Uptake of CO₂ in Layered P2-Na_0.67Mn_0.5Fe_0.5O₂: Insertion of Carbonate Anions

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-Na 0.67 [Mn 0.5 Fe 0.5 ]O 2 on exposure to CO 2 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-Na 0.67 [Mn 0.5 Fe 0.5 ]O 2; a detailed study by online electrochemistry mass spectroscopy reveals that the inserted carbonate ions decompose during electrochemical charging, accounting for the differences observed between the first and second cycles. Furthermore, we show that Ni-substituted materials P2-Na 0.67 [Ni x Mn 0.5+ x Fe 0.5−2 x ]O 2 are less prone to such reactivity and thus are more promising candidates for scalable processing. Understanding these mechanisms provides a vital guide for future sodium metal oxide battery research.