What is a Battery Dry Room and Why is it Critical for Production?
Battery dry rooms are an often-overlooked component of battery production, yet any battery company would attest to the fact that dry rooms are extremely important to high-quality cell manufacturing. Whether you are making battery prototypes at lab-scale or churning cells out by the millions in a gigafactory, you will need to control the moisture level of your environment with a high degree of accuracy and consistency. This is where dry rooms come into play. This article dives into the reasons that dry rooms are so important, how dry rooms actually work, and why they will remain a key aspect of battery production well into the future.
Why Do Batteries Need to be Manufactured in Dry Conditions in the First Place?
Battery production relies heavily on the manufacturer’s ability to accurately control environmental conditions – specifically humidity. Most battery cells require extremely dry conditions during production. Even a small amount of moisture in the air during cell production can have a profound impact on cell quality. This is due to the fact that batteries often contain hygroscopic elements, like lithium and sodium. This essentially means these elements have an affinity for moisture. A lithium-based electrolyte, for instance, will react with water to form hydrogen gas and aqueous lithium hydroxide. The chemical formula for this reaction is as follows:
2Li + 2H2O 🡪 2LiOH + H2
In practice, this reaction causes cell swelling, decreased cycle life, and reduced energy storage capacity. In extreme cases, excess humidity can result in battery explosion. This is why it is critical to keep conditions as dry as possible, especially during operations where highly moisture-sensitive battery components are exposed to the environment. These conditions can only be maintained with specialized dry rooms with heavy-duty industrial dehumidification equipment.
Understanding Humidity Control in a Battery Dry Room: Dew Point, Relative Humidity, and Their Impact on Battery Dry Rooms
Before we quantify just how moisture-sensitive battery components are, we must first define a few terms:
- Dew Point is an environmental property that is correlated to humidity. The formal definition of dew point is the temperature at which water vapor begins to condense at a given state of humidity and pressure as temperature is being reduced. The lower the dew point, the lower the humidity.
- Relative Humidity (RH) is another term that is commonly used to quantify humidity. RH is defined as the amount of water vapor in air as a percentage of the amount of water vapor in saturated air at the same temperature and pressure. With that said, dew point is often considered a more robust measure of humidity than RH, since RH is dependent on the temperature of the air while dew point is not.
Typically, lithium-ion battery makers require their dry rooms to be maintained at a dew point of approximately -40°F. For reference, the average dew point in Las Vegas (a notoriously dry city) is approximately 30-40°F. Despite the massive difference in dew point of approximately 100°F, humans aren’t really equipped to sense this difference. Both environments just feel… dry.
Being in a battery dry room at -40°F dew point is by no means dangerous to operators, but it is important that they take scheduled breaks to rehydrate. It is also important that these operators avoid exhaling directly onto moisture sensors or battery components, as their breath contains quite a bit of moisture. Many battery manufacturers will mandate the use of face masks in their dry rooms for this very reason.
Newer technologies such as sulfide-based solid-state batteries often need even drier conditions than lithium-ion batteries – as low as -100°F dew point! This is because many solid-state electrolytes are very easily hydrolyzed by moisture, resulting in the formation of hydrogen sulfide (H2S). This is a colorless, flammable, and highly toxic gas with a strong odor of rotten eggs – definitely something to be avoided!
Whether it is to prevent gas formation, maximize cycle life, or optimize energy storage capacity, the need for effective environmental controls is paramount in cell production. Now that we know a bit more about why dry conditions are so important, let’s dive into how these conditions are actually created and maintained.
Dry Rooms 101: How do Battery Dry Rooms Create Such Dry Conditions?
Dry rooms are sealed areas that have a positive pressure relative to the outside environment. This positive pressure ensures that outside air does not get into the controlled space, carrying excess moisture with it. In order to maintain this positive pressure, it is important that there is sufficient air flow (expressed in cubic feet per minute, or CFM) of conditioned air into the dry room.
The flow of conditioned air comes from one or more dehumidification units (DHUs) that send air at a set temperature and humidity into the battery dry room. DHUs often rely on desiccants, or materials with a high moisture adsorption capacity, to extract moisture from air. These DHUs are referred to as desiccant dehumidifiers.
The process by which desiccant dehumidifiers condition air depends on the type of desiccant dehumidifier used. With that said, the process flow below shows a high-level overview of how desiccant dehumidifiers generally work:
- Intake air is continuously pulled in from the outside environment.
