Oxygen-Deficient P2-Na_0.7Mn_0.75Ni_0.25O_2−x Cathode by a Reductive NH₄HF₂ Treatment for Highly Reversible Na-Ion Storage

Oxygen-deficient P2-Na0.7Mn0.75Ni0.25O2-x (OD-NMNO) for sodium-ion batteries is prepared by a modified high-temperature solid-state reaction using NH4HF2 to induce oxygen vacancies inside. OD-NMNO has a reduced lattice parameter along the c-axis and thereby a reduced unit cell volume compared with that of pristine P2-Na0.7Mn0.75Ni0.25O2 (P-NMNO) owing to the presence of oxygen vacancies (leading to the highly ionic character of Mn, as well as shorter transition metal–oxygen (TM–O) bonds) in the layered crystal structure of P-NMNO. The resultant OD-NMNO demonstrates a higher initial discharge capacity (>150 mA h g–1 at 10 mA g–1), superior rate capability, and improved cycling performance (69.4% after 100 cycles at 50 mA g–1) in comparison with P-NMNO because of the optimum oxygen vacancies in its whole lattice. These oxygen vacancies reinforce the TM–O bonds in OD-NMNO, preventing irreversible phase transitions during cycling, as well as facilitating Na+-ion diffusion from the surface to the bulk. These results break up the conventional preconception that defects generally deteriorate the layered structure of sodium transition metal oxides due to the induced structural instability.