Facile Synthesis and Very Stable Cycling of Polyvinylidene Dichloride Derived Carbon: Sulfur Composite Cathode

Abstract

Activated microporous carbon with narrow pores up to 1 nm, high surface area of about 1000 m 2 /g and pore volume of 0.43 cc/g was synthesized by facile one-step carbonization of polyvinylidene dichloride (PVDC) resin at high temperature without any additional activation process and was used for the preparation of sulfur-carbon (S/C) composite electrodes with sulfur content of 40 wt% in the composite S/C powder and 32 wt% in the composite electrode. The electrodes thus obtained, demonstrate a very stable cycling performance with more than 2000 charge-discharge cycles delivering about 600 mAh/g at a current rate of 1.04 A/g with Coulombic efficiency close to 100% at 30 • C. Stable and highly reversible behavior was also obtained at 45 • C for hundreds of cycles. Quasi-solid state type of behavior with single reduction plateau was observed for these Li-S cells using organic carbonates based electrolyte solutions. The formation of solid electrolyte interphase (SEI) on the surface of the cycled S/C electrodes was demonstrated using scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS). For the composite sulfur electrodes prepared with PVDC-derived carbon the shuttle phenomena are fully avoided due to appropriate encapsulation, surface protection and quasi-solid state operation mechanism.Rechargeable lithium/sulfur batteries have received increasing attention due to the high theoretical capacity of sulfur cathodes -1675 mAh/g, abundance, environmental friendliness, low toxicity and low cost of elemental sulfur. 1-5 Despite of these advantages, commercialization of this type of batteries is hampered by the fact that lithium polysulfides Li 2 S n (3 ≤ n ≤ 8) which are formed during sulfur reduction are soluble in the electrolyte solutions and react with the Li anode. 6 This parasitic process known as the shuttle phenomenon, leads to a fast capacity fading and low Coulombic efficiency of Li-S cells. Many efforts were made to mitigate the shuttle process in these systems such as encapsulation of sulfur into the porous carbon matrices, 7-9 using solutions with LiNO 3 as an additive in which Li anodes develop effective passivation 10-11 and the use of ionic liquid based electrolytes which suppress the solubility of polysulfide species. 12-14 Another approach is the use of a conductive-polymer coating on the exterior of the sulfur cathodes 15 or the addition of a porous carbon-based interlayer between the separator and the cathode to prevent the migration of polysulfide species to Li anode. 16 In order to enhance the stability of sulfur cathodes core-shell structures of the active mass were developed, in which sulfur cores are encapsulated by ceramic, Li-ion conductive shells. 17 Adsorption or chemisorption of polysulfide anions by polar hydrophilic cathode substrates such as metal oxides, 18-21 silica, 22 2D MXene conductive nanosheets 23 were also proposed for preventing the dissolution of lithium polysulfides int...

Nanostructured Sulfur and Sulfides for Advanced Lithium/Sulfur Cells

“…To explain the absence of this discharge voltage plateau, the authors proposed a hypothesis that the Gibbs free energy of the reaction between sulfur and lithium ions could change because sulfur in the micropores could be mixed with carbon at the atomic level rather than as a form of sulfur (S 8 ). Similar discharge behavior of sulfur-microporous composite electrodes in organic liquid electrolyte [36][37][38][39][40] has been reported by other workers with excellent cell cycling durability demonstrated.…”

“…Since the use of CMK-3 [6] and other mesoporous carbons [28][29][30][31] made a significant advance in improving specific capacity and durability of the sulfur electrode, researchers' attention has moved to carbons with different pore-sizes (e. g. microporous [32][33][34][35][36][37][38][39][40] and macroporous carbon [41,42] ). In 2010, Zhang et al demonstrated a specific capacity of~890 mAh/gS at 200 mA/gS using a sulfur-microporous carbon composite with a narrow pore size distribution of carbon (~0.7 nm).…”

Review—On the Mechanism of Quasi-Solid-State Lithiation of Sulfur Encapsulated in Microporous Carbons: Is the Existence of Small Sulfur Molecules Necessary?

“…This type of behavior schematically presented in Fig. 1b is generally observed for organic carbonatebased electrolyte solutions, [13][14][15][16][17][18][19][20][21][22]24,25,27,31,33,35,36,39,40 as well as for bis(fluorosulfonyl)imide (FSI) anion-based ionic liquid electrolyte solutions. 28,37,38 For this type of the operation of Li-S cells voltage profiles differ from those commonly measured for conventioanal Li-S composite electrodes and are characterized by a single galvanostatic charge/discharge voltage plateau observed around 2.0 V vs. Li/Li + .…”

“…18,[42][43][44] In the case of liquid electrolytes Wang et al proposed that this type of unique behavior (namely, the single plateau observation) relates to the reaction of Li ions after desolvation with sulfur encapsulated in the micropores under solvent deficient conditions and called this mechanism "quasi-solid-state reaction". 15,45 In most cases this mechanism is observed for composite S/C electrodes with very narrow micropores which width is lower than 1 nm, [14][15][16]19,20,22,23,28,40 but in some cases this type of operation was described also for larger pore size. 13,29,[37][38][39] Several explanations for this single-plateau behavior can be found in the literature.…”

“…The realization of quasi-solid-state reaction is governed by the formation of solid electrolyte interphase (SEI) on the surface of sulfur cathodes during the initial discharge. 29,[38][39][40] Surface films when formed, force desolvation of Li ions that migrate through them, and hence, induce possible quasi-solid-state operation of the cells. Voltage hysteresis and irreversible capacity observed during the first cycles are the result of side reactions of electrolyte solution components with Li 2 S n leading to SEI formation on the surface of S/C composite electrodes, what prevents further irreversible reactions.…”