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–O2/CO2 (O2 and CO2 mixture) and Na–O2 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–O2/CO2–CNT nanobattery, the discharge reactions occurred in two steps: (1) 2Na+ + 2e– + O2 → Na2O2; (2) Na2O2+ CO2 → Na2CO3 + O2; concurrently a parasitic Na plating reaction took place. The charge reaction proceeded via (3) 2Na2CO3 + C → 4Na+ + 3CO2 + 4e–. In the Na–O2/CO2–Ag nanobattery, the discharge reactions were essentially the same as those for the Na–O2/CO2–CNT nanobattery; however, the charge reaction in the former was very sluggish, suggesting that direct decomposition of Na2CO3 is difficult. In the Na–O2 battery, the discharge reaction occurred via reaction 1, but the reverse reaction was very difficult, indicating the sluggish decomposition of Na2O2. Overall the Na–O2/CO2–CNT nanobattery exhibited much better cyclability and performance than the Na–O2/CO2–Ag and the Na–O2–CNT nanobatteries, underscoring the importance of carbon and CO2 in facilitating the Na–O2 nanobatteries. Our study provides important understanding of the electrochemistry of the Na–O2/CO2 and Na–O2 nanobatteries, which may aid the development of high performance Na–O2/CO2 and Na–O2 batteries for energy storage applications.