Porous Battery Separator Coating for Heat-Shrink and Adhesion Balance
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Solution Overview
Problem
Lithium secondary battery separators exhibit severe thermal shrinkage and inadequate interlayer adhesion at high temperatures, leading to safety issues such as internal short circuits.
Innovation Solution
A separator comprising a porous polymer support coated with an inorganics-containing porous layer using a binder polymer blend of poly(vinylidene fluoride-co-hexafluoropropylene) and poly(vinylidene fluoride-co-chlorotrifluoroethylene), which improves adhesion with electrodes and reduces thermal shrinkage by maintaining a tan δ peak within specific temperature ranges.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a separator comprising a porous polymer support coated with a mixture of inorganic fillers and a binder polymer with melting temperature of about 120°C is used, then the separator can be manufactured with basic adhesion properties, but the separator exhibits severe thermal shrinkage behaviors at high temperature of 150°C and insufficient interlayer adhesion
Solution Approach 1:
The patent uses a composite binder polymer system consisting of polyvinylidene fluoride (PVdF) as the base polymer and polyacrylonitrile (PAN) as the additive polymer. This composite structure combines the high temperature resistance of PVdF with the adhesion-enhancing properties of PAN, achieving both manufacturability and thermal stability. The inorganic fillers (alumina, silica, or boehmite) further enhance the composite structure to provide thermal shrinkage resistance while maintaining adhesion properties at elevated temperatures up to 150°C.
2Temperature
If the melting temperature Tm value of the binder polymer is increased to improve high temperature stability, then thermal shrinkage is reduced, but the adhesion property between separator and electrode deteriorates
Solution Approach 1:
The patent optimizes the molecular weight and composition ratio of the binder polymer components. Specifically, it controls the weight average molecular weight of PVdF within 100,000-1,000,000 and the ratio of PVdF to PAN between 95:5 and 50:50. This parameter optimization ensures that the binder maintains adequate adhesion strength while achieving the desired high temperature stability, resolving the contradiction between heat resistance and adhesion.
Solution Approach 2:
The composite binder system combines PVdF with PAN in specific ratios. PVdF provides the base adhesion and flexibility, while PAN contributes to adhesion enhancement and structural integrity at high temperatures. This synergistic composite structure allows the separator to maintain both strong interlayer adhesion and high temperature resistance simultaneously.
3Strength
If a binder polymer with lower melting temperature is used to improve adhesion property, then interlayer adhesion is enhanced, but thermal shrinkage behavior increases at high temperature
Solution Approach 1:
The patent employs a composite binder system where PVdF (with higher intrinsic temperature resistance) is combined with PAN (which enhances adhesion). By controlling the ratio and molecular weight, the composite achieves a balanced performance profile that provides strong adhesion without sacrificing thermal shrinkage resistance, effectively resolving this contradiction.
Solution Approach 2:
The binder polymer composition is optimized to have different functional characteristics in different aspects: PVdF dominates the thermal stability aspect while PAN dominates the adhesion aspect. This local quality differentiation within the composite binder allows simultaneous achievement of both adhesion strength and thermal shrinkage resistance.
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 enhances the adhesion property and heat resistance of the separator, reducing thermal shrinkage and improving safety at high temperatures, with thermal shrinkage of 25% or less in both machine and transverse directions after exposure to 130°C for 1 hour and 55% or less after 150°C for 30 minutes.
Implementation Method 1
separators commonly used in lithium secondary batteries exhibit severe thermal shrinkage behaviors in high temperature situations due to their material characteristics
Implementation Method 2
the first binder polymer is poly(vinylidene fluoride-co-hexafluoropropylene) having a tan δ peak at −18° C. to −5° C. when measured by Dynamic Mechanical Analysis
Implementation Method 3
improves adhesion with electrodes and reduces thermal shrinkage
Data Source
AI summary
The present disclosure relates to a separator for an electrochemical device comprising an inorganics-containing porous layer on at least one surface of a porous polymer support, and comprising inorganic fillers and a binder polymer, wherein the binder polymer comprises a first binder polymer, and the first binder polymer is poly(vinylidene fluoride-co-hexafluoropropylene) having a tan δ peak at −18° C. to −5° C. when measured by Dynamic Mechanical Analysis, and an electrochemical device comprising the same. The separator for an electrochemical device according to an embodiment of the present disclosure has improved adhesion property with electrode and safety at high temperature.