Carbon Nanotube Arrays And Pyrrolidinium-based Ionic Liquids For Rechargeable Li–air Batteries

Rechargeable Li–air batteries have recently been considered as a potential form of energy storage and conversion for novel electric-vehicle applications because of their unrivaled energy density.However,the practical development of this battery remains hindered by various issues: the low discharge–charge capacities and poor cycling performance of Li–air batteries.Lately,many efforts have been actively made to enhance the air-cathode performances of Li–air batteries.Thus,studies on preparing the support materials of cathode catalysts and the electrolyte materials are carried out with an expectation to explore the electrochemical performances for the cathode materials and build a new battery system with other materials.Herein,the research work was focused on the preparation of cathode supports and the correlation of electrolyte properties and battery performances.The main results are described as follows:?1?The preparation of well aligned-carbon nanotube?WA-CNT?arrays onto a carbon paper substrate with a conductive carbon-black layer was firstly reported by the catalyst seed-impregnated chemical vapor deposition method.The WA-CNT arrays/carbon-black layer/carbon paper substrate composites could be used as catalyst supports in air electrodes.The prepared WA-CNT arrays were then characterized by scanning electron microscopy.Results indicated that the optimal WA-CNT arrays of length 40–50 ?m and diameter 30–40 nm were obtained at the reaction temperature range of 750–800°C,linear velocity of feed vapors of 1.40 cm s^-1,and ferrocene-to-xylene molar ratio of 1:50–1:30.To achieve growth of WA-CNT arrays on the carbon-black layer/carbon paper substrate in a controlled manner,a new insight into the growth mechanism of CNT arrays was provided,which involved cross-coupling of chemical reaction process and mass transfer process based on reaction engineering theory.Electrochemical measurement indicated that the prepared WA-CNT arrays/carbon-black layer/carbon paper substrate composites achieved relatively high first discharge capacity of 2930 mAh g^-1?CNTs?at a current density of 0.05 mA cm^-2 in Li-air batteries,which far exceeded other carbonaceous materials in Li-air batteries.?2?The pyrrolidinium-based ionic liquids?ILs?,such as N-propyl-N-methyl pyrrolidinium bis?trifluoromethanesulfonyl?imide(PYR₁₃TFSI),N-butyl-N-methyl pyrrolidinium bis?trifluoromethanesulfonyl?imide(PYR₁₄TFSI),N-methoxyethyl-N-methyl pyrrolidinium bis?trifluoromethanesulfonyl?imide [PYR_1?2O1?TFSI],as electrolytes were explored for application to rechargeable Li–air batteries.Several fundamental properties of three ILs were measured: the ionic conductivity,oxygen solubility,and oxygen diffusion coefficient as a function of the different LiTFSI molality.The oxygen electro-reduction kinetics was characterized using cyclic voltammetry.Results showed that the ionic conductivities and oxygen diffusion coefficients of the PYR₁₃ TFSI and PYR_1?2O1?TFSI were obviously higher than those of the PYR₁₄ TFSI.The oxygen solubility in the PYR₁₄ TFSI was clearly higher than that in the PYR₁₃ TFSI and PYR_1?2O1?TFSI.At 0.6 mol kg^-1 LiTFSI additive,PYR_1?2O1?TFSI showed the lowest reaction overpotential,followed by PYR₁₃ TFSI,and then by PYR₁₄ TFSI,at uniform current density.The performances of Li–air batteries with these IL electrolytes were also investigated using electrochemical impedance spectroscopy and galvanostatic discharge–charge tests.At a current density of 0.05 mA cm^-2,the PYR_1?2O1?TFSI electrolyte battery had a higher first-discharge voltage than the PYR₁₃ TFSI electrolyte and PYR₁₄ TFSI electrolyte batteries.Both PYR₁₃TFSI-and PYR_1?2O1?TFSI-based batteries exhibited higher first-discharge capacities and better cycling stabilities than the PYR₁₄TFSI-based battery.The results demonstrated that the oxygen diffusion coefficient of the electrolyte could effectively facilitate the electrochemical oxygen electro-reduction reaction and oxygen concentration distribution in the air electrodes.The high-oxygen diffusion coefficient and large-oxygen solubility of IL electrolytes could enhance the discharge–charge performances of Li–air batteries.?3?The prepared MnO₂/WA-CNT arrays/carbon-black layer/carbon paper substrate composites as air cathodes,and the PYR₁₄ TFSI and TEGDME respectively as electrolytes have been employed for use in Li-air batteries.Through the combination of the MnO₂/WA-CNT arrays/carbon-black layer/carbon paper substrate air cathode and PYR₁₄ TFSI electrolyte,the discharge–charge performances of Li–air batteries could be improve effectively.Furthermore,the performances of Li–air batteries with the PYR₁₄ TFSI and TEGDME electrolytes were also investigated by using cyclic voltammetry,electrochemical impedance spectroscopy and galvanostatic discharge–charge tests.The results showed that the oxygen electro-reduction reaction activity of the TEGDME electrolyte battery was obviously higher than that of the PYR₁₄ TFSI electrolyte battery.At a current density of 0.05 mA cm^-2,the PYR₁₄ TFSI electrolyte battery had a higher discharge capacity and average coulombic efficiency,whereas the TEGDME electrolyte battery possessed the higher discharge voltage.The analysis results indicated the IL electrolyte with stronger stability and larger Li2O₂ solubility was beneficial to improve the discharge capacity and cycling stability of the Li–air battery.The low reaction overpotential of the oxygen electro-reduction in the battery with the organic solvent electrolyte could bring about its higher discharge voltage.