ABSTRACT
The Na-O2 system holds great potential as a low-cost, high-energy-density battery, but under normal operating conditions, the discharge is limited to sodium superoxide (NaO2), whereas the high-capacity peroxide state (Na2O2) remains elusive. Here, we apply density functional theory calculations with an improved error-correction scheme to determine equilibrium potentials and free energies as a function of temperature for the different phases of NaO2 and Na2O2, identifying NaO2 as the thermodynamically preferred discharge product up to â¼120 K, after which Na2O2 is thermodynamically preferred. We also investigate the reaction mechanisms and resulting electrochemical overpotentials on stepped surfaces of the NaO2 and Na2O2 systems, showing low overpotentials for NaO2 formation (ηdis = 0.14 V) and depletion (ηcha = 0.19 V), whereas the overpotentials for Na2O2 formation (ηdis = 0.69 V) and depletion (ηcha = 0.68 V) are found to be prohibitively high. These findings are in good agreement with experimental data on the thermodynamic properties of the Na xO2 species and provide a kinetic explanation for why NaO2 is the main discharge product in Na-O2 batteries under normal operating conditions.
ABSTRACT
The effects of Li2CO3 like species originating from reactions between CO2 and Li2O2 at the cathode of non-aqueous Li-air batteries were studied by density functional theory (DFT) and galvanostatic charge-discharge measurements. Adsorption energies of CO2 at various nucleation sites on a stepped (11Ì 00) Li2O2 surface were determined and even a low concentration of CO2 effectively blocks the step nucleation site and alters the Li2O2 shape due to Li2CO3 formation. Nudged elastic band calculations show that once CO2 is adsorbed on a step valley site, it is effectively unable to diffuse and impacts the Li2O2 growth mechanism, capacity, and overvoltages. The charging processes are strongly influenced by CO2 contamination, and exhibit increased overvoltages and increased capacity, as a result of poisoning of nucleation sites: this effect is predicted from DFT calculations and observed experimentally already at 1% CO2. Large capacity losses and overvoltages are seen at higher CO2 concentrations.
