Anode Properties of Si–FeS Films Prepared By Pulsed Laser Deposition in Solid-State Lithium Batteries

Lithium–silicon alloys are one of the most attracting anode materials for next-generation lithium-ion batteries due to the high theoretical capacity, low electrode potential, and the second largest Clark number.  However, they have exerted their high performance only in nano-sized form.  This study reveals that Si-based anodes fabricated into films exhibit excellent performance even in a bulky state, when they are in a solid electrolyte.

The Si-based films were deposited on stainless steel plates used as current collectors by pulsed laser deposition.  Because it is impossible to ablate pure Si by the KrF excimer laser used in this study, 10wt% of FeS, which was reported to be effective in enhancing electrode activity in solid electrolytes [1], was added to Si powder before pressed into a target in order to allow the ablation.  Film thickness was varied from 30 nm to 1 µm.  Electrode properties of the films were investigated in a solid electrolyte, 70Li₂S–30P₂S₅ glass ceramics, with an In–Li alloy as a counter electrode.  The films were galvanostatically lithiated (charged) down to 0.01 V vs. Li⁺/Li and then delithiated (discharged) up to 2.62 V vs. Li⁺/Li at various discharge rates. 

The Si–FeS films exhibit excellent performance in the solid electrolyte, as shown in the figure.  The rate dependence of the discharge capacity clearly indicates that a film with thickness of 30 nm delivers a high capacity approaching the theoretical value at 0.1 C-rate discharge and maintains a high capacity of 2300 mAh g⁻¹ at 100 C.  Although such high rate capability may have been reported only for nano-sized Si materials, the film keeps it against increasing thickness.  Even a 1-µm-thick film anode delivers a capacity of 3100 mAh g⁻¹ at a discharge rate of 0.1 C, and 2500 mAh g⁻¹is kept at a high rate discharge of 10 C. 

It should be emphasized that such high performance is realized in a solid-state cell, which will be free from safety issues.  In addition, the use of solid electrolytes is effective in improving cycling performance, which is a great drawback of Si anodes, by preventing detachment of the active material from current collectors.  Capacity retention observed for a 400 nm-thick film over 120 cycles exceeds 80% in the solid electrolyte, while it decreases to 38% in an organic liquid electrolyte. 

Acknowledgement

This study was partly supported by the Li-EAD project of the New Energy and Industrial Technology Development Organization (NEDO), Japan

Reference

[1] B. T. Hang, T. Ohnishi, M. Osada, X. X. Xu, K. Takada, and T. Sasaki, J. Power Sources 195 (2010) 3323. 

Figure.  Rate dependence of discharge capacity from the Si−FeS films.