Enhancement of cycling performance of Li-oxygen batteries by using surface-modified lithium electrode and dual-doped graphene catalyst

Abstract

Rechargable lithium-oxygen batteries receive great attention among the post lithium ion batteries(post-LIB) due to their high theoretical energy density. To achieve commercialized long-term lithium-oxygen batteries in practice, appropriate developments of the electrode and electrolyte are necessary. The major target of this study is to develop the cycling performance of lithium-oxygen batteries. To accomplish this purpose, various type of electrochemical materials and method were applied to electrode and electrolyte. In charter 2, ester-functionalized ionic liquids with different cations (imidazolium, pyrrolidinium, piperidinium, morpholinium) and a bis(trifluoromethanesulfonyl)imide anion were synthesized and subsequently mixed with tetra(ethylene glycol)dimethylether (TEGDME) at different concentrations. The mixed solutions were ultimately investigated for their applicability as the electrolyte in non-aqueous lithium–oxygen batteries. Among the ionic liquids investigated, the pyrrolidinium-based ionic liquid, ethyl-N-methylpyrrolidinium-N-acetate bis(trifluoromethanesulfonyl)imide (MEEsPyr-TFSI), exhibited a lower viscosity, higher ionic conductivity and wider electrochemical stability window than any other ionic liquids. The addition of an appropriate amount of MEEsPyr-TFSI to the TEGDME-based organic electrolyte had a positive effect on the ionic conductivity, electrochemical stability and oxygen radical stability. A mixed ionic liquid-based solution with an optimum composition was successfully employed as a promising electrolyte for lithium–oxygen batteries with good cycling stability. In chapter 3, mesoporous carbon on nitrogen and sulfur co-doped graphene nanosheets (NSGC) was synthesized and its bi-functional catalytic activity toward oxygen reduction reaction and oxygen evolution reaction was investigated. The NSGC material showed high bi-functional catalytic activity due to the synergistic effect of co-doping of sulfur and nitrogen, as well as the presence of a hierarchical porous structure. The enhanced bi-functional catalytic activity of NSGC facilitated the efficient formation and decomposition of Li2O2 on the air cathode. The lithium-oxygen cell assembled with the NSGC-based air cathode delivered a high initial discharge capacity of 11 431 mA h g-1 and exhibited good cycling stability. The hybrid structure consisting of mesoporous carbon with co-doped graphene nanosheets can be an effective strategy to improve the round-trip efficiency and cycle life of lithium-oxygen batteries. In charter 4, we demonstrate that the protective layer comprising conductive polymer and AlF3 particles on lithium metal stabilized the lithium electrode by effectively reducing the reductive decomposition of the liquid electrolyte and suppressing the growth of lithium dendrite. As a result, the cycling performance of a lithium−oxygen cell assembled with a surface-modified lithium electrode was remarkably improved as compared to a cell with a pristine lithium electrode. In charter 5, electrochemical performance of lithium-oxygen cell assembled with lithium electrode modified by bi-functional protective layer was evaluated. The composite of couducting polymer and aluminum iodide was applied as protective layer for lithium electrode to reduce charging overpotential with iodide ion redox mediator and to prevent self-discharge of redox mediator with lithium metal anode. Also, formation of Li-Al alloy by dissolve aluminum ion could reduce effectively increase of interfacial resistance. The cells with lithium anode protected by bi-functional protective layer exhibit high charge-discharge efficiency and good cycling retention due to reduced interfacial resistance and charging overpotential. In these studies, many electrochemical functionalized materials were applied to electrolyte and electrode. It was demonstrated that these application of advanced materials to lithium-oxygen batteries can be one of the positive approach to improve the electrochemical characterization and cycling performance of lithium-oxygen batteries.