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.