In this study, an FeS2@carbon fiber electrode is developed with FeS2 nanoparticles either embedded in or attached to carbon fibers by using an electrospinning method. By applying this binder -free, metal-current,collector-free FeS2@carbon fiber electrode, both the redox reaction and capacity decay mechanisms for the Li FeS2 system are revealed by changing the electrolyte (conventional carbonate electrolyte and a "solvent -in -salt" -type Li S battery electrolyte) and working voltage ranges (1.0-3.0 V and 1.5-3.0 V vs Li/Li+). The FeS2@carbon fiber electrode shows stable cycling performance in both the conventional' carbonate electrolyte and the solvent -in -salt -type Li S battery.electrolyte in the voltage range of 1.5-3.0 V. Electrochemical tests in the solvent -in -salt -type Li S battery electrolyte indicate that the Li-FeS2 system becomes a hybrid of the Li S cell and Li iron sulfide cell after the initial cycle. Based on the understanding on the capacity decay mechanisms, the cycling stability of the Li FeS2 system in the voltage range of 1.0-3.0 V is then significantly enhanced by coating the FeS2@carbon fiber electrode with a thin layer of Al2O3. The Al2O3-coated electrode demonstrates excellent cycling performance with high discharge energy densities at both the material level (similar to 300 Wh/kgFeS(2)) and the electrode level (similar to 000 Wh/kg-FeS2 electrode).