Low electrical resistance microporous battery separator membranes, separators, cells, batteries, and related methods
Microporous battery separators, batteries including such separators, and/or methods of making such separators, and/or methods of using such separators. In accordance with at least certain embodiments, an improved or novel battery separator for a secondary or rechargeable lithium battery may have low electrical resistance of less than 0.95 ohm-cm 2 , or in some cases, less than 0.8 ohm-cm 2 . Furthermore, the inventive battery separator membrane may provide a means to achieve an improved level of battery performance in a rechargeable or secondary lithium battery based on a possibly synergistic combination of low electrical resistance, low Gurley, low tortuosity, and/or a unique trapezoid shaped pore. In accordance with at least certain multilayer embodiments (by way of example only, a trilayer membrane made of two polypropylene layers with a polyethylene layer in between), the inventive microporous membrane or battery separator may have excellent onset and rate of thermal shutdown performance.
1. A battery separator comprising:
a microporous polyolefin separator membrane with non-round, trapezoid shaped pores, and having
an electrical resistance less than 0.95 ohm-cm 2 ,
a Gurley less than 500 sec/100 cc, and
a tortuosity less than 1.5.
2. The battery separator of claim 1 wherein the microporous polyolefin separator membrane has
an electrical resistance less than 0.8 ohm-cm 2 ,
a Gurley less than 150 sec/100 cc,
a tortuosity less than 1.3, and/or
said microporous polyolefin separator membrane consists of a polypropylene, polyethylene, mixtures thereof, or co-polymers thereof.
3. The battery separator of claim 1 where said microporous polyolefin separator membrane is a monolayer membrane or a multilayer membrane with a thermal shutdown function.
4. The battery separator of claim 1 where said microporous polyolefin separator membrane is a multilayer membrane consisting of a trilayer of polypropylene/polyethylene/polypropylene.
5. The battery separator of claim 4 , wherein the trilayer of polypropylene/polyethylene/polypropylene is made by a process comprising:
extruding a polyethylene which has a melt flow index less than 1.0 g/10 minutes to form a monolayer nonporous polyethylene precursor membrane, and
extruding a polypropylene which has a melt flow index less than 1.0 g/10 minutes to form a monolayer nonporous polypropylene precursor membrane, and
laminating two plies of polypropylene precursor membrane as outer plies or layers sandwiching one inner ply or layer of polyethylene precursor membrane to form a trilayer polypropylene/polyethylene/polypropylene nonporous precursor,
machine direction stretching a nonporous polypropylene/polyethylene/polypropylene precursor membrane to form a semi-porous intermediate membrane having a puncture strength >350 gf and TD elongation >600%,
transverse direction stretching the semi-porous intermediate membrane using a stretch ratio of 15 to 400% to form the trilayer of polypropylene/polyethylene/polypropylene which is microporous.
6. The battery separator of claim 5 wherein the stretch ratio is 25 to 100%.
7. The battery separator of claim 1 wherein the said microporous polyolefin separator membrane has a thickness less than 25 μm.
8. The battery separator of claim 7 wherein said microporous polyolefin separator membrane has pores with a non-round, trapezoid shape on each surface of said membrane.
9. The battery separator of claim 1 , wherein the microporous polyolefin separator membrane is made by a process comprising:
extruding a polypropylene which has a melt flow index less than 1.0 g/10 minutes to form a monolayer nonporous polypropylene precursor membrane, and
machine direction stretching the monolayer nonporous polypropylene precursor membrane to form a semi-porous intermediate membrane having a puncture strength >350 gf and TD elongation >600%, and
transverse direction stretching the semi-porous intermediate membrane using a stretch ratio of 15 to 400% to form the microporous polyolefin separator membrane of the battery separator.
10. The battery separator of claim 9 wherein the stretch ratio is from 25 to 100%.