between the anode and cathode will be blocked and the available cathode surface will be decreased, enlarging both RSEI and Rct [53]. As shown in Fig. 4b, it is
found that the depressed semi-circles representing the interfacial and charge transfer resistances can be decreased after the overcharging process, which are very close to the EIS result measured from the battery after the initial 2 cycles (Fig. S7). The fitted results
Fig. 5. X-ray photoelectron spectroscopy (XPS) analysis of Li anodes. (a, b) S 2p spectra of Li anodes obtained from the batteries after charged (20 cycles at 0.5 C): (a) without overcharging; (b) with overcharging. (c) Comparison of I 3d spectra of Li anodes obtained from batteries with the addition of InI3 (with overcharging, without overcharging). (d) Comparison of In 3d spectra of Li anodes obtained from batteries with the addition of InI3 (with overcharging, without overcharging).
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as shown in Table 1 further indicate that the decrease of the RSEI should be the main attribute, while the charge transfer resistance shows a decrease to some extent. In this regard, it can be deduced that overcharging might lower the interfacial resistance by decomposing side product. Also, the charge transfer process involved in Li stripping/plating or sulfur redox reactions might be improved, as the removal of nonconductive side product allows for more available electrode surface for electrochemical reactions. Though useful for analyzing the interfacial resistance, the twoelectrode EIS measurement result cannot allow us to calculate the anodic or cathodic exchange current density as pointed out by the reviewer. To address this issue, a reference eletrode setup, which is stable in the corrosive electrolyte environment and has minor effect on the original battery configuration, should be developed and thus it might be possible to depict the anodic and cathodic reaction kinetics separately. In line with the achieved electrochemical performance, we further conducted X-ray photoelectron spectroscopy (XPS) analysis to characterize the chemical compositions of the Li anodes with and without overcharging. Fig. 5(a and b) show a comparison of S 2p spectra of the SEIs formed on the anodes with and without the overcharging process. Overcharging was carried out after the initial 20 cycles (the battery was charged to 3.4 V and rested for 30 min). Oxidized species including -NSO2CF3 and -SO3 exist for both
samples, which can be attributed to the LiTFSI decomposition [23,29]. As shown in Fig. 5a, two peaks exist at 161.3 and 162.5 eV for the SEI formed in the battery without overcharging, reflecting the accumulation of Li2S and Li2S2 in the SEI layer [24]. In contrast, as can be found in Fig. 5b, there are minor signals of Li2S and Li2S2 species for the battery with overcharging. It can be thus deduced that the overcharging process successfully removes the side products (Li2S and Li2S2) deposited onto the Li anode. Moreover, as shown in Fig. 5c, it is found that for the battery with overcharging, there exist more apparent peaks of I 3d 5/2 and I 3d 3/2, indicating that polyiodide has been partially oxidized into elemental iodine [37]. If we further look into the In 3d spectra in Fig. 5d, the In 3d 5/2 and In 3d 3/2 peaks maintain unchanged for the batteries with and without overcharging, confirming the chemical stability of In layer under the existence of corrosive iodine and polyiodide [54,55]. Eventually, we provide the SEM images as the evidence of side product removal. As shown in Fig. 6a, at the initial cycle, the deposited Li shows a rather smooth morphology, because intrinsically the In layer can perform as a preferential Li deposition substrate, lowering the possibility of dendrite growth [56]. After 20 cycles, for the Li anode deposited with the In layer, a layer of protruding particles, which are composed of insoluble reduced sulfur species (Li2S and Li2S2), is found to emerge. Though the
Fig. 6. (aed) SEM images and the corresponding EDX mappings of sulfur on the Li anodes: (a) after the initial cycle; (b) after 20 cycles without overcharging; (c) after 20 cycles with overcharging; (d) after 80 cycles with overcharging. SEM images of separators: (e) after 20 cycles without overcharging; (f) after 20 cycles with overcharging.
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accumulation of side product is considerably mitigated than the case with bare Li metal (Fig. S8), the detrimental effects of side products still need to be addressed for performance enhancement. After overcharging, the anode surface demonstrates a smooth and plain morphology, and the surface sulfur content is also found to be considerably decreased (Fig. 6c), which confirms the successful removal of reduced sulfur species. Interestingly, we observe that, after overcharging after 80 cycles, a smooth polymer-like film is found to be coated onto the Li-In alloy layer on the anode surface (Fig. 6d), which might formed from the iodide induced polymerization of DME [37]. The neutral I$ radical, which is an intermediate