To meet growing need for large-scale energy storage, rechargeable batteries that reversibly cycle sodium ions are an attractive alternative to the Li-based technology. In particular, Na-intercalating oxides favorably form energy-dense layered structures with almost all transition metals, allowing a wide selection of redox centers to obtain desirable electrochemical activities. However, desodiation in the layered materials is often limited by structural instability due to transition metal migration at high voltage. To address this issue, we formulate a quaternary-mixed transition metal system with the O3-type layered structure.

We propose Ti as a structural stabilizer and report a composition of NaTi_0.25Fe_0.25Co_0.25Ni_0.25O₂ in this work. Synthesized by a solid-state reaction, the compound obtained is phase-pure with the R-3m symmetry. X-ray absorption spectroscopy reveals that transition metal oxidation states are Ti⁴⁺, Fe³⁺, Co³⁺, and Ni²⁺, suggesting that Ti remains electrochemically inert while others are redox-active. Our results highlight promising electrochemical performance of the material; 504 Wh kg^-1 at C/20 and 200 Wh kg^-1 at 30C at room temperature. According to in situ X-ray diffraction, the structural evolution upon desodiation differs from that upon resodiation. Interestingly, compared with the pristine O3 structure, we find that the discharged structure at 2 V after cycle is unchanged, leading to respectable cyclability. We will discuss the possible origin of this path-dependency of desodiated structures and its implication on electrochemical properties. Also, perspectives on the multi-transition metal system for layered cathode materials will be provided in this talk.