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Journal of Power Sources 361 (2017) 203e210

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Journal of Power Sources

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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. JiangDepartment 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

Article history:Received 6 March 2017Received in revised form22 June 2017Accepted 27 June 2017

Keywords:Lithium-sulfur batteryRedox mediatorSide reactionsSolid electrolyte interphase

* Corresponding author.E-mail address: metzhao@ust.hk (T.S. Zhao).

http://dx.doi.org/10.1016/j.jpowsour.2017.06.0830378-7753/© 2017 Elsevier B.V. All rights reserved.

a b s t r a c t

The polysulfide shuttle effect and lithium dendrite growth in lithium-sulfur (Li-S) batteries canrepeatedly breach the anodic solid electrolyte interphase (SEI) over cycling. As a result, irreversible short-chain sulfide side products (Li2Sx, x ¼ 1, 2) keep depositing on the Li anode, leading to the active materialloss, increasing the Liþ transport resistance, and thereby reducing the cycle life. In this work, indiumiodide (InI3) is investigated as a bifunctional electrolyte additive for Li-S batteries to protect the Li anodeand decompose the side products spontaneously. On the one hand, Indium (In) is electrodeposited ontothe Li anode prior to Li plating during the initial charging process, forming a chemically and mechanicallystable 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 redoxmediator is capable of chemically transforming side products deposited on the Li anode and separatorinto soluble polysulfides, which can be recycled by the cathode. It is shown that the battery with the InI3additive exhibits a prolonged cycle life, and is capable of retrieving its capacity by a facile overchargingprocess.

© 2017 Elsevier B.V. All rights reserved.

1. Introduction

With the depletion of fossil fuel, there is a large demand forhigh-energy rechargeable batteries, for portable electronics, elec-tric vehicles and grid-scale stationary storage systems. Lithium orsodium-based batteries with high-energy alkaline metal anodeshave thus been developed for these purposes [1e3]. Among thesystems studied, Li-S batteries are especially attractive due to thestriking merits including the high specific energy of sulfur cathode(2600 Wh kg�1, 2800 Wh L�1, respectively), reasonably fast ki-netics, environmental benignity and low cost [1,2,4e8]. However,the dissolution of intermediate polysulfides in the electrolyte, theso called “shuttle effect”, leads to the active material loss, anodedegradation and thus a shortened cycle life, which is a formidable

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 theLiþ transport but inhibits the diffusion of polysulfides, can beexploited for use [23e26]. However, in most cases, irreversiblecapacity loss can be still observed over cycling due to the insuffi-cient ion selectivity. In another approach, a chemically and me-chanically stable passivation layer (solid electrolyte interphase) isdesirable to form onto the Li anode, which can shield the Li anodefrom reacting with polysulfide shuttles [27,28]. Electrolyte addi-tives such as LiNO3 and FEC (Fluoroethylene carbonate) have beenproposed to facilitate the formation of a more stabilized solidelectrolyte interphase (SEI) on the Li metal anode surface [29e32].However, a critical issue facing Li metal is the volume change overthe repeated stripping/plating cycles, leading to the collapse of SEIover cycling [16,33,34]. In consequence, short-chain sulfides

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Y.X. Ren et al. / Journal of Power Sources 361 (2017) 203e210204

formed by the side reactions between Li metal and polysulfides canbe continuously deposited onto the anode surface over cycling asshown in Figs. 1(a, b), leading to the irreversible capacity loss andconsiderably lowered Liþ conductivity [35,36].

