We synthesize Sb/Sb₂O₃ nanoparticles by the oxidation of Sb nanoparticles at 100, 200, and 300 °C. The half sodium-ion batteries with Sb/Sb₂O₃-200 exhibit the optimal performance with a charge capacity of 540 mAh g⁻¹ after 100 cycles at 0.1 A g⁻¹, maintaining up to six times more capacity than pure Sb, and superior rate performance with 95.7% retention after cycling at varied current densities. One reason for this is that Sb/Sb₂O₃-200 is at exactly the optimum ratio of Sb₂O₃:Sb and the particle size of Sb/Sb₂O₃ to ensure both high capacity for Na⁺ and small stress during sodiation/desodiation, which is confirmed by the diffusion–stress coupled results. It indicates that increasing the ratio of Sb₂O₃:Sb causes a decrease of Mises equivalent stress, radial stress, and tangential stress in the range of 1:1–3.5:1, and an increase in the range of 3.5:1–4:1. These stresses decrease with a particle radius in the range of 30–50 nm and increase with a particle radius in the range of 50–70 nm. Additionally, another reason is related to the formation of cycling-induced coral-like Sb, which can promote Na⁺ diffusion, relieve cycling-induced volume changes, and provide exceptional Na⁺ storage.