The separator is a thin polymer membrane, typically around nine micrometers thick, that sits between the cathode and anode. Its job sounds simple: keep the two electrodes from touching each other electrically while allowing lithium ions dissolved in the electrolyte to pass freely through its microscopic pores. But that porous architecture carries consequences. A separator with high porosity offers abundant channels for ion transport, reducing the internal resistance of the cell. A low-porosity separator provides better mechanical stability but throttles the flow of ions. For decades, engineers have treated this as a straightforward trade-off, and most studies have optimized separators in isolation from the rest of the cell.

The Korean team, led by Seungyeop Choi, Yeseo Lim, Jaejin Lim, and corresponding author Yong Min Lee, decided to ask a more subtle question: what happens when you pair separators of different porosities with electrodes whose intrinsic lithium diffusion kinetics differ dramatically? To find out, they built coin cells using a nickel-rich NCM622 cathode paired with an artificial graphite anode, and swapped in two polyethylene separators of identical thickness but very different microstructures. The high-porosity separator, dubbed HPPE, had a porosity of 58.2 percent, while the low-porosity LPPE came in at 37.2 percent. Every other design variable, from electrode composition and loading to electrolyte volume, was held constant, ensuring that any performance differences could be traced back to the separator alone.

The physical measurements told a clear story. Scanning electron microscopy revealed that the LPPE possessed a denser fiber network, which translated into a Gurley air-permeability number of 159.3 seconds per 100 milliliters, nearly three times the 58.9 seconds recorded for HPPE. Impedance measurements on stainless steel/separator/stainless steel cells confirmed the consequence: the LPPE carried roughly 60 percent more internal resistance, 0.855 ohms compared with 0.538 ohms for HPPE. Since both separators are made of the same polyethylene with equivalent wettability, the difference in ionic transport stemmed purely from microstructure, not surface chemistry.

Then came the surprise. When the cells were cycled under fast-discharge conditions, with a 0.5C charge followed by a 3C discharge, the HPPE cell clearly outperformed its low-porosity counterpart, delivering 136.0 mAh per gram of cathode material against 123.0 mAh per gram for the LPPE cell, a gap of 9.6 percent. But when the test was flipped to fast charging, a 3C charge followed by a 0.5C discharge, the two cells behaved almost identically. Rate capability tests reinforced the asymmetry: the LPPE cell’s capacity fell off a cliff above 3C during discharge, yet both cells tracked each other closely through 3C charging, with only a modest divergence appearing at 5C and beyond. Direct-current internal resistance measurements painted the same picture, showing a pronounced separator effect during discharge but only a faint one during charge.

Why would a component that sits symmetrically between the two electrodes behave so differently depending on the direction of current flow? The answer, the researchers found, lies in the electrodes themselves. Using electrochemical impedance spectroscopy combined with galvanostatic intermittent titration technique measurements across the full state-of-charge range, they calculated the chemical lithium-ion diffusion coefficients of both active materials. The result was striking: NCM622 diffused lithium at roughly 3.28 times ten to the minus twelve square centimeters per second, while graphite managed 2.28 times ten to the minus nine, nearly three orders of magnitude faster. During discharge, lithium ions must travel through the separator and then insert into the sluggish NCM622 particles, so any extra resistance imposed by a dense separator compounds the cathode’s inherent slowness. During charging, lithium ions leave the fast-diffusing graphite, and the anode’s rapid internal redistribution masks the separator’s limitations.

To test this coupling hypothesis directly, the team fabricated four cells with bilayer separators, stacking high- and low-porosity films in different orientations: HP-HP, HP-LP, LP-HP, and LP-LP, where the first layer faced the cathode and the second faced the anode. Although the HP-LP and LP-HP configurations should have similar total cell resistance, the HP-LP cell, with its porous layer adjacent to the slow-diffusing cathode, delivered better high-rate discharge capacity than the LP-HP cell. The explanation is interfacial: when a restrictive separator sits next to an electrode that already struggles to redistribute lithium, local ion accumulation and concentration polarization build up, inflating the discharge overpotential. Placing the high-porosity layer against the cathode smooths the lithium flux at precisely the point where the cell is most vulnerable.

The quantitative details sharpen the picture further. Under 3C charging, the constant-current portion of the total charge capacity was highest for the HP-HP cell at 65.9 percent and lowest for the LP-LP cell at 55.1 percent, while the HP-LP and LP-HP cells landed nearly tied at 60.4 and 59.5 percent. Under 3C discharge, however, the discharge capacity contribution ranked strictly by configuration: HP-HP beat HP-LP, which beat LP-HP, which beat LP-LP. In other words, the direction of ion flow through a porosity gradient matters, and the gradient should be arranged to favor the electrode with the weaker diffusion kinetics.

