Polyolefin Separator Dendritic Crystals Puncture Resistance
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Solution Overview
Problem
Lithium-ion battery separators face challenges in maximizing mechanical properties such as tensile strength and puncture resistance, which are critical for safety in high-capacity batteries, particularly in new energy vehicle applications.
Innovation Solution
A polyolefin porous separator with dendritic crystals and micropores is developed, featuring a specific crystal structure and preparation method involving mixed melting, extrusion, stretching, and heat-setting, using ultra-high molecular weight and high molecular weight polyethylene resins with mineral oil, to enhance mechanical properties.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If conventional polyolefin separator structures are used, then manufacturing is simple, but mechanical properties (tensile strength and puncture resistance) are insufficient
Solution Approach 1:
The patent changes the crystal structure parameters by controlling dendritic crystal formation through specific processing conditions (temperature, stretching ratios, mineral oil content) to achieve enhanced mechanical properties. The dendritic crystals with widths of 0.25-0.5 μm and specific distribution densities (2-8 per square micron) provide improved tensile strength and puncture resistance compared to conventional structures.
Solution Approach 2:
The patent creates a composite structure combining dendritic crystals, micropores, and polyolefin matrix. This composite architecture where dendritic crystals intersect with micropores on the surfaces and penetrate through the separator provides synergistic effects, improving mechanical strength while maintaining porosity for ion transport.
2Strength
If dendritic crystals with specific structure are formed, then mechanical strength is improved, but manufacturing process complexity increases
Solution Approach 1:
The patent achieves dendritic crystal formation by optimizing processing parameters including temperature control during extrusion (above melting temperature of polyethylene), stretching ratios (1.2-1.8 for adjacent rollers, 9-12 times transverse stretching), and mineral oil content (70-80% by mass). These parameter changes enable controlled crystal growth without requiring complex additional processing steps.
Solution Approach 2:
The patent uses mineral oil as an intermediary substance during the extrusion and stretching processes. The mineral oil facilitates the formation of dendritic crystals by acting as a plasticizer and processing aid, enabling the polyethylene to form the desired crystal structure during stretching, and is subsequently removed to create micropores.
3Strength
If dendritic crystals with dendritic structure are formed, then puncture resistance is improved, but manufacturing precision requirements increase
Solution Approach 1:
The patent controls dendritic crystal width (0.25-0.5 μm) by precisely managing processing parameters: extrusion temperature above polyethylene melting point, stretching temperature (150-170°C), and stretching ratios (1.2-1.8 between adjacent rollers). These parameter controls ensure consistent crystal dimensions for optimal puncture resistance.
Solution Approach 2:
The patent implements feedback control in the stretching process by monitoring and adjusting the speed ratios of stretching rollers (1.2-1.8) and temperature (150-170°C) to maintain consistent dendritic crystal formation. The feedback mechanism ensures that the crystal width and distribution remain within the specified ranges for optimal mechanical properties.
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 solution significantly improves the mechanical tensile strength and puncture resistance of the separator, ensuring safety and performance in lithium-ion batteries.
Implementation Method 1
setting a temperature above the melting temperature of the polyethylene resin to form a mineral oil/polyethylene resin molten mixture
Implementation Method 2
stretching of the thick sheet in a machine direction (MD)... stretching the separator transversely to 9-12 times... secondary stretching of the separator in the TD: stretching the separator transversely to 1.2-1.4 times to form longitudinal crystals
Implementation Method 3
immersing the separator into a solvent to extract the mineral oil
Implementation Method 4
subjecting the separator having the longitudinal crystals to a heat-setting treatment
Data Source
AI summary
A polyolefin porous separator includes a first surface and a second surface corresponding to the first surface. The surfaces of the polyolefin porous separator contain dendritic crystals and micropores, the dendritic crystals intersect with the micropores on the first surface or/and the second surface, and the dendritic crystals penetrate through the second surface from the first surface. A preparation method of the polyolefin porous separator includes: (1) a mixed melting of polyethylene resin and a mineral oil; (2) an extrusion of the mineral oil/polyethylene resin molten mixture; (3) a stretching of a thick sheet in a machine direction (MD); (4) a stretching of the separator in a transverse direction (TD); (5) immersing the separator into a solvent to extract the mineral oil; (6) a secondary stretching of the separator in the TD; and (7) subjecting the separator having the longitudinal crystals to a heat-setting treatment and then rolling up.