Considering the detrimental effects of side product accumula-tion, it occurs to us that a potential strategy is to allow for a timelyremoval of side products, which can be possibly revealed byoxidizing redox mediators [37e39]. Specifically, over charging,reduction state redox species can be charged into the oxidationstate at the cathode, which is capable of migrating out of thecathode and oxidizing the side products (solid Li2S and Li2S2 spe-cies) into soluble long-chain polysulfides, which also results in theregeneration of reduction state redox species. The redox mediatorthus acts as an electron-hole transfer agent that facilitates efficientoxidation of side products. However, just like polysulfide species,usually the redox mediator can react with the Li anode, whiledecomposing those deposited Li2S and Li2S2 species [38]. In thatcase, in the corrosive electrolyte containing polysulfides and redoxmediator, anode engineering approaches are critically needed toreinforce 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 andLi2S2) decomposition and Li anode protection. We demonstratethat, on the one hand, a stable and uniform solid electrolyteinterphase (SEI) can be enabled by a deposited metallic In layer,which adheres onto the Li anode surface and functions as a pro-tective shield for the bulk phase of Li anode as shown in Fig. 1c. Onthe other hand, we propose that a timely decomposition of sideproducts can be realized by overcharging the batteries into thevoltage window of iodide/triiodide redox reaction, which chemi-cally transforms the short-chain Li2Sx (x ¼ 1, 2) deposited outsidethe cathode into soluble and electrochemically active polysulfidesas shown in Fig. 1d. Owing to the side product decomposition byovercharging, the Liþ transport resistance can be lowered, and thebattery's capacity can be retrieved, enabling a Li-S battery with theself-cleaning capability.

Fig. 1. (a, b) A schematic diagram of side product (Li2S and Li2S2) accumulation. Over cydeposited onto the Li anode. (c, d) A schematic diagram illustrates the merits of the bifunctbetween Li metal and polysulfides, reducing the accumulated side products; (d) by overcharsoluble polysulfides.

2. Experimental

2.1. Material preparation

The Li2S8 solution was prepared by dissolving a desired amountof stoichiometric S and Li2S in 1,3-dioxolane (DOL)/1,2-dimethoxyethane (DME) solution (1:1 in volume) with the addi-tion of 1 M LiTFSI and LiNO3 additive (1 wt%). For the typicalpreparation of 250 mM Li2S8 solution, 280 mg of S and 57.5 mg ofLi2S were added to 5 mL of DOL/DME (1:1) based electrolyte. Theobtained suspension was stirred at 80 �C overnight to yield red-brown Li2S8 solution. The battery's specific capacity was calcu-lated based on the mass of S2�8 in Li2S8 in consistence with theprevious literature [22,24].

2.2. Cell assembly and test

The electrochemical performance was tested in the Swagelok-type lithium-sulfur battery fixture as shown in Fig. S1, whichwere assembled in an argon-filled glove box with oxygen and H2Omaintained below 0.1 ppm. All tests were performed at the tem-perature of 23þ 2� C. A piece of Li foil (16mmdiameter) was placedonto the bottom Cu cell body and a piece of Celgard 2500 separator(18 diameter) was placed onto the Li foil, followed by the additionof 50 mL 50 mM InI3 electrolyte (theoretical capacity of In3þ/In is0.2 mAh) to saturate the separator. Subsequently, a piece of hy-drophilic carbon cloth (12 mm, 1.13 cm�2) was employed as thecathode and 25 mL 250 mM Li2S8 catholyte was uniformly droppedonto the carbon cloth cathode, with a theoretical areal capacity of2.37 mAh cm�2 (S8/Li2S). The galvanostatic discharge and chargetests were conducted on a battery cycling system (Neware, CT-3008W) at room temperature (298 K).

The electrochemical impedance spectroscopy (EIS) measure-ments were conducted on a potentiostat (Princeton AppliedResearch, PARSTAT M2273) via the two-electrode setup, where thelithium (Li) metal anode performs as both the reference andcounter electrode and the cathode performs as the working and

cling, soluble polysulfides can be transformed to insulating short-chain sulfides andional InI3 electrolyte additive: (c) the electrodeposited In layer mitigates side reactionsging, I� can be charged into I�3 , which can chemically decompose the side products into

Y.X. Ren et al. / Journal of Power Sources 361 (2017) 203e210 205

sensing electrodes. Because Li stripping and plating is regarded asreversible and exhibits a stable potential, in most of the existing Li-S battery research, the Li anode can serve as the reference electrode[22,40e42]. Here, the EIS measurement using a frequency rangefrom 100 kHz to 100 mHz with a wave amplitude of 5 mV wasapplied to the charged batterie at the open circuit voltage. Besides,the cyclic voltammetry (CV) of the Li-S batteries was tested at ascanning rate of 0.1 mV s�1 with the carbon cloth electrode as theworking electrode and Li foil as both reference and counterelectrodes.

