Synergetic catalysis of bimetallic sites in CoNi alloys in-situ encapsulated N-doped CNTs to accelerate sulfur redox kinetics for lithium-sulfur batteries

Transition metal nanomaterials are a kind of promising catalysts for lithium-sulfur batteries (LSBs), but a single metal component is difficult to satisfy the demends simultaneously catalyzing the multi-step sulfur redox reactions (SRRs). Moreover, the agglomeration of nanometal materials leads to the decrease of catalytic sites during long-term cycling. Herein, we propose a in-situ encapsulation strategy to fabricate a composite featuring CoNi alloy wrapped in nitrogen-doped carbon nanotubes (denoted as CoNi@NCNT). Compared with a single metal Co/Ni, CoNi alloy provides multiple active sites for strong chemisorption of lithium polysulfides (LiPSs), and acts as the bidirectional catalyst of SRRs by integrating the preferred synergetic catalysis of the reduction of LiPSs by Co sites and the opposite process by Ni sites. The batteries based on CoNi@NCNT interlayer delivers a high discharge capacity of 1482.0 mAh g−1 at 0.1C, superior rate performance with a high capacity of 712.9 mAh g−1 at 5C, and good cycling stability. The battery can also provide a maximal specific capacity of 823.9 mAh g−1 when operated under the sulfur loading of 5.0 mg cm−2 and the electrolyte/sulfur ratio of ∼ 7.1 μL. In addition, the CNT-encapsulation structure effectively inhibits the agglomeration of CoNi nanoparticles, thus enabling its durable catalytic activity for SRRs. This work provides an insight to understand the specific catalysis of transition metal in SRRs and also develop an effective strategy to explore stable catalysts for high-performance LSBs.