Cathode Contact Resistance: The Overlooked Lever Behind Single-Crystal NMC811 Longevity

Quick facts

Study: WMG (University of Warwick), Faraday Institution, University of Cambridge and University of Southampton; single-layer pouch cells, SC-NMC811//graphite
Variable tested: cathode porosity after calendering: 38% (light), 31% (moderate), 25% (aggressive)
Headline result: the point where extended-voltage cycling starts losing to conventional-voltage cycling shifted from ~40 cycles (31% porosity) to ~300 cycles (25% porosity)
Second lever: 1 MPa external stack pressure roughly quadrupled cycle life at 1C/1C versus a loosely jigged cell
Status: preprint, posted 6 August 2026, not yet peer reviewed (chemrxiv.org/doi/10.26434/chemrxiv.15007133/v1)

The prompt for this piece

Prof Louis Piper (WMG/Warwick, Faraday Institution) posted a summary of this paper on LinkedIn, describing work from the Faraday Institution’s LEAP project led by Eleni Fiamegkou and Daniela Proprentner. His framing of the result is worth repeating directly, because it’s the thesis the underlying data set out to support:

“manufacturing variables such as calendaring and stack pressure are no longer simply process optimisation steps. Instead, they should be recognised as fundamental design parameters that control contact resistance, suppress the impact of oxygen-loss degradation, and enable stable high-voltage operation without chemical modification of the cathode or electrolyte.”

That’s a strong claim to hang on two process levers most people file under “manufacturing throughput,” not “degradation mechanism.” The paper backs it up with pouch-cell data built specifically to separate the effect of calendering from every other variable: same active material, same electrolyte, same cell format, only the roll gap and the stack pressure changed. Here’s what’s actually in it, and where I think it points next.

Why the aluminium foil is the weak link

Every NMC cathode coating sits on an aluminium current collector, and the electrical path from that foil into the particle network isn’t as clean as a schematic makes it look. Aluminium forms a thin native oxide the moment it’s exposed to air, and that oxide layer sits directly at the interface the coating is pressed onto. Electrons crossing from the foil into the cathode composite have to get through it, and depending on how much of the particle surface is actually touching bare or well-contacted foil, that crossing can dominate the cell’s total electronic resistance rather than being a rounding error next to the “real” chemistry happening inside the particles.

The paper separates this interfacial resistance from the bulk resistance of the coating itself using two independent methods: a four-point Hioki probe on pristine electrodes, and impedance spectroscopy on assembled cells, where the high-frequency semicircle (with a capacitance signature around 4 µF/cm², consistent with a thin oxide film) is fitted separately from the medium-frequency arc associated with charge transfer at the NMC/electrolyte interface. Both methods agree: for a lightly calendered cathode, contact resistance isn’t a minor contributor, it’s the largest single resistance in the whole cathode.

Single crystals let you calender your way out of it

Calendering, which means compressing the coated electrode between rollers to push density up and porosity down, is standard practice on every production line, but it has a ceiling. Push a polycrystalline NMC particle too hard and the internal grain boundaries crack, opening fresh unpassivated surface to the electrolyte and creating new degradation pathways. That ceiling is why most academic degradation studies leave calendering out of the picture: it’s treated as a manufacturing nuisance rather than a variable worth studying.

Single-crystal particles don’t have that failure mode. With no internal grain boundaries to crack, SC-NMC811 can be compressed much further before anything breaks, and the WMG team pushed three cathode sheets to 38%, 31%, and 25% porosity to see what that extra compression actually buys.

Cross-sectional imaging showed no detectable microcracking even at the densest point (3.45 g/cm³), and, more importantly for the contact-resistance question, it showed a steadily growing fraction of the foil surface with particles physically pressed into it: 5.9% embedded at 38% porosity, rising to 15.6% at 31%, and 36.6% at 25%. That embedment is what’s actually closing the electrical gap. It tracked almost exactly with the Hioki measurements, where contact resistance dropped roughly fourfold going from 38% to 31% porosity (1.7 to 0.4 Ω·cm²) before flattening out toward 25%, while the bulk resistance of the coating itself barely moved regardless of porosity. Calendering, in other words, isn’t improving the cathode. It’s improving the joint between the cathode and the foil.

The sevenfold delay is the number that matters

None of that would matter much if it only showed up in a fresh cell. The part of this paper that’s actually new is what happens to contact resistance once cycling starts, and specifically what happens once the upper cutoff voltage is pushed past the oxygen-loss onset around 4.33 V vs. Li, the regime that unlocks meaningfully more capacity from a Ni-rich cathode but has historically been avoided because surface reconstruction and rock-salt formation degrade the cathode faster than it can be used.

