Unraveling the contribution of water to the discharge capacity of Li-O2 batteries from a modelling perspective

Adding water (H2O) to the electrolyte improves discharge capacity by enhancing the solution mechanism, but the evolution of discharge capacity with H2O content growth remains divergent. In view of this, the contribution of H2O to discharge capacity is revealed by modelling a lithium-oxygen (Li-O2) battery coupled with the H2O reaction mechanism. With increasing H2O content in the electrolyte, the discharge capacity first rises thanks to the alleviation of surface passivation and then declines owing to the limitation of O2 diffusion. Although the promotion of solution mechanism is most pronounced with a small amount of H2O (below 2000 ppm) in the dimethoxyethane (DME)-based electrolyte, the enhancement of solution mechanism in the tetraethylene glycol dimethyl ether (TEGDME)-based electrolyte is more sensitive to changes in H2O contents (above 500 ppm) than DME- and DMSO (dimethyl sulfoxide)-based electrolytes. Hence the H2O contents corresponding to the maximum discharge capacity (defined as "optimized water content") of TEGDME- and DMSO-based electrolytes are 2000 ppm and 10,500 ppm based on the Super P carbon cathode, respectively. The evolutions of "optimized water content" and discharge capacity are more sensitive to changes in the porosity and initial carbon particle radius of the carbon cathode. Compared to the absence of H2O, the discharge capacities with the "optimized water content" increase by 360% and 346% at a porosity of 0.9, as well as by 268% and 290% for TEGDME- and DMSO-based electrolytes at an initial carbon particle radius of 70 nm, respectively. In consequence, the electrolyte composition and cathode structure codetermine the "optimized water content" and the maximum promotion of H2O to the discharge capacity.