generated when triiodide is reduced to iodide, may react with electrolyte components to form a linear or comb-branched oligoethers. Such a Li-permeable layer is capable of protecting the Li anode from the direct contact with the corrosive electrolyte and greatly suppressing side reactions, partially explaining the exceptional cycling stability [37]. In addition to the anode side, it is found that the separators obtained from the cycled batteries have been partially covered by the precipitated side products formed from the chemical disproportion of polysulfides (Fig. 6e), which also can lower the active material utilization ratio, as found in the previous experiment [53]. After overcharging, interestingly, the accumulated side products on the separator are also found to be well dissolved by the oxidative polyiodide and the resultant separator (Fig. 6f) shows minor difference compared with the pristine one, further confirming the role of I =I 3 redox mediator in decomposing side products. As a further discussion, in addition to InI3, we also exploit InBr3 as an alternative because Br =Br3 is of a stronger oxidizability than I =I 3 . Though stable cycling of the redox mediator alone is achieved as shown in Fig. S9a, it is realized that Br is an anion with high donor number (DN) as previously reported in Li-O2 battery [57]. Specially, when being added into the electrolyte with low DN such as Dimethyl ether (DME), the existence of Br promotes the solution mechanism [58]. In consequence, as shown Figs. S9b and a sloping lower voltage plateau is demonstrated in contrast with the flat lower voltage plateau observed in conventional Li-S batteries. Such a phenomenon can be attributed to the promoted short-chain (poly)sulfide dissolution during the precipitation process, leading to less irreversible discharge product formed by disproportion [46,59,60]. In this regard, the redox mediator's effect on the electrolyte system's donor number is supposed to be an essential criterion for choosing adequate redox mediators. In brief summary, an appropriate redox mediator for side product removal in Li-S batteries should possess the following properties: (i) excellent stability and fast redox kinetics; (ii) high redox potential to chemically oxidize the side product and (iii) minor or positive effects on the sulfur reduction/oxidation processes. In addition to I =I 3 redox couple, further efforts can be directed towards the exploration of organic redox mediators (e.g. TEMPO and ferrocene) as well [61,62]. In addition to Li-S batteries, Li-O2 batteries suffer from the cathode/ anode surface passivation induced by the side products such as lithium carbonate (Li2CO3) and lithium hydroxide (LiOH), in that case, identifying suitable redox mediators to decompose side products should be also desirable for prolonging Li-O2 batteries' cycle life [63,64]. 4. Conclusion To conclude, we propose a self-cleaning Li-S battery enabled by the bifunctional electrolyte additive InI3. One is the protection of Li anode induced by the electrodeposited In layer. The other concerns for the effective decomposition of insoluble side product (shortchain polysulfide) outside the cathode, exploiting I =I 3 redox mediator. The resultant battery using a Li anode with a deposited In
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layer shows an improved cycling performance in comparison with the one with the bare Li metal. On the other hand, by timely overcharging the battery, it is demonstrated that the battery's capacity can be retrieved and the battery's cycle life can be effectively prolonged. The facile and efficient strategy that protects Li anode and decomposes side products thus opens a new avenue for prolonging the Li-S battery's cycle life. Acknowledgment The work described in this paper was fully supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (project no. 16213414). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2017.06.083. References [1] P.G. Bruce, S.A. Freunberger, L.J. Hardwick, J.M. Tarascon, Nat. Mater. 11 (2012) 19e29. [2] F. Wu, G. Yushin, Energy & Environ. Sci. 10 (2017) 435e459. [3] S. Chu, Y. Cui, N. Liu, Nat. Mater. 16 (2017) 16e22. [4] S. Evers, L.F. Nazar, Accounts Chem. Res. 46 (2013) 1135e1143. [5] Y. Yang, G. Zheng, Y. Cui, Chem. Soc. Rev. 42 (2013) 3018e3032. [6] H.R. Jiang, Z. Lu, M.C. Wu, F. Ciucci, T.S. Zhao, Nano Energy 23 (2016) 97e104. [7] H. Jiang, W. Shyy, M. Liu, L. Wei, M. Wu, T. Zhao, J. Mater. Chem. A 5 (2017) 672e679. [8] Y. Ren, T. Zhao, M. Liu, P. Tan, Y. Zeng, J. Power Sources 336 (2016) 115e125. [9] P.P.R. Harks, C.B. Robledo, T.W. Verhallen, P.H. Notten, F.M. Mulder, Adv. Energy Mater. 7 (2016) 1601635. [10] X.-B. Cheng, J.-Q. Huang, H.-J. Peng, J.-Q. Nie, X.-Y. Liu, Q. Zhang, F. Wei, J. Power Sources 253 (2014) 263e268. [11] Y. Sun, Z.W. Seh, W. Li, H. Yao, G. Zheng, Y. Cui, Nano Energy 11 (2015) 579e586. [12] A. Manthiram, Y. Fu, S.-H. Chung, C. Zu, Y.-S. Su, Chem. Rev. 114 (2014) 11751e11787. [13] A. Manthiram, Y. Fu, Y.