NOVELTY - The microporous polymer film (10) comprises a polymer matrix (14) binding an inorganic filler material (16), and an unfilled microporous polyolefin film having thermal shutdown characteristics and positioned to form a multilayer separator assembly. The microporous polymer film has first and second opposite surfaces (24, 26). The polymer matrix is characterized by a melting point temperature, and includes an ultrahigh molecular weight polyolefin, which provides sufficient molecular chain entanglement to form three-dimensional interconnecting and interpenetrating pore and polymer networks.    USE - The freestanding, heat resistant microporous polymer film is useful for an energy storage device such as a lithium battery, and in an electric motor vehicle power train (all claimed), where the energy storage device such as capacitor, supercapacitor or fuel cell is useful for hybrid electric vehicle or plug-in hybrid electric vehicle application.    ADVANTAGE - The freestanding, heat resistant microporous polymer film has excellent high temperature mechanical and dimensional stability, low resistance, low thermal shrinkage, sufficient mechanical properties such as high puncture resistance, excellent wettability and enhanced thermal shutdown characteristics, improves the performance and thermal safety of energy storage devices, prevents internal short circuits such as electronic conduction in the battery, and provides high rate discharge characteristics to the energy storage devices.    DETAILED DESCRIPTION - The microporous polymer film (10) comprises a polymer matrix (14) binding an inorganic filler material (16), and an unfilled microporous polyolefin film having thermal shutdown characteristics and positioned to form a multilayer separator assembly. The microporous polymer film has first and second opposite surfaces (24, 26). The polymer matrix is characterized by a melting point temperature, and includes an ultrahigh molecular weight polyolefin, which provides sufficient molecular chain entanglement to form three-dimensional interconnecting and interpenetrating pore and polymer networks through which the bound inorganic filler material is uniformly dispersed from the first surface to the second surface. The inorganic filler material is present at high loading levels to achieve low thermal shrinkage of the microporous polymer film at temperatures exceeding the melting point temperature of the polymer matrix. The three-dimensional interconnecting and interpenetrating pore network establishes a porosity of the microporous polymer film. The polymer matrix is a biaxially oriented, heat-treated type that exhibits reduced residual stress while maintaining high porosity after exposure to temperatures exceeding the melting point temperature of the polymer matrix. The interconnecting and interpenetrating pore network has pores (18) encompassing a volume fraction of greater than 60%, and contributes to an electrical resistance represented by a MacMullin number N(Mac) of less than 4.5. INDEPENDENT CLAIMS are included for:    (1) improved energy storage device;    (2) a battery pack; and    (3) an electric motor vehicle power train.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of a freestanding, inorganic material-filled microporous film.    Microporous polymer film (10)    Polymer matrix (14)    Inorganic filler particles (16)    Pore (18)    First and second surfaces (24, 26)    Film thickness. (28)