In the production of lithium-ion batteries, the mixing processes for cathode and anode materials are quality-determining and safety-relevant. Suitable mixers must achieve the required mixing quality, protect the particles as far as possible, limit the ingress of unwanted foreign substances, and be integrated into a suitable explosion protection concept.

Cathode materials such as NMC, NCA, LFP or LCO, as well as anode materials such as graphite, silicon composites or lithium titanate, differ considerably in particle size, bulk weight, abrasiveness, moisture sensitivity and electrical conductivity. The permissible contamination and the required mixing intensity must therefore be defined for each recipe and process stage. Not every battery product is automatically dust-explosible or particularly moisture-sensitive. This must be assessed on the basis of the actual material properties.

Several mixing principles can be considered for the dry mixing of active materials, conductive additives and other powder components. Conical screw mixers are particularly suitable for shear-sensitive, coated or fragile particles. Their slow mixing motion enables gentle, three-dimensional product circulation. They are especially useful where low mechanical stress and good dischargeability are the priority.

Ploughshare and intensive mixers offer high mixing intensity and short mixing times. They can be suitable for well to moderately flowing powders as well as for recipes with additives and agglomerates. Additional size-reduction tools, however, may only be used once the required energy input, possible abrasion and the effect on particle structure have been sufficiently investigated.

Twin-shaft paddle mixers can be a good solution for larger batches and high throughputs. They generate intensive convective product movement and can be used as batch mixers or, in adapted concepts, also for continuous processes. Uniform charging, a suitable discharge design and control of possible segregation are decisive here.

Container and drum mixers are particularly interesting for small batches, development quantities and frequent product changes. If the container serves simultaneously as a transport and mixing vessel, transfer steps can be reduced. This lowers open interfaces, cleaning effort and potential cross-contamination. Acoustic resonance mixers can be an alternative for small batches and special mixing tasks. Since no product-contact mixing tools rotate inside the vessel, mechanical tool abrasion can be reduced. Their suitability for throughput, container size and recipe must, however, be assessed on a case-by-case basis.

Different requirements apply to slurry production than to dry mixing. Active material, conductive additives, binder and solvent must be wetted, dispersed and, where applicable, degassed. Planetary and double-planetary mixers are suitable for highly viscous electrode pastes, especially where work is to be carried out under vacuum. Dispersers, dissolvers or rotor-stator systems can support wetting and agglomerate breakdown. For very high dispersion requirements, a downstream wet milling step may be necessary.

Contamination control begins with the selection of all product-contact materials. Stainless steel can be suitable for many applications, but is not automatically the best choice. Where particular metals or metallic particles are critical, materials, surfaces and permissible wear rates must be specifically matched to the product specification. Electropolished stainless-steel surfaces can improve cleanability and reduce build-up. Coatings, technical ceramics or polymer linings can also be suitable, but must be assessed for abrasion, particle release, chemical resistance and cleanability.

Seals, bearings and shaft feed-throughs are also important sources of contamination. Sealing systems must prevent product from entering bearing areas and prevent lubricants or abrasion from entering the mixing chamber. Depending on the process, gas-purged seals, barrier-gas systems or low-contact sealing concepts can be useful. Magnetic separators in the product stream can capture ferromagnetic foreign particles. However, they do not detect all metallic or non-metallic contaminants and therefore do not replace a preventive contamination strategy.

Good cleanability reduces cross-contamination between different cathode and anode chemistries. The suitable cleaning method depends on the product, binder, solvent and required purity. Dry cleaning, manual cleaning, WIP or CIP concepts must each be checked for suitability. For moisture-sensitive materials, wet cleaning can require additional drying and release steps. Cleaning effectiveness should be verified with a documented procedure.

The ATEX design is based on a risk assessment for the specific product and process. This assesses, among other things, dust explosibility, minimum ignition energy, maximum explosion pressure, minimum ignition temperature, electrical conductivity and, where applicable, solvent atmospheres. The interior of a mixer can be classified as Zone 20 for dust-explosible powders. Areas at charging, emptying or filters can be classified as Zone 21 or Zone 22, depending on emission behaviour. For solvents, an assessment of possible gas or vapour zones is additionally required.

An effective protection concept can combine inerting, ignition-source avoidance and constructional explosion protection. In inerting, the oxygen content is kept below the limiting oxygen concentration determined for the product, using nitrogen or argon. Pressure-resistant or vacuum-resistant designs may be required for this. A pressure-shock-resistant design, explosion pressure relief, explosion suppression or decoupling should be considered where an explosion event cannot be reliably ruled out.

All conductive plant components must be integrated into an earthing and equipotential-bonding concept. This includes the mixer, tools, piping, filters, containers and mobile packaging. Temperature, speed, vibration, bearings and seals can be monitored to detect mechanical ignition sources such as friction or overheating components at an early stage. The required temperature class follows from the actual material properties and the maximum surface temperatures that can occur.

There is no universally valid recommendation for a single mixer type. For gentle dry mixing, conical screw mixers or other slow-operating vertical systems are often suitable. For intensive dry mixing and higher throughputs, ploughshare, intensive or twin-shaft mixers can be considered. For highly viscous electrode slurries, planetary mixers, dissolvers and other dispersing systems are suitable.

The best selection results from trials with the original material. Mixing quality, particle protection, abrasion, contamination, dischargeability, cleanability, energy input and ATEX requirements should be assessed together. Only in this way can the mixer be designed to match the recipe, the production scale and the safety requirements.