Long-life Na–O2 batteries with high energy efficiency enabled by electrochemically splitting NaO2 at a low overpotential

Abstract

Metal-air batteries are thought to be the ultimate solution for energy storage systems owing to their high energy density. Here we report a long-life Na-O2 battery with a high capacity of 750 mA h g(carbon)(-1) by manipulating the nucleation and growth of nano-sized NaO2 particles in a vertically aligned carbon nanotubes (VACNTs) network with a large surface area. Benefiting from the kinetically favorable formation of NaO2 reaction with a low overpotential of ~0.2 V, the electrical energy efficiency is as high as 90% for up to 100 cycles. A good rate performance (~1500 mA h g(carbon)(-1) at 667 mA g(carbon)(-1)) can be achieved through pre-deposition of a thin NaO2 layer. This study encourages the exploration of the key factors influencing the performance of metal-air batteries, as well as Na-based batteries characterized by phase transformation or conversion reactions.

Toward a Sodium–“Air” Battery: Revealing the Critical Role of Humidity

“…The results in regards to the electrochemical Na−O 2 reactions (2.26 V for NaO 2 and 2.33 V for Na 2 O 2 under pure oxygen) are consistent with the values calculated in previous reports, 12,13,25 while the corresponding EMFs have a slight difference when changed from pure oxygen to simulated air (20% O 2 ). However, the EMFs for electrochemical formation of NaOH (4Na + + 4e + 2H 2 O(g) + O 2 (g) → 4 NaOH(s)) varied from 2.63 to 2.58 V, assuming that the air's RH ranges from 100% to 1% at 25°C.…”

“…In contrast, it has recently been experimentally proven that the NaOH phase can be decomposed around 3.15 V in SABs. 25 It is also worthy to note that Kang's group recently reported that the decomposition energy barrier for Na 2 O 2 and NaO 2 are theoretically calculated to be lower than Li 2 O 2 . 20 Thus, the difference between the decomposition of sodium and lithium hydroxides may be further explained based on theoretical calculation or subsequent experiments in future.…”

Revealing instability and irreversibility in nonaqueous sodium–O2 battery chemistry

“…This highlights again the sensitivity of NaO 2 oxidation to electrolyte reactivity, vide infra. We propose that the plateau at 3.20 V vs. Na/Na + found in the PITT measurements corresponds to oxidation of NaOH (thermodynamic potential of NaOH Solid is 3.11 V vs. Na/Na + ) 18 . The decomposition of NaOH has been observed at 3.15V vs. Na/Na + by Zhao et al and was related to the decomposition of Na 2 O 2 .2H 2 O→ 2NaOH + H 2 O + 1/2O 2 .…”

“…The decomposition of NaOH has been observed at 3.15V vs. Na/Na + by Zhao et al and was related to the decomposition of Na 2 O 2 .2H 2 O→ 2NaOH + H 2 O + 1/2O 2 . 18 In order to examine the effect of NaO 2 -electrolyte exposure time on the charging kinetics, we discharged Na-O 2 cells to 500 mAh/g c at rates of 1000 and 10 mA/g c ( Figure 2) and aged them in contact with the electrolyte at OCV from 1 to 20 days, followed by charging at a galvanostatic rate of 100 mA/g c . For electrodes discharged at 1000 and 10 mA/g c , aging up to 20 days led to significant loss in the charging capacity for the low-voltage plateau (<3.00 V vs. Na/Na + ) characteristic of NaO 2 oxidation.…”

Investigating dendrites and side reactions in sodium–oxygen batteries for improved cycle lives

“…[5][6][7][8] Inspired by the complex by-product formation reactions at the cathode side of Li-O 2 batteries, 5 most of the research studies have been focused on the characterization of cathodes, e.g., effects of cathode morphology and by-products on cyclability. 7 However, McCloskey et al pointed out that the cleaner cathode reactions in Na-O 2 compared to Li-O 2 were less concerned and the low cycle life was perhaps caused by the Na anode. 6 Zhao et al detected sodium hydroxide and sodium carbonate as decomposition products on vertically aligned carbon nanotubes.…”