Lithium ion battery

1. A lithium ion battery comprising:

a positive electrode including a positive electrode current collector and a positive electrode active material layer that contains a positive electrode active material capable of absorbing and desorbing lithium;

a negative electrode including a negative electrode current collector and a negative electrode active material layer that contains an alloy-formable active material;

an ion permeable insulating layer that is interposed between said positive electrode and said negative electrode; and

an ion conductive polymer layer that is interposed between said negative electrode and said ion permeable insulating layer,

wherein said ion conductive polymer layer includes a negative electrode-side portion and an ion permeable insulating layer-side portion that have different compositions, and said ion permeable insulating layer-side portion has higher ion conductivity than said negative electrode-side portion.

2. The lithium ion battery in accordance with claim 1 , wherein said ion permeable insulating layer-side portion has an ion conductivity level of 0.05 mS/cm or more when measured by an alternating current impedance method.

3. The lithium ion battery in accordance with claim 1 , wherein the adhesion strength between said negative electrode active material layer and said ion conductive polymer layer is 100 to 300 N/cm 2 .

4. The lithium ion battery in accordance with claim 1 , wherein:

said ion conductive polymer layer is a laminate including a first polymer layer that adheres to said alloy-formable active material and a second polymer layer that is in contact with said ion permeable insulating layer, and

said second polymer layer has higher ion conductivity than said first polymer layer.

5. The lithium ion battery in accordance with claim 4 , wherein the ratio T N /T I of thickness T N of said first polymer layer to thickness T I of said second polymer layer is 0.01 to 10.

6. The lithium ion battery in accordance with claim 4 , wherein each layer forming said laminate contains at least one polymer selected from the group consisting of polyvinylidene fluoride and a copolymer of vinylidene fluoride and hexafluoropropylene.

7. The lithium ion battery in accordance with claim 4 , wherein said first and second polymer layers contain a copolymer of vinylidene fluoride and hexafluoropropylene, and said first polymer layer has a smaller hexafluoropropylene unit content in the copolymer than said second polymer layer.

8. The lithium ion battery in accordance with claim 7 , wherein said first polymer layer has a hexafluoropropylene unit content R N in the copolymer of 0.1 to 15 wt %, and the difference R I -R N between R N and a hexafluoropropylene unit content R I in the copolymer of said second polymer layer is 0.1 to 25 wt %.

9. The lithium ion battery in accordance with claim 1 , wherein said ion conductive polymer layer contains an inorganic filler.

10. The lithium ion battery in accordance with claim 1 , wherein said ion conductive polymer layer has a thickness of 0.01 to 20 μm.

11. The lithium ion battery in accordance with claim 1 , wherein said negative electrode active material layer is a film formed by a vapor deposition method.

12. The lithium ion battery in accordance with claim 1 , wherein said negative electrode active material layer extends outward from the surface of said negative electrode current collector, and is an array of a plurality of columnar bodies that are spaced apart from each other.

13. The lithium ion battery in accordance with claim 1 , wherein said alloy-formable active material is at least one selected from an alloy-formable active material that contains silicon and an alloy-formable active material that contains tin.