Dry Processing vs. Solution Processing Electrodes: A Comparison

Electrode manufacturing is a crucial step in the production of lithium-ion batteries, directly impacting the performance and lifespan of battery cells. Generally, the electrode manufacturing process involves preparing electrode slurries, coating and drying, calendaring, electrode slitting, separator slitting, winding, electrolyte filling, packaging, aging, formation, sorting, and inspection, as depicted in Figure 1.

Currently, solution processing techniques dominate electrode manufacturing, extensively employed in various lithium-ion battery production processes. However, the use of toxic solvent NMP in slurry preparation is environmentally unfriendly. In some electrode preparation processes, NMP can be substituted with water. Equipment developers have also designed solvent recovery systems. Nonetheless, the subsequent drying process remains lengthy, consuming significant energy and time.

Dry processing electrode preparation directly eliminates the coating and drying steps. Let's delve into the dry electrode manufacturing process.

1. Dry Mixing

The primary method involves mechanical and physical means to uniformly mix active materials, conductive agents, binders, etc., while avoiding clumping during dry mixing to ensure uniform mixing. Advanced techniques include dual-blade grinding or advanced gas-assisted mixing technology, both aiding in uniform particle mixing and preventing excessive clumping. Dry mixing is a key technology in solid-state battery production, determining battery electrode quality and production efficiency.

2. Dry Coating

There are several categories, with the first being Maxwell dry electrodes. Patents in this layout include dry mixing, dry fiber-making, dry feeding, dry compaction/rolling, and bonding, processes compatible with roll-to-roll production lines. In the mixing process, a dough mixture is formed by shearing the binder, followed by dry fiberization of the dough mixture, then thermal rolling/compaction to form a film. In cases requiring high binder demand, Maxwell employs PTFE with a molecular weight of 106 - 107 g/mol, processing at 80°C, and adds rolling processes to control electrode thickness and effectively reduce porosity. The second type is dry mix spraying. After dry mixing, the mixture is made into flowable particles, which are then sprayed onto the current collector to form a film.

The second category involves electrostatic spray coating technology, aiding in improving adhesion and manufacturing efficiency. Additionally, by using special adhesives, adhesion can be enhanced through UV curing. The main challenge of dry spraying is controlling loading, thickness, and uniformity. The third type is hot pressing and melt extrusion, used in the production of all solid-state polymer electrolyte (SPE) batteries. The third category is 3D printing. Electrodes based on thermoplastic polymer SSEs can be manufactured using 3D printing technology, but for inorganic materials, it is currently immature, with uncertain commercial prospects. Currently, Maxwell's dry electrode process is the most commercially viable.

Mechanism of Dry Electrode Binding

The microstructure and binding mechanism of dry electrodes differ from traditional liquid cells. The microstructure of the electrode is influenced by the distribution of the binder. Additionally, the type and distribution of the binder also affect the electrode's strength and battery performance. This distribution is largely influenced by the surface energy of solid particles. Studies show that PTFE is compatible with most cathode and anode materials, while PVDF has poor compatibility with solid electrolytes (such as LLZO). Currently, PTFE is the most widely used binder in dry electrodes. Compared with oxide solid electrolytes, sulfide electrolytes are easier to contact with active materials.

Advantages and Challenges of Dry Electrodes

Advantages:

• Cost advantage: According to laboratory statistics, considering the total material cost and battery manufacturing process, spraying, drying, and solvents in the electrode manufacturing process account for over 48% of the electrode manufacturing cost. Therefore, the entire electrode manufacturing, spraying, drying, and solvent recovery steps account for over 19.56% of the total cost, especially drying and solvent recovery, which not only require increased investment in solvent recovery equipment but also consume enormous energy. Using dry electrodes can effectively reduce electrode manufacturing costs, estimated to decrease by approximately 10% to 15%.
• Improved electrode performance: Reduced electrode porosity effectively increases electrode thickness, thereby improving battery energy density, increasing electrode strength, and improving electrode rate performance by reducing inactive components.

Challenges:

• Firstly, the working mechanism of dry electrode binders in different processes has not been fully studied, and research on binder selection is also accelerating. Different processes, especially different binders' compatibility with different materials, should be fully studied and different binders' compatibility with different materials should be fully demonstrated.
• Secondly, the changes of dry electrodes in long-term cycling processes have not been fully studied. It is necessary to fully understand the evolution of electrodes prepared by dry electrode preparation during the cycling process.
• Thirdly, the production efficiency of large-scale dry electrodes needs to be discussed. Currently, the uniformity and film-forming properties of large batches of dry electrodes still need continuous improvement.

Conclusion

In summary, the dry electrode preparation process is suitable for the preparation of all solid-state battery electrodes, avoiding compatibility issues between solvents and solid electrolytes in traditional battery electrode preparation processes. However, at the current stage, dry electrodes cannot fully meet the requirements of battery electrode preparation. In the application of all solid-state batteries, the feasibility of technology cannot be fully verified due to the influence of the current industrial scale of all solid-state batteries. For dry electrodes, depending on the development of the all solid-state battery industry, the outbreak of the industry still requires some time.

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