Polyolefin Microporous Membrane Structure Against Separator Crushing
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Solution Overview
Problem
Conventional polyolefin microporous membranes used in lithium-ion secondary cells face challenges in achieving high output and cycle characteristics while preventing short-circuit defects due to membrane crushing and voltage reduction during the cell production process.
Innovation Solution
A polyolefin microporous membrane with specific properties, including a film thickness of 1 μm to 30 μm, air permeability of 500 sec/100 cm³ or less, a withstand voltage reduction rate between 1.0% and 28.0% under controlled pressing conditions, and a polyethylene crystal long period of 35.0 nm or more, is developed to enhance compression resistance and inhibit crystal structure changes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional polyolefin microporous membrane is used as a separator, then the cell can be manufactured with basic functionality, but membrane crushing and withstand voltage reduction occur during pressing, leading to short-circuit defects and poor cycle characteristics
Solution Approach 1:
The patent applies parameter changes by optimizing the crystal structure of polyethylene in the microporous membrane. Specifically, it controls the long period of polyethylene crystals to 30 nm or more and adjusts the ratio of orthorhombic to monoclinic crystal forms. This crystalline parameter optimization enables the membrane to maintain its withstand voltage above 15 V even after pressing at 3.4 MPa, thereby preventing short-circuit defects while improving compression resistance.
2Ease of manufacture
If pressing is applied during cell production to insert electrodes and separator into the exterior can, then the cell assembly is completed, but membrane crushing occurs causing withstand voltage reduction and short-circuit defects
Solution Approach 1:
The patent applies preliminary action by pre-optimizing the crystal structure of the polyolefin microporous membrane before the pressing process. The membrane is manufactured with controlled polyethylene crystal long period (30 nm or more) and specific orthorhombic/monoclinic crystal ratios. This preliminary structural preparation ensures the membrane can withstand the subsequent pressing operation without crushing, maintaining manufacturing precision while enabling efficient cell assembly.
3Reliability
If the polyolefin microporous membrane has high porosity for good ion permeability, then ion transport is improved, but the membrane becomes more susceptible to crushing and withstand voltage reduction during pressing
Solution Approach 1:
The patent resolves this contradiction by changing the crystalline parameters of polyethylene in the membrane. By controlling the long period to 30 nm or more and optimizing the orthorhombic/monoclinic crystal ratio, the membrane achieves a structure that maintains high porosity for ion permeability while simultaneously improving structural stability to resist crushing during pressing operations.
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 achieves high output and cycle characteristics and prevents short-circuit defects by optimizing the crystal structure and compression resistance, ensuring stable voltage performance even with expanding electrodes.
Implementation Method 1
a polyethylene crystal long period before compression of 35.0 nm or more, measured by a small-angle X-ray scattering (SAXS) method; and a diffraction peak-to-peak distance of 2.410° or more and 2.600° or less derived from a (110) plane and a (200) plane of a polyethylene crystal before compression
Data Source
AI summary
A polyolefin microporous membrane having a film thickness of 1 μm to 30 μm; an air permeability of 500 sec/100 cm3 or less; and a withstand voltage reduction rate of 1.0% or greater and 17.0% or less due to pressing under conditions of a temperature of 60° C., a pressure of 3.4 MPa, and a compression time of 1 sec or a withstand voltage reduction rate of 1.0% or greater and 28.0% or less due to pressing under conditions of a temperature of 70° C., a pressure of 8 MPa, and a compression time of 3 min, or having a polyethylene crystal long period before compression of 35.0 nm or more, measured by a small-angle X-ray scattering (SAXS) method, and a diffraction peak-to-peak distance of 2.410° or more and 2.600° or less derived from a (110) plane and a (200) plane of a polyethylene crystal before compression is provided.