-S. Su, Accounts Chem. Res. 46 (2013) 1125e1134. [14] Z. Liu, X.H. Zhang, C.S. Lee, J. Mater. Chem. A 2 (2014) 5602e5605. [15] S. Evers, T. Yim, L.F. Nazar, J. Phys. Chem. C 116 (2012) 19653e19658. [16] S. Zheng, F. Yi, Z. Li, Y. Zhu, Y. Xu, C. Luo, J. Yang, C. Wang, Adv. Funct. Mater. 24 (2014) 4156e4163. [17] L.C. Gerber, P.D. Frischmann, F.Y. Fan, S.E. Doris, X. Qu, A. Scheuermann, K.A. Persson, Y.-M. Chiang, B.A. Helms, Nano letters (2015). [18] Z. Li, Y. Jiang, L. Yuan, Z. Yi, C. Wu, Y. Liu, P. Strasser, Y. Huang, ACS Nano 8 (2014) 9295e9303. [19] Y. Fu, Y.-S. Su, A. Manthiram, ACS Appl. Mater. Interfaces 4 (2012) 6046e6052. [20] Y. Fu, Y.-S. Su, A. Manthiram, Angew. Chem. Int. Ed. 52 (2013) 6930e6935. [21] C. Zu, Y. Fu, A. Manthiram, J. Mater. Chem. A 1 (2013) 10362e10367. [22] Y. Ren, T. Zhao, M. Liu, L. Wei, R. Zhang, Electrochimica Acta 242 (2017) 137e145. [23] M. Liu, D. Zhou, Y.-B. He, Y. Fu, X. Qin, C. Miao, H. Du, B. Li, Q.-H. Yang, Z. Lin, Nano Energy 22 (2016) 278e289. [24] M. Liu, Y. Ren, D. Zhou, H. Jiang, F. Kang, T. Zhao, ACS Appl. Mater. Interfaces 9 (2016) 2526e2534. [25] M. Liu, D. Zhou, H. Jiang, Y. Ren, F. Kang, T. Zhao, Nano Energy 28 (2016) 97e105. [26] M. Liu, H. Jiang, Y. Ren, D. Zhou, F. Kang, T. Zhao, Electrochimica Acta 213 (2016) 871e878. [27] S. Xiong, K. Xie, Y. Diao, X. Hong, J. Power Sources 236 (2013) 181e187. [28] X.B. Cheng, R. Zhang, C.Z. Zhao, F. Wei, J.G. Zhang, Q. Zhang, Adv. Sci. 10 (2015) 1500213. [29] W. Li, H. Yao, K. Yan, G. Zheng, Z. Liang, Y.-M. Chiang, Y. Cui, Nat. Commun. 6 (2015). [30] S.S. Zhang, J. Electrochem. Soc. 159 (2012) A920eA923. [31] M. Kazazi, M.R. Vaezi, A. Kazemzadeh, Ionics 20 (2014) 1291e1300. [32] J.T. Lee, K. Eom, F. Wu, H. Kim, D.C. Lee, B. Zdyrko, G. Yushin, ACS Energy Lett. 1 (2016) 373e379. [33] P. Bai, J. Li, F.R. Brushett, M.Z. Bazant, Energy & Environ. Sci. 9 (2016) 3221e3229. [34] M. Ishikawa, H. Kawasaki, N. Yoshimoto, M. Morita, J. power sources 146 (2005) 199e203. [35] Z. Liu, S. Bertolini, P.B. Balbuena, P.P. Mukherjee, ACS Appl. Mater. interfaces 8 (2016) 4700e4708. [36] Z. Liu, D. Hubble, P.B. Balbuena, P.P. Mukherjee, Phys. Chem. Chem. Phys. 17 (2015) 9032e9039.
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A self-cleaning Li-S battery enabled by a bifunctional redox mediator Y.X. Ren, T.S. Zhao*, M. Liu, Y.K. Zeng, H.R. Jiang Department of Mechanical and Aerospace Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong SAR, China
h i g h l i g h t s Indium iodide as a self-defense redox mediator is proposed for Li-S batteries. The deposited indium (In) layer protects the Li anode from side reactions. I =I 3 redox mediator is capable of decomposing Li2Sx (x ¼ 1, 2) side products.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 March 2017 Received in revised form 22 June 2017 Accepted 27 June 2017
The polysulfide shuttle effect and lithium dendrite growth in lithium-sulfur (Li-S) batteries can repeatedly breach the anodic solid electrolyte interphase (SEI) over cycling. As a result, irreversible shortchain sulfide side products (Li2Sx, x ¼ 1, 2) keep depositing on the Li anode, leading to the active material loss, increasing the Liþ transport resistance, and thereby reducing the cycle life. In this work, indium iodide (InI3) is investigated as a bifunctional electrolyte additive for Li-S batteries to protect the Li anode and decompose the side products spontaneously. On the one hand, Indium (In) is electrodeposited onto the Li anode prior to Li plating during the initial charging process, forming a chemically and mechanically stable SEI to prevent the Li anode from reacting with soluble polysulfide species to form Li2Sx (x ¼ 1, 2) side products. On the other hand, by adequately overcharging the battery, the triiodide/iodide redox mediator is capable of chemically transforming side products deposited on the Li anode and separator into soluble polysulfides, which can be recycled by the cathode. It is shown that the battery with the InI3 additive exhibits a prolonged cycle life, and is capable of retrieving its capacity by a facile overcharging process. © 2017 Elsevier B.V. All rights reserved.
Keywords: Lithium-sulfur battery Redox mediator Side reactions Solid electrolyte interphase
1. Introduction With the depletion of fossil fuel, there is a large demand for high-energy rechargeable batteries, for portable electronics, electric vehicles and grid-scale stationary storage systems. Lithium or sodium-based batteries with high-energy alkaline metal anodes have thus been developed for these purposes [1e3]. Among the systems studied, Li-S batteries are especially attractive due to the striking merits including the high specific energy of sulfur cathode (2600 Wh kg1, 2800 Wh L1, respectively), reasonably fast kinetics, environmental benignity and low cost [1,2,4e8]. However, the dissolution of intermediate polysulfides in the electrolyte, the so called “shuttle effect”, leads to the active material loss, anode degradation and thus a shortened cycle life, which is a formidable
* Corresponding author. E-mail address: [email protected] (T.S. Zhao). http://dx.doi.org/10.1016/j.jpowsour.2017.06.083 0378-7753/© 2017 Elsevier B.V. All rights reserved.
