In Situ Electrochemical Study of Na-O2/CO2 Batteries in an Environmental Transmission Electron Microscope

Metal-air batteries are potential candidates for post-lithium energy storage devices due to their high theoretical energy densities. However, our understanding of the electrochemistry of metal-air batteries is still in its infancy. Herein we report in situ studies of Na-O-2/CO2 (O-2 and CO2 mixture) and Na-O-2 batteries with either carbon nanotubes (CNTs) or Ag nanowires as the air cathode medium in an advanced aberration corrected environmental transmission electron microscope. In the Na-O-2/CO2-CNT nanobattery, the discharge reactions occurred in two steps: (1) 2Na(+) + 2e(-) + O-2 -> Na2O2; (2) Na2O2+ CO2 -> Na2CO3 + O-2; concurrently a parasitic Na plating reaction took place. The charge reaction proceeded via (3) 2Na(2)CO(3) + C -> 4Na(+) + 3CO(2) + 4e(-). In the Na-O-2/CO2-Ag nanobattery, the discharge reactions were essentially the same as those for the Na-O-2/CO2-CNT nanobattery; however, the charge reaction in the former was very sluggish, suggesting that direct decomposition of Na2CO3 is difficult. In the Na-O-2 battery, the discharge reaction occurred via reaction 1, but the reverse reaction was very difficult, indicating the sluggish decomposition of Na2O2. Overall the Na-O-2/CO2-CNT nanobattery exhibited much better cyclability and performance than the Na-O-2/CO2-Ag and the Na-O-2-CNT nanobatteries, underscoring the importance of carbon and CO2 in facilitating the Na-O-2 nanobatteries. Our study provides important understanding of the electrochemistry of the Na-O-2/CO2 and Na-O-2 nanobatteries, which may aid the development of high performance Na-O-2/CO2 and Na-O-2 batteries for energy storage applications.