Structural and dynamic behavior of lithium iron polysulfide Li8FeS5 during charge-discharge cycling

Lithium sulfide (Li2S) is one of the promising positive electrode materials for next-generation rechargeable lithium batteries. To improve the electrochemical performance of electronically resistive Li2S, a Fe-doped Li2S-based positive electrode material (Li8FeS5) has been recently designed and found to exhibit excellent discharge capacity close to 800 mAh g−1. In the present study, we investigate the structural and dynamic behavior of Li8FeS5 during charge-discharge cycling. In Li8FeS5, Fe ions are incorporated into the Li2S framework structure. The Li2S-based structure is found to transform to an amorphous phase during the charge process. The delithiation-induced amorphization is associated with the formation of S-S polysulfide bonds, indicating charge compensation by S ions. The crystalline to non-crystalline structural transformation is reversible, but Li ions are extracted from the material via a two-phase reaction, although they are inserted via a single-phase process. These results indicate that the delithiation/lithiation mechanism is neither a topotactic extraction/insertion nor a conversion-type reaction. Moreover, the activation energies for Li ion diffusion in the pristine, delithiated, and lithiated materials are estimated to be in the 0.30-0.37 eV range, which corresponds to the energy barriers for local hopping of Li ions along the Li sublattice in the Li2S framework.