Keywords: Powder rheology, Lithium-Ion Batteries, Dry powder coating, NMC, Cathode
RH154
Abstract
Dry powder coating is gaining attention for optimizing lithium-ion battery electrode manufacturing. This solvent-free method reduces volatile organic compound emissions and lowers processing costs compared to traditional slurry methods. Dry powder mixing and coating involves distinct flow conditions, making powder rheology an essential tool for evaluating flowability, cohesion and compressibility under different processing conditions. Additionally, understanding the impact of conductive network material and polymer binder content is crucial for optimizing the coating process. Thus, unconfined and confined flow energy, shear-dependency, cohesion, unconfined yield strength, flow functions, wall friction, and compressibility were assessed for NMC-based cathode powders. 96 wt% of lithium nickel manganese cobalt (NMC) were combined with different ratios of carbon black (CB) and polyvinylidene fluoride (PVDF) to explore the impact of formulation on powder behavior. The results indicate that the ratio of CB:PVDF impacts cohesion, compression, and flow energy: higher compression, cohesion, and unconfined yield strength were measured in samples with higher CB:PVDF ratios. Temperature‑controlled powder rheology enables evaluation of how temperature influences processing conditions, particularly during powder handling, mixing, calendaring, and hot pressing.
The transition toward sustainable lithium-ion battery (LIB) manufacturing has intensified interest in solvent-free electrode fabrication methods, particularly dry powder coating. Unlike conventional slurry-based techniques, which rely on toxic solvents such as N-methyl-2-pyrrolidone (NMP), dry coating eliminates solvent use, thereby reducing environmental impact, energy consumption, and production costs [1, 2, 3]. Techniques such as extrusion, binder fibrillation, and roll-to-roll dry coating have demonstrated scalability and compatibility with high-energydensity battery designs, including thick and dense electrodes for next-generation systems [4, 5].
However, the success of dry coating processes hinges on a deep understanding of powder behavior under varying shear and compressive conditions. Powder rheology has emerged as a critical tool for characterizing flowability, cohesion, compressibility, and unconfined yield strength – properties that directly influence mixing, transport, and coating performance [6, 7]. Cathode formulations based on lithium nickel manganese cobalt oxide (NMC) are sensitive to the ratios of conductive carbon black (CB) and polymer binder polyvinylidene fluoride (PVDF). CB contributes to the electrical network, while PVDF provides mechanical integrity and adhesion. Their interplay affects not only the electrochemical performance but also the rheological response of the powder during processing.
Understanding these rheological dynamics is essential for optimizing dry coating parameters and achieving uniform, defectfree electrodes. The integration of rheological characterization into formulation development and process control offers a pathway to more efficient, scalable, and environmentally responsible battery manufacturing.
Three powder formulations were prepared with fixed NMC content (96 wt%) and variable CB and PVDF ratios:
- Sample 3-1: 3 wt% CB, 1 wt% PVDF (Highest CB content)
- Sample 2-2: 2 wt% CB, 2 wt% PVDF
- Sample 1-3: 1 wt% CB, 3 wt% PVDF
The TA Instruments™ Discovery™ Hybrid Rheometer series offers a comprehensive range of powder rheology tests on a single platform while still being easy to use and integrate into existing workflows. Figures 1 and 2 show the Powder Rheology Accessory and HR 30 rheometer.
Powder Compressibility with Temperature Control
The powder compressibility test evaluates compressibility using a flat upper plate and the flow cup from the Powder Rheology accessory (Figure 1A). During this test, a sequence of increasing normal stresses is applied without torque. At each stress level, the system equilibrates for a user-defined duration to allow the gap to stabilize, enabling measurement of gap change. From there, calculates compressibility based on the relationship between gap reduction and the corresponding volume change. Following the flow conditioning protocol, the initial gap is set to 30 mm. Normal stress is incrementally applied in 2 kPa steps up to 20 kPa, with each increment maintained for 60 seconds to ensure gap stabilization. Tests were conducted at room temperature and 80 °C.
Powder Shear Cell with Temperature Control
Shear measurements under applied consolidation stress were conducted using the Shear Cell accessory (Figure 1B), featuring a serrated upper plate and cup to minimize slippage. Powder samples were introduced using the supplied slide and funnel, then consolidated at pressures of 6 kPa, 9 kPa, and 15 kPa. Excess material was trimmed to create a flat, even surface.
All tests followed ASTM D7891. For each consolidation level, a pre-shear was performed at an angular velocity of 1×10−3 rad/s until the shear stress reached a steady state. The normal stress was then reduced, and shear continued until incipient yield was observed, marked by a peak in shear stress. Five test cycles were carried out, with normal stresses ranging from 12 kPa down to 3 kPa. These tests were conducted at 9 °C, 37 °C, and 80 °C.
Powder Wall Friction with Temperature Control
The powder wall friction geometry is a quick-change plate holder into which plates of different materials and surface finishes can be installed for use with powder shear cups to perform powder wall friction tests. Wall friction testing is performed by applying an axial stress to the powder bed and inducing a very slow rotational velocity until a steady-state shear peak is achieved, indicating shear strength of the powder-plate interface, as determined by friction between the powder bed and the surface of the plate.
