The coating process and innovative technologies for Lithium Batteries

In the production process of lithium batteries, slurry coating is a crucial process. Its main purpose is to uniformly coat the slurry with good stability, viscosity, and fluidity onto the positive and negative current collectors, which is of great significance for the capacity, consistency, and safety of lithium batteries. However, according to incomplete statistics, the proportion of battery failures caused by the electrode coating process among all lithium battery failures due to various reasons reaches 10% - 15%.

I. The Key Impact of the Coating Process on the Performance of Lithium Batteries

(A) Key Control Factors

1. Control of coating drying temperature: If the drying temperature is too low, the electrode sheets cannot be completely dried; if the temperature is too high, the organic solvents inside the electrode sheets will evaporate too quickly, and the surface coating is prone to phenomena such as cracking and peeling.
2. Coating areal density: If the areal density is too small, the battery capacity may not reach the nominal capacity; if the areal density is too large, it will not only cause waste of materials in the batching process, but, in severe cases, if the capacity of the positive electrode is excessive, lithium dendrites may form due to the precipitation of lithium, piercing the battery separator and causing a short circuit, resulting in potential safety hazards.
3. Coating size: If the coating size is too small or too large, it may lead to the situation where the positive electrode inside the battery cannot be completely wrapped by the negative electrode. During the charging process, lithium ions are extracted from the positive electrode and move into the electrolyte that is not completely wrapped by the negative electrode, affecting the actual capacity utilization of the positive electrode and even forming lithium dendrites that pierce the separator and cause an internal short circuit in the battery.
4. Coating thickness: If the coating thickness is too thin or too thick, it will have an impact on the subsequent electrode sheet rolling process, making it difficult to ensure the performance consistency of the battery electrode sheets.

(B) Safety Significance

It is crucial to do a good job in 5S (Sort, Set in order, Shine, Standardize, Sustain) before coating to ensure that no particles, debris, dust, etc. are mixed into the electrode sheets, otherwise it will cause a micro short circuit inside the battery, and in severe cases, lead to the battery catching fire or exploding.
II. Main Reasons for the Instability of Coating Areal Density
(A) Coating Materials

1. Slurry problems:
   (1) Uneven mixing will lead to uneven dispersion of active materials, binders, and conductive additives, resulting in inconsistent solid content at different positions of the slurry and ultimately affecting the coating areal density.
   (2) Agglomeration and sedimentation of active material particles will lead to uneven distribution of materials during the coating process.
   (3) Changes in the viscosity or composition of the slurry over time, such as solvent evaporation or reaction with air, will directly affect the coating quality.
2. Substrate problems: Variations in the surface roughness and thickness of the coating substrate will lead to uneven coatings and affect the uniformity of electrode quality.

(B) Coating Equipment

1. Old equipment will lead to a decline in the performance of the coating machine and affect the coating quality.
2. Wear or damage to mechanical components caused by insufficient equipment maintenance will also result in poor coating quality.

(C) Coating Process

1. Uneven drying will cause uneven solvent evaporation, resulting in the redistribution of materials and affecting the coating areal density.
2. Fluctuations in the coating speed will lead to inconsistent slurry deposition and differences in mass loading.
3. Wrinkling or stretching of the foil will cause more slurry to be coated in some areas and less in others, resulting in fluctuations in areal density.
4. Changes in the foil tension will affect the thickness and uniformity of the coating and, in turn, affect the stability of the areal density.
5. Blockage of the discharge port will lead to uneven coating and fluctuations in areal density.

(D) Coating Environment

1. Temperature fluctuations will affect the slurry viscosity, drying rate, and solvent evaporation, resulting in changes in the coating thickness.
2. Humidity changes will affect the drying process, especially the solvent evaporation rate, making the coating areal density uneven.

(E) Monitoring and Adjustment during the Coating Process

1. Lack of real-time monitoring makes it difficult to detect and correct coating inconsistency problems in a timely manner.
2. Interference from human factors: Manual adjustment of parameters may introduce interference from human factors, resulting in fluctuations in the coating areal density.

III. Innovation and Challenges of the Dry Electrode Process
(A) Process Characteristics

The dry electrode process is an innovation in the coating process in electrode sheet production. It uses a dry coating method to produce finished electrodes. Unlike the wet coating method, the dry electrode process mixes PTFE binder with electrode powder and forms an electrode film through an extruder, and then uses a calender for hot pressing and forming. It eliminates solvents, simplifies the process, and does not require wet solvent recovery steps and equipment. The wet method uses NMP solvent, which is toxic, environmentally unfriendly, and has a large carbon emission.
(B) Application Advantages
The production process of dry electrodes shows inclusiveness for electrode materials, can better match with high-nickel positive electrodes and silicon-based negative electrodes, and is adaptable to the solid-state battery system. It can serve as a technological reserve for the next-generation battery system.
(C) Challenges Faced
The dry coating method encounters huge challenges in practice. Taking Elon Musk's application of the first principles as an example, when the dry positive electrode process presses the positive electrode powder to a specific thickness by rolling, it is flattened like pizza dough but becomes very hard, causing the pressure on the rolling rollers to overload, resulting in position offset and even deformation. After the fibrillation of the positive electrode material is completed, it is in a viscous and flocculent state and cross-links with each other, with poor self-lubricating properties. During the continuous transmission process, it is extremely prone to phenomena such as segregation, bridging, and agglomeration. The production of self-supporting films is extremely difficult. As can be seen from Tesla's videos, when using horizontal opposed rollers to prepare electrode sheet films, phenomena such as uneven thickness, inconsistent areal density, and even film breakage and holes will occur. The locally overly thick powder material will also widen the gap between the rollers, causing the roller body to deform and become unable to be driven.
IV. Innovative Design and Advantages of the 4680 Battery

The 4680 battery adopts a full-tab (tabless) design. The core design concept is to directly connect the positive and negative current collectors with the cover plate/case, which multiplies the current conduction area and shortens the current conduction distance, thereby significantly reducing the internal resistance of the battery, reducing heat generation, prolonging the battery life, and increasing the peak power of charging and discharging. Its advantages are mainly reflected in the following aspects:
(A) Improvement in Output Power
The full-tab design broadens the current path, reduces the internal resistance, and lowers the internal losses, thus significantly increasing the battery power.
(B) Improvement in Safety
The full-tab structure improves the heat dissipation effect and enhances the thermal stability of the battery.
(C) Improvement in Fast Charging Performance
Since the current can move more easily inside the battery, the charging and discharging speed is faster.
(D) Improvement in Production Efficiency
It eliminates the process and time of adding tabs on the production line, saves equipment space, and reduces the possibility of manufacturing defects.

In conclusion, the coating process of lithium batteries is crucial for battery performance. Although the dry electrode process, as an innovative technology, has many advantages, it also faces huge challenges. The full-tab design of the 4680 battery performs excellently in improving battery performance and production efficiency. In the future, with the continuous progress of technology, it is expected that greater breakthroughs will be made in the coating process and dry electrode technology, promoting the continuous development of the lithium battery industry.