A Lithium/Polysulfide Battery with Dual-Working Mode Enabled byLiquid Fuel and Acrylate-Based Gel Polymer ElectrolyteMing Liu,⊥,†,‡,§ Yuxun Ren,⊥,† Dong Zhou,‡,§ Haoran Jiang,† Feiyu Kang,*,‡,§ and Tianshou Zhao*,†
†Department of Mechanical and Aerospace Engineering, Hong Kong University of Science and Technology, Clear Water Bay,Kowloon, Hong Kong‡Engineering Laboratory for the Next Generation Power and Energy Storage Batteries, Graduate School at Shenzhen, TsinghuaUniversity, Shenzhen 518055, China§Laboratory of Advanced Materials, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China
ABSTRACT: The low density associated with low sulfur areal loadingin the solid-state sulfur cathode of current Li−S batteries is an issuehindering the development of this type of battery. Polysulfide catholyteas a recyclable liquid fuel was proven to enhance both the energy densityand power density of the battery. However, a critical barrier with thislithium (Li)/polysulfide battery is that the shuttle effect, which is thecrossover of polysulfides and side deposition on the Li anode, becomesmuch more severe than that in conventional Li−S batteries with a solid-state sulfur cathode. In this work, we successfully applied an acrylate-based gel polymer electrolyte (GPE) to the Li/polysulfide system. TheGPE layer can effectively block the detrimental diffusion of polysulfidesand protect the Li metal from the side passivation reaction. Cathode-static batteries utilizing 2 M catholyte (areal sulfur loading of 6.4 mgcm−2) present superior cycling stability (727.4 mAh g−1 after 500 cyclesat 0.2 C) and high rate capability (814 mAh g−1 at 2 C) and power density (∼10 mW cm−2), which also possess replaceable andencapsulated merits for mobile devices. In the cathode-flow mode, the Li/polysulfide system with catholyte supplied from anexternal tank demonstrates further improved power density (∼69 mW cm−2) and stable cycling performance. This novel andsimple Li/polysulfide system represents a significant advancement of high energy density sulfur-based batteries for future powersources.
KEYWORDS: Li/polysulfide battery, redox flow battery, gel polymer electrolyte, energy density, liquid fuel
1. INTRODUCTION
Energy density and power density perform critical roles inportable devices, electric vehicles (EVs), and energy storagesystems (EESs).1−6 Recently, lithium−sulfur (Li−S) batterieshave drawn particular interest owing to the natural abundance,nontoxicity, and especially high energy density lithium storagenature of sulfur.7−11 The lithiation of sulfur involves two phasetransformations including both dissolution and precipitation.On the higher voltage plateau (∼2.35 V) where dissolutiondominates, solid sulfur is first reduced to long-chainpolysulfides (Li2S8), which are inclined to dissolve in theelectrolyte. Then, they are subsequently reduced to lower orderpolysulfides (Li2S6 and Li2S4) also in the liquid phase. On thelower voltage plateau (∼2.15 V) where precipitation occurs,Li2S4 is reduced to insoluble Li2S2 and Li2S, whichreprecipitates in the electrode.12−17
A great challenge facing all categories of Li−S batteries is theshuttle effect caused by the dissolution of highly mobilepolysulfides, which can migrate to the metallic lithium (Li)anode and spontaneously form an inactive layer thatpronouncedly degrades the battery performance.18−22 Intensiveefforts have therefore been paid to modifying all components of
the battery system (cathode, anode, electrolyte, and interlayer)and exploring corresponding strategies, including physicalblocking, chemical binding, and electrocatalytic traps.18,19,23 Amajority of recent research efforts have been directed towardcarbon materials for trapping polysulfide intermediates withinthe cathode.18,24−26 On the other hand, designing microporousinterlayers27 and solid state electrolytes28 that prevent themigration of polysulfides as well as surface coatings on Lianode29 to avoid deterioration are all favored for theelectrochemical performances. The solid-state Li−S batteryhas been regarded as one of the most promising emergingenergy storage technologies because it possesses merits of highenergy density, stable electrochemical performance, andextraordinary safety.28,30 Recently, there emerged a series ofworks related to the all solid-state Li−S battery with glass-ceramic electrolytes and fast ionic conductors.30−32 Thoughthese batteries show considerable electrochemical performance,the polysulfide can corrode the grain boundaries, causing
Received: November 9, 2016Accepted: December 27, 2016Published: December 27, 2016
