Electronic Supplementary Information Sulfur-Infiltrated Porous Carbon Microspheres with Controllable Multi-Modal Pore Size Distribution for High Energy Lithium- Sulfur Batteries Cunyu Zhao, a Lianjun Liu, a Huilei Zhao, a Andy Krall, a Zhenhai Wen, a Junhong Chen, a Patrick Hurley, b Junwei Jiang, b and Ying Li a * a Mechanical Engineering Department, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin, 53211, USA b Johnson Controls Inc., Milwaukee, Wisconsin, 53209, USA *Corresponding Author Email: liying@uwm.edu 1
Scheme 1. Schematic of experimental setup and process for synthesizing porous carbon/sulfur microspheres. 2
(d) Figure S1. SEM image (a), and X-ray elemental mapping of PMC/S-40: (b) Carbon element, (c) Sulfur element, and (d) EDX analysis of the surface of PMC/S-40 (To obtain clear SEM images, 4 nm thick iridium was coated on PMC/S-40). 3
(a) (b) (c) Figure S2. SEM image of the cross-section of PMC/S-40 (a), and X-ray elemental mapping images of the cross section of PMC/S-40: (b) carbon element, (c) sulfur element. 4
Figure S3. Thermogravimetric analysis (TGA) of PMC/S samples. The sulfur content for PMC/S-40, PMC/S-40:10, and PMC/S-10 is 60.5%, 63.9%, and 63.2%, respectively. 5
(a) (b) (c) Figure S4. t-plot of PMC: (a) PMC-40 (b) PMC-40:10 and (c) PMC-10. Note: Harkins-Jura equation was used as the thickness equation. The density conversion factor is 0.0015468, and the thickness range is 3.5 Å~5.0 Å. Fig. S4 shows the t-plot results for PMC. The t-plot method is attributed to Lippens and deboer. Harkins and Jura equation is used in most applications to calculate the thickness of adsorbed gas t, as a function of nitrogen relative pressure. For multi-molecular adsorption, the experimental points should fall in a straight line and pass through the origin for a non-porous material if plotting the volume of nitrogen adsorbed at different P/P 0 values as a function of t value. For porous material, the line will have a positive intercept indicating micropores existence. The micropore volume can be calculated as: V micropore =Intercept value/0.001547. The slope of this line can be used to calculate the external pores surface area (mesopores and macropores). S external =15.47 slope value. The micropore surface area can be calculated as: S micropore =S BET -S external. 6
Figure S5. Cyclic voltammetry curves of PMC/S-40 between 1.5 V and 2.8 V recorded at a potential sweep rate of 0.1 mv s -1. Fig. S5 shows the cyclic voltammetry (CV) curves of PMC/S-40 in the potential range of 1.5-2.8 V with a 0.1 mv s -1 scan rate. Two reduction peaks positioned around 2.3 V and 2.0 V were observed during the cathodic scan, attributing to a two-step reduction of sulfur. The first step around 2.3 V is ascribed to the elemental sulfur conversion to lithium polysulfides (Li 2 S x, 4 < x < 8). The second step around 2.0 V corresponds to the conversion of polysulfides to Li 2 S 2 and then to Li 2 S. One oxidation peaks around 2.5 V was observed during anodic scan, which corresponds to the oxidation of Li 2 S and Li 2 S 2 to LiS 8. 1-3 In the following scan cycles, both cathodic and anodic peaks are positively shifted, which can be ascribed to the polarization of the electrode materials in the first cycle. 4 From 2nd scan to 5th scan cycle, there is no obvious change for redox peak currents and potentials, which indicates that the electrode material is with good reactive reversibility and cycling stability. The CV results also indicate PMC/S matrix can effectively restrain the dissolution of the lithium polysulfides in the organic electrolyte during charge-discharge process. 7
Figure S6. Electrochemical impedance spectroscopy (EIS) curves of PMC/S-40, PMC/S- 40:10, and PMC-10 before initial discharge. Electrochemical impedance spectroscopy (EIS) was utilized to investigate the differences in the composite materials of PMC/S. Fig. S6 shows that all of the Nyquist plots of the PMC/S cathodes are composed of a semicircle at high frequencies due to the contact and charge transfer resistance, with a short inclined line in low frequency regions corresponding to the ion diffusion within the cathode. The much smaller semicircle of PMC/S-40:10 indicates a lower charge transfer resistance, which is assumed to be attributed with the interconnected porous carbon skeleton with bridging small mesopores in between the large mesopores/macropores. References 1. Ding, B.; Yuan, C. Z.; Shen, L. F.; Xu, G. Y.; Nie, P.; Zhang, X. G. Chem. Eur. J. 2013, 19, (3), 1013-1019. 2. Elazari, R.; Salitra, G.; Garsuch, A.; Panchenko, A.; Aurbach, D. Adv. Mater. 2012, 23, (47), 5641-5644. 3. Ji, X. L.; Lee, K. T.; Nazar, L. F. Nature Mater. 2009, 8, (6), 500-506. 4. Wang, Y. X.; Huang, L.; Sun, L. C.; Xie, S. Y.; Xu, G. L.; Chen, S. R.; Xu, Y. F.; Li, J. T.; Chou, S. L.; Dou, S. X.; Sun, S. G. Journal of Materials Chemistry 2012, 22, (11), 4744-4750. 8