Polyolefin Microporous Separator Balancing Shutdown and Thermal Strength
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Solution Overview
Problem
Polyolefin-based microporous membranes used as separators in secondary batteries suffer from shrinkage and breakage at high temperatures due to their melting point, compromising safety and performance.
Innovation Solution
A polyolefin-based microporous membrane with specific molecular weight ranges and ratios of polyethylene and polypropylene, combined with controlled manufacturing processes, achieves enhanced puncture strength, gas permeability, porosity, and surface roughness, ensuring high thermal stability and safety.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a polyolefin-based microporous membrane is heated to a high temperature close to its melting point, then the membrane provides hole-closing function for safety, but the membrane undergoes shrinking and breakage
Solution Approach 1:
The patent applies parameter changes by carefully controlling the molecular weight distribution of polyethylene (weight average molecular weight of 30×10^4 to 60×10^4) and polypropylene (viscosity average molecular weight of 100×10^4 to 230×10^4), along with their specific weight ratios, to optimize both the hole-closing temperature and thermal stability. This resolves the contradiction by tuning material parameters to achieve reliable hole-closing function while preventing shrinkage and breakage.
Solution Approach 2:
The patent uses composite materials by combining polyethylene and polypropylene in specific ratios (85:15 to 95:5) to create a microporous membrane that leverages the complementary properties of both polymers. Polyethylene provides low-temperature hole-closing capability while polypropylene contributes high-temperature structural stability, resolving the contradiction between hole-closing reliability and thermal strength.
2Strength
If the molecular weight of polyethylene is increased to improve puncture strength, then the membrane strength increases, but the gas permeability and ion transport may be affected
Solution Approach 1:
The patent applies parameter changes by optimizing the weight average molecular weight of polyethylene to a specific range (30×10^4 to 60×10^4) rather than simply increasing it. This controlled parameter adjustment ensures sufficient puncture strength while maintaining appropriate pore structure for gas permeability and ion transport.
Solution Approach 2:
The patent applies local quality by creating heterogeneous molecular weight distribution within the polymer matrix. The specific molecular weight range of polyethylene and polypropylene creates localized regions with different mechanical and transport properties, allowing the membrane to simultaneously achieve high puncture strength and adequate gas permeability.
3Reliability
If the porosity of the membrane is increased to improve ion permeability, then the gas permeability increases, but the mechanical strength decreases
Solution Approach 1:
The patent uses composite materials where polyethylene and polypropylene work synergistically. The polypropylene component provides structural reinforcement that maintains mechanical strength even when porosity is optimized for high ion and gas permeability, resolving the contradiction between transport efficiency and mechanical integrity.
Solution Approach 2:
The patent applies parameter changes by controlling the molecular weight and ratio of constituent polymers to achieve an optimal pore structure. The specific molecular weight ranges and weight ratios create a pore architecture that allows high ion permeability while maintaining sufficient mechanical strength through controlled pore size and distribution.
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 membrane maintains structural integrity and performance at elevated temperatures, enhancing battery safety and efficiency by preventing internal short circuits and maintaining capacity retention.
Implementation Method 1
when the polyolefin-based microporous membrane is heated to a high temperature close to its melting point or even higher, it undergoes shrinking
Data Source
Figure 1~2
AI summary
Provided are a polyolefin-based microporous membrane or a separator for a secondary battery, a method for manufacturing the same, and a secondary battery including the same. The polyolefin-based microporous membrane or the separator for a secondary battery has a puncture strength of 0.3 N/µm or more, a gas permeability of 0.8×10-5 Darcy or more, a porosity of 30.0% or more, and a surface roughness of 2.1 µm or less, the surface roughness being a value obtained by selecting 5 random points on a front surface and 5 random points on a back surface, each having an area of 284 µm (width) × 220 µm (length), measuring a maximum height difference (µm, Rmax) which is a difference between the highest surface height and the lowest surface height in the area of each point, and adding an average value of values measured in each of the 5 points on the front surface and an average value of values measured in each of the 5 points on the back surface.