Powder Flow Cell
The Powder Flow Cell (Figure 1C) was used to evaluate powder flowability by driving an impeller rotor through the powder bed along a helical path. Upward movement of the impeller assessed unconfined flow, while downward motion measured confined flow behavior. Prior to testing, powders were conditioned with two impeller passes, first at a tip speed of 100 mm/s, followed by 60 mm/s. After conditioning, samples were trimmed to ensure a consistent surface.
Flowability tests were then performed at tip speeds of 140, 100, 60, 40, 30, 20, and 10 mm/s to investigate flow characteristics and rate dependence. Each speed was tested across seven independent cycles using fresh samples to ensure reproducibility. During each test, normal force and torque were recorded to calculate flow energy.
Compressibility
Compressibility determines how powders compact under pressure, directly affecting electrode density and coating thickness. Figure 3 illustrates that among the samples tested, the formulation with the highest CB content exhibited approximately three times greater compressibility than the sample with the lowest CB content. This enhanced compressibility is attributed to the fine, high surface area-to-volume CB particles, which effectively fill the voids between larger NMC particles, thereby increasing packing density under applied stress. In contrast, increasing the PVDF content did not appear to improve compressibility. This is likely due to PVDF’s binder-like properties, which resist deformation and limit the material’s ability to compact under pressure. Sample 1-3 was tested twice to assess the repeatability of the data.
These findings highlight the critical role of conductive additive concentration in influencing powder bed mechanics.
As shown in Figure 4, the test was repeated at 80 °C to simulate calendaring and hot press conditions encountered during electrode manufacturing, where compressibility is critical for achieving uniform density and mechanical integrity. While this temperature may vary across different processes, it is used here as a representative example [8]. Sample 1‑3 was selected due to its higher PVDF content, which is expected to exhibit a more pronounced response under elevated temperature.
PVDF is a semi-crystalline polymer with a glass transition temperature (Tg) around 35 °C and a melting point near 170–180 °C [8]. At 80 °C, PVDF is well above its Tg but below its melting point, making it more flexible and rubbery, which can increase particle deformability. As seen in Figure 4, this enhanced softness leads to greater compressibility as PVDF particles conform more easily under pressure.
Shear Test
The shear test provides key parameters such as cohesion, unconfined yield strength (UYS), and the flow function coefficient (FFc), defined as:
which influence hopper discharge and coating spreading, ensuring consistent coating application. The formulation with the highest CB content, shown in Figure 5, exhibited twice the cohesion and UYS compared to the sample with the lowest CB content, shown in Figure 6. This suggests greater resistance to flow initiation under consolidation force, likely due to the fine, high surface area-to-volume CB particles enhancing contact points and mechanical interlocking.
Despite these differences, both samples were classified within the easy-flowing region of the flow function chart [9]. However, the high-CB sample was positioned closer to the cohesive flow region, while the low-CB sample approached the free-flowing region. This shift reflects the impact of increased cohesion on flowability, with potential implications for hopper discharge and coating uniformity in battery electrode processing.
Shear tests were performed at 37 °C and 9 °C to simulate manufacturing conditions in different geographical regions. Within this temperature range, the effect on shear flow was found to be negligible (See Figures 5 and 6).
Sample 1-3 with higher PVDF content was subjected to a shear test at 80 °C to evaluate the flow function as a critical factor during calendaring. A specific range of flow function is desirable: if it’s too low, the material resists flow, which can cause uneven calendaring or edge defects. Conversely, if the flow function is too high, the powder may over-flow or delaminate, leading to poor mechanical integrity.
In Figure 7, the flow function is observed to increase at 80 °C. At elevated temperatures, PVDF softens (well above its Tg). This reduces interparticle friction, improving overall flow. However, if PVDF becomes excessively sticky, cohesion may increase and reduce flowability. Additionally, any moisture loss, if present, can further enhance flow by eliminating capillary bridges between particles.
Table 1 compares the results of two shear cell tests in Figures 6and 7 side by side.
Table 1. Shear cell tests of 1-3 at 9 °C, 37 °C and 80 °C, 0.009 MPa
| Temperature | Cohesion (Pa) | UYS (Pa) | MPS (Pa) | FFc |
|---|---|---|---|---|
| 9 °C | 459.76 | 1533.57 | 14622.3 | 9.53 |
| 37 °C | 460.95 | 1529.68 | 14906.8 | 9.75 |
| 80 °C | 351.64 | 1192.10 | 15471 | 12.98 |
The shear flow behavior of Sample 3‑1, the sample with the highest CB and the highest compressibility, was evaluated across a range of consolidation stresses. The results are shown in Figure 8. A reduction in consolidation stress led to a lower flow function coefficient. This trend suggests that under low consolidation, the powder becomes more prone to flow obstructions such as arching or bridging. This behavior is particularly relevant for very fine, cohesive powders, where inter-particle forces—such as van der Waals, electrostatic, or capillary interactions—are strong relative to the weak gravitational forces acting on the small particles. At low consolidation stresses, inherent cohesive forces dominate, leading to the formation of low‑density aggregates. These “fluffy” structures are easily disrupted yet prone to creating stable arches or ratholes that obstruct flow. Increased air voids further diminish the effectiveness of consolidation, while adhesive interactions cause particles to preferentially adhere to one another rather than flow freely.
