Cathode design of lithium-sulfur battery

Kurzfassung

The rapid development of wireless electronic devices, electric vehicles and grid storage has brought increasing demands for the energy storage systems with high energy density. However, the overall energy density of the state-of-art lithium-ion batteries is limited to only about 300 Wh (kg)-1, which is not sufficient to satisfy the demands of high energy density. Therefore, new, alternative energy storage systems with higher theoretical specific energy are required. Besides various systems, lithium-sulfur battery (Li-S) has been regarded as one of the most promising candidates due to its higher theoretical gravimetric energy density of 2600 Wh(kg)-1, much higher than those of the conventional lithium-ion battery. Moreover, the low cost, natural abundance and environmental friendliness of elemental sulfur are beneficial. Despite the advantages of the Li-S battery, some obstacles still exist in sulfur-based cathode, which hinder its practical development. The poor electronic conductivity of sulfur, significant volume changes during electrochemical reaction and the high solubility of polysulfide intermediates in electrolyte may lead to poor cycle performance and crack formation caused by volume change. Our strategy to address these problems includes design of novel aerogel based cathode: 1. synthesis of elastic deformable cathode material to prevent fracturing in the structure while volume change, 2. embedding of sulfur into the microporous (pore size < 2 nm), “cage-like” structure of carbon aerogel leading to hold-on of sulfur inside of cathode and reduces the loss of active material (sulfur). In our work we synthesized elastic deformable resorcinol-formaldehyde aerogels via a sol-gel process. Afterwards they have been carbonized in argon atmosphere to carbon aerogels. Carbonized aerogels have been milled in a ball mill and the powder was embedded with sulfur in gas phase under vacuum. After sulfur infiltration a Li-S cell was prepared by using 1M LiPF6 EC/DEC/FEC as electrolyte. The N2-adsorption analysis showed that the surface area of flexible carbon aerogel is about 560 m² g-1 and micropore volume 0.176 cm³ g-1. The coulombic efficiency exhibits nearly 100 % after 200 cycles. The specific capacity is slowly decreasing but even after 200 cycles it is very high about 600 mAh g-1. These first results are very promising. High coulombic efficiency which were reached with elastic deformable carbon aerogels approved our assumption, that reversible flexibility and high microporosity of cathode material are crucial for high cycle stability of a Li-S battery.