Flame Retarded Functional Electrolyte For Rechargeable Lithium Sulfur Battery With Sulfur Based Composite Cathode Materials

Sulfur cathode material based rechargeable lithium battery has theadvantage of high energy density over traditional lithium ion battery. However,lithium sulfur battery will undergo the same thermal runaway that has caused thesafety problems as lithium ion batteries, which is even more serious. Reasonsare as below:1) the growth of lithium dendrite results in the innerbreaking-down and heat generation of the battery;2) carbonate or ethersolventsof high volatility and flammabilityare thermal unstable and easy to beignited, and produce additional heat due to the reactions with the anode andcathode materials under high temperature;3) the cathode materials are usuallythe mixtures of conductive carbon material and sulfur, which are very unsafe asthey are inflammable and explosive materials.On one side, we prepared polyacrylonitrile and sulfur composite by thermaltreatment and found it achieved a better thermal stability than sulfur alone. The composite itself was nonflammable and cannot be ignited in the air. On the otherside, we introduced flame retardant additives (FRs) into the normal electrolytewhich not only effectively reduced the flammability of the normal electrolyteand but improved the power rate capability of the battery. The detailed contentsand results are as follows.We screened several kinds of liquid flame retardants with differentsubstituent groups. Firstly, alkyl phosphate-trimethyl phosphate (TMP), trimethyphosphite (TMP(i)), and dimethyl methylphosphonate (DMMP); secondly,phenyl phosphate-triphenyl phosphite (TPP(i)) and finally fluorinated alkylphosphate-tris(2,2,2-trifluoroethyl)phosphite. The influence of FRs on theflammability of the electrolytes was studied by combustion experiments andSET tests and the effects on the electrochemical performance of the electrodeswere researched by the electrochemical tests.TMP and TMP(i) cannot be used as additives for lithium sulfur battery foronly5wt.%of each do a heavy damage to the electrochemical performance ofthe PAN-S composite electrodes with poor charge and discharge performance.With11wt.%DMMP added, the electrolyte of1M LiPF6/EC+EMC (1:1, v/v) isnearly nonflammable and its thermal stability is obviously improved.Cycleperformance and electrochemical stability are little affected with an appropriateDMMP addition of7~11wt.%. TPP(i) with phenyl substituent shows excellent electrochemical stability when used as additive. The DSC results show thatTPP(i) with phenyl substituent helps delay the exothermic reaction between theelectrolyte and the electrode material for20℃, indicating an enhanced thermalstability of the electrolyte in the appearance of electrode. The electrochemicaltests show that5wt.%TPP(i) can effectively reduce the interfacial resistancebetween the electrode and the electrolyte and improve the rate capability andcycle stability of the PAN-S composite electrode. Therefore, TPP(i) is a goodcandidate for a versatile electrolyte additive for rechargeable lithium sulfurbatteries.When TTFP was used as an additive for the normal electrolyte, it exhibits acomparable flame retarding performance with DMMP. The assembled cellsachieve stable cycle performance even in a high TTFP addition (20wt.%). WhenTTFP amount is confined within10wt.%, the rate capability of the PAN-Selectrode is apparently improved. CV results indicate that the addition of TTFPcan increase the lithium ion diffusion coefficient by one order of magnitude. EISdata show that TTFP effectively reduces the charge transfer resistance and theinterfacial resistance. Furthermore, a co-solvent electrolyteLiPF6/EC-DMC-TTFP (1:2:1, v/v/v) was prepared by replacing some of the ECwhich has a high viscosity with an equal amount of TTFP. PAN-S electrodesachieve excellent power rate performance and long-term cycle stability in the proved nonflammable new type electrolyte. In summary, TTFP can be applied inthe co-solvent composition with EC and DMC, which helps bring out amultifunctional electrolyte with both flame retarding and high power rate effects.The innovative electrolyte is applicable for the lithium sulfur cell fabrication.Through comparing different kinds of flame retardants, we advanced themost suitable FRs for the lithium sulfur battery in the paper, which haveeffectively improved the thermal stability of the electrolyte and the safety oflithium sulfur batteries. What’s more important, we successfully prepared a typeof flame retarding and high power rate electrolyte by optimizing the ratio of FR.Additionally, this is quite a simple, economic and effective method forstrengthening the rate performance of electrode materials.