Microporous Polyethylene Membrane Gradient Pore Structure
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Solution Overview
Problem
Microporous polyethylene membranes used as battery separators lack sufficient compression resistance and electrolytic solution absorbability, leading to poor battery performance and capacity, especially with the expansion and contraction of lithium ion battery electrodes during charging and discharging.
Innovation Solution
A microporous polyethylene membrane with a dense-structure layer and a coarse-structure layer having a controlled pore size ratio is produced by extruding a melt blend of ultra-high-molecular-weight polyethylene and a membrane-forming solvent, stretching the gel-like sheet, and treating it with a heat roll to create a gradient structure that enhances deformability and electrolytic solution absorption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If microporous polyethylene membranes are used as battery separators, then battery safety and performance are improved, but compression resistance is insufficient leading to poor cycle properties
Solution Approach 1:
The patent applies local quality by creating a gradient pore size distribution within the membrane structure. The membrane contains both small pores (providing compression resistance) and large pores (providing electrolyte absorption), with the pore size varying through the membrane thickness. This allows different regions of the membrane to perform different functions: the dense region provides mechanical strength while the porous region provides electrolyte absorption, resolving the contradiction between compression resistance and battery performance.
2Reliability
If microporous polyethylene membranes are used as battery separators, then battery safety is improved, but electrolytic solution absorbability is insufficient leading to poor productivity
Solution Approach 1:
The patent creates a gradient pore size distribution where large pores are present in certain regions of the membrane to enhance electrolytic solution absorbability and battery productivity, while maintaining overall structural integrity for safety. The large pores allow rapid electrolyte absorption, directly addressing the productivity issue while the membrane's overall structure maintains safety requirements.
3Productivity
If microporous polyethylene membranes with large surface pore size are used, then electrolytic solution absorbability is improved, but mechanical strength becomes insufficient
Solution Approach 1:
The patent resolves this contradiction by creating a spatially varying pore size distribution. Large pores are localized in specific regions to provide electrolyte absorption, while other regions maintain smaller pores and higher density to provide mechanical strength. This gradient structure allows the membrane to simultaneously achieve good electrolyte absorbability and maintain sufficient mechanical strength, unlike uniform large-pore structures that would be mechanically weak.
Solution Approach 2:
The patent effectively creates a composite structure within the polyethylene membrane by having regions with different pore characteristics. The membrane combines dense regions (small pores) and porous regions (large pores) into a unified structure, allowing it to exhibit both mechanical strength and electrolyte absorption properties that neither region could achieve alone.
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 exhibits small air permeability change by compression, high electrolytic solution absorbability, and excellent deformability, improving battery productivity and safety by maintaining capacity and cycle properties.
Implementation Method 1
stretching the gel-like sheet while heating to provide a temperature distribution in a thickness direction
Implementation Method 2
bringing at least one surface of the stretched gel-like sheet into contact with a heat roll controlled to a temperature in a range of the crystal dispersion temperature of the polyethylene resin +10° C. or higher and lower than the melting point of the polyethylene resin
Implementation Method 3
extruding the resultant melt blend through a die, cooling the resultant extrudate to provide a gel-like sheet
Data Source
AI summary
A microporous polyethylene membrane made of a polyethylene resin having a ratio (mass-average molecular weight/number-average molecular weight) of 5 to 300 and comprising 1% or more by mass of ultra-high-molecular-weight polyethylene having a mass-average molecular weight of 7×105 or more, the microporous polyethylene membrane comprising (a) a coarse-structure layer having an average pore diameter of more than 0.04 μm, which is formed on at least one surface, and (b) a dense-structure layer having an average pore diameter of 0.04 μm or less, an area ratio of the coarse-structure layer to the dense-structure layer in a membrane cross section being 0.1 to 0.8.