Electrochemical Properties and Microstructural Analysis of Li2feso Cathode Material for All-Solid-State Battery
Authors: Hikima Kazuhiro , Nishimoto Maro , Matsuda Atsunori 
Published: December 2024

All-solid-state battery configurations using a solid electrolyte provide the promise of next-generation high-performance batteries. One of the main problems with all-solid-state lithium-ion batteries is their low cathode capacity, which requires the use of conductive carbon additives. The oxy-sulfide Li2FeSO with a cubic anti-perovskite structure has been investigated as a new cathode material due to its high theoretical capacity (455 mAhg-1). We constructed an all-solid-state battery with a Li2FeSO electrode that demonstrated a relatively high discharge capacity of approximately 270 mAh g-1 at a high electrode loading ratio (90 wt%) [1]. Hence, we demonstrated that Li2FeSO is suitable for all-solid-state batteries. However, the reason for its superior performance remains unclear. Therefore, in this study, Li2FeSO was synthesized via mechanical milling, and the charge compensation mechanism during the battery reaction was analyzed via its cross-sectional microstructure.
Li2O (FujiFilm Wako > 95%), S (Aldrich > 99.98%), and Fe (FujiFilm Wako > 99.9%) were mixed in a 1:1:1 molar ratio for the synthesis of Li2FeSO using an agate mortar. Planetary ball-milling was performed at 650 rpm for 72 h, and the precursor powder was pressed via uniaxial pressing and sintered in a tube furnace under an Ar flow at 650 °C for 2 h. The crystal structure was then characterized using X-ray diffraction (XRD), and the electrochemical properties were measured using direct current polarization and impedance spectroscopy. The cross-sectional microstructure of the cathode composite was observed using field-emission scanning electron microscopy. The intrinsic mechanical properties of the samples were examined using the indentation method [2]. The battery performance was characterized by charge–discharge measurements, and hard X-ray photoelectron spectroscopy (HAXPES) was used to analyze valence changes.
The XRD pattern was attributed to the cubic anti-perovskite structure with a Pm3̅m space group. The Li2FeSO all-solid-state battery showed an approximate 270 mAh g-1 reversible discharge capacity (Fig. 1). According to scanning electron microscopy imagery, a dense body was observed, even at a high Li2FeSO active material loading ratio (90 wt%). From an indentation test, the elastic modulus, Mayer hardness, and yield point were found to be 24.9, 0.46, and 0.82 GPa, respectively, which are similar to those of sulfide solid electrolyte all-solid-state batteries. The excellent low modulus and high formability due to the presence of sulfur contribute to superior battery performance. Finally, HAXPES measurements were conducted to analyze the charge compensation mechanism. Fe oxidation in the low-voltage region and S oxidation in the high-voltage region occurred during charging. In contrast, S reduction in the high-voltage region and Fe reduction in the low-voltage region occurred during discharge.

Acknowledgments：This work was supported by JSPS KAKENHI (Grant Number JP 21K14716), Tokai Foundation for Industrial Technology, and Hibi Science Foundation. The HAXPES experiments were conducted at BL6N1 at the Aichi Synchrotron Radiation Center, Aichi Science & Technology Foundation, Aichi, Japan (Proposal No. 202205052 and No. 202302108).

References:

[1] M. Miura, K. Hikima, A. Matsuda, Fabrication and Electrochemical Characterization of an All-solid-state Battery with an Anti-perovskite Electrode Material (Li2Fe)SO. Chemistry Letters
,
2022, 51, 690-692.
[2] K. Hikima, M. Totani, S. Obokata, H. Muto, A. Matsuda, Mechanical Properties of Sulfide-Type Solid Electrolytes Analyzed by Indentation Methods. ACS Applied Energy Materials
2022, 5, 2349-2355.

Figure 1