Microporous Polyolefin Film Compression Resistance via Controlled Stretching
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Solution Overview
Problem
Microporous polyolefin membranes used as battery separators lack sufficient compression resistance, leading to issues with air permeability changes and electrode expansion, which affects battery capacity and cyclability.
Innovation Solution
A method involving melt-blending polyolefin with a membrane-forming solvent, followed by uniaxial stretching, solvent removal, drying, and re-stretching at controlled temperatures and speeds, with optional heat treatment and cross-linking to stabilize crystals and improve lamella uniformity, resulting in a membrane with enhanced compression resistance and balanced properties.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If microporous polyethylene membrane is used for battery separators, then permeability and mechanical strength are improved, but compression resistance is insufficient
Solution Approach 1:
The patent applies parameter changes by optimizing the molecular weight distribution of polyethylene (controlling Mw/Mn ratio between 10-300) and adjusting processing parameters including stretching temperature (below crystal dispersion temperature), stretching magnification (λ1t/λ2m and λ1m/λ2t between 1-10), and heat treatment conditions. These parameter optimizations simultaneously improve mechanical strength and compression resistance of the microporous membrane.
Solution Approach 2:
The patent uses composite material approach by creating a polyethylene composition with specific molecular weight distribution characteristics, combining different molecular weight components to achieve both good mechanical properties and compression resistance. The controlled Mw/Mn ratio creates a composite-like structure within the polyethylene that balances permeability, strength, and compression resistance.
2Quantity of substance
If stretching temperature is increased to improve porosity, then air permeability is improved, but heat shrinkage resistance deteriorates
Solution Approach 1:
The patent applies parameter changes by precisely controlling stretching temperature to be below the crystal dispersion temperature of polyethylene, rather than using higher temperatures. This temperature parameter optimization allows achieving adequate porosity through controlled stretching while maintaining the crystalline structure's stability, thus preserving heat shrinkage resistance.
3Quantity of substance
If stretching magnification is increased to improve porosity, then air permeability is improved, but mechanical strength deteriorates
Solution Approach 1:
The patent applies parameter changes by optimizing stretching magnification ratios (λ1t/λ2m and λ1m/λ2t between 1-10) rather than using extreme magnifications. This moderate stretching approach creates sufficient porosity for air permeability while maintaining mechanical integrity. The balanced stretching ratios prevent excessive structural degradation that would occur with higher magnifications.
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 method produces a microporous polyolefin membrane with improved compression resistance, air permeability, mechanical strength, and heat shrinkage resistance, suitable for battery separators, enhancing battery cyclability and productivity.
Implementation Method 1
melt-blending a polyolefin and a membrane-forming solvent
Implementation Method 2
cooling the extrudate to form a gel molding
Implementation Method 3
stretching the gel molding at least uniaxially
Implementation Method 4
removing the membrane-forming solvent
Implementation Method 5
re-stretching the resultant membrane at least uniaxially, wherein the re-stretching temperature is equal to or lower than the crystal dispersion temperature of the polyolefin +20°C
Implementation Method 6
heat treatment and cross-linking to stabilize crystals
Implementation Method 7
optional heat treatment and cross-linking to stabilize crystals and improve lamella uniformity
Data Source
AI summary
A microporous polyolefin membrane having excellent compression resistance is obtained by stretching a gel molding comprising a polyolefin and a membrane-forming solvent, removing the membrane-forming solvent, and stretching the resultant membrane again at least uniaxially at a speed of 3%/second or more at a temperature equal to or lower than the crystal dispersion temperature +20°C.