Laminated Polyolefin Separator Shrinkage Control
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Solution Overview
Problem
Conventional polyolefin microporous membranes used in lithium ion secondary batteries often experience distortion and reduced permeability and cycle characteristics due to shrinkage during hot pressing, leading to increased defect ratios and safety concerns, particularly in in-vehicle applications where short-circuit resistance is critical.
Innovation Solution
A laminated polyolefin microporous membrane structure with specific polypropylene content ranges in each layer, controlled thermal shrinkage, and optimized melt indices, ensuring the A layer has a higher polyethylene content and the B layer has a higher polypropylene content, along with limited inorganic particle content, to prevent distortion and maintain permeability and safety.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If conventional polyolefin microporous membranes are used as separators, then battery assembly is simplified, but distortion and reduced permeability occur due to shrinkage during hot pressing
Solution Approach 1:
The invention uses a laminated structure combining polyethylene microporous membrane and polypropylene microporous membrane. The polyethylene layer provides shutdown function at lower temperatures while the polypropylene layer provides structural stability and resistance to shrinkage during hot pressing, solving the contradiction between assembly simplicity and distortion control.
Solution Approach 2:
The invention controls the thickness ratio between polyethylene layer (5-20 μm) and polypropylene layer (20-50 μm), and adjusts polypropylene content (1-30% by weight) to optimize the balance between shutdown responsiveness and dimensional stability during hot pressing, preventing distortion while maintaining manufacturability.
2Manufacturing precision
If separator shrinkage is reduced to prevent distortion, then manufacturing precision improves, but permeability and cycle characteristics may deteriorate
Solution Approach 1:
The invention assigns different functions to different layers: the polyethylene layer (thinner, 5-20 μm) provides shutdown function and ion permeability, while the polypropylene layer (thicker, 20-50 μm) provides dimensional stability. This local differentiation maintains permeability through the polyethylene layer while preventing distortion through the polypropylene layer's shrinkage resistance.
Solution Approach 2:
The laminated composite structure allows the polyethylene layer to maintain high porosity (30-80%) for ion permeability and shutdown function, while the polypropylene layer provides mechanical stability and shrinkage resistance, achieving both dimensional stability and reliable cycle characteristics.
3Temperature
If polypropylene content is increased to reduce shrinkage, then thermal stability improves, but shutdown response time may increase
Solution Approach 1:
The invention optimizes polypropylene content to 1-30% by weight (not exceeding 30%) to provide sufficient shrinkage resistance while maintaining fast shutdown response. The polyethylene layer (melting point 100-150°C) ensures rapid shutdown, while the limited polypropylene content prevents excessive thermal stability that would delay shutdown.
Solution Approach 2:
The invention segments the separator into two functional layers: polyethylene layer for rapid shutdown response and polypropylene layer for shrinkage resistance. This segmentation allows each layer to optimize its specific function without compromising the other, achieving both fast shutdown and thermal stability.
4Weight of moving object
If separator thickness is reduced to increase energy density, then battery weight decreases, but mechanical strength and short-circuit resistance deteriorate
Solution Approach 1:
The laminated structure combines thin polyethylene layer (5-20 μm) for shutdown function with polypropylene layer (20-50 μm) for mechanical strength. The polypropylene layer's higher tensile strength and puncture resistance compensate for the reduced overall thickness, maintaining safety while enabling thinner design for lower weight.
Solution Approach 2:
The invention concentrates the mechanical strength function in the polypropylene layer (providing puncture strength and tensile strength) while keeping the polyethylene layer thin for shutdown responsiveness. This local functional differentiation achieves high strength-to-thickness ratio, reducing battery weight without compromising safety.
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 effectively prevents battery distortion and maintains high short-circuit resistance, ensuring safety and performance under severe conditions by controlling thermal shrinkage and permeability while enhancing mechanical strength and elongation.
Implementation Method 1
Separators prevent direct contact between positive electrodes and negative electrodes
Implementation Method 2
permeate ions through electrolytic solutions held in their microporous membrane
Implementation Method 3
permeate ions through electrolytic solutions held in their microporous membrane
Implementation Method 4
permeate ions through electrolytic solutions held in their microporous membrane
Data Source
AI summary
A polyolefin microporous film having a laminated structure provided with at least one layer A containing a polyolefin and at least one layer B containing a polyolefin. 0 mass % to less than 3 mass % of polypropylene is contained in layer A and 1 mass % to less than 30 mass % of polypropylene is contained in layer B. When the proportion of polypropylene contained in layer A is represented by PPA (mass %) and the proportion of polypropylene contained in layer B is represented by PPB (mass %), PPB>PPA. In the polyolefin microporous film, the heat shrinkage ratio in TD at 120° C. measured upon applying, in MD, a constant load determined on the basis of the relationship: load (gf)=0.01×piercing strength (gf) of polyolefin microporous film×length (mm) in TD of polyolefin microporous film, is 10 to 40% inclusive.