2.3. Characterization

The battery components (anode and separator) after cyclingwere rinsed in pure DME for 10 min and then dried before SEMobservation in an argon-filled glove box with oxygen and H2Omaintained below 0.1 ppm. JSM-6700F field emission SEM in-struments were used for micrograph observation at an accelerationvoltage of 5.0 kV.

The UV-Vis spectra were collected by SEC2000 UVevisiblespectrophotometer (ALS Co., Ltd.) An SCE-C thin layer quartz glasscell with an optical path length of 4.5 mm was used as the holder.

X-ray photoelectron spectroscopy (XPS) measurements wereconducted on a Physical Electronics PHI5802 instrument using anX-rays magnesium anode (monochromatic Ka X-rays at 1253.6 eV)as the source. C 1s region was used as the reference and set at285 eV.

3. Results and discussion

The first step to validate the self-cleaning concept is to investi-gate the electrochemical and chemical compatibility betweeniodine and sulfur based redox species in the ether based electrolyte.The Li-polysulfide battery containing 25 mL 250 mM Li2S8 as the

Fig. 2. (a) Cyclic voltammetry (CV) of Li-S battery with the addition of InI3; (b) UV-Vis spectrthe inset shows the photography of Li2S suspension (0.20 mg mL�1 in DME), 10 mM LiI3 indeposited In layer.

activematerial and 50 mL 50mM InI3 as the electrolyte additivewasassembled and the as-prepared battery was charged firstly to 3.4 V.A layer of protective In layer (with a theoretical areal loading of0.072 mg cm�2) can be deposited onto the Li anode following thereaction of In3þþ3e�/In (�0.375 V vs SHE), which occurs prior toLi plating Liþþe�/Li (�3.04 V vs SHE). The battery was rested for30 min before discharge and the corresponding voltage profile forthe 1st charge-discharge cycle can be found in Fig. S2. Separateddischarge 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 be-tween the Li metal and corrosive polyiodide and polysulfide spe-cies, owing to the protective effects of the deposited In layer. Cyclicvoltammetry (CV) was also exploited for the Li-S battery with theaddition of InI3 as the electrolyte additive as shown in Fig. 2a. It isfound that the sulfur part and iodine part were well separated andconsistent with the reported CVs for Li-S and Li-I2 batteriesrespectively [43e45]. The anodic peak of I�=I�3 redox reaction is at3.0 V, which is considerably higher than the anodic peaks for Li2Soxidation (~2.4 V). It is thereby deduced that I�3 is capable ofoxidizing Li2S. To confirm this issue, we dispersed the Li2S powders(commercially available from Sigma-Aldrich) into the DOL/DMEelectrolyte via sonication and further added excessive LiI3. Simplyafter 30-min rest at the room temperature, those sediment Li2Spowders were found to be well dissolved, as can be confirmed inthe UV-Vis spectra in Fig. 2b, two absorption peaks at around 295and 366 nm, representing I�3 species, decreased with the existenceof Li2S6 species [43]. The peak representing Li2S6 was found toemerge at around 470 nm [46]. This result suggests that LiI3 issufficiently oxidative to transform the insoluble Li2S to solublepolysulfide species.

On the other hand, as shown in Fig. 2(c and d), we show thatduring the initial charging process, a coarse layer of deposited Incan be observed to passivate the Li surface uniformly. Also, from the

a of the 5 mM LiI3 in DME solution and the mixture of 5 mM LiI3 þ 0.50 mg Li2S in DME,DME solution and their mixture. (c) Pristine Li anode; (d) passivated Li anode with the

Y.X. Ren et al. / Journal of Power Sources 361 (2017) 203e210206

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 canbe clearly seen. Over the subsequent cycles, Li-In alloy can beformed as found in the previous literature, which allows for facile Listorage and Liþ transport [34,47e49]. Moreover, a notable feature ofIn is that it is highly inertial to the oxidative iodine, polyiodide aswell as polysulfides, thus it can be hypothesized that the Li anodecan be protected from the side reactions with the In layer, whichwill be presented in the following section [50].

