Unveiling the Role and Mechanism of Nb Doping and In Situ Carbon Coating on Improving Lithium-Ion Storage Characteristics of Rod-Like Morphology FeF3 ·0.33H2 O.

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).