Unveiling the Role and Mechanism of Nb Doping and In Situ Carbon Coating on Improving Lithium‐Ion Storage Characteristics of Rod‐Like Morphology FeF₃·0.33H₂O

Abstract

Given the inherent characteristics of transition metal fluorides and open tunnel-type frameworks, intercalation-conversion-type FeF₃ ·0.33H₂ O has attracted widespread attention as a promising lithium-ion battery cathode material with high operating voltage and high energy density. However, its low electronic conductivity and poor structural stability impede its practical application in high-rate capacity and long-lifetime batteries. Herein, rod-like Nb-substituted FeF₃ ·0.33H₂ O (Nb-FeF₃ ·0.33H₂ O@C) nanocrystals with a carbon coating derived from in situ carbonization in an ionic liquid are deliberately designed and prepared. Based on first-principles calculations and electrochemical analysis, it is shown that substitution of Nb into a proportion of Fe sites can dramatically reduce the total energy of the system and the bandgap, thus boosting the structural stability and electronic conductivity of FeF₃ ·0.33H₂ O. Simultaneously, the combination of a surface conductive carbon coating and assembly of the nanoparticles into a rod-like mesoporous architecture can produce an omni-directional ion/electron transmission network and a robust 3D composite structure. The Nb-FeF₃ ·0.33H₂ O@C composite with 3% Nb-doping displays high capacity (583.2 mAh g^-1 at 0.2 C), good rate capacity (187.8 mAh g^-1 at a high rate of 5.0 C), and excellent long-term cycle stability (160.4 mAh g^-1 after 300 long cycles).