Effect of Dew Point on Lithium Battery Workshop and Dehumidification Treatment
What is Dew Point?
Dew point is the temperature at which moisture condenses.
In a constant air pressure and with the water vapor content unchanged, the temperature at which air becomes saturated is referred to as the dew point temperature (Td), simply known as dew point. It can also be understood as the temperature at which water vapor and water reach an equilibrium state.
The difference between the actual temperature (t) and the dew point temperature (Td) indicates the degree of air saturation.
When t > Td, it indicates unsaturated air.
When t = Td, it is saturated,
and when t < Td, it is oversaturated.
Relative Sizing | Water Vapor Content in the Air |
Environmental Temperature > Dew Point Temperature | Under-saturated |
Environmental Temperature = Dew Point Temperature | Saturated |
Environmental Temperature < Dew Point Temperature | Over-saturated |
Generally, temperatures above 0°C are referred to as the ‘dew point,’ and temperatures below 0°C are referred to as the ‘frost point’.
Why monitor the dew point in the lithium battery manufacturing environment?
Lithium-ion batteries have very high humidity requirements during production, mainly because uncontrolled moisture or coarsening control can have an adverse effect on the electrolyte.
The electrolyte is the carrier for ion transport in the battery, composed of lithium salt and organic solvents.
And it also ensures the advantages of high voltage and high specific energy in lithium-ion batteries. Excessive moisture can have an adverse effect on the electrolyte:
① Electrolyte Degradation
The electrolyte is the carrier for ion transport in the battery, composed of lithium salt and organic solvents.
The electrolyte plays a role in conducting ions between the positive and negative electrodes of lithium-ion batteries, ensuring the advantages of high voltage and high specific energy.
When injecting the battery, it must be done in an environment with less than 1% humidity. And after injection, it should be sealed quickly to prevent contact between the internal battery and the air.
If the moisture content is too high, the electrolyte reacts with water to produce trace amounts of harmful gases, which have an adverse effect on the injection room environment.
This can also affect the quality of the electrolyte itself, leading to poor battery performance.
② Reduced Battery Capacity
The first discharge capacity of the battery decreases with an increase in the moisture content inside the battery.
Excessive moisture can damage the effective components of the electrolyte and deplete lithium ions, resulting in irreversible chemical reactions at the negative electrode of the battery.
The consumption of lithium ions reduces the energy of the battery.
③ Increase in Internal Resistance
With the increase in moisture within the battery, the internal resistance tends to rise.
In the process of battery usage, low internal resistance allows for high-current discharge, resulting in high battery power.
If the internal resistance is high, high-current discharge is not possible, and the battery’s power output is comparatively lower.
Excessive moisture can affect the quality of the Solid Electrolyte Interphase (SEI) film in lithium-ion batteries, which, in turn, affects the battery’s internal resistance.
④ Excessive Internal Battery Pressure
Moisture reacts with LiPF6 in the electrolyte to generate harmful gases.
When there is too much moisture, the internal pressure within the battery increases, leading to mechanical deformation.
In the case of phone batteries, this can result in a bulging battery casing.
If the internal pressure becomes too high, it poses a danger to the battery, as it may explode, causing electrolyte splatter and potentially injuring individuals.
⑤ Battery Leakage
The reaction between LiPF6 in the electrolyte and moisture not only generates gas but also produces hydrofluoric acid, which is a highly corrosive acid. It can corrode the metal components inside the battery, leading to eventual leakage.
If the battery leaks, its performance rapidly deteriorates, and the electrolyte can also corrode the user’s device.
Electrolyte and positive and negative electrode materials are highly sensitive to moisture.
To ensure battery quality, it is essential to strictly control the moisture levels in the workshop and glovebox, especially during critical processes such as cell baking, injection, and sealing. These processes must be conducted in a low-humidity environment, typically below 1%, to prevent moisture from entering the electrolyte.
To achieve this, variations in dew point temperature values are used to reflect humidity fluctuations.
Dew point temperatures generally need to be controlled below -45°C or even drier.
Using the dew point to indicate water vapor concentration at this level is due to the corresponding relative humidity values being less than 1%.
For most instruments used to measure relative humidity, even if they can convert and display values as dew point temperatures, they lack the required resolution and accuracy for meaningful measurements at this level.
