Porous PVDF Separator Coating for Bonding and Gas Permeability
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Solution Overview
Problem
Existing lithium battery separators face challenges in balancing coating thickness, bonding strength, thermal shrinkage, and ionic conductivity, which affect the performance and energy density of lithium batteries.
Innovation Solution
A lithium battery separator with a porous PVDF-based resin coating, comprising high and low molecular weight PVDF and inorganic particles, is designed to achieve a specific ratio of bonding strength to Gurley gas permeability and surface density, with controlled slurry and drying conditions to enhance bonding and thermal stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If a crosslinking catalyst master batch is used during the extrusion step to promote silane-modified polyethylene crosslinking reaction, then crosslinking reaction efficiency is improved, but resin aggregates are generated and homogeneity of physical properties is lowered
Solution Approach 1:
The crosslinking process is divided into two distinct stages: first, silane grafting during extrusion without catalyst to avoid aggregates; second, post-extrusion crosslinking in a controlled environment (water immersion or steam treatment) to achieve the desired gel fraction. This segmentation prevents resin aggregation during extrusion while still achieving efficient crosslinking ultimately.
Solution Approach 2:
The silane modification is performed preliminarily during the extrusion step without catalyst, creating silane-grafted polyethylene that maintains good processability and homogeneity. The actual crosslinking reaction is then activated later under controlled conditions, ensuring uniform physical properties while achieving the required crosslinking degree.
2Temperature
If gel fraction is increased to improve heat resistance and membrane rupture temperature, then high-temperature performance is improved, but processing difficulty increases due to high elasticity
Solution Approach 1:
The patent applies different gel fraction ranges for different product applications: 5-60% for general high-temperature applications, and 30-80% specifically for hot press applications. This localized optimization allows processing considerations to be addressed while still achieving the necessary heat resistance for each specific use case.
Solution Approach 2:
The patent optimizes the gel fraction parameter within specific ranges (5-60% or 30-80%) to balance heat resistance improvement with processing feasibility. By controlling the gel fraction within these optimized ranges rather than maximizing it indefinitely, the patent achieves adequate high-temperature performance while maintaining manageable elasticity for processing.
3Temperature
If silane crosslinked sections are formed in polyolefin resin to improve high-temperature properties, then membrane rupture temperature is improved, but resin aggregates are generated and homogeneity is lowered
Solution Approach 1:
The patent extracts the crosslinking catalyst application from the extrusion process itself, performing silane modification without catalyst during extrusion to maintain homogeneity, then applying crosslinking catalyst separately in a post-processing step. This separation prevents aggregate formation during extrusion while still achieving the desired crosslinked structure for high-temperature performance.
4Power
If polyethylene crosslinking is promoted during extrusion to reduce resin aggregation, then crosslinking efficiency is improved, but gel fraction becomes uneven and high-temperature resistance is lowered
Solution Approach 1:
The patent segments the crosslinking process into two phases: Phase 1 during extrusion without catalyst for controlled silane grafting with good homogeneity; Phase 2 after extrusion with catalyst addition for achieving target gel fraction. This ensures both homogeneous structure and adequate crosslinking for high-temperature resistance.
Solution Approach 2:
The silane modification is performed as a preliminary step during extrusion without catalyst, creating a homogeneous silane-grafted polyethylene intermediate. The actual crosslinking that determines gel fraction and high-temperature resistance is then performed in a controlled post-processing step, ensuring both homogeneity and adequate crosslinking.
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 separator exhibits excellent bonding performance, high temperature dimensional stability, and good gas permeability, resulting in lithium batteries with high energy density and improved cycling performance.
Implementation Method 1
silane crosslinking reaction between polyolefin and silane
Implementation Method 2
crosslinking reaction between polyolefin and silane in the presence of an organometallic catalyst
Implementation Method 3
gel structure formed by silane crosslinked sections
Data Source
Figure 1~2
Figure 3
AI summary
Disclosed is a lithium battery separator including a porous PVDF-based resin coating, wherein the porous PVDF-based resin coating is located on at least one surface of a base film, and the porous PVDF-based resin coating on a single side has a thickness of 0.5-3.5µm, and has a ratio of a bonding strength (N/m) to coating gas permeability increment (s/100cc) of greater than or equal to 0.25, and a ratio of a bonding strength (N/m) to a surface density per unit coating (g/m2/µm) of greater than or equal to 10, resulting in a porous PVDF-based resin coating with excellent thickness, coating gas permeability increment, bonding strength and thermal shrinkage; and the formed lithium battery separator is also excellent in cycling performance and heat resistance.