Enhanced oxygen redox reversibility and capacity retention of titanium-substituted Na-4/7[1/7Ti1/7Mn5/7]O-2 in sodium-ion batteries

Anion redox reactions offer a means of enhancing the capacity of layered sodium transition metal oxide positive electrode materials. However, oxygen redox reactions typically show limited reversibility and irreversible structural changes upon cycling, resulting in rapid capacity loss. Here, the Ti-substituted Na-4/7[1/7Ti1/7Mn5/7]O-2 (where represents a transition metal vacancy) is presented as a positive electrode material for sodium-ion batteries. Na-4/7[1/7Ti1/7Mn5/7]O-2 delivers a reversible capacity of 167 mA h g(-1) after 25 cycles at 10 mA g(-1) within the voltage range of 1.6-4.4 V and presents enhanced stability compared with Na-4/7[Mn-1/7(6/7)]O-2 over the voltage range 3.0-4.4 V. The structural and electronic structural changes of this Ti4+ substituted phase are investigated by powder X-ray diffraction, X-ray absorption spectroscopy, electron paramagnetic resonance and Raman spectroscopy, supported by density functional theory calculations. These results show that the Na-4/7[Mn-1/7(6/7)]O-2 structure is maintained between 3.0 and 4.4 V, and the presence of TiO6 octahedra in Na-4/7[1/7Ti1/7Mn5/7]O-2 relieves structural distortions from Jahn-Teller distorted Mn3+O6 between 1.6 and 4.4 V. Furthermore, Ti4+ substitution stabilises the adjacent O 2p orbitals and raises the ionicity of the Mn-O bonds, increasing the operating potential of Na-4/7[1/7Ti1/7Mn5/7]O-2. Thereby providing evidence that the improved electrochemical performance of Na-4/7[1/7Ti1/7Mn5/7]O-2 can be attributed to Ti4+ substitution. This work provides insight and strategies for improving the structural stability and electrochemical performance of sodium layered oxides