Flowability
The dynamic flowability of the three powder formulations was evaluated by measuring both confined and unconfined flow energy (CFE & UFE) across a range of tip speeds. This methodology simulates how powders respond to shear flow conditions relevant to some manufacturing processes, including mixing blades in slurry preparation and feeder screws in dry powder handling.
It is worth mentioning that shear tests assess a material’s resistance to deformation under relatively high consolidation stress, simulating conditions where the powder is compacted. In contrast, flow tests evaluate flowability under near-zero to relatively low stress, using unconfined and confined flow energy measurements to represent how the powder behaves in freeflowing or loosely packed conditions.
Samples 3‑1 and 2‑2 exhibited a pronounced reduction in confined flow energy with increasing tip speed, as shown in Figure 9, resembling shear‑thinning behavior commonly observed in soft materials. This effect may be partly attributed to higher tip speeds disrupting the three‑dimensional percolated network of carbon black. The trend indicates that these powders become more mobile and easier to spread under elevated shear, which could be advantageous for some coating applications requiring efficient powder distribution.
In contrast, Sample 1-3 displayed a distinct response: CFE plateaued at lower tip speeds and declined only modestly at higher speeds (Figure 10). The relative reductions in confined flow energy were 46% for Sample 3-1, 40% for Sample 2-2, and 22% for Sample 1-3.
These results suggest that the higher polymer binder content in Sample 1-3 may dampen its responsiveness to shear flow, thereby limiting its dynamic flowability enhancement. While this reduced rate-dependency could hinder powder mobility during high-speed processing, on the other side it may mitigate issues such as powder overflow, depending on the specific application requirements. Thus, the optimal flow behavior is context-dependent and should be aligned with the desired coating strategy.
As shown in Figures 9 and 10, differences in unconfined flow energy among the samples were less pronounced, reinforcing that tip speed effects are more significant under confined conditions, which more accurately reflect the constraints encountered in real-world electrode manufacturing environments.
Wall Friction
Wall friction tests measure powder slippage against calendar rolls or press plates, a critical factor for achieving uniform adhesion and smooth electrode surfaces. Low wall friction can lead to excessive slipping of the powder against the calendar rolls or press plates, reducing the material’s ability to adhere uniformly to the current collector. This may cause uneven coating thickness, edge defects, or delamination during subsequent handling. Maintaining an optimal wall friction level ensures controlled powder movement and proper bonding, which is critical for electrode integrity and performance.
Figure 11 illustrates the wall friction of the NMC-based powder Sample 2-2 and shows a clear temperature dependence. At 25 °C, PVDF remains rigid, and wall friction is primarily governed by the extremely fine and porous carbon black and NMC particles, resulting in the highest friction values. As the temperature increases to 80 °C, PVDF softens and begins to act as a compliant layer, reducing interfacial resistance and lowering wall friction. At 130 °C, near the onset of PVDF melting, the binder becomes highly deformable, further decreasing friction; however, slight tackiness may introduce localized adhesion, potentially causing a minor increase. Overall, the trend indicates that wall friction decreases with temperature due to PVDF’s thermoplastic behavior, with the most pronounced reduction occurring between 25 °C and 80 °C.
Compressibility, shear, flow, and wall friction tests are essential tools for qualifying powder formulations for LIB electrodes. By linking these rheological properties to processing steps, manufacturers can better control powder handling and coating quality, ultimately improving electrode performance and reliability.
Compressibility assesses the powder’s resistance to deformation and its ability to compact under applied pressure. Powder rheology demonstrates that carbon black’s role extends beyond enhancing electrical conductivity. It also affects the compressibility and cohesion of NMC-based cathode powders, thereby influencing packing density and surface coating quality.
Shear testing quantifies cohesion, unconfined yield strength and the flow function coefficient, providing insight into powder flowability, hopper discharge behavior, and coating uniformity during electrode processing. The flow function of the sample with the highest PVDF content approached the free‑flowing region, which may lead to uneven coating or overspray. Flow testing tracks changes in flow energy at varying tip speeds, simulating practical processes such as mixing blades in slurry preparation and feeder screws in dry powder handling.
Wall friction testing determines the wall friction angle, reflecting the potential for powder slippage against calendar rolls or press plates.
Maintaining balanced wall friction is critical for achieving uniform electrode coatings. Wall friction decreased with increasing temperature, primarily due to PVDF softening, with the most pronounced reduction occurring between 25 °C and 80 °C. An important consideration is whether, at a given temperature, the binder’s tackiness or its compliant layer behavior predominates at the interface.
Temperature‑controlled powder rheology enables evaluation of how processing temperature impacts powder behavior. At 80 °C, the formulation with the highest PVDF content exhibited increased compressibility. Under reduced humidity, PVDF softening further elevated the flow function, underscoring the importance of carefully controlling flow behavior to ensure reliable calendaring.
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Acknowledgement
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This paper was written by Behbood Abedi, PhD.
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