Separator for lithium ion secondary battery and lithium ion secondary battery provided therewith
A separator for a lithium ion secondary battery, comprising a porous base material containing polyolefin, and a porous layer containing a vinylidene fluoride resin as a main component provided on at least one surface of the porous base material is excellent in electrolytic solution retention properties, adhesion and bondability to electrodes, and dimensional stability, and also has high and uniform ionic conductivity, reduced interfacial resistance to electrodes, and shutdown properties. A lithium ion secondary battery having excellent capacity characteristics, charge and discharge characteristics, cycle characteristics, safety and reliability can be provided by using the separator.
1. A separator for lithium ion secondary battery, comprising a porous base material containing polyolefin, and a porous layer containing a vinylidene fluoride resin as a main component provided on at least one surface of the porous base material, wherein
the porous layer contains at least one vinylidene fluoride resin having a weight-average molecular weight of 150,000 to 500,000 in an amount of 50% by weight or more based on the overall vinylidene fluoride resin,
the average pore size of an external surface is less than that of an interior in the porous layer, and
the external surface has an average pore size of 0.1 to 5 μm and the interior has an average pore size of 0.5 to 10 μm in the porous layer.
2. The separator for lithium ion secondary battery according to claim 1 , wherein
air permeability, as measured by a Gurley air permeability tester, of the porous base material is 1000 sec/100 ml or less, and
the porous base material has a porosity of 20 to 80% by volume.
3. The separator for lithium ion secondary battery according to claim 2 , wherein the porous layer has a thickness of 0.1 to 5 μm.
4. The separator for lithium ion secondary battery according to claim 2 , wherein the porous layer has an average pore size of 0.01 to 10 μm.
5. The separator for lithium ion secondary battery according to claim 1 , wherein the porous layer has a weight of 0.5 to 10 g/m 2 .
6. The separator for lithium ion secondary battery according to claim 1 , wherein the porous layer has a thickness of 0.5 to 8 μm.
7. The separator for lithium ion secondary battery according to claim 1 , wherein the porous base material has a thickness of 5 to 50 μm.
8. The separator for lithium ion secondary battery according to claim 1 , wherein the vinylidene fluoride resin is a homopolymer of vinylidene fluoride, or a copolymer of at least one of ethylene tetrafluoride, propylene hexafluoride and ethylene, and vinylidene fluoride, or a mixture of the homopolymer and the copolymer.
9. The separator for lithium ion secondary battery according to claim 1 , wherein air permeability as measured by a Gurley air permeability tester is 1000 sec/100 ml or less.
10. A lithium ion secondary battery provided with the separator for lithium ion secondary battery according to claim 1 .
11. A lithium ion secondary battery comprising:
a positive electrode obtained by bonding a positive electrode active material to a positive electrode current collector,
a negative electrode obtained by bonding a negative electrode active material to a negative electrode current collector,
the separator for lithium ion secondary battery according to claim 1 disposed and bonded between the positive electrode and the negative electrode, and an electrolytic solution containing lithium ions held in the separator.