Membranes, calendered microporous membranes, battery separators, and related methods
Novel or improved microporous single or multilayer battery separator membranes, separators, batteries including such membranes or separators, methods of making such membranes, separators, and/or batteries, and/or methods of using such membranes, separators and/or batteries are provided. In accordance with at least certain embodiments, a multilayer dry process polyethylene/polypropylene/polyethylene microporous separator which is manufactured using the inventive process which includes machine direction stretching followed by transverse direction stretching and a subsequent calendering step as a means to reduce the thickness of the multilayer microporous membrane, to reduce the percent porosity of the multilayer microporous membrane in a controlled manner and/or to improve transverse direction tensile strength. In a very particular embodiment, the inventive process produces a thin multilayer microporous membrane that is easily coated with polymeric-ceramic coatings, has excellent mechanical strength properties due to its polypropylene layer or layers and a thermal shutdown function due to its polyethylene layer or layers. The ratio of the thickness of the polypropylene and polyethylene layers in the inventive multilayer microporous membrane can be tailored to balance mechanical strength and thermal shutdown properties.
1. A process comprising the steps of:
extruding a polypropylene nonporous precursor membrane and,
extruding a polyethylene nonporous precursor membrane, and
stacking and bonding the polypropylene nonporous precursor membrane and polyethylene nonporous precursor membrane to form a polyethylene/polypropylene/polyethylene nonporous multilayer membrane, and
annealing the polyethylene/polypropylene/polyethylene nonporous multilayer membrane, and
machine direction stretching the polyethylene/polypropylene/polyethylene nonporous multilayer membrane to form an intermediate uniaxial stretched multilayer microporous membrane, and
transverse direction stretching the intermediate uniaxial stretched multilayer microporous membrane to form a second intermediate machine direction (MD) and transverse direction (TD) stretched polyethylene/polypropylene/polyethylene multilayer microporous membrane, and
calendering the second intermediate machine direction (MD) and transverse direction (TD) stretched polyethylene/polypropylene/polyethylene multilayer microporous membrane, after the machine direction (MD) stretching and the transverse direction (TD) stretching, to form a calendered polyethylene/polypropylene/polyethylene multilayer microporous membrane, and
recovering the calendered polyethylene/polypropylene/polyethylene multilayer microporous membrane with at least one outer surface or surface layer having a pore structure, with a pore being an opening or a space between adjacent lamellae that may be bounded or not bounded on one or on two sides by a fibril or a bridging structure between the adjacent lamellae and wherein at least a portion of the calendered polyethylene/polypropylene/polyethylene multilayer microporous membrane contains respective groups of pores between the adjacent lamellae with the lamellae oriented substantially along a transverse direction and the fibrils or the bridging structures between the adjacent lamellae oriented substantially along a machine direction and an outer surface of at least some of the lamellae being flattened or planar, and wherein the pore structure has at least one of: substantially trapezoidal or rectangular pores, pores with rounded corners, lamellae across a width direction or the transverse direction, randomly ordered pores, groups of pores with areas of missing or broken fibrils, a densified lamellar skeletal structure, groups of pores with a transverse direction (TD)/machine direction (MD) length ratio of at least 4, groups of pores with a TD/MD length ratio of at least 6, groups of pores with a TD/MD length ratio of at least 8, groups of pores with a TD/MD length ratio of at least 9, groups of pores with at least 10 fibrils, groups of pores with at least 14 fibrils, groups of pores with at least 18 fibrils, groups of pores with at least 20 fibrils, pressed or compressed stacked lamellae, a uniform surface, a non-uniform surface, or combinations thereof.
2. The process according to claim 1 , wherein the calendered polyethylene/polypropylene/polyethylene multilayer microporous membrane has a thickness less than 10 μm.
3. The process according to claim 1 wherein a ceramic coating is incorporated into the calendering process.