Improved electrochemical kinetics and rate performance of lithium-ion batteries by Li₂FeS_2−xF_x cathode materials

Lithium iron sulfide (Li₂FeS₂) 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 Li₂FeS₂ 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 Li₂FeS_2−xF_x. Theoretical calculations confirm that F substitution occurs at the S site, enhancing charge mobility. After 100 cycles, the optimized Li₂FeS_2-xF_x cathode exhibits a specific capacity of 250 mAh g⁻¹, higher than pristine Li₂FeS₂. 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.