Spin polarization in the phase diagram of a Li-Fe-S system.

Divalent and trivalent states of Fe ions are known to be stable in inorganic compounds. We focus a novel LiFeS cathode, in which the Li content (x) changes from 2 to 10 by an electrochemical technique. As x increases from 2, a Pauli paramagnetic conductive LiFeS phase changes into a superparamagnetic insulating LiFeS phase. Density functional theory calculations suggest that Fe ions in a high-x phase are responsible for ferromagnetic spin polarization. Reaching the monovalent Fe ion is significant for understanding microscopic chemistry behind operation as Li-ion batteries and the original physical properties resulting from the unique local structure.