The implications reach well beyond the laboratory coin cell. The authors note that the coupling effect they identified is expected to be even more pronounced in material systems where the diffusion disparity between cathode and anode is large. High-nickel layered oxides such as NCM811 and NCA, which power many of today’s long-range electric vehicles, diffuse lithium even more slowly than NCM622 relative to graphite, making them especially sensitive to separator resistance. Olivine-structured lithium iron phosphate, with its restrictive one-dimensional diffusion channels, is another candidate for strong separator-coupled behavior. For battery designers chasing fast-charging and high-power applications, this means separator selection cannot be treated as a generic materials choice; it must be matched to the specific electrochemical fingerprints of the electrodes it separates.

The study also reframes how the field should think about so-called inactive components. A separator’s properties, the researchers conclude, may either emerge as a limiting factor or remain effectively invisible, depending on the diffusion kinetics of the surrounding electrodes. That duality helps explain why previous studies have reached seemingly conflicting conclusions about how much separators matter: the answer depends on what is on either side of the membrane. As the industry pushes toward extreme fast charging and high-power discharge in electric vehicles, grid storage, and electrified aviation, this work offers a practical design principle, orient and engineer separator porosity gradients to shield the slowest-diffusing electrode, and a conceptual one: in a lithium-ion battery, no component is truly inactive. Every layer participates in the intricate choreography of ion transport, and the weakest link is revealed only when the current flows in the right direction.

Subject of Research: Coupled effects of separator porosity and electrode lithium-ion diffusion kinetics on the fast-charge and fast-discharge performance of lithium-ion batteries

Article Title: Coupled effects of separator microstructure and active material diffusion kinetics on high-power performance of lithium-ion batteries

Article References: Choi, S., Lim, Y., Lim, J., Kang, D., Park, K. T., Hong, R., & Lee, Y. M. (2025). Coupled effects of separator microstructure and active material diffusion kinetics on high-power performance of lithium-ion batteries. Advances in Industrial and Engineering Chemistry, 1(1), Article 32. https://doi.org/10.1007/s44405-025-00033-w

Image Credits: AI Generated

DOI: 10.1007/s44405-025-00033-w

Keywords: lithium-ion batteries, separator, porosity, polyethylene separator, NCM622 cathode, graphite anode, diffusion coefficient, fast charging, fast discharging, ionic resistance, electrochemical impedance spectroscopy, high-power batteries

The motivation for the work stems from two of the most consequential applications of coal gas science. Gas diffusion behavior in coal particles underpins the development of coalbed methane, an increasingly important energy resource, and it also governs how quickly methane can desorb from freshly exposed coal during mining operations, which is central to predicting and preventing gas outbursts and explosions in underground mines. Because laboratory measurements of gas desorption are usually performed on crushed particles, the mathematical interpretation of those measurements depends directly on the assumed particle geometry. If that geometry is wrong, every diffusion coefficient extracted from the data inherits the error.

To confront the problem, the team began not with equations but with images. They obtained two-dimensional contours of real coal particles and extracted quantitative shape parameters, allowing them to characterize particle morphology across different scales in a statistically meaningful way. The results were unambiguous about the gap between idealization and reality. Coal particles exhibited median roundness values of 0.73 to 0.80, well below the value of 1.0 that a perfect circle would show, and median axial ratios ranging from 1.34 to 1.49, meaning the particles are substantially longer in one direction than another. Taken together, these measurements indicate that the actual particle shape is much closer to an ellipsoid than to a sphere.

That distinction might sound like a matter of academic pedantry, but the mathematics of diffusion is acutely sensitive to boundary geometry. In the standard spherical model, derived from the classical analytical framework established by Crank in 1975, gas molecules are assumed to diffuse radially inward from a spherical surface toward the center, with the surface-to-volume ratio determined entirely by the particle radius. An ellipsoid has a different distribution of distances between its surface and its interior, with shorter diffusion path lengths along the minor axis and longer ones along the major axis. As a result, the characteristic time for gas to escape from, or enter, an ellipsoidal particle differs from that of a sphere of equivalent volume, and the magnitude of that difference scales with how elongated the particle actually is.

The researchers coupled this geometric insight with a rigorous experimental and computational pipeline. They performed gas desorption experiments on coal particles and then used numerical forward modeling together with parameter optimization to infer the diffusion coefficients that best explained the observed desorption data. Crucially, they ran this inversion twice, once with a spherical model and once with an ellipsoidal model, so that the two geometric frameworks could be compared head to head against identical experimental evidence. This design isolates the effect of particle shape on the estimated diffusion coefficient, which is precisely the quantity that previous studies had left insufficiently quantified.

The evolution laws of the diffusion coefficients and gas pressures were then analyzed across varying particle sizes and adsorption pressures, revealing a consistent pattern. The spherical model generally overestimates gas diffusion coefficients and diffusion rates relative to the ellipsoidal model. More importantly, the deviation is not constant. It grows under lower adsorption pressures and for larger particle sizes, and in the most extreme case examined, the difference between the two models reached 8.94 × 10⁻¹³ m²/s. This means that engineers using spherical assumptions in low-pressure or coarse-particle conditions, which are common in field-relevant scenarios, would systematically misjudge how fast methane is released from the coal matrix.

