Microporous Polyolefin Film Separator with Cellulose Nanofibers
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Solution Overview
Problem
Conventional lithium ion battery separators face challenges in achieving both high puncture strength and high-temperature resistance while maintaining insulating properties, as they are prone to melting and short-circuiting due to the trade-off between shut-down characteristics and heat resistance, and the use of conventional reinforcing fibers like glass or aramid fibers results in uneven films and increased production costs.
Innovation Solution
A method involving the production of a microporous stretched polyolefin film by melt-kneading monoesterified cellulose nanofibers with a polyolefin resin, followed by extrusion molding, stretching, and thermal fixation, which enhances mechanical and thermal characteristics, and includes a blend ratio of cellulose nanofibers from 0.01 wt % to 5 wt % to ensure adequate entanglement and insulation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If conventional reinforcing fibers (glass or aramid) are used to improve puncture strength, then mechanical strength is improved, but film uniformity deteriorates and production cost increases
Solution Approach 1:
The patent changes the fiber dimension parameter from micrometer-scale (conventional fibers) to nanometer-scale (cellulose nanofibers with diameter 1-100 nm). This parameter change enables the fibers to be dispersed uniformly in the polyolefin matrix without causing film unevenness, while still providing reinforcement. The nanoscale dimension allows the fibers to act as nucleating agents for micropore formation and provides reinforcement at the molecular level without creating surface defects.
Solution Approach 2:
The patent creates a composite material system consisting of polyolefin resin matrix combined with cellulose nanofibers. This composite structure leverages the hydrophobic polyolefin for chemical resistance and the hydrophilic cellulose nanofibers for mechanical reinforcement and micropore formation. The synergistic combination resolves the contradiction by using naturally occurring nanofibers that disperse well in the matrix while providing superior strength-to-weight ratio compared to conventional synthetic fibers.
2Reliability
If polyethylene is used to achieve good shut-down characteristics, then low-temperature shut-down performance is improved, but high-temperature resistance deteriorates due to melting
Solution Approach 1:
The patent creates a composite material system consisting of polyolefin resin matrix combined with cellulose nanofibers. This composite structure leverages the hydrophobic polyolefin for chemical resistance and the hydrophilic cellulose nanofibers for mechanical reinforcement and micropore formation. The synergistic combination resolves the contradiction by using naturally occurring nanofibers that disperse well in the matrix while providing superior strength-to-weight ratio compared to conventional synthetic fibers.
Solution Approach 2:
The patent changes the fiber dimension parameter from micrometer-scale (conventional fibers) to nanometer-scale (cellulose nanofibers with diameter 1-100 nm). This parameter change enables the fibers to be dispersed uniformly in the polyolefin matrix without causing film unevenness, while still providing reinforcement. The nanoscale dimension allows the fibers to act as nucleating agents for micropore formation and provides reinforcement at the molecular level without creating surface defects.
3Strength
If cellulose nanofiber content is increased to improve mechanical strength, then puncture resistance is improved, but insulating properties may deteriorate
Solution Approach 1:
The patent optimizes the concentration parameter of cellulose nanofibers to a specific range (0.01-5 wt%). At this optimized concentration, the nanofibers provide sufficient mechanical reinforcement and micropore formation while maintaining adequate insulation. The nanoscale dimension and high aspect ratio of the nanofibers allow effective reinforcement at low concentrations, preventing the onset of conductive pathways that would occur with higher loadings of conventional fibers.
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 method produces a separator with improved mechanical strength, high puncture resistance, and excellent shut-down and high-temperature characteristics, reducing production costs and environmental impact, while maintaining sufficient air permeability for charge-discharge efficiency.
Implementation Method 1
dispersing a cellulose nanofiber in a polyolefin resin
Implementation Method 2
extracting the plasticizer from the film
Implementation Method 3
the polyolefin resin with the cellulose nanofiber dispersed therein in a form of slurry is thermally fixed while the film is stretched
Data Source
AI summary
The present invention provides a production method and the like of a microporous stretched film having a high puncture strength and the like. The method includes: a first step of melt-kneading a cellulose nanofiber and a polyolefin resin to thereby disperse the nanofiber in the resin; a second step of removing water from a kneaded mixture obtained in the first step; a third step of mixing a plasticizer in the nanofiber and the resin and melt-kneading them to prepare a polyolefin resin composition; a fourth step of extrusion-molding the polyolefin resin composition; a fifth step of stretching an extrusion-molded article obtained in the fourth step to form a film; and a sixth step of extracting the plasticizer from the film.