P-doped carbon quantum dot graft-functionalized amorphous WO₃ for stable and flexible electrochromic energy-storage devices

The intrinsic structural instability of amorphous WO₃ (a-WO₃) following repeated Li ion insertion/extraction and bending hinders its application in stable and flexible electrochromic (EC) energy-storage devices. To resolve this problem, developing an elaborate heterostructure of a-WO₃ is essential for high-performance flexible EC energy-storage devices. Herein, a novel structure is proposed to introduce P-doped carbon quantum dots (P-CQDs) into a-WO₃ through a low-temperature process to induce robust chemical linkages with a-WO₃. The functional groups of P-CQDs form strong hydrogen bonds with a-WO₃ in the form of quantum dot grafts, which effectively tolerate structural deformation following long, repetitive cycling electrochemical reactions and bending. Moreover, the functional groups and graphitic carbon of P-CQDs improve the Li ion wettability and electrical conductivity, respectively, thereby enhancing the electrochemical activity and kinetics. As a result of the impacts, P-CQD graft-functionalized a-WO₃ (P-CQD/a-WO₃) electrodes exhibit excellent EC energy-storage performances. Furthermore, a flexible EC energy-storage device fabricated with the P-CQD/a-WO₃ electrode exhibits a remarkably enhanced long-term cycle stability (transmittance retention of 87.3% after 4,000 cycles) and flexibility (transmittance retention of 84.1% and specific capacitance retention of 85.3% after 500 bending cycles). Therefore, we believe that the proposed heterostructure-inducing quantum dot graft functionalization with a-WO₃ is a breakthrough in the realization of high-performance flexible EC energy-storage devices.