Provided is a separator including a first layer which consists of a porous polyolefin and a secondary battery utilizing the separator. A first layer exhibits a minimum height equal to or more than 50 cm and equal to or less than 150 cm when a ball having a diameter of 14.3 mm and a weight of 11.9 g located over the first layer is allowed to free fall causing a split in the first layer. A tearing strength of the first layer in a width direction, measured with an Elmendorf tearing method, is equal to or more than 1.5 mN/m. A tensile elongation of the first layer is equal to or longer than 0.5 mm until a load decreases to 25% of a maximum load in a load-elongation curve in machine direction measured by a rectangular tearing method.

Provided is a separator including a first layer which consists of a porous polyolefin and a secondary battery utilizing the separator. A first layer exhibits a minimum height equal to or more than 50 cm and equal to or less than 150 cm when a ball having a diameter of 14.3 mm and a weight of 11.9 g located over the first layer is allowed to free fall causing a split in the first layer. A tearing strength of the first layer in a width direction, measured with an Elmendorf tearing method, is equal to or more than 1.5 mN/m. A tensile elongation of the first layer is equal to or longer than 0.5 mm until a load decreases to 25% of a maximum load in a load-elongation curve in machine direction measured by a rectangular tearing method.

Provided are a separator capable of being used for a secondary battery such as a nonaqueous electrolyte-solution secondary battery and a secondary battery including the separator. A separator having a first layer including a porous polyethylene and an organic additive is provided. A white index of the first layer is equal to or more than 85 and equal to or less than 98, and a reduction rate of diethyl carbonate dropped on the first layer is equal to or higher than 0.048 mg/s and equal to or lower than 0.067 mg/s. The to separator may further include a porous layer over the first layer.

Provided is a separator including a separator which consists of a porous polyolefin of 95 wt % or more and an organic additive. A temperature-increase convergence time of the first layer is equal to or longer than 2.9 s.Math.m.sup.2/g and equal to or shorter than 5.7 s.Math.m.sup.2/g when the first layer is irradiated with a microwave having a frequency of 2455 MHz and an output power of 1800 W after dipping the first layer in N-methylpyrrolidone containing 3 wt % of water, whereas a white index is equal to or more than 86 and equal to or less than 98.

Provided is a separator including a separator which consists of a porous polyolefin of 95 wt % or more and an organic additive. A temperature-increase convergence time of the first layer is equal to or longer than 2.9 s.Math.m.sup.2/g and equal to or shorter than 5.7 s.Math.m.sup.2/g when the first layer is irradiated with a microwave having a frequency of 2455 MHz and an output power of 1800 W after dipping the first layer in N-methylpyrrolidone containing 3 wt % of water, whereas a white index is equal to or more than 86 and equal to or less than 98.

Provided is a separator capable of suppressing an increase in internal resistance and a decrease in a battery performance. A separator having a first layer consisting of a porous polyolefin and an organic antioxidant and a secondary battery including the separator are provided. The first layer has a parameter X, defined by the following equation, equal to or more than 0 and equal to or less than 20,

[00001]$X=100.Math..Math.\frac{\frac{.Math.\mathrm{MD}.Math..Math.\tan .Math..Math.-\mathrm{TD}.Math..Math.\tan .Math..Math..Math.}{.Math.\mathrm{MD}.Math..Math.\tan .Math..Math.+\mathrm{TD}.Math..Math.\tan .Math..Math..Math.}}{2}$

where MD tan and TD tan are respectively a loss tangent in a flow direction and a loss tangent in a width direction which are obtained by a viscoelasticity measurement of the first layer at a temperature of 90 C. and a frequency of 10 Hz. A white index of the first layer is equal to or more than 85 and equal to or less than 98.

Provided is a separator capable of suppressing an increase in internal resistance and a decrease in a battery performance. A separator having a first layer consisting of a porous polyolefin and an organic antioxidant and a secondary battery including the separator are provided. The first layer has a parameter X, defined by the following equation, equal to or more than 0 and equal to or less than 20,

[00001]$X=100.Math..Math.\frac{\frac{.Math.\mathrm{MD}.Math..Math.\tan .Math..Math.-\mathrm{TD}.Math..Math.\tan .Math..Math..Math.}{.Math.\mathrm{MD}.Math..Math.\tan .Math..Math.+\mathrm{TD}.Math..Math.\tan .Math..Math..Math.}}{2}$

where MD tan and TD tan are respectively a loss tangent in a flow direction and a loss tangent in a width direction which are obtained by a viscoelasticity measurement of the first layer at a temperature of 90 C. and a frequency of 10 Hz. A white index of the first layer is equal to or more than 85 and equal to or less than 98.

Porous polymer composites and methods of preparing porous polymer composites are provided herein. In some embodiments, a method for preparing porous polymer composites may include mixing a first polymer with a solvent and a particulate filler to form a first polymer composition, wherein the amount of particulate filler in the first polymer composition is below a mechanical percolation threshold; and removing the solvent from the first polymer composition to concentrate the first polymer and particulate filler into a second polymer composition having a porous structure, wherein the particulate filler concentration in the second polymer composition is increased above the mechanical percolation threshold during solvent removal.

Porous polymer composites and methods of preparing porous polymer composites are provided herein. In some embodiments, a method for preparing porous polymer composites may include mixing a first polymer with a solvent and a particulate filler to form a first polymer composition, wherein the amount of particulate filler in the first polymer composition is below a mechanical percolation threshold; and removing the solvent from the first polymer composition to concentrate the first polymer and particulate filler into a second polymer composition having a porous structure, wherein the particulate filler concentration in the second polymer composition is increased above the mechanical percolation threshold during solvent removal.

A polyimide precursor solution includes an aqueous solution that contains water; a resin particle that does not dissolve in the aqueous solution; an inorganic particle; and a polyimide precursor.