Microporous Battery Separator Membrane with High Meltdown Temperature
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Solution Overview
Problem
Microporous membranes used as battery separator films in lithium ion polymer batteries face challenges in maintaining high meltdown temperature and pin puncture strength while avoiding film tearing and yield reduction, especially when incorporating high amounts of polypropylene.
Innovation Solution
A microporous membrane composition comprising a first polyethylene with a weight average molecular weight less than 1.0×10^6, a second polyethylene with a weight average molecular weight greater than or equal to 1.0×10^6, and polypropylene with a weight average molecular weight greater than 5.0×10^5 and a melting enthalpy of 80.0 J/g, where the combined weight percentage of polypropylene and the second polyethylene is at least 15%, resulting in a membrane thickness of no more than 12.0 μm.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If polypropylene is included in the membrane to increase meltdown temperature, then thermal stability is improved, but film tearing and yield reduction occur
Solution Approach 1:
The patent changes the molecular weight parameters of the polymers used. Specifically, it uses polypropylene with Mw ≥ 5.0×10^5 and melting enthalpy ≥ 80.0 J/g, combined with polyethylene having Mw ≥ 1.0×10^6. This parameter optimization allows the membrane to achieve high meltdown temperature (≥145°C) while preventing film tearing during production, thus resolving the contradiction between thermal stability and productivity
Solution Approach 2:
The patent creates a composite membrane system combining multiple polymers (polypropylene and polyethylene) with specific molecular weight characteristics. This composite approach leverages the high melting point of polypropylene for thermal stability while the polyethylene component contributes to film strength and processability, eliminating the yield reduction typically caused by high polypropylene content
2Length of moving object
If membrane thickness is reduced for lithium ion polymer batteries, then energy density is improved, but pin puncture strength decreases
Solution Approach 1:
The patent optimizes the molecular weight parameters of the constituent polymers, using polyethylene with Mw ≥ 1.0×10^6 and polypropylene with Mw ≥ 5.0×10^5. These high molecular weight polymers provide exceptional strength-to-thickness ratio, enabling the membrane to achieve thickness ≤12.0 μm while maintaining pin puncture strength ≥320 mN/μm, thus resolving the contradiction between thinness and strength
3Temperature
If high amounts of polypropylene are incorporated to increase meltdown temperature, then thermal stability is improved, but membrane stretching becomes necessary which reduces yield
Solution Approach 1:
The patent specifies precise molecular weight parameters: polypropylene with Mw ≥ 5.0×10^5 and melting enthalpy ≥ 80.0 J/g, combined with polyethylene having Mw ≥ 1.0×10^6. This parameter optimization creates a membrane with inherent strength that eliminates the need for stretching operations, allowing high polypropylene content (≥15 wt%) to be used without causing yield reduction from stretching-related film tearing
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 solution enables the production of thin, high-strength membranes with elevated meltdown temperatures and enhanced pin puncture strength without the need for membrane stretching, thereby improving the thermal stability and safety of lithium ion polymer batteries.
Implementation Method 1
a polypropylene having an Mw≧5.0×105 and a ΔHm≧80.0 J/g
Implementation Method 2
increase the BSFs meltdown temperature to a value ≧145° C.
Data Source
AI summary
A process of producing a membrane includes extruding diluent and polymer to form an extrudate, the polymer includes a first polyethylene having an Mw<1.0×106, a second polyethylene having an Mw≧1.0×106, and a polypropylene having an Mw≧5.0×105 and a ΔHm≧80.0 J/g; wherein the sum of the polypropylene having an Mw≧5.0×105 and a ΔHm≧80.0 J/g and the second polyethylene is ≧15.0 wt. % and processing the extrudate into a membrane having a thickness ≦12.0 μm by stretching the extrudate in at least one planar direction at about 108.0 to 116.0° C. after removing the solvent to a magnification factor of ≦1.1 and excludes any stretching of the extrudate after removing the solvent at a magnification factor or >1.1 and removing at least a portion of the diluent from the extrudate.