Li-O2 batteries have attracted extensive attention recently due to the extremely huge specific energy. Similar to research mode of Li-ion batteries, nowadays specific capacity based on the mass of cathode material is widely adopted to evaluate the electrochemical performance of Li-O2 batteries. However, the prerequisite of linear correlation between the delivered capacity and active mass is easily neglected. In this paper, we demonstrate the rationality of specific capacity adopted in Li-ion batteries with classic LiCoO2 cathode by confirming the linear correlation between cell capacity and LiCoO2 mass. Delivered capacities of Li-O2 batteries with different cathode masses are simultaneously measured and nonlinear correlation is obtained. The discharge and charge products are identified by X-ray diffraction and in situ gas chromatography-mass spectrometry analysis to ensure reaction mechanism. Discharge capacities of Li-O2 batteries with various areas of oxygen window are further studied, which shows that cell capacity increases linearly with the area of oxygen window. Scanning electron microscopy is employed to observe the discharged electrode and shows that Li2O2 deposition during discharge mainly occurs in the electrode area exposure to the oxygen, which is consequently defined as effective area for accommodating Li2O2. Moreover, a plausible route for formation of effective area in the oxygen electrode is proposed. These results provide evidence that effective area is an equally important factor determining cell capacity.
Keywords: cell capacity; effective area; lithium−oxygen battery; oxygen diffusion; oxygen electrode.