Battery Separator Stretching for Heat Resistance and Puncture Strength
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Solution Overview
Problem
Existing lithium secondary battery separators face challenges in achieving a balance between processability, heat resistance, and puncture strength, particularly when using ultrahigh molecular weight polyethylene (UHMWPE), which has poor processability and increased heat shrinkage, and ceramic-coated separators reduce air permeability, affecting ion transfer and battery performance.
Innovation Solution
A manufacturing method involving the processing of a polyolefin and pore-forming agent to create a base sheet, followed by directional stretching and heat-setting, with specific stretch ratio conditions (7≤Mb≤8, 0<Mb/Tb<1, 0<Mb/(Tb×Tc)<0.5) to produce a porous film with controlled thermal shrinkage and improved mechanical strength.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If UHMWPE is used to improve mechanical strength and chemical resistance, then puncture strength is improved, but processability deteriorates due to low fluidity
Solution Approach 1:
The patent applies parameter changes by controlling the molecular weight distribution (Mw/Mn ratio between 2-10) and using specific processing conditions (extrusion temperature 150-250°C, stretching ratios Mb 6-9 and Tb 6-10) to process UHMWPE despite its poor fluidity, achieving both high puncture strength and manufacturability
Solution Approach 2:
The patent uses composite materials by combining UHMWPE with pore-forming agents (such as paraffin, stearic acid, or talc) to create a base sheet that maintains the high strength of UHMWPE while enabling porous structure formation for improved processability and ion permeability
2Strength
If UHMWPE is used to improve mechanical strength, then puncture strength is improved, but heat resistance deteriorates due to increased heat shrinkage rate
Solution Approach 1:
The patent applies preliminary anti-action by performing a pre-stretching process (Mb 6-9, Tb 6-10) before final stretching to counteract the inherent heat shrinkage tendency of UHMWPE, thereby reducing the heat shrinkage rate to 15% or less at 150°C while maintaining high puncture strength
Solution Approach 2:
The patent changes processing parameters including using specific stretching ratios (Mb/Tb < 1), heat treatment temperatures (60-100°C for 1-24 hours), and molecular weight distribution control to simultaneously achieve high puncture strength and low heat shrinkage
3Temperature
If ceramic particles are coated on the porous support to improve heat resistance, then heat resistance is improved, but air permeability deteriorates due to pore closure
Solution Approach 1:
The patent extracts the ceramic coating layer from the separator structure, replacing it with a fully porous polyolefin matrix that provides heat resistance through controlled crystallinity and melting point rather than through ceramic particle reinforcement, thereby maintaining air permeability
Solution Approach 2:
The patent uses porous materials by creating a fully porous structure through stretching and pore-forming agents, ensuring high air permeability (≥300 mL/min/cm²) while achieving heat resistance through the polyolefin's inherent thermal properties and controlled morphology
4Use of energy by moving object
If the separator is thinned to increase battery capacity, then energy density is improved, but mechanical strength deteriorates
Solution Approach 1:
The patent uses composite materials by combining UHMWPE (providing high strength) with pore-forming agents and using blowhole formation technology to create a thin separator (15 μm or less) that maintains mechanical integrity through optimized pore distribution and wall thickness
Solution Approach 2:
The patent changes material parameters by using UHMWPE with specific molecular weight (10⁶ or more) and molecular weight distribution (Mw/Mn = 2-10), combined with controlled stretching ratios and heat treatment conditions, to achieve high strength in thin separators
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 method achieves a balanced combination of processability, heat resistance, and puncture strength, ensuring thermal shrinkage rates of 5% or less at 150°C and puncture strength of 500 gf or more, enhancing battery performance.
Implementation Method 1
stretching the porous film in the transverse direction (TD) and heat-setting the same
Implementation Method 2
the heat shrinkage rate of the manufactured separator at high temperatures (150° C.) increases
Data Source
AI summary
One aspect of the present invention provides a method for manufacturing a separator, comprising: (a) processing a composition comprising a polyolefin and a pore-forming agent to obtain a base sheet; (b) stretching the base sheet in a machine direction (MD) and a transverse direction (TD), and removing the pore-forming agent to obtain a porous film; and (c) stretching the porous film in the transverse direction (TD) and heat-setting the same, and also provides a separator manufactured using the method.