Application and Research Directions of Microporous Foils in the Lithium-Ion Battery Industry

Lithium-ion batteries, (super)capacitors, flexible electronics, and electromagnetic shielding materials are key application areas for microporous copper foil. Its development is driven by emerging technologies such as electric vehicles, foldable screen phone batteries, and 5G base station electromagnetic shielding, occupying a significant and rapidly growing share of the industrial chain.

Currently, common processes for preparing microporous copper foil mainly include laser drilling, mechanical punching, electrochemical etching, and solution etching. As shown in the table below, solution etching offers distinct advantages in manufacturing microporous copper foil.

The approaches to increasing the specific energy of lithium-ion batteries essentially involve using higher-capacity cathode and anode materials, thinner separator paper, thinner copper and aluminum foils, and minimizing other auxiliary additives as much as possible.

The undisputed focus of research and development lies in higher-weight-capacity cathode and anode materials (collectively accounting for over 50% of the total weight). Lithium iron phosphate has little potential left to tap, while ternary materials are progressing towards higher nickel content (NCM 111 -> 523 -> 622 -> 811 -> NCA?). On the anode side, progress is largely dependent on the maturation of silicon-carbon materials. What about their high expansion coefficient? What about insufficient cycle life? Another method is to use thinner separator paper, but separators only account for 4~5% of the battery weight, and excessively thin separators increase the risk of cathode-anode short circuits, often resulting in more losses than gains.

Currently, conventional copper foil thickness used in lithium-ion battery production is 8μm~12μm (6~7μm copper foil is already used in some 3C digital batteries), and aluminum foil thickness is 12μm~20μm. As conductive substrates for the cathode and anode, they constitute 15%~20% of the lithium-ion battery's weight. How can we further reduce the weight proportion of copper and aluminum foils to boost specific energy? It is under such circumstances that microporous copper and aluminum foils were conceived.

Existing specifications of microporous copper/aluminum foils (pores created by mechanical processing, maintaining the foil's original physical properties, ensuring no breakage during coating, and being burr-free and non-leaking)

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What advantages do microporous foils offer in lithium-ion battery applications? (Taking microporous foil with 17% porosity as an example)

• Directly and effectively increases lithium-ion battery specific energy: For foils of the same specification, microporous foil with 17% porosity reduces weight by 17%; at the same areal density, the compaction density of the cathode and anode increases (as some active material fills the pores).

• Effectively improves lithium-ion battery rate capability: In batteries using conventional foil, lithium-ion migration diffuses towards the tab in a two-dimensional manner along the foil. After perforation, the lithium-ion diffusion path transforms into three-dimensional omnidirectional penetration. Furthermore, the increased contact area between the active material entering the pores and the foil reduces the lithium-ion migration radius, enhancing conductive efficiency. (Personal opinion: The bottleneck limiting lithium-ion rate performance lies not in electron conduction but in lithium-ion transfer efficiency. For instance, the application of porous Ketjen black as a conductive agent in rate-type batteries yields better experimental results than non-porous conductive agents.)

• Effectively reduces lithium-ion battery internal resistance: Comparative tests using identical foil materials show that simultaneously using perforated copper and aluminum foils can reduce internal resistance by 8%---20%.

The theoretical basis is presumed to be the combined effect of increased contact area between the conductive foil and the active materials, coupled with a reduction in the foil's own internal resistance. Personal opinion: If the coating thickness of the cathode/anode electrode is less than the radius of the foil's micropores, internal resistance may increase; conversely, it decreases. The distance from the lithium ions at the outermost layer of the coating to the foil surface is related to rate performance. In cell design, higher areal density may potentially lead to lower achievable rate performance.

• Significantly improves electrolyte infiltration efficiency after injection and ensures 100% infiltration uniformity: In batteries with conventional foil, electrolyte diffuses and infiltrates from the periphery towards the center longitudinally. After perforation, infiltration becomes a three-dimensional permeation diffusion process, completely eliminating issues of incomplete infiltration at the center of some electrode sheets. Within the industry, inconsistent infiltration has been identified as one reason for insufficient consistency among individual cells.

• Increases the surface adhesion of the foil: Through the material within the pores, the coatings on both sides of the cathode/anode electrode sheet form an "I-beam" interlocking structure, significantly reducing the probability of electrode material delamination.