Cells built with 31% and 25% porosity cathodes were cycled in both a conventional 3.0–4.2 V window and an extended 2.5–4.4 V window. In the conventional window, porosity barely mattered: both retained over 93% of capacity after 500 cycles, because degradation there is dominated by graphite-side SEI growth rather than anything happening at the cathode. Push the same cells into the extended window and the picture inverts. The 31% porosity cells started with a capacity advantage from the wider window, but that advantage eroded fast: the point where their extended-window capacity dropped below what a conventional-window cell would deliver arrived at around 40 cycles. For the 25% porosity cells, that same crossover didn’t arrive until roughly 300 cycles: a sevenfold delay, and one achieved without touching the active material, the electrolyte, or the cell chemistry. After 500 cycles, the heavily calendered cells retained 87.5% of their initial extended-window capacity against 76.2% for the moderately calendered ones.

The impedance data explains the mechanism. In the 31% porosity cells, contact resistance already accounted for roughly half the total cell impedance right after formation, and its share climbed to about 87% after 400 cycles of extended-voltage operation: the interface, not the bulk cathode, was doing most of the damage. In the 25% porosity cells the same fraction started at only ~22% and reached ~58% after the same 400 cycles. Crucially, the underlying oxygen-loss degradation wasn’t prevented in either case: the authors are explicit that calendering doesn’t stop the chemistry activated above the O-loss onset. What it does is stop that chemistry from being compounded by a current collector interface that’s simultaneously getting worse, which is a genuinely different failure mode than the ones most surface-coating and electrolyte-additive work targets.

Stack pressure does the same job from the outside

Calendering fixes the interface once, at manufacture. The paper’s second experiment asks whether you can keep improving it during life, using nothing but mechanical force. Cells with 30% porosity cathodes were cycled at a faster 1C/1C in a 3.0–4.3 V window, one set under a loose 0.1 MPa Perspex jig and one set under 1 MPa in a purpose-built high-pressure jig. The 1 MPa cells extended projected cycle life from around 190 cycles to over 700, roughly a fourfold improvement, and the impedance breakdown attributes most of that to the same mechanism: pressure suppresses the growth of contact resistance during cycling, with charge-transfer resistance only moderately affected. It’s an independent confirmation, using a completely different lever, that keeping the cathode physically pinned against the foil is doing the real work.

Our view

Louis Piper’s post frames calendering and stack pressure as design parameters rather than process steps, and the data supports that reframing better than most degradation papers manage.

This complements a fast-moving thread on stack pressure generally: Wang et al.’s recent Nature Energy paper on stack pressure, mechanical expansion, and degradation pathways is cited directly in this work. It also sits alongside the same group’s broader case that single-crystal Ni-rich cathodes need to be understood on their own degradation terms rather than as a drop-in swap for polycrystalline material (Ogley et al., Chemical Reviews, 2025). Contact resistance isn’t a new idea in electrode engineering, but treating its cycling evolution, not just its fresh-cell value, as the thing that decides whether extended-voltage operation is usable is the part of this paper worth taking seriously.

References

Fiamegkou, E. et al. Improving cell longevity of high nickel single crystal NMC by reducing the contact resistance. ChemRxiv preprint, posted 6 August 2026. doi.org/10.26434/chemrxiv.15007133/v1
Louis Piper, LinkedIn post summarising the above study, WMG/Faraday Institution LEAP project.
Raju, K. et al. Influence of Cathode Calendering Density on the Cycling Stability of Li-Ion Batteries Using NMC811 Single or Poly Crystalline Particles. Journal of The Electrochemical Society 171, 080519 (2024). doi.org/10.1149/1945-7111/ad6378
Wang, H. et al. The interplay between stack pressure, mechanical expansion and degradation pathways in lithium-ion batteries. Nature Energy (2026). doi.org/10.1038/s41560-026-02087-6 See also our own coverage of this paper: What’s the Right Stack Pressure?
Páez Fajardo, G. J. et al. Synergistic Degradation Mechanism in Single Crystal Ni-Rich NMC//Graphite Cells. ACS Energy Letters 8, 5025–5031 (2023). doi.org/10.1021/acsenergylett.3c01596
Ogley, M. J. W. et al. Understanding Degradation in Single-Crystalline Ni-Rich Li-Ion Battery Cathodes. Chemical Reviews 125, 9774–9806 (2025). doi.org/10.1021/acs.chemrev.5c00330
Seidl, C. et al. Comparison of Electronic Resistance Measurement Methods and Influencing Parameters for LMFP and High-Nickel NCM Cathodes. Batteries 10, 105 (2024). doi.org/10.3390/batterie s 10030105

See also: Chemistry Mechanics

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