Functional Mechanism And Electrochemical Performance Of The Blended Electrolytes For Lithium Air Batteries

The lithium-air battery is attracting a lot of attention due to the fact that its highest theoretical energy density which holds promise for all-electric vehicle and stationary energy storage applications. All of instability of the electrolyte, the poor dissolution of oxygen and the low solubility of discharge products are the key factors which is restricted the commercialize of lithium-air battery, but the instability and cathode passivation are the main reason that restrict the development of lithium air batteries at present stage. The instability of electrolyte, such as organic carbonate, can not reversibly form Li2O2 in the operation of battery and decompose irreversible byproducts such as lithium carbonate, lithium formate, lithium acetate and other lithium alkylcarbonates, meanwhile many insoluble byproducts will deposit in the positive pore and block O2 transport channel and aggravate polarization. The cathode passivation will magnify high polarization(overvoltage) and electrochemical instability, therefore the transformation in electrolyte system, result in generates that the battery reaction mechanism changed.The organic electrolyte is also known as aprotic polar solvent and can not give proton in the reaction system, and will not release H+ to corrode the lithium cathode; It can make cation solvated, especially metal cations(Li+). Li+ migrate by the form of solvation in the electrolyte, while it have the advantages of a wide electrochemical window, high melting point, less volatile and low viscosity. It is excepted to solve the commercialization of lithium air batteries. In this paper, 1M Li TFSI was used for lithium salt, meanwhile the organic solvent sulfolane(TMS), Dimethylacetamide(DMA) and tetraethylene glycol dimethyl ether(TEGDME) were studied by the basic physical and chemical properties, such as melting point, flash point, dielectric constant, viscosity, conductivity, dissolved oxygen and electrochemical window, and electrochemistry performance of three groups single electrolytes was compared by using AC impedance, CV, charge/discharge tests. The idea of the DMA/TMS as a lithium air battery electrolyte solution is proposed. It was confirmed that the Li TFSI-20DMA: 80 TMS was suitable as lithium air battery electrolyte. According to the coordinating role between DMA and TMS, the two kinds of electrolytes were used for the mixing modification research of TEGDME, respectively.(1) LITFSI-TMS has a high viscosity(25.26 m Pa·s@28 °C) and a low ionic conductivity(1.91×10-3 S·cm-1) and the value of viscosity change is 43.56 m Pa·s from 4? to 44 °C, so that the temperature has a great effect on the ionic conductivity of the TMS electrolyte. Li TFSI-TEGDME has a acceptable viscosity, but the ionic conductivity of TEGDME is lower(2.06×10-3 s·cm-1), because of its structure. The variation of viscosity with temperature change is more obvious. Li TFSI-DMA has a low visitoy(2.70 m Pa·s @ 28°C) and the variation of viscosity with temperature change is little. Compared with TMS and TEGDME,the ionic conductivity of Li FTSI-DMA(8.87×10-3 s·cm-1) is more four times. The starting potential of the OER process of the TMS reach 3.2 V between DMA(3 V) and TEGDME(3.3V), and the cycle performance of the battery were TMS(200 cycles), TEGDME(80 cycles) and DMA(50 cycles) at a specific capacity of 1000 m Ah gcarbon-1 and a current density of 0.3 m A cm-2, but TEGDME and TMS reveal the serious polarization during charge-discharge process.(2) We proposed to prepare lithium air battery electrolytes by using the blend of DMA and TMS. After the analysis and optimization of DMA/TMS electrolytic electrolyte system by analyzing the properties of viscosity, ionic conductivity, gas chromatography- mass spectrometry, and the electrochemical performances of AC impedance, cyclic voltammetry and cycle performance, the lithium air battery using TMS electrolyte had high stability performance. We showed that the TMS electrolytes with ca. 20%(in volume) DMA content reduce the overvoltage range from 0.2 to 0.6V in the cycle life, charge-discharge tests exhibited excellent cyclability of 200 cycles and Coulombic efficiency of 100% with a current density of 0.3 m A cm-2 and a specific capacity of 1000 m Ah gcarbon-1 by optimizing the electrolyte in lithium-air batteries, and the DMA improved poor basic physical and chemical properties of the TMS.(3) The properties of discharge product were investigated by scanning electron microscopy(SEM), X-ray diffraction(XRD) and proton nuclear magnetic resonance spectroscopy(1H NMR) tests. The product of TMS lithium air battery discharge was dense and the surface of the cathode covers many the block of Li2O2.The products of lithium air battery using 20DMA:80TMS electrolyte were tiny particles, and poor conductivity of Li2O2 made cathode passivated. The higher specific surface area of Li2O2 for the product of the cathode of lithium air battery using 20DMA:80TMS electrolyte accelerated the decomposition of Li2O2. The double function of DMA/TMS electrolyte was firstly proposed, due to TMS stability performance and the improvement of the DMA for the battery polarization. To sum up, the cycle performance of the lithium battery was obviously improved by the synergy of DMA and TMS.(4)According to the modification research of DMA and TMS, the lithium air battery using TEGDME electrolyte was modified by TMS and DMA, respectively. The results showed that the TMS for the performance of battery using TEGDME electrolyte was no significant improvement, and the DMA for the overvoltage of battery using TEGDME electrolyte obviously reduced.