Date: 2026-05-15 hits: 105
The first stage is preheating and temperature rise. After wet slurry is coated onto the aluminum foil current collector via the die and enters the oven, high‑speed hot air rapidly heats the foil and wet coating. The overall coating temperature rises quickly, while solvent evaporation is minimal, with only slight vaporization at the surface. The core goal is to establish an initial temperature field; drying rate is dominated by convective heat transfer efficiency. Hot‑air velocity and initial temperature determine the heating rate. Excessively fast heating impairs slurry leveling and causes surface defects such as orange peel.
The second stage is the accelerated evaporation stage, also known as the “fast‑drying phase.”
During this stage, the wet slurry on the aluminum foil is continuously heated by hot air in the oven, with temperature rising close to the boiling point of the oily solvent NMP. Solvent vaporization driving force surges, and evaporation rate increases exponentially. As large amounts of solvent escape, the wet coating undergoes noticeable bulk shrinkage. Positive active material particles, previously separated by solvent, gradually draw closer together.
A key feature: solvent inside the coating remains a continuous liquid phase (not isolated droplets between particles). Internal solvent can flow rapidly to the surface through continuous liquid pathways, sustaining steady evaporation. Thus, drying speed in this stage depends only on heat input from the oven—more heat = faster evaporation.
The third stage is the constant‑rate drying stage, the core solvent‑removal phase; over 70% of NMP is removed here.
During this stage, a continuous liquid film is maintained on the coating surface. Internal solvent is continuously drawn to the surface through capillary action (like water wicking in paper). With abundant solvent always available at the surface, drying rate remains constant, determined solely by surface vaporization rate (independent of residual solvent inside the coating). Key control parameters: hot‑air temperature, air velocity, and NMP concentration in return air.
This stage is also high risk for binder migration: solvent moving upward carries dissolved PVDF binder toward the surface. Uncontrolled migration causes binder accumulation at the top surface, weakening adhesion at the foil interface, leading to electrode powder shedding and performance failure.
The fourth stage is the falling‑rate drying stage, the final critical drying phase.
When the solvent content in the coating drops to a critical threshold and scattered dry spots appear on the surface, the constant‑rate drying stage ends and this stage begins. The solvent, previously a continuous liquid phase, becomes isolated droplets trapped in the gaps between active material particles. It can no longer continuously replenish the surface via capillary action. Drying rate is now governed entirely by molecular diffusion through tortuous pores—the less solvent left, the slower the drying, with the overall rate declining steadily.
In this stage, heat transfers from the coating surface inward, while solvent molecules must navigate dense pores to evaporate. Excessively high temperature causes sudden, violent vaporization of residual internal solvent, rupturing the surface and forming defects such as pinholes and blisters. Thorough removal of residual solvent in this stage directly determines the final quality of the electrode.
Drying of cathode slurry is essentially a heat‑transfer‑driven mass‑transfer process, accompanied by irreversible formation of the coating’s microstructure. Heat transfer occurs via hot‑air convection and conduction, while mass transfer relies primarily on capillary convection, molecular diffusion, and thermal convection.