fissures and resultant cracks in the ceramic and leavingundesirable cracks on the all solid-state electrolyte.32 Therefore,gel polymer electrolyte becomes another suitable choice forsolving the aforementioned problems in solid-state Li−Sbatteries. In a recent study, we elaborately developed anacrylate-based gel polymer electrolyte (GPE)33 for Li−Sbatteries. The acrylate-based GPE shows a high ionicconductivity of 1.13 × 10−2 S cm−1 at 25 °C, achievingexcellent compatibility with the solid sulfur cathode andsuccessfully inhibiting the diffusion of polysulfides with apolysulfide-binding ester functional group and rational cellconfiguration design. This in situ method avoids the sacrifice ofenergy density and reserves the premium nature of the Li−Sbattery.34
Despite these efforts, the drawback of low areal sulfur loadingcaused by poor electronic conductivity of sulfur (∼10−30 S cm−1
at 25 °C), volumetric expansion (76%) during the lithiation/delithiation process, and continous dissolution−precipitation inthe solid-state sulfur electrode of current Li−S batteries is stillan inevitable issue hindering the development of Li−Sbatteries.35,36 Recently, the sulfur cathode with high activematerial loading has attracted great attention due to theupcoming capitalization of the Li−S battery. In general, thereare two featured strategies for preparing high areal loadingsulfur electrodes. One approach is to synthesize a three-dimensional layer-by-layer cathode that directly splintscommercial sulfur powders between porous carbon nanofiberlayers.37,38 In addition, polysulfide catholyte can be used asactive material, which also results in a high sulfur areal loadingdue to the improved ion and electron transportation.13,14,39,40
The Li/polysulfide battery was proven to be an efficient andpromising system to increase the areal sulfur loading andenhance both the energy density and power density.13,41 Thecomparison of the configuration and electrochemical perform-ance of recent static working mode Li/polysulfide batteries isshown in Table 1. As shown in Table 1, the development of Li/polysulfide batteries mainly focused on the electrode design.Tin-doped indium oxide-decorated CNF paper,42 Pt/gra-phene,23 nitrogen-doped graphene paper,43 and TiO2 nano-wire/graphene hybrid membranes44 were introduced asbifunctional polysulfides immobilizers and current collectorsinto this system. Commercial separators work as the
compromised choice showing weak ion selectivity, and theimmobilization function is mainly dependent on the decoratedcarbon-based current collector with multidimensional pores.Manthiram et al.44 believed that a second protection, such asthe graphene coating, on the electrolyte/separator interface tofurther block the diffusion of polysulfides is urgent andeffective. In this sense, developing an applicable GPE layerfor the Li/polysulfide battery is essential for the advancementof this configuration.On the other hand, this configuration can further derive Li/
polysulfide redox flow batteries with further improved energydensity and power density, potentially for large-scale electricvehicles.41,45 With the employment of polysulfide and theaggravation of flow convection, the shuttle effect, which is thecrossover and side deposition of polysulfides on Li anode,becomes much more severe.46 The configuration and electro-chemical performance of recent redox flow Li/polysulfidebatteries are shown in Table 2. As verified, adding a large ratioof lithium salt (LiTf and LiTFSI) or percolating nanoscaleconductor suspension (Ketjen Black) in the electrolyte enablesthe cycling of a polysulfide cathode at the cathode-flow modeto improve the capacity utilization.40,41,45 However, thesemethods of adding a large ratio of additives generally suffersfrom high viscosity and flow resistance of the catholyte. Thus,the main direction Li/polysulfide batteries should turn towardis blockage of polysulfide diffusion and stabilization of the Limetal without sacrificing flow resistance.41 These efforts furtherpromote the exploration of an appropriate electrolyte layer,especially for Li/polysulfide devices.In this paper, we applied an acrylate-based gel polymer
electrolyte to the Li/polysulfide system. The GPE layer caneffectively block the diffusion of polysulfides and protect the Limetal from the side deposition reaction. The static Li/polysulfide batteries utilizing 2 M Li2S8 catholyte achieve ahigh sulfur areal loading (6.4 mg cm−2) and exhibit superiorrate capability (∼814 mAh g−1 at 2 C) and cycling stability(727.4 mAh g−1 after 500 cycles at 0.2 C). In addition, theapplication of acrylate-based GPE in Li/polysulfide redox flowbatteries assembled with a polysulfide catholyte tank achievesfurther improved power density (∼69 mW cm−2) and stabilizedcycling capability.
