Ultra-Thin Polyolefin Separator Film With Uniform Pore Tortuosity
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Solution Overview
Problem
Existing polyolefin-based films used to prepare ultra-thin separators in secondary batteries cannot achieve both small thickness and uniform pore structure, leading to significant performance deviations.
Innovation Solution
A polyolefin-based film with a thickness of ≥7 μm and a pore structure tortuosity of 7-10, optimized for uniform pore structure and air permeability, is developed. This film is prepared using polyolefin with a high weight-average molecular weight and a specific polydispersity index, combined with biaxial drawing and pore-forming agents.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Volume of moving object
If the thickness of polyolefin-based film is reduced to create ultra-thin separators, then energy density of battery cells increases, but uniformity of pore structure deteriorates
Solution Approach 1:
The patent applies parameter changes by optimizing the molecular weight distribution of polyolefin (specifically setting Mw/Mn ratio between 2.0-4.0) and controlling processing parameters during biaxial stretching to achieve uniform pore structure in ultra-thin films. This resolves the contradiction by finding the optimal parameter range that enables both thinness and uniformity.
Solution Approach 2:
The patent implements local quality control by ensuring uniform pore distribution throughout the entire film thickness and area through controlled biaxial stretching. The pore structure achieves local uniformity in each region of the film while maintaining overall consistency, allowing the film to be both ultra-thin and uniformly porous.
2Volume of moving object
If the thickness of polyolefin-based film is reduced, then energy density increases, but performance deviation increases
Solution Approach 1:
The patent reduces performance deviation through parameter changes by controlling the polyolefin molecular weight distribution (Mw/Mn = 2.0-4.0) and processing conditions during biaxial stretching. These parameter optimizations ensure consistent pore structure formation across multiple films, reducing performance variation while maintaining ultra-thin thickness for high energy density.
3Reliability
If tortuosity of pore structure is increased to improve ion transmission, then air permeability improves, but film thickness must increase
Solution Approach 1:
The patent resolves this contradiction through parameter changes by optimizing the biaxial stretching ratios and temperatures to create a pore structure with tortuosity of 7-10. This specific tortuosity range, achieved through controlled processing parameters, enables effective ion transmission pathways while maintaining ultra-thin film geometry.
Solution Approach 2:
The patent utilizes porous material design by creating a controlled pore network through biaxial stretching of polyolefin. The pore structure with optimized tortuosity provides efficient ion transmission channels without requiring increased thickness, as the porous architecture itself creates the necessary transport pathways.
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 optimized polyolefin-based film achieves improved consistency and reduced performance deviation of the base film, enhancing air permeability and energy density of battery cells. When used as a separator, it reduces battery internal resistance, direct current resistance, self-discharge rate, and short-circuit rate.
Implementation Method 1
subjecting the film to biaxial drawing
Implementation Method 2
removing the pore-forming agent from the biaxially drawn film to form pores
Data Source
AI summary
A polyolefin-based film is described. Through optimization of tortuosity of a pore structure in the polyolefin-based film, the polyolefin-based film has a uniform pore structure, thereby improving consistency of the base film, reducing performance deviation of the base film, and effectively ensuring air permeability of the base film. Additionally, the polyolefin-based film of this application is ultra-thin. For every 1 μm reduction in thickness, energy density of a battery cell can increase by approximately 0.7%. Therefore, with the thickness controlled to be below 7 μm, the energy density of the battery cell is improved. When the polyolefin-based film is used as a separator in a battery, electrical performance of the battery can be improved, including reducing battery internal resistance, direct current resistance, self-discharge rate, short-circuit rate, and the like. This application further relates to a preparation method of the polyolefin-based film, a separator, a secondary battery, and an electric apparatus.