An Amorphous Si Anode: A High Capacity Anode for All-Solid-State Lithium Battery

Lithium ion batteries (LIBs) are widely used as a versatile power supply in various portable devices because of their high energy densities. On the other hand, further enlargement of their capacity is still necessary to apply them in electric vehicles in the future. One way for the enlargement is employment of high capacity electrodes. Si is one of the most promising potential candidates for the anodes in next-generation LIBs, because its theoretical capacity is 4200 mAh/g for the alloying reaction to Li_4.4Si, and the value is more than 10 times higher compared to that of the graphite anodes.¹ However, the Si has to undergo severe volume change during the electrode reaction to take advantage of the exceptionally high lithium storage capacity; this change leads to some drawbacks including rapid capacity fading.² To circumvent this difficulty, many efforts have been focused on nanostructured Si in the last decade.³ On the other hand, even a bulky FeS‑doped a‑Si film has been reported to exhibit excellent anode performance in a solid electrolyte recently.⁴ In the present work, we prepared a‑Si films without FeS and investigated the electrode properties, because the effects of FeS on the electrode performance in the previous study were not clear.

a‑Si films were deposited by RF‑magnetron sputtering onto stainless steel substrates (10 mm diameter) using CFS‑4EP‑LL (Shibaura Mechatronics). High-purity (99.999%) Si target was purchased from Kojundo Chemical Laboratory. In the cell assembling, In‑Li alloy was used as a counter electrode. 70Li₂S‑30P₂S₅ glass ceramic⁵was selected as the solid electrolyte. The electrode performance of the films were investigated in the all-solid-state cell by using multichannel potentio-galvanostats, PS‑08 (Toho Technical Research) and VSP (Biologic).

Fig. 1(a) shows discharge curves for a 300‑nm‑thick a‑Si film at various current densities. The discharge capacity exceeds 2400 mAh/g even at the high discharge current density of 10 mA/cm². In Fig. 1(b), the plateau voltages are plotted as a function of the current density, revealing linear relationship with current density. The slope derived from the linear relationship is 107 µÙ, which is almost the same as the resistance of the electrolyte layer in the solid-state cell. Therefore, it can be concluded that the overvoltage is predominantly originated from iR drop in the electrolyte layer and the kinetics of the electrode reaction is very fast. On the other hand, the large volumetric change in Si anodes was reported to promote growth of SEI layer, which raises the interfacial resistance to the order of 10³ Ω cm².⁶The low electrode resistance observed in this study suggests absence of SEI layer in this all-solid-state electrochemical system, which is considered to have positive effects on other electrode properties. They will be also discussed in the meeting.

AcknowledgementsFinancial support by the NEDO Japan and the Toyota Motor Corp. for a project entitled “Applied and Practical LiB Development for Automobile and Multiple Application” is gratefully acknowledged. We also thank the MANA Foundry and the NIMS Molecule & Material Synthesis Platform in “Nanotechnology Platform Project” operated by the MEXT Japan for the use of research facilities.

 

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