Publication date: 28th August 2024
The lithium-oxygen (Li-O2) battery, with its extremely high theoretical energy density (> 3500 Wh/kg), is considered one of the most promising candidates for next-generation energy storage.[1] However, it suffers from serious side reactions due to the instability of both the carbon cathode and the electrolytes to reactive intermediates such as lithium superoxide (LiO2), singlet oxygen (1O2), and even the discharge product lithium peroxide (Li2O2) formed during ORR/OER processes.[2] In addition, neither carbon cathodes nor electrolytes can withstand a high overpotential and decompose under the operating conditions of a Li-O2 battery.[3] To deepen the fundamental understanding of the failure mechanisms in Li-O2 batteries, it is crucial to decouple the side reactions of the cathode and electrolyte.
As reported in our previous work, topological-defect-rich graphene mesosponge (GMS) synthesized by chemical vapor deposition (CVD) using Al2O3 as a template is a promising carbon cathode for Li-O2 batteries, exhibiting a large capacity.[4] Moreover, the large surface area of GMS makes it a good substrate for solid catalyst loading.[5] In this work, isotope 13C-GMS was first synthesized using high-purity 13C-CH4 as a carbon source for CVD. Then, we unraveled the critical influence of overpotential in Li-O2 batteries by using 13C-GMS cathodes with hexagonal close-packed (hcp) and face center cubic (fcc) Ru crystals as catalysts. Under the monitoring of in situ differential electrochemical mass spectrometry (DEMS), side reactions caused by isotopic labeling of 13C-based cathodes and conventional 12C-based electrolytes were first decoupled. Our result shows that the lower overpotential of fcc-Ru compared to hcp-Ru only inhibits carbon cathode degradation but accelerates electrolyte degradation, which severely limits the cycling performance of Li-O2 batteries, especially when a limited amount of electrolyte was used. Eventually, the cyclability of Li-O2 batteries can be described as the liquid in a barrel. With sufficient Li anode, cyclability is determined by the shortest rod of cathode stability or electrolyte stability. We would like to draw attention to the critical evaluation of side reactions in Li-O2 batteries. And our 13C-GMS can also be useful to decouple the side reaction in other batteries.
This presentation was supported by JST ASPIRE (grant no. JPMJAP2309) and JSPS KAKENHI (grant no. 24K17761).
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