Atomic-scale mechanism of alloy anodes mitigating polysulfide-induced degradation in lithium–sulfur batteries

Abstract

Polysulfide reactions at the lithium anode critically degrade the performance of Li–S batteries. Using density functional theory and molecular dynamics simulations, we unveil atomic-scale mechanisms of how alloying the lithium anode with Mg, Al, or Zn suppresses this degradation. In the lithium anode, weak Li–Li and Li–S bonds promote lithium migration into the electrolyte, accelerating polysulfide decomposition and structural collapse, along with the formation of a porous, unstable solid-electrolyte interphase (SEI). However, in Li–Mg alloy anodes, Mg atoms co-migrate with Li, forming a uniform, chemically stable Mg-rich SEI due to moderate Mg–S bonding. Li–Al and Li–Zn alloys have strong metal–metal bonds, leading to surface segregation of Al and Zn atoms that act as physical barriers to limit polysulfide access. S–S bond analysis shows that Li_1−xMg_x alloys suppress polysulfide decomposition most effectively at low concentrations (x = 0.05), while the suppression effects of Li_1−xAl_x and Li_1−xZn_x alloys are significantly enhanced at higher concentrations, eventually surpassing those of Li_1−xMg_x (x = 0.5). These distinct protection mechanisms offer design strategies for corrosion-resistant alloy anodes that enhance the long-term stability and performance of Li–S batteries.