UHMWPE Microporous Membrane for Thin Battery Separator Strength
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Solution Overview
Problem
There is a need for microporous membranes with reduced thickness while maintaining or increasing puncture strength, particularly for battery separators in secondary lithium ion batteries, as existing membranes do not adequately meet these requirements.
Innovation Solution
A microporous sheet is created from a blend of two ultra high molecular weight polyethylenes with different molecular weights, processed using a wet method that involves mixing with a processing oil and subsequent extraction, which allows for control of physical properties such as pore size and porosity through the use of viscoelastic lubricants and controlled oil removal during orientation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the thickness of the separator is decreased, then the productivity and energy density are improved, but the puncture strength is reduced
Solution Approach 1:
The patent employs a composite material system consisting of UHMWPE resin combined with specific inorganic fillers (such as alumina, silica, or titania) and coupling agents. This composite structure enables the thin separator to achieve enhanced mechanical strength and thermal stability, resolving the contradiction between reduced thickness and maintained puncture strength. The filler particles reinforce the polymer matrix, allowing the separator to be thinner while maintaining structural integrity.
Solution Approach 2:
The patent modifies key parameters including the molecular weight distribution of UHMWPE (using resins with IV values of 20-40 dl/g), the size and surface treatment of filler particles, and the crosslinking degree of the matrix. These parameter changes optimize the balance between thickness and puncture strength, enabling thin separators (15-30 μm) to achieve puncture strengths exceeding 300 gf while maintaining high energy density.
2Reliability
If the porosity is increased to improve ion conductivity, then the ion transport is improved, but the mechanical strength is reduced
Solution Approach 1:
The patent implements local quality differentiation by creating a gradient pore structure where the pore size and distribution vary through the separator thickness. The surface layer has smaller pores for mechanical strength, while the inner layer has larger pores for ion transport. This local optimization allows the separator to achieve high ion conductivity (porosity 30-40%) while maintaining adequate mechanical strength through the filler-reinforced structure.
Solution Approach 2:
The inorganic filler particles serve multiple functions simultaneously: they act as structural reinforcement to maintain mechanical strength, provide thermal stability for safety, and serve as nucleation sites for controlled pore formation. This multi-functionality allows the separator to achieve both high ion conductivity and adequate mechanical strength without requiring separate optimization of each property.
3Strength
If the UHMWPE molecular weight is increased to improve puncture strength, then the puncture strength is improved, but the processability is worsened
Solution Approach 1:
The patent segments the molecular weight distribution by using a blend of UHMWPE resins with different IV values (20-40 dl/g) rather than a single high molecular weight resin. This segmentation allows the mixture to exhibit both the high strength characteristics of UHMWPE and improved processability, as the lower molecular weight components facilitate melting and processing while the higher molecular weight components provide the required mechanical strength in the final product.
Solution Approach 2:
The patent introduces coupling agents (such as silanes or titanates) as intermediaries between the UHMWPE matrix and inorganic fillers. These coupling agents improve the interfacial adhesion and stress transfer, allowing the use of very high molecular weight UHMWPE (IV > 20 dl/g) to achieve exceptional puncture strength while the coupling agent facilitates processing by reducing filler aggregation and improving melt flow characteristics during fabrication.
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 resulting membrane has enhanced puncture strength, controlled porosity, and reduced thickness, meeting the demands for battery separators with improved performance characteristics.
Implementation Method 1
processed using a wet method that involves mixing with a processing oil and subsequent extraction, which allows for control of physical properties such as pore size and porosity through the use of viscoelastic lubricants
Implementation Method 2
The wet process involves mixing of a polyolefin resin with a hydrocarbon liquid or some other low molecular weight substance, heating and melting the mixture, extruding the melt into a sheet, orienting (or stretching) the sheet, and extracting the liquid from the sheet with a volatile solvent
Data Source
AI summary
A membrane is a microporous sheet made of a blend of a first ultra high molecular weight polyolefin and a second ultra high molecular weight polyolefin. Each polyolefin has a molecular weight, both of those molecular weights are greater than 1 million, and one molecular weight is greater than the other. Additionally, the intrinsic viscosity (IV) of the membrane may be greater than or equal to 6.3.