By increasing active material coating thickness (areal density) on electrode substrates, this technology enhances battery capacity and energy density – a crucial solution for high-capacity demands in EVs and energy storage systems.
The thick coating technology of lithium batteries literally means coating the electrode “thickly”, that is, coating a thicker layer of active material on the positive or negative current collector of the lithium battery, that is, increasing the surface density, so as to achieve high capacity and high energy density.
Existing technical challenges
1. Cracking in Wet Coating Process
Conventional wet coating shows amplified cracking, delamination and flexibility degradation with thick electrodes, critically limiting active material loading capacity.
2. Stress Gradient in Single-Layer Coating
Inconsistent drying rate: After the slurry is coated, the surface solvent (such as NMP or water) will evaporate first, causing the surface to solidify quickly and form a dense structure, while the bottom solvent evaporates more slowly. Because the evaporation rates of the top and bottom layers are inconsistent, stresses differ, resulting in a shrinkage stress gradient inside the coating (tensile stress on the surface and compressive stress on the bottom layer). During the drying process, the binder (such as PVDF or CMC) will migrate to the surface due to its low density, resulting in insufficient binder content in the bottom layer. At the same time, the binder-rich area has high elasticity, while the binder-poor area has low elasticity but increased brittleness, which further causes the stress to be dispersed unevenly.
3. Uneven porosity and inconsistent drying volatilization rates
After coating, the surface of the electrode slurry is directly exposed to the air, and the solvent evaporates first, forming a dense structure on the surface, resulting in low porosity. However, the solvent in the bottom layer evaporates slowly, and it is easy to shrink in the later stage of drying, which will form relatively large pores. Because the volatilization rates of the top and bottom layers are inconsistent, stress differences occur, tearing into some irregular pores.
Particle stratification and aggravated by weight effect: During the static or drying process of the slurry, the active materials with higher density (such as NCM and graphite) settle downward due to gravity, resulting in a tighter accumulation of particles at the bottom, forming low pores, while the upper layer has higher pores due to the relatively small amount of active materials. Lightweight conductive agents (such as carbon black) are easily suspended in the slurry, aggravating the uneven distribution of pores. At the same time, due to the fast drying rate, the porosity of the edge area is lower than that of the center area, which is more obvious than that of thin electrodes.
4.Wetting problem
The increase in electrode thickness means that the electrolyte needs a longer path to penetrate into the electrode, especially in the case of complex pore structure, the tortuosity increases, which will hinder the flow of electrolyte. In addition, the pore distribution of thick electrodes is uneven. When the pore diameter in the thick electrode is too small, the surface tension of conventional electrolyte (such as about 30 mN/m in EC/DMC system) is difficult to overcome the capillary resistance, resulting in infiltration.
5. Increased interfacial impedance
Thick electrodes are prone to poor electrolyte wetting, which reduces the effective electrochemical reaction area and increases interfacial impedance. Thick electrodes increase the diffusion distance of lithium ions inside the electrode. This results in a longer diffusion path for lithium ions, which causes concentration polarization and further affects interfacial impedance.
Summary: Although there are some technical problems with thick coatings, we can still make some optimizations, such as
- reducing cracking with dry electrodes;
- optimizing porosity with pore formers;
- solving stress with double-layer coating.
- improving wetting with special electrolyte additives
These methods xiaowei will be analyzed separately later.
