Shortening the charging time of electric vehicles takes more than higher-output charging equipment. It also requires reducing the resistance lithium ions encounter as they move inside the battery. Lithium-metal batteries, which use metallic lithium as the anode, are a leading candidate for next-generation batteries, with the potential to exceed the energy density of conventional lithium-ion cells. During fast charging, however, the supply of lithium ions to the electrode surface tends to become uneven. This can cause the growth of "dendrites," tree-like crystals of metallic lithium, which risk degrading the battery and causing internal short circuits.
A joint research team led by Professors Dong Ki Yoon and Hye Ryung Byon of the Department of Chemistry at the Korea Advanced Institute of Science and Technology (KAIST) has developed a new way to address this problem by controlling the molecular alignment of a liquid crystal polymer. By orienting the nanopores in the polymer perpendicular to the electrode surface, the team says it reduced the tortuosity of the lithium-ion pathway to about one-2.5th of that in a horizontally aligned film. The results were published online in the journal ACS Nano on September 28, 2026, and appeared in Volume 20, Issue 40, dated October 13. On November 9, KAIST announced the findings as a foundational technology for improving the fast-charging performance of next-generation batteries.
What stands out in this work is not just that a new polymer material was developed. The team prepared films with equivalent chemical composition and pore structure, changed only the direction of molecular alignment between "planar alignment," parallel to the film surface, and "homeotropic alignment," aligned through the film thickness, and compared their ion transport properties. The results confirmed that the alignment direction strongly affects not only ion transport efficiency but also how lithium is deposited on the electrode surface.
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| 有効曲折率（相対値） (相対値（垂直配向＝1）) | |
|---|---|
| 水平配向（プラナー） | 2.5 |
| 垂直配向（ホメオトロピック） | 1 |
The vertically aligned separator has been reported to show an ionic conductivity of 1.77 mS/cm in a carbonate-based electrolyte and a low activation energy of 0.12 eV for ion migration. However, the roughly 2.5-fold difference in tortuosity is a relative comparison, and the values in the chart above are not absolute tortuosity figures. Nor do these favorable properties mean that 15-minute charging of electric vehicles has been realized. This result is a laboratory-scale proof of principle, and evaluating its significance requires distinguishing between the performance that was confirmed and the challenges that remain before practical use.
Vertical Alignment of a Liquid Crystal Polymer Straightens the Maze-Like Ion Path
Conventional lithium-ion batteries mainly use microporous membranes made by stretching polyolefin or by phase separation as separators. Inside these membranes is a complex network of fine voids, and lithium ions travel between the cathode and anode through these winding paths. The index of this path complexity is "tortuosity," and in general, the higher the tortuosity, the greater the resistance to ion transport through the thickness of the membrane. In a lithium-metal anode, if the ion supply becomes locally uneven, deposition becomes nonuniform, which can lead to dendrite growth.
The KAIST team focused on the self-assembly property of liquid crystals, whose molecules line up in a particular direction. Liquid crystals combine the fluidity of a liquid with the order of a crystal, and the alignment direction of their molecules can be controlled through methods such as surface treatment. The team synthesized liquid crystal molecules incorporating polyethylene glycol (PEG) chains, which aid ion transport through coordination interactions with lithium ions, and then cross-linked and fixed them with the alignment controlled. This produced a liquid crystal polymer (PEG-LCP) membrane with nanopore channels about 19 nm in diameter.
The defining feature of this membrane is that the direction of its nanopores can be controlled. In a horizontally aligned film, the channels run mainly along the film surface, whereas in a vertically aligned film, they form through the film thickness. The latter matches the direction in which lithium ions travel between the electrodes, providing a more direct transport path. The team kept the chemical composition and pore structure of the two films equivalent and compared how alignment direction affects transport properties.
| Property | Horizontally aligned PEG-LCP | Vertically aligned PEG-LCP | Notes for evaluation |
|---|---|---|---|
| Molecular alignment | Parallel to the electrode surface | Perpendicular to the electrode surface | Alignment controlled by surface treatment |
| Nanopore channels | About 19 nm, mainly in the in-plane direction | About 19 nm, through the film thickness | Pore size and basic structure are equivalent |
| Effective tortuosity | Relatively high | About 1/2.5 that of horizontal alignment | Relative comparison, not an absolute value |
| Ionic conductivity | Lower than vertical alignment | 1.77 mS/cm | Measured in a carbonate-based electrolyte |
| Lithium deposition morphology | Prone to nonuniform deposition | Promotes uniform deposition | Suppresses dendrites but does not completely prevent them |
| Capacity retention at 4C | Control group lost capacity early | 94% after 100 cycles | Laboratory-scale full cell using an NCM811 cathode |
According to the team, vertical alignment reduced the effective tortuosity to about one-2.5th of that of the horizontal case. The significance of the study lies in showing that ion transport efficiency can be improved by changing molecular alignment rather than by searching one after another for materials with different chemical compositions. It suggests that molecular alignment can be used as an independent control element in materials design. However, changes in microstructure and interfacial conditions that accompany alignment control can also affect battery performance, so not every observed improvement can necessarily be explained by a difference in path length alone.
