Developing the Tandem Structure to Regulate Interfacial Chemistry and Promote Ion Transport Kinetics Toward High‐Voltage Lithium Metal Batteries

The chemical properties of the solid electrolyte interphase (SEI) layer and cathode electrolyte interphase (CEI) are crucial for achieving high-energy-density lithium metal batteries, especially under extreme operating conditions. Herein, we propose a delicately designed tandem separator (CYANO-COF|PP|SnF₂) to regulate the chemical stability of dual interfaces. The cyano group in CYANO-COF induces a stable CN-enriched CEI on the surface of high-nickel LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) cathode through the adsorption/coordination effect with transition metals (TMs), inhibiting irreversible phase transitions, TMs dissolution, and other side reactions. Meanwhile, a straightforward in-situ conversion is used to construct an artificial hybrid SEI layer comprising LiF and Li-Sn alloy. As demonstrated theoretically and experimentally, the hybrid SEI with enhanced electron-blocking ability and rapid transmission characteristics can decrease the electron from the Li anode into the SEI and allow Li⁺ to rapidly diffuse through the SEI layer, achieving even dendrite-free lithium plating at the SEI/Li interface. With the synergistic effect of dual interfaces, the NCM811||Li battery maintains a capacity retention of 81.8% within 200 cycles at 4.5 V and 55 °C. This work emphasizes the significance of regulating the chemical properties of double interfaces and provides new insights into the rational design for tandem separators.