Multilayer Polyolefin Separator Film for Battery Shutdown Safety
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Solution Overview
Problem
There is a need to enhance the safety and capacity of power-storage devices, particularly in the separator films used in lithium ion secondary batteries and lithium ion capacitors, to prevent accidents such as ignition due to excessive temperature rises.
Innovation Solution
A multilayered polyolefin micro-porous film structure is developed, comprising a first layer of polypropylene resin, a second layer of polyethylene resin, and a third layer of polypropylene resin, with specific thickness ratios and porosity levels, where the thickness of the multilayered structure is 16 μm or less, and the porosity is between 40 to 70%, to effectively shut down ion mobility and prevent short circuits.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If the separator film is thinned to increase capacity, then the capacity of the power-storage device is improved, but the safety and reliability of the separator are worsened
Solution Approach 1:
The separator film is divided into multiple layers (first layer, second layer, third layer) with different materials and functions. The first and third layers provide mechanical strength and thermal stability, while the second layer provides shutdown function, allowing each layer to be optimized for its specific function while maintaining overall thinness and safety
Solution Approach 2:
The separator uses a composite structure combining different polyolefin materials (polypropylene and polyethylene) with different melting points and properties in a multilayer configuration, achieving both thinness for high capacity and enhanced safety through functional differentiation of materials
2Quantity of substance
If the thickness of the separator is reduced, then the capacity is increased, but the ability to prevent short circuits and accidents is worsened
Solution Approach 1:
The separator is segmented into functional layers where the first and third layers (polypropylene-based) provide mechanical integrity and short circuit prevention, while the second layer (polyethylene-based) provides shutdown function, allowing thin overall thickness without compromising safety
Solution Approach 2:
The multilayer structure provides redundant safety mechanisms: the first and third layers serve as backup barriers against short circuits, while the second layer provides preemptive shutdown before thermal runaway can occur, cushioning against potential harmful events
3Quantity of substance
If the porosity is increased to improve ion mobility, then the capacity is improved, but the mechanical strength and safety are worsened
Solution Approach 1:
Different layers are assigned different porosity levels suited to their functions: the first and third layers have lower porosity for mechanical strength, while the second layer has higher porosity for ion transport during shutdown, resolving the contradiction between porosity and strength
Solution Approach 2:
Each layer of the multilayer separator has locally optimized properties: the polypropylene layers have specific porosity for mechanical support, while the polyethylene layer has different porosity for shutdown function, allowing the system to achieve both high capacity and strength through localized property optimization
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 multilayered polyolefin micro-porous film structure increases the capacity of power-storage devices while ensuring excellent safety by effectively shutting down ion mobility and preventing short circuits, even at high temperatures, thus preventing accidents like ignition.
Implementation Method 1
when a temperature in the power-storage device becomes equal to or more than a predetermined temperature due to an abnormal current, pores of the microporous membrane are closed to prevent a mobility of ions between the electrodes
Data Source
AI summary
This polyolefin micro-porous film includes a multilayered structure, wherein the multilayered structure contains a first layer composed of polypropylene resin, a second layer composed of polyethylene resin and provided on the first layer, and a third layer composed of polypropylene resin and provided on the second layer. Furthermore, a thickness of the first layer is thinner than a thickness of the second layer, a thickness of the third layer is thinner than the thickness of the second layer, and in the multilayered structure, a thickness is 16 μm or less, a porosity is 40 to 70%, and a surface opening ratio is 10 to 30%.