Uptake of CO2 in Layered P2-Na0.67Mn0.5Fe0.5O2: Insertion of Carbonate Anions

Abstract

Batteries based on sodium layered transition metal oxides are a promising alternative to current state-of-the-art lithium-ion systems for large-scale energy storage, resulting in recent intensive efforts to develop high-energy density, low cost, stable cathode materials. Some of the most promising degrade on exposure to ambient atmosphere; however, the process is not understood. Here, using neutron/X-ray diffraction coupled with mass spectroscopy and thermal analysis, we reveal the nature of the reactivity. We demonstrate the unprecedented insertion of carbonate ions in the vacancy-rich layered structure of P2-Na0.67[Mn0.5Fe0.5]O2 on exposure to CO2 and moisture, concomitant with oxidation of Mn(III) to Mn(IV). The material exhibits much higher charge/discharge polarization and lower capacity than rigorously air-protected P2-Na0.67[Mn0.5Fe0.5]O2; a detailed study by online electrochemistry mass spectroscopy reveals that the inserted carbonate ions decompose during electrochemical charging, accounting for ...