Unravelling Ultra‐Stable Conversion‐Type Zinc‐Ion Storage in Copper Selenides for Flexible Aqueous Batteries

Abstract Rocking‐chair configurations based on advanced anodes with reliable zinc‐ion storage can intrinsically avoid the deterioration of flexible zinc‐ion energy devices by corrosion, dendrites, and inadequate mechanical stability of zinc metal, yet the identification of durable anode materials is still challenging. Herein, the first reported conversion‐type Cu 2‐x Se is proposed as an ultra‐stable anode for flexible rocking‐chair aqueous zinc‐ion batteries. Differing from these well‐recognized intercalation‐type copper selenide anodes, the unique Cu 2‐x Se structure features a preferred thermodynamic deep transformation path, and the high‐reversible conversion reaction is fully illuminated by in situ synchrotron X‐ray diffraction and substantial ex situ characterization, demonstrating an outstanding performance combination of high capacity (150 mAh g −1 at 0.5 A g −1 ) and record‐high stability (91% capacity retention after 20 000 cycles at 5 A g −1 in Cu 2‐x Se||Zn x MnO 2 full cells). Consequently, fiber‐shaped batteries using Cu 2‐x Se as the anode are successfully assembled and exhibited an ultra‐long life (89.8% capacity retention after 900 cycles) and extraordinary flexibility (98% capacity retention after 4500 cycles of bending), far exceeding those of representative flexible batteries previously reported. The findings provide novel insights into the energy storage mechanism of copper selenides and, as an elegant forerunner, offer a plausible path for the development of rocking‐chair flexible aqueous zinc‐ion batteries.