Microporous Polyolefin Separator Structure for Porosity-Strength Balance
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Solution Overview
Problem
Current polyolefin microporous membranes face challenges in achieving both high output characteristics and strength, particularly in lithium ion batteries, where increased porosity for better ion conductivity compromises mechanical strength and safety, and reduced thickness for improved capacity leads to potential short circuits and rupture.
Innovation Solution
The development of a polyolefin microporous membrane with a fine and uniform fibril structure achieved through controlled stretching and molecular weight optimization, increasing the number of pores per unit volume and surface pores, while maintaining high puncture strength and thermal stability, using ultrahigh molecular weight polyethylene and specific stretching processes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If porosity is increased to improve ion conductivity, then output characteristics are improved, but mechanical strength decreases
Solution Approach 1:
The patent applies parameter changes by precisely controlling the molecular weight of polyethylene (2,000,000 or more) and the stretching ratio (40 times or more area magnification) to achieve optimal porosity and strength balance. This resolves the contradiction by finding the optimal parameter range where both ion conductivity and mechanical strength are maximized simultaneously.
Solution Approach 2:
The patent uses composite material structure by creating a microporous membrane with specific fibril network architecture. The controlled stretching process creates a composite structure of crystalline regions and amorphous regions with interconnected pores, achieving both high ion conductivity through porous structure and high strength through the fibril network.
2Quantity of substance
If separator thickness is reduced to improve battery capacity, then energy density increases, but safety decreases due to increased risk of short circuit and rupture
Solution Approach 1:
The patent changes the parameter of molecular weight to ultrahigh molecular weight polyethylene (2,000,000 or more), which provides exceptional mechanical strength even at reduced thickness. This allows the separator to be thinner for higher capacity while maintaining safety through the enhanced strength-to-thickness ratio.
Solution Approach 2:
The patent applies preliminary action by performing extensive stretching (40 times or more area magnification) during manufacturing to pre-establish a robust fibril network structure. This preliminary structural reinforcement ensures that even thin separators maintain high safety margins against short circuits and rupture during battery operation.
3Productivity
If stretching ratio is increased to improve porosity, then ion permeability improves, but mechanical integrity may deteriorate
Solution Approach 1:
The patent changes the parameter of base material molecular weight to ultrahigh molecular weight (2,000,000 or more), which provides sufficient mechanical integrity to withstand extreme stretching ratios (40 times or more area magnification). This enables achieving high porosity for excellent ion permeability while the ultrahigh molecular weight polymer maintains structural coherence during and after stretching.
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
This approach enhances both the safety and output characteristics of the membrane by improving ion permeability and mechanical strength, allowing for higher power consumption and rapid charging without compromising battery safety.
Implementation Method 1
Lithium ion secondary batteries are used in in-vehicle applications... rapid charging (large-current charging) and an increase in power consumption (large-current discharging) are required
Implementation Method 2
a method of increasing the stretching ratio... the stretched blend is washed and dried and then is stretched at 1.2 times (area magnification: 54 times)
Data Source
AI summary
The present invention addresses the problem of providing a microporous polyolefin film which is superior in output characteristics and strength to conventional ones. A first aspect of the present invention relates to a microporous polyolefin film which satisfies the relationship [Y (N)] ≥ [-6.7×10-3×(X(/µm3))+4.5] where Y (N) is the piercing strength in terms of 10-µm film thickness and X (/µm3) is the number of pores per unit volume, the number of pores X being 40 /µm3 or larger.