Interfacial polarity modulation of positive electrode active materials for high-potential lithium metal batteries

A central challenge in battery electrochemistry is achieving stable electro-chemo-mechanical interphases at the electrode|electrolyte interfaces across a wide range of operating conditions. Despite advances in electrolyte solution engineering aimed at optimizing interphase chemistry, the fundamental understanding of the interfacial environment remains limited, preventing rational designs and molecular-level manipulation of the interface. Here we introduce a molecular engineering strategy that uses dipolar self-assembled monolayers (SAMs) on the positive electrode active material to modulate interfacial stability. By tuning the electronic structure of the SAM terminal group, we establish interfacial polarity as a descriptor governing interactions between electrodes and the liquid electrolyte solution. Via in situ nanoscale depth-sensitive surface-enhanced infrared absorption spectroscopy, we directly probe the Coulombic interactions between SAMs and liquid electrolyte's molecular components, revealing how SAM terminal groups modulate electrolyte solution behaviour, offering a scientific basis for rational battery electrode interface molecular engineering. We also show that SAM-modified positive electrodes tested in Li metal coin cells with a glyme-based non-aqueous electrolyte solution enable improved cycling stability compared with their unmodified analogues, retaining 80% of their initial specific discharge capacity after 200 cycles at 0.15 mA cm^-2 within the cell's potential range 2.8-4.7 V at 25 °C.