Battery Separator Coating for Thin-Film Withstand Voltage
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Solution Overview
Problem
Lithium-ion secondary batteries face challenges in achieving high capacity and energy density due to reduced separator thickness, leading to potential defects in high-potential tests and increased risk of low-voltage phenomena, as the thinner dielectric barrier compromises discharge start voltage and insulation breakdown voltage.
Innovation Solution
A separator with a porous polymer material and an inorganic coating layer, where the ratio of average to maximum pore size is controlled between 0.5 to 0.99, and surface roughness is maintained at 2.5 µm or less, enhancing withstand voltage characteristics and air permeability, thereby improving battery assembly and safety.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If the separator thickness is reduced to increase energy density, then the spacing distance between anode and cathode decreases, but the withstand voltage capability and insulation performance deteriorate
Solution Approach 1:
The patent uses a composite structure consisting of a polyolefin porous substrate combined with an inorganic coating layer (alumina, silica, or boehmite). This composite material provides both the mechanical integrity needed for thin separators and the electrical insulation properties required for high withstand voltage capability, resolving the contradiction between reduced thickness and maintained reliability.
Solution Approach 2:
The patent employs a porous polyolefin substrate with controlled pore size (30-80 nm) and porosity (30-80%). The porous structure allows for ion transport while the controlled pore dimensions prevent direct electrical breakdown paths, enabling thin separator design that maintains both energy density and withstand voltage performance.
2Productivity
If the separator thickness is reduced, then the spacing distance decreases for higher capacity, but the discharge start voltage may be reduced due to decreased dielectric barrier thickness
Solution Approach 1:
The inorganic coating layer (alumina, silica, or boehmite) with thickness of 1-10 µm provides enhanced dielectric strength that compensates for the reduced overall separator thickness. This composite structure maintains the dielectric barrier strength necessary for preserving discharge start voltage while enabling thinner design for higher capacity.
Solution Approach 2:
The inorganic coating is applied specifically on the inner surface of the separator facing the electrodes, creating a localized region of high dielectric strength where it is most needed for electrical insulation, while the rest of the separator can be optimized for ion transport and mechanical properties.
3Reliability
If a Hi-pot test is performed to detect separator defects, then battery safety is improved, but assembly processability decreases due to higher defect rates
Solution Approach 1:
The inorganic coating layer is applied in advance during separator manufacturing, creating a pre-protected structure that is inherently more resistant to high voltage stress. This preliminary protective action reduces the likelihood of breakdown during subsequent Hi-pot testing, improving pass rates without compromising safety verification.
Solution Approach 2:
The inorganic coating layer serves as a cushioning protective layer that absorbs and distributes electrical stress before it can reach the polyolefin substrate. This beforehand cushioning effect prevents localized breakdown that would cause Hi-pot test failures, thereby improving assembly processability while maintaining rigorous safety testing.
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 controlled pore structure and surface flatness of the separator ensure improved withstand voltage, preventing low-voltage defects and enabling batteries to pass high-potential tests, ensuring stable assembly and high capacity, energy density performance.
Implementation Method 1
the separator may act as a dielectric barrier. Therefore, the reduction in the thickness of the separator may indicate that the thickness of the dielectric barrier may decrease
Implementation Method 2
a porous material; and a coating layer including an inorganic material and a binder on one or both surfaces of the porous material
Data Source
AI summary
A separator for a secondary battery includes a porous polymer material; and a coating layer including an inorganic material and a binder on one or both surfaces of the porous polymer material, wherein a ratio of average pore size of the separator to a maximum pore size, measured by a bubble point method, is 0.5 to 0.99, and surface roughness Rz is 2.5 or less, and the separator may have excellent withstand voltage characteristics as a thin film and may exhibit suitable evaluation characteristics in a Hi-pot test performed during a battery assembly process.