For example, at a dew point temperature of -50°C, raising it by 5°C to reach -45°C represents a mere 0.1% change in relative humidity – a value that is challenging to distinguish from other interfering information.
Dew Point Measurement Principle and Methods
Common solutions for dew point measurement include chilled mirror hygrometers, aluminum or silicon sensors, and polymer moisture sensors. Each solution has its own advantages and disadvantages.
Chilled mirror hygrometers use optical reflection to detect the condensation temperature on a reflective surface (mirror). These devices are highly precise under laboratory conditions. However, they can be affected by measurement errors called the Raoult effect when the sampled gas contains a solvent that will condense on the mirror. Strong acids or bases can also damage the mirror.
Aluminum and silicon oxide sensors can measure extremely low dew point temperatures but are susceptible to damage and contamination from environmental acids and bases.
Polymer sensors are relatively resistant to various chemical pollutants.
Currently, in the context of lithium battery environmental monitoring, polymer thin film sensors are the primary means for dew point monitoring. Laser-based sensors are the future direction of development.
Common Dew Point Testing Methods
NO. | Methods | Principles |
1 | Mirror-Based Dew Point Meter | Different gas with varying moisture content condenses on a mirror at different temperatures. Using optoelectronic detection technology, the dew layer is detected, and the temperature at the time of condensation is measured, directly displaying the dew point. |
2 | Capacitive Sensor Dew Point Meter | Using hydrophilic or hydrophobic materials as the medium, these constitute the dew point meter’s capacitance or resistance. After moist air passes through, the dielectric constant or conductivity changes correspondingly. The gas’s moisture content is indirectly determined based on the measured capacitance or resistance values. |
3 | Electrolytic Dew Point Meter | Using materials like phosphorus pentoxide, which absorb moisture and decompose into polar molecules, accumulating charge on the electrodes. This design establishes an electrolytic method dew point meter based on a good moisture unit. |
4 | Crystal Oscillation Dew Point Meter | Utilizing the characteristic of a wet crystal changing its oscillation frequency, a crystal oscillation dew point meter can be designed. |
5 | Infrared Dew Point Meter | Exploiting the property of water in gas to absorb infrared spectrum, an infrared dew point meter can be designed. |
6 | Semiconductor Sensor Dew Point Meter | Every water molecule has its natural vibrational frequency. When it enters the gaps of a semiconductor lattice, it resonates with the lattice that has received excitation. Its resonance frequency is directly proportional to the number of water molecules, and this resonance effect causes the semiconductor to release free electrons, thereby increasing the lattice’s electrical conductivity and reducing impedance. |
Selection and Determination of Dehumidification Methods for Lithium Battery Production Workshops
Current air dehumidification methods include cooling dehumidification, liquid desiccant dehumidification, solid desiccant dehumidification, and membrane dehumidification.
① Cooling Dehumidification
Cool the air to below the dew point and then remove the condensed moisture. It is effective in situations where the dew point is above 10°C.
② Compression Dehumidification
Compress and cool humid air to separate its moisture. It is effective in low airflow situations but not suitable for high airflow.
③ Solid Adsorption Dehumidification
Use capillary action to adsorb moisture onto solid desiccants. It can lower the dew point, but equipment size increases with a larger adsorption area.
④ Liquid Absorption Dehumidification
Utilize lithium nitrate water solution to spray and absorb moisture. It can lower the dew point to around 0°C but requires larger equipment and periodic absorption liquid replacement.
⑤ Adsorption Wheel Dehumidification
Perforated plates soaked with desiccants are processed into honeycomb-like rotating wheels, which are ventilated. This dehumidification method has a simple structure and, with special assembly, can achieve a dew point of below -70°C.
To meet the humidity requirements of lithium battery production workshops, a two-stage adsorption wheel dehumidifier is necessary.
Considering dew point control in different areas on-site, two dehumidifiers are used to meet the requirements. One dehumidifier is used for controlling the -30°C dew point area, while the other is used to control the -45°C and -60°C dew point areas.