Electrochemical test was conducted to confirm the protectiveeffect of the deposited In layer. The battery's performance overcycling was presented in Fig. 3a-c. Without the addition of InI3, theaverage capacity decay rate for the assembled Li-polysulfide batteryis as high as 0.82% for 100 cycles. On the other hand, with theaddition of InI3 as the electrolyte additive, after an initial cyclesimilar with the process in Fig. S2, the battery was cycled within thevoltage windowof 1.7e2.8 V at 0.5 C. The overall discharge capacitydecay rate was lowered to 0.38% for 100 cycles, indicating that thedeposited In layer, which anchors on the Li surface, can function asa stable shield for the bulk phase of Li anode against the dissolvedpolysulfide species. With minor side reactions occurring on theanode, the dissolved polysulfides would diffuse back into thecathode and could be utilized in the subsequent electrochemicalprocesses. In this regard, by mitigating side reactions, the activematerial loss induced by polysulfide dissolution can be welldecreased.

Moreover, interestingly, if we assembled a Li-polyiodide batterywith the addition of InI3 (50 mL 50 mM) as the active material, it isfound that there is minor capacity decay. As shown in Fig. S4,though the coulombic efficiency is averaged only around ~90%, thebattery's capacity can be well stabilized over cycling, implying thatthe side reaction actually occurs in an electrochemical approach:I�3 þ 2e�43I�, withminor degradation of the Li anode [51]. Thus, itis hypothesized that the deposited In layer is capable of suppressingside chemical reactions. However, for the Li-polysulfide battery,electrodeposition of sulfides from polysulfides can keep occurringon the anode over the repeated cycles. Unlike I�=I�3 redox speciesthat are both highly soluble in the ether-based electrolyte, short-

Fig. 3. (a) Cycling performance of Li-polysulfide batteries with and without the addition ofvoltage profiles of Li-polysulfide batteries without the addition of InI3. (d) Cycling perforovercharged to 3.4 V and rested for 30 min for every 20 cycles; (eef) corresponding voltag

chain sulfides are insoluble and can continuously accumulate onthe 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 inFig. 3d, overcharging (the upper limit is 3.4 V also) was conceivedfor every 20 cycles for overall 100 cycles. With this overchargingstrategy, the average capacity decay rate was lowered to 0.24% anda discharge capacity around ~850 mAh g�1 was maintained over100 cycles (Fig. 3d). Interestingly, as shown in Fig. 3(e and f), it isfound that after overcharging and rest, the battery's capacity can beconsiderably retrieved, implying that polysulfides derived from theside products can be utilized in the subsequent cycle, also the ionictransport 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 foundthat by resting the batteries for 60 min after 20 cycles, the battery'scyclability doesn't exhibit any substantial improvement (Fig. S5).Also, in the control cell without I�=I�3 redox mediator, if the batteryis overcharged, from 2.8 V to 3.4 V, the cell voltage climbs uprapidly, with minor increase of the charge capacity. Unlike thebattery with the addition of InI3, the battery's capacity keepsdecreasing (Fig. S6). In this regard, it can be testified that ourstrategy of introducing a redox mediator to timely clean (decom-pose) the side products in Li-S batteries shows a promisingperspective for prolonging the battery's cycle life.

The positive role of such an overcharging process was confirmedin the electrochemical impedance spectroscopy (EIS) measure-ment. A two-electrode setup is exploited, where Li metal performsas both the counter and reference electrodes. As shown in theequivalent circuit model in Fig. 4a, the high-frequency intercept isattributed to the battery's ohmic resistance and the semi-circle isattributed to the interfacial resistance (RSEI) and charge transferresistance (Rct) [35]. The charge transfer process involves both Listripping/plating reaction at the anode and sulfur redox reactionsat the cathode. Considering that these two processes might havesimilar frequency response, a single Rct//CPE2 module was used todepict the charge transfer process [40,52]. Over cycling, as the side

InI3 at 0.5 C; (b) voltage profiles of Li-polysulfide batteries with the addition of InI3; (c)mance of Li-polysulfide batteries at 0.5 C with the addition of InI3, the battery wase profiles at 20e22 cycles and 80e82 cycles.

Fig. 4. (a) Equivalent circuit model and (b) the electrochemical impedance spectroscopy (EIS) results of batteries after charged at different cycle indexes.

Table 1Summary of fitted EIS results from Fig. 4.

