Identifying reaction interface of next-generation Li-O₂ secondary batteries

抄録

Aprotic Li-O₂ batteries are one of the emerging next-generation batteries, boasting a much higher theoretical energy density compared to the state-of-the-art Li-ion batteries. However, before Li-O₂ batteries can be practically applied, it is essential to address the high charging overpotentials of these devices, which inevitably lead to irreversible parasitic reactions. Utilizing a redox mediator (RM) is an effective strategy to reduce the charging overpotential and suppress these parasitic reactions. The Br^-/Br₃^- redox couple is particularly interesting as it is less prone to decomposition compared to organic RMs. Nonetheless, the mitigating effect of RMs is currently inadequate, making it imperative to elucidate the Li₂O₂ reductive growth and oxidative decomposition mechanisms. Specifically, identifying the true reactive interfaces is crucial. In this study, Nano-secondary ion mass spectrometry (Nano-SIMS) isotopic three-dimensional imaging and differential electrochemical mass spectrometry (DEMS) analyses of individual Li₂O₂ particles revealed the reactive interface in a system containing the Br^-/Br₃^- redox couple as the RM. By combining the Nano-SIMS analyses and the DEMS data acquired using ¹⁸O₂, both discharging and charging reactions take place at the Li₂O₂/electrolyte interrace. Results of similar examinations in other electrolyte systems (amide-based electrolyte without RM) will also be reported. Briefly, in the amide-based electrolyte systems, it was also revealed that both charging and discharging reactions are proceeding at the Li₂O₂/electrolyte interface despite not using RM. Such characteristics are important for reducing the charging voltage and consequently improving the cycle characteristics.