Rate-dependent structure-electrochemistry relationships and origins of capacity fading in P2-type Na2/3Fe2/3Mn1/3O2†

Abstract

Sodium transition metal oxides with layered structures have generated significant research interest as promising cathode materials for use in ambient temperature sodium-ion batteries. In this study, the structure and magnetic properties of P2-Na_2/3Fe_2/3Mn_1/3O₂ are investigated, in tandem with operando diffraction studies to resolve the structural changes taking place in the material when subject to variable current cycling in the range 1.5–4.2 V vs. Na⁺/Na⁰. Complementary diffraction studies are used to provide insight into the mechanism of sodium de-intercalation in P2-Na_2/3Fe_2/3Mn_1/3O₂ at low rates, as well as high current densities up to 1 C, enabled by the excellent time resolution allowed by high intensity synchrotron radiation. The structural evolution is found to differ markedly depending on the applied current density which illustrates the need to perform such structural studies under various applied current rates to better understand processes taking place in the electrode. The results obtained shed new light on the reaction mechanism of P2-type layered oxides and provide insight into some of the causes for their capacity fading.