Why does this systematic bias arise? Physically, the elongation of coal particles changes the effective diffusion path network within each grain. A sphere of equivalent volume concentrates its interior points at larger average distances from the surface than an ellipsoid does along its short axes, and the inversion of desorption data compensates for the mismatched geometry by inflating the fitted diffusion coefficient. In practical terms, a spherical model forces the data to fit a shape the particles do not have, so the estimated diffusivity absorbs the geometric error. The ellipsoidal model, by encoding the measured axial ratios, removes much of this compensating distortion and yields diffusion coefficients that better reflect the intrinsic transport properties of the coal matrix itself rather than artifacts of the assumed shape.

The implications ripple outward across the coal gas research community. Diffusion coefficients measured in the laboratory are routinely embedded into larger reservoir-scale simulations of coalbed methane recovery, into models of gas emission from mine working faces, and into indices used to assess the risk of coal and gas outbursts. If the input diffusivities are systematically inflated by spherical assumptions, reservoir productivity predictions may be overly optimistic, gas drainage designs may be misconfigured, and safety margins in mines may be thinner than intended. The authors argue that incorporating nonspherical morphology is essential for accurately characterizing gas diffusion in coal particles and that their ellipsoidal model offers improved theoretical rationality and engineering applicability compared with the entrenched spherical convention.

The study also fits into a broader movement toward geometric realism in porous media science. Related work has documented how grinding methods alter coal particle morphology, how three-dimensional X-ray computed tomography reveals the complex shapes of micron-sized coal grains, and how particle shape influences processes from flotation kinetics to sorption behavior. What sets this research apart is that it quantifies, with specific numbers, the price of ignoring shape in gas diffusion analysis, and then demonstrates a practical remedy. By first measuring roundness and axial ratios, then building forward and inverse numerical models on that measured geometry, the team provides a reproducible workflow that other laboratories can adopt without exotic instrumentation.

For an industry that depends on precise predictions of how methane behaves in coal, the message is straightforward. The sphere has been a useful fiction, but it is a fiction nonetheless. As coalbed methane development expands and gas disaster prevention remains a life-or-death concern in mining regions worldwide, models that respect the true, irregular, ellipsoid-like character of coal particles offer a path to more trustworthy science and, ultimately, safer and more efficient operations. The new results make clear that when it comes to gas moving through coal, shape is not a detail. It is part of the physics.

The findings also connect to a persistent puzzle in coal gas research: the observation that diffusion in coal powders behaves as a multi-rate process rather than a single, constant-coefficient phenomenon. Because coal contains pore systems spanning multiple scales, gas molecules encounter different transport regimes as they migrate through the matrix. Geometry adds another layer to this complexity, since the distribution of path lengths within an elongated particle naturally produces a spread of diffusion timescales. An ellipsoidal framework therefore aligns more naturally with the heterogeneous character of coal’s pore architecture than a sphere does.

The methodological approach deserves attention as well. Rather than deriving a closed-form analytical solution, the team relied on forward modeling paired with parameter optimization, an inverse-problem strategy in which model outputs are iteratively adjusted until they match measured desorption data. This approach has become increasingly common in coal permeability and diffusion studies, and it allows researchers to work with geometries that lack elegant analytical expressions. The trade-off is that results depend on the quality of the optimization, which makes the careful quantification of particle shape parameters a necessary foundation rather than an optional refinement.

Pressure dependence is another thread worth noting. Earlier experimental work has shown that gas diffusion coefficients in coal vary with adsorption pressure, and the present results indicate that the error introduced by spherical assumptions is itself pressure-dependent, growing as pressure falls. This coupling of geometric and pressure effects suggests that laboratory conditions must be matched carefully to field conditions when transferring measured diffusivities into reservoir or mine-safety models, since a coefficient calibrated at one pressure may mislead at another even before shape effects are considered.

Subject of Research: Effects of nonspherical coal particle morphology on gas diffusion behavior, quantified through desorption experiments and numerical modeling

Article Title: Effects of Nonspherical Morphology on Gas Diffusion in Coal Particles: Experiments and Numerical Modeling

Article References: Liu, Q., Li, Z., Wang, E., Sang, S., Mao, Z., Liu, X., Feng, X., Deng, S., Wang, D., & Zhang, X. (2026). Effects of Nonspherical Morphology on Gas Diffusion in Coal Particles: Experiments and Numerical Modeling. Natural Resources Research. https://doi.org/10.1007/s11053-026-10769-x

Image Credits: AI Generated

DOI: 10.1007/s11053-026-10769-x

Keywords: gas diffusion, coal particles, particle morphology, ellipsoidal model, diffusion coefficient, gas desorption, coalbed methane, numerical modeling, spherical model, adsorption pressure, particle size, mine safety