Built-in Electric Field Within CoSe₂-FeSe₂ Heterostructure for Enhanced Sulfur Reduction Reaction in Li-S Batteries

The conversion of Li₂S₄ to Li₂S is the most important and slowest rate-limiting step in the complex sulfur reduction reaction (SRR) for Li-S batteries, the adjustment of which can effectively inhibit the notorious "shuttle effect". Herein, a CoSe₂-FeSe₂ heterostructure embedded in 3D N-doped nanocage as a modified layer on commercial separator is designed (CoSe₂-FeSe₂@NC//PP). The CoSe₂-FeSe₂ heterostructure forms a built-in electric field at the two-phase interface, which leads to the optimized adsorption force on polysulfides and the accelerated reaction kinetics for Li₂S₄-Li₂S evolution. Density functional theory (DFT) calculations and experimental results combine to show that the liquid-solid reaction (Li₂S₄-Li₂S₂/Li₂S) is significantly enhanced in terms of thermodynamics and electrodynamics. Consequently, the batteries assembled with CoSe₂-FeSe₂@NC//PP delivered an excellent rate capability (606 mAh g^-1 under 8.0 C) and a long cycling lifespan (only 0.056% at 1.0 C after 1000 cycles). In addition, the cells can provide high initial capacity of 887 mAh g^-1 at sulfur loading of 5.8 mg cm^-2 and 0.1 C. This work would provide valuable insights into binary metal selenide heterostructures for liquid-solid conversion in Li-S batteries.