Cycle index 2 20 21 80 81

Rb (U) 4.88 8.66 5.89 9.81 6.67RSEI (U) 2.94 11.64 2.75 12.31 3.79Rct (U) 6.26 9.12 5.25 8.46 7.15CPE1 (F) 3.42 � 10�6 2.73 � 10�5 4.13 � 10�6 1.19 � 10�5 5.65 � 10�5CPE2 (F) 9.78 � 10�6 1.19 � 10�4 9.16 � 10�6 1.08 � 10�4 1.29 � 10�4WR (S�1/2) 8.20 13.17 16.57 13.31 7.791WP 0.41 0.36 0.28 0.36 0.43

Y.X. Ren et al. / Journal of Power Sources 361 (2017) 203e210 207

products passivate the anode/separator as well as cathode/sepa-rator interphases, Liþ transport pathway between the anode andcathode will be blocked and the available cathode surface will bedecreased, enlarging both RSEI and Rct [53]. As shown in Fig. 4b, it is

Fig. 5. X-ray photoelectron spectroscopy (XPS) analysis of Li anodes. (a, b) S 2p spectra ofovercharging; (b) with overcharging. (c) Comparison of I 3d spectra of Li anodes obtained fComparison of In 3d spectra of Li anodes obtained from batteries with the addition of InI3

found that the depressed semi-circles representing the interfacialand charge transfer resistances can be decreased after the over-charging process, which are very close to the EIS result measuredfrom the battery after the initial 2 cycles (Fig. S7). The fitted results

Li anodes obtained from the batteries after charged (20 cycles at 0.5 C): (a) withoutrom batteries with the addition of InI3 (with overcharging, without overcharging). (d)(with overcharging, without overcharging).

Y.X. Ren et al. / Journal of Power Sources 361 (2017) 203e210208

as shown in Table 1 further indicate that the decrease of the RSEIshould be the main attribute, while the charge transfer resistanceshows a decrease to some extent. In this regard, it can be deducedthat overcharging might lower the interfacial resistance bydecomposing side product. Also, the charge transfer processinvolved in Li stripping/plating or sulfur redox reactions might beimproved, as the removal of nonconductive side product allows formore available electrode surface for electrochemical reactions.Though useful for analyzing the interfacial resistance, the two-electrode EIS measurement result cannot allow us to calculate theanodic or cathodic exchange current density as pointed out by thereviewer. To address this issue, a reference eletrode setup, which isstable in the corrosive electrolyte environment and has minor ef-fect on the original battery configuration, should be developed andthus it might be possible to depict the anodic and cathodic reactionkinetics separately.

In line with the achieved electrochemical performance, wefurther conducted X-ray photoelectron spectroscopy (XPS) analysisto characterize the chemical compositions of the Li anodes withand without overcharging. Fig. 5(a and b) show a comparison of S2p spectra of the SEIs formed on the anodes with and without theovercharging process. Overcharging was carried out after the initial20 cycles (the battery was charged to 3.4 V and rested for 30 min).Oxidized species including -NSO2CF3 and -SO3 exist for both

Fig. 6. (aed) SEM images and the corresponding EDX mappings of sulfur on the Li anodes: (aovercharging; (d) after 80 cycles with overcharging. SEM images of separators: (e) after 20

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 eVfor the SEI formed in the battery without overcharging, reflectingthe 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 Li2S2species for the battery with overcharging. It can be thus deducedthat the overcharging process successfully removes the side prod-ucts (Li2S and Li2S2) deposited onto the Li anode.

Moreover, as shown in Fig. 5c, it is found that for the batterywith overcharging, there exist more apparent peaks of I 3d 5/2 and I3d 3/2, indicating that polyiodide has been partially oxidized intoelemental iodine [37]. If we further look into the In 3d spectra inFig. 5d, the In 3d 5/2 and In 3d 3/2 peaks maintain unchanged forthe batteries with and without overcharging, confirming thechemical stability of In layer under the existence of corrosive iodineand polyiodide [54,55].

Eventually, we provide the SEM images as the evidence of sideproduct removal. As shown in Fig. 6a, at the initial cycle, thedeposited Li shows a rather smooth morphology, because intrin-sically the In layer can perform as a preferential Li depositionsubstrate, lowering the possibility of dendrite growth [56]. After 20cycles, for the Li anode deposited with the In layer, a layer of pro-truding particles, which are composed of insoluble reduced sulfurspecies (Li2S and Li2S2), is found to emerge. Though the

) after the initial cycle; (b) after 20 cycles without overcharging; (c) after 20 cycles withcycles without overcharging; (f) after 20 cycles with overcharging.