Fresh air is pre-cooled before entering the first-stage wheel treatment area. Moisture in the air is absorbed by the desiccant inside the rotating wheel. The dried air then passes through a middle-stage cooler and enters the second-stage wheel. After another round of drying, the air passes through a rear cooler (with optional heating) to reach the required temperature before being supplied to the workspace. This cycle continues to meet the requirements of humidity and temperature control.
* Functions of Each Section:
The rotating wheel is responsible for adjusting the humidity of the mixed air to the extreme working humidity required, transforming dry air into the necessary humidity level. The rear cooler or rear heating section is then used to adjust the dry air temperature to the required temperature point in the workspace.
The rotating wheel absorbs water molecules from the air within the treatment area, becoming saturated.
It will automatically rotate to the regeneration area for high-temperature regeneration treatment to restore the dehumidification capacity.
Lithium Battery Production Workshop Dehumidification Air Conditioning System
(1) Classification of Dehumidification Air Conditioning Systems for Lithium Battery Production Workshops
① Normal Humidity
Air-conditioned rooms with humidity control requirements of ≤60%, such as raw material storage rooms, earthenware material rooms, copper and aluminum foil unpacking areas, etc. This requirement can be achieved through cooling dehumidification.
② Low Humidity Requirement
Air-conditioned rooms with humidity control requirements of ≤20%, such as sampling rooms, earthenware slurry rooms, positive electrode die-cutting rooms, etc., as well as rooms like negative electrode roll-cutting rooms, negative electrode laser die-cutting rooms with humidity control requirements of ≤45%. Achieving this requirement through cooling dehumidification is difficult, so it is often met by processing with a rotary dehumidification unit, which includes a first-stage rotary dehumidification section.
③ Low Dew Point Humidity
Air-conditioned rooms with humidity control requirements of dew point ≤-30°C, or even lower, such as battery disassembly rooms, drying rooms, and injection rooms.
The existing first-stage rotary dehumidification units have difficulty meeting the process requirements. Therefore, in the engineering setup, two-stage rotary dehumidification sections are typically used to treat the room air and meet the usage requirements.
(2) Moisture Control in Lithium Battery Production Workshops and Its Characteristics
① Efficient Dehumidification
Lithium battery production workshops need to maintain relative humidity between 30% and 50%.
Therefore, it is necessary to choose efficient dehumidification equipment, such as desiccant dehumidifiers and refrigeration dehumidifiers.
② Precise Control
The dehumidification air conditioning system should have precise humidity control capabilities.
It should be able to automatically adjust the dehumidification level based on changes in humidity within the workshop to maintain a constant level of humidity.
③ High-Efficiency Filtration
Lithium battery production workshops need to maintain air quality, so it is essential to select high-efficiency air filters that can remove fine particles and harmful gases.
④ Energy Efficiency and Environmental Friendliness
Dehumidification air conditioning systems should be characterized by energy efficiency and environmental friendliness.
They should meet production requirements while minimizing energy consumption and their impact on the environment.
⑤ Safety and Reliability
Dehumidification air conditioning systems should have safety and reliability features.
They should automatically monitor parameters such as temperature and humidity within the workshop, promptly detect and address any anomalies, ensuring a safe and stable production environment.
(3) Special Requirements for Dehumidification Air Conditioning Systems in Lithium Battery Production Workshops
① Air Duct Material Requirements
In lithium battery production workshops, the supply and return air ducts of the low dew point air conditioning system often use stainless steel plates with a thickness of no less than 1.0 mm, welded with argon arc welding.
Since the air humidity in the supply and return ducts of the low dew point air conditioning system is extremely low, this approach is taken to avoid secondary contamination of the air inside the ducts by external air.
This minimizes air duct leakage rates, ensuring that the leakage rate is not greater than 0.5% or even lower.
② Airflow Organization in High-Temperature Formation Rooms
Formation is a critical process in lithium battery production.
During formation, a passivation layer is formed on the surface of the battery’s negative electrode, known as the Solid Electrolyte Interface (SEI) film.
The quality of the SEI film directly affects the electrochemical performance of the battery, including its cycle life, stability, self-discharge rate, safety, and more.
The process requirements for high-temperature formation rooms typically include a ceiling height of over 4 meters, a temperature requirement of 45 ± 3°C, and a humidity requirement with a dew point of ≤ -30°C.
Conventional air conditioning system designs rarely encounter the high-temperature requirement of 45±3°C.