challenge facing the wide deployment of Li-S batteries [9e13]. To suppress the polysulfide shuttle effect, strategies have been proposed to localize the polysulfide species in the cathodic side [14e22]. Specifically, an ion selective separator, which allows the Liþ transport but inhibits the diffusion of polysulfides, can be exploited for use [23e26]. However, in most cases, irreversible capacity loss can be still observed over cycling due to the insufficient ion selectivity. In another approach, a chemically and mechanically stable passivation layer (solid electrolyte interphase) is desirable to form onto the Li anode, which can shield the Li anode from reacting with polysulfide shuttles [27,28]. Electrolyte additives such as LiNO3 and FEC (Fluoroethylene carbonate) have been proposed to facilitate the formation of a more stabilized solid electrolyte interphase (SEI) on the Li metal anode surface [29e32]. However, a critical issue facing Li metal is the volume change over the repeated stripping/plating cycles, leading to the collapse of SEI over cycling [16,33,34]. In consequence, short-chain sulfides
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formed by the side reactions between Li metal and polysulfides can be continuously deposited onto the anode surface over cycling as shown in Figs. 1(a, b), leading to the irreversible capacity loss and considerably lowered Liþ conductivity [35,36]. Considering the detrimental effects of side product accumulation, it occurs to us that a potential strategy is to allow for a timely removal of side products, which can be possibly revealed by oxidizing redox mediators [37e39]. Specifically, over charging, reduction state redox species can be charged into the oxidation state at the cathode, which is capable of migrating out of the cathode and oxidizing the side products (solid Li2S and Li2S2 species) into soluble long-chain polysulfides, which also results in the regeneration of reduction state redox species. The redox mediator thus acts as an electron-hole transfer agent that facilitates efficient oxidation of side products. However, just like polysulfide species, usually the redox mediator can react with the Li anode, while decomposing those deposited Li2S and Li2S2 species [38]. In that case, in the corrosive electrolyte containing polysulfides and redox mediator, anode engineering approaches are critically needed to reinforce the SEI and shield the Li anode. Motivated by these issues, we investigated the indium iodide (InI3) additive as a bifunctional redox mediator for Li-S batteries, which can spontaneously fulfill the roles of side product (Li2S and Li2S2) decomposition and Li anode protection. We demonstrate that, on the one hand, a stable and uniform solid electrolyte interphase (SEI) can be enabled by a deposited metallic In layer, which adheres onto the Li anode surface and functions as a protective shield for the bulk phase of Li anode as shown in Fig. 1c. On the other hand, we propose that a timely decomposition of side products can be realized by overcharging the batteries into the voltage window of iodide/triiodide redox reaction, which chemically transforms the short-chain Li2Sx (x ¼ 1, 2) deposited outside the cathode into soluble and electrochemically active polysulfides as shown in Fig. 1d. Owing to the side product decomposition by overcharging, the Liþ transport resistance can be lowered, and the battery's capacity can be retrieved, enabling a Li-S battery with the self-cleaning capability.
2. Experimental 2.1. Material preparation The Li2S8 solution was prepared by dissolving a desired amount of stoichiometric S and Li2S in 1,3-dioxolane (DOL)/1,2dimethoxyethane (DME) solution (1:1 in volume) with the addition of 1 M LiTFSI and LiNO3 additive (1 wt%). For the typical preparation of 250 mM Li2S8 solution, 280 mg of S and 57.5 mg of Li2S were added to 5 mL of DOL/DME (1:1) based electrolyte. The obtained suspension was stirred at 80 C overnight to yield redbrown Li2S8 solution. The battery's specific capacity was calculated based on the mass of S2 8 in Li2S8 in consistence with the previous literature [22,24]. 2.2. Cell assembly and test The electrochemical performance was tested in the Swageloktype lithium-sulfur battery fixture as shown in Fig. S1, which were assembled in an argon-filled glove box with oxygen and H2O maintained below 0.1 ppm. All tests were performed at the temperature of 23 þ 2 C. A piece of Li foil (16 mm diameter) was placed onto the bottom Cu cell body and a piece of Celgard 2500 separator (18 diameter) was placed onto the Li foil, followed by the addition of 50 mL 50 mM InI3 electrolyte (theoretical capacity of In3þ /In is 0.2 mAh) to saturate the separator. Subsequently, a piece of hydrophilic carbon cloth (12 mm, 1.13 cm2) was employed as the cathode and 25 mL 250 mM Li2S8 catholyte was uniformly dropped onto the carbon cloth cathode, with a theoretical areal capacity of 2.37 mAh cm2 (S8/Li2S). The galvanostatic discharge and charge tests were conducted on a battery cycling system (Neware, CT3008W) at room temperature (298 K). The electrochemical impedance spectroscopy (EIS) measurements were conducted on a potentiostat (Princeton Applied Research, PARSTAT M2273) via the two-electrode setup, where the lithium (Li) metal anode performs as both the reference and counter electrode and the cathode performs as the working and
Fig. 1. (a, b) A schematic diagram of side product (Li2S and Li2S2) accumulation. Over cycling, soluble polysulfides can be transformed to insulating short-chain sulfides and deposited onto the Li anode. (c, d) A schematic diagram illustrates the merits of the bifunctional InI3 electrolyte additive: (c) the electrodeposited In layer mitigates side reactions between Li metal and polysulfides, reducing the accumulated side products; (d) by overcharging, I can be charged into I 3 , which can chemically decompose the side products into soluble polysulfides.
