NOVELTY - The method comprises forming a film having a cross-section with a non-uniform thickness from a polymer composition (20-95 wt.%) that includes a polyolefin, and deriving a porous polymer separator layer (10) from the film. The porous polymer separator layer has a cross-section that has a first edge (12) and a second edge (14) opposite to the first edge. Each of the first and second edges has a height that defines a thickness of the porous polymer separator layer at the first and second edges, respectively, where the height of the first edge is 2% greater than the height of the second edge.    USE - The method is useful for making and using a porous polymer separator layer that is useful as all or part of a separator used in an electrochemical battery cell of a secondary liquid electrolyte battery such as a lithium ion battery (all claimed), a nickel-cadmium battery, a nickel-metal hydride battery and a lead-acid battery, where the battery is useful in a electrical device such as a electrical load including: an electric motor for a hybrid electric vehicle or an extended-range electric vehicle in an automobile industry; and a power-generating device including an alternating wall outlet, an internal combustion engine and/or a vehicle alternator.    ADVANTAGE - The porous polymer separator layer can be formed with uniform thickness and increased reliability thus maintaining evenly distributed current density of the cell and enhancing lifespan and dimensional stability of the cell, and physically separates the confronting inner face surfaces of the electrodes thus preventing a short-circuit in the electrochemical cell.    DETAILED DESCRIPTION - The method comprises forming a film having a cross-section with a non-uniform thickness from a polymer composition (20-95 wt.%) that includes a polyolefin, and deriving a porous polymer separator layer (10) from the film. The porous polymer separator layer has a cross-section that has a first edge (12) and a second edge (14) opposite to the first edge. Each of the first and second edges has a height that defines a thickness of the porous polymer separator layer at the first and second edges, respectively, and where the height of the first edge is 2% greater than the height of the second edge such that the porous polymer separator layer has a thickness at the cross-section that decreases from the first edge to the second edge. The porous polymer separator layer further has a porosity of 30-90% that is distributed evenly throughout the separator layer such that, when the separator layer is infiltrated with a liquid electrolyte, lithium ions are communicated through the thickness of the separator layer coextensively between the first and second opposed faces. The heights of each of the first and second edges of the porous polymer separator layer are 10-50 mu m. The forming step comprises heating the polymer composition to form a melt of the polyolefin during formation of the film, and the deriving step comprises annealing the film and uniaxially stretching the film. The polymer composition further comprises particles having a higher melting temperature than the polyolefin, and a plasticizer (5-70 wt.%). The particles are dispersed within the melt of the polyolefin during formation of the film. The forming step further comprises heating the polymer composition to form a mixture of a melt of the polyolefin and the plasticizer during formation of the film, and the deriving step further comprises biaxially stretching the film and extracting the plasticizer from the film. The plasticizer is an organic liquid that is absorbed into the polyolefin and lowers the glass transition temperature of the polyolefin. The method further comprises disposing the porous polymer separator layer between a negative electrode (42) and a positive electrode (44) to assemble an electrochemical battery cell of a secondary liquid electrolyte battery, and extruding the film along an extrusion direction such that a width of the film oriented transverse to the extrusion direction is defined by the cross-section. An INDEPENDENT CLAIM is included for an electrochemical battery cell of a lithium ion battery.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic cross-sectional view of an electrochemical battery cell for a lithium ion battery.    Porous polymer separator layer (10)    First edge (12)    Second edge (14)    Negative electrode (42)    Positive electrode. (44)