NOVELTY - An electrochemical cell comprises a housing divided into first and second chambers; and a symmetric, strong, highly porous, microporous polymer membrane. The first chamber contains protogenous ion-conducting liquid and a positive high density electrode surrounded by a surface having a porosity level. The second chamber contains an aprotic, ion conducting liquid and a negative high density electrode surrounded by a surface having a porosity level. The polymer membrane has a porosity level that is 25% greater than the porosity level at the surface of the high density electrodes.    USE - As a secondary electrochemical cell for a battery (claimed) such as lithium-ion battery.    ADVANTAGE - The electrochemical cell having positive and negative high density electrodes and symmetric, strong, highly porous, microporous polymer membrane, improves the battery capacity. The use of highly densified electrode with a symmetric, strong, highly porous, microporous polymer film that functions as a separator between the electrodes within the electrochemical cell, improves the battery capacity and efficiency. The high strength and uniform porosity of the polymer film allow for more efficient travel of ions, resulting in maximum current flow within the electrochemical cell. The improved ion availability resulting from the highly densified electrodes with unimpeded flow of ions through the symmetric, strong, highly porous, microporous polymer film provides an electrochemical cell of improved capacity and efficiency.    DETAILED DESCRIPTION - An electrochemical cell comprises a housing divided into first and second chambers; and a symmetric, strong, highly porous, microporous polymer membrane dividing the housing into first and second chambers. The first chamber contains protogenous ion-conducting liquid and a positive high density electrode including a first active material and a porous binder, surrounded by a surface having a porosity level, where the porosity level increases towards the surface. The second chamber contains an aprotic, ion conducting liquid and a negative high density electrode including a second active material and a porous binder, surrounded by a surface having a porosity level, where the porosity level increases towards the surface. The polymer membrane has a porosity level that is 25% greater than the porosity level at the surface of the positive and negative high density electrodes. An INDEPENDENT CLAIM is included for improving battery capacity using the electrochemical cell, involving making a positive high density electrode; making a negative high density electrode; producing a symmetric, strong, highly porous, microporous polymer membrane; discharging ions from second active material of the negative high density electrode through the aprotic ion conducting liquid of the second chamber; increasing rate capability of the ion flow by reducing interfacial resistance to ion flow by increasing porosity of the polymer membrane by controlling pore size and solids distribution; and accepting ions into the first active material of the positive high density electrode of the first chamber through the protogenous ion conducting liquid.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic cross-sectional view of an electrode with porous or conductive binder with a separator applied to the electrode.