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sensing electrodes. Because Li stripping and plating is regarded as reversible and exhibits a stable potential, in most of the existing LiS battery research, the Li anode can serve as the reference electrode [22,40e42]. Here, the EIS measurement using a frequency range from 100 kHz to 100 mHz with a wave amplitude of 5 mV was applied to the charged batterie at the open circuit voltage. Besides, the cyclic voltammetry (CV) of the Li-S batteries was tested at a scanning rate of 0.1 mV s1 with the carbon cloth electrode as the working electrode and Li foil as both reference and counter electrodes. 2.3. Characterization The battery components (anode and separator) after cycling were rinsed in pure DME for 10 min and then dried before SEM observation in an argon-filled glove box with oxygen and H2O maintained below 0.1 ppm. JSM-6700F field emission SEM instruments were used for micrograph observation at an acceleration voltage of 5.0 kV. The UV-Vis spectra were collected by SEC2000 UVevisible spectrophotometer (ALS Co., Ltd.) An SCE-C thin layer quartz glass cell with an optical path length of 4.5 mm was used as the holder. X-ray photoelectron spectroscopy (XPS) measurements were conducted on a Physical Electronics PHI5802 instrument using an X-rays magnesium anode (monochromatic Ka X-rays at 1253.6 eV) as the source. C 1s region was used as the reference and set at 285 eV. 3. Results and discussion The first step to validate the self-cleaning concept is to investigate the electrochemical and chemical compatibility between iodine and sulfur based redox species in the ether based electrolyte. The Li-polysulfide battery containing 25 mL 250 mM Li2S8 as the
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active material and 50 mL 50 mM InI3 as the electrolyte additive was assembled and the as-prepared battery was charged firstly to 3.4 V. A layer of protective In layer (with a theoretical areal loading of 0.072 mg cm2) can be deposited onto the Li anode following the reaction of In3þþ3e/In (0.375 V vs SHE), which occurs prior to Li plating Liþþe/Li (3.04 V vs SHE). The battery was rested for 30 min before discharge and the corresponding voltage profile for the 1st charge-discharge cycle can be found in Fig. S2. Separated discharge voltage plateaus representing the triiodide reduction (~2.8 V), sulfur dissolution (~2.3 V) and Li2S precipitation (~2.1 V) can be found, indicating that there are minor side reactions between the Li metal and corrosive polyiodide and polysulfide species, owing to the protective effects of the deposited In layer. Cyclic voltammetry (CV) was also exploited for the Li-S battery with the addition of InI3 as the electrolyte additive as shown in Fig. 2a. It is found that the sulfur part and iodine part were well separated and consistent with the reported CVs for Li-S and Li-I2 batteries respectively [43e45]. The anodic peak of I =I 3 redox reaction is at 3.0 V, which is considerably higher than the anodic peaks for Li2S oxidation (~2.4 V). It is thereby deduced that I 3 is capable of oxidizing Li2S. To confirm this issue, we dispersed the Li2S powders (commercially available from Sigma-Aldrich) into the DOL/DME electrolyte via sonication and further added excessive LiI3. Simply after 30-min rest at the room temperature, those sediment Li2S powders were found to be well dissolved, as can be confirmed in the UV-Vis spectra in Fig. 2b, two absorption peaks at around 295 and 366 nm, representing I 3 species, decreased with the existence of Li2S6 species [43]. The peak representing Li2S6 was found to emerge at around 470 nm [46]. This result suggests that LiI3 is sufficiently oxidative to transform the insoluble Li2S to soluble polysulfide species. On the other hand, as shown in Fig. 2(c and d), we show that during the initial charging process, a coarse layer of deposited In can be observed to passivate the Li surface uniformly. Also, from the
Fig. 2. (a) Cyclic voltammetry (CV) of Li-S battery with the addition of InI3; (b) UV-Vis spectra of the 5 mM LiI3 in DME solution and the mixture of 5 mM LiI3 þ 0.50 mg Li2S in DME, the inset shows the photography of Li2S suspension (0.20 mg mL1 in DME), 10 mM LiI3 in DME solution and their mixture. (c) Pristine Li anode; (d) passivated Li anode with the deposited In layer.
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cross-sectional view of the Li anode deposited with the In layer (Fig. S3), the border between the In surface layer and the Li foil can be clearly seen. Over the subsequent cycles, Li-In alloy can be formed as found in the previous literature, which allows for facile Li storage and Liþ transport [34,47e49]. Moreover, a notable feature of In is that it is highly inertial to the oxidative iodine, polyiodide as well as polysulfides, thus it can be hypothesized that the Li anode can be protected from the side reactions with the In layer, which will be presented in the following section [50]. Electrochemical test was conducted to confirm the protective effect of the deposited In layer. The battery's performance over cycling was presented in Fig. 3a-c. Without the addition of InI3, the average capacity decay rate for the assembled Li-polysulfide battery is as high as 0.82% for 100 cycles. On the other hand, with the addition of InI3 as the electrolyte additive, after an initial cycle similar with the process in Fig. S2, the battery was cycled within the voltage window of 1.7e2.8 V at 0.5 C. The overall discharge capacity decay rate was lowered to 0.38% for 100 cycles, indicating that the deposited In layer, which anchors on the Li surface, can function as a stable shield for the bulk phase of Li anode against