Effects of the Electrode Wettability on the Deep Discharge Capacity of Li-O₂ Batteries

The wettability of customized Li-O₂ battery electrodes is altered by mixing acetylene black carbon particles with various binders. The wettability of the electrode can be characterized by the static contact angles between the electrode surface and nonaqueous electrolyte, which is 1 M bis(trifluoromethane)sulfonimide lithium salt (LiTFSI) dissolved in tetraethylene glycol dimethyl ether, and the double-layer capacitance measured by the cyclic voltammetry. Results show that electrodes containing poly(vinylidene difluoride) (PVDF) binder are lyophilic and increasing the fraction of poly(tetrafluoroethylene) (PTFE) increases the lyophobicity of electrodes. Li-O₂ batteries are discharged at 0.1 mA/cm² with the cut-off voltage of 2.0 V. The discharge capacity of the electrode with 15% PVDF (36.5°) carbon coatings is 1665.8 mAh/g, whereas the customized electrode with 15% PTFE (128.4°) carbon coatings obtains the discharge capacity of 4160.8 mAh/g. However, the discharge capacity decreases to 3109.5 and 2822.9 mAh/g as the PTFE content further increases to 25% (135.5°) and 35% (138.5°), respectively. The electrode composed of two lyophobic carbon coatings on top and bottom and one lyophilic carbon coating in the middle has the static contact angle of 118.8° and acquires the highest specific discharge capacity of 5149.5 mAh/g.