Effect of Ti-substitution on the properties of P3 structure Na₂/3Mn_0.8Li_0.2O₂ showing a ribbon superlattice

Oxygen anion redox offers an effective strategy to enhance the energy density of layered oxide positive electrodes for sodium- and lithium-ion batteries. However, lattice oxygen loss and irreversible structural transformations over the first cycle may result in large voltage hysteresis, thereby impeding practical application. Herein, ribbon superstructure ordering of Li/transition-metal-ions was applied to suppress the voltage hysteresis combined with Ti-substitution to improve the cycling stability for P3-Na_0.67Li_0.2Ti_0.15Mn_0.65O₂. When both cation and anion redox reactions are utilized, Na_0.67Li_0.2Ti_0.15Mn_0.65O₂ delivers a reversible capacity of 172 mA h g⁻¹ after 25 cycles at 10 mA g⁻¹ between 1.6–4.4 V vs. Na⁺/Na. Ex-situ X-ray diffraction data reveal that the ribbon superstructure is retained with negligible unit cell volume expansion/contraction upon sodiation/desodiation. The performance as a positive electrode for Li-ion batteries was also evaluated and P3-Na_0.67Li_0.2Ti_0.15Mn_0.65O₂ delivers a reversible capacity of 180 mA h g⁻¹ after 25 cycles at 10 mA g⁻¹ when cycled vs. Li+/Li between 2.0–4.8 V.