Engineering Transition Metal Layers for Long Lasting Anionic Redox in Layered Sodium Manganese Oxide

Abstract

Oxygen-redox-based layered cathode materials are of great importance in realizing high-energy-density sodium-ion batteries (SIBs) that can satisfy the demands of next-generation energy storage technologies. However, Mn-based layered materials (P2-type Na-poor Nay[AxMn1−x]O2, where A=alkali ions) still suffer from poor reversibility during oxygen-redox reactions and low conductivity. Herein, we introduce P2-Na0.75[Li0.15Ni0.15Mn0.7]O2 and P2-Na0.6[Li0.15Co0.15Mn0.7]O2, an oxygen-redox-based layered oxide cathode materials for SIBs. The effect of Ni and Co doping on the electrochemical performance was investigated by comparison with Ni-free and Co-free P2-Na0.67[Li0.22Mn0.78]O2 material. Combined experiments (galvanostatic cycling, neutron powder diffraction (NPD), X-ray absorption spectroscopy (XANES), X-ray photoelectron spectroscopy (XPS), and nuclear magnetic resonance (7Li NMR)) and theoretical studies (density functional theory, DFT calculations) confirmed that Ni substitution not only increases the operating voltage and decreases voltage hysteresis but also improves the cycling stability by reducing Li migration from transition metal (TM) to Na layers. It is also anticipated that having Na–O–Li configuration in a Mn4+-based layered material is important to active oxygen redox and that Co doping is crucial for improving the electrical conductivity. This research demonstrates the effect of Ni and Co doping in P2-type layered materials and suggests a new strategy of using Mn-rich cathode materials via oxygen redox with optimization of doping elements for SIBs.