An Advanced Separator for Li–O₂ Batteries: Maximizing the Effect of Redox Mediators

Abstract

Although Li–O2 batteries are promising next‐generation energy storage systems with superior theoretical capacities, they have a serious limitation regarding the large overpotential upon charging that results from the low conductivity of the discharge product. Thus, various redox mediators (RMs) have been widely studied to reduce the overpotential in the charging process, which should promote the oxidation of Li2O2. However, RMs degrade the Li metal anode through a parasitic reaction between the RM and the Li metal, and a solution for this phenomenon is necessary. In this study, an effective method is proposed to prevent the migration of the RM toward the anode side of the lithium using a separator that is modified with a negatively charged polymer. When DMPZ (5,10‐dihydro‐5,10‐dimethylphenazine) is used as an RM, it is found that the modified separator suppresses the migration of DMPZ toward the counter electrode of the Li metal anode. This is investigated by a visual redox couple diffusion test, a morphological investigation, and an X‐ray diffraction study. This advanced separator effectively maximizes the catalytic activity of the redox mediator. Li–O2 batteries using both a highly concentrated DMPZ and the modified separator exhibit improved performance and maintained 90% round‐trip efficiency up to the 20th cycle.

Redox Mediators for Li–O2 Batteries: Status and Perspectives

“…Recently, another method to suppress the crossover phenomenon of RMs in Li–O 2 batteries was reported . In this article, it was shown that DMPZ, an organic RM used for Li–O 2 batteries, migrates to the Li anode and reacts with it.…”

“…c,d) Schematic illustrations of the different charge operating mechanisms of Li–O 2 batteries using a pristine GF/C separator (c) and a PEDOT‐PSS‐coated separator (d). Reproduced with permission . Copyright 2017, Wiley‐VCH.…”

“…It is impossible to prevent the migration of RM molecules to the Li‐metal anode side, without using appropriate separation . Using solid electrolyte membranes (Li ions conductor) based on LATGP or PVdF‐HFP between the anode and cathode in Li–O 2 batteries can completely prevent the RM shuttle reaction.…”

Bifunctional Oxygen Electrocatalysts for Lithium−Oxygen Batteries

“…The concept of soluble catalysts is that a species that is dissolved in an electrolyte and thus freely moves around drives the catalytic reaction sites into the electrolyte solution rather than to the surface of the electrode or solid catalysts. Many different types of soluble catalysts have been proposed, including organic redox‐active species (e. g., organosulfur, nitroxyradical, phenazine, quinone and halide (e. g., iodide, bromide,) as well as organometallic compounds (e. g., phthalocyanine,, protoporphyrin).…”

“…Such shuttle behavior can significantly reduce the efficacy of soluble catalysts and even result in the continuous loss of catalysts owing to undesirable parasitic reactions with the lithium anode. The detrimental effects of shuttle phenomena have been demonstrated in several model studies in which interlayers such as inorganic solid electrolyte and advanced separator, were introduced between the anode and cathode to selectively permit the penetration of mobile species. The introduction of protection layers on the lithium anode has also been proposed as a potential solution to prevent shuttle reactions ,.…”