Investigation Of Functional Ionic Liquid Based Electrolytes In Lithium Sulfur Batteries

Lithium-sulfur batteries have been one of the most promising candidate of the rechargeable batteries in the next 20 years because of their superior energy density, lower cost and extensive source of sulfur. However, they also have several practical drawbacks:the shuttle effect, deposition of the discharge products, lithium dendritic crystal, electrolyte combustible and so on, which seriously affect the electrochemical properties of Li/S batteries, resulting in a rapid capacity fading and safety hazards. This paper attempts to search for novel ionic liquid based electrolyte systems to improve the performance of Li-S batteries. Lithium bis(trifluoromethane sulfonamide)(Li TFSI), different kinds of ionic liquids and organic ethers were prepared. Moreover, the spectroscopy performances and thermal stability performance of the electrolytes were characterized. Meanwhile, the related electrochemical performance of the Li/S coin cell with these electrolytes were evaluated by the tests of cycle performance, alternating-current impedance, cyclic voltammetry and so on.1. Ionic liquid based electrolytes for lithium-sulfur battery. N-butyl-methyl piperidinium bis(trifluoromethylsulfonyl) imide(PP14TFSI),N-methyl-N-propyl-pyrrolidinium bis(trifluoromethanesulfonyl) imide(Pyr13TFSI) and diethylene glycol dimethyl ether(DEGDME) were prepared. They composed of 0.5M Li TFSI+ PP14TFSI/DEGDME(X:Y,v:v) and 0.5M Li TFSI+Pyr13TFSI/DEGDME(X:Y,v:v)electrolyte systems. The results showed that the ionic conductivity of these electrolytes could meet the application requirements of Li/S battery when the temperature was above 0℃. It was worth mentioning that the lithium ion transference number of the Pyr13TFSI/DEGDME system was 0.2. The thermal stability of the two electrolytes was better than common DOL/DME electrolyte system, and the thermal decomposition temperature was 87.3 ℃,90.4 ℃ respectively. The cycle performance results showed that the best volume ratio of these two kinds of ionic liquids and DEGDME is 2:1. And the electrolytes exhibited better cycle and rate performance. In 0.5M Li TFSI+PP14TFSI/DEGDME(2:1, v:v) system, the capacity was 1042.9m Ahg-1 in the first cycle and maintained 446.5m Ahg-1 after 100 cycles at current density of 0.1C.2. The effect of organic ether co-solvent on ionic liquid based electrolytes system. To explore the ether chain length on the influence of the electrolytes, the Pyr13TFSI-based ionic liquid electrolyte system with 1, 2-dimethoxyethane(DME), DEGDME, triethylene glycol dimethyl ether(TEGDME) were prepared, respectively. With increasing ether chain length, the thermal ability of electrolyte systems gradually increased, it was 88.7 ℃,90.4℃,135.76 ℃ respectively. And the longer the ether chain length, the higher the ionic conductivity. Moreover, the discharge capacities of the electrolytes Li TFSI-Pyr13TFSI/DME(2:1, v:v), Li TFSI-Pyr13TFSI/TEGDM(2:1,v:v) were attenuated slowly with the increasing cycle number, showing better cycle performance. The study showed that the ionic liquid/organic ether co-solvent electrolyte system as a novel type electrolyte material can be used in lithium-sulfur batteries.