Top 10 Key Equipment for Lithium Batteries: Lithium Battery Drying Oven

01 Introduction to Lithium Battery Drying Oven

Drying is a chemical unit operation that provides energy to a wet material to vaporize and remove its contained moisture, thereby obtaining a dry material. Currently, there are numerous industrial drying equipment with different classification methods, as shown in Figure 1. Based on the operation mode, drying equipment can be classified as continuous drying equipment or batch drying equipment. Based on operating pressure, it can be classified as atmospheric drying equipment or vacuum drying equipment. Based on heat transfer method, it can be classified into conductive drying equipment, convective drying equipment, radiative drying equipment, and dielectric drying equipment, among others.

Figure 1 Different classifications of drying equipment

Moisture in batteries mainly comes from moisture contained in raw materials (including anode and cathode sheets, separators, electrolyte, and other metal components) as well as moisture from the factory environment. For environmental moisture, a dry room can be established, where a dryer generates dry air that is continuously supplied to the dry room to displace humid air, thereby removing environmental moisture. For internal moisture within the battery, the drying standard is very high, typically requiring a moisture content between 100 and 300 ppm (parts per million). Therefore, vacuum drying equipment is generally used for dehumidification. After drying, the battery is tested to verify whether it has been baked to an acceptable level. Several processes in battery manufacturing require vacuum drying, such as drying of cathode/anode powders, drying of electrode rolls, and drying of cells before electrolyte filling. Hence, vacuum drying equipment is critical for battery production.

02 Principle of Battery vacuum drying box

The basic principle of vacuum drying is that the boiling point of water varies under different atmospheric pressures. The variation pattern is shown in Figure 2.

Figure 2: The boiling point of water as a function of atmospheric pressure curve

As can be seen from Figure 2, under atmospheric pressure (i.e., one standard atmosphere), the boiling point of water is 100°C. However, as the pressure decreases, the boiling point of water also continuously drops. Under a vacuum environment of around 100 Pa, the boiling point of water has already decreased to approximately -20°C. This is the fundamental principle by which a vacuum environment promotes the drying process.

Therefore, vacuum drying is the process of removing moisture from a material under environmental conditions below one standard atmosphere. The basic kinetic principle of vacuum drying is the theory of heat and mass transfer. During vacuum drying, while the vacuum system evacuates the chamber, the material being dried is continuously heated. This causes moisture inside the material to diffuse to the surface through pressure or concentration differences. Water molecules on the surface of the material gain sufficient kinetic energy to overcome intermolecular attractive forces, escape into the low‑pressure space of the vacuum chamber, and are then removed by the vacuum pump.

The rate at which moisture is removed during vacuum drying – i.e., the mass of water vaporized and expelled from the dry material per unit drying area per unit time – is defined as the drying speed:

However, in battery drying, the focus is generally on moisture content rather than the absolute mass of moisture. Moreover, the moisture content in batteries is extremely small, making the mass of evaporated water difficult to measure. Therefore, it is considered to use the change in moisture content instead of the change in moisture mass per unit area. The above equation can then be transformed as follows:

Where:
v = drying speed, mg/(kg·h)
m = mass of expelled water, mg
M = total mass of material, kg
C = moisture content, mg/kg
t = drying time, h

A typical vacuum drying speed curve is shown in Figure 3. The moisture removal process is divided into three stages: the acceleration drying stage, the constant‑speed drying stage, and the deceleration drying stage.

Segment AB is the acceleration drying stage. At this point, the moisture content inside the material is constant. Due to vacuum evacuation and heating, the material is heated to the vaporization temperature at the corresponding pressure within the allowable temperature range, causing substantial vaporization, and the drying speed continuously increases.

Due to the limitations of heat and mass transfer characteristics, the drying speed reaches a maximum value and enters segment BC – the constant‑speed drying stage. During this stage, the material temperature remains constant. The heat supplied is used for latent heat of vaporization and various heat losses. The vaporized steam is continuously removed, maintaining a pressure difference between the evaporation surface and the chamber space, allowing drying to proceed continuously.

When the moisture content of the material decreases to a certain level, the amount of evaporated moisture decreases, and the pressure difference between the evaporation surface and the chamber space diminishes. The process then enters segment CD – the deceleration drying stage, during which the drying speed gradually decreases and approaches zero.

