Theoretical predictions for low-temperature phases, softening of phonons and elastic stiffnesses, and electronic properties of sodium peroxide under high pressure

High-pressure phase stabilities up to 600 K and the related properties of Na₂O₂ under pressures up to 300 GPa were investigated using first-principles calculations and the quasi-harmonic approximation. Two high-pressure phases of Na₂O₂ that are thermodynamically and dynamically stable were predicted consisting of the Amm2 (distorted P6̄2m) and the P2₁/c structures, which are stable at low temperature in the pressure range of 0-22 GPa and 22-28 GPa, respectively. However, the P6̄2m and Pbam structures become the most stable instead of the Amm2 and P2₁/c structures at the elevated temperatures, respectively. Interestingly, the softening of some phonon modes and the decreasing of some elastic stiffnesses in the Amm2 structure were also predicted in the pressure ranges of 2-3 GPa and 9-10 GPa. This leads to the decreasing of phonon free energy and the increasing of the ELF value in the same pressure ranges. The HSE06 band gaps suggest that all phases are insulators, and they increase with increasing pressure. Our findings provide the P-T phase diagram of Na₂O₂, which may be useful for investigating the thermodynamic properties and experimental verification.

On the Stability of NaO2 in Na-O2 Batteries

Na-O₂ batteries are regarded as promising candidates for energy storage. They have higher energy efficiency, rate capability, and chemical reversibility than Li-O₂ batteries; in addition, sodium is cheaper and more abundant compared to lithium. However, inconsistent observations and instability of discharge products have inhibited the understanding of the working mechanism of this technology. In this work, we have investigated a number of factors that influence the stability of the discharge products. By means of in operando powder X-ray diffraction study, the influence of oxygen, sodium anode, salt, solvent, and carbon cathode were investigated. The Na metal anode and an ether-based solvent are the main factors that lead to the instability and decomposition of NaO₂ in the cell environment. This fundamental insight brings new information on the working mechanism of Na-O₂ batteries.

Growth and dissolution of NaO2 in an ether-based electrolyte as the discharge product in the Na-O2 cell

The deposition and dissolution of sodium superoxide (NaO₂) was investigated by atomic force microscopy. Rectangular prisms consisting of 8 smaller sub-structures grew from NaO₂ platelets, when discharged in 0.5 M NaClO₄, diethylene glycol dimethyl ether on highly ordered pyrolytic graphite. During oxidation the 8 sub-structures are conserved. Ring-like structures of Na₂CO₃ of 200 nm diameter remain at the end of oxidation.