8782 | Chem. Commun., 2017, 53, 8782--8785 This journal is©The Royal Society of Chemistry 2017

Cite this:Chem. Commun., 2017,

53, 8782

Unveiling the synergistic effect of polysulfideadditive and MnO2 hollow spheres in evolving astable cyclic performance in Li–S batteries†

Syed Abdul Ahad,ab P. Ragupathy,*ac Soojy Ryu,a Hyun-Wook Lee b andDo Kyung Kim *a

Herein, we demonstrate a synergistic approach involving polar-based

oxide and polysulfide additives for effectively suppressing polysulfide

dissolution during cycling. The MnO2 hollow spheres not only provide

physical confinement for the polysulfide species but also enable

strong chemical interactions between polysulfide species and oxides,

while the added polysulfide furnishes a mass buffering effect and

compensates for the capacity losses due to partial cathode dissolu-

tion during discharge. The capacity retentions of S/KB, S/KB/LiPS,

S/KB/MnO2, and S/KB/MnO2/LiPS composite cathodes are 31%, 45%,

59%, and 91% respectively. The remarkable capacity retention of

the S/KB/LiPS/MnO2 composite electrode is mainly attributed to the

synergistic effect between MnO2 and polysulfide additives.

The development of high performance rechargeable batteriesthat can outperform the state-of-the-art lithium-ion (Li-ion)technology is highly desired to meet the ever increasing energydemand for various sectors such as personal electronic devices,electric vehicles, and large-scale energy storage.1–3 In this regard,the lithium–sulfur (Li–S) battery has attracted much interest asone of the major rechargeable energy storage systems owing toits high theoretical specific capacity of 1675 mA h g�1 and hightheoretical specific energy density of 2600 W h kg�1.4 Moreover,sulfur is less expensive, abundant, less-toxic and environmen-tally benign. However, the commercialization of Li–S technologyis still impeded due to multitude issues such as the inherentlypoor electrical conductivity of sulfur, dissolution of intermediatelithium polysulfides (LiPSs) and large volumetric change duringcharge–discharge, resulting in instability of the electrodes.5 For anumber of decades, strenuous efforts have been devoted towardsaddressing these issues to improve the performance of Li–S

batteries including composite sulfur electrodes by integratingwith conductive carbon materials or conducting polymers,6–10

designing new architectures to buffer the volumetric expansion/shrinkage,8 and modifying the electrode to confine the lithiumpolysulfides suppressing the shuttling effect.11,12 Among thesemethods, physically trapping LiPSs within the sulfur electrode byvarious carbonaceous materials such as porous carbon, carbonfibers, carbon spheres, carbon nanotubes and conducting poly-mers has been proven to be promising.6,7,9,13,14 However, thecycling stability of Li–S batteries still remains a significantchallenge for practical applications due to the fact that nonpolarcarbon based materials can provide only weak physical inter-action with LiPSs15 owing to their poor capacity retention uponcycling. In the case of conducting polymers, a portion of LiPSscan be trapped, limiting the cycling stability due to the structuralchanges that originate from the volume change of the sulfurelectrode. Therefore, some strong chemical interactions betweenLiPSs and host materials are essential to trap LiPSs.

It has been widely recognized that some metal oxides/sulfideswith polar surfaces can significantly increase the chemical inter-action between LiPSs and substrates.15,16 Various metal oxidessuch as TiO2,3 Ti4O7,17 La2O3,18 MgO,19 MnO2,2 Fe2O3,20 VO2,21

SiO2,22 Al2O3,23 and Nb2O5,24 and metal hydroxides includinglayered double hydroxides25 have been observed to form strongchemical interaction with polysulfides. Thus, the binding betweenpolysulfide species and the surface of host materials results in theenhanced cycling stability of Li–S batteries. Recently, addition ofpolysulfides to sulfur electrodes as stabilizers to suppress cathodedissolution has become more popular. However, to the best of ourknowledge, there are no reports on the combination of polysulfideadsorbents with polysulfide additives in Li–S batteries. Herein, wedemonstrate an innovative and simple strategy to effectively trappolysulfides using high surface area MnO2 hollow spheres and asmall amount of polysulfide as a stabilizer. This simple syntheticprotocol alleviates the use of templates and polymers as previouslyemployed in preparing various architectures of MnO2. We pre-sume that hollow MnO2 provides strong adsorption to polysulfidesformed during cycling. Moreover, the shell structure can act as a

a Department of Materials Science and Engineering, Korea Advanced Institute of

Science and Technology (KAIST), Daejeon 305-701, Republic of Korea.

