Erdite NaFeS₂ as a New Anode Material for Lithium-Ion Batteries

Anode materials with simultaneously large capacity and low working voltage have always been one of the pursuing goals in the development of lithium-ion batteries. In this report, erdite NaFeS2 was synthesized by phase conversion of Fe-saponite for the first time, which displays attractive lithium-storage performance as an anode material. Taking into consideration that NaFeS2 can be regarded as sodium pre-embedded FeS2, a thorough comparison of lithium-storage performance between NaFeS2 and FeS2 was carried out. Theoretical calculation reveals that NaFeS2 has metallic conductivity and thus faster electron transfer than FeS2. The main oxidation and reduction peaks of the NaFeS2/Li battery reduce by 0.4–0.8 V compared with those of FeS2/Li, which are qualitatively supported by density functional theory calculations. Additionally, NaFeS2 delivers higher capacity, longer cycle life (1157 mAh·g–1 after 500 cycles), and better rate performance (618 mAh·g–1 at 5 A·g–1) than FeS2. Significantly decreased (de)lithiation voltage and increased capacity through changing the electrochemical reaction mechanism are favorable for improving the energy and power density of the batteries. This work develops a facile method of transforming silicate into sulfide and puts forward a strategy to reduce the (de)lithiation voltage of high-capacity anode materials, which is meaningful for boosting energy/power densities of energy storage devices.