Detailed Structural and Electrochemical Comparison between High Potential Layered P2-NaMnNi and Doped P2-NaMnNiMg Oxides
Cornelius Gauckler, Manuel Dillenz, Fabio Maroni, Lukas Fridolin Pfeiffer, Johannes Biskupek, Mohsen Sotoudeh, Qiang Fu, Ute Kaiser, Sonia Dsoke, Holger Euchner, Peter Axmann, Margret Wohlfahrt‐Mehrens, Axel Groß, Mario Marinaro
Abstract
Rechargeable sodium-ion batteries are viable candidates as next-generation energy storage devices. Nonetheless, the development of high-potential and stable cathode materials is still one among the open tasks. Here, we propose a combined experimental/theoretical approach to shed light on the effect of magnesium doping on the layered P2-Na0.67Mn0.75Ni0.25O2 cathode material. The P2-Na0.67Mn0.75Ni0.25O2 baseline material and doped P2-Na0.67Mn0.75Ni0.20Mg0.05O2, synthesized via coprecipitation route followed by thermal treatment, have been physically and chemically characterized via X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), as well as electrochemically via galvanostatic cycling and galvanostatic intermittent titration technique (GITT). The Mg-doped material showed stabilization of the high potential plateau and improved cycle life. The analysis of the phase transition with synchrotron operando XRD (SXRD) shows multiple possible intermediate phases ("Z-phase") rather than a pure OP4-like structure. Based on our experimental data and periodic density functional theory (DFT) calculations, the stability of the O2, P2, and OP4 phases for the pristine and Mg-doped systems was investigated to elucidate the origin of the "Z"-phase formation in the Mg-doped material.