為研發具競爭力之低成本與高能量密度的先進儲能設備，以因應現今社會逐漸迫切之能源需求，鋰硫電池作為一具有豐富材料資源、高理論電容量 (1,675 mA·h g-1) 與高能量密度 (2,600 W·h kg-1) 之可充電電池設計，可作為未來最具潛力之儲能系統發展選項；然而，商業化鋰硫電池需同時提升活性物質載量並下降電解液使用量以提升其能量密度，而上述條件則成為鋰硫電池發展中難以突破之瓶頸。
為追求可達商業化鋰硫電池需求之性能，本研究共提出四種鋰硫電池結構，分別針對鋰硫電池之陰極材料與固態電解質進行討論。陰極設計方面，分別以純硫、多硫化物與硫化鋰等硫還原衍生產物作為鋰硫電池陰極活性材料，藉由使用選擇性化學吸附材料 (環安息香酯)、無微孔之集流體設計 (商用奈米碳管/奈米碳纖維) 與可降低硫化鋰活化電位之材料 (硫化磷)，於三種不同之陰極設計中皆達成高硫載量、高硫含量與寡電解液之條件，並同時達到高放電容量、高能量密度與長循環壽命之電化學性能；此外，更延伸硫化物之應用，提出以多硫化物陰極搭配具高導離子性之硫化物固態電解質的固態鋰硫電池設計，改善過往文獻中固態鋰硫電池陰極與固態電解質間之固相介面電阻問題，並有效提升陰極硫載量與含量，達成商業化鋰硫電池所需之高活性物質載量與寡電解液環境需求。

The Material Development and Electrochemical Mechanisms of Sulfides in Designing High-energy-density Lithium-sulfur Batteries
Author: Yin-Ju Yen
Advisor: Sheng-Heng Chung
Department of Materials Science and Engineering, College of Engineering
National Cheng Kung University

INTRODUCTION

Lithium-sulfur battery is an electrochemical system featuring a high theoretical charge-storage capacity of 1,675 mA·h g-1 and a low material cost of $150 per ton. The high abundance of sulfur turns the lithium-sulfur batteries into one of the most expectedly rechargeable batteries in the next generation. To realize the commercialization of lithium-sulfur batteries, high energy density is required for the current studies, which may be achieved by increasing the sulfur loading and content, and decreasing the added amount of electrolyte. However, the insulating nature of the sulfur cathode with a high loading and content, and the lean electrolyte condition leads to the difficulty in the lithium-ion transfer. The repeated conversion between the solid/liquid active materials is thus blocked and contributes to the deterioration of the cell performance.
In pursuit of high-energy-density lithium-sulfur batteries, we demonstrate four different designs of the cell structures, including applications of three cathode materials and one cell design with the solid electrolyte material. For the cathode designs, we adopt sulfur, polysulfide and lithium sulfide, as the active materials. In the modification of sulfur cathode, sulfur is incorporated with a porous molecular crystal for its selective adsorption property. In the polysulfide cathode, a non-microporous current collector is applied for the prevention of fast electrolyte consumption. In the lithium sulfide cathode, phosphorous pentasulfide is added to form ionically-conductive materials. With the three cathode designs, the high sulfur loading, the high sulfur content, and the lean electrolyte-to-sulfur ratios are all achieved. In addition, they all attain desirable electrochemical performances of a high charge-storage capacity, a high energy density, and a long cycle life. Moreover, we further extend the application of sulfide materials to sulfide solid electrolytes. With the novel incorporation of a highly ionically conductive sulfide solid electrolyte and a polysulfide cathode, the high solid/solid interface resistance between the sulfur cathode and the solid electrolyte in common solid-state lithium-sulfur batteries can be significantly improved. The enhanced sulfur loading and sulfur content brought by the polysulfide cathode and the improved contact interface provides a new research direction for developing all-solid-state lithium-sulfur batteries with high energy densities.

MATERIALS AND METHODS
The first cathode design is to incorporate sulfur with a porous molecular crystal material, cyclobenzoin ester, by a sulfur-melting method. The second cathode design is the application of a non-microporous carbon nanotube/nanofiber (CNT/CNF) as the polysulfide cathode substrate by a drop-casting method. The third cathode design is the addition of phosphorus pentasulfide (P2S5) in the lithium sulfide (Li2S) catholyte to synthesize the Li2S-P2S5-based cathodes at a low temperature. The last cell design is the adoption of the polysulfide cathode with the ball-milled sulfide solid electrolyte to fabricate a solid-state lithium-sulfur battery.

RESULTS AND DISCUSSION
In the cathode design of sulfur with cyclobenzoin ester, the abundant carbonyl groups in cyclobenzoin ester provide an efficient accommodation for polysulfide adsorption, despite of its porous structure, as shown in Figure 1 (a). With this merit, we improve the cell-fabrication parameters to a high sulfur loading of 4 mg cm-2, a high sulfur content of 80 wt%, and a lean electrolyte-to-sulfur ratio of 4 μL mg-1. The selective adsorption behavior of cyclobenzoin ester between the polysulfide and electrolyte prevents the fast electrolyte consumption and enhances cycling performances.
In the cathode design of the CNT/CNF-polysulfide cathode, the non-microporous CNT/CNF substrate maintains well electrochemical conversion of polysulfides by inhibiting fast electrolyte adsorption by micropores. Therefore, we simultaneously achieve a high sulfur loading of 8.64 mg cm-2, a high sulfur content of 68 wt%, and low electrolyte-to-sulfur ratios of 7–4 μL mg-1. Besides, we propose the cell failure mechanism under high-loading and lean-electrolyte condition with the observation of dropped Coulombic efficiency and overcharge behavior at a slow rate, as shown in Figure 1 (b).
In the cathode design of Li2S-P2S5-based cathodes, we present a facile and low-temperature synthesis of ionically-conductive thiophosphates during the cathode fabrication, as shown in Figure 1 (c). With the improved ionic conductivity in the cathode, we elevate the cell-fabrication parameters to a high Li2S loading of 3.75 mg cm-2, a high Li2S content of 71 wt%, and a low electrolyte-to-Li2S ratio of 10 μL mg-1. In addition, we confirm the reduction of the activation overpotential of Li2S with the help of P2S5 to prevent the electrolyte decomposition at high working voltages and thus improve the electrochemical performance.
In the cell design of the solid-state lithium-sulfur cell, we declare the application of polysulfide cathode with the sulfide solid electrolyte to replace the traditional solid/solid interface between the sulfur cathode and the solid electrolyte to the liquid/solid interface, as shown in Figure 1 (d). This improved interface contact betters the charge transfer and thus enhances the loading of active material and the cycle life. Therefore, we increase the cell-fabrication parameters to high sulfur loading and content of 3–5 mg cm-2 and 66 wt%, with a long cycle life of 80–100 cycles at a C/20 rate.

Figure 1. The schematic mechanism of the proposed cathode and cell structures: (a) The polysulfide adsorption by the cyclobenzoin ester, (b) the conversion of active materials and the indicators of the cell failure, (c) the lithium-ion transfer promoted by ionically conductive thiophosphates in the Li2S-P2S5-based cathodes, and (d) the lithium-ion diffusion in the solid-state lithium-sulfur cell.

CONCLUSION
In this study, we discuss the different cathode designs based on the materials generated in the electrochemical conversion of the sulfur active material. With sulfur, polysulfide, and lithium sulfide as the active material, we all have excellent cell-fabrication parameters and cell performances, respectively. Furthermore, we extend the use of the sulfide material to sulfide solid electrolyte. Both the novel cell design and cathode structures attain high energy densities and long cycle life.

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