The composite cathode of a solid-state battery is a dense, non-porous blend of active material, conductive additive and a portion of solid electrolyte known as the catholyte. Unlike conventional lithium-ion cells, where a liquid electrolyte seeps into every pore, the solid-state version has no pore-filling liquid to rescue poor contacts. Every particle must touch its neighbour intimately, and both ions and electrons need homogeneous, percolating pathways through the electrode. The conductive additive in this study is Super C65, a carbon black from Imerys, which must form an unbroken electronic network threading through the cathode. The active material is carbon-coated lithium iron phosphate, or LFP, whose own carbon coating enhances intrinsic electronic conductivity and interfacial lithium transfer. The researchers were careful to distinguish between these two carbons: the factory-applied coating on the LFP particles and the C65 introduced during electrode processing behave very differently under the forces of mixing.

To isolate the effects of processing, the team prepared slurries under dry-room conditions with a dew point below minus 60 degrees Celsius, using a bead mill to first dissolve LiTFSI in acetonitrile and then mix in high molecular weight PEO to form the catholyte. The LFP and C65 powders were dry-mixed in a planetary vacuum mixer at 200, 750 or 1500 revolutions per minute, and the resulting powder blend was then wet-mixed into the catholyte for 60, 120 or 180 minutes. Five slurries were produced in total, each cast onto carbon-coated aluminium current collectors and dried under vacuum. Crucially, the researchers deliberately skipped calendering, the mechanical compaction step common in electrode manufacturing, so that the morphological fingerprints left by the mixing process itself would remain visible in the finished electrodes.

One of the study’s most elegant methodological contributions is the introduction of a relative viscosity parameter. Batch-to-batch variability in the PEO raw material, stemming from differences in molecular weight distribution, meant that absolute viscosity measurements could not be compared reliably across samples. By normalizing the slurry viscosity to the viscosity of the corresponding catholyte at the end of its own mixing, the team could decouple the true effects of mixing time and speed from uncontrolled fluctuations in the polymer feedstock. Validation experiments with four separate preparations, including three different PEO batches, confirmed that this normalization substantially reduced variability, offering a practical tool that other laboratories can adopt to make their slurry comparisons meaningful.

The rheological results revealed a counterintuitive trend. Longer wet mixing times and higher dry mixing speeds both reduced slurry viscosity, and the researchers traced this not to further polymer degradation but to the progressive deagglomeration of the carbon black. Gel permeation chromatography showed that the PEO molecular weight decreased exponentially toward a plateau, with an apparent scission-rate constant of 7.49 times ten to the minus seven per minute, but since total mixing times stayed below the point where degradation becomes negligible, the viscosity drop had to come from elsewhere. As C65 agglomerates break apart, the number of carbon particles rises, promoting immobilization of polymer chains and reducing chain entanglement, a mechanism previously described by Hoffman and colleagues. More effective deagglomeration also increases interparticle volume, allowing more solvent molecules to occupy the spaces between carbon particles and thereby lowering the suspension viscosity.

These rheological differences translated directly into electrode microstructure. Scanning electron microscopy of surfaces and argon ion-milled cross-sections, analysed with ImageJ, showed that longer wet mixing produced electrodes with lower porosity, higher active material loading and fewer surface lumps, because lower viscosity allows better particle rearrangement and compaction during drying. In the dry mixing series, the sample processed at 1500 rpm displayed the most homogeneous morphology, while the 200 rpm sample exhibited a highly cracked surface, suggesting that large C65 agglomerates weaken the mechanical integrity of the composite cathode and hinder uniform polymer distribution. Porosity values ranged from about 1.65 percent for the longest wet mixing time to nearly 3 percent for the mildest conditions, differences that proved consequential for transport properties.

Electronic and ionic conductivity measurements told a more nuanced story. Reference pellets containing as little as 5 weight percent C65 achieved electronic conductivities of roughly ten to the minus two siemens per centimetre, confirming that this modest carbon fraction is sufficient for percolation. Yet after full electrode processing, bulk electronic conductivity dropped by nearly two orders of magnitude at both 25 and 70 degrees Celsius. Wet mixing time had little effect on this bulk conductivity, but in-plane resistivity measurements revealed that excessive wet mixing stresses the carbon particles and degrades the conductive network. In the dry mixing series, the intermediate 750 rpm condition delivered the lowest interfacial resistance and the highest ionic conductivity, while the 1500 rpm sample, despite its denser packing, showed signs of carbon particles being over-stressed and unable to form an optimal network.

