Bifunctional Role of LiNO₃ in Li-O₂ Batteries: Deconvoluting Surface and Catalytic Effects

Out of the many challenges in the realization of lithium-O₂ batteries (LOB), the major is to deal with the instability of the electrolyte and the cathode interface under the stringent environment of both oxygen reduction and evolution reactions. Lithium nitrate was recently proposed as a promising salt for LOB because of its capability to stabilize the lithium anode by the formation of a solid electrolyte interphase, its low level of dissociation in aprotic solvents, and its catalytic effect toward oxygen evolution reaction (OER) in rechargeable LOB. Nevertheless, a deeper understanding of the influence of nitrate on the stability and electrochemical response of the cathode in LOB is yet to be realized. Additionally, it is well accepted that carbon instability toward oxidation is a major reason for early failure of LOB cells; therefore, it is essential to investigate the effect of electrolyte components on this side of the battery. In the present work, we show that nitrate leads to interfacial changes, which result in the formation of a surface protection domain on the carbon scaffold of LOB cathode, which helps in suppressing the oxidative damage of the carbon. This effect is conjugated with an additional electrocatalytic effect of the nitrate ion on the OER. Using in operando online electrochemical mass spectroscopy, we herein deconvolute these two positive effects and show how they are dependent on nitrate concentration and the potential of cell operation. We show that a low amount of nitrate can exhibit the catalytic behavior; however, in order to harness its ability to suppress the oxidative damage and passivate the carbon surface, an excess of LiNO₃ is required.