One-Step Fabrication of Sulfur Composite Electrode for All-Solid-State Lithium-Sulfur Battery Using High-Temperature Mechanical Milling Method

Abstract

Since higher energy and power density than those of lithium ion batteries are required for next generation battery system, lithium-sulfur battery attracts attention because of its large theoretical capacity of sulfur: 1672 mAh g–1. All-solid-state battery system incorporating solid electrolyte can solve serious issue of dissolution of lithium polysulfide during battery reaction and contribute higher power and energy density [1]. However, ionic/electronic-insulating properties of sulfur prevents practical use of sulfur as electrode. Therefore, composite electrodes incorporating sulfur, conductive carbons and solid electrolytes are fabricated for use in solid-state battery system. In this study, sulfur, acetylene black and Li2S-GeS2-P2S5based solid electrolyte were mixed to obtain the composite electrode using a planetary ball milling apparatus, which has temperature control system. Structures and electrochemical properties of the composites and their milling-temperature dependences were investigated. Solid electrolyte of Li4–x Ge1–x P x S4 (x = 0.75) was prepared by a solid-state reaction method [2]. Ionic conductivity of the solid electrolyte was evaluated by ac-impedance method at room temperature. Sulfur, acetylene black and solid electrolyte were milled with a weight ratio of 25:25:50 (wt.%). Various mixing time (10–600 min) and temperature during mixing (298–443 K) were examined; their structure and electrochemical properties in all-solid-state battery were evaluated using scanning electron microscopy (SEM), XRD measurement, Raman spectroscopy and galvanostatic charge-discharge test. All-solid-state batteries were constructed in an Ar-filled glove box for charge-discharge measurements [3]. Electrochemical properties of the cells were evaluated by galvanostatic charge-discharge measurements using multi-channel galvanostat. A constant current of 83.75 mA g–1(1/20 C) was applied for charging and discharging at room temperature. Since the composite electrode prepared by 300 min-milling at room temperature showed highest initial discharge capacity about 1200 mAh g-1, milling time was determined at 300 min for high temperature milling process. Composite electrodes were prepared by high temperature milling at 403, 423 and 443 K; in this temperature region, the sulfur exists as liquid phase. Increase in milling temperature contributed to larger capacity and better capacity retention during charge-discharge reactions for 20 cycles. Raman spectroscopy indicated that novel structure unit including Ge-S-Ge bonding was generated in the composite via milling process. The novel unit could be due to the reaction between sulfur and solid electrolyte. Differential capacity plots of the charge-discharge curves and ex situRaman spectroscopy revealed that the novel structure units might contribute to the charge-discharge reaction and phase transition reaction during charge-discharge process occurred reversibly for the composite electrode with high temperature milling. On the other hand, the composited prepared at room temperature showed capacity fading during cycling due to irreversible phase transition of the electrode. Liquid phase sulfur during milling process could contribute to construct the electronic and ionic conduction pathways in the composite matrix because the liquid phase is suitable for nano/atomic scale distribution and better contact to the each component. One-step high-temperature milling process can be a promising candidate for sulfur composite electrode fabrication. Acknowledgements This work was supported by a Grant-in-Aid from the Advanced Low Carbon Technology Research and Development Program, Specially Promoted Research for Innovative Next Generation Batteries (ALCA- SPRING) of the Japan Science and Technology Agency (JST).