Figure 3 Typical Vacuum Drying Rate Curve

Many factors influence the drying speed during vacuum drying. First, the physical properties of the material being dried – such as its shape, size, stacking method, moisture content, and density – affect the drying speed. Second, the operating vacuum level of the drying equipment influences the drying speed. A higher vacuum allows moisture to vaporize at a lower temperature, but excessive vacuum can hinder heat transfer and reduce heating efficiency. Finally, the structural design of the drying equipment, the heating method, and the drying process parameters all affect the drying speed. Consequently, calculating drying time and drying speed is quite challenging.

Currently, measuring moisture variation during battery drying is difficult, so experimental research in this area remains limited. Nevertheless, studying the mechanism of moisture evaporation in batteries is of great significance for guiding battery drying processes. Guan Yuming et al. used computational fluid dynamics (CFD) simulation software to analyze the moisture evaporation rate inside a cell. By loading a compiled language subroutine based on Fick’s law to calculate moisture diffusion within the cell, the simulation revealed that the moisture evaporation rate on the cell surface is highest after about 10 minutes of baking, whereas the evaporation rate at the bottom of the cell is very low initially and reaches its peak at around 50 minutes, as shown in Figure 4.

Figure 4: Graph of the internal water content of the battery cell changing over time

03 Battery Vacuum Drying Process

In the lithium battery production process, several basic steps are required: mixing, coating, calendering, slitting, winding/stacking, welding, sealing, electrolyte filling, formation, grading, and assembly, as shown in Figure 5. To control the moisture content of the final product, moisture control points must be established at key stages throughout the production process. Typical designs include moisture control for cathode powder materials, moisture control for electrode rolls/sheets, and the most critical moisture control for cells before electrolyte filling.

Figure 5 Battery Production Process Flow

In the lithium‑ion battery production process, cathode and anode powder materials generally require moisture control before mixing, which is done simultaneously during the final stage of powder manufacturing. During the mixing process, the anode slurry is typically water‑based, while the cathode slurry is oil‑based. After the slurry is coated, a preliminary drying step is performed. The main purpose of this step is to remove the solvent from the slurry, forming a battery electrode with a micro‑porous structure. After this drying step, a significant amount of residual moisture remains in the electrode. There are then two main drying processes to remove this residual moisture:

a. Before cell winding or stacking, the battery electrodes undergo vacuum drying, typically at a temperature of 120–150°C. The electrodes are often dried in rolls or stacks.

b. Before electrolyte filling, the assembled cells undergo vacuum drying. Since the cell now contains components such as the separator, the drying temperature is generally 60–90°C.

The selection of drying temperature is not arbitrary; it is related to the forms in which moisture exists within the solid materials before electrolyte filling. Based on the nature and strength of the interaction between solid material molecules and water molecules, there are three main forms of moisture, as shown in Figure 6:

1. Surface moisture – Water simply and mechanically adheres to the surface of the material.

2. Adsorbed moisture – Water is bound to the solid material through physical or chemical adsorption.

3. Chemically combined water – Water is bound to the material in the form of crystalline hydrates.

For surface moisture, it can naturally evaporate at room temperature and atmospheric pressure. For adsorbed moisture, evaporation typically occurs at around 105°C under atmospheric pressure. For chemically combined water, evaporation under atmospheric pressure generally requires temperatures above 150°C. Under vacuum conditions, the temperature required for moisture removal can be significantly reduced.

Higher temperatures yield better moisture removal, but the temperature cannot be set too high because many of the components that make up the lithium battery separator are polymer materials, such as high‑density polyethylene and high‑density polypropylene. These polymer materials can degrade at excessively high temperatures, causing serious safety issues. Therefore, properly setting the drying temperature for lithium batteries is an extremely important issue and must be appropriately adjusted according to the specific material system.

The battery drying process generally consists of three stages: preheating, vacuum drying, and cooling. Because heat transfer is relatively slow during the vacuum stage, preheating is typically performed under atmospheric pressure or at a higher pressure first. Once the battery reaches a certain temperature, a vacuum is applied to remove moisture. After drying is complete, the battery is cooled to room temperature to prevent oxidation of the battery materials. The dried battery should be kept away from contact with the atmospheric environment as much as possible. Process parameters such as temperature, vacuum level, preheating time, and vacuum holding time have a significant impact on the drying results. Selecting appropriate process parameters helps improve drying efficiency.

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