Table 1. Comparison of the Configuration and Cycling Performance of Recent Static Working Li/Polysulfide Batteries
percolating nanoscale conductor networks 2.5 M S L−1 13 nL s−1 2.5 mA cm−2, 400 h 40sulfur-impregnated carbon composite 20S-26KB 4 mA cm−2, 30h 453 M LiTFSI in DMSO 0.4 M 40 mL min−1 1.5/0.25 mA cm−2, 55 cycles 41carbon cloth/acrylate-based GPE 0.2 M 7.5 mL min−1 2.4 mA cm−2, 30 h this work
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2. EXPERIMENTAL SECTION2.1. Preparation and Characterization of Acrylate-Based
GPE-Filled Separator. Separators filled with acrylate were preparedby gelation of a precursor solution in a sealed CR2032-type coin cell.The precursor solution was composed of 1.5 wt % of pentaerythritoltetraacrylate (PETEA, C17H20O8; Tokyo Chemical Industry Co., Ltd.)monomer and 0.1 wt % of azodiisobutyronitrile (AIBN, C8H12N4,Aldrich) initiator dissolved in a liquid electrolyte (LE) consisting of 1M bis(trifluoromethane) sulfonamide lithium (LiTFSI) salt in anonaqueous mixture of 1,2-dioxolane (DOL)/dimethoxymethane(DME) (1:1 by volume) with 1 wt % of LiNO3 additive. Theprecursor solution was injected to infiltrate the separator (SD216;Shenzhen Senior Technology Material Co., LTD). For the accuracy ofthe experiment to be ensured, the precursor solution in each separatorwas uniformly set as ∼0.02 mL. After covering the stainless steel sheetsand sealing the coin cells, the cells were quickly heated at 70 °C for 0.5h in a glovebox to ensure the complete polymerization of monomers.All procedures were carried out in an Ar-filled glovebox (MBRAUN)with the concentrations of moisture and oxygen below 1 ppm. Theseparators filled with acrylate-based GPE were dissembled from coincells for further examination, which were repeatedly rinsed with DMEand vacuum dried at 50 °C for 6 h to remove any residual solvent. Theair-sensitive samples were rapidly transferred into the vacuum chamberunder the protection of a vacuum box before the following test. Themorphology of acrylate-based GPE was examined by field emissionscanning electron microscopy (FE-SEM, HITACH S4800) at 5 kV.2.2. Electrochemical Performance Measurements of the Li/
Polysulfide Cell. Li/polysulfide batteries were assembled by ahomemade setup in an Ar-filled glovebox. The acrylate-based polymercells were fabricated by direct polymerization. The cell was comprisedof the carbon cloth (ELAT, hydrophilic) as the cathodic currentcollector, polyolefin separator (SD216; Shenzhen Senior TechnologyMaterial Co., LTD), and lithium (Li) foil as the anode. The precursorsolution containing 1.5 wt % of PETEA and 0.1 wt % of AIBNdissolved in 1 M LiTFSI/DOL:DME (1:1 by volume) with 1 wt % ofLiNO3 electrolyte was first injected to infiltrate the separator, whichwas covered on the top of the Li anode. For the accuracy of the
experiment to be ensured, the precursor solution in each separator wasuniformly set as ∼0.02 mL. After covering the carbon cloths andstainless steel sheets, the cells were quickly sealed and heated at 70 °Cfor 0.5 h in a glovebox to ensure the complete polymerization ofmonomers. Then, the Li2S8 catholyte was injected into the cathodicside of the cell with a pipet. For the polysulfide catholyte to beprepared, stoichiometric amounts of sulfur powder (Dk NanoTechnology, Beijing) and lithium sulfide (Li2S; Sigma-Aldrich) weremixed in a proper amount of the electrolyte (1 M LiTFSI/DOL:DME(1:1 by volume) with 1 wt % of LiNO3) by magnetic stirring overnightat room temperature to render a 2 M Li2S8 solution. The electrolyteand polysulfides solution were prepared in an argon-filled glovebox.The mass loading of the active material (based on the weight of sulfur)on the electrode is ∼6.4 mg/cm2. The assembled Li/polysulfide cellswere cycled at various rates between 1.7−2.8 V on a Land 2001Abattery testing system at 25 °C.
