Construction of Low‐Softening‐Point Coal Pitch Derived Carbon Nanofiber Films as Self‐Standing Anodes Toward Sodium Dual‐Ion Batteries

Abstract To achieve high‐quality hard carbon nanofibers (HCNFs), and particularly flexible HCNFs films is the eternal pursuit from low‐cost coal pitch (CP). However, it is still trapped seriously by the inborn bottleneck of low‐softening‐point (LSP) characteristics of CP itself. Herein, an efficient Bi(NO₃)₃·5H₂O‐assisted electrospinning‐carbonization methodology is creatively devised to obtain flexible HCNFs films directly from LSP CP. The essential roles of Bi(NO₃)₃·5H₂O and pre‐oxidation in constructing flexible films are rationally proposed. With further regulation in Bi(NO₃)₃·5H₂O dosage and calcination temperatures, specific micro‐structures/morphologies of flexible HCNFs films are finely optimized. The optimum HCNFs‐1.2 film is endowed with robust structural flexibility/stability, high‐content active oxygen/nitrogen groups, abundant graphic microcrystalline zones of large interlayer spacing, and convenient ion‐diffusion channels. Thanks to such remarkable merits, HCNFs‐1.2 retains a large reversible capacity of 125.3 mAh g^‒1 over 1000 cycles at 1.0 A g^‒1, when evaluated as a self‐supporting film anode for sodium dual‐ion batteries (SDIBs). Furthermore, the HCNFs‐1.2‐based SDIBs deliver a specific capacity of 90.9 mAh g^‒1 at 0.1 A g^‒1, along with a capacity retention of 78.4% after 1500 cycles at 1.0 A g^‒1. The insightful understanding here will provide meaningful guidance for rational design of advanced flexible film electrodes toward next‐generation SDIBs and beyond.