Beyond Equimolarity: Entropy-tuned Stoichiometric Design of Co-free Earth-abundant P2-type Cathode for Sodium-ion Batteries

Abstract

Layered oxide cathodes for sodium-ion batteries (SIBs) face a stubborn trade-off: high voltage boosts capacity but triggers Jahn–Teller distortion, Na+/vacancy ordering, and irreversible phase transitions. We show that deliberate, nonequimolar entropy-tuning partly resolves this trade-off, suppressing distortion without sacrificing capacity, while exposing its limits at high voltage. We report a cobalt-free, earth-abundant P2-type cathode, Na0.67(Mn0.45Ni0.15Cu0.10Ti0.10Mg0.10Zn0.10)O2, built from low-cost, supply secure elements for scalable, commercial SIBs. Six cations satisfy the high-entropy criterion (ΔSconfig ≥ 1.5R) and entropy-tuning rule (Mn+Ni > 50%), yielding a Mn/Ni/Cu-dominant redox framework via two-step solid-state synthesis. Rietveld-refined X-ray diffraction, electron microscopy, and inductively coupled plasma mass spectrometry confirm structural fidelity, while X-ray photoelectron spectroscopy resolves active Mn3+/4+, Ni2+/3+/4+, and Cu2+/3+ couples delivering 139.8 mAh/g and 461.3 Wh/kg at 0.1 C. The galvanostatic intermittent titration technique and electrochemical impedance spectroscopy reveal Na+ diffusion coefficients of 10–10 to 10–15 cm2/s. Entropy suppresses low-voltage distortion but not the high-voltage transition, showing that synthesis-driven homogeneity, not entropy alone, is the decisive lever for durable SIB cathodes.