In Situ XRD Investigations on Structural Change of P2-Layered Materials during Electrochemical Sodiation/Desodiation

Sodium layered oxides (Na_xMO₂) are attractive as positive electrode materials for rechargeable sodium-ion batteries (SIBs) due to high capacity, fast ionic diffusion and simple synthetic process. O3-layered lithium compounds have led successful commercialization of current lithium-ion batteries; as a result, rich experiences for structural studies of O3-layered compounds have been accumulated over the past decades. For sodium layered oxides, however, P2-layered compounds have been reported for better cyclability and structural stability during electrochemical reactions than O3-structure. Therefore, systematic studies on P2-layered materials for SIBs are highly required. In this study, we report the structural and electrochemical property of P2-Na_xFe_yMn_yCo_1-2yO₂ synthesized by simple solid state reaction. The X-ray diffraction pattern of as-synthesized powder is indexed as a hexagonal lattice (P6₃/mmc, No.194), which is identical to P2-layered structure. The structural changes in hexagonal P2-layered oxides have been investigated during electrochemical sodiation/desodiation by in-situ synchrotron X-ray diffractions of a capillary based micro battery cell. From the result of in-situ studies, the initial layered structure is maintained from 2.0 to 4.0 V vs. Na⁺/Na during first desodiation. The phase transformation is observed over the 4.0 V, but the original P2 structure is completely restored at the following sodiation process. The relationship between structural and electrochemical properties of this P2-layered material will be discussed.