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Unraveling the Preference of Discharge Products in Na-O2 Batteries Using ab initio Computation

Strong Inter-layer Coupling in van der Waals Heterostructures Built from Individual Monolayers
(A) The phase diagram of NaO2 and Na2O2 at 300 K as a function of O2 partial pressure and particle size, and (B) the nucleation energy barrier and (C) the critical nucleus size of NaO2 and Na2O2 nanoparticles as a function of O2 partial pressure at the electrochemical potential of 2.1 V

Scientific Achievement
Researchers with the Materials Project have investigated thermodynamic stability of Na−O particles as a function of temperature, O2 partial pressure, and particle size using first-principles calculations.

Significance and Impact
The particle-size-dependent stability and nucleation kinetics of Na−O compounds set the conditions governing the formation and growth of discharge products in Na−O2 batteries.

Summary
Recent experiments on Na-air batteries reveal that the performance of Na-air batteries strongly depends on the type of discharge product, either Na2O2 or NaO2. When Na2O2 is formed as a discharge product, the battery cells exhibit poor reversibility and high overpotentials, while the charging and discharging overpotentials are reduced to less than 200 mV when NaO2 is formed.

In this work, the thermodynamic stability of Na-O compounds and their nucleation kinetics have been investigated using ab initio computation. ShinYoung Kang and colleagues in the Materials Project, a joint collaboration which Kristin Persson leads at Berkeley Lab, that bulk NaO2 is stable only at O2 partial pressure PO2 > 8.5 atm under room temperature. On the other hand, the particle-size-dependent phase diagram (A) constructed by combining the bulk formation energies with surface energies revealed that the low surface energy stabilizes NaO2 over Na2O2. Given the distinct structures and the small energy difference between Na2O2 or NaO2, the NaO2 nanoparticles may grow further to micrometer-size.

In addition, the lower nucleation energy barrier (B) and critical nucleus size (C) for NaO2 nanoparticles indicate that the nucleation of NaO2 is kinetically favored. Therefore, they conclude that PO2 is a key parameter to determine the thermodynamic and kinetic preference of the formation of discharge product, and in turn, the performance of Na-air batteries.

“Nanoscale Stabilization of Sodium Oxides: Implications for Na−O2 Batteries,” ShinYoung Kang, Yifei Mo, Shyue Ping Ong, and Gerbrand Ceder, Nano Letters 14 (2014). doi:10.1021/nl404557w