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