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

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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 stabilityof dual interfaces. The cyano group in CYANO-COF induces a stable CN-enriched CEI on the surface of high-nickel LiNi0.8Co0.1Mn0.1O2 (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 4.5V 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.