Polyolefin Separator Thermal Shrinkage Control
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Solution Overview
Problem
High-capacity lithium ion secondary batteries face challenges with safety and cycle characteristics due to thinner separators, increased risk of internal short circuits, and inadequate resistance against foreign matters and high temperatures, leading to reduced stability and capacity retention.
Innovation Solution
A polyolefin microporous membrane with specific physical properties, including a tensile strength ratio of 0.75 to 1.25, thermal shrinkage rate less than 10% at 120°C, and controlled aluminum content, is developed to enhance resistance against foreign matters and maintain cycle characteristics at high temperatures.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If the separator is formed to be thinner to increase battery capacity, then the battery capacity increases, but the risk of internal short circuit increases due to electrode active material penetration
Solution Approach 1:
The invention uses a composite microporous membrane comprising a polyolefin base layer combined with a heat-resistant porous layer containing inorganic particles (such as alumina, silica, or boehmite) dispersed in a binder resin. This composite structure provides both the thin profile needed for high capacity and the enhanced mechanical strength and thermal stability required to prevent penetration by electrode active materials, thereby resolving the contradiction between capacity and safety.
Solution Approach 2:
The invention applies different material properties to different regions/layers of the separator. The base layer provides porosity and ion conductivity, while the heat-resistant porous layer provides enhanced mechanical strength and thermal stability. This localized functional differentiation allows the separator to simultaneously achieve thinness for high capacity and sufficient strength to prevent short circuits.
2Quantity of substance
If the separator thickness is reduced to increase battery capacity, then the battery capacity increases, but the resistance against foreign matters and high temperatures decreases
Solution Approach 1:
The composite microporous membrane combines a polyolefin base layer with a heat-resistant porous layer containing inorganic particles (alumina, silica, boehmite) and binder resin. This composite structure provides enhanced resistance against foreign matters and high temperatures while maintaining thin thickness, thereby resolving the contradiction between capacity and resistance to harmful factors.
Solution Approach 2:
The heat-resistant porous layer is designed with controlled porosity (30-80%) and specific pore size (0.01-10 μm) to maintain ion conductivity while providing mechanical strength and thermal stability. The porous structure of the inorganic particle-binder composite provides both the necessary permeability for battery operation and the structural integrity to resist foreign matters and high temperatures.
3Quantity of substance
If the separator is made thinner to increase battery capacity, then the battery capacity increases, but the cycle characteristics at high temperature deteriorate due to separator shrinkage
Solution Approach 1:
The composite microporous membrane combines a polyolefin base layer with a heat-resistant porous layer containing inorganic particles and binder resin. The inorganic particles (alumina, silica, boehmite) have low thermal expansion coefficients and high thermal stability, which constrain the separator from shrinking at high temperatures. This composite structure maintains dimensional stability during cycling at elevated temperatures while keeping the separator thin for high capacity.
Solution Approach 2:
The invention addresses thermal expansion issues by incorporating inorganic particles with low thermal expansion coefficients (such as alumina, silica, and boehmite) into the heat-resistant porous layer. These inorganic components counteract the thermal shrinkage tendency of the polyolefin base layer at high temperatures, maintaining separator dimensional stability and preventing short circuits during high-temperature cycling.
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 provides improved resistance against foreign matters and maintains high cycle characteristics, ensuring safety and capacity retention in high-capacity lithium ion secondary batteries.
Implementation Method 1
a thermal shrinkage rate in the width direction at 120° C. of less than 10%
Implementation Method 2
the possibility that the electrode active material penetrates a separator to cause short circuit is increased because the separator is formed so as to be thinner
Data Source
AI summary
A polyolefin microporous membrane that can realize a lithium ion secondary battery having favorable resistance against foreign matters or the like, and high cycle characteristics at a high temperature is provided. The present invention provides a polyolefin microporous membrane having a ratio of tensile strength in a length direction to that in a width direction of 0.75 to 1.25, and a thermal shrinkage rate in the width direction at 120° C. of less than 10%.