Polysulfide solution effects on LiS batteries performances G. Tarquini a , A. Dell'Era b, ⁎ , P.P. Prosini a , F.A. Scaramuzzo b , C. Lupi c , M. Pasquali b a ENEA, Energy and Sustainable Economic Development, Casaccia Research, Centre, Via Anguillarese 301, 00123 Santa Maria di Galeria, Rome, Italy b Department of Basic and ApplScifor Engineering (SBAI), Sapienza University of Rome, Via del Castro Laurenziano 7, I-00161 Roma, Italy c Department Chemical Engineering Materials Environment DICMA, University Sapienza Rome, Via Eudossiana 18, 00184 Rome, Italy ABSTRACT ARTICLE INFO Article history: Received 12 February 2020 Received in revised form 30 April 2020 Accepted 7 May 2020 Available online 19 May 2020 Recently, rechargeable LiS batteries, a next-generation energy storage system, are deeply studied due to their theoret- ical specific energy density. However, to produce batteries comparable to those already available on the market some drawbacks must be overcome, including essentially self-discharge, high internal resistance and rapid capacity fading upon cycling. In this work the use of polysulfide solutions either as additives or as active material in LiS batteries is proposed. The addition of polysulfides to the electrolytic solution improves the cell performances in terms of specific capacity and coulombic efficiency, passing from a capacity of about 150 mAh/g with a coulombic efficiency of about 0.85 to a capacity of 600 mAh/g with a coulombic efficiency of about 0.99 after 10 cycles. In batteries where polysul- fide solutions are used as cathodic material, the obtained performances are even higher, reaching specific capacities of 420–450 mAh/g after about 70 cycles. Moreover, the cells tested with carbon paper as electrode support shows a greater reversibility, with a coulombic efficiency very close to 1. Finally, reducing the potential window from 3 to 1.5 V to 2.8–1.7 V, the cells show high stability and efficiency, reaching specific capacity values of about 600 mAh/g after 200 cycles. Keywords: Lithium-sulfur battery Polysulfide solutions Hybrid semi-flow batteries 1. Introduction In the post-Li-ion batteries era, looking for cathodes with always higher capacity values, sulfur represent one of the main objective of in-depth study as electrode material [1]. Indeed, the conversion reaction from S to Li 2 S, in- volving 2 mol of electrons per mole of sulfur, shows a specific theoretical capacity of 1675 mAh/g. LiS batteries have a theoretical specific energy of 2600 Wh/kg. Even if it is practically possible to obtain specific energy performances of 500–600 Wh/kg, this value would still be greater than the performances offered by the lithium-ion currently on the market [2]. Among the various problems related to the sulfur use as a cell active mate- rial, some are intrinsically associated to the sulfur nature whose electrical conductivity is extremely low, others are instead connected to the sulfur use in presence of lithium. The polysulfides formed during cell operations can migrate from the cathode to the anode, through a shuttle-like mecha- nism, causing a series of parasitic reactions. The conversion from sulfur to lithium sulfide also involves morphological and structural changes. In fact, the transformation from sulfur to sulfide causes a significant increase in volume; upon battery cycling, such increases and decreases in volume de- termine the crushing of the electrode and the consequent contact loss. The shuttle effect strongly limits the Coulombic efficiency and the battery func- tioning over time; on the other hand, the solubility of the polysulfides in the electrolyte allows a more efficient use of the sulfur. Conventional sulfur electrodes with non-nanometric particles often show very low capacitance values, because the formation of polysulfides occurs only at the interface with the electrolyte, while most of the sulfur is inactive. To address these issues, in recent years, different strategies have been proposed, all aiming to obtain cathode materials with high performances. Numerous suggestions for developing the “lithium‑sulfur” technology consist in tuning the chemical-physical properties and morphology of sulfur. First of all, the aim was to reduce particle size by synthesizing nanometric sulfur. An alter- native approach is the use of nanocomposites with different components like carbon [3–11] or conductive polymers, besides sulfur. This approach intends to increase both the amount of electrochemically active sulfur and the conductivity of the cathode, by forming networks of conductive material in intimate contact with the sulfur particles [4–7]. The addition of conductive materials obviously limits the sulfur content in the nanocomposite, reducing the sulfur load in the cathode itself. There- fore, current research aims to achieve the necessary balance between sulfur load and battery performance [12]. To reduce the concentration of polysulfides in the electrolyte, so as to limit the parasitic reactions, many porous trapping materials have been proposed, minimizing the diffusion of the polysulfides produced outside the cathode [6].Therefore the predom- inant strategy involves the encapsulation of sulfur in conductive and porous structures, that allow the electrolyte reaching the most sulfur possible amount. Thus, trapping the produced polysulfides into conductive network Journal of Electroanalytical Chemistry 870 (2020) 114239 ⁎ Corresponding author. E-mail address: alessandro.dellera@uniroma1.it. (A. Dell'Era). http://dx.doi.org/10.1016/j.jelechem.2020.114239 1572-6657/© 2020 Elsevier B.V. All rights reserved. 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