Full-cell testing in monolayer pouch cells with lithium metal anodes, cycled at 70 degrees Celsius between 2.6 and 3.7 volts, exposed the trade-offs with striking clarity. At the gentle C/20 rate, all cells delivered similar capacities between 156 and 158 mAh per gram, but at higher rates the mildest processing conditions won out: the 60-minute wet mix and the 200 rpm dry mix, both with the highest slurry viscosities and highest porosities, achieved the highest capacities at C/5. The researchers attribute this to the higher viscosity limiting conductive additive redistribution, thereby preserving a favourable electronic network, while mud cracking in the 200 rpm electrode may have created preferential channels for solid electrolyte infiltration during cycling. However, these high-capacity electrodes also showed the poorest cycle life, with the 200 rpm sample lasting only 65 cycles before coulombic efficiency fell below 98 percent, compared to 120 cycles for the balanced 750 rpm condition.

The degradation mechanism was revealed through X-ray computed micro-tomography of intact pouch cells, a technique that avoids the impossible task of disassembling strongly adhesive cycled cells. Cross-sections of a pristine cell showed homogeneous layers with only minor pores at the cathode-electrolyte interface, but after 130 cycles the lithium metal anode had thinned from 500 to 470 micrometres, voids had multiplied in the cathode, and dark regions appeared within the solid electrolyte layer, signatures of lithium dendrites growing from the anode through the polymer. Grain boundaries in the electrolyte, attributed to solid electrolyte interphase formed by soft dendrites, were also visible. Impedance spectroscopy of the aged cell confirmed diminished ionic diffusion, tying the loss of coulombic efficiency directly to dendrite propagation.

The broader lesson of this work is that in solid-state batteries, processing is not a secondary consideration but a first-order design variable. The optimal wet mixing time of 120 minutes balanced electrode compaction against preservation of the conductive network, while an intermediate dry mixing speed of 750 rpm achieved the best cyclability without compromising lithium-ion transport. Even a small amount of residual porosity, the authors suggest, may serve as stress-relief space accommodating the volume changes of lithium insertion and extraction in a mechanically rigid system. As solid-state batteries edge toward commercialization, this study demonstrates that the path to better cells may run through the mixing vessel, where the fate of every carbon black agglomerate ultimately determines whether electrons and ions can find their way home.

Subject of Research: Effect of slurry mixing parameters on carbon black distribution and performance of solid polymer electrolyte composite cathodes

Article Title: Carbon black distribution in low viscosity polymer slurries

Article References: Carbon black distribution in low viscosity polymer slurries. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: solid-state batteries, carbon black, polymer electrolyte, slurry rheology, composite cathode, lithium metal anode, PEO, LiTFSI, electrode processing, lithium dendrites, X-ray tomography, LFP

The new work, published in the open-access journal iScience, describes a low-frequency bidirectional pulsed current heating method that operates at frequencies of one hertz or below. The central innovation is adaptivity: rather than applying fixed pulse parameters, the system continuously adjusts the pulse currents according to the battery’s real-time state, ensuring that the anode potential never crosses the lithium-plating threshold throughout the entire heating process. The result, according to the authors, is the fastest possible heating that a given battery chemistry can safely tolerate, a genuine non-destructive approach validated on both lithium iron phosphate and nickel-cobalt-manganese cells.

To understand why this matters, it helps to consider the landscape of existing solutions. External heating methods, which rely on thermal fluids, heat pipes, or phase change materials placed outside the cell, tend to produce uneven temperature distributions and accelerate aging. Internal heating, by contrast, exploits the battery’s own internal resistance to generate Joule heat directly where it is needed, minimizing losses. Alternating current heating and mutual pulse heating, in which battery modules alternately charge and discharge one another, have both been explored extensively. Bidirectional pulsed current strategies, in which positive and negative pulses alternate to keep the state of charge nearly constant, have shown particular promise, but earlier high-frequency implementations demanded costly power electronics that limited real-world adoption.

The research team began by systematically mapping how pulse parameters influence heat generation. Using 2 ampere-hour LFP soft-pack cells placed in a temperature chamber, they varied the ratio of positive to negative pulse amplitudes from 1:1 up to 1:5 while holding the pulse period fixed at six seconds. Across both low and high states of charge, a larger amplitude ratio produced a greater temperature rise, a consequence of the fundamental relationship that heat generation scales with the square of current. They then varied the pulse period from fractions of a second up to 36 seconds and found that longer periods heated the battery more effectively, although the benefit plateaued beyond roughly ten seconds. That plateau defined their operating point: a ten-second pulse period, well within the low-frequency regime that avoids expensive high-frequency excitation hardware.

