MXenes with entropy-derived mitigation of shuttle effect in sodium-sulfur batteries

Abstract

Room-temperature sodium-sulfur batteries (NaSBs) hold great promise for large-scale energy storage due to their high energy density and resource abundance. However, the notorious shuttle effect, caused by the dissolution of sodium polysulfides (Na₂S_n; n = 1–8) in electrolyte, significantly undermines the performance and lifespan of NaSBs. To address this, we investigate high-entropy (HE) Ti₂CO₂ MXenes, where Ti is partially substituted with Mn, V, and Cr, as cathode additives. The configurational entropy stabilizes crystal structure, enhances electronic properties, and enables strong adsorption of sodium polysulfides. Ab-initio molecular dynamics confirm the stability of HE-MXenes with reduced Cr, e.g., (Ti_0.25Mo_0.5V_0.125Cr_0.125)₂CO₂, which exhibit significantly higher binding energies for polysulfides (−1.672 to −3.99 eV) compared to pristine MXene (−0.987 to −1.43 eV) and common electrolytes like DOL and DME (−0.18 to −0.98 eV). These systems reduce Na₂S dissociation energy barrier (∼ 0.92 eV), promoting efficient Na⁺ ion diffusion. HE-MXenes also show enhanced density of electronic states at the Fermi level, facilitating faster electrochemical processes. The combination of strong polysulfide binding, low dissociation barriers, and stable electronic conductivity effectively mitigates the shuttle effect while improving overall electrochemical performance of NaSBs. HE-MXenes present a robust strategy for advancing NaSB technology, offering superior cycling stability and longer operational lifespans.