J. Huang, J. Zhou, B.G. Sumpter, J. Jakowski, P.R.C. Kent, C. Liang, S.C. Smith
Oak Ridge National Laboratory,
Keywords: Li-S batteries, electrolyte additives, cycle life, nanoporous carbon, ab initio molecular dynamics, catalytic mechanism
Summary:Li-S batteries are under the spotlight in the energy storage area due to their high theoretical energy density of ~2600 Wh/kg and demonstrated energy density of ~500 Wh/kg. Cathode materials typically consist of chemically active but electrically insulating S8 (fully charged) or Li2S (fully discharged) for redox reactions that are infiltrated into nanoporous carbons for electrical conduction. Despite of the great potential, the capacity and cycle life of Li-S batteries are usually less than 400 mAh/g and 50 times. Experiments at CNMS of ORNL showed that by adding homogeneous LiX additives to the organic electrolyte, both the capacity and the cycle life were significantly improved (see figure). In this work, we carried out theoretical analysis and ab initio molecular dynamics (AIMD) simulations, aiming to reveal the catalytic mechanism of electrolyte additives for cycle life improvement of the Li-S batteries. Li2S clusters of different sizes were adopted to simulate the solid deposit after the battery was fully discharged. We considered THF in our implicit and explicit solvation model. For the redox reactions during the charging process, multiple active intermediates were identified, showing the vehicle role of electrolyte additive LiX to carry electrons from the electrode to electrically insulating Li2S to produce S2 and S3 units leading to co-crystals with LiX and Li2S. Except for Li2S itself, other components of the co-crystals should be at least partially soluble in the electrolyte, and therefore the X component in the co-crystals could return to its original catalyst form of LiX, and the solid deposit Li2S could be gradually broken down. After the solid deposit was dissolved, the battery would go back to its normal operation without needing the LiX catalyst anymore. Although the catalysis was eventually found to be very corrosive to the battery components, this work could provide an enlightenment for catalyst design to enable performance enhancement of highly potential Li-S batteries.