Microporous Polyolefin Battery Separator with High Meltdown Temperature
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Solution Overview
Problem
Microporous polyolefin membranes comprising polyethylene and polypropylene for battery separators face challenges in achieving high meltdown temperatures while maintaining suitable permeability and mechanical properties, particularly when the polypropylene content exceeds 20% by mass, leading to issues such as short-circuiting and poor production yield due to thickness uniformity and compression resistance problems.
Innovation Solution
A microporous polyolefin membrane is produced by extruding a molten blend of polyethylene and polypropylene resins with specific molecular weight ranges and heat of fusion characteristics, followed by cooling and stretching to form a gel-like sheet, and subsequent removal of the solvent, allowing for membranes with 20% or more polypropylene content to achieve improved meltdown properties without compromising permeability and mechanical strength.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If the polypropylene content is increased to improve meltdown properties, then meltdown temperature is improved, but thickness uniformity and mechanical strength deteriorate
Solution Approach 1:
The patent applies parameter changes by carefully controlling the molecular weight of polypropylene (weight-average molecular weight of 5×10^5 or more) and optimizing the composition ratio (polypropylene content of 20-80% by mass). This allows achieving high meltdown temperature while maintaining thickness uniformity and mechanical strength through precise parameter optimization rather than simply increasing polypropylene content.
2Temperature
If the polypropylene content is increased to improve meltdown properties, then meltdown temperature is improved, but pin puncture strength deteriorates
Solution Approach 1:
The patent resolves this contradiction by changing the molecular weight parameter of polypropylene to weight-average molecular weight of 5×10^5 or more, and optimizing the composition ratio. This parameter optimization enables the membrane to achieve high meltdown temperature (175°C or higher) while maintaining adequate pin puncture strength, overcoming the trade-off between thermal stability and mechanical strength.
3Strength
If ultra-high-molecular-weight polyethylene is added to improve mechanical strength, then tensile strength is improved, but thickness uniformity deteriorates
Solution Approach 1:
The patent applies parameter changes by specifying the molecular weight of polypropylene (weight-average molecular weight of 5×10^5 or more) and optimizing the composition ratio of polyethylene and polypropylene. This precise parameter control allows achieving improved mechanical strength through controlled crystallization and phase separation during membrane formation, while maintaining thickness uniformity by preventing excessive viscosity increases that would cause shark-skin surface texture.
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 membranes exhibit enhanced meltdown temperatures, pin puncture strength, and air permeability, addressing the limitations of previous membranes and ensuring improved battery performance, including capacity, cyclability, and heat resistance.
Implementation Method 1
the microporous polyolefin membrane should have suitable mechanical properties, heat resistance, permeability, dimensional stability, shutdown properties, meltdown properties
Implementation Method 2
forming a gel-like molding (or sheet) comprising a composition containing a membrane-forming solvent and ultra-high-molecular-weight polyolefin... then removing the solvent
Data Source
Figure 1~2
Figure 3
AI summary
A microporous polyolefin membrane comprising a polyethylene resin, and polypropylene having a weight-average molecular weight of 6 x 105 or more and a heat of fusion of 90 J/g or more (measured by a differential scanning calorimeter), a fraction having a molecular weight of 1.8 x 106 or more being 10% or more by mass of the polypropylene.