Enhanced oxygen redox reversibility and capacity retention of titanium-substituted Na4/7[□1/7Ti1/7Mn5/7]O2 in sodium-ion batteries

Abstract

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[□_1/7Ti_1/7Mn_5/7]O₂ (where □ represents a transition metal vacancy) is presented as a positive electrode material for sodium-ion batteries. Na_4/7[□_1/7Ti_1/7Mn_5/7]O₂ 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[□_1/7Mn_6/7]O₂ 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[□_1/7Mn_6/7]O₂ structure is maintained between 3.0 and 4.4 V, and the presence of TiO₆ octahedra in Na_4/7[□_1/7Ti_1/7Mn_5/7]O₂ 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[□_1/7Ti_1/7Mn_5/7]O₂. Thereby providing evidence that the improved electrochemical performance of Na_4/7[□_1/7Ti_1/7Mn_5/7]O₂ can be attributed to Ti⁴⁺ substitution. This work provides insight and strategies for improving the structural stability and electrochemical performance of sodium layered oxides.