Regulating Li+ Coordination and Interfacial Stability in Poly(ε-caprolactone) Electrolytes: A First-Principles Study

Abstract Solid polymer electrolytes based on poly(ε-caprolactone) (PC) provide abundant carbonyl coordination sites for Li+ transport, yet the atomistic effects of carbonyl-adjacent different halogen substituents remain unclear. Herein, first-principles calculations and AIMD simulations are combined to compare dihalogenated PC matrices at site 2 (PC-2X2 , X = F, Cl, or Br) from the perspectives of Li+ affinity, local electronic redistribution, structural compactness, LiTFSI adsorption dynamics and interfacial compatibility with Li metal. All halogen substitutions enhance PC-Li+ interactions, although their effects differ substantially. Cl and Br substitutions induce stronger out-of-plane distortion and more diffuse halogen-derived frontier states, whereas F substitution produces a more compact and electronically balanced coordination environment. AIMD-based RDF, RMSD and relaxed Li–O scanning further show that PC-2F2 maintains a stable O/F mixed coordination shell around Li+ and provides an energetically favorable Li+ capture pathway. At the Li-metal interface, PC-2F2 enhances adsorption and charge transfer, whereas Cl and Br substitutions strengthen polarization and interfacial coupling at the expense of structural compactness. These results reveal that the advantage of fluorination originates from the balanced optimization of Li+ affinity, molecular compactness, electronic stability and interfacial charge regulation, rather than from stronger Li+ binding alone, providing molecular-level guidance for halogen-modified polyester electrolytes.