Molecular-level polymer design and interface engineering enable 4.5 V high-voltage Li metal batteries

The inadequate room-temperature ionic conductivity and unstable electrode-electrolyte interfaces have encumbered the widespread adoption of high-voltage solid-state Li metal batteries (SSLMBs). Here, we report a molecularly engineered polymer electrolyte synthesized by grafting poly(ethylene glycol) diacrylate (PEGDA) and 2,2,2-trifluoroethyl methacrylate (TFEMA) onto a tri(2-hydroxyethyl) isocyanurate triacrylate (THEICTA) framework, coupled with interface engineering strategies. The hierarchical coordination competition within the polymer matrix facilitates rapid Li⁺ transport, achieving an ionic conductivity of 0.95 mS cm⁻¹ at room temperature. Advanced characterization techniques including X-ray photoelectron spectroscopy (XPS), in situ X-ray diffraction (XRD), and high-resolution transmission electron microscopy (HRTEM) reveal the formation of a robust, nitrided and boronized cathode electrolyte interphase (CEI) on LiNi 0.6 Co 0.2 Mn 0.2 O 2 (NCM622), effectively suppressing oxidative decomposition and mitigating structural degradation at high voltages (≥4.5 V). As a result, the Li||NCM622 cell delivers a high initial discharge capacity of 188.6 mAh g⁻¹ with 70.4 % capacity retention after 700 cycles. Furthermore, a Li metal pouch cell featuring an energy density of 456 Wh kg⁻¹ demonstrates excellent cycling stability, maintaining a capacity retention rate of 87.8 % after 130 cycles. This study provides a scalable molecular design strategy for polymer electrolytes, advancing the development of high-voltage SSLMBs for next-generation energy storage.