Polymeric Selenium Sulfides As Promising Cathode Materials for High-Energy Lithium Batteries
Authors: Dong Panpan , Han Kee Sung , Lee Jung-In , Zhang Xiahui , Cha Younghwan , Song Min-Kyu 
Published: February 2020

Lithium/sulfur (Li/S) batteries have been widely studied for high-energy applications during the past few decades owing to the high theoretical specific capacity (1675 mAh g-1), low cost, natural abundance and environmentally benign nature of sulfur.1 However, the unceasing dissolution/diffusion (shuttle effects) of reaction intermediates (polysulfides, S
x

2-, 4 ≤ x ≤ 8) during battery operation and the insulating nature of sulfur lead to low Coulombic efficiency (CE), short cycle life and poor rate capability, critically limiting the commercial deployment of Li/S batteries. To address these issues, extensive efforts have been conducted by utilizing physical or chemical confinement of S as well as polysulfides within conductive hosts.2 Recently, chemically stable S-rich copolymers were reported as novel active materials for Li/S batteries, exhibiting effective suppression of the shuttle effects of polysulfides within cathodes.3, 4 However, the poor electrical conductivity of polymeric materials leads to lower utilization of sulfur in copolymer matrix as well as unsatisfactory rate capability of Li/S batteries.5

Selenium (Se), as a congener of S, has been reported as an alternative cathode for Li batteries due to its higher electronic conductivity (Se: 1 × 10−3 S m−1 vs. S: 5 × 10−28 S m−1) and comparable theoretical volumetric capacity (Se: 3,253 Ah L−1­ vs. S: 3,467 Ah L−1) than sulfur.6 Herein, to combine the high conductivity of Se and excellent confinement capability of S-rich copolymers, we prepared polymeric selenium-sulfides, utilizing chemical bonds between S, Se and C atoms, by the inverse vulcanization method with S, SeS2 and 1,3-diisopropenylbenzene (DIB) as comonomers (noted as poly(S-SeS­2-DIB)). We first investigated the effects of chemical incorporation of Se in S-rich copolymers on the cycling performance of Li/S batteries. To further enhance the electrochemical utilization and rate capability of poly(S-SeS­2-DIB), porous carbon (KetjenBlack600, KB600) network was utilized as the conductive host within copolymers. Among all the copolymer composite samples that were systematically investigated, the poly(S-SeS­2-DIB) with an optimal S/SeS2 mass ratio of 9:1 exhibited the highest initial specific capacity (1218 mAh g−1 at 100 mA g−1) and superior rate capability (537 mAh g−1 at 2000 mA g−1). The novel molecular structures of poly(S-SeS­2-DIB) composites were revealed by 13C cross polarization and magic angle spinning nuclear magnetic resonance spectra as well as X-ray photoelectron spectroscopy. The effects of SeS2 contents in poly(S-SeS­2-DIB) on the electrochemical properties of Li/S batteries such as rate capability and CE were further analyzed by combined electrochemical and microstructural characterization methods.
Figure 1. (a) The molecular structures of DIB, S copolymer (noted as PS10) and S-SeS2 copolymer (noted as PSS90-10); (b) Cycling performances of PS10, PSS90-10 and PSS90-10@KB600 cathodes under the current density of 500 mA g–1; (c) Rate capabilities of poly(S-SeS2-DIB) composites with different contents of SeS2 under various current densities.

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