Furthermore, high-temperature formation rooms occupy a very small area within lithium battery production workshops and are often overlooked in the design.
Organizing the airflow with a downward-to-upward or upward-to-downward pattern should be closely integrated with the layout of process equipment, which requires a considerable amount of design effort.
As a result, the airflow organization in high-temperature formation rooms in lithium battery production workshops typically follows a conventional pattern, such as upward supply and upward return or upward supply and downward return.
However, these arrangements do not consider the physical phenomenon of hot air naturally rising, leading to highly uneven room temperature distribution with significant temperature gradients (up to 10°C or more), excessively high temperatures near the room’s ceiling (>60°C), and safety risks associated with lighting fixtures and nozzles exposed to prolonged high-temperature conditions. These issues significantly impact the production process.
(4) Low Dew Point Air Conditioning System Design
Currently, in most of the operating lithium battery production workshops, there is a problem of low temperatures during transitional seasons (<18°C). Low room temperatures can lead to issues such as the condensation and crystallization of the electrolyte. Therefore, it is essential to prevent the occurrence of low room temperatures.
The situation mentioned above arises because the air temperature, after undergoing secondary wheel processing, remains around 15-16°C. Additionally, during transitional seasons, the outdoor air temperature is relatively low, resulting in low room cooling load, which is insufficient to raise the room temperature above 18°C. Meanwhile, the concentrated heat sources are not in use during this time.
Hence, to address the issue of low room temperatures (<18°C) during transitional seasons and to ensure strict compliance with the process requirements in practical use, electrical heating measures should be added to the end of the low dew point air conditioning unit.
Analysis of Workshop Dew Point Influencing Factors
During the battery production process, there are many factors that affect the environmental dew point, as mentioned in the previous design proposal. These factors can be summarized and categorized into the following aspects:
(1) Dehumidification Units
Dehumidification units are the core factors influencing the environmental dew point.
To ensure they meet usage requirements, it is essential to perform regular maintenance and cleaning of the equipment’s primary and secondary filters. Setting a reasonable humidity level, preventing frequent starts and stops of the compressor, and avoiding extended periods of heating operation are all measures that maximize the effectiveness of dehumidification units.
(2) Cleanroom
The installation of air ducts is crucial. Proper room and pipeline sealing is essential to ensure effective dehumidification.
(3) Operators
The number of operators directly affects the room’s dew point. It is important to minimize personnel entry, and those entering the cleanroom must wear protective clothing.
(4) External Environment
Changes in the external environment can directly increase the load on the dehumidification unit. Whenever possible, connect the dehumidification unit’s fresh air intake to areas with temperature control to reduce fluctuations caused by external environmental changes.
(5) On-Site Management
Minimize the frequency of opening and closing doors and, at the same time, reduce the introduction of other sources of humidity into the cleanroom.
This helps maintain stable room dew points.
Due to the extremely stringent requirements of the battery production environment, any deviation in design or construction can lead to incalculable losses for the entire system.
Furthermore, considering energy efficiency, reducing equipment dimensions, and lowering costs, it is essential to take into account the overall production process on-site to strike a balance between these factors and provide valuable assistance for the subsequent production line optimization.
The conversion between relative humidity and dew point temperature can refer to their respective tables, but the lookup process can be cumbersome. Below, we will introduce two methods for quickly converting relative humidity and dew point temperature.
① The Antoine equation is the simplest three-parameter vapor pressure equation, derived from engineering experience, with its general form as follows:
log P = A – B / (t + C)
Where P is in units of mmHg (1 kPa = 7.5 mmHg), t is in degrees Celsius, and A, B, and C are Antoine constants. Antoine constants for different temperatures are as follows:
Temperature Range | A | B | C |
0 – 60℃ | 8.10765 | 1750.286 | 235 |
60 -150℃ | 7.96681 | 1668.21 | 226 |
Here is the table of relative humidity and dew point temperatures calculated based on the Antoine equation at different temperatures:
Dew Point Temperature | Environmental 15°C | Environmental 25℃ | Environmental 35℃ |
0℃ | 35.7% | 19.2% | 10.8% |
5℃ | 51.1% | 27.5% | 15.5% |
10℃ | 72.0% | 38.7% | 21.8% |
15℃ | 100.0% | 53.8% | 30.3% |
20℃ | / | 73.8% | 41.6% |
25℃ | / | 100.0% | 56.3% |
30℃ | / | / | 75.5% |
35℃ | / | / | / |
② Origin Fit Formula.