The cathode flow cell was also assembled using the same proceduredescribed above. For the pressure drop through the flow cell to bereduced, the flow-by fixture was exploited here, where the flow channelhas a depth of 0.75 mm and a width of 2 mm. Five milliliters of 0.2 MLi2S8 catholyte was stored in the tank and pumped into the flow cellwith a BT-100 2J peristaltic pump (Longer Precision Pump Co. Ltd.,China). The Li flow cell assembly process was conducted in an Ar-filled glovebox (<0.1 ppm of H2O and <0.1 ppm of O2; Etelux). Thegalvanostatic charge and discharge tests were carried out on a batterytest system (Neware, CT-3008 W). Using the VMP3 multichannelelectrochemical station, rate capability of the assembled Li/polysulfidecell was measured with a sweeping rate of 5 mV s−1; at the chargedstate, the EIS measurement was conceived in the frequency range of10−2 to 105 Hz by applying a 5 mV ac oscillation.
The Li/polysulfide cells after designated cyclic tests were transferredinto the glovebox and dissembled for further examination. Theircarbon cloths and Li anodes were repeatedly rinsed with DME andvacuum-dried at 50 °C for 6 h to remove the residual solvent. The air-sensitive samples were rapidly transferred into the vacuum chamber ofSEM/XPS under the protection of a vacuum box before the followingtest. The morphology and microstructure of electrodes were
Figure 1. (a) Schematic illustration of the Li/GPE/polysulfide battery; (b) morphology of the commercial separator employed in the Li/polysulfidebattery; (c) FESEM image of the acrylate-based GPE (the optical image of acrylate-based GPE is shown in inset); (d) FESEM image of the acrylate-based GPE filled in the pores of the commercial separator; (e) FESEM image of the carbon cloth employed in the Li/polysulfide battery; (f, g)polysulfide crossover across the GPE-modified separator and (h) control separator in H-cell discharged at a current density of 1 mA/cm2.
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characterized by FE-SEM at 5 kV and transmission electronmicroscopy (TEM, JEOL-2100F) at 20 kV. X-ray photoelectronspectroscopy (XPS) measurements were conducted on a PhysicalElectronics PHI5802 instrument using an X-ray magnesium anode(monochromatic Kα X-ray at 1253.6 eV) as the source. The C 1sregion was used as reference and set at 284.8 eV.