It is also necessary to distinguish between suppressing dendrite growth and completely preventing its occurrence. What the team confirmed is that more uniform ion supply improved the deposition of metallic lithium and suppressed dendritic and porous growth. The risks of side reactions at the interface between metallic lithium and the electrolyte, and of internal short circuits, have not been fundamentally eliminated.
Understanding the Transport Mechanism Through Ionic Conductivity and Activation Energy
KAIST likens the vertically aligned structure to a "highway" along which lithium ions can travel efficiently toward the electrode. The metaphor describes a mechanism in which aligning the ion transport paths inside the separator with the direction of travel between electrodes reduces resistance to movement. However, evaluating improved transport efficiency requires checking not only the shape of the paths but also the actual ionic conductivity and the activation energy needed for migration.
According to the paper published in ACS Nano, the vertically aligned PEG-LCP separator showed an ionic conductivity of 1.77 mS/cm in a carbonate-based electrolyte. The activation energy for lithium-ion migration was reported as 0.12 eV. This is favorable conductivity even compared with the roughly 1 mS/cm typically reported for electrolyte-soaked conventional polyolefin separators. However, ionic conductivity varies with electrolyte composition, temperature, and membrane structure, so this is not a simple indication of superiority among materials measured under different conditions.
Resistance to ion transport inside the separator is not the only factor hindering fast charging. In a lithium-metal anode, uneven ion supply promotes localized deposition and tends to roughen the electrode surface. As surface area increases, side reactions with the electrolyte progress, and excessive formation of the solid electrolyte interphase (SEI) consumes lithium and electrolyte. These reactions reduce the reversibility of charging and discharging and shorten battery life.
In this study, the directional ion flux created by the vertically aligned PEG-LCP is reported to have suppressed not only dendritic and porous lithium growth but also excessive formation of the interfacial layer. This suggests that what matters is not only how efficiently ions move through the separator but also how they are distributed when they reach the electrode surface. If ions are supplied relatively uniformly, localized lithium deposition and the side reactions that accompany it may be suppressed.
In other words, the significance of this research is not simply that ions were made to move faster. It is that controlling molecular alignment organized the ion transport paths and improved both deposition behavior at the electrode surface and interfacial stability. However, lower tortuosity, higher ionic conductivity, dendrite suppression, and longer battery life are separate evaluation items. The experimental results support links among them, but they do not guarantee that lowering tortuosity alone will extend the life of batteries under all conditions.
94% Capacity Retained After 100 Cycles of 4C Fast Charging
The team incorporated the developed separator into an experimental lithium-metal battery and evaluated its charge-discharge performance. The cell was a full cell with metallic lithium as the anode and the high-nickel material NCM811 () as the cathode. This allowed the team to verify performance in an operating battery in addition to measuring the properties of the separator alone.
The cell retained 94% of its initial capacity even after a 100-cycle test under 4C fast-charging conditions. The average Coulombic efficiency reached 99.86%, and better cycling characteristics were confirmed compared with the control group using a horizontally aligned film. Coulombic efficiency indicates the proportion of the charge stored during charging that can be extracted during discharging, and it offers a clue to side reactions inside the battery and irreversible consumption of lithium.
One point to note is the meaning of the "4C" charge rate. The C-rate expresses the magnitude of the charge/discharge current relative to the battery's rated capacity: 1C is the current that moves the charge equivalent to the rated capacity in one hour, and 4C is four times that current. A 4C condition therefore means a current that can, in calculation, supply the charge equivalent to the rated capacity in 15 minutes. However, the actual time to full charge depends on voltage limits, the charging method, temperature, and the state of the battery, so these experimental results do not directly demonstrate a technology that can fully charge an electric vehicle in 15 minutes.
A 100-cycle test is also short for judging whether a battery is ready for automotive use. Although it varies with application and design goals, a common benchmark for long life in EV batteries is maintaining 80% or more of capacity over 1,000 to 1,500 cycles or more. In addition to degradation from charging and discharging, evaluation must cover degradation from high-temperature environments and long-term storage at full charge, tolerance to temperature changes, and safety against impact and crushing. A high capacity retention after 100 cycles is a promising result, but it alone does not prove practical lifespan or safety.
The reported average Coulombic efficiency of 99.86% and uniform lithium deposition may be more important results than fast-charging performance itself. In lithium-metal batteries, slight irreversible reactions that occur with each charge and discharge accumulate and lead to long-term capacity loss. If interfacial reactions can be stabilized by organizing the ion transport paths, it could lead to designs that combine fast charging and long life. However, it has not yet been confirmed whether equivalent performance can be maintained under practical cell conditions.
