A Lithiophilic–Lithiophobic Gradient Solid Electrolyte Interface Toward a Highly Stable Solid-State Polymer Lithium Metal Batteries

The functional lithiophilic−lithiophobic gradient solid electrolyte interphase (SEI) between Li-metal anode and solid-state polymer electrolytes may be effective in addressing the long-standing issue of side reactions and Li-dendrite growth during repeated deposition or dissolution in solid-state polymer-based high-energy-density batteries. Herein, a reliable lithiophilic–lithiophobic gradient SEI (G-SEI) of LiAg-LiF/Li₃N is in situ formed by AgTFSI, used as an additive for polyethylene oxide-based electrolyte (PEO-Ag). The upper layer consists of a lithiophobic LiF/Li₃N-rich layer, wherein LiF possesses a high interfacial energy, while Li₃N enables fast Li⁺ diffusion, which synergistically facilitates the uniform deposition of Li⁺. Lithiophilic Li–Ag alloy can effectively reduce the nucleation overpotential and promote more planar growth of lithium. Furthermore, such G-SEI possesses a high mechanical modulus, mitigating the penetration of dendrites through the SEI and thereby preventing the continuous degradation of the PEO-based electrolyte. As a result, an over three times improvement in the lifespan of Li | PEO-Ag | LFP cell is achieved, demonstrating an 81.4% capacity retention rate after 500 cycles at 1C, as compared to Li | PEO | LFP cell with common SEI. Thus, the developments of the lithiophilic-lithiophobic gradient SEI provide a substantial path toward high-performance solid-state lithium batteries.