E-mail: dkkim@kaist.ac.krb School for Energy and Chemical Engineering, Ulsan National Institute of Science

and Technology, Ulsan 44919, Republic of Koreac Electrochemical Power Sources Division, Fuel Cells Section Central Electrochemical

Research Institute, Karaikudi-630 003, India. E-mail: ragupathyp@cecri.res.in

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7cc04229a

Received 2nd June 2017,Accepted 12th July 2017

DOI: 10.1039/c7cc04229a

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protective layer to entrap the LiPSs within the electrode. Inaddition, added polysulfide acts as a buffering agent to preventcathode dissolution.

To confirm the crystal structure and phase purity of the synthe-sized materials, XRD analysis was carried out. Accordingly, Fig. S1(ESI†) shows the powder XRD patterns of the as prepared MnO2,S/KB and S/KB/MnO2 composites. The poor crystalline nature of theas-synthesized MnO2 is revealed by the presence of a single broadpeak around a 2y value of 371. The formation of d-MnO2 is indicatedby the presence of amorphous phases in the XRD pattern asreported earlier.26 Further, a broad peak around a 2y value of 251of S/KB/MnO2 composites is evident from the amorphous nature ofthe composites. Mostly, the peaks observed for S/KB and S/KB/MnO2

correspond to the orthorhombic phase of sulfur (JCPDS-008-0247).The change in the morphology of MnO2 and S/KB/MnO2 was

characterized using SEM and TEM analysis. Fig. 1a and b show theSEM and TEM images of the as-prepared uniform sized MnO2

hollow spheres. It is apparent that the MnO2 hollow spheres consistof hierarchical and well-interconnected spherical particles. It is veryclear from the SEM image that the particle size varies from about300 nm to 500 nm, as seen in Fig. 1(a and b). As is evident in Fig. 1b(inset), the wall thickness of the hollow sphere is between 50 nm and60 nm, while the inside cavity ranges from 300 to 400 nm, givingenough space for sulfur infiltration. After sulfur infiltration, it isclearly seen that MnO2 spheres are well distributed and mixed with Sand KB particles, forming a S/KB/MnO2 composite (Fig. 1c and d).The well mixed sulfur with MnO2 spheres and KB particles ensuresgood contact among the three components and with the electrolyte.Also, the good structural stability and integrity of the structures areevident from the absence of any damage to the sulfur infiltratedMnO2 spheres as is evident in the TEM images of S/KB/MnO2

presented in Fig. 1e and f. The uniform distribution of S in andaround the MnO2 spheres is further confirmed by STEM results(Fig. 1g). Other major elements like carbon (C), manganese (Mn) andoxygen (O) present in the S/KB/MnO2 composite are also evident inthe elemental mapping supported by the STEM image. The sulfurcontent in the S/KB and S/KB/MnO2 composites was found tobe B71 wt% using an elemental analyzer. The MnO2 contentwas B8%, which was calculated by deducting the amounts of C,H and S from 100% as presented in Table S1 (ESI†). This simple butthoughtful design of the S/KB/MnO2 composite along with its keyfeatures is presented in Scheme 1. The presence of MnO2 sphereswill not only inhibit polysulfides by physically entrapping theminside the spheres but will also chemically entrap them within thecathode itself. The conventional addition of KB ensures effectivetransport of electrons through the cathode structure, thereby activat-ing sulfur which is present outside the MnO2 spheres.

In order to understand the electrochemical performance of theS/KB composite with the addition of MnO2 and LiPS electrolyte,cells made with S/KB and S/KB/MnO2 composites were subjectedto galvanostatic charge–discharge cycling. Moreover, to investigatethe role of MnO2 and LiPS electrolyte [i.e. Li2S8 additive electrolyte]that influence the cycling behavior, a few control experiments wereperformed. The initial discharge capacities of the S/KB, S/KB/MnO2, S/KB/LiPS and S/KB/MnO2/LiPS composite electrodes arefound to be 1111, 1131, 900 and 967 mA h g�1 at 0.2C, respectively,

as shown in Fig. S2 (ESI†). The variation in the initial capacity isdue to the addition of hollow MnO2 and LiPSs. However, thecapacities of all the electrodes are almost the same at the 10th cycle.