Particularly important are the amount of metallic lithium in the anode, the amount of electrolyte, and the cathode active material loading. In laboratory coin cells, larger amounts of metallic lithium or electrolyte than in practical batteries are sometimes used to obtain good cycling characteristics. High-energy-density practical cells, by contrast, must secure high cathode loading under "lean electrolyte" conditions, with less excess lithium and less electrolyte. For this paper as well, it is difficult to judge performance under conditions close to commercial cells without checking the cathode loading, the ratio of electrolyte volume to capacity, and the N/P ratio (the capacity ratio of anode to cathode), including in the supplementary materials.
Challenges Ahead: Large-Area Manufacturing and Mechanical and Thermal Stability
The research involved KAIST doctoral student Geunjung Lee and Dr. Rak Hyeon Choi as co-first authors, with Professors Dong Ki Yoon and Hye Ryung Byon as co-corresponding authors. The work was supported by programs including the mid-career researcher support program of Korea's Ministry of Science and ICT (MSIT) and the National Research Foundation of Korea (NRF), and the paper declares no financial conflicts of interest. KAIST also presents the result not as a mass-producible battery product but as a foundational technology applicable to next-generation batteries.
One of the biggest challenges for practical use is whether vertical alignment of the nanopores can be formed uniformly over large areas. The alignment of liquid crystal molecules changes with external conditions such as substrate surface state, alignment treatment, and electric and magnetic fields. Making small-area thin films in a laboratory is very different, in terms of the difficulty of keeping alignment uniform, from continuously producing long films on a factory roll-to-roll line. Localized alignment defects or pinholes could cause nonuniform ion transport or internal short circuits, making quality control across the entire film essential.
Balancing film thickness and mechanical strength is also important. Separators for automotive batteries must have tensile strength to withstand the tension applied during winding and stacking, as well as puncture strength so they are not damaged by protrusions or minute burrs on electrode surfaces. Whether the nanoporous film formed by liquid crystal polymer self-assembly has mechanical properties equal to or better than existing stretched polyolefin films will need to be verified. And even if thinner films reduce ion transport resistance, practical use would be difficult if mechanical durability or manufacturing yield declines.
Thermal stability during abnormal heat generation must also be confirmed. If a separator shrinks or breaks at high temperature, the cathode and anode can come into direct contact, risking internal short circuits and thermal runaway. The heat resistance of the cross-linked liquid crystal polymer, and whether the alignment structure changes as temperature rises, are important items for evaluating safety. The paper's supplementary materials include an evaluation of thermal stability, but confirming the safety required of practical cells calls for verification under a wider range of conditions.
Given these challenges, it would not be appropriate to regard this result as a technology that can immediately replace existing automotive separators. On the other hand, the design principle that ion transport and electrode interface stability can be improved by controlling molecular alignment, not just chemical composition, may be applicable to future materials development. It offers useful insights especially for research on polymer electrolytes and composite separators, where the directionality of ion transport paths is important.
What to Watch: Long-Term Life Under Practical Conditions and Establishing Mass-Production Technology
What matters in this KAIST study is not just the capacity retention figure after 100 cycles. It shows that controlling the molecular alignment of a separator can lower the tortuosity of ion transport paths and improve the deposition of metallic lithium and the stability of the electrode interface. It reveals that designing the directionality of the internal structure, not just making the material's chemical composition more complex, could lead to better performance in next-generation batteries.
In judging the prospects for practical use, reproducibility under conditions close to actual automotive batteries will be important first. It must be confirmed whether the average Coulombic efficiency of 99.86% and good capacity retention can be reproduced with a combination of a thin metallic lithium anode, high cathode loading, and a small amount of electrolyte. In particular, with less electrolyte, the conditions for ion transport and interfacial reactions change, so it will be of interest how well the effect of the vertically aligned structure confirmed here is maintained.
The next requirement is long-term testing over 500 to 1,000 cycles or more. Because the shape and volume of a lithium-metal anode change with charging and discharging, the separator is subjected to repeated mechanical loads. It must be confirmed that, even under these conditions, the nanopore structure and alignment are maintained and the ion transport paths do not become blocked or uneven. Even if initial charge-discharge characteristics are excellent, the advantage could be lost if the alignment structure breaks down over long use.
Finally, establishing a manufacturing process that can stably produce large-area, uniform vertically aligned films is essential. Whether existing separator production lines can be used or new continuous film-forming technology is needed will greatly change the mass-production cost and the outlook for commercialization. This result is promising basic research showing that molecular-level structural design can improve the performance of lithium-metal batteries. Whether that potential can be developed into a practical technology depends on whether long-term performance under practical cell conditions can be combined with safety and manufacturability.