Characterization of Sulfur-Phosphorus Sulfide Composite Electrodes in All-Solid-State Na/S Batteries
Authors: Tanibata Naoto , Hayashi Akitoshi , Tatsumisago Masahiro 
Published: February 2020

Sodium-sulfur batteries (NAS batteries) have a high energy density (760 Wh kg-1) and are used in stationary energy storages [1]. The operating temperature is so high (>300oC) where the active materials are melting. We have reported that bulk-type all-solid-state sodium-sulfur batteries worked at room temperature [2]. Na3PS4 glass-ceramics with high sodium ion conductivity (σ25 = 4.6x10-4 S cm-1) [3] were used for the solid electrolyte. Sulfur electrodes were prepared by mechanical milling of sulfur with Na3PS4 glass-ceramics and electron conductive additives because sulfur is an insulator. The all-solid-state sodium-sulfur batteries using the solid electrolyte and the sulfur composite electrode showed a high reversible capacity of 1106 mAh (g-sulfur)-1 at room temperature. Sulfur content of the sulfur composite electrodes was only 25 wt%. In general, lower proportion of ion and electron conductors in composite electrodes decreases the utilization of sulfur active materials. Recently, Nagata et al. have reported all-solid-state sodium-sulfur batteries using P2S5 instead of Na3PS4 glass-ceramic solid electrolyte in the sulfur composite electrode [4]. The cell with 50 wt% of sulfur active material showed a high capacity of 1456 mAh (g-sulfur)-1 at the first discharge process. The coulomb efficiency was low but the reversible capacity was 671 mAh g-1. The discharge-charge mechanism in the cell seems to be different from that of the cells using conventional solid electrolytes. In this study, all-solid-state sodium-sulfur batteries using sulfur composite electrode with P2S5 were prepared and structure of the electrodes before and after discharge-charge process was examined.
Ketjen black (KB) was used for an electron conductive additive. S-KB-P2S5 composite electrodes were prepared by a mechanical milling technique using a planetary ball mill apparatus. The composition of S : KB : P2S5 was 45 : 19 : 36 (wt%). The structure of the S-KB-P2S5 composite electrodes was analyzed by XRD measurement and Raman spectroscopy. Sodium-tin alloy and Na3PS4 glass-ceramic powders were used for a negative electrode and a separator for bulk-type all-solid-state batteries. The batteries were prepared by pressing these powders. Galvanostatic measurement was done at the current density of 0.13 mA cm-2. A sulfur composite electrode (25% discharge electrode) was prepared by discharging 25% capacity against the theoretical capacity of sulfur (1672 mAh g-1). In addition to 25% discharge electrode, the sulfur composite electrodes after full discharge and charge processes were analyzed by SEM observation, EDX elemental mapping, XRD measurement and Raman spectroscopy.
The S-KB-P2S5 composite electrode exhibited a XRD halo pattern and Raman bands attributable to P2S5+x unit [5]. All-solid-state sodium-sulfur batteries using the S-KB-P2S5 composite electrodes showed an initial discharge capacity of 1955 mAh g-1 and an initial charge capacity of 796 mAh g-1, respectively. EDX mapping results of the cross section of the 25% discharge electrode suggested that sodium element was only observed at the side of a solid-electrolyte layer. The XRD measurement and Raman spectroscopy of 25% discharge electrode showed that amorphous compounds with PS4 tetrahedral unit [6] and polysulfide anion unit [7] were formed. Each element (P, S, Na, C) evenly existed in the sulfur composite electrodes after full discharge and charge process. Crystalline Na2S and Na3PS4 were precipitated in the electrode after the full discharge process. The crystalline Na3PS4 also existed in the electrode after the charge process. The residual of crystalline Na3PS4 seems to be one reason of irreversible capacity in the cells.

Acknowledgement: This research was partially supported by Kyoto University, “Elements Strategy Initiative for Catalysts and Batteries (ESICB)”.
[1] X. Lu et al., J. Power Sources, 195 (2010) 2431.
[2] N. Tanibata et al., Abstracts of the 81th Electrochemistry Symposium in Japan, p. 425 (2014).
[3] A. Hayashi et al., Nat. Commun., 3 (2012) 856.
[4] H. Nagata et al., Chem. Lett., 43 (2014) 1333–1334.
[5] P. Boolchand et al.,
J. Non-Cryst.
Solids, 355 (2009) 1773.
[6] G. J. Janz et al., Inorg. Chem., 15 (1976) 1759.
[7] C. Bischoff et al., J.Non-Cryst.Solids,
358 (2012) 3216.