Further, Fig. 2a presents the cycling performance of theS/KB, S/KB/MnO2, S/KB/LiPS and S/KB/MnO2/LiPS compositeelectrodes at 0.2C. As expected, with the reference electrolyte,

Fig. 1 (a) SEM analysis of MnO2 hollow spheres. (b) TEM analysis with insetsrepresenting a TEM magnified image and a pictorial representation of MnO2

hollow spheres. The inset scale bar represents 50 nm length. (c) & (d) SEMimages of S/KB/MnO2 composites. (e) & (f) TEM images of S/KB/MnO2 compo-sites showing sulfur infiltrated MnO2 as well and (g) STEM and the correspondingelemental mapping of the selected region of the S/KB/MnO2 composite.

Scheme 1 Schematic representation of the S/KB/MnO2 composite withthe possible advantages of the components involved in the system.

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8784 | Chem. Commun., 2017, 53, 8782--8785 This journal is©The Royal Society of Chemistry 2017

the S/KB composite delivered a very poor discharge capacity of345 mA h g�1 after 100 cycles, while with the addition of LiPSelectrolyte, the capacity stabilized at 400 mA h g�1 at the 100th cycle.Here, in the S/KB composite with LiPS electrolyte, no significantchange in the capacity was observed. This might be due to theexcessive dissolution of polysulfides in the S/KB composite. Moreimportantly, the capacity retention was increased to 665 mA h g�1

after 100 cycles upon incorporating the MnO2 hollow spheres as apolysulfide entrapping agent. However, the capacity utilized was onlyhalf of the theoretical specific capacity of sulfur. To overcome thisproblem, a synergistic approach was adopted to capture and inhibitpolysulfide dissolution by utilizing MnO2 and LiPSs within thecathode composite. Very interestingly, upon the integration ofMnO2 hollow spheres with S/KB in the presence of LiPSs, a dischargecapacity of 881 mA h g�1 was achieved after 100 cycles with acoulombic efficiency of B99%. The enhanced capacity retention ismainly attributed to the hollow structure of MnO2 and LiPS electro-lyte which not only favor the structural restriction of polysulfide butalso preferentially diffuse inwardly due to the steric hindrance.27

Also, the variation in capacity retention is ascribed to the polysul-fides added in the cathode, which ensures full material utilization,delivering maximum specific capacity. The role of Li2S8 electrolyte(LiPS) additives is clearly emphasized as follows. Firstly, dissolvedLi2S8 in LiPS electrolyte would provide a mass buffering effect,

thereby reducing the polysulfide dissolution according toLe Chatelier’s Principle. Secondly, the polysulfide additive dissolvedin the electrolyte would deposit on the cathode to compensate forthe dissolved sulfur during cycling and also prevent the formation ofLi2S.28,29 Further, as shown in Fig. 2b, the capacity retention in S/KB/MnO2 is the highest and amounts to 90.96% at the 100th cycle ascompared to a capacity retention of 58.75% without the addition ofLiPS electrolyte. This corresponds to only 0.09% capacity loss percycle in the case of S/KB/MnO2 using LiPS electrolyte. The capacityretentions of the S/KB composite electrodes with and without theuse of LiPS electrolyte is estimated to be 44.63% and 31.4%,respectively, thus re-affirming the beneficial and synergistic effectsof MnO2 and LiPS electrolyte. Further, the postmortem TEManalysis of S/KB/MnO2/LiPS after cycling shows a well retainedmorphology of MnO2, indicating the structural stability duringcharge–discharge cycling (Fig. S3, ESI†).

