How the microstructure of cathode material influences the performance of lithium-sulfur batteries

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. Therefore, efficient energy storage systems with high performance are required. Besides others, lithium-sulfur battery is one of the most promising candidates due to its higher theoretical gravimetric energy density of 2600 Wh/kg. Moreover, the natural abundance and environmental friendliness of elemental sulfur and low cost are beneficial. Despite the advantages of the Li-S battery, some challenges still exist in sulfur-based cathode, which hinder its practical development. The limited amount of accessible sulfur, significant volume changes during electrochemical reaction and the high solubility of polysulfide intermediates in electrolyte lead to poor capacity and short life time. To address these challenges novel aerogel-based cathode materials were developed. [1,2] In this work, we synthesized and investigated highly microporous carbon aerogels with different degree of flexibility and particle sizes [3] to study their influences on the capacity in lithium-sulfur batteries. We have performed physisorption, electrical conductivity, SEM, and TEM measurements to investigate the microstructure, and compression test in order to study the mechanical properties. TGA measurements were used to evaluate the sulfur loading. The specific capacity was measured in DOL/DME with 0.1 C. The results show, that: • Micropores can encapsulate sulfur and thus reduce the polysulfide shuttle effect • High micropore volume and high electrical conductivity are not the only decisive factors for high performance • Due to long diffusion paths in large particles, the utilization of sulfur is limited • Large primary particles show channels as a result of activation, promoting the shuttle effect • High flexibility is required to compensate volume changes during cycling and to avoid cracks in the electrode The conversion reaction of sulfur and the formation of Li2S cause a large volume expansion, which results in a reduction of electron transport paths and thus a decrease in kinetics. Flexible carbon aerogels with tailored microstructure can accommodate volume expansion due to their ability for reversible deformation up to a certain degree during cycling. The investigations show that the combination of mechanical and structural properties play a key role in the performance of lithium-sulfur batteries