the dissolved polysulfide species. With minor side reactions occurring on the anode, the dissolved polysulfides would diffuse back into the cathode and could be utilized in the subsequent electrochemical processes. In this regard, by mitigating side reactions, the active material loss induced by polysulfide dissolution can be well decreased. Moreover, interestingly, if we assembled a Li-polyiodide battery with the addition of InI3 (50 mL 50 mM) as the active material, it is found that there is minor capacity decay. As shown in Fig. S4, though the coulombic efficiency is averaged only around ~90%, the battery's capacity can be well stabilized over cycling, implying that the side reaction actually occurs in an electrochemical approach: I 3 þ 2e 43I , with minor degradation of the Li anode [51]. Thus, it is hypothesized that the deposited In layer is capable of suppressing side chemical reactions. However, for the Li-polysulfide battery, electrodeposition of sulfides from polysulfides can keep occurring on the anode over the repeated cycles. Unlike I =I 3 redox species that are both highly soluble in the ether-based electrolyte, short-
chain sulfides are insoluble and can continuously accumulate on the Li anode, leading to the active material loss, enlarged Liþ transport resistance and thus a shortened cycle life. To decompose the insoluble short-chain sulfide side products, we carried out the proposed overcharging strategy. As shown in Fig. 3d, overcharging (the upper limit is 3.4 V also) was conceived for every 20 cycles for overall 100 cycles. With this overcharging strategy, the average capacity decay rate was lowered to 0.24% and a discharge capacity around ~850 mAh g1 was maintained over 100 cycles (Fig. 3d). Interestingly, as shown in Fig. 3(e and f), it is found that after overcharging and rest, the battery's capacity can be considerably retrieved, implying that polysulfides derived from the side products can be utilized in the subsequent cycle, also the ionic transport might be improved via removing the side products. Also, to further testify the effectiveness of the overcharging strategies. We try to elucidate the effects of resting the batteries. It is found that by resting the batteries for 60 min after 20 cycles, the battery's cyclability doesn't exhibit any substantial improvement (Fig. S5). Also, in the control cell without I =I 3 redox mediator, if the battery is overcharged, from 2.8 V to 3.4 V, the cell voltage climbs up rapidly, with minor increase of the charge capacity. Unlike the battery with the addition of InI3, the battery's capacity keeps decreasing (Fig. S6). In this regard, it can be testified that our strategy of introducing a redox mediator to timely clean (decompose) the side products in Li-S batteries shows a promising perspective for prolonging the battery's cycle life. The positive role of such an overcharging process was confirmed in the electrochemical impedance spectroscopy (EIS) measurement. A two-electrode setup is exploited, where Li metal performs as both the counter and reference electrodes. As shown in the equivalent circuit model in Fig. 4a, the high-frequency intercept is attributed to the battery's ohmic resistance and the semi-circle is attributed to the interfacial resistance (RSEI) and charge transfer resistance (Rct) [35]. The charge transfer process involves both Li stripping/plating reaction at the anode and sulfur redox reactions at the cathode. Considering that these two processes might have similar frequency response, a single Rct//CPE2 module was used to depict the charge transfer process [40,52]. Over cycling, as the side
Fig. 3. (a) Cycling performance of Li-polysulfide batteries with and without the addition of InI3 at 0.5 C; (b) voltage profiles of Li-polysulfide batteries with the addition of InI3; (c) voltage profiles of Li-polysulfide batteries without the addition of InI3. (d) Cycling performance of Li-polysulfide batteries at 0.5 C with the addition of InI3, the battery was overcharged to 3.4 V and rested for 30 min for every 20 cycles; (eef) corresponding voltage profiles at 20e22 cycles and 80e82 cycles.
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Fig. 4. (a) Equivalent circuit model and (b) the electrochemical impedance spectroscopy (EIS) results of batteries after charged at different cycle indexes.
Table 1 Summary of fitted EIS results from Fig. 4. Cycle index
2
20
21
80
81
R b ( U) RSEI (U) Rct (U) CPE1 (F) CPE2 (F) WR (S1/2) WP
4.88 2.94 6.26 3.42 106 9.78 106 8.20 0.41
8.66 11.64 9.12 2.73 105 1.19 104 13.17 0.36
5.89 2.75 5.25 4.13 106 9.16 106 16.57 0.28
9.81 12.31 8.46 1.19 105 1.08 104 13.31 0.36
6.67 3.79 7.15 5.65 105 1.29 104 7.791 0.43
products passivate the anode/separator as well as cathode/separator interphases, Liþ transport pathway between the anode and cathode will be blocked and the available cathode surface will be decreased, enlarging both RSEI and Rct [53]. As shown in Fig. 4b, it is
found that the depressed semi-circles representing the interfacial and charge transfer resistances can be decreased after the overcharging process, which are very close to the EIS result measured from the battery after the initial 2 cycles (Fig. S7). The fitted results
Fig. 5. X-ray photoelectron spectroscopy (XPS) analysis of Li anodes. (a, b) S 2p spectra of Li anodes obtained from the batteries after charged (20 cycles at 0.5 C): (a) without overcharging; (b) with overcharging. (c) Comparison of I 3d spectra of Li anodes obtained from batteries with the addition of InI3 (with overcharging, without overcharging). (d) Comparison of In 3d spectra of Li anodes obtained from batteries with the addition of InI3 (with overcharging, without overcharging).
