📄 chemistry

6FAP-Derived Hydroxyl-Containing Polyimide Microsphere-Coated PE Separator for Lithium-Ion Batteries

This study demonstrates that coating polyethylene separators with hydroxyl-containing polyimide (HPI) microspheres via electrospraying significantly enhances thermal stability, electrolyte wettability, and electrochemical performance, with the 8 wt% HPI-coated separator achieving optimal lithium-ion transference and cycling stability.

Original authors: Danni Jia, Chenying Li, Hongyan Wang

Published 2026-07-29

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Danni Jia, Chenying Li, Hongyan Wang

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Tiny Gatekeepers of Your Phone's Power

Imagine your smartphone, electric car, or laptop as a bustling city. Inside this city, energy is constantly shuttling back and forth between two districts: the positive side and the negative side. The currency of this city is the lithium-ion, a tiny, energetic traveler that zips back and forth to charge and power your devices. But for this city to function without collapsing into chaos, there needs to be a very specific kind of traffic controller: a separator.

Think of the separator as a high-tech fence or a bouncer standing between the two districts. Its job is twofold: it must keep the two sides from touching (which would cause a short circuit and a fiery disaster), while still letting the lithium-ion travelers pass through freely. Most modern devices use a fence made of polyethylene (PE), a type of plastic. It's cheap and works well, but it has a fatal flaw: it's a bit of a coward when it gets hot. If the battery gets too warm, this plastic fence shrinks and melts, letting the two sides crash into each other. To fix this, scientists have been trying to coat this flimsy fence with stronger materials, like ceramic or special plastics, to make it heat-resistant and better at soaking up the liquid "fuel" (electrolyte) that helps the ions move. The big question in this corner of science is: how do we make this fence not just stronger, but also a better host for our lithium travelers?

The Paper's Story: A New Kind of Micro-Sponge

In this study, a team of researchers from Tianjin University of Technology decided to upgrade the standard plastic fence by coating it with tiny, microscopic balls made of a special plastic called "hydroxyl-containing polyimide" (HPI). You can think of these HPI microspheres as microscopic, porous sponges that are incredibly good at holding onto liquid and standing up to heat.

The scientists didn't just dump these sponges on the fence; they used a technique called electrospraying, which is like using a high-tech spray gun to paint the PE separator with a layer of these tiny balls. They created three different versions of this new fence, with different amounts of microspheres: 8%, 10%, and 12% by weight. They also made a control group using regular polyimide (PI) microspheres without the special "hydroxyl" ingredient, just to see if the extra chemical group made a difference.

What they found was quite promising. The paper suggests that adding these HPI microspheres turned the flimsy plastic fence into a much tougher, more heat-resistant barrier. When they tested how much the separators shrank under heat, the HPI-coated ones held their shape much better than the plain plastic ones. In fact, the version with 8% HPI microspheres (called HPI-8) was the star of the show. It didn't just resist shrinking; it became a super-absorbent sponge for the battery's liquid fuel.

The researchers measured how well the liquid "wet" the surface of the separator. The plain plastic fence had a contact angle of 33.4°, meaning the liquid didn't spread out well. The HPI-8 separator, however, had a contact angle of just 7.6°, which is practically flat. This means the liquid spreads out instantly, soaking into the tiny pores of the microspheres. Because of this, the HPI-8 separator could hold 159.3% of its own weight in liquid electrolyte, far outperforming the plain plastic (43.5%) and even the ceramic-coated versions.

This improved soaking ability had a direct impact on how fast the battery could charge and discharge. The paper reports that the HPI-8 separator allowed lithium ions to move more easily, achieving a lithium-ion transference number of 0.58 (a measure of how efficiently the ions move compared to other particles). In real-world testing, batteries with this separator maintained a high discharge specific capacity of 145.7 mAh·g⁻¹ at a 1C rate, which is faster and more efficient than the batteries using the plain plastic or the regular polyimide coating.

However, the paper also rules out the idea that "more is always better." When the researchers increased the microsphere content to 10% and 12%, the performance actually started to drop. The HPI-12 separator had a lower discharge capacity (130.9 mAh·g⁻¹) than the plain plastic separator. The authors suggest that packing too many microspheres together clogs the pores, making it harder for the ions to get through.

Furthermore, while the HPI microspheres were great at soaking up liquid, the paper notes a slight trade-off in extreme heat stability compared to regular polyimide. The hydroxyl groups that made the HPI so good at absorbing liquid also made it slightly more reactive at very high temperatures, causing the HPI-8 separator to shrink a bit more (73%) than the regular PI separator (61%) when heated to 180°C. Despite this, the HPI-8 separator was still vastly superior to the plain plastic, which melted completely at that temperature.

In the long run, the paper suggests that this HPI-8 coating is a winning strategy for battery safety and performance. Batteries using this separator were able to cycle (charge and discharge) for over 300 times while keeping their efficiency above 97%, whereas the plain plastic batteries failed much earlier. The study concludes that by choosing the right chemical ingredients—specifically those with hydroxyl groups—and finding the perfect balance of how much to add, we can create battery separators that are safer, soak up more fuel, and keep our devices running longer.