3. RESULTS AND DISCUSSION
3.1. Configuration of the Li/Polysulfide Battery. Figure1a presents the schematic configuration of the Li/polysulfidebattery employing acrylate-based GPE (defined as Li/GPE/polysulfide battery). For the cathodic side, polysulfide (Li2S8)dissolved in liquid electrolyte (LE) (1 M bis(trifluoromethane)sulfonamide lithium (LiTFSI)/1,2-dioxolane (DOL):dime-thoxymethane (DME) with 1 wt % of LiNO3) is adsorbed inthe pores of commercial carbon cloth, together constituting theworking electrode. As shown in Figure 1e, the carbon cloth isweaved with regularly oriented and smooth carbon fibers withan average diameter of approximately 10 μm, which works asthe electron mobile and the solid-state products (sulfur orsulfides) precipitation substrate. Carbon cloth is found to be anexcellent sulfur host, as it can take up the polysulfide catholytewell and adapt to the volume change during cycling owing to itswell-developed and uniformly distributed pores (porosity of∼80%) and freestanding and flexible nature.46 As for theelectrolyte layer, the acrylate-based GPE was thermal-initiatingly polymerized in the commercial separator as shownin Figure 1b−d. The detailed molecule structure andelectrochemical/mechanical stability were characterized in ourprevious work.33 A representative field emission scanning
electron microscopy (FESEM) image of the commercialseparator is shown in Figure 1b, displaying that a typicalporous structure with an average pore size of ∼200 nm wasuniformly distributed in the polymer matrix. Interestingly, theacrylate-based GPE can fully fill the pores of the commercalseparator and inhibit the diffusion of polysulfides out of thecatholyte (Figure 1c,d). The GPE scaffold acts as a mechanicalsupport for the liquid electrolyte and limits the crossover ofpolysulfides while the liquid electrolyte offers the pathway forion transport. For an intuitive way to represent the function ofGPE, the visible durability measurement was carried out in anH-type glass cell (cathodic side with 0.2 M polysulfide dissolvedin LE and carbon cloth acting as current collector and theanodic side with pure LE and Li metal) to present the stabilityof the Li/polysulfide battery under amplified workingconditions (Figure 1f−h). A separator fully filled with GPE(detailed preparation process is shown in the ExperimentalSection) was inserted between the glass cell, and the test wasconducted in a glovebox to exclude the influence of water andoxygen. The practical discharge current was applied on themodel H-cell to induce polysulfide crossover. We compared thepolysulfide crossover across the GPE-modified separator andthe control separator in Figure 1f−h. The color of the anodicside in Figure 1g slightly changed to faint yellow after 8 h ofdischarge, indicating that most of the polysulfide was blockedby the GPE-modified separator. As a control group, thedurability test after the replacement of the commercialseparator is shown in Figure 1h. The anodic side quicklyvisibly evolved to a bright yellow color after only 1 h of
Figure 2. (a) Electrochemical performance (rate performances from 0.2 to 2 C and the following cycling performances at 0.2 C) of the Li/polysulfide static batteries employing LE and acrylate-based GPE (the current density of 1 C is 1675 mA g−1); typical charge/discharge voltagecurves of Li/polysulfide static batteries employing (b) LE and (c) acrylate-based GPE for the first 10 cycles at 0.2 C; typical charge/discharge voltagecurves of Li/polysulfide static batteries employing (d) LE and (e) acrylate-based GPE at various C-rates. Specific capacity values were calculatedbased on the mass of sulfur.
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discharge, verifying the poor ion selectivity of the commercialseparator. In this regard, the stability test confirms therationality of applying acrylate-based GPE in the Li/polysulfidebatteries.3.2. Electrochemical Performance of Li/Polysulfide
Battery under Static-Working Mode. Li/polysulfide bat-teries employing acrylate-based GPE (Li2S8/GPE/Li) cellscontaining Li metal anode, polysulfide cathode, and acrylate-based GPE were fabricated according to the ExperimentalSection. As a comparison, the Li/polysulfide batteries employ-ing LE (defined as Li2S8/LE/Li) were also prepared under thesame conditions, both of which were assembled with a highsulfur mass loading of 6.4 mg cm−2 (based on the weight ofsulfur). Figure 2a compares the rate performances of the Li2S8/LE/Li and Li2S8/GPE/Li cells at various rates from 0.2 to 2 Cand the following cycling performances at 0.2 C (the currentdensity of 1 C is 1675 mA g−1). As a result, the Li2S8/LE/Li cellperforms with a