Multi‐Level Design and Irreversible Ion Exchange Involved Sodium‐Storage Mechanism of Zero‐Strain K ₂ Ti ₆ O ₁₃ Toward Sodium‐Ion Capacitors

ABSTRACT K 2 Ti 6 O 13 (KTO) is recognized as a highly promising anode for sodium‐ion capacitors (SICs) due to its 3D tunable framework, low cost, and environmental compatibility. However, its modest electronic conductivity and controversial sodium‐storage mechanism constrain commercial applications. In response, a multi‐level optimization strategy from materials to electrodes is proposed to construct a self‐standing KTO@carbon nanofibers (KTO@CNFs) hybrid film anode for SICs. The KTO nanobelts (NBs), like “seed sprouting,” are in situ grown and well dispersed on/within the porous CNFs, resulting in abundant exposed active sites and an orthogonal conductive network composed of axial electron transport in CNFs and radial ion permeation through KTO NBs. Comprehensive (ex)in situ physicochemical characterizations and theoretical calculations unveil the formation of “zero‐strain” K 2‐x Na x Ti 6 O 13 with superb ionic transportability and solid‐solution chare‐storage mechanism, induced by the irreversible “Na + /K + ‐exchange,” which is the intrinsic electroactive phase for reversible sodium storage. The KTO@CNFs film delivers exceptional sodium‐storage properties in both half and full cells. Particularly, an ultrahigh material‐level energy density of 96.1 Wh kg ‒1 (10.2 kW kg ‒1 ) and 10,000‐cycle stability with a capacitance retention of 92.4% at 4 A g– 1 are achieved by our SICs. More essentially, our findings offer constructive guidance for rational design and mechanism elucidation of advanced electrodes for next‐generation SICs.