Improvement of Cycle Capability of FeS2Positive Electrode by Forming Composites with Li2S for Ambient Temperature Lithium Batteries

Abstract

FeS₂-Li₂S composite positive electrode material, consisted of mainly Li₃Fe₂S₄ and Li_2.33Fe_0.67S₂, was prepared for improving the cycle capability of FeS₂ in rechargeable lithium batteries with liquid electrolytes operated at ambient temperature. The electrochemical tests demonstrated that the FeS₂-Li₂S composite cells showed a relatively higher capacity retention after 15 cycles (ca. 64%) as compared with that of the FeS₂ cells (ca. 26%), though both sample cells showed comparable initial discharge capacities (ca. 740 and 790 mAh·g⁻¹ (4.2 and 4.5 mAh·cm⁻²), respectively). The improved cycle capability of the FeS₂-Li₂S cells was ascribed particularly to rather unchanged discharge plateau at ca. 2.0 V, which was reported to correspond to the reduction of sulfur. The ex-situ S K-edge X-ray absorption fine structure measurements indicated that the redox reaction of sulfur ions (S₂²⁻ + 2e⁻ ⇔ 2S²⁻), associated with Li extraction / insertion reactions, was still active after 15 cycles in the FeS₂-Li₂S electrode, while such redox reactions almost disappeared in the FeS₂ electrode. The atomic displacements required in the initial delithiation / lithiation were reduced in the FeS₂-Li₂S composite as compared with FeS₂, which would give rise to the suppression of the structural irreversibility in the following charge / discharge, resulting in the improved cycle capability.