For battery materials, there is no universally best mixing system. The suitable technology depends decisively on whether dry electrode powders, pre-mixes or dry blends, or already solvent- or water-based electrode slurries are being processed. For very fine, cohesive conductive additives such as conductive carbon black and CNTs, targeted de-agglomeration is usually the critical step. For the final homogenisation of active material, conductive additive and binder, by contrast, a defined addition sequence, controlled energy input, closed process control and protection against metallic contamination are often more important than maximum mixing intensity.

Dry electrode powders

For dry cathode and anode mixtures, horizontally operating intensive mixers, ploughshare mixers with switchable size-reduction tools, and vertical or conical mixers with controlled convective circulation can be used. What matters is that the entire batch volume repeatedly reaches the active mixing zone and that agglomerates of the fine conductive additives can be broken up in a targeted manner.

Intensive mixers based on the high-shear or fluidised-bed principle are particularly suitable where conductive carbon black agglomerates must be reliably dispersed. They generate high local stress and can distribute carbon black over the surface of the active material. An industrial study on cathode slurry production, for example, describes a short dry-mixing phase at moderate tool speed to homogenise active material and binder and break up coarse conductive-carbon-black agglomerates. However, the required shear intensity must be tightly limited so that particle morphology, crystal structure and the desired surface of the active material are not adversely altered.

Ploughshare mixers with size-reduction or chopper tools are a robust option for larger batches. The ploughshares generate strong convective circulation, while choppers are only switched on when agglomerates need to be specifically broken up. This separation of base mixing and locally confined de-agglomeration is advantageous, because it avoids permanently subjecting the whole batch to high mechanical stress. For abrasive cathode materials, the plant must be designed to be low-wear and contamination-compatible, since tool or wall abrasion can generate critical metallic foreign particles.

Conical and vertical mixers with ribbon or helical tools are suitable for gentle dry mixing, premixing, and the homogenisation of sensitive active materials. They provide three-dimensional convective product movement with comparatively low mechanical stress. For strongly cohesive nanoparticles, CNTs or firm conductive-carbon-black agglomerates, however, they are not always sufficient on their own. In such cases, an upstream intensive dispersion step, a local intensive mixing stage, or a matched pre-treatment of the fine additives is required.

Continuous twin-shaft or twin-screw mixers can be useful at high throughputs, provided dosing, mixing action and residence time are precisely controlled. They offer closed process control and reduce the intermediate storage of already mixed materials. The actual homogeneity, however, strongly depends on gravimetric dosing accuracy, residence time distribution, the design of the mixing elements and the stability of the raw material feed. A twin-screw extruder is particularly relevant where the mixture is further compounded or converted into a plastic or pasty state; for pure, gentle dry mixing, it is not automatically the best choice.

Slurry and paste

For cathode and anode slurries, planetary mixers, double planetary mixers, planetary dissolvers and planetary centrifugal mixers are particularly widespread. They combine macroscopic circulation with high local shear action and can often be operated under vacuum. Vacuum reduces entrapped air and supports a low-bubble coating. Planetary mixers are regarded as an established solution for battery slurries, because they can break up conductive-carbon-black agglomerates and, in an intensive kneading phase, also process active material.

The mixing sequence has a strong influence on microstructure and electrode performance. In one study on NMC cathode slurries, the stepwise addition of powders to a binder solution, with correspondingly lower shear stress in the early process stages, led to better mechanical and electrochemical properties than a process in which all the powders were first mixed and the solvent added only afterwards. Other investigations have shown that the sequence of carbon black, binder and NMC affects the distribution of the conductive network, the binding to the active material particles, and the high-current capability. The optimal sequence must therefore be determined experimentally for each material system – for example NMC, NCA, LFP, graphite, silicon-graphite, PVDF/NMP, or water-based CMC/SBR systems.

With very fine conductive additives, an upstream dispersion step can be useful. High-shear or thin-film mixers are used to break up carbon black agglomerates before the active material is added, because planetary mixing alone does not always fully resolve nanoscale agglomerates. A concept frequently used industrially therefore combines intensive pre-dispersion of the conductive additive with subsequent planetary final homogenisation and degassing.

Contamination and safety

For all battery materials, closed, dust-tight and, where necessary, inertable systems are important. Cathode and anode powders can be moisture-sensitive, prone to oxidation, hazardous to health, abrasive, or dust-explosible. The mixer should therefore be matched to the specific material system. This includes the appropriate choice of materials, low-wear product-contact surfaces, as few product-contact seals and bearing points as possible, controlled charging and emptying, and the avoidance of unnecessary drop heights and vibration.

Metallic abrasion must be minimised especially with high-purity cathode and anode materials. Abrasion-resistant surfaces, suitable ceramic coatings, low effective rotational speeds and a constructionally controlled tool-to-wall clearance can limit the ingress of metallic foreign particles. The entire periphery – dosing, screening, conveying equipment, filters, containers, valves and maintenance tools – is likewise part of the contamination concept. Suitability cannot be derived from mixing quality alone; it must be verified through elemental analytics and particle analytics after realistic operating periods.

Selection criteria

For dry cathode mixtures with conductive carbon black and binder, a combination of controlled convective base mixing and targeted de-agglomeration is usually the most effective approach. An intensive mixer or a ploughshare mixer with a chopper can be suitable where conductive-carbon-black agglomerates need to be broken up. For sensitive or abrasion-critical dry mixtures, a gentle vertical or conical mixer with an upstream conductive-additive premix can be the better choice. The same basic logic applies to anode materials with graphite or silicon-graphite, where the sensitivity of the particles, the desired surface structure and the risk of abrasion must be examined with particular care.

For slurries, planetary mixers and planetary dissolvers are often the preferred solution. Where conductive carbon black or CNT dispersion is particularly difficult, a separate high-shear or thin-film stage can be added upstream. Continuous twin-screw processes are of interest where high throughputs, short and narrow residence time distributions, constant dosing and a consistently closed production line are required. However, they require careful process development and robust gravimetric dosing.

Mixing quality should not be assessed from a single overall sample alone. Relevant tests include the distribution of conductive additive and binder, agglomerate size, particle size distribution, rheology and sedimentation stability for slurries, residual moisture, oxygen content, metallic contamination, and, where applicable, the electrical properties of the resulting electrode. What ultimately matters is not only a homogeneous mixture, but a reproducible electrode microstructure and the resulting cell performance.