High-valent cation-mediated inorganic-rich gradient SEI for highly stable solid state polymer lithium metal batteries

Solid-state polymer lithium metal batteries (SSPLMBs) are widely regarded as the most promising next-generation energy storage technologies due to the high energy density and intrinsic safety. However, common organic-rich solid electrolyte interphase (SEI) exhibits the inhomogeneous and sluggish Li⁺ transport at the lithium anode interface, and the high nucleation barrier for lithium atoms, collectively promoting Li dendritic growth, thereby hindering the large-scale deployment of SSPLMBs. Herein, we propose a high-valence cation-mediated strategy to construct an inorganic-rich gradient (SEI) by introducing In(NO₃)₃ as an additive into a polyethylene oxide-based electrolyte (PEO-In(NO₃)₃). Specifically, trivalent In³⁺ reacts with lithium to form a lithiophilic Li-In alloy underlayer, while simultaneously mediating anion enrichment at the lithium anode interface via enhanced electrostatic attraction, which facilitates formation of a lithiophobic top layer enriched with anion-derived LiF/Li₃N components. The inorganic-rich gradient SEI architecture is established, and superior Li⁺ diffusion kinetics and reduced energy barrier for lithium atom nucleation are achieved, synergistically enabling homogeneous lithium deposition morphology. Consequently, over one order of magnitude improvement in the lifespan of Li|PEO-In(NO₃)₃|LFP cell is achieved, demonstrating a 78.3 % capacity retention after 1000 cycles at 1 C, as compared to Li|PEO|LFP cell with common SEI. This study presents a novel avenue to the rational design of inorganic-rich gradient SEI.