Hierarchically Porous and Heteroatom Doped Carbon Host Matrix for Long Life Li/S Battery: A Mechanistic Investigation By in-Situ Raman Spectroscopy and Ex-Situ XPS

Lithium-sulfur (Li/S) batteries are considered as one of the promising rechargeable battery technology due to their low cost, ecofriendly and high theoretical energy density (~2500 Whkg -1 ). 1, 2 The high theoretical specific capacity (~1672 mAh g -1 ) of Li/S battery is the result of reversible two electron conversion reactions between sulfur and lithium (S 8 + 16Li + + 16e − ↔ 8Li 2 S) at an average voltage of 2.1 V with respect to Li/Li + and finally form solid lithium sulfide (Li 2 S) through a series of intermediate liquid polysulfides. 2, 3 The physiochemical properties of the carbon host matrix and their sulfur loadings at the cathode side play a major role in the electrochemical performance of lithium-sulfur (Li/S) batteries. We have designed a highly sulfur loaded (~ 75 wt. %) carbon matrix (S/NGC), with hierarchically organized micro/meso pore structures and containing nitrogen and oxygen functional groups, using metal oxide nano templates. The S/NGC electrodes give the reversible capacities of 868 and 666 mAh g -1 at C/5 current rates, with the sulfur loadings of 2.2 and 3.4 mg Sulfur cm -2 , respectively. Based on the advantages of the hierarchical porous structure and nitrogen doping, S/NGC electrode gives a long cycling stability (0.03% capacity decay per cycle up to 1000 cycles) and very good coulombic efficiency (Fig. 1a). Further, the charge/discharge mechanism of the cell is investigated in detail by insitu Raman spectroscopy (Fig. 1b) and ex-situ X-ray photoelectron spectroscopy (XPS). In-situ Raman spectroscopy analysis of the Li/S cell shows the formation of short chain polysulfides (S 3 .- , Li 2 S x , x = 4-1) already at higher discharge potentials (> 2 V).The presence of nitrogen on carbon support is found to enhance the bonding between sulfur and oxygen functional groups present in the carbon support, which is supposed to take a major role in the prevention of sulfur/polysulfides species dissolution to anode side. References 1. Y. V. Mikhaylik, I. Kovalev, R. Schock, K. Kumaresan, J. Xu and J. Affinito, ECS Transactions , 2010, 25, 23-34. 2. L. F. Nazar, M. Cuisinier and Q. Pang, MRS Bulletin , 2014, 39, 436-442. 3. P. G. Bruce, S. A. Freunberger, L. J. Hardwick and J.-M. Tarascon, Nat Mater , 2012, 11, 19-29. Figure 1