The Polymer Society of Korea

Abstract

Although sodium-ion supercapacitors are promising for next-generation energy storage, their performance is often limited by the large ionic radius of Na⁺, which hinders ion insertion and suppresses capacity. Here, we report colloidally synthesized monoclinic WO₂.₇₂ nanocrystals that accommodate Na⁺ within hexagonal sites. After removing insulating organic ligands, the nanocrystals were uniformly deposited onto hydrophilic carbon cloth (CC) via a controllable, additive-free dip-coating process. A conformal 1 nm Al₂O₃ overlayer deposited by ALD improved mechanical robustness and suppressed delamination, enabling highly reversible charge storage. A symmetric device based on ALD-treated WO₂.₇₂/CC with a PVA/Na₂SO₄ gel electrolyte maintained stable performance under bending and delivered an areal energy density of 10.08 μWh cm⁻² at 0.3724 mW cm⁻², highlighting promise for wearable electronics.