Effect of surface bonding PEI with electrospun carbon nanofiber on the dissolution of polysulfide intermediates for lithium-sulfur batteries

Abstract

It is essential to examine the behavior of the dissolution and migration of long-chain lithium polysulfide (LiPS) intermediates (Li2Sx, 4 ≤ x ≤ 8) in the sulfur cathode to promote the performance of Li-S batteries. In this work, PEI functionalized electrospun carbon nanofibers (CNFs-PEI) were prepared to host sulfur to form carbon nanofibers polyethylenimine/sulfur (CNFs-PEI/S) composites for high performance Li-S batteries. The as-prepared CNFs-PEI/S cathode with a sulfur content of 67% delivers an initial discharge capacity of 1279 mAh g−1 at 0.1 C and retains a cycling stability for 80 cycles with a low capacity fading rate of 0.26% per cycle. Further, compared with CNFs/S, the CNFs-PEI/S revealed an enhanced rate capability and cyclic stability primarily attributed to the formation of the electrostatic interaction between amino groups in the PEI chains and LiPS intermediates, leading to suppression of LiPS dissolution and restraint of the shuttle effect during the charge-discharge cycling process. An in situ rotating ring-disk electrode (RRDE) collection experiment is designed to further examine the LiPS dissolution extent from a sulfur electrode during cycling. A maximum slope of ring current change with time (voltage) occurred at the major LiPS intermediate dissolution step over a voltage range between 2.4 and 2.0 V during disk negative scan (reduction). The CNFs-PEI/S showed a lower maximum slope value, which can be attributed to a lower dissolution of LiPS intermediates from the sulfur electrode. The present study also demonstrated the proposed simple RRDE experiment is a useful method to evaluate the dissolution properties of LiPS intermediates from various sulfur electrode materials.