context: Then Li–O2 batteries were fabricated with PFOSF treated Li anodes. A carbon nanotube film was used as the air-cathode and 1 M LiTFSI in TEGDME as the electrolyte. The fabricated LOBs were charged and discharged repeatedly. SEM and XRD results (Fig. S12†) show that the cathode side reaction is reversible. As shown in Fig. 5, the bare Li based cell can only cycle for 43 cycles with the discharge voltage suddenly dropped to 2.1 V at a current density of 300 mA g−1 with the capacity limited at 1000 mA h g−1. To find out the reason, the cell was disassembled after cycling for 10 cycles, the bare Li foil became black and many cracks and pulverizations were observed under SEM (Fig. S13a and c†). In contrast, LOBs with the PFOSF based Li anode exhibited largely improved cycling performance which could be enhanced to 185 cycles without using any cathode catalyst (Fig. 5). The Li surface was flat and kept its pristine silvery color after 10 cycles with about 40 μm thickness of the protective film covered on the Li surface (Fig. S13b and d†). LOBs with different SEI thicknesses were assembled and tested. As shown in Fig. S14,† the cycling performance was improved. The cycle life was elongated to 104, 92, 140, 185 and 167 cycles after PFOSF treatment for 10, 20, 30, 60 and 180 min, respectively, so the optimized PFOSF treatment time is 60 min. The EIS of the LOBs further explained this phenomenon. In Fig. S15,† the ohmic resistance (Rs) and charge transfer resistance (Rc) gradually increased within 30 cycles for LOBs with the pristine Li anode, which was induced by the pulverization of the Li anode and accumulation of side products due to the severe side reactions of Li metal with the O2-rich electrolyte. In contrast, for PFOSF-Li based LOBs, the values of Rohm and Rctr almost remained stable, which were far smaller than those of bare LOBs, indicating the well protecting effect of the LiF coating on the Li surface in suppressing the side reactions of Li metal with the electrolyte.
answer:  Li