Porous Battery Separator Coating for Adhesion and Thermal Shrinkage
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Solution Overview
Problem
Lithium secondary batteries face challenges with separator adhesiveness towards electrodes, interfacial resistance, and thermal stability due to thermal contraction, leading to potential internal shorts and performance degradation.
Innovation Solution
A separator with a porous coating layer comprising inorganic particles and a terpolymer binder, specifically a VDF-HFP-CTFE terpolymer, which provides improved adhesiveness, reduced interfacial resistance, and enhanced thermal stability through controlled phase separation and interstitial volumes, ensuring uniform micropore formation and increased air permeability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If an organic-inorganic composite porous separator with inorganic particles and binder polymer is used, then thermal stability is improved, but adhesiveness towards electrode deteriorates causing separation
Solution Approach 1:
The patent changes the chemical composition parameters of the binder polymer by using a specific copolymer with controlled comonomer content (5-30 mol% of HFP and/or CTFE units) to achieve optimal balance between adhesiveness and thermal stability. This parameter optimization allows the binder to maintain strong electrode adhesion while preserving the thermal shutdown function of the separator.
Solution Approach 2:
The patent creates a composite coating layer combining inorganic particles (alumina, silica, titania, zirconia, or magnesia) with a specifically designed copolymer binder (PVDF-HFP-CTFE). This composite structure provides both the thermal stability from inorganic particles and the adhesiveness from the copolymer, resolving the contradiction between these two properties.
2Strength
If PVDF-CTFE binder polymer is used to increase adhesiveness, then adhesiveness towards electrode is improved, but solubility in solvent and processability deteriorate
Solution Approach 1:
The patent optimizes the comonomer composition parameters by incorporating HFP units (5-30 mol%) alongside CTFE units. The HFP component provides solubility in common solvents and good processability, while CTFE contributes to adhesiveness. This compositional parameter control resolves the contradiction between adhesiveness and processability.
Solution Approach 2:
The patent creates local quality differentiation within the binder polymer structure by having specific regions with different comonomer ratios. The copolymer contains both HFP-rich segments (providing solubility) and CTFE-rich segments (providing adhesiveness), allowing the material to exhibit both properties simultaneously through its molecular structure.
3Quantity of substance
If comonomer content in PVDF-based copolymer is increased to improve solubility and wetting property, then solubility in solvent and electrolyte solution wetting property are improved, but phase transition characteristics for non-solvent deteriorate making pore structure formation difficult
Solution Approach 1:
The patent precisely controls the comonomer content parameters within the range of 5-30 mol% to maintain the right balance. Below this range, adhesiveness is insufficient; above this range, phase transition characteristics deteriorate. This parameter optimization enables both good solubility and effective pore structure formation through controlled phase separation.
Solution Approach 2:
The patent utilizes phase transition phenomena during the coating process where the copolymer undergoes phase separation upon contact with non-solvent (electrolyte solution or water). This phase transition creates the desired porous structure while the controlled comonomer content ensures the transition occurs at appropriate conditions, resolving the contradiction between solubility and phase transition characteristics.
4Strength
If two types of PVDF-based copolymers are mixed to combine advantages, then adhesiveness and phase transition characteristics are improved, but process complexity increases and pore structure becomes non-uniform
Solution Approach 1:
The patent merges the functions of multiple copolymers into a single ternary copolymer (PVDF-HFP-CTFE) that contains all necessary components: VDF for backbone structure, HFP for solubility and phase transition, and CTFE for adhesiveness. This consolidation eliminates the need to mix separate copolymers, reducing process complexity while maintaining uniform pore structure and optimal adhesiveness.
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 results in improved adhesiveness towards electrodes, reduced interfacial resistance, and enhanced thermal stability, leading to better performance and lifespan of lithium secondary batteries by maintaining electrode contact and preventing shorts.
Implementation Method 1
there is a method that forms an adhesive layer by inducing the binder polymer migration to the separator surface through wet phase separation
Implementation Method 2
the separator has ion conduction paths due to the uniform micropores formed on the surface
Implementation Method 3
the separator commonly used in the lithium secondary battery show severe thermal contraction behaviors in a high temperature
Data Source
AI summary
The present disclosure relates to a separator for a lithium secondary battery comprising a porous substrate, and a porous coating layer disposed on at least one surface of the porous substrate and comprising inorganic particles and binder polymer, wherein the binder polymer is terpolymer including 65 to 90 weight % of a repeat unit derived from vinylidenefluoride (VDF), 1 to 28 weight % of a repeat unit derived from hexafluoropropylene (HFP) and 5 to 28 weight % of a repeat unit derived from chlorotrifluoroethylene (CTFE), and the separator has adhesiveness towards electrode ranging from 30 gf/25 mm to 150 gf/25 mm and machine direction (MD) thermal shrinkage of 1 to 18% and transverse direction (TD) thermal shrinkage of 1 to 17%, and a lithium secondary battery comprising the same, wherein the separator has uniform micropores on the surface, and thus has the increased adhesive surface area with electrode and consequential improved adhesiveness towards electrode.