Lithium-oxygen Battery Cathode Interface Construction And Lithium-oxygen Reaction Mechanism Research

Li-O₂ batteries have attracted extensive attention because of their high theoretical energy density based on the lithium peroxide(Li₂O₂)as the discharge product.However,the realization of applicable Li-O₂batteries is hindered by the low energy efficiency,poor cycle life,and inferior rate capability.To overcome these shortcomings,the deep fundamental understanding of the Li-O₂reaction is of great importance.Previously,numerous researches on the Li-O₂batteries were conducted in the liquid electrolytes.However,due to the nucleophilic attack by the oxidizing products,the decomposition of electrolyte is unavoidable.In addition,with the carbon-based cathodes,the Li-O₂reactions are further complicated by the parasitic reactions from the carbon oxidation during charge.To unveil the reaction mechanism,it is necessary to construct the Li-O₂batteries without the liquid electrolytes as well as the carbon cathodes.Hence,starting from the material selection and structure design of cathode,based on the preparation of all solid carbon-free Li-O₂batteries,the Li-O₂reactions mechanism at the all-solid interfaces was studied by various characterization methods.The conclusions are draw as follows:(1)Silver nanowires(AgNWs)were coated on the garnet Li_6.4La₃Zr_1.4Ta_0.6O₁₂(LLZTO)electrolyte pellets to fabricate porous cathode.The interface contact between AgNWs cathode and garnet electrolyte were improved by the gold(Au)thin film as intermediate layer.The results show that the discharge capacity of Au/AgNWs cathode is50-fold higher than that of Au interlayer cathode,and the charging overpotential is reduced by 0.5 V.Finally,it is proved that the charge and discharge process of the batteries are the growth mechanism of Li₂O₂controlled by surface path reaction through a variety of material characterization methods.(2)The growth and decomposition of Li₂O₂on AgNWs cathode were observed in situ by environmental transmission electron microscope(TEM).Upon discharge,film-like Li₂O₂is formed on the surface of AgNWs with homogenous Li⁺/e^-distribution,and hollow disk-like Li₂O₂formed at the joints in between the AgNWs as well as at the garnet/AgNWs interfaces with uneven Li⁺/e^-distribution.Upon charge,the Li₂O₂began to decompose,the film-like Li₂O₂shrinks,and the hollow disk-like Li₂O₂at the joints of AgNWs and Li₂O₂releases gas and then collapses.(3)Based on the characterization and analysis of various experiments,the mechanism of Li-O₂batteries reaction at the solid interface was clarified clearly:Upon discharge,the formation of film and hollow disks is accordance with the surface path reaction process ofO₂+Li⁺+e^-?LiO₂and 2LiO₂?Li₂O₂+O₂.Upon charge,the decomposition processes are Li₂O₂?LiO₂+Li⁺+e^-and LiO₂?Li⁺+e^-+O₂.