Journal of The Electrochemical Society, 166 (8) A1355-A1362 (2019) A1355 0013-4651/2019/166(8)/A1355/8/$38.00 © The Electrochemical Society MnO 2 -Coated Sulfur-Filled Hollow Carbon Nanosphere-Based Cathode Materials for Enhancing Electrochemical Performance of Li-S Cells Zheng Yue, 1 Hamza Dunya, 1 Kamil Kucuk, 2, * Shankar Aryal, 2, * Qiang Ma, 1 Stoichko Antonov, 3 Maziar Ashuri, 3, * Bader Alabbad, 3 Yiwei Lin, 1 Carlo U. Segre, 2, ** and Braja K. Mandal 1, z 1 Department of Chemistry, Illinois Institute of Technology, Chicago, Illinois 60616, USA 2 Department of Physics & CSRRI, Illinois Institute of Technology, Chicago, Illinois 60616, USA 3 Department of Mechanical, Material and Aerospace Engineering, Illinois Institute of Technology, Chicago, Illinois 60616, USA When comes to very high energy density energy storage systems, the prospect of lithium-sulfur battery (LSB) technology is very promising. The major problem that prevents the commercial production of LSBs is their poor cycling life caused by the migration of polysulfide intermediates from the cathode structure to the anode. Confining sulfur and the discharged polysulfide intermediates inside conductive porous carbon matrices is regarded as a promising and effective strategy to overcome this problem. In this study, a new class of MnO 2 -coated sulfur-filled hollow carbon nanosphere (HCN)-based cathode materials has been prepared to improve the electrochemical performance of Li-S Cells. The cathode materials constructed with silica nanoparticle hard templates displayed a high initial capacity of 834 mAh g -1 with a capacity fading rate of 0.080% per cycle. This material also showed superior redox kinetics, which enabled the Li-S cells to run at a higher charging-discharging current density. © 2019 The Electrochemical Society. [DOI: 10.1149/2.0321908jes] Manuscript submitted February 19, 2019; revised manuscript received April 12, 2019. Published April 23, 2019. In the past decade, the lithium-sulfur battery (LSB) chemistry has been extensively investigated because of high theoretical specific ca- pacity (1675 mAh g -1 ) for the sulfur cathode and energy density (2500 Wh kg -1 based on Li metal anode) 1–4 . Despite the signifi- cant progress of LSBs, 5–7 their commercialization is still hampered by several challenges: the insulating nature of sulfur, the great volume change (up to 80%) of S cathodes during discharge (lithiation) and the polysulfide shuttle (PSS) effect—which lead to rapid capacity fading, insufficient sulfur utilization, and low Coulombic efficiency. 8 The most widely applied strategy to address these challenges is the use of high specific surface area (SSA) and high porosity carbona- ceous materials as the sulfur host material. This strategy provides good electrical conductivity, manages the volume expansion using the void spaces in the host matrix, and suppresses the migration of polysulfide (PS) intermediates by physical entrapment. 4,8–10 Notable carbonaceous materials include carbon black, 11–14 graphene, 15,16 car- bon nanowires/nanotubes 17 and hollow nanospheres. 18–20 However, the physical adsorption of PS intermediates by the carbonaceous ma- terials is not sufficient to completely eliminate the PSS effect, be- cause of the weak interactions between the polar S x 2- anions and the nonpolar carbon matrix. 21 Consequently, various chemical adsorption strategies have been explored to further improve the long-term cycling performance. Two of the most effective approaches include doping of the carbon matrix with heteroatoms, such as N, 22 B 23 and P, 24,25 and coating an inorganic metal-oxide layer 26 on the surface of the car- bon host materials, including TiO 2 , 27 Fe 3 O 4 , 28 MoO 2 29,30 and alucone (aluminum oxide polymers). 31 The polysulfide trapping effect of N-doped carbon through the co- ordination interactions between polysulfides and N atoms has been examined in various carbon materials. Qiu et al. reported a cathode design based on N-doped graphene, which performed 2000 cycles with an extremely low capacity decay rate (0.028% per cycle). They also re- vealed by ab initio calculations that N-doped graphene possesses much stronger binding energy to Li 2 S x species than the primitive graphene. 21 Song et al. reported sulfur binding mechanism in a N-doped meso- porous carbon cathode by density functional theory calculations. 12 Furthermore, Sun et al. demonstrated that a proper amount of N-doping also improves the conductivity of the carbon material. 22 * Electrochemical Society Student Member. ** Electrochemical Society Member. z E-mail: mandal@iit.edu Among the metal-oxides for cathode coating, MnO 2 is the most extensively investigated because of its low cost and ease of deposition in the nanoscale. 26,32,33 Most importantly, MnO 2 has been proven to suppress PS diffusion by strong polysulfide binding through chemical interactions 34,35 and by the fast catalytic effect to polysulfide redox reactions through the conversion of soluble long-chain polysulfide in- termediates to insoluble Li 2 S. 36 Based on the combination of nanos- tructured carbon materials and MnO 2 coating, Li-S cells with im- proved performance have been developed. 34–38 For example, Lou et al. reported a sulfur cathode based on hollow carbon nanofibers filled with MnO 2 nanosheets, which displayed an initial capacity of 890 mAh g -1 and ∼74% retention after 300 cycles at 0.5C. 37 In this study, we report the performance of a new cathode ma- terial, S@HCN@MnO 2 , in which MnO 2 is coated over sulfur-filled hollow carbon nanospheres (HCNs). The MnO 2 layer was synthe- sized through the in-situ reduction of KMnO 4 . HCNs were derived from polyaniline-polypyrrole (PANi-PPy) nanoparticles as the pre- cursor, which were prepared by the oxidative polymerization of the monomers in an aqueous solution in the presence of a surfactant, Triton X-100. To further increase the SSA and pore volume of the cathode material, we applied SiO 2 nanoparticle impregnation in the PANi-PPy precursor. The SiO 2 was subsequently removed by HF etch- ing after the carbonization step. The modified cathode with increased porosity not only displayed higher discharging capacity, but also of- fered much improved redox kinetics that enabled the Li-S cells to run at a higher charging-discharging current density. Experimental Materials.—Trimethoxy(octadecyl)silane (C 18 -silane) (90%), ammonium persulfate (APS) (98+%), pyrrole (98%), aniline (99.5+%), Triton X-100, SiO 2 nanopowder (10-20 nm particle size), HF solution (48%) were purchased from Sigma Aldrich Chem- ical Co., Ltd (USA). Anhydrous ethanol (94-96%), elemental S (sublimed, 99.5+%), tetraethoxysilane (TEOS) (99.9%), N-methyl- 2-pyrrolidone (NMP) and KMnO 4 (99.5+%) were purchased from Alfa Aesar (USA). All chemicals were used as received with no fur- ther purification. Synthesis of SiO 2 @ PANi-PPy and PANi-PPy nanoparticles.— SiO 2 @PANi-PPy and PANi-PPy nanoparticles were synthesized by copolymerization of aniline and pyrrole monomers, with APS as the oxidant, SiO 2 nanopowder with 10–20 nm average diameter as the