Y.X. Ren et al. / Journal of Power Sources 361 (2017) 203e210 209

accumulation of side product is considerably mitigated than thecase with bare Li metal (Fig. S8), the detrimental effects of sideproducts still need to be addressed for performance enhancement.After overcharging, the anode surface demonstrates a smooth andplain morphology, and the surface sulfur content is also found to beconsiderably decreased (Fig. 6c), which confirms the successfulremoval of reduced sulfur species. Interestingly, we observe that,after overcharging after 80 cycles, a smooth polymer-like film isfound to be coated onto the Li-In alloy layer on the anode surface(Fig. 6d), which might formed from the iodide induced polymeri-zation of DME [37]. The neutral I$ radical, which is an intermediategenerated when triiodide is reduced to iodide, may react withelectrolyte components to form a linear or comb-branched oli-goethers. Such a Li-permeable layer is capable of protecting the Lianode from the direct contact with the corrosive electrolyte andgreatly suppressing side reactions, partially explaining the excep-tional cycling stability [37].

In addition to the anode side, it is found that the separatorsobtained from the cycled batteries have been partially covered bythe precipitated side products formed from the chemical dispro-portion of polysulfides (Fig. 6e), which also can lower the activematerial utilization ratio, as found in the previous experiment [53].After overcharging, interestingly, the accumulated side products onthe separator are also found to be well dissolved by the oxidativepolyiodide and the resultant separator (Fig. 6f) shows minor dif-ference compared with the pristine one, further confirming the roleof I�=I�3 redox mediator in decomposing side products.

As a further discussion, in addition to InI3, we also exploit InBr3as an alternative because Br�=Br�3 is of a stronger oxidizability thanI�=I�3 . Though stable cycling of the redox mediator alone is ach-ieved as shown in Fig. S9a, it is realized that Br� is an anion withhigh donor number (DN) as previously reported in Li-O2 battery[57]. Specially, when being added into the electrolyte with low DNsuch as Dimethyl ether (DME), the existence of Br� promotes thesolution mechanism [58]. In consequence, as shown Figs. S9b and asloping lower voltage plateau is demonstrated in contrast with theflat 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, leadingto less irreversible discharge product formed by disproportion[46,59,60]. In this regard, the redox mediator's effect on the elec-trolyte system's donor number is supposed to be an essential cri-terion for choosing adequate redox mediators. In brief summary, anappropriate redox mediator for side product removal in Li-S bat-teries should possess the following properties: (i) excellent stabilityand fast redox kinetics; (ii) high redox potential to chemicallyoxidize the side product and (iii) minor or positive effects on thesulfur reduction/oxidation processes. In addition to I�=I�3 redoxcouple, further efforts can be directed towards the exploration oforganic redoxmediators (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 aslithium carbonate (Li2CO3) and lithium hydroxide (LiOH), in thatcase, identifying suitable redox mediators to decompose sideproducts 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 bythe bifunctional electrolyte additive InI3. One is the protection of Lianode induced by the electrodeposited In layer. The other concernsfor the effective decomposition of insoluble side product (short-chain polysulfide) outside the cathode, exploiting I�=I�3 redoxmediator. The resultant battery using a Li anodewith a deposited In

layer shows an improved cycling performance in comparison withthe one with the bare Li metal. On the other hand, by timelyovercharging the battery, it is demonstrated that the battery's ca-pacity can be retrieved and the battery's cycle life can be effectivelyprolonged. The facile and efficient strategy that protects Li anodeand decomposes side products thus opens a new avenue for pro-longing the Li-S battery's cycle life.

Acknowledgment

The work described in this paper was fully supported by a grantfrom the Research Grants Council of the Hong Kong SpecialAdministrative 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.

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A self-cleaning Li-S battery enabled by a bifunctional redox mediator1. Introduction2. Experimental2.1. Material preparation2.2. Cell assembly and test2.3. Characterization

3. Results and discussion4. ConclusionAcknowledgmentAppendix A. Supplementary dataReferences