Cathode Design and Evaluation for the High-Sulfur-Loading, Lean-Electrolyte Lithium–Sulfur Battery
Authors: Wu Cheng-Che , Chung Sheng-Heng 
Published: February 2024

Sulfur is a promising active material for the lithium rechargeable batteries because of its high theoretical capacity, a relatively low cost, and an environmentally friendly nature. Lithium–sulfur battery with a high-energy-density is regarded as a third-generation rechargeable battery with high potential for commercialization. The well charged and discharged lithium–sulfur battery can generate specific energy density of up to 600 W∙h kg−1. However, the charging product, sulfur, and the discharging product, lithium sulfide, both have low conductivity, which lead to low active material utilization and excessive addition of conductive materials, lowering the active material content and performance in the cathode region. The liquid active material, polysulfides, formed in the middle of the cycling reaction will dissolve in the electrolyte, causing leakage to the lithium anode to form a non-reactive lithium sulfide layer, causing severe capacity fading. Furthermore, the repeated transformation of sulfur (solid-state), polysulfides (liquid-state), and lithium sulfur (solid-state) active materials led to the sluggish battery reaction and irreversible electrolyte consumption. All and all, the chemical and physical properties of the sulfur and the combination with lithium resulted in low active material utilization, low cycling ability, low capacity-retention rate, and shortened cycle life.

To fully realize the potential of the high-energy-density lithium–sulfur battery, the cathode material, configuration, and fabrication must be carefully considered. Herein, a sulfur cathode fabricated by a hot-pressing method and the evaluation of different porous substrate was also conducted. Porous conductive network is a common cathode material, which provides a large surface area and great polysulfides absorbability. However, as a result of high-sulfur-loading and lean-electrolyte cathode batteries, a low specific capacity and short cycle life are discovered, which is affected by the high electrolyte consumption. Accordingly, a carbon electrospun substrate with a non-nanoporous structure was introduced as the conductive network to achieve the high-energy-density battery design. To fabricate the free-standing sulfur cathode, sulfur powder was loaded between two porous carbon electrospun substrate by a hot-pressing technique. The cathode consists of three layers: carbon current collector, melt-in sulfur layer, and carbon interlayer, forming a sandwich-like cathode.

With this design, we can accommodate high areal loading of sulfur (8 mg cm−2) in the cathode area while lessening the conductive additive (sulfur content of 73 wt%). The hot-pressed electrospun cathode, conducted under low electrolyte usage (i.e., the low electrolyte-to-sulfur ratio of 7 µL mg−1) can achieve excellent energy density (11.8 mW∙h cm−2) with a stable areal capacity up to 5.9 mA∙h cm−
2 let alone with an extended cycle life over 200 cycles. Furthermore, this devised cathode can also retain excellent cycling ability with lower ratios of electrolyte to sulfur (6–4 µL mg−1) and great rate dependent performance up to C/2 cycling rate.

Figure 1