NOVELTY - A microporous membrane comprises polyethylene and polypropylene and having a thickness fluctuation standard deviation in at least one planar direction of greater than or equal to 0.7 mu m and a melt down temperature greater than or equal to 150 degrees C.    USE - In battery, particularly lithium ion secondary battery that is used as a power source for electric vehicle or hybrid electric vehicle (claimed), computers, mobile telephones, electronic games, and power tools. Also useful for producing microporous polyolefin membrane films and sheets.    ADVANTAGE - The microporous polyolefin membrane films and sheets provide a good balance of key properties, including high meltdown temperature, improved surface smoothness and improved electrochemical stability while maintaining high permeability, good mechanical strength and low heat shrinkage with good compression resistance. The microporous membranes when used as a battery separator, exhibit excellent heat shrinkage, melt down temperature and thermal mechanical properties, i.e. reduced maximum shrinkage in the molten state.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) producing the microporous membrane involving: combining a polyolefin composition and at least one diluent to form a mixture, where the composition comprises at least a first polyethylene having a crystal dispersion temperature (Tcd) and polypropylene; extruding the mixture through an extrusion die to form an extrudate; cooling the extrudate to form a cooled extrudate having a first area; orienting the cooled extrudate in at least a first direction by about one to about ten fold at a temperature of about Tcd plus minus 15 degrees C; and further orienting the cooled extrudate in at least a second direction by about one to about five fold at a temperature about 10-40 degrees C higher than the temperature employed in previous orienting step to a second area at least ten fold larger than the first area; and    (2) a battery comprising an anode, a cathode, and electrolyte, and at least one separator located between the anode and the cathode, where the separator comprises polyethylene and polypropylene and has the thickness fluctuation standard deviation in at least one planar direction of greater than or equal to 0.7 mu m and the melt down temperature of greater than or equal to 150 degrees C.