Improved electrochemical kinetics and rate performance of lithium-ion batteries by Li2FeS2−xFx cathode materials

Lithium iron sulfide (Li 2 FeS 2 ) exhibits unique characteristics, including multielectron redox behavior and abundant valence states, making it a promising candidate for electrode material in lithium-ion batteries. However, the sluggish charge transfer kinetics, low stability, and slow rate performance hamper its practical application. Herein, we propose a strategy to boost the electrochemical performance of Li 2 FeS 2 by substituting F dopants with S sites through a two-step solid-state process. The effects of F dopants on material characteristics and electrochemical behaviors are investigated. Experimental results show that F dopants significantly enhance diffusion kinetics and rate performance, indicating improved interfacial activity in Li 2 FeS 2−x F x . Theoretical calculations confirm that F substitution occurs at the S site, enhancing charge mobility. After 100 cycles, the optimized Li 2 FeS 2-x F x cathode exhibits a specific capacity of 250 mAh g −1 , higher than pristine Li 2 FeS 2 . The improved electrochemical properties, diffusion kinetics, capacity, and rate performance are attributed to the enhanced structural stability from a stronger metal–fluorine bond compared to metal–sulfur, and increased Li + ion diffusion due to a greater electronegativity difference.