rather unstable rate capability and cyclingperformance. The poor electrochemical performance could beattributed to the severe shuttling effect of liquid polysulfides,details of which would be verified later. Interestingly, the Li2S8/GPE/Li cell presents a much better rate performance than thatof the Li2S8/LE/Li cell (comparison of the electrochemicalperformance of recent cathode-static Li/polysulfide batteries isshown in Table 1). The specific discharge capacities (the fifthcycle of rate performance under different current densities) ofthe Li2S8/GPE/Li at 0.2, 0.4, 0.8, 1, 1.5, and 2 C are 1172.5,1086.9, 1048.4, 1008.6, 969.3, and 814 mAh g−1, respectively,whereas the corresponding capacities of Li2S8/LE/Li are only919.7, 567.4, 369.7, 365.2, 226.3, and 141.3 mAh g−1. Theincrease of the rate capability can be attributed to the fact thatthe GPE layer is capable of retaining most of the active materialon the cathodic side, which can be repeatedly utilized withminor active material loss. The corresponding charge/dischargecurves of the first 10 cycles are shown in Figure 2b and c. It is asexpected that both cells exhibit an initial discharge voltage of∼2.15 V with an inconspicuous first platform and lower firstcycle discharge specific capacity than charge capacity, which arerepresentative properties of Li/polysulfide batteries due to thepartial prelithiation nature of the polysulfide.44 In comparison,
Li2S8/GPE/Li shows a longer platform, reduced polarization,and slower decay rate than those of Li2S8/LE/Li for the first 10cycles. Furthermore, the corresponding charge/dischargecurves under different current densities are presented in Figure2d and e. The Li2S8/GPE/Li cells can maintain the merits ofthe first 10 cycles to all of the current densities and enable ahigh rate performance under high sulfur areal loading of 6.4 mgcm−2. The following cycling performances of the Li2S8/LE/Liand Li2S8/GPE/Li cells under low current densities of 0.2 Cafter the rate test are presented in Figure 2a. The Li2S8/GPE/Licell is able to maintain capacity retention above 60% up to 500cycles at 0.2 C with a final discharge capacity of 727.4 mAh g−1
(Figure 2a). In sharp contrast, the corresponding retention forthe Li2S8/LE/Li cell is 52.3% after only 155 cycles, and thebattery failed. During the test, the Li2S8/LE/Li cell exhibits lowand fluctuant Coulombic efficiency, unstable charge/dischargecurves, and worse polarization. The above results indicate thatthe acrylate-based GPE layer is able to retain the active materialin the cathodic side even for the liquid polysulfides, which canbe repeatedly utilized under different current densities in theLi2S8/GPE/Li cells.
3.3. Electrochemical Performance of Li/PolysulfideBattery under Cathode-Flow Working Mode. This Li/polysulfide configuration can further derive out Li/polysulfideredox flow batteries (the configuration of the setup is shown inFigure 3a), improving the energy density and power density forthe equipment of polysulfide catholyte tank and the reductionof concentration polarization. However, a critical barrier withthis Li/polysulfide flow battery is that the shuttle effect, whichis the crossover of polysulfides and the side deposition on theLi anode, becomes much more severe. As a control group, aredox flow battery with only commercial separator was firsttested at 0.6 mA cm−2 for a fixed capacity of 0.9 mAh (Figure3b). Unfortunately, the battery failed within only 3 cycles,showing a greatly increased internal resistance during operation(Figure 3f and Table 3). Unlike the battery working at a staticmode, the cathode-flow mode results in the dissolved long-chain polysulfides’ convection toward the Li anode. Con-sequently, long-chain polysulfides are reduced to insoluble andinsulating products precipitating on the Li anode surface,
Figure 3. (a) Schematic illustration for the configuration of the redox flow Li/GPE/polysulfide battery setup; (b) typical charge/discharge voltagecurves of Li/LE/polysulfide redox flow battery for the first 3 cycles at a current density of 2.4 mA cm−2 and flow rate of 7.5 mL min−1; (c) cyclingperformance of the Li/GPE/polysulfide redox flow battery at a current density of 2.4 mA cm−2 and flow rate of 7.5 mL min−1; (d) I−V curve of theLi/GPE/polysulfide redox flow battery under a flow rate of 7.5 mL min−1; (e) typical charge/discharge voltage curves of a Li/GPE/polysulfide redoxflow battery at current densities of 0.6, 1.8, and 2.4 mA cm−2; and (f) electrochemical impedance spectroscopy (EIS) plots of Li/polysulfide redoxflow batteries with LE and GPE for the fresh cell and after cycling. Specific capacity values were calculated based on the mass of sulfur.
