Carbon quantum dot modified W 18 O 49 electrode enables smart energy storage device with a self‐diagnosis character

Abstract In Monitoring the health state of energy storage devices, including batteries and supercapacitors, is of significance to ensure safety and potential endurance for electric vehicles or other usage. However, electrochemical parameters measured from a battery or supercapacitor usually bring very less information about potential fault or failure of electrodes at the material level. Herein, W 18 O 49 is synthesized to build a smart electrode with high specific capacity, which is also used as an indicator with self‐diagnosis character. The flexible carbon quantum dot modified W 18 O 49 electrode is loaded on carbon cloth, revealing failure by irreversibility of discoloration. An irreversible electrochromism from indigo to cyan or dark, along with cycling, is observed when the electrode is used in a supercapacitor or in a lithium battery. The mechanism is revealed as cations preferentially diffuse in tunneling of W 18 O 49 rather than the (010) face. This irreversibly changes the crystal structure and valence state of W, inducing the electrochromic effect with decayed performance of the electrode. The smart electrode is further demonstrated in a transparent pouch‐type full cell, delivering excellent flexibility, good performance, and visual chromogenic effect. Our work provides a novel perspective on the design of smart energy storage devices with a function of self‐diagnosis for future applications.