NOVELTY - Preparation of porous polymer separator layer involves forming a film from polymer solution containing polymer material dissolved in polymer solvent, and precipitating the polymer material in the obtained film into porous polymer separator layer having crosssection containing edge (12) and edge (14) opposite to the edge (12). The height (400) of the edge (12) is more than the height of the edge (14). The porous polymer separator layer has porosity of 30-90%, which is distributed evenly throughout the separator layer. The film has crosssection with non-uniform thickness.    USE - Preparation of porous polymer separator layer used for electrochemical battery cell of lithium ion battery (claimed). Uses include but are not limited to electric motor for hybrid electric vehicle and extended-range electric vehicle, alternating current wall outlet, internal combustion engine and vehicle alternator.    ADVANTAGE - The method efficiently provides porous polymer separator layer. The electrochemical battery cell of lithium ion battery obtained by the electrochemical battery cell of lithium ion battery porous polymer separator layer has excellent durability.    DETAILED DESCRIPTION - Preparation of porous polymer separator layer involves forming a film from polymer solution containing polymer material dissolved in polymer solvent, and precipitating the polymer material in the obtained film into porous polymer separator layer having crosssection containing edge (12) and edge (14) opposite to the edge (12). Each of the edge (12) and edge (14) of crosssection has height which defines thickness of the porous polymer separator layer at the edge (12) and edge (14), respectively. The height of the edge (12) is more than the height of the edge (14), such that the porous polymer separator layer has thickness at the crosssection which decreases from the edge (12) to the edge (14). The porous polymer separator layer has porosity of 30-90%, which is distributed evenly throughout the separator layer. The film has crosssection with non-uniform thickness. An INDEPENDENT CLAIM is included for electrochemical battery cell of lithium ion battery.    DESCRIPTION OF DRAWING(S) - The drawing shows a cross-sectional view of the porous polymer separator layer.    Porous polymer separator layer (10)    Edges (12,14)    Film (110)    Substrate (130)    Height (400)