• Enhances the bending flexibility of the electrode sheet, making it more suitable for flexible battery applications. (Some companies have already mass-produced it for wearable lithium batteries, showing significant performance improvements.)

• Improves battery safety: Tests conducted by multiple battery manufacturers have verified its role in safety, and it has also received testing recognition from European customers.
• Directly and effectively increases lithium-ion battery specific energy: For foils of the same specification, the weight of microporous foil is reduced; at the same areal density, the compaction density of cathode and anode increases (as some material fills the pores).
• Effectively improves lithium-ion battery rate capability: In batteries using conventional foil, lithium-ion migration diffuses towards the tab in a two-dimensional manner along the foil. After perforation, the lithium-ion diffusion path transforms into three-dimensional omnidirectional penetration, and the increased contact area between the material entering the pores and the foil reduces the lithium-ion migration radius, enhancing conductive efficiency.
• Effectively reduces lithium-ion battery internal resistance: Using microporous foil can effectively lower internal resistance.
• Improves electrolyte wettability: The efficiency of electrolyte infiltration after injection into lithium batteries can be significantly increased, potentially improving production efficiency by over 50%, while ensuring 100% infiltration uniformity.
• Enhances slurry adhesion to the current collector: Increases the surface adhesion of the foil. Through the material within the pores, the coatings on both sides of the electrode sheet form an "H"-shaped interlocking state, greatly reducing the probability of electrode material delamination.
• Enhances the bending flexibility of the electrode sheet, making it more suitable for flexible battery applications.
• Reduces battery gas generation: During the processing of microporous aluminum foil, the foil surface undergoes secondary cleaning to remove residual oils, reducing side reactions with the electrolyte.
• Requires no changes to production processes or equipment: The use of microporous foil meets the requirements of existing production processes like coating and calendering, with no need for additional or replaced equipment.

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• Coating Leakage Prevention: During the coating process of microporous copper/aluminum foil, it is essential to prevent slurry leakage through the foil pores due to excessively low slurry viscosity during extrusion/spraying. Foils with different pore sizes and porosity levels have different viscosity requirements for the slurry. Taking microporous aluminum foil with 17% porosity and 0.35mm pore diameter as an example, tests indicate that the cathode material viscosity should be around 8000 mPa·s, preferably not lower than 6000 mPa·s. The conveyor speed needs appropriate adjustment during the extrusion spraying process. (If the slurry sits for too long, minor seepage to the other side may occur; rapid drying can solve this.)
• Burr Control during Electrode Slitting: Finally, we hope colleagues who have already conducted experiments using microporous copper or aluminum foil in lithium-ion batteries can share data and engage in mutual exchange.
• Additional note: The performance improvement using microporous copper foil in lithium capacitors, supercapacitors, Ni-Cd, and Ni-MH batteries is very significant. The reason for not achieving large-scale adoption is cost. Using mechanical processing for pore creation offers extremely high production efficiency. It is estimated that the cost after scale mass production will be only slightly higher than that of conventional double-sided smooth copper foil.

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• Power Batteries

Used in new energy vehicles, energy storage power stations, etc., as the core material for the negative electrode current collector in lithium batteries. The microporous structure enhances lithium-ion transport efficiency, reduces internal resistance, meets high-rate charge/discharge demands, and significantly improves battery cycle stability and energy density.

• Flexible Electronics

Used in flexible printed circuit boards and wearable device circuit substrates. Leveraging its thin, lightweight, and porous characteristics, it adapts to scenarios involving repeated bending and rolling, ensuring stable circuit conduction and meeting the flexible design requirements of products like smartwatches and flexible screens.

• Electromagnetic Shielding

Used in communication base stations, precision instruments, military electronics, and other scenarios. The porous structure enhances electromagnetic wave absorption and attenuation capabilities, effectively blocking electromagnetic interference, and ensuring operational accuracy and safety of equipment in complex electromagnetic environments.

• Thermal Management Components

As a core material for heat dissipation substrates in high-power devices, the high specific surface area accelerates heat conduction and dissipation. It is suitable for high heat density scenarios, addressing overheating issues in high-power chips, LED lighting, 5G RF devices, and preventing performance degradation.

The performance advantages of this microporous copper foil are significant, offering (superior cost-performance). Compared to related products from leading companies like Nuode Investment Co., Ltd. , Fukuda, and Mitsui, it boasts broad market prospects. Hard technology brings new markets! New opportunities! A new future!

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