Improvement of cycle capability of FeS ₂ positive electrode by forming composites with Li ₂S for ambient temperature lithium batteries

FeS ₂-Li ₂S composite positive electrode material, consisted of mainly Li ₃Fe ₂S ₄ and Li _2.33Fe _0.67S ₂, was prepared for improving the cycle capability of FeS ₂ in rechargeable lithium batteries with liquid electrolytes operated at ambient temperature. The electrochemical tests demonstrated that the FeS ₂-Li ₂S composite cells showed a relatively higher capacity retention after 15 cycles (ca. 64%) as compared with that of the FeS ₂ 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 FeS ₂-Li ₂S 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 (S ₂ ^2- + 2e ^- 2S ^2-), associated with Li extraction / insertion reactions, was still active after 15 cycles in the FeS ₂-Li ₂S electrode, while such redox reactions almost disappeared in the FeS ₂ electrode. The atomic displacements required in the initial delithiation / lithiation were reduced in the FeS ₂-Li ₂S composite as compared with FeS ₂, which would give rise to the suppression of the structural irreversibility in the following charge / discharge, resulting in the improved cycle capability.