Microporous film battery separators are used in various batteries, particularly rechargeable batteries, such as lithium batteries. Such battery separators allow electrolytes to cross through the battery separators while preventing any contact between electrodes of opposite polarity. Typically, the microporous film comprises one or more layers of microporous membranes.
In lithium batteries, particularly secondary lithium batteries (rechargeable lithium batteries), overheating problems can occur and cause thermal runaway in the battery. Thus, shutdown separators were developed to prevent thermal runaway. See e.g., U.S. Pat. No. 4,650,730 and U.S. Pat. No. 4,731,304. A shutdown battery separator has a microporous membrane that closes its pores at a temperature substantially lower than the temperature that could cause thermal runaway in the lithium battery.
Some multilayer shutdown separators are known in the art. For example, U.S. Pat. No. 4,650,730 discloses a bilayer battery separator having an unfilled microporous sheet and a filled microporous sheet. Each sheet is formed separately by an extraction process using a suitable solvent. The two microporous sheets are then laminated together to form the shutdown separator.
The Celgard® battery separators, which have been commercially available for a number of years, are typically formed by a stretch method. For example, a non-porous tubular polypropylene film is first formed by blown film extrusion. The tubular film is collapsed onto itself to form a non-porous flat sheet having two polypropylene plies. Optionally, the die assembly is rotated slowly twisting the tubular film somewhat to prevent and remove wrinkles and uneven distribution so that the surface of the film is substantially smooth. The flat sheet is then annealed and stretched to impart microporosity therein. The two microporous flat sheets are then de-plied into two layers of microporous battery separator. Normally, the adhesion force between the two plies in the flat sheet must be sufficiently low such that the two plies can be separated without damaging the plies. However, when a separator having two layers of the microporous polypropylene film is desired, the adhesion force can be higher, e.g., 5 grams/inch to about 35 grams/inch, which can be caused by, e.g., bonding the plies after collapsing the tubular film.
U.S. Pat. No. 5,691,077 discloses a trilayer battery separator. In a preferred embodiment disclosed therein, the separator has a polypropylene-polyethylene-polypropylene construction, and is made by laminating and bonding microporous layers. Each microporous layer is formed by a Celgard® process described above involving a de-plying step.
U.S. Pat. No. 5,691,047 also discloses a microporous trilayer battery separator having a polypropylene-polyethylene-polypropylene construction. A plurality of non-porous single-layered precursors are first extruded by cast extrusion. The non-porous single layers are laminated and bonded together into a precursor of a polypropylene-polyethylene-polypropylene structure. The precursor is then annealed and stretched to form a microporous trilayer battery separator.
A number of co-extrusion processes for making multilayer battery separators have also been proposed. For example, UK Patent Publication No. GB 2,298,817 describes a microporous trilayer battery separator made by co-extruding a trilayer film precursor having a non-porous polypropylene-polyethylene-polypropylene construction using a T-die, annealing the trilayer precursor, and then stretching the annealed trilayer precursor to form the porous trilayer battery separator.
A porous trilayer separator is also proposed in Japanese Patent Application No. 56320/1995 (JP8-250097A) filed by Kureha Chemical Industry Co. Ltd. The Kureha separator is prepared by a process that includes the steps of co-extruding a trilayer precursor containing a solvent extractable material as pore forming aid, and forming pores in the precursor by solvent extraction of the extractable material in the precursor.
U.S. Pat. No. 6,346,350 discloses making a multilayer battery separator by co-extrusion in a blown film process. The co-extruded molten film is rapidly quenched such that it is in a substantially solidified state. The co-extruded film is then annealed and stretched to impart microporosity therein.
A multilayer microporous shutdown separator should be as thin as possible in order to minimize the space it occupies in a battery and also to reduce electrical resistance (ER). Nevertheless the shutdown separator must also have sufficient strength to resist puncture. Punctured battery separators are ineffective in preventing the contact between the electrodes of opposite polarity. Under overheating conditions, a punctured battery separator cannot shut down effectively to prevent the electrolytes from crossing the battery separator, and thus is ineffective in preventing thermal runaway. Battery separators with low puncture strength are difficult to handle especially in the battery separator manufacturing processes. Once punctured, battery separators are prone to splitting, i.e., tearing.
Thus, it is an objective in the art to further develop efficient methods for making relatively thin battery separators with improved puncture strength.