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as shown in Table 1 further indicate that the decrease of the RSEI should be the main attribute, while the charge transfer resistance shows a decrease to some extent. In this regard, it can be deduced that overcharging might lower the interfacial resistance by decomposing side product. Also, the charge transfer process involved in Li stripping/plating or sulfur redox reactions might be improved, as the removal of nonconductive side product allows for more available electrode surface for electrochemical reactions. Though useful for analyzing the interfacial resistance, the twoelectrode EIS measurement result cannot allow us to calculate the anodic or cathodic exchange current density as pointed out by the reviewer. To address this issue, a reference eletrode setup, which is stable in the corrosive electrolyte environment and has minor effect on the original battery configuration, should be developed and thus it might be possible to depict the anodic and cathodic reaction kinetics separately. In line with the achieved electrochemical performance, we further conducted X-ray photoelectron spectroscopy (XPS) analysis to characterize the chemical compositions of the Li anodes with and without overcharging. Fig. 5(a and b) show a comparison of S 2p spectra of the SEIs formed on the anodes with and without the overcharging process. Overcharging was carried out after the initial 20 cycles (the battery was charged to 3.4 V and rested for 30 min). Oxidized species including -NSO2CF3 and -SO3 exist for both
samples, which can be attributed to the LiTFSI decomposition [23,29]. As shown in Fig. 5a, two peaks exist at 161.3 and 162.5 eV for the SEI formed in the battery without overcharging, reflecting the accumulation of Li2S and Li2S2 in the SEI layer [24]. In contrast, as can be found in Fig. 5b, there are minor signals of Li2S and Li2S2 species for the battery with overcharging. It can be thus deduced that the overcharging process successfully removes the side products (Li2S and Li2S2) deposited onto the Li anode. Moreover, as shown in Fig. 5c, it is found that for the battery with overcharging, there exist more apparent peaks of I 3d 5/2 and I 3d 3/2, indicating that polyiodide has been partially oxidized into elemental iodine [37]. If we further look into the In 3d spectra in Fig. 5d, the In 3d 5/2 and In 3d 3/2 peaks maintain unchanged for the batteries with and without overcharging, confirming the chemical stability of In layer under the existence of corrosive iodine and polyiodide [54,55]. Eventually, we provide the SEM images as the evidence of side product removal. As shown in Fig. 6a, at the initial cycle, the deposited Li shows a rather smooth morphology, because intrinsically the In layer can perform as a preferential Li deposition substrate, lowering the possibility of dendrite growth [56]. After 20 cycles, for the Li anode deposited with the In layer, a layer of protruding particles, which are composed of insoluble reduced sulfur species (Li2S and Li2S2), is found to emerge. Though the
Fig. 6. (aed) SEM images and the corresponding EDX mappings of sulfur on the Li anodes: (a) after the initial cycle; (b) after 20 cycles without overcharging; (c) after 20 cycles with overcharging; (d) after 80 cycles with overcharging. SEM images of separators: (e) after 20 cycles without overcharging; (f) after 20 cycles with overcharging.
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accumulation of side product is considerably mitigated than the case with bare Li metal (Fig. S8), the detrimental effects of side products still need to be addressed for performance enhancement. After overcharging, the anode surface demonstrates a smooth and plain morphology, and the surface sulfur content is also found to be considerably decreased (Fig. 6c), which confirms the successful removal of reduced sulfur species. Interestingly, we observe that, after overcharging after 80 cycles, a smooth polymer-like film is found to be coated onto the Li-In alloy layer on the anode surface (Fig. 6d), which might formed from the iodide induced polymerization of DME [37]. The neutral I$ radical, which is an intermediate generated when triiodide is reduced to iodide, may react with electrolyte components to form a linear or comb-branched oligoethers. Such a Li-permeable layer is capable of protecting the Li anode from the direct contact with the corrosive electrolyte and greatly suppressing side reactions, partially explaining the exceptional cycling stability [37]. In addition to the anode side, it is found that the separators obtained from the cycled batteries have been partially covered by the precipitated side products formed from the chemical disproportion of polysulfides (Fig. 6e), which also can lower the active material utilization ratio, as found in the previous experiment [53]. After overcharging, interestingly, the accumulated side products on the separator are also found to be well dissolved by the oxidative polyiodide and the resultant separator (Fig. 6f) shows minor difference compared with the pristine one, further confirming the role of I =I 3 redox mediator in decomposing side products. As a further discussion, in addition to InI3, we also exploit InBr3 as an alternative because Br =Br3 is of a stronger oxidizability than I =I 3 . Though stable cycling of the redox mediator alone is achieved as shown in Fig. S9a, it is realized that Br is an anion with high donor number (DN) as previously reported in Li-O2 battery [57]. Specially, when being added into the electrolyte with low DN such as Dimethyl ether (DME), the existence of Br promotes the solution mechanism [58]. In consequence, as shown Figs. S9b