Concurrent hetero-/homo-geneous electrocatalysts to bi-phasically mediate sulfur species for lithium-sulfur batteries

Expediting redox kinetics of sulfur species on conductive scaffolds with limited charge accessible surface is considered as an imperative approach to realize energy-dense and power-intensive lithium-sulfur (Li-S) batteries. In this work, the concept of concurrent hetero-/homo-geneous electrocatalysts is proposed to simultaneously mediate liquid-solid conversion of lithium polysulfides (LiPSs) and solid lithium disulfide/sulfide (Li2S2/Li2S) propagation, the latter of which suffers from sluggish reduction kinetics due to buried conductive scaffold surface by extensive deposition of Li2S2/Li2S. The selected model material to verify this concept is a two-in-one catalyst: carbon nanotube (CNT) scaffold supported iron-cobalt (Fe-Co) alloy nanoparticles and partially carbonized selenium (C-Se) component. The Fe-Co alloy serves as a heterogeneous electrocatalyst to seed Li2S2/Li2S through sulphifilic active sites, while the C-Se sustainably releases soluble lithium polyselenides and functions as a homogeneous electrocatalyst to propagate Li2S2/Li2S via solution pathways. Such bi-phasic mediation of the sulfur species benefits reduction kinetics of LiPS conversion, especially for the massive Li2S2/Li2S growth scenario by affording an additional solution directed route in case of conductive surface being largely buried. This strategy endows the Li-S batteries with improved cycling stability (836 mA h g−1 after 180 cycles), rate capability (547 mA h g−1 at 4 C) and high sulfur loading superiority (2.96 mA h cm−2 at 2.4 mg cm−2). This work hopes to enlighten the employment of bi-phasic electrocatalysts to dictate liquid-solid transformation of intermediates for conversion chemistry batteries.