Microporous Membrane Composite Polyolefin Battery Separator
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Solution Overview
Problem
Existing microporous membranes for battery separators face challenges in achieving a balance between low shutdown temperature, high air permeability, and mechanical strength, often compromising on one or more of these properties.
Innovation Solution
A microporous membrane is developed using a specific composition of polyethylenes with controlled molecular weights and branching, combined with a diluent, and processed to achieve a shutdown temperature ≤ 130°C, normalized pin puncture strength ≥ 150 gf/20µm, and air permeability ≤ 6 x 10^2 seconds/100cm^3/20µm, while maintaining low heat shrinkage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If microporous membranes containing only polyethylene are used, then shutdown temperature is low (about 140°C), but meltdown temperature is low (about 150°C) and mechanical strength is insufficient
Solution Approach 1:
The patent uses a composite polyolefin composition containing both polyethylene (providing low shutdown temperature) and polypropylene (providing high meltdown temperature and mechanical strength). This composite approach allows the membrane to achieve both low shutdown temperature (≤130°C) and adequate mechanical strength, resolving the contradiction between these two properties.
2Strength
If microporous membranes containing only polypropylene are used, then mechanical strength is improved, but shutdown temperature is high (about 155°C)
Solution Approach 1:
The patent combines polypropylene (providing mechanical strength) with polyethylene (providing low shutdown temperature). The composite composition enables the membrane to achieve both high mechanical strength and low shutdown temperature (≤130°C), resolving the contradiction between these properties.
3Quantity of substance
If membranes are designed for high air permeability, then battery capacity is improved, but mechanical strength decreases
Solution Approach 1:
The patent optimizes the composition parameters of the polyolefin mixture, specifically controlling the ratio of polyethylene to polypropylene and the molecular weight distribution. By adjusting these parameters, the membrane achieves both high air permeability (≥300 seconds/100cm²/20µm) and adequate mechanical strength, resolving the contradiction between permeability and strength.
4Temperature
If membranes are designed for low shutdown temperature, then battery safety is improved, but air permeability and mechanical strength deteriorate
Solution Approach 1:
The patent uses a composite polyolefin system where polyethylene provides the low shutdown temperature (≤130°C) and polypropylene maintains air permeability and mechanical strength. This composite approach resolves the contradiction by allowing each polymer to contribute its advantageous properties without compromising the others.
Solution Approach 2:
The patent carefully controls the composition parameters, including the weight ratio of polyethylene to polypropylene and the molecular weight distribution, to achieve the optimal balance between low shutdown temperature and high air permeability. This parameter optimization allows simultaneous achievement of safety (low shutdown temperature) and performance (high permeability).
Data Source
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AI summary
The invention relates to microporous membranes comprising polymer and having well-balanced permeability, shutdown temperature, and pin puncture strength. The invention also relates to methods for making such membranes, and the use of such membranes as battery separator film in, e.g., lithium ion secondary batteries. The membrane has a shutdown temperature < 130.5°C.