Solid\nPolymer Electrolytes with Sacrificial End Groups\nfor a Wide Oxidative Potential and Stable Interface in Lithium Metal\nBatteries

Poly(ethylene glycol) (PEG), despite\nbeing the most studied polymer\nelectrolyte, suffers from serious drawbacks, which require fundamental\nstudies behind its underperformance in lithium batteries. Here, we\nreport the effect of the terminal group on triarm PEG stars bearing\neither hydroxyl (TPEG-OH) or carbonate-ketone (TPEG-Carb-ket) terminal\ngroups. The latter is synthesized by a ring-opening reaction triggered\nby the −OH end group of TPEG-OH and results in a carbonate-ketone\nfunctionality. Indeed, the modified chain end is found to act as a\nsacrificial group by focusing the reactivity of the chain on the terminal\ngroup, protecting the rest of the TPEG molecule, which significantly\nreduces interfacial degradation and achieves a broader electrochemical\nstability window of up to 4.47 V, high Coulombic efficiency, and capacity\nretention. It furthermore demonstrates a stable interface with lithium\nmetal after more than 1200 h of stripping and plating. When those\nelectrolytes are investigated in reference cells based on LiFePO<sub>4</sub> cathodes and Li anodes, the change in discharge capacity\nis observed from 118.7 to 113.8 and 108.9 to 5.03 mAh g<sup>–1</sup> for TPEG-Carb-ket and TPEG-OH electrolytes, respectively, from the\n1st to 100th cycle. The experimental results are further supported\nby density functional theory calculations and ab initio molecular\ndynamics simulations.