Segment Engineering Homogenizes Ion Flux to Overcome Microphase-Separation-Induced Failure in Solid Polymer Electrolytes

Solid polymer electrolytes (SPEs) are promising for practical solid-state batteries, but their chemically distinct hard and soft segments intrinsically undergo microphase separation, creating transport heterogeneity that induces nonuniform Li+ flux and unstable Li deposition. Here, we show that this intrinsic disparity can be mitigated through segment engineering. By introducing ion-conductive diethylene glycol (DEG) into the hard domains and employing poly(caprolactone)-b-poly(ethylene glycol)-b-poly(caprolactone) (PCL–PEG–PCL) triblock soft segments to improve domain compatibility, continuous and homogeneous Li+ pathways are established without sacrificing mechanical robustness. Molecular dynamics simulations confirm a more uniform Li+ distribution in the modified SPE than in conventional counterparts. The resulting electrolyte delivers a high ionic conductivity of 2.1 × 10–3 S cm–1, elongation of ∼1890%, and tensile strength of ∼17.5 MPa. Consequently, Li|LiNi0.8Co0.1Mn0.1O2 (NCM811) cells retained 78.7% capacity after 300 cycles. This work establishes molecular-level ion-flux homogenization as a new design principle for SPEs.