Preparation And Mechanism Study Of Cathode Catalysts And Electrolytes For Lithium Air Batteries

Nonaqueous Li air battery has become a potential energy storage device due to its ultra-high theoretical energy density.However,there still exist many problems,such as poor cycle stability and rate performance.This subject mainly focuses on the basic problems faced by air electrodes and electrolytes.The non-carbon cathodes,soluble redox mediator molecule catalysts and novel amide based electrolytes for lithium-air batteries are deeply investigated.Binder-free and freestanding Ir nano cathodes are prepared by replacement deposition method with ethanol as solvent.The results show that Ir nano cathode could effectively catalyze the ORR/OER process.The discharge and charge overpotentials decrease by 76 and 210 m V,respectively.And the Li-air battery with Ir nano cathode can matain 67 cycles with a limitted capacity of 800 m Ah g^-1,exhibiting superior cycling stability.Experiments and DFT calculations reveal that ORR mainly follows a“surface-solution”coupled process:at the initial stage,Li O₂ will strongly adsorb on Ir catalyst surface and then follows the surface electrochemical route to form Li₂O₂;as the charge transport decreases,the adsorbed Li O₂ species would tend to desorb and dissolve into the electrolyte,aand then disproportionates to form Li₂O₂ from solution.Furthermore,the holey IrNi alloy nanosheet cathodes are prepared by using water as the alternative reaction solvent.Cyclic voltammetry and scanning electron microscope analyses indicate that IrNi alloy nanosheets can promote the formation of pooly crystalline lithium peroxide containing lithium deficient Li_2-xO₂ component,which can be ascribed to the strong adsorbtion of IrNi alloy toward O₂ and Li O₂ intermediate.Compared with the carbon-based air cathodes,the porous IrNi alloy nanosheet cathodes exhibit better cycling stability under full discharge/charge protocol（2.3-4.3 V）.After 50 cycles,the specific capacity could still reach 448 m Ah g^-1,corresponding to a capacity retention of 40%.X-ray Photoelectron Spectroscopy（XPS）and Fourier Transform Infrared Spectroscopy（FTIR）measurements reveal that the content of by-products on IrNi alloy cathode after 50 cycles is much lower than that on carbon electrode,suggesting the superior stability of non-carbon based IrNi alloy cathode and its good compatibility with electrolyte.The effect of cobaltocene as a soluble molecule catalyst on the electrochemical performance of Li-air batteries and corresponding catalytic mechanism are studied.The research results show that cobaltocene can effectively improve the discharge capacity of Li-air batteries and improve the kinetics of ORR process.Experiments,molecular orbital theory and DFT calculations reveal that cobaltocene will first combine with oxygen molecules to form a（Cp₂Co）₂^II-O₂^2-complex,which then undergoes electrochemical reduction to form Li₂O₂,where the cobaltocene molecule acts as an electron transfer medium for the ORR reaction process.The unique ORR reaction pathway avoids the generation of superoxide intermediate,and thus effectively inhibits related side reactions caused by superoxide-driven nucleophilic attack.The yield of Li₂O₂ increases from 71%to 82%with the addition of cobaltocene.UV-Vis spectra and DFT calculations reveal that Li₂O₂will preferentially interact with Cp₂Co molecules and dissolve into the electrolyte,which then recrystallizes,grows into large-sized toroidal structure through Ostwald ripening process.This solution phase deposition mode effectively delays the passivation of the air cathode and significantly improves the discharge capacity of Li-air batteries.The application of electrolyte based on N-methylformamide（C₂H₅NO,NMF）solvent in Li-air batteries is studied.When N-methylformamide（NMF）-based electrolyte is used,the discharge and charge overpotentials of Li-air batteries reach as low as 0.17 and 0.32 V(200 m A g^-1),respectively.The battery life is increased to 108 cycles when highly concentrated NMF electrolyte is employed due to the enhanced stability of Li anode and air cathode.DFT calculations indicate that solvated protons will participate in the ORR/OER process in NMF based electrolyte.During discharge,oxygen will preferentially react with protons to form HOO intermediate;while OER doesn’t follow the reverse ORR pathway as usual,Li O₂ is found to be the preferred intermediate.The proton mediated process significantly lowers the ORR energy barrier during discharge,and provides a forceful impetus for converting the solid-solid toward partial liquid-solid OER reaction,dramatically accelerating Li₂O₂ decomposition kinetics.