Laminated Microporous Battery Separator with Viscosity-Optimized Layers
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Solution Overview
Problem
Laminated microporous films used in lithium ion batteries for hybrid electric vehicles face challenges in achieving a balance between good air permeability and high film rupture temperature, with existing solutions having insufficient film rupture resistance and air permeability under severe conditions.
Innovation Solution
A laminated microporous film is developed with a first microporous layer composed of a high-melting point polypropylene resin and a second microporous layer of a lower-melting point polyethylene-based resin, where the melt viscosity ratio between the two layers is optimized to achieve a high film rupture temperature and air permeability, using a specific resin composition and manufacturing process involving stretching and heat setting.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If a polypropylene microporous film is laminated on a polyethylene microporous film to raise the film rupture temperature, then the film rupture temperature is improved, but the film rupture resistance is insufficient under severe conditions due to relatively low molecular weight polypropylene resin
Solution Approach 1:
The patent changes the molecular weight parameter of the polypropylene resin from conventional lower molecular weight to specifically high molecular weight (weight-average molecular weight of 100,000 to 1,000,000), which fundamentally alters the film's thermal stability and rupture resistance properties while maintaining the shutdown function at lower temperatures
Solution Approach 2:
The patent creates a composite microporous film structure combining polyethylene (for shutdown function at lower temperature) and high molecular weight polypropylene (for high temperature structural integrity), where each layer contributes different functional properties to achieve both shutdown capability and high film rupture resistance
2Reliability
If the molecular weight of polypropylene resin is increased to improve film rupture resistance, then the film rupture temperature is improved, but the air permeability and film rupture resistance balance is still insufficient
Solution Approach 1:
The patent applies local quality by giving each layer different functional characteristics: the polyethylene layer provides shutdown function with appropriate porosity for ion permeability, while the high molecular weight polypropylene layer provides thermal stability and mechanical strength, with each layer's properties optimized for its specific function rather than uniform properties throughout
Solution Approach 2:
The patent optimizes multiple parameters simultaneously including the molecular weight of polypropylene (100,000-1,000,000), the thickness ratio between layers, and the porosity of each layer to achieve the optimal balance between air permeability and film rupture resistance
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 laminated microporous film provides a good balance between air permeability and film rupture temperature, enhancing safety and performance as a battery separator by maintaining structural integrity and ion permeability even at elevated temperatures.
Implementation Method 1
the first microporous layer has a melt viscosity ηA of 10,000 Pa·s or higher
Implementation Method 2
subjecting a laminated film of a specific resin film having a relatively high melting point and a resin film having a relatively low melting point to stretching under a specific condition
Implementation Method 3
a hot stretch step of hot stretching the laminate cold stretched in the step (1)
Data Source
Figure 1(A)~1(C)
AI summary
There is provided a laminated microporous film having an excellent balance between air permeability and film rupture temperature obtained by laminating a first microporous layer comprising a first resin composition and a second microporous layer comprising a second resin composition having a lower melting point than the first resin composition, wherein the first microporous layer has a melt viscosity ηA of 10,000 Pa·s or higher, and a ratio ηA/ηB of the melt viscosity ηA to a melt viscosity ηB of the second microporous layer is 0.01 to 10.