Wednesday, 4 October 2017
Prince George's Exhibit Hall D/E (Gaylord National Resort and Convention Center)
The low oxygen diffusion rate in electrolyte causes great capacity loss in Li-O2 batteries. It is desirable to adjust the wettability of electrode such that more gas phase channels can contribution to increasing the discharge capacity. Lyophobic (or lyophilic) electrodes were fabricated by mixing binders such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF) with Acetylene Black carbon particles. Customized electrodes were madd by coating the mixture on one side of a piece of untreated carbon cloth (210 µm thick) until the carbon loading reached about 2mg/cm2. 1 mol/L bis(trifluoromethane)sulfonimide lithium salt (LiTFSI) dissolved in tetraethylene glycol dimethyl ether (TEGDME) was utilized to characterize the surface wettability of customized electrodes by contact angle measurement. The contact angle between the coated surface of electrode with 15% PTFE and electrolyte (5µL) was 128.8o while it was 36.5o (unstable) for electrode with 15% PVDF. Discharge capacities of the customized electrodes in Li-O2 batteries were first measured with coated layers facing the lithium metal. Discharge capacities with 2V cut-off voltage were 4160.8 mAh/g (15% PTFE) and 1665.8 mAh/g (15% PVDF) at 0.1 mA/cm2, respectively. Then, discharge capacities of the customized electrodes were measured with the coated layer facing the oxygen channel. The discharge capacity of electrode with 15% PVDF increased to 2778.8 mAh/g while that of electrode with 15% PTFE decreased to 3401.8 mAh/g. Concerning the electrode with PVDF 15%, more deep discharge capacity was obtained when the coated layer was placed facing O2 due to the fact that more reaction sites were created in the lyophilic electrode and the O2 diffusion path to reaction sites (in the coated layer) was shorter in this configuration. The opposite trend observed with the electrode with PTFE 15% may be attributed to the fact that lyophobic coated layer repelled electrolyte and decreased the number of reaction sites. Under the same discharge current rate, the electrode was blocked by the generated solid Li2O2 more quickly as the reaction continued.