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By contrast with LIB， SPEs due to their physical nature do not display polysulfide shuttling， but exhibit poor ionic conductivities at room temperature， making them unattractive for Li-S batteries ［37］．
A conventional liquid electrolyte uses lithium bis （ trifluoromethyl sulfonyl） imide （LiTFSI） dissolved in ethers， such as dimethoxyethane （DME） and 1，3-dioxolane （DOL） ［38］． All essential liquid electrolytes share the following characteristics： a low lattice energy （ LLE） lithium salt， a solvent with high relative permittivity， and a strong interaction between the solvent and lithium ions ［39］［40］． However， two major problems exist in all currently used liquid electrolytes： （i） dissolution of polysulfide （PS） intermediates， and （ii） depletion and oxidation of polysulfides through side reactions with the electrolyte； both issues lead to rapid capacity fading ［16］．
For a practical Li-S battery electrolyte， the following criteria must be met：
•Ionic conductivity in excess of 10 mS ／ cm at temperatures ranging from－ 30℃ to ＋ 60℃ ．
•Electrochemical stability of at least 4V．
•Complete inhibition of the PSS， allowing for 1，000 cycles minimum with very high capacity retention．
•Non-flammable for safety and cost reduction．
•Lithium ion transference number at least 0. 6 （ essential for high rate capability） ［41］．
Up to now， ethers， sulfones， and fluorinated ethers are the three categories of preferred organic solvents applied in the electrolyte for Li-S batteries．However， a single solvent is rarely used because it cannot meet all of the requirements for battery electrolytes． As for binary or trinary solvent mixtures，different components play specific roles and are complement with each other．
It is well known that lithium cells with carbonate only electrolytes always show notable capacity fading and voltage decline when they work at extremely low temperatures （below 243 K）， due to the poor ionic conductivity of electrolyte and low diffusion coefficient of lithium ions in the SEI layer． More strategies should be developed， on the road to achieving electrolytes with wide operating temperatures， such as decreasing EC content and adding co-solvents with low melting point and high boiling point ［42］． After modeling and calculating， we have found that nitrogen and lithium ion can make an interaction（Li.N） which is weaker than that of oxygen and lithium ion． Nitrile groups have a high dielectric constant which can increase the polarity of the molecule to increase the solubility of lithium salt． Also， terminated polar groups of the linear molecule can facilitate the flexibility of the linear chain to improve further ioniczation． Moreover， the asymmetric chelation structure may increase the lithium transportation and ionic conductivity ［43］．
In our group research， two different series of ether based solvents have been synthesized as a potential co-solvent for the electrolyte system： （i） the nitrile terminated ethers can be prepared via the Michael addition reaction of an appropriate alcohol with excess acrylonitrile． The donating ability of the nitrile group is lower than that of the ether group so that the binding force of nitrile groups is weaker than those of ethers， which will allow lithium ions to have much faster ligand exchange in triglyme （G3）， leading to faster transport of lithium ions and an increase in ionic conductivity． We used amino ／ ether-based ligands （AELs） and siloxane core to synthesize a series of nitrile groups terminated polar small molecular compounds to play as liquid electrolyte solvents． （ii） The trifluoromethyl terminated ethers can be prepared via the Williamson ether reaction ［44］． New fluorinated solvents are being investigated as nonflammable solvents． A solvent with an F to H ratio ＞ 4 appears to have improved thermal properties， fluoro ether may be an alternative in conjunction with cyclic carbonates which could improve the thermal properties of the solvent system ［45］．
For this part of the research， varied amounts of LiTFSI will be dissolved in a certain weight of different product solvents to make liquid electrolyte solutions． A separator is dipped into the electrolyte solution． The soaked separator is ready to use as a liquid electrolyte． Ionic conductivities， electrochemical windows and thermal properties of each electrolyte solution will be tested．
The research of our designed linear nitrile-ester ＆ fluoro-ester as appropriate co-solvents for carbonate-based electrolytes will be presented in my colleague： Zheng Yue's PH． D dissertation．
The fully oxidized state （S） and the fully reduced state （Li 2 S） are solid， and they are difficult to be dissolved in the electrolyte systems． Nevertheless， lithium polysulfide is highly soluble in the liquid phase． Thus， the electrochemical reaction of the cell is compared with the solid-liquid-solid phase changing， which leads to its unique kinetics ［46］［47］．
Battery electrolytes should meet the following fundamental requirements：an excellent conductivity for Li ＋ cations， which is determined by the diffuse coefficient， chemical， and electrochemical stability； a real affinity for the electrode material so that it could penetrate the matrix and is distributed to the active material． However， traditional electrolytes that meet these requirements are also highly soluble to long chain polysulfide， which causes polysulfide shuttle （PSS） ［48］．
Figure 1. 3. Mechanism of polysulfide chain dissolves in DMC
Figure 1. 4. Electrochemistry on the cathode of Li-S Batteries
In traditional electrolytes applied in rechargeable Li-ion batteries， S 8 ，Li 2 S 2 and Li 2 S have very slight solubility． However， long chain polysulfide，Li 2 S 8 ， Li 2 S 6 ，and Li 2 S 4 are highly soluble in most of the organic solvents commonly employed in electrolyte systems． These dissolved sulfur species cause not only capacity fading but also more severe problems． Polysulfide anions diffuse to the anode and be reduced to insoluble Li 2 S 2 and Li 2 S， lead to selfdischarging and deposit on the surface of the anode forming an insulate layer that blocked the transport of Li ＋ ． As a result， the battery undergoes electric capacity fading and decreasing Columbia efficiency． This process is named redox PSS effect．