Lithium Ion Capacitor Separator Thermal Shrinkage Safety
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Solution Overview
Problem
Lithium ion capacitors face challenges with lower energy density compared to lithium ion batteries and lack sufficient safety features to prevent rupturing or igniting, especially at high temperatures, which can lead to short circuits and safety issues.
Innovation Solution
A lithium ion capacitor design incorporating a specific separator made of polyolefin resin with controlled thermal shrinkage and porosity, which melts down at high temperatures to safely short-circuit the electrodes and prevent rupture or ignition, while maintaining high energy density and output characteristics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a lithium ion capacitor uses a conventional separator without controlled thermal shrinkage, then the structure is simpler and manufacturing is easier, but the safety is insufficient at high temperatures leading to potential rupture or ignition
Solution Approach 1:
The separator's thermal shrinkage rate is specifically controlled within 3-10% in the machine direction and 2-10% in the transverse direction at 100°C. This parameter change enables the separator to maintain dimensional stability during normal operation while preventing catastrophic failure at elevated temperatures, thus improving safety without excessive complexity
Solution Approach 2:
The separator is constructed as a composite structure combining a base fabric layer with a heat-resistant coating layer. This composite design provides both the mechanical strength needed for structural integrity and the thermal resistance properties required for high-temperature safety, resolving the contradiction between simplicity and safety
2Reliability
If the separator has high thermal shrinkage to prevent high-temperature failure, then safety improves, but the structural stability during normal operation deteriorates
Solution Approach 1:
The thermal shrinkage rate is precisely controlled within specific ranges (3-10% in machine direction, 2-10% in transverse direction at 100°C). This controlled parameter change ensures the separator contracts enough to prevent thermal runaway while maintaining sufficient structural stability during normal operating conditions
Solution Approach 2:
The separator exhibits dynamic behavior where its dimensional stability is maintained at operating temperatures but it undergoes controlled contraction at elevated temperatures. This dynamic response allows the separator to adapt to temperature changes, providing structural stability when needed and safety activation when required
3Strength
If the separator uses lower porosity to improve mechanical strength, then strength improves, but the ion transport efficiency and output characteristics deteriorate
Solution Approach 1:
The separator combines a strong base fabric providing mechanical strength with a heat-resistant coating layer that maintains porosity for ion transport. This composite structure resolves the contradiction by allowing the base layer to provide strength while the coating layer preserves ion transport pathways
Solution Approach 2:
Different regions of the separator have different properties: the base fabric provides mechanical strength and structural integrity, while the heat-resistant coating layer provides thermal stability and maintains controlled porosity for ion transport. This local differentiation allows simultaneous optimization of strength and output characteristics
4Reliability
If the separator has high heat resistance to prevent melting, then safety improves, but the ability to short-circuit and discharge accumulated energy deteriorates
Solution Approach 1:
The separator's thermal decomposition temperature is controlled within 150-250°C, which is high enough to maintain structural integrity during normal operation and moderate thermal events, but low enough to allow controlled decomposition and energy release during severe thermal runaway conditions. This parameter optimization balances thermal stability with safety discharge capability
Solution Approach 2:
The separator is designed to undergo controlled thermal decomposition at moderate temperatures (150-250°C) rather than catastrophic failure at high temperatures. This converts the potentially harmful effect of thermal runaway into a beneficial controlled energy release mechanism, preventing more severe damage while still allowing energy discharge
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 enhanced separator design achieves high energy density, high output, and improved safety by ensuring safe meltdown and short-circuiting at high temperatures, preventing damage and ensuring reliable operation.
Implementation Method 1
a microporous membrane made of a polyolefin, when maintained at 100°C for one hour in an unconstrained state, has a thermal shrinkage rate of 3% to 10% in a first direction and 2% to 10% in a second direction which is orthogonal to the first direction... melts down at high temperatures to safely short-circuit the electrodes
Implementation Method 2
a microporous membrane made of a polyolefin, when maintained at 100°C for one hour in an unconstrained state, has a thermal shrinkage rate of 3% to 10% in a first direction and 2% to 10% in a second direction which is orthogonal to the first direction
Implementation Method 3
carrying out charge-discharge by a non-faradaic reaction based on adsorption / desorption of a negative ion similar to that in the electric double layer capacitor, in a positive electrode, and by a faradaic reaction based on intercalation / deintercalation of a lithium ion similar to that in the lithium ion battery
Data Source
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AI summary
Provided is a lithium ion capacitor that is safe and has a high energy density and a high output. Specifically provided is a lithium ion capacitor comprising, accommodated within an outer casing: an electrode stack obtained by stacking a negative electrode in which a negative-electrode active material layer including a carbon material as the negative-electrode active material is disposed on a negative-electrode collector, a separator comprising a polyethylene-containing polyolefin resin, and a positive electrode in which a positive-electrode active material layer including a positive-electrode active material layer comprising a carbon material or a carbonaceous material is disposed on a positive-electrode collector; and a non-aqueous electrolyte solution including a a lithium ion-containing electrolyte. The lithium ion capacitor is characterized in that the thermal contraction/expansion rate of the separator falls within a prescribed range, the surface area of the separator is greater than the surface area of the positive-electrode active material layer of the positive electrode or the surface area of the negative-electrode active material layer of the negative electrode, and the length of a section where the electrode surface and the separator do not overlap has a prescribed relationship to the size of the separator.