Inhabiting Inactive Transition by Coupling Function of Oxygen Vacancies and Fe─C Bonds achieving Long Cycle Life of an Iron‐Based Anode

Abstract Fe‐based battery‐type anode materials with many faradaic reaction sites have higher capacities than carbon‐based double‐layer‐type materials and can be used to develop aqueous supercapacitors with high energy density. However, as an insurmountable bottleneck, the severe capacity fading and poor cyclability derived from the inactive transition hinder their commercial application in asymmetric supercapacitors (ASCs). In this work, driven by the “oxygen pumping” mechanism, oxygen‐vacancy‐rich Fe@Fe 3 O 4 (v) @Fe 3 C@C nanoparticles that consist of a unique “fruit with stone”‐like structure are developed, and they exhibit enhanced specific capacity and fast charge/discharge capability. Experimental and theoretical results demonstrate that the capacity attenuation in conventional iron‐based anodes is greatly alleviated in the the Fe@Fe 3 O 4 (v) @Fe 3 C@C anode because the irreversible phase transition to the inactive γ ‐Fe 2 O 3 phase can be inhibited by a robust barrier formed by the coupling of oxygen vacancies and Fe─C bonds, which promotes cycle stability (93.5% capacity retention after 24 000 cycles). An ASC fabricated using this Fe‐based anode is also observed to have extraordinary durability, achieving capacity retention of 96.4% after 38 000 cycles, and a high energy density of 127.6 W h kg −1 at a power density of 981 W kg −1 .