Technical Summary: 6FAP-Derived Hydroxyl-Containing Polyimide Microsphere-Coated PE Separator for Lithium-Ion Batteries

Problem Statement
Commercial polyolefin separators, specifically polyethylene (PE), are widely used in lithium-ion batteries (LIBs) but suffer from inherent limitations that compromise safety and performance. These include high thermal shrinkage rates at elevated temperatures, poor thermal stability, and low melting points, which can lead to internal short circuits. Furthermore, traditional PE separators exhibit poor electrolyte wettability and limited lithium-ion transport efficiency. While surface modifications using inorganic nanoparticles or polymer coatings (such as polyimide, PI) have been explored, many existing methods are constrained by the substrate's inherent properties or face challenges related to structural stability and pore control when using techniques like electrospinning. There is a need for a functionalized separator that enhances thermal stability, electrolyte wettability, and ionic conductivity without compromising the mechanical integrity of the cell.

Methodology
The study employed an electrospraying technique to synthesize hydroxyl-containing polyimide (HPI) microspheres and coat them onto commercial PE separators.

Synthesis: HPI microspheres were derived from a polyamic acid (PAA) precursor synthesized via low-temperature polycondensation of 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane (6FAP) and phthalic anhydride (PMDA). The PAA was converted to HPI microspheres through thermal imidization at 300°C. For comparison, traditional PI microspheres were synthesized using 4,4'-diaminodiphenyl ether (ODA) and PMDA.
Coating: The resulting microspheres were dispersed in a slurry containing binders and surfactants, then coated onto PE separators at varying loadings (8, 10, and 12 wt%). The samples were designated HPI-8, HPI-10, and HPI-12.
Characterization: The separators were evaluated using scanning electron microscopy (SEM) for morphology, differential scanning calorimetry (DSC) for thermal transitions, and thermal shrinkage tests at temperatures up to 180°C. Electrolyte wettability was assessed via contact angle measurements and electrolyte uptake tests. Electrochemical performance was analyzed using electrochemical impedance spectroscopy (EIS), linear sweep voltammetry (LSV), and cycling tests on LiFePO₄||Li cells.

Key Contributions and Results
The research demonstrates that HPI microsphere coatings significantly improve the physicochemical and electrochemical properties of PE separators compared to pristine PE, commercial Al₂O₃-coated separators, and traditional PI-coated separators.

Thermal Stability: While the introduction of hydroxyl groups in HPI initially presented a theoretical risk of reduced thermal stability due to potential dehydration reactions, the composite separators exhibited superior thermal performance compared to unmodified PE. At 180°C, the HPI-12 separator showed the lowest thermal shrinkage rate (26%), outperforming the PI separator (61%) and the commercial PE separator (which melted completely). The study notes that increasing the HPI microsphere content correlates with enhanced thermal stability.
Electrolyte Wettability and Absorption: The incorporation of polar hydroxyl (-OH) groups significantly enhanced electrolyte affinity. The HPI-8 separator achieved the lowest electrolyte contact angle (7.6°) and the highest electrolyte uptake (159.3%), surpassing the PI separator (11° and 153.1%, respectively). The porosity of the HPI-8 separator was also the highest at 71.3%.
Ionic Conductivity and Lithium-Ion Transference: The HPI-8 separator exhibited the highest lithium-ion transference number ( $t_{Li+}$ ) of 0.58, compared to 0.57 for PI and 0.40 for pristine PE. The authors attribute this to the hydroxyl groups forming hydrogen bonds with lithium salt anions (e.g., PF₆⁻), which partially immobilizes the anions and reduces migration resistance for lithium ions.
Electrochemical Performance: In LiFePO₄||Li half-cells, the HPI-8 separator delivered the highest discharge specific capacity at 1C (145.7 mAh·g⁻¹) and maintained superior rate capability at high currents (e.g., 118.6 mAh·g⁻¹ at 5C). Over 300 cycles at 1C, the HPI-8 separator maintained a discharge capacity of 148 mAh·g⁻¹ with a Coulombic efficiency of 97.4%, demonstrating excellent cycling stability compared to the rapid failure of the pristine PE separator.

Significance
The paper claims that the fabrication of HPI microsphere-coated separators represents an effective strategy for improving the safety and electrochemical performance of lithium-ion batteries. The study highlights that the specific selection of monomers containing functional groups (such as the hydroxyl groups in 6FAP) is pivotal in determining the overall performance of the polymer membrane. By leveraging the unique microstructure of microspheres and the chemical properties of hydroxyl-containing polyimides, the authors successfully addressed the trade-offs between thermal stability, electrolyte wettability, and ionic transport. The findings suggest that HPI-8 coated separators offer a viable pathway to developing high-safety, high-performance LIBs, particularly by mitigating the thermal shrinkage issues of polyolefin separators and enhancing lithium-ion migration efficiency.

The Tiny Gatekeepers of Your Phone's Power

The Paper's Story: A New Kind of Micro-Sponge

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