Microporous membrane, lithium ion secondary battery and method of producing the microporous membrane

1. A microporous membrane, wherein average membrane thickness is 15 μm or less, and relative impedance A after a heat compression treatment under a pressure of 4.0 MPa at 80° C. for 10 minutes is 140% or less, said relative impedance A being obtained by the equation below:

Relative impedance A=(impedance measured at 80° C. after said heat compression treatment)/(impedance measured at room temperature prior to said heat compression treatment) ×100, and

wherein pin puncture strength corresponding to a membrane thickness of 12 μm is 4000 mN or more.

2. The microporous membrane according to claim 1 , wherein said relative impedance A is 135% or less.

3. The microporous membrane according to claim 1 , wherein the relative impedance ratio C obtained by the equation below is 135% or less:

Relative impedance ratio C=(relative impedance A/relative impedance B)×100,

wherein, the relative impedance B is determined by the equation below:

Relative impedance B=(impedance measured at room temperature (25° C.) after said heat compression treatment)/(impedance measured at room temperature prior to said heat compression treatment)×100.

4. A microporous membrane, wherein average membrane thickness is 15 μm or less, and relative impedance B after a heat compression treatment under a pressure of 4.0 MPa at 80° C. for 10 minutes is 120% or less, said relative impedance B being obtained by the equation below:

Relative impedance B=(impedance measured at room temperature (25° C.) after said heat compression treatment)/(impedance measured at room temperature prior to said heat compression treatment)×100, and

wherein pin puncture strength corresponding to a membrane thickness of 12 μm is 4000 mN or more.

5. The microporous membrane according to claim 4 , wherein said relative impedance B is 110% or less.

6. The microporous membrane according to claim 4 , wherein the relative impedance ratio C obtained by the equation below is 135% or less:

Relative impedance ratio C=(relative impedance A/relative impedance B)×100,

wherein, said relative impedance A is determined by the equation below:

Relative impedance A=(impedance measured at 80° C. after said heat compression treatment)/(impedance measured at room temperature prior to said heat compression treatment) ×100.

7. The microporous membrane according to claim 3 , wherein said relative impedance ratio C is 130% or less.

8. The microporous membrane according to claim 1 , wherein air resistance is 400 seconds or less.

9. The microporous membrane according to claim 1 , wherein heat shrinkage rate after an exposure at 105° C. for 8 hours is 5% or less, and average tensile rupture elongation is 130% or less.

10. The microporous membrane according to claim 1 , wherein at least one of: the shutdown temperature is 140° C. or less, and the temperature difference between the shutdown temperature and the meltdown temperature obtained by a temperature-increasing air permeability method is 10° C. or more is satisfied.

11. The microporous membrane according to claim 1 , wherein average pore size is 0.1 μm or less.

12. The microporous membrane according to claim 1 , comprising 2% or more of an ultra high molecular weight polyethylene component having a weight average molecular weight of 1,000,000 or more, or 5% or more of a polyethylene component having a molecular weight of 1,000,000 or more.

13. The microporous membrane according to claim 1 , wherein formation of a hybrid structure of a ladder structure in a submicron region and a three-dimensional network structure in a micron region is observed on at least one surface of the microporous membrane.

14. A lithium ion secondary battery, wherein the microporous membrane according to claim 1 is used.

15. A method of producing a polyolefin microporous membrane, comprising:

(a) an extrusion step of melt blending and extruding a resin composition containing a polyolefin resin and a pore-forming material,

(b) a sheet forming step of sheet forming the extrudate obtained in said step (a) into a sheet,

(c) a first stretching step of stretching the sheet-shaped product obtained in said step (b) at least twice in at least different axial directions,

(d) an extraction step of extracting the pore-forming material from the stretched sheet obtained in said step (c), and

(e) a second stretching step of stretching the sheet obtained in said step (d) at least once in at least one axial direction,

wherein at least one of (i) and (ii): and (iii) are satisfied:

(i) said step (c) is a first stretching step of stretching the sheet-shaped product at least once in a sheet transport direction (MD direction) and at least once in a sheet width direction (TD direction),

and the MD stretching magnification and the TD stretching magnification in said step (c) satisfy Equations (1-1) and (1-2):

TD stretching magnification≥MD stretching magnification−α  (1-1)

α=2.0  (1-2),

(ii) the stretching temperature (T 1 ) of a first axial stretching performed first in said step (c) and the maximum stretching temperature (T 2 ) of a second stretching performed after the first axial stretching satisfy Equations (2-1) and (2-2),

T 1− T 2≥β  (2-1)

β=0  (2-2),

(iii) the stretching temperature (D (T)) in the step (e) satisfies Equations (3-1) and (3-2):

SDT-D( T )≤γ  (3-1)

γ=12  (3-2),

wherein SDT represents shutdown temperature.

16. The method according to claim 15 , wherein said (i) to said (iii) are satisfied simultaneously.