Microporous Polyolefin Membrane for Battery Separators
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Solution Overview
Problem
Existing microporous polyolefin membranes used as battery separators lack optimal permeability, mechanical strength, heat shrinkage resistance, compression resistance, and electrolytic solution absorption properties, which are crucial for improved battery safety and productivity, particularly in lithium-ion batteries.
Innovation Solution
A microporous membrane with a bimodal pore size distribution, manufactured by combining polyethylene and polypropylene resins, and processed through extrusion, cooling, stretching, and heat-setting, to achieve enhanced heat shrinkage resistance, compression resistance, and electrolytic solution absorption characteristics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If a microporous polyolefin membrane is used as a battery separator, then permeability and mechanical strength are improved, but heat shrinkage resistance and compression resistance deteriorate
Solution Approach 1:
The invention uses a composite structure consisting of a microporous polyolefin membrane and a heat-resistant porous layer. The heat-resistant porous layer contains inorganic particles (such as alumina, silica, or titania) dispersed in a porous structure, which provides heat shrinkage resistance and compression resistance. This composite structure allows the separator to maintain mechanical strength while resisting thermal deformation and compression during battery operation.
Solution Approach 2:
The heat-resistant porous layer is applied locally to the polyolefin membrane, specifically forming a coating on one or both surfaces. This localized application provides heat shrinkage resistance at the critical interfaces where electrodes contact the separator, while maintaining the permeability and mechanical properties of the bulk membrane structure.
2Productivity
If porosity is increased to improve permeability, then electrolytic solution absorption is improved, but mechanical strength deteriorates
Solution Approach 1:
The invention employs a microporous polyolefin membrane with controlled porosity (30-80%) to achieve optimal permeability for electrolyte transport. The porous structure is maintained through controlled phase separation during membrane formation, creating interconnected pores that facilitate ion transport while the polyolefin matrix provides mechanical integrity.
Solution Approach 2:
The heat-resistant porous layer containing inorganic particles reinforces the membrane structure, compensating for the mechanical strength reduction caused by high porosity. The inorganic particles form a rigid scaffold within the porous structure, providing structural support while allowing electrolyte penetration through the interconnected pores.
3Productivity
If membrane thickness is reduced to improve battery energy density, then productivity is improved, but mechanical strength and safety deteriorate
Solution Approach 1:
The invention uses an ultra-thin microporous polyolefin membrane (1-20 μm) as the base separator structure, which minimizes the distance for ion transport and reduces overall battery thickness. The thin film design improves energy density by increasing the volume available for active materials while maintaining adequate mechanical properties through the reinforced composite structure.
Solution Approach 2:
The heat-resistant porous layer with inorganic particles provides enhanced mechanical strength and thermal stability to the ultra-thin membrane. This composite reinforcement allows the separator to maintain structural integrity and safety functions (such as shutdown behavior and resistance to thermal runaway) despite the reduced thickness, enabling high energy density batteries without compromising safety.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The resulting membrane exhibits improved air permeability, pin puncture strength, tensile strength, and electrolytic solution absorption, ensuring better battery safety, cyclability, and productivity with minimal thickness variation and air permeability changes under heat compression.
Implementation Method 1
cooling the extrudate to form a cooled extrudate having a high polyolefin content
Implementation Method 2
heat-setting the stretched membrane to form the microporous membrane
Data Source
AI summary
A microporous membrane having a structure in which its pore size distribution curve obtained by mercury intrusion porosimetry has at least two peaks, which is produced by extruding a combination of a diluent or solvent and a polyolefin resin composition comprising from about 75 to about 99% of a polyethylene resin having a weight average molecular weight of from about 2.5×105 to about 5×105 and a molecular weight distribution of from about 10 to about 100, and from about 1 to about 25% polypropylene resin having a weight average molecular weight of from about 1×104 to about 4×106, a heat of fusion of 80 J/g or higher, and a molecular weight distribution of from about 1 to about 100, percentages based on the mass of the polyolefin composition; cooling the extrudate to form a high polyolefin content cooled extrudate; stretching the cooled extrudate at a high stretching temperature to form a stretched sheet; removing the diluent or solvent from the stretched sheet to form a membrane; stretching the membrane to a high magnification in at least one direction to form a stretched membrane; and heat-setting the stretched membrane to form the microporous membrane.