Inner Helmholtz Plane Engineering via In Situ Electrolyte Polymerization to Enhance the Lithium Metal Anode Stability

The interfacial instability of lithium metal anodes severely impedes the practical application of lithium metal batteries. Regulating the inner Helmholtz plane (IHP) structure adjacent to the lithium metal offers a promising way to enhance the stability of lithium metal, although it is insufficiently explored. Specifically, in conventional electrolytes, the IHP often contains few anions, while solvent molecules occupy interfacial reaction sites. This structure hinders the formation of the anion-derived stable solid electrolyte interphase (SEI). In this study, we report an anion-rich IHP engineering strategy enabled by in situ polymerization of the liquid electrolyte (LE) via pentaerythritol tetraacrylate (PETEA), which provides noncovalent driving force to anchor the PF₆^- anions in the IHP. Experimental characterizations and theoretical simulations jointly reveal that the PETEA-based electrolyte (SPE) system offers PF₆^- anion-rich IHP structure and then biases interfacial reactions toward more anion-involved pathways, leading to a spatially continuous LiF-rich SEI. Meanwhile, the SPE system reduces solvent accessibility to the IHP and maintains a stable, anion-rich IHP structure, even under an anion-repulsion electric field, which is more conducive to the formation of anion-derived SEI. Benefiting from this interfacial engineering, the resulting SEI shows enhanced resistance to dissolution versus LE. The Li||Li symmetric cell displays excellent cycling stability for over 1700 h at 0.2 mA·cm^-2. The Li||NCM811 full cell enables outstanding performance, retaining more than 80% capacity retention after 600 cycles at 1 C and an average Coulombic efficiency (CE) above 99.9% after 2000 cycles. This in situ polymerization-enabled IHP modulation strategy offers a promising path toward high-performance semisolid-state lithium metal batteries.