Tailoring Ti Content for Lattice Stable High-Entropy O3-Type Cathode for Sodium-Ion Batteries

O3-type layered oxides are promising cathodes for sodium-ion batteries (SIBs), but their practical deployment is hindered by structural degradation and voltage decay during cycling. Here, we investigate a high-entropy, O3-type layered oxide series with systematically varied Ti contents to examine how a nominally electrochemically inactive cation influences framework stability and electrochemical reversibility. The optimized composition, NaNi_0.4Mn_0.3Fe_0.1Ti_0.1Li_0.05Sb_0.05O₂ (HEO-Ti10), delivers long-cycle stability (∼80% capacity retention after 200 cycles at 150 mA g^-1) and high rate capability (113 mAh g^-1 at 1.5 A g^-1). Structural and spectroscopic analyses─including in situ/ex-situ XRD, XAFS, and TEM─indicate that moderate Ti incorporation enhances lattice robustness, suppresses irreversible phase evolution, and mitigates instability associated with Mn-site octahedral distortion signatures. These findings highlight a compositional tuning strategy within high-entropy layered oxides and may inform the rational design of stable and high-performance cathodes for practical SIBs applications.