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leading to increased active material loss and resistance for Li+
diffusion, which will be discussed in the following section.Therefore, toward the design of flow batteries to retard thecrossover induced by convection, developing appropriate GPEto enable the immobolization of polysulfides is an efficientapproach. In this regard, acrylate-based GPE with ultrahighionic conductivity and ideal inhibition ability was introducedinto this system.The Li/polysulfide redox flow battery was assembled with
GPE to mitigate the crossover induced by convection in thecathode flow mode. Figure 3c shows eight iterations of charge/discharge profiles in continuous flow mode at 1.8 mA cm−2,demonstrating stable cycling performance (the optical image ofthe homemade setup during operation is shown in the inset ofFigure 3c). The power capability and energy efficiency of theLi/polysulfide system are also measured. As shown in Figure3d, the battery operated under a flow rate of 7.5 mL min−1
delivers a peak power density around 69 mW cm−2 at a currentdensity as high as 45 mA cm−2. In comparison, for a Li/polysulfide battery working at static mode, a power density of10 mW cm−2 was reached. In this regard, it is worth noting thata cathode-flow working mode can efficiently improve the poweroutput of Li/polysulfide batteries, which is attributed to thereduction of concentration polarization. Moreover, when cycledat a fixed charge/discharge capacity of 6 mAh under flow-modeworking mode, it was found that the Li/GPE/Li2S8 batteryshows a stable performance at cycling rates ranging from 0.6 to2.4 mA cm−2 with reasonably high energy efficiency (92.5, 85.7,and 82.4%), which is due to the improved mass transport forsulfur species under flow-working mode. The improved masstransportation can further result in the high power output. Inthis regard, the as-prepared GPE can successfully retard thepolysulfides’ shuttle effect induced by convection when thebattery is operated at cathode-flow mode. Electrochemicalimpedance spectroscopy (EIS) is used to evaluate the interfacialresistance and reversibility of Li/polysulfide flow batteries. TheEISs of the battery with LE and GPE after different cycles aresimulated (Figure 3f) using an equivalent circuit shown in the
inset of Figure 3f, and the simulation results are summarized inTable 3.47,48 It is seen that the interfacial resistance (Rsei) in thecell with only a commercial separator increases from 14.65 to34.69 Ω after only 3 cycles. Notably, the Rsei for the cell withGPE exhibits an obviously lower increase after 8 cyles (from18.52 to 19.36 Ω), demonstrating a stable surface layer withoutsevere side deposition on the Li electrode, which would bevisually validated in Figure 4. In our experiment, the mostcritical issue in the Li/polysulfide battery under flow-workingmode is the polysulfide corrosion of the metallic lithium,especially under high sulfur loading and large practical capacity.The lower Rsei for the cell with GPE contributes to themarkedly improved performance as shown in Figure 3. Fromthese results, for the Li/polysulfide flow batteries, the shuttleeffect through the intermediate region of the polysulfides isattributed to the convection and diffusion caused by theconcentration gradient and electric migration of the poly-sulfides.49 With increased flow rate, the effect of convectionbecomes dominant and aggravates the migration of polysulfidestoward the anode, implying a higher criteria for suppressing theshuttle effect.41,45 For a Li/polysulfide flow battery to beendowed with a practical power density, the following twoissues should be ensured: (a) high in-plane mass transfer in theflow cathode and large specific surface area and (b) sufficientthrough-plane mass transfer and selective blockage ofpolysulfides. The morphology of the Li anode plays a ratherimportant role in the electrochemical performance.12,50,51
3.4. Surface Characterization of Electrodes in the Li/Polysulfide Battery. For the electrochemical process of Li/polysulfide batteries, dissolved polysulfides were first depositedas lithium sulfides during the initial discharge period and thencharged back to polysulfides and finally reprecipitated assulfur.14,36 Panels a−d in Figure 4 reveal the morphologies ofthe discharged and charged state carbon cloths dissembled fromthe Li/GPE/polysulfide static battery after 10 cycles at 0.2 C.After discharge to 1.7 V, the lithium sulfides, including Li2S2and Li2S, were uniformly deposited on the surface of everysingle carbon fiber of the carbon cloth (Figure 4a,b). Theprecipitated lithium sulfides show the particle-like structure,which is different from the carbon cloth under a charged state(Figure 4c,d). After charging to 2.8 V, the precipitated sulfur onthe carbon cloth demonstrates a filmlike structure. The benignkinetics for dissolution−precipitation of sulfur species and thehigh electric conductivity of the carbon cloth enabled highsulfur areal loading Li/polysulfide batteries (6.4 mg cm−2).
