On the stability of nitrate ion in rechargeable Li-O₂ batteries

A major challenge in realization of Lithium-O₂ batteries (LOB), is the instability and reactivity of metallic lithium under the stringent environment of the LOB cell. LiNO₃ was proposed as a promising salt for the LOB, due to its catalytic behavior toward OER. However, previous effects are reported to arise from its reaction with metallic lithium. In the present work, we investigated the behavior of LiNO₃ in the LOB, under conditions, where, it cannot undergo direct reactions on metallic lithium. Moreover, the stability of NO₃^- under the electrochemical conditions of LOB, specifically on the cathode side of the cell remained an unvisited territory. We show here that nitrate is unstable on the surface of the carbon scaffold in presence of superoxide ions. We observed the decomposition of nitrate by XPS and electrochemical means and monitored its occurrence in-operando using online electrochemical mass spectroscopy (OEMS). We also illustrate how the reversible nitrate reduction pathway and the regeneration of nitrate are dependent on the presence/absence of metallic lithium in the cell. This work investigates and discusses the importance of chemical/electrochemical stability of the salt used in LOB, and illustrate how this (generally overlooked) aspect is crucial when choosing LOB electrolyte.