Synergistic Regulation of Lattice Chemistry and Interfacial Kinetics in High-Entropy O3 Layered Oxides for High-Rate Sodium Storage

O3-type layered oxides are promising cathodes for sodium-ion batteries, yet their practical performance is often limited by phase-transition-induced lattice strain and sluggish Na+ transport. Herein, we report a rare-earth-enabled high-entropy O3 layered oxide, NaNi0.3Cu0.1Fe0.2–xMn0.3Ti0.1GdxO2 (x = 0–0.10), prepared by a solid-state route. X-ray diffraction and selected area electron diffraction confirm an as-prepared single-phase O3 framework, with subtle lattice modulation after Gd substitution. The optimized NCFMTG02 delivers reversible capacities of 128.43 mAh g–1 at 0.1 C and 101.33 mAh g–1 at 5 C and retains 101.3 mAh g–1 after 100 cycles at 1 C, together with an average discharge voltage of 3.210 V, higher than that of the undoped NCFMTG0 electrode. The composition-dependent trend indicates that performance does not increase monotonically with configurational entropy; instead, an appropriate low-level Gd substitution within the high-entropy O3 framework is required to balance lattice-chemistry modulation and reaction kinetics. Galvanostatic intermittent titration technique and scan-rate-dependent cyclic voltammetry indicate faster Na+ transport and lower kinetic overpotential. X-ray photoelectron spectroscopy shows suppressed Jahn–Teller-active Mn3+, increased Mn4+, slightly more oxidized Ni, and an increased high-binding-energy O 1s component associated with changes in the surface/local oxygen environment. These results suggest that minor rare-earth substitution can complement the high-entropy O3 framework by regulating lattice chemistry, enhancing electrochemical robustness, and improving Na-storage kinetics.