Entropy-mediated layered oxide cathodes: Synergistic channel expansion and strain control for sodium-ion batteries at cryogenic conditions

Abstract

O3-type layered oxide cathodes for sodium-ion batteries are promising owing to high theoretical capacity and broad temperature adaptability, yet hindered by structural degradation and sluggish Na⁺ diffusion kinetics. Herein, we present a sodium-deficient high-entropy layered oxide cathode (Na_0.85Ni_0.3Mn_0.3Fe_0.1Co_0.15Ti_0.1Cu_0.05B_0.02O₂, denoted as Na0.85-HEO), combining sodium content optimization and high-entropy composition design. Incorporating six transition metals and light element boron creates a unique high-entropy configuration, effectively mitigating local lattice distortion and internal strain through chemical disorder effects, thereby enabling highly reversible phase transitions (O3-P3-O3) and smaller volume change (0.6 Å³) during the initial cycle. The sodium-deficient high-entropy design effectively increases the sodium interlayer spacing to 0.322 nm, facilitating the Na⁺ diffusion kinetics. Moreover, this high-entropy strategy enables the cathode to have a completely solid solution charge curve and significantly reduces the proportion of (O₂)ⁿ⁻, thereby suppressing gas release during the cycling process. The resultant cathode demonstrates exceptional cyclability (80% capacity retention after 400 cycles at 100 mA g⁻¹ in a full cell), and remarkable low-temperature performance (108.6 mAh g⁻¹ at −40 °C). This work guides the design of high-entropy electrode materials with tailored ionic transport channels for extreme-temperature energy storage applications.