Spin polarization in the phase diagram of a Li\u2013Fe\u2013S system

Tsuyoshi Takami,Toshiharu Fukunaga,Tomonari Takeuchi

doi:10.1038/s41598-019-56244-x

Tsuyoshi Takami, Toshiharu Fukunaga + Show 1 more

Open Access

https://doi.org/10.1038/s41598-019-56244-x

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Abstract

Divalent and trivalent states of Fe ions are known to be stable in inorganic compounds. We focus a novel LixFeS5 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 Li2FeS5 phase changes into a superparamagnetic insulating Li10FeS5 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.

Highlights

Divalent and trivalent states of Fe ions are known to be stable in inorganic compounds
The recent discovery of the Li8FeS5 system with an exceptionally high capacity of 800 mAh/g8, which is a fingerprint for up to eight transferrable Li ions (Fig. 1a), gives opportunities to achieve an unusual valence state of the Fe ions owing to a wide range of Li contents (x = 2 –10 in LixFeS5)
The precise structure of the amorphous phase has remained unknown, and X-ray diffraction (XRD), extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) measurements indicated the formation of Fe–S bond and the presence of discrete sulfer ions[8]

Summary

Introduction

Divalent and trivalent states of Fe ions are known to be stable in inorganic compounds. The recent discovery of the Li8FeS5 system with an exceptionally high capacity of 800 mAh/g8, which is a fingerprint for up to eight transferrable Li ions (Fig. 1a), gives opportunities to achieve an unusual valence state of the Fe ions owing to a wide range of Li contents (x = 2 –10 in LixFeS5). This means that the valence state of the Fe ions naturally decreases with increasing x in order to keep charge neutrality.

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