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The solid components deposited in sulfur cathode during cycling for Li-S battery is studied in this work. ROLi, HCO 2 Li, Li x SO y and Li 2 S (or Li 2 S 2 ) are proved to be the main components by the methods of Fourier transform infrared (FTIR), Raman spectra and X-ray photoelectron spectroscopy (XPS). ROLi and HCO 2 Li are solvent degradation products existed in electrolyte. The reversibility of Li 2 S and Li 2 S 2 are not serious as in previous reports. ROLi, HCO 2 Li and Li x SO y co-deposited with Li 2 S or Li 2 S 2 in discharge process lead to the cathodes performance deterioration. Lithium salts such as LiNO 3 and LiTFSI can oxidize sulfur compounds to higher oxidation states, and Li x SO y species increased with cycling indicates the active mass irreversible oxidation that may be another important reason for the capacity fading of Li-S battery.Rechargeable Li-S battery possesses more advantages over the conventional lithium ion battery, but the practical use faces with a variety of problems such as serious capacity fading and poor cycle performance. The most acceptable electrode reactions of the sulfur cathode include multistep redox reactions. During discharge process, elemental sulfur in solid phase S 8 (s) is firstly dissolved in electrolyte as S 8 (l) , and then reduced to lithium polysulfide gradually. Intermediate products of high ordered lithium polysulfide (Li 2 S n , 3 ≤ n ≤ 8) are soluble in electrolyte, but low ordered lithium polysulfide (Li 2 S 2 and Li 2 S) are insoluble. Different from conventional lithium ion battery, electrolyte just takes the role of Li + transfer medium. In Li-S system, large amount of electrochemical products dissolve in electrolyte and make it difficult to isolate lithium anode from active mass absolutely. Thus, capacity fading mechanisms of Li-S battery should be considered from different aspects.Firstly, lithium anode reacted with high ordered lithium polysulfide (Li 2 S n , 3 ≤ n ≤ 8) described as the shuttle phenomenon in electrolyte resulting in active mass diminishing. At the same time, metallic lithium reacted with solvent causing electrolyte degradation. All of these extra reactions could lead to capacity fading. 1-3 Great efforts had been made to study the electrolyte, both the organic solvent and the lithium salts how to react with lithium anode. Various inorganic and organic lithium compounds such as Li 2 CO 3 , Li 2 O, LiOH, LiF, alkyl carbonate species (ROCO 2 Li), alkoxy species ROLi, and alkyl carboxylate species RCOOLi were reportedly detected in lithium surface film. 4-10 Mikhaylik et al. argued that in Li-S battery the most common solvent 1,2-dimethoxyethane (DME) and 1,3-dioxolane (DOL) decomposed into lithium 2-methoxyethoxide. 11 Aurbach 12 described all the possible reduction processes and the various functional groups appearing in the lithium surface films formed in DOL/LiTFSI, Li 2 S n , and LiNO 3 solutions. It was proved that the formation of insoluble Li x NO y species and Li x SO y species could passivate the Li electrodes and thus prevent...

In this paper, the qualitative and semi-quantitative analysis of polysulfide species dissolved in electrolyte during the dischargecharge process and cycling of Li-S batteries was reported. ICP-OES (Inductively coupled plasma-Optical emission spectrometer) measurement was used to estimate the total sulfur content dissolved in electrolyte. Lithium polysulfide with different order were separated and confirmed by LC-MS (Liquid chromatography coupled with mass spectrometry). Li 2 S 4 and Li 2 S 6 were proved to be the most stable form of lithium polysulfide species. At the end of discharge process, total sulfur content in the form of Li 2 S 4 and Li 2 S 6 remained in electrolyte was about 20% of the active material in initial cathode. At the end of charge process, 45% total sulfur content was preserved in electrolyte mainly in the form of Li 2 S 6 . The partial transformation of active material from liquid phase to solid phase resulted in relatively low practical specific capacity than the theoretical. In cycles, active material transferred between liquid and solid phase kept a balance, and the content of total sulfur, Li 2 S 4 and Li 2 S 6 were changed slightly at the end. Consequently, polysulfide dissolved in electrolyte just took limited responsibility for the capacity fading with cycles of Li-S batteries.