Analyses of Charge/Discharge Mechanism of Fe-Containing Li2S-Based Positive Electrode Material Using X-Ray Absorption and Scattering Measurements
Authors: Takeuchi Tomonari , Kageyama Hiroyuki , Nakanishi Koji , Ogawa Masahiro , Kawaguchi Tomoya , Ohara Koji , Fukuda Katsutoshi , Sakuda Atsushi , Ohta Toshiaki , Fukunaga Toshiharu , Sakaebe Hikari , Kobayashi Hironori , Matsubara Eiichiro 
Published: February 2020

1. Introduction
Lithium sulfide (Li2S) is one of the promising cathode active materials for high-energy rechargeable lithium batteries because of its high theoretical capacity (ca. 1170 mAh・g-1) [1]. However, this material shows both electronically and ionically resistive property, which gives rise to relatively low electrochemical performance in the cells [2]. Several attempts have been performed to enhance the conductivity of Li2S, such as forming composites with several metals (Li2S-Fe, Li2S-Cu) [1,2] or carbon [3,4]. Recently, we have tried to prepare Fe-containing Li2S-based positive electrode materials (Li8FeS5) and found that the cells showed the discharge capacity of ca. 330 mAh・g-1 and it was enlarged to ca. 730 mAh・g-1after the pre-cycling treatment [5]. However, its charge/discharge mechanism and the effect of the pre-cycling treatment were still unclear.
In the present work, we analyzed the structure of Li8FeS5for Li extraction/insertion reactions with/without pre-cycling treatment using X-ray absorption and scattering measurements.
2. Experimental
Li2S and FeS with a molar ratio of 4 : 1 were blended thoroughly, and the mixture was then treated by the SPS process at 600oC [5]. The resulting pellet was ground and mixed with carbon (acetylene black) powder with a weight ratio of 9 : 1, and then it was mechanically milled for 8 h to yield the Li8FeS5sample.
The obtained Li8FeS5 sample was characterized by X-ray diffraction, as well as inductively coupled plasma (ICP) and atomic absorption spectroscopy. Electrochemical lithium extraction / insertion reactions (charge/discharge) were carried out using lithium coin-type cells with 1M LiPF6 / (EC + DMC) electrolyte at a current density of 46.7 mA・g-1(corresponding to 0.04C) initially with charging with and without stepwise pre-cycling treatment [5]. Fe K-edge X-ray absorption fine structure (XAFS) of the composite cathode materials before and after charge/discharge cycling, sealed in argon filled aluminum laminate bags, were measured using a laboratory-type spectrometer (Technos EXAC-820) and S K-edge XAFS of the materials were measured in vacuum chamber by the beamline BL-10 in SR center of Ritsumeikan University, respectively. We also carried out the X-ray scattering measurements at the beamline BL28XU in SPring-8, and the obtained data were conducted by pair distribution function (PDF) analysis [6].
3. Results and Discussion
The obtained Li8FeS5 sample was black in color, and the XRD pattern showed that it consisted of low-crystalline Li2S (antifluorite structure) [5]. The atomic ratio, estimated by ICP and atomic absorption spectroscopy, was Li : Fe : S = 7.8 : 1.0 : 4.8 [5].
The radial structure functions around S atoms for the Li8FeS5 sample, calculated from EXAFS spectra, showed irreversible changes without pre-cycling treatment, while they showed reversible changes after the pre-cycling treatment. And the radial structure functions around Fe atoms showed that the first neighbor peak shifted to shorter distance and the amplitude decreased significantly after the initial charge without pre-cycling treatment. Although the peak position and the amplitude reversibly changed to the initial ones after the subsequent discharge, actually little change was observed in the second cycling. These are consistent with the charge/discharge performances of the cells, where they showed relatively low initial capacity and very rapid cycle degradation. On the contrary, the reversible shift of the distances and change of amplitudes of the first neighbor peaks were observed for the charge/discharge cycling after the pre-cycling treatment. Such stabilized local structure around Fe atoms for Li extraction/insertion reactions might be assigned to the Fe atoms that occupy partially Li site of low-crystalline Li2S lattice with antifluorite structure, which would be formed during the pre-cycling treatment. Probably it would result in the relatively higher capacity and improved cycle performance of the cells.
We also carried out the PDF analyses of the Li8FeS5sample with/without pre-cycling treatment, and the calculated radial distribution functions were compared with those obtained by the EXAFS analyses, which will be presented in the Conference.
References
[1] A. Hayashi et al., J. Power Sources, 183, 422 (2008).
[2] M. N. Obrovac and J. R. Dahn, Electrochem. Solid-State Lett., 5, A70 (2002).
[3] M. Nagao et al., J. Mater. Chem., 22, 10015 (2012).
[4] T. Takeuchi et al., J. Electrochem. Soc., 157, A1196 (2010).
[5] T. Takeuchi et al., J. Electrochem. Soc., 162, A1745 (2015).
[6] K. Ohara et al., Sci. Rep., 6, 21302 (2016).
Acknowledgment
This work was financially supported by R&D project for lithium batteries by METI and NEDO.