The invention discloses a separator medium for electrochemical cells that contains at least one nonwoven sheet of polymeric fibers. The nonwoven sheet has a surface area of about 0.5 to about 1.5 m2/g and has a maximum pore size that is equal to or more than 2.5 times the mean flow pore size and more than 11 times the minimum pore size. The sheet may be sulfonated to a level of 0.67% and demonstrates superior tensile properties after sulfonation and relative to previously known separators.