- That intake air is sent through a desiccant wheel, which contains a high adsorption capacity desiccant such as silica gel.
- The desiccant adsorbs moisture from the intake air.
- That desiccant, which has now adsorbed a substantial amount of moisture, is rotated into a different airstream.
- Heat is applied to drive off the moisture, which is expelled to the outside environment.
- The desiccant, which is now dry, must be cooled to restore its high adsorption capacity.
- At this point, the desiccant has been rotated to the original airstream, where it can continue to adsorb moisture from the intake air.
- Steps 3-7 repeat indefinitely.
As mentioned above, there are various DHU configurations, some involving multiple rotors, pre-heating coils, etc. DHU design is application-specific and the DHU manufacturer should be consulted for proper guidance.
When sizing DHUs for a particular battery dry room, the most critical parameter to be considered is moisture load. This is essentially the amount of moisture that is introduced to a system over time. There are two key elements that make up a majority of the moisture load:
- People in the room. An average person will release 1,500 to 2,000 grains of moisture per hour through breathing and perspiration. A grain is a unit of weight applied to water – there are 7,000 grains of water in a pound.
- Moisture ingress due to permeation. Despite dry rooms being theoretically sealed environments with positive pressure to the outside air, there will be some non-negligible moisture entry from the surrounding air.
Other factors such as the amount and type of equipment, number of entryways, and how often those entryways are used also play a large role in the sizing of battery dry room equipment. Ultimately, it is highly recommended that battery manufacturers consult directly with dry room / DHU providers to ensure the design of their battery dry room is sound.
It is critical that there are proper controls and sensors in place throughout the DHU and the dry room itself. Only with accurate temperature and humidity readings at critical points in the system can important parameters like rotor speed, coolant flow rate, etc. be effectively adjusted to optimize system performance.
One last feature of battery dry rooms that should be noted is the presence of airlocks, which are specialized entryways that limit the amount of outside air ingress as people enter and leave the dry room. Dry rooms typically do not have normal doors for good reason – as soon as you open the door, there will be some outside air that seeps into the dry room, even if the dry room has a positive pressure. Instead, an airlock consists of one or two antechambers, or small rooms that lead to the larger dry room. This allows for less outside air ingress, as the controlled space within the dry room is never directly exposed to the outside air.
Future Trends and Developments: Where is the Battery Dry Room Industry Heading?
The future of dry rooms is bright. As the battery world starts focusing more and more on next-gen technologies like solid-state, it is clear that there will be some sweeping changes in this industry. What will not change is the need for highly effective environmental controls.
As mentioned earlier in this article, solid-state batteries not only require low dew point conditions, they require dew points that are far lower than those demanded by their lithium-ion counterparts – as low -100°F. This is by no means unachievable, but it requires some heavy-duty DHUs (or multiple, depending on the moisture load). In all likelihood, there will not only be a lot of new dry rooms built for solid-state batteries, but there will also be quite a bit of augmentation to existing battery dry rooms to accommodate this highly moisture-sensitive technology.
Another trend we are expecting is the increased use of segmentation. This essentially means subdividing battery dry rooms into different sections held at varying dew points to optimize the amount of “drying power” needed to maintain required humidity conditions. If you have certain processes that require ultra-low dew point and others that only require moderately-low dew point, it makes sense to group those processes into separate subdivisions held at different dew points. For larger facilities, the number of subdivisions typically gets larger.
Although segmentation is the focus of more and more discussion these days, it has been common practice for some time. In 2021, Bry-Air installed a 55,000 square foot battery dry room for a prismatic cell pilot line. This dry room was split into two sections, the Fill Room and the Assembly Room, maintained at -40°F dew point and -22°F dew point respectively. The drier room is reserved exclusively for highly moisture-sensitive operations like electrolyte fill, since that is the more valuable real estate. The less-dry room was then free to house other operations like cathode coating and cell assembly.
The Critical Role of Battery Dry Rooms in Long-Term Industry Success
Now we know why dry rooms are so important to battery manufacturing. We’ve also learned how a battery dry room works and we have some insight as to where the state of dry room technology is trending. The battery industry is evolving at such a rapid pace that it’s difficult to predict what the landscape will look like in 5, 10, or 15 years. What is certain is that environmental controls systems will be featured prominently in that landscape.
We are in an exciting industry with some incredible people. We are looking forward to working with other leaders in the space now and for many years to come. If you have questions, or if you’re looking for more information, please don’t hesitate to reach out using the form below to discuss batteries, dry rooms, or anything in between!
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