and a sloping lower voltage plateau is demonstrated in contrast with the flat lower voltage plateau observed in conventional Li-S batteries. Such a phenomenon can be attributed to the promoted short-chain (poly)sulfide dissolution during the precipitation process, leading to less irreversible discharge product formed by disproportion [46,59,60]. In this regard, the redox mediator's effect on the electrolyte system's donor number is supposed to be an essential criterion for choosing adequate redox mediators. In brief summary, an appropriate redox mediator for side product removal in Li-S batteries should possess the following properties: (i) excellent stability and fast redox kinetics; (ii) high redox potential to chemically oxidize the side product and (iii) minor or positive effects on the sulfur reduction/oxidation processes. In addition to I =I 3 redox couple, further efforts can be directed towards the exploration of organic redox mediators (e.g. TEMPO and ferrocene) as well [61,62]. In addition to Li-S batteries, Li-O2 batteries suffer from the cathode/ anode surface passivation induced by the side products such as lithium carbonate (Li2CO3) and lithium hydroxide (LiOH), in that case, identifying suitable redox mediators to decompose side products should be also desirable for prolonging Li-O2 batteries' cycle life [63,64]. 4. Conclusion To conclude, we propose a self-cleaning Li-S battery enabled by the bifunctional electrolyte additive InI3. One is the protection of Li anode induced by the electrodeposited In layer. The other concerns for the effective decomposition of insoluble side product (shortchain polysulfide) outside the cathode, exploiting I =I 3 redox mediator. The resultant battery using a Li anode with a deposited In
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layer shows an improved cycling performance in comparison with the one with the bare Li metal. On the other hand, by timely overcharging the battery, it is demonstrated that the battery's capacity can be retrieved and the battery's cycle life can be effectively prolonged. The facile and efficient strategy that protects Li anode and decomposes side products thus opens a new avenue for prolonging the Li-S battery's cycle life. Acknowledgment The work described in this paper was fully supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (project no. 16213414). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2017.06.083. References [1] P.G. Bruce, S.A. Freunberger, L.J. Hardwick, J.M. Tarascon, Nat. Mater. 11 (2012) 19e29. [2] F. Wu, G. Yushin, Energy & Environ. Sci. 10 (2017) 435e459. [3] S. Chu, Y. Cui, N. Liu, Nat. Mater. 16 (2017) 16e22. [4] S. Evers, L.F. Nazar, Accounts Chem. Res. 46 (2013) 1135e1143. [5] Y. Yang, G. Zheng, Y. Cui, Chem. Soc. Rev. 42 (2013) 3018e3032. [6] H.R. Jiang, Z. Lu, M.C. Wu, F. Ciucci, T.S. Zhao, Nano Energy 23 (2016) 97e104. [7] H. Jiang, W. Shyy, M. Liu, L. Wei, M. Wu, T. Zhao, J. Mater. Chem. A 5 (2017) 672e679. [8] Y. Ren, T. Zhao, M. Liu, P. Tan, Y. Zeng, J. Power Sources 336 (2016) 115e125. [9] P.P.R. Harks, C.B. Robledo, T.W. Verhallen, P.H. Notten, F.M. Mulder, Adv. Energy Mater. 7 (2016) 1601635. [10] X.-B. Cheng, J.-Q. Huang, H.-J. Peng, J.-Q. Nie, X.-Y. Liu, Q. Zhang, F. Wei, J. Power Sources 253 (2014) 263e268. [11] Y. Sun, Z.W. Seh, W. Li, H. Yao, G. Zheng, Y. Cui, Nano Energy 11 (2015) 579e586. [12] A. Manthiram, Y. Fu, S.-H. Chung, C. Zu, Y.-S. Su, Chem. Rev. 114 (2014) 11751e11787. [13] A. Manthiram, Y. Fu, Y.-S. Su, Accounts Chem. Res. 46 (2013) 1125e1134. [14] Z. Liu, X.H. Zhang, C.S. Lee, J. Mater. Chem. A 2 (2014) 5602e5605. [15] S. Evers, T. Yim, L.F. Nazar, J. Phys. Chem. C 116 (2012) 19653e19658. [16] S. Zheng, F. Yi, Z. Li, Y. Zhu, Y. Xu, C. Luo, J. Yang, C. Wang, Adv. Funct. Mater. 24 (2014) 4156e4163. [17] L.C. Gerber, P.D. Frischmann, F.Y. Fan, S.E. Doris, X. Qu, A. Scheuermann, K.A. Persson, Y.-M. Chiang, B.A. Helms, Nano letters (2015). [18] Z. Li, Y. Jiang, L. Yuan, Z. Yi, C. Wu, Y. Liu, P. Strasser, Y. Huang, ACS Nano 8 (2014) 9295e9303. [19] Y. Fu, Y.-S. Su, A. Manthiram, ACS Appl. Mater. Interfaces 4 (2012) 6046e6052. [20] Y. Fu, Y.-S. Su, A. Manthiram, Angew. Chem. Int. Ed. 52 (2013) 6930e6935. [21] C. Zu, Y. Fu, A. Manthiram, J. Mater. Chem. A 1 (2013) 10362e10367. [22] Y. Ren, T. Zhao, M. Liu, L. Wei, R. Zhang, Electrochimica Acta 242 (2017) 137e145. [23] M. Liu, D. Zhou, Y.-B. He, Y. Fu, X. Qin, C. Miao, H. Du, B. Li, Q.-H. Yang, Z. Lin, Nano Energy 22 (2016) 278e289. [24] M. Liu, Y. Ren, D. Zhou, H. Jiang, F. Kang, T. Zhao, ACS Appl. Mater. Interfaces 9 (2016) 2526e2534. [25] M. Liu, D. Zhou, H. Jiang, Y. Ren, F. Kang, T. Zhao, Nano Energy 28 (2016) 97e105. [26] M. Liu, H. Jiang, Y. Ren, D. Zhou, F. Kang, T. Zhao, Electrochimica Acta 213 (2016) 871e878. [27] S. Xiong, K. Xie, Y. Diao, X. Hong, J. Power Sources 236 (2013) 181e187. [28] X.B. Cheng, R. Zhang, C.Z. Zhao, F. Wei, J.G. Zhang, Q. Zhang, Adv. Sci. 10 (2015) 1500213. [29] W. Li, H. Yao, K. Yan, G. Zheng, Z. Liang, Y.-M. Chiang, Y. Cui, Nat. Commun. 6 (2015). [30] S.S. Zhang, J. Electrochem. Soc. 159 (2012) A920eA923. [31] M. Kazazi, M.R. Vaezi, A. Kazemzadeh, Ionics 20 (2014) 1291e1300. [32] J.T. Lee, K. Eom, F. Wu, H. Kim, D.C. Lee, B. Zdyrko, G. Yushin, ACS Energy Lett. 1 (2016) 373e379. [33] P. Bai, J. Li, F.R. Brushett, M.Z. Bazant, Energy & Environ. Sci. 9 (2016) 3221e3229. [34] M. Ishikawa, H. Kawasaki, N. Yoshimoto, M. Morita, J. power sources 146 (2005) 199e203. [35] Z. Liu, S. Bertolini, P.B. Balbuena, P.P. Mukherjee, ACS Appl. Mater. interfaces 8 (2016) 4700e4708. [36] Z. Liu, D. Hubble, P.B. Balbuena, P.P. Mukherjee, Phys. Chem. Chem. Phys. 17 (2015) 9032e9039.
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