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In this work we analyzed the phenomenon of quasi solid state (QSS) lithiation of sulfur-carbon (S/C) composite electrodes with sulfur confined in the micropores of carbon matrices based on our recent studies and data published in literature. We demonstrated that the existence of sulfur in the form of small molecules is not a necessary condition for the realization of QSS mechanism. QSS operation behavior was demonstrated both for carbons with small up to 1nm micropores and for carbons with larger pore size up to 2-3 nm. A key role in the operation of S/C electrodes via a QSS mechanism plays surface electrolyte interphase (SEI) which is formed on the surface of S/C composite during the initial discharge. The formation of SEI was supported by X-ray photoelectron spectroscopy and by scanning electron microscopy. Small pore size (up to 1 nm) of the carbon matrices has a positive effect on the cycling of S/C electrodes. A superior cycling performance for more than 3500 charge-discharge cycles was demonstrated for S/C composite electrodes based on carbons synthesized by carbonization of polyvinylidene dichloride (PVDC) resin. In recent years lithium-sulfur batteries have been the subject of very intensive research due to the high theoretical specific capacity of sulfur cathodes (1672 mA h g −1 ) which is an order of magnitude higher than that of lithiated transition-metal oxides and phosphates cathode materials used in commercial Li-ion batteries (140-200 mA h g −1 ). [1][2][3][4][5][6] This high capacity relates to the ability of sulfur atoms to accept two electrons resulting in the conversion of elemental sulfur to lithium sulfide (Li 2 S). Furthermore, sulfur is naturally abundant, environmentally friendly and relatively cheap.Due to the low electrical conductivity of elementary sulfur the addition of conductive additives or the use of conductive host materials it is necessary to ensure good performance of sulfur electrodes. Besides, during the discharge process elemental sulfur S 8 accepts electrons to give a chain of electroactive Li-polysulfides (Fig. 1a). The long-chain polysulfides Li 2 S n (4 ≤ n ≤ 8) are soluble in commonly used ethereal solvents and diffuse freely throughout the cell to the anode side where they are chemically reduced. This phenomenon known as the shuttle effect presents one of the main problems of Li-S cells. 7 The shuttle reactions prevent the possibility of extracting the full capacity of sulfur cathodes and lead to low Coulombic efficiency. Encapsulation of sulfur within activated carbons with high pores volume is one of the most effective approaches to mitigate the detrimental shuttle mechanism and stabilize composite sulfur cathodes during prolong cycling. 8-10The typical cyclic voltammetry and galvanostatic charge-discharge curves of the Li-S cell with C/S encapsulated cathode are shown in Fig. 1a. Two peaks observed in the cathodic voltammetric response of sulfur electrodes correspond to two plateaus observed in the voltage profiles of the discharge processes of these cathodes upon ...

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We report on a rigorous comparative study of nano-and microparticles of LiMn 1.5 Ni 0.5 O 4 spinel as cathode materials for Li-ion batteries. The stability of these materials in LiPF 6 /alkyl carbonate solutions in temperatures up to 70°C was explored. Capacity, cycling, rate capabilities, and impedance behavior were also studied. The methods included X-ray diffraction, Raman, X-ray photelectron, Fourier transform infrared, and electron paramagnetic resonance spectroscopies, and electron microscopy, in conjunction with standard electrochemical techniques: voltammetry, chronopotentiometry, and impedance spectroscopy. These materials show an impressive stability in solutions at elevated temperature. The use of nanomaterials was advantageous for obtaining a better rate capability of LiMn 1.5 Ni 0.5 O 4 electrodes. LiMn 1.5 Ni 0.5 O 4 particles develop a unique surface chemistry in solutions that passivates and protects them from detrimental interactions with solution species at elevated temperatures.

a Composite sulfur-carbon electrodes were prepared by encapsulating sulfur into the micropores of highly disordered microporous carbon with micrometer-sized particles. The galvanostatic cycling performance of the obtained electrodes was studied in 0.5M Li bis(fluorosulfonyl)imide (FSI) in methylpropyl pyrrolidinium (MPP) FSI ionic-liquid (IL) electrolyte solution. We demonstrated that the performance of Li-S cells is governed by the formation of solid electrolyte interphase (SEI) during the initial discharge at potentials lower than 1.5V vs. Li/Li + . Subsequent galvanostatic cycling is characterized by one plateau voltage profile specific to quasi-solid-state reaction of Li ions with sulfur encapsulated in the micropores in solvent deficient conditions. The stability of the SEI thus formed, is critically important for the effective desolvation of Li ions participating in quasi-solid-state reactions. We proved that realization of the quasi-solid-state mechanism is controlled not by the porous structure of the carbon host but rather by the nature of the electrolyte solution composition and the discharge cut off voltage value. The cycling behavior of these cathodes is highly dependent on sulfur loading. The best performance can be achieved with electrodes in which the sulfur loading was 60% by weight, when sulfur filled micropores are not accessible for N2 molecules according to gas adsorption isotherm data. A limited contact of the confined sulfur with the electrolyte solution results in the highest reversible capacity and initial Coulombic efficiency. This insight into the mechanism provides a new approach in the development of new electrolyte solutions and additives for Li-S cells.

We report on the highly stable lithium metal|LiNiCoMnO (NCM 622) cells with practical electrodes' loading of 3.3 mA h g, which can undergo many hundreds of stable cycles, demonstrating high rate capability. A key issue was the use of fluoroethylene carbonate (FEC)-based electrolyte solutions (1 M LiPF in FEC/dimethyl carbonate). Li|NCM 622 cells can be cycled at 1.5 mA cm for more than 600 cycles, whereas symmetric Li|Li cells demonstrate stable performance for more than 1000 cycles even at higher areal capacity and current density. We attribute the excellent performance of both Li|NCM and Li|Li cells to the formation of a stable and efficient solid electrolyte interphase (SEI) on the surface of the Li metal electrodes cycled in FEC-based electrolyte solutions. The composition of the SEI on the Li and the NCM electrodes is analyzed by X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. A drastic capacity fading of Li|NCM cells is observed, followed by spontaneous capacity recovery during prolonged cycling. This phenomenon depends on the current density and the amount of the electrolyte solution and relates to kinetic limitations because of SEI formation on the Li anodes in the FEC-based electrolyte solution.

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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...