Functional Electroltes For Rechargeable Lithium Sulfur Batteries

Lithium-sulfur batteries has received intensive attention due to theirhigh capacity, environment friendliness. However, the practical applicationof lithium-sulfur batteries has been impeded by their innate shortages: thesolubility of polysulfides, the lithium dendrite growth brought by lithiumanode, an unstable Solid Electrolyte Interface (SEI) ect. The development inthe sulfur based cathodes has significantly curbed the dissolution ofpolysulfides in the electrolytes and granted the Li-S batteries with betterelectrochemical performances. The safety issues of Li-S batteries remainunresolved, however. The S-PAN composite cathode materials arenonflammable in air. Hence the improvement of safety performancesconcerning lithium sulfur batteries lies in the advancement of electrolytesand the negative electrodes.In this thesis, we dedicate to compose the electrolytes that are capableof enhancing the lithium sulfur battery performances in both electro andthermo aspects. The synopsis of this thesis is presented as follows:The Self-healing Electrostatic Shield Mechanism was employed tosuppress the growth of the lithium dendrites. SEM images proved theexistence of the SHES mechanism in the functional electrolyte containingCsPF6. In the repeating lithium deposition-dissolution experiment, thebatteries with the functional electrolytes illustrated a much longer cyclelife. The electrochemical performance of the batteries using CsPF6dopedelectrolytes had been enhanced. After100discharge-recharge cycles at1C,the capacity retention of Li-S batteries with functional electrolytes was94.2%on average while conventional electrolyte,86.4%. The combinationof this CsPF6and FEC endowed the Li-S batteries a much better cyclic stabilityTriethyl phosphite was chosen as the flame retardant additive in theelectrolyte. Self-Extinguish Time was employed to evaluate the thermoimpact on combustibility brought by TEPi. It was found that the SET hadplummet from136smL-1to86smL-1with the addition of wt.15%TEPi.Moreover, the electrochemical performances of the electrolytes wereenhanced by TEPi’s presence. With the addition of TEPi, the solidelectrolyte interface was thickened, resulting in a larger impedance.However, as the viscosity of TEPi is relatively low, the activity of thecathode was improved, resulting in an overall improvement inelectrochemical performance.LATP is mechanically strong and non-flammable, hence selected as aprobable candidate for Li-S batteries. The room-temperature conductivityof LATP was. At present, Li-S batteries with LATPexhibited less satisfactory electrochemical performances. Much work areto be done to apply LATP in Li-S batteries.To sum up, CsPF6functional electrolytes are capable of inhibitingdendrite growth and elongating Li-S battery life. TEPi functionalelectrolytes improve safety performances by reducing the inflammability.Meanwhile TEPi functional electrolytes enhance the electrochemicalperformance. LATP has the potential of improving safety performances ofLi-S batteries fundamentally. However, its relatively high impedance needsto be altered before it is applied in Li-S batteries.