Improvement of Rate Performance for All-Solid-State Na15Sn4 / Amorphous TiS3 Cells Using 94Na3PS4･6Na4SiS4 Glass-Ceramic Electrolytes
Authors: Tanibata Naoto 
Published: February 2020

All-solid-state batteries have been attracting attention because they do not suffer from leakage, volatilization, and flammability of electrolytes. Sodium-ion batteries are considered to be a promising low-cost alternative to common lithium-ion batteries because of the abundance of sodium sources. All-solid-state sodium rechargeable batteries are expected to be highly safe and low-cost. We have found that all-solid-state sodium batteries with a Na3PS4 glass-ceramic electrolyte successfully operated as a rechargeable battery at 25oC [1]. The Na3PS4 glass-ceramic exhibited a high conductivity of 4.6 x 10-4 S cm-1 at room temperature. The all-solid-state Na15Sn4 / amorphous TiS3 (a-TiS3) cell using Na3PS4 glass-ceramic showed a high capacity of about 300 mAh g-1 and good cycle performance [2]. Development of rate performance of the cell is next issue to be solved. We have recently found that 94Na3PS4･6Na4SiS4 (mol%) glass-ceramic showed a higher conductivity of 7.4 x 10-4 s cm-1 at room temperature than the Na3PS4 glass-ceramic [3]. In this study, the addition of acetylene black (AB) as a conductive additive to Na15Sn4 negative electrode and the application of 94Na3PS4･6Na4SiS4 glass-ceramic as a solid electrode were conducted to improve the rate performance of the all-solid-state Na15Sn4 / a-TiS3 cells.
All-solid-state Na15Sn4 / a-TiS3 cells using Na3PS4 glass-ceramic or 94Na3PS4･6Na4SiS4 glass-ceramic solid electrolyte (SE) were fabricated. Na15Sn4 powder was prepared by ball milling of Na and Sn using a planetary ball milling apparatus. Na15Sn4-AB composite was also prepared by ball milling. Na3PS4 glass-ceramic and 94Na3PS4･6Na4SiS4 glass-ceramic were prepared by ball milling and consecutive heat treatment [1, 3]. a-TiS3 was prepared by ball milling of an mixture of crystalline TiS2 and sulfur [4]. The a-TiS3 positive electrode was prepared by well-mixing a-TiS3, AB and Na3PS4 or 94Na3PS4･6Na4SiS4 glass-ceramics SE with the weight ratio of 20 : 3 : 30.
AC impedance measurement of the Na15Sn4 / a-TiS3 cells using Na3PS4 glass-ceramic showed that the resistance attributable to the Na15Sn4 negative electrode became bigger with charge-discharge cycles. The resistance was decreased by adding AB as a conductive additive to Na15Sn4 negative electrode. Rate performance of the Na15Sn4-AB / a-TiS3 cell was better than that of the Na15Sn4 / a-TiS3
cell.
The cell using the 94Na3PS4･6Na4SiS4 glass-ceramic electrolyte showed better rate performance than the cell using the Na3PS4 glass-ceramic electrolyte. The AC impedance measurement showed that the cell resistances attributable to the separator SE and the positive electrode were decreased by using the 94Na3PS4･6Na4SiS4 glass-ceramic.
The rate performance of all-solid-state Na15Sn4 / a-TiS3 cells was improved by adding AB to the Na15Sn4 electrode and using the 94Na3PS4･6Na4SiS4 glass-ceramic with higher conductivity than Na3PS4 glass-ceramic.
References
[1] A. Hayashi et al.,
Nat. Commun. 3 (2012) 856.
[2] N. Tanibata et al., Abstracts of the 54th Battery Symposium in Japan, p. 281 (2013).
[3] N. Tanibata et al., submitted.
[4] A. Hayashi et al., Chem. Lett., 9 (2012) 886.

Acknowledgement: This research was partially supported by JST, “Advanced Low Carbon Technology Research and Development Program (ALCA)”.