Solid Polymer Electrolytes with Sacrificial End Groups for a Wide Oxidative Potential and Stable Interface in Lithium Metal Batteries

Poly(ethylene glycol) (PEG), despite being the most studied polymer electrolyte, suffers from serious drawbacks, which require fundamental studies behind its underperformance in lithium batteries. Here, we report the effect of the terminal group on triarm PEG stars bearing either hydroxyl (TPEG-OH) or carbonate-ketone (TPEG-Carb-ket) terminal groups. The latter is synthesized by a ring-opening reaction triggered by the -OH end group of TPEG-OH and results in a carbonate-ketone functionality. Indeed, the modified chain end is found to act as a sacrificial group by focusing the reactivity of the chain on the terminal group, protecting the rest of the TPEG molecule, which significantly reduces interfacial degradation and achieves a broader electrochemical stability window of up to 4.47 V, high Coulombic efficiency, and capacity retention. It furthermore demonstrates a stable interface with lithium metal after more than 1200 h of stripping and plating. When those electrolytes are investigated in reference cells based on LiFePO₄ cathodes and Li anodes, the change in discharge capacity is observed from 118.7 to 113.8 and 108.9 to 5.03 mAh g^-1 for TPEG-Carb-ket and TPEG-OH electrolytes, respectively, from the 1st to 100th cycle. The experimental results are further supported by density functional theory calculations and ab initio molecular dynamics simulations.