Investigation of Li-Ion Sulfur Battery Using Highly Composed S8 Positive Electrode and Li-Doped C6 Negative Electrode

Abstract

Recently, demands of rechargeable batteries are increasing with applying to load-leveling systems for in/output natural energy, such as solar / wind power. Thus, lithium-sulfur (Li-S) batteries are attracted attention because of sulfur-based positive electrodes have high theoretical reversible capacity (1,672 mAh/g-S8), low active material cost and so on. On the other hand, Li-S batteries still have some issues for practical use. For instance, dissolution of intermediate products (Li2S n ) into electrolyte and deposition of Li-dendrite during charge process are serious problems for Li-S batteries. Then, we applied sulfolane (SL) based electrolyte because of their low-solubility of Li2S n into electrolyte, and prepared Li-doped graphite (Li x C6) negative electrodes by electrochemical process to prevent deposition of Li-dendrite. However, Li x C6 electrodes should be handled under inert atmosphere. Therefore, we used to disassemble-type cell, which can easily pick up Li x C6 electrodes. In addition, increasing for loading amount of elemental S8 positive electrodes are required for actual high-energy-density systems. Therefore, we fabricated 3D-S8 electrode prepared by using 3D-Al current collector, which can easily maintain electron conductive pathway. In this study, we fabricated Li-S battery using 3D-S8 positive electrode and Li x C6 negative electrode (litihium-ion-sulfur battery), and evaluated by constant current charge-discharge cycle test. S8 and ketjen black (KB) were mixed in mortar, and mixed compound was heated at 438K into sealed bottle for compositing S8 and KB. Slurry was prepared by the mixed compounds and binders (CMC+SBR) into H2O. 3D-S8 electrodes were fabricated by putting on the slurry to 3D-Al cermet current collector. Prepared 3D-S8 electrodes were dried, punched out φ13 mm and pressed at 0.3t. First, [Li | electrolyte | C6] cells were prepared using disassemble-type cell, and measured with constant current charge-discharge tests. Li x C6 (x<1) negative electrodes were picked out by disassemble-type cell in the inert Ar-filled glove box, and washed with DMC. [Li x C6 | SL based electrolyte | 3D-S8] cells were fabricated with 2032 type coin cell, and evaluated by constant current charge-discharge cycle test. Fig. 1(a) shows charge-discharge profiles of [Li x C6 | SL-based electrolyte | 3D-S8] cell. Obtained first discharge capacity was ca. 1,000 mAhg-1. Similar capacity and low over potential compared with same-type [Li | electrolyte | 3D-S8] cells, because of low resistance of Li x C6 electrode than Li-metal electrode. In addition, reaction resistances of second plateau at discharge process and initial charge process gradually decreased with cycles. Those phenomenon are showed frequently normal Li-S battery using Li-metal negative electrode, and regard as constitution of ionic conductive pathway on positive electrode. In addition, capacity changes of 1st plateaus were quite lower than 2nd plateaus during discharge process at any cycles. From the above, low capacity degradation of [Li x C6 | SL-based electrolyte | 3D-S8] cell should be caused by constitution of conductive pathway and slightly capacity degradation. Fig. 1(b) shows cycle number dependences of charge-discharge capacity and coulombic efficiency of [Li x C6 | SL based electrolyte | 3D-S8] cell. Quite high coulombic efficiencies (ca. 98.5 %) were confirmed except for first cycle. Also, electrochemically stable charge- discharge operation, in other words, high performance lithium-ion-sulfur battery was achieved. Figure 1