Table 3. Summary of Simulation Results from Figure 3f
Figure 4.Morphologies of the (a, b) discharged and (c, d) charged state carbon cloths dissembled from the cycled Li/GPE/polysulfide static battery;FESEM images of the Li anodes dissembled from the cycled Li/polysulfide static batteries employing (e) LE and (f) acrylate-based GPE; (g) S 2pXPS spectra of the Li anodes dissembled from the cycled Li/polysulfide static battery employing LE and acrylate-based GPE.
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The morphology of the Li anode plays a rather importantrole in the electrochemical performance.12,50 For intuitivelyverifying the proposed concern about polysulfide diffusion anddeposition, field emission scanning electron microscopy (FE-SEM) and X-ray photoelectron spectroscopy (XPS) are used toexamine the surface morphology and constitution of the Lianode obtained from the cycled Li2S8/LE/Li and Li2S8/GPE/Li. The batteries (after 10 cycles at 0.2 C) were disassembledunder fully charged conditions (2.8 V) to exclude the influenceof the polysulfides. It is seen in Figure 4e that the morphologyof the Li anode from Li2S8/LE/Li shows an uneven surface,demonstrating a serious deposition of sulfur species and furtherconfirming the increased impedance from EIS plots (Figure 3f).For the Li anode from the Li2S8/GPE/Li cell, as expected, asmooth and clear surface proves that the side effects of thedetrimental reaction have been significantly inhibited (Figure4f). The clean anodic solid electrolyte interface (SEI) is criticalbecause it acts as a passive layer between the electrolyte andelectrode that allows facile transport of ions but avoids extraresistance caused by an uneven interface.51−55 In addition, theXPS of Li anodes dissembled from Li2S8/LE/Li and Li2S8/GPE/Li was examined to investigate their surface compositionafter 10 cycles. As shown in Figure 4g, the S 2p region spectrareveals that the peak intensity of lithium sulfides (Li2S at 161.2and Li2S2 at 162.4 eV) and electrochemically irreversibleoxidized sulfur species (−SO3, 165.2 eV) on the Li electrode ofthe Li2S8/LE/Li cell are much stronger than that in the Li2S8/GPE/Li cell.56−58 These results demonstrate that many fewerside reaction products appeared on the Li anode, and thebattery was successfully stabilized by the acrylate-based GPE.
4. CONCLUSIONSIn conclusion, we have successfully applied an acrylate-basedgel polymer electrolyte (GPE) to the Li/polysulfide systemworking in dual mode. A static working battery utilizing 2 Mcatholyte (sulfur areal loading of 6.4 mg cm−2) presentsultrastable cycling stability and high rate capability. In addition,at the cathode flow mode, the Li/polysulfide system withaccessorial catholyte supplied from an external tank demon-strates further improved power density (∼69 mW cm−2) andcyclability, indicating that acrylate-based GPE can reduce wellthe convective transport of polysulfides from cathode to anode.The Li/polysulfide battery has the potential to be a promisingdevice for future energy convection and storage, which has toensure the stability of the system by the corresponding GPElayer that can effectively block the detrimental diffusion ofpolysulfides and protect the Li metal from the side passivationreaction. This Li/GPE/polysulfide battery design solves the lowconductivity and huge volumetric change of sulfur andsimultaneously restrains the shuttle effect with appropriateacrylate-based GPE, and the mature lithium ion and redox flowbatteries accessories accomplishes the production design forfuture power sources.
■ AUTHOR INFORMATIONCorresponding Authors*E-mail: [email protected].*E-mail: [email protected] Zhao: 0000-0003-4825-2381Author Contributions⊥M.L. and Y.R. contributed equally to this work.
NotesThe authors declare no competing financial interest.
■ ACKNOWLEDGMENTSThis work was fully supported by the Research Grants Councilof the Hong Kong Special Administrative Region of China(Project No. 16213414) and the National Key Basic ResearchProgram of China (No. 2014CB932400).
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