Concentration Polarization Compromises Oxidation Tolerance in Polymer Electrolytes

Polyethylene oxide (PEO)-based polymer electrolytes are promising for enabling safe and cost-effective lithium metal batteries, owing to their non-combustibility, processability, and compatibility with Li metal. However, their oxidative stability deteriorates rapidly under high-voltage conditions, restricting their practical deployment. Previous approaches to extend electrochemical stability have primarily relied on interphase engineering or molecular modifications, under the assumption that oxidative limits are intrinsic and fixed. In this study, we demonstrate that oxidation tolerance is dynamically compromised by lithium-ion (Li⁺) concentration polarization during the charging process. In situ X-ray diffraction reveals localized Li⁺ depletion at the cathode interface, which disrupts Li–O coordination in the PEO matrix. This depletion increases the electron density on ether oxygen atoms, elevating their HOMO energy and accelerating oxidative degradation. Resonant inelastic X-ray scattering (RIXS) directly confirms the correlation between Li⁺ desolvation and HOMO energy elevation. It is further shown that incorporating immobile Mg²⁺ ions stabilizes Li⁺ coordination, suppresses this polarization-induced HOMO shift and restores oxidative resilience. The resulting Mg2+ doped membrane extends the electrochemical stability window from 4.2 V to 4.6 V and supports over 200 cycles in NMC811||PEO||Li cells with suppressed interfacial degradation. These findings uncover a previously overlooked degradation pathway and establish a new framework for designing oxidation-tolerant polymer electrolytes via ionic coordination engineering.