Porous and thermally stable membranes are attractive materials as separators in lithium-ion batteries. This work reports a new class of high-performance separators for Li-ion batteries based on a crosslinked polybenzoxazine electrospun fiber mat (CR-PBz-FbM). The CR-PBz-FbM separator shows a high ion conductivity of 2.92mScm(-1) (5.2times the value recorded for Celgard 2300), a low MacMullin number of 3.35, ultrahigh electrolyte uptake (approximate to 825%), and near-zero shrinkage at 150 degrees C. Consequently, the CR-PBz-FbM separator shows a high C-rate capacity and good cycle retention in the half-cell test. At a C-rate of 2.0, the CR-PBz-FbM-based Li-ion cell only exhibits a capacity loss of 16% compared to a 40% capacity loss found with the cell based on Celgard 2300. The results demonstrate that CR-PBz-FbM is an inexpensive, non-shutdown-type, thermally stable, and highly ion-conductive separator for Li-ion batteries.