The Origin Fit Formula is a polynomial fit to the relationship between relative humidity and dew point temperature. It does not have any physical meaning itself. Its general form is as follows: 1gP = A + B1 t + B2t2 + B3t3 +B4t4
Where P is in mmHg (1Kpa=7.5mmHg), t is in °C, and B1, B2, B3, B4 are fitting constants. Different fitting constants at different temperatures:
Temperature Range | A | B1 | B2 | B3 | B4 |
-75 – 0℃ | 0.66288 | 0.03417 | -1.09706 × 10-4 | 8.75923 × 10-7 | 0 |
0 – 200℃ | 0.66294 | 0.03128 | -1.19235 × 10-4 | 3.24888 × 10-7 | -4.27065 × 10-10 |
210 – 273 0℃ | 1.26044 | 0.02022 | -3.69148 × 10-4 | 2.98082 × 10-7 | 0 |
Relative Humidity and Dew Point Reference Table Calculated Based on the Origin Fit Formula at Different Temperatures:
Dew Point Temperature | Environmental 15℃ | Environmental 25℃ | Environmental 35℃ |
-70℃ | 0.02% | 0.01% | 0.01% |
-65℃ | 0.03% | 0.02% | 0.01% |
-60℃ | 0.06% | 0.03% | 0.02% |
-55℃ | 0.1% | 0.07% | 0.04% |
-50℃ | 0.2% | 0.1% | 0.07% |
-45℃ | 0.4% | 0.2% | 0.1% |
-40℃ | 0.8% | 0.4% | 0.2% |
-35℃ | 1.3% | 0.7% | 0.4% |
-30℃ | 2.2% | 1.2% | 0.7% |
-25℃ | 3.7% | 2.0% | 1.1% |
-20℃ | 6.1% | 3.3% | 1.8% |
-15℃ | 9.7% | 5.2% | 2.9% |
-10℃ | 15.2% | 8.2% | 4.6% |
-5℃ | 23.6% | 12.7% | 7.1% |
0℃ | 36.0% | 19.4% | 10.9% |
5℃ | 51.3% | 27.6% | 15.5% |
10℃ | 72.1% | 38.8% | 21.8% |
15℃ | 100.0% | 52.8% | 30.3% |
20℃ | / | 73.8% | 41.5% |
25℃ | / | 100.0% | 56.3% |
30℃ | / | / | 75.4% |
35℃ | / | / | 100.0% |
Lithium Battery Manufacturing Process Relative Humidity Control Standards
Finally, we would like to share the humidity control standards for various manufacturing processes in lithium battery production.
It can be observed that different manufacturers have slightly different control standards.
The general principle is that lower humidity control yields better performance for lithium batteries, but it also increases energy consumption and manufacturing costs.
Therefore, humidity control should be adjusted according to the actual conditions.
Process | Relative Humidity (RH) or Dew Point (°C) Control Standards | ||
Company X | Company Y | Company Z | |
Positive Electrode Ingredient Mixing | ≤30%RH | ≤-20℃ | ≤10%RH |
Negative Electrode Ingredient Mixing | ≤75%RH | ≤35%RH | ≤90%RH |
Positive Electrode Coating | ≤30%RH | ≤-20℃ | ≤10%RH |
Negative Electrode Coating | ≤75%RH | ≤35%RH | ≤90%RH |
Positive Electrode Calendering | ≤30%RH | ≤-20℃ | ≤10%RH |
Negative Electrode Calendering | ≤40%RH | ≤35%RH | ≤30%RH |
Positive Electrode Slitting | ≤30%RH | ≤-20℃ | ≤10%RH |
Negative Electrode Slitting | ≤40%RH | ≤35%RH | ≤30%RH |
Assembly Stage | ≤30%RH | ≤-20℃ | ≤-34℃ |
Baking Room | ≤-40℃ | ≤-50℃ | ≤-34℃ |
Injection Glovebox | ≤-40℃ | ≤-50℃ | ≤-34℃ |