Electrolyte-Driven Cu Substitution in MoSe: Synergy of an Inorganic-Rich Solid Electrolyte Interphase and Thermal Activation for Sodium-Ion Batteries.

Transition metal chalcogenides (TMCs) have garnered significant attention as high-capacity anode materials, yet the unconventional role of the Cu collector meditating atomic-level substitution of metal-site cations by Cu ions during electrochemical cycling remains mechanistically unclear. To address this, herein, Cu-doped MoSe@C ultrathin nanosheets were synthesized via the solvothermal process and carbonization strategies. A systematic investigation was conducted to elucidate the underlying driving forces for Cu substitution at Mo sites and the crucial regulatory effects of solid electrolyte interphase (SEI) formation. The substitution mechanism was elucidated through the Hard and Soft Acid-Base principle, where Cu (classified as a soft acid) demonstrates significantly stronger coordination affinity with Se anions (soft bases) compared to the native Mo cations (hard acids). This electrochemical transition is mediated by ether-based electrolytes coupled with the Cu collector, where the in situ formation of a thin, inorganic-rich SEI layer establishes synergistic ion-transport highways for accelerated Na/Cu co-diffusion. Temperature-dependent studies reveal Arrhenius-type kinetics: charge transfer is kinetically hindered at ≤ 0 °C but thermally activated at 50-70 °C, confirming that interfacial charge transfer requires thermal energy to overcome activation barriers. This work provides a fundamental guideline for designing stable metal chalcogenide electrodes through interface engineering and electrolyte optimization.