Li-S batteries: effect of uniaxial compression on the electrical conductivity and electrochemical performance of graphene aerogel cathodes

Abstract

Porous cathodes are preferred to be used in lithium-sulfur (Li-S) batteries for better impregnation of the active material. On the other hand, the electrochemical performances of Li-S cells are mostly correlated with the free pore volume of the cathode hosts; however, their electrical conductivities were rarely reported in the existing literature. In this work, lightweight, 3D spongy boron-doped graphene aerogel cathodes were prepared and their electrical conductivity values were found to increase under the uniaxial compression through the vertical axis. The variation of the uniaxial compression corresponds to the fluctuations in the pressure of the springs during the assembly of Li-S cells, which resulted in the diversity of the cell performance containing the same cathode material. The electrical conductivity of the cathode was determined to be nearly constant up to a compressive strain of 60% but increased significantly from 60 to 80%. In parallel, electrical conductivity of the cathode increased with the accompanying increase of the aerogel density from 0.04 to 0.08 g.cm(-3). In addition, the electrochemical performances of the aerogel cathodes having the same amount of active material were investigated in Li-S batteries when different uniaxial compressions were applied. It was found that the aerogel cathode having 40% uniaxial compression exhibited the best stable discharge capacity (similar to 440 mAh.g(-1) over 100 cycles). These results indicate that both electrical conductivity and porosity properties of graphene cathodes should be considered simultaneously to obtain reliable Li-S cell performances.