Polyamine‐Mediated Proton/TFSI ⁻ Dual Capture Enables High‐Voltage PEO‐Based All‐Solid‐State Li Batteries

All-solid-state lithium batteries (ASSLBs) employing poly(ethylene oxide) (PEO)-based solid polymer electrolytes (SPEs) experience severe degradation of hydroxyl/ether groups within the PEO matrix at high voltages (>3.8 V vs. Li⁺/Li), thereby limiting their energy density. The origin of this breakdown is essentially induced by in situ generated corrosive acids (mainly HTFSI). Conventional passive strategies aim at protecting the PEO matrix by creating physical isolations; however, the generation of HTFSI in the system has not been effectively inhibited. Herein, we propose a proactive strategy for the dual capture of H⁺ (protons) and TFSI^- anions via a polyamine-based agent. Featuring a high density of strong Brønsted‑base sites and H-bond donors, this agent is capable of capturing free proton/TFSI^- through electrostatic/H-bonding interactions, respectively, effectively mitigating acid-catalyzed chain scission in the PEO matrix by significantly suppressing HTFSI formation at high-voltage. When implemented in 4.2 V LiCoO₂-based ASSLBs, the system achieves exceptional cycling stability (>600 cycles at 1.0 C, 65°C) with 95.5% capacity retention, outperforming state-of-the-art high-voltage polymer-based ASSLBs. This study pioneers a dual capture active strategy that simultaneously targets protons and TFSI^- ions, mitigating both interfacial and bulk degradation in high-voltage, which provides new insights for the design of high-energy-density ASSLBs.