The truly distinctive element of the study is its use of three-electrode cells to police the lithium-plating boundary in real time. The researchers modified commercial cells by implanting a lithium titanate reference electrode at the center of the electrode stack, allowing them to measure the anode potential directly during operation, something impossible with conventional two-terminal cells. Through hybrid pulse power characterization tests conducted at temperatures from minus ten to twenty-five degrees Celsius and across a range of states of charge, they constructed a three-dimensional map of the maximum charging current the battery could tolerate at each condition without lithium deposition. This boundary current map became the adaptive brain of the heating algorithm, dictating the positive pulse amplitude as temperature and state of charge evolved.

The preheating experiments themselves were conducted under conditions mimicking a real electric vehicle arriving at a charger on a frigid winter day. With cells adjusted to ten percent state of charge and stabilized at minus fifteen degrees Celsius, the bidirectional pulses were applied. For the LFP cell, heating from minus fifteen to ten degrees took 2,068 seconds, an average temperature rise of 0.73 degrees Celsius per minute. Crucially, the measured anode potential, corrected against the reference electrode’s equilibrium potential of 1.565 volts versus lithium, remained above zero volts throughout the entire process, the electrochemical signature that no metallic lithium was plating onto the graphite anode at any moment.

Durability is where many preheating schemes have quietly failed, so the team subjected cells to one hundred repeated heating cycles at fixed state of charge. Because the bidirectional pulses deliver equal charge and discharge capacity within each cycle, the state of charge never drifts, allowing every cycle to begin from identical conditions. The authors note that with typical weekly charging and a three-month winter, one hundred heating cycles correspond to roughly seven years of real-world service, comparable to the lifetime of a traction battery. Capacity measurements taken before the test, after fifty cycles, and after one hundred cycles showed essentially no degradation. Post-mortem analysis sealed the case: transmission electron microscopy revealed an intact, uncracked SEI film, scanning electron microscopy showed an orderly lamellar electrode structure with no dendritic deposits, and energy-dispersive spectroscopy confirmed the elemental composition was unchanged.

To test whether the framework generalizes beyond one chemistry, the researchers repeated the entire procedure with a 1.75 ampere-hour commercial NCM811 pouch cell, independently recalibrating its lithium-plating boundary rather than simply copying LFP parameters. The results were striking. The NCM811 cell heated from minus fifteen to ten degrees in just 865.8 seconds, an average rise of 1.75 degrees Celsius per minute, nearly double the LFP rate. The reason lies in the interplay of resistance and allowable current: the NCM cell’s internal resistance is roughly half that of the LFP cell, but its lithium-plating boundary current is nearly twice as large, so its heating power, proportional to current squared times resistance, is consistently higher across all temperatures. The NCM cell likewise survived one hundred heating cycles with no measurable capacity loss and no lithium plating detected.

The physics underlying these results rewards aggressive current within safe limits. Because heat generation is proportional to the square of current, the alternating charge and discharge pulses of the bidirectional strategy allow larger amplitudes than constant-current heating would, since the alternating polarity alleviates anode polarization and prevents the anode potential from collapsing below zero volts. The authors point out an encouraging trend for the future: as battery technology improves fast-charging capability, the allowable lithium-plating boundary current grows, which in turn permits even larger pulse currents and faster heating, even as falling internal resistance reduces the Joule heating contribution per ampere.

The study is candid about its limitations. The pulse period was optimized at zero degrees and extrapolated to minus fifteen, an assumption supported but not exhaustively characterized across the full temperature range. The long-term stability of the implanted reference electrodes under repeated pulsing remains an open question, the durability test did not combine heating with subsequent fast charging, and the hardware costs and system-level energy efficiency of the required bidirectional power electronics were not quantified. Even so, the demonstration that a low-frequency, adaptive, chemistry-agnostic pulse strategy can warm frozen batteries at up to 1.75 degrees per minute without any detectable damage represents a meaningful step toward electric vehicles and stationary storage that shrug off winter. For the millions of drivers who watch their range evaporate on cold mornings, the prospect of a battery that heats itself safely and swiftly may be the most welcome news of the season.

Subject of Research: Non-destructive low-temperature pulse preheating of lithium-ion batteries

Article Title: Low-frequency non-destructive adaptive fastest pulse preheating for lithium-ion batteries

Article References: Li, C., Mao, S., Gao, M., He, M., Sun, Y., Wang, D., Han, X., Xia, C.-J., Wang, X., Lu, Y., & Gao, W. (2026). Low-frequency non-destructive adaptive fastest pulse preheating for lithium-ion batteries. iScience, 29(10), Article 117715. https://doi.org/10.1016/j.isci.2026.117715

Image Credits: AI Generated

DOI: 10.1016/j.isci.2026.117715

Keywords: lithium-ion batteries, preheating, bidirectional pulse current, lithium plating, cold climate, electric vehicles, anode potential, three-electrode cells, LFP, NCM811, fast charging, battery degradation