With the aim of investigating the structural stability andfeasibility for practical application, the rate performance ofLi–S cells made with S/KB/MnO2/LiPS composite electrodeswas evaluated (Fig. 2c and d). The discharge capacities of theS/KB/MnO2 composite with LiPS electrolyte was found to be 1035,989, 874, 694 and 426 mA h g�1 at a C rate of 0.1C, 0.2C, 0.5C, 1Cand 2C respectively. When the current density increased from0.1C to 0.2C, the discharge capacity decreased from 1035 to989 mA h g�1. The obtained specific capacities at 0.2, 0.5, 1Cand 2C correspond to 95.56, 84.46, 67.06 and 41.16% of thecapacity achieved at 0.1C. When the cells were operated at 0.1Cafter being subjected to a high C rate, it was possible to recover99.2% (1026 mA h g�1) of the original discharge capacity at0.1C, indicating the excellent rate performance of the S/KB/MnO2

composite with LiPS electrolyte. Following that, the S/KB/MnO2/LiPScell showed robust performance once again when the cell was cycledat a higher C-rate of 0.5C. At 0.5C, a capacity of 800 mA h g�1 wasachieved after 80 cycles, showing a capacity retention of 91.54%.The charge–discharge curves of S/KB/MnO2 with LiPS electrolyte(Fig. 2c) represent the typical charge–discharge profiles of theS/KB/MnO2 composite electrodes at various C-rates, showing twoplateaus during the discharge process which correspond to thetwo step reaction mechanism of Li–S batteries. The two plateausare attributed to the solid sulfur and soluble lithium polysulfide(Li2Sx, 4 r x o 8) and further reduction to insoluble Li2S2 andLi2S. Similarly, during charging, voltage plateaus appeared dueto the conversion of lithium polysulfide and lithium sulfide backto elemental sulfur.

Lastly, a UV-Vis test was conducted for 10 mM bare Li2S8

solution and also with the addition of KB and MnO2 hollowspheres. Upon the addition of KB and MnO2 hollow spheres,the Li2S8 solution changes its colour, showing the adsorption ofLi2S8 polysulfides with KB and MnO2. The extent of adsorptionis shown in Fig. 3a and is further characterized by UV-Visspectroscopy. As presented in Fig. 3b, the bare Li2S8 solutionshows minimal transmittance around 600 nm and almost0% transmittance around 400 nm, signifying the presence ofLi–polysulfides.30 The KB–Li2S8 solution displayed better butdecreased transmittance as compared to the bare Li2S8 solutionaround 600 nm and 400 nm wavelengths, revealing that KB

Fig. 2 (a) Cyclic performances of S/KB and S/KB/MnO2 composites with andwithout LiPS electrolyte at 0.2C between 3.0 V and 1.7 V. (b) Correspondingcapacity retentions of the composites after 100 cycles at 0.2C. (c) & (d) Voltage–capacity curves and the corresponding rate capabilities of the S/KB/MnO2

composite.

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particles played a minimal role in polysulfide entrapment.However, the Li2S8–MnO2 solution displayed 100% transmittancearound both 600 nm and 400 nm wavelengths, re-affirming theeffective role of MnO2 and LiPS additives in trapping the poly-sulfide and preventing the formation of Li2S respectively.

In summary, we have successfully demonstrated a uniquepairing of MnO2 and LiPS additives in S/KB composites to delivercapacity for Li–S batteries with remarkable capacity retention uponcycling. The remarkable capacity retention is mainly attributed tothe synergistic effect of the hollow structure of MnO2 and LiPSelectrolyte. The inbuilt MnO2 hollow spheres trap the polysulfidespecies effectively, while the polysulfide additive provides massbuffering to compensate for the capacity loss and prevent theformation of Li2S. We believe that this work may open a new avenueto build promising Li–S cells for sustainable energy technology.

P. R. is grateful to the Korean Federation of Science andTechnology Societies for financial support through the BrainPool Program and Dr V. K. Pillai, Director, CSIR-CECRI, for his

support and granting him the sabbatical leave. This project wassupported by the National Research Foundation of Korea (NRF)(MSIP) (No. 2017R1A2B2010148) and the Climate ChangeResearch Hub of EEWS from KAIST (Grant No. N11170059).S. A. A. and H.-W. L. acknowledge support from the Ministryof Trade, Industry & Energy/Korea Evaluation Institute ofIndustrial Technology (MOTIE/KEIT) (10067185).

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Fig. 3 (a) Photographic evidence of the colour change after addition ofKB and MnO2 hollow spheres to a 10 mM Li2S8 solution. (b) Transmittancevs. wavelength graph of the bare Li2S8 solution, KB–Li2S8 solution andMnO2–Li2S8 solution obtained from UV-Vis spectroscopy data.

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