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 Li(x)FeS(5) 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 Li(2)FeS(5) phase changes into a superparamagnetic insulating Li(10)FeS(5) 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.