Chemical compatibility of polymer binders with a reversible anionic redox reaction in lithia-based cathodes

Abstract

Lithia-based cathodes, operated by oxygen-anion redox reactions, can be an alternative to air cathodes in Li–O₂ batteries, owing to their high energy density in a sealed environment. However, the realization of lithia-based cathodes has been hindered by their low cycling stability owing to the low reversibility of the anion-redox reaction. We recently demonstrated a successful strategy for improving this reversibility through oxygen-vacancy enhancement in lithia-based cathodes. However, the reactive/reduced oxygen species inherently present in oxygen-vacancy-enhanced lithia can result in the decomposition of organic materials, such as binders and solvents. A suitable binder–solvent pair is thus essential to harness both a high energy density derived from enhanced oxygen vacancies and facile electrode preparation via solution-based casting. Herein, we explored binder candidates with different chemical structures paired with various solvents to develop a robust binder system for long-term operation of lithia-based cathodes. Through screening binders and solvents, we found that low polarity (and thus the dipole moment of the solvent) is the determining factor for the preparation of workable lithia-based cathodes and stable high-performance operation. Among the various binder–solvent pairs used for cathode slurry preparation, the SEBS–cyclohexane combination appeared to be the most feasible because of the non-polar nature of SEBS, the low dipole moment of cyclohexane, and the high dispersibility of SEBS in cyclohexane. We also proposed degradation mechanisms for each polymer based on spectroscopic analysis results. The lithia-based cathode prepared with the chosen SEBS–cyclohexane binder solution demonstrated good electrochemical characteristics, with cycling stability at a capacity limit of 300 mA h g⁻¹ with respect to other polymer–solvent combinations.