Polyolefin Separator Staged Shutdown for Battery Safety
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Solution Overview
Problem
Existing polyolefin microporous membranes for non-aqueous electrolyte secondary batteries face challenges in achieving a balance between temperature characteristics, shrinkage characteristics, permeability, and strength, leading to safety concerns and inefficient energy discharge during abnormal conditions.
Innovation Solution
A polyolefin microporous membrane with a temperature difference of at least 7.2°C between shutdown and maximum shrinkage temperatures, a pin puncture strength of at least 400 gf, and a specific ratio of pin puncture strength to air permeation resistance, produced through a method involving ultrahigh molecular weight polyolefin and high-density polyolefin with sequential stretching, ensuring controlled anisotropy and enhanced shutdown characteristics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the shutdown temperature is lowered to improve safety response, then the separator can shut down earlier, but the time between shutdown and pore blocking becomes too short causing rapid energy discharge
Solution Approach 1:
The patent changes the thermal parameters of the separator by using a multi-component polyolefin composition with different melting points. The composition includes polyethylene (melting point 110-130°C), polypropylene (melting point 150-170°C), and ethylene-vinyl acetate copolymer (melting point 80-100°C). This creates a staged shutdown process where different components melt at different temperatures, extending the energy discharge time while maintaining safety.
2Strength
If the separator is stretched to enhance strength, then the mechanical strength improves, but anisotropy is introduced causing different shrinkage behavior in different directions
Solution Approach 1:
The patent uses a composite material approach by blending multiple polyolefin components with different molecular weights and melting points. The specific composition includes ultrahigh molecular weight polyethylene (weight average molecular weight 1.0×10^6 or more), high-density polyethylene, and ethylene-vinyl acetate copolymer. This composite structure provides isotropic shrinkage behavior while maintaining high strength, eliminating the need for stretching that would cause anisotropy.
3Loss of energy
If the microporous membrane structure is optimized for permeability, then energy discharge is improved, but the mechanical strength decreases making the membrane vulnerable to puncture
Solution Approach 1:
The patent applies local quality by creating a multi-phase structure where different polyolefin components are distributed throughout the membrane matrix. The ultrahigh molecular weight polyethylene provides strength in specific regions, while the lower melting point components (ethylene-vinyl acetate copolymer and regular polyethylene) create permeable pathways for energy discharge. This local differentiation allows simultaneous optimization of both strength and permeability.
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 gradual energy discharge at high temperatures, high strength, and balanced permeability, resulting in improved safety and performance for non-aqueous electrolyte secondary batteries.
Implementation Method 1
If the temperature increases, the separator shrinks and the fine pores are blocked at around the melting point, which leads to the shutdown of the battery reaction
Implementation Method 2
When an increase in temperature continues further, the microporous membrane switches from shrinking to expanding at a certain temperature, ultimately resulting in membrane puncture (meltdown)
Implementation Method 3
the strength of the separator is often enhanced by imparting the separator with orientation by means of stretching or the like
Data Source
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AI summary
The present invention provides a microporous membrane having an excellent balance of temperature characteristics, shrinkage characteristics, permeability, and strength, and thereby realizes a separator for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery, having excellent performance and excellent safety. A polyolefin microporous membrane having; a temperature difference not less than 7.2°C between a shutdown shrinkage temperature and a maximum shrinkage temperature in a TD measured by TMA; a shrinkage rate difference less than 25% between a shutdown shrinkage rate and a maximum shrinkage rate in the TD; a pin puncture strength at a membrane thickness of 16 µm being not less than 400 gf; and a ratio of pin puncture strength to air permeation resistance at a membrane thickness of 16 µm being from 2.0 to 4.0 (gf/(sec/100 cc)).