Self-established rapid magnesiation/de-magnesiation pathways in binary selenium–copper mixtures with significantly enhanced mg-ion storage reversibility

Rechargeable magnesium/sulfur (Mg/S) and magnesium/selenium (Mg/Se) batteries are characterized by high energy density, inherent safety, and economical effectiveness, and therefore, are of great scientific and technological interest. However, elusive challenges, including the limited charge storage capacity, low Coulombic efficiency, and short cycle life, have been encountered due to the sluggish electrochemical kinetics and severe shuttles of ploysulfides (polyselenide). Taking selenium as model paradigm, a new and reliable Mg-Se chemistry is proposed through designing binary selenium-copper (Se-Cu) cathodes. An intriguing effect of Cu powders on the electrochemical reaction pathways of the active Se microparticles is revealed in a way of forming Cu₃Se₂ intermediates, which induces an unconventional yet reversible two-stage magnesiation mechanism: Mg-ions first insert into Cu₃Se₂ phases; in a second step Cu-ions in the Mg_2xCu₃Se₂ lattice exchange with Mg-ions. As expected, binary Se-Cu electrodes show significantly improved reversibility and elongated cycle life. More bracingly, Se/C nano-structures fabricated by facile blade coating Se nanorodes onto copper foils exhibit high output power and capacity (696.0 mAh g⁻¹ at 67.9 mA g⁻¹), which outperforms all previously reported Mg/Se batteries. This work envisions a facile and reliable strategy to achieve better reversibility and long-term durability of selenium (sulfur) electrodes.