Electrolyte-Driven Cu⁴⁺ Substitution in MoSe₂: 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 Cu4+ ions during electrochemical cycling remains mechanistically unclear. To address this, herein, Cu-doped MoSe2@C ultrathin nanosheets were synthesized via the solvothermal process and carbonization strategies. A systematic investigation was conducted to elucidate the underlying driving forces for Cu4+ substitution at Mo4+ 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 Cu4+ (classified as a soft acid) demonstrates significantly stronger coordination affinity with Se2- anions (soft bases) compared to the native Mo4+ 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+/Cu4+ 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.