Influence of Electrolyte\nSaturation on the Performance\nof Li–O<sub>2</sub> Batteries

Electrolyte saturation can strongly affect the Li–O<sub>2</sub> battery performance. However, it is unclear to what extent\nsaturation reduction will impact the battery capacity. In this study,\nwe investigated the influence of electrolyte saturation and distribution\nwithin a porous positive electrode on the deep discharge–charge\ncapacities and cycling stability. The study used both models and experiments\nto investigate the change of electrolyte distribution, double-layer\ncapacitance, ohmic resistance, and O<sub>2</sub> concentration in\nthe positive electrode at different electrolyte saturations. Results\nrevealed that electrodes with 60% electrolyte saturation achieved\nalmost the same maximum discharge (6.38 vs 6.76 mAh/cm<sup>2</sup>) and charge (5.52 vs 5.65 mAh/cm<sup>2</sup>) capacities with fully\nsaturated electrodes. The partially wet positive electrode (40% saturation)\nobtained more cycles than the electrode with 100% saturation before\nthe discharge capacity dropped below the cutoff point. However, the\nelectrode with 40% saturation had a low average charging efficiency\nof 88.76%, whereas the fully saturated electrode obtained 98.96% charging\nefficiency. Moreover, the fully wet positive electrode had the lowest\noverpotential during cycling (1.26–1.39 V). The measured electrochemically\nactive surface areas showed that even 40% saturation could sufficiently\nwet the positive electrode surface and obtain a double-layer capacitance\n(18.12 mF) similar to that with 100% saturation (20.4 mF). Furthermore,\na considerable increase in O<sub>2</sub> concentration at wetted surface\nareas was observed for the electrolyte saturation of less than 60%\ndue to the significantly higher O<sub>2</sub> diffusivity in the gas\nphase.