Minienvironment as local purity solution
Large, high-quality cleanrooms are often planned to create permanently clean production conditions. The purchase of a cleanroom generates...
5 min read
In recent decades, the topic of ‘dry rooms for battery cell production’ has increasingly become a key technology for the manufacture of lithium-ion cells. Other products also require low humidity for their manufacture in order to guarantee their quality.
Besides battery cells, they can also be OLEDs, fuel cells or even effervescent tablets, for example. Unfortunately, there is still no binding definition for dry rooms. In English-speaking countries, the term ‘dry cleanroom’ is used, which could or should also be adopted in German terminology. The term dry cleanroom is also appropriate because almost all of these applications are cleanrooms with a defined (extremely) low humidity level. The range in which the humidity must be set is determined by the respective product and the associated manufacturing processes.
The following requirements for a dry cleanroom can be found on the Internet for the production of lithium-ion batteries [1]:
"Pure lithium metal is extremely sensitive to even tiny amounts of moisture in the air, and leads to reduced performance and reliability over the battery life cycle.
With the latest LI chemistries, lithium salts are used to keep the electrolytes stable. In some chemistries, moisture leads to the generation of hydrogen fluoride as an undesired by-product, which leads to gassing and bloating up of Prismatic and Pouch batteries, thereby reducing performance and life expectancy.
Lithium-ion batteries are affected by uncontrolled temperature and humidity during manufacturing stages, in particular electrolyte filling and then battery assembly leading to quality issues around:
- Reduced performance (charging capacity)
- Reduced product lifetime
- Safety issues, including chances of explosion
Lithium-ion battery production, takes place in controlled environment rooms now commonly referred to as ‘dry cleanrooms’ which are hermetically sealed with ultra-low humidity control.
Battery cleanrooms require low humidity (moisture) levels of cleanliness for LI battery manufacturing processing to ensure product quality in terms of yield, reliability, and safety. Energy management and smart design is critical to manage costs.
When there are people present, comfort conditions must be maintained. The average person releases 1500 to 2000 grains per hour through breathing, and perspiration. This must be addressed by the HVAC system. Therefore, modern Giga Battery dry cleanrooms use high levels of robotics and automation."
The installation of cleanrooms is a not insignificant cost factor when setting up production under cleanroom conditions. Realising the whole thing as a dry room also drives up the costs - not only the investment costs, but also increasingly the running costs for operating the dry rooms.
Joint planning of clean and dry rooms
Battery cells must be produced under dry and clean conditions. Therefore, the combination of clean and dry room technology cannot be considered independently of each other, as both technologies influence each other. The planning of the machine and production processes must be carried out across all components, taking into account the clean and dry room technology.
Battery cell production has currently reached a point where the semiconductor industry was in the mid/end of the 1990s. A decision had to be made to invest in ever higher quality cleanrooms (air purity classes) or to switch to decentralised minienvironment solutions. This new path was then consistently followed, which was reflected in the standards and guidelines of the semiconductor industry [2]:
Mini-environments: 300 mm process/metrology equipment must have minienvironments integrated into the equipment design at a fundamental level, so that first generation equipment includes it as a standard, and wafer and operational cleanliness is universally maintained. The clean wafer environment must be maintained inside pods, at the load ports, and during all wafer handling and processing. (6.3.3)
It is therefore more than logical to use minienvironment technology to realise an extremely dry environment and thus to extend the advantages in terms of cleanliness to the parameters of a dry room.
But what does 'dry' mean?
There are various ways to specify the water concentration in the air. The absolute humidity (aF) describes the content of water or water vapour in the air and is given in g/m³. It is important to know that the water vapour content depends on the respective temperature. At 20 °C the air can absorb 14.68 g of water vapour per 1 m³ of air, at 10 °C it is only 7.62 g/m³. The relative humidity (RH) indicates the percentage of water in the air in relation to the absolute humidity at the respective temperature. At 100 % RH is the point at which the air is saturated with water and the water begins to condense. This point is called the dew point. Due to the dependence of the water content of the air on the temperature, the dew point is given in °C. At this temperature we have a dew point in the air. At this temperature, we have a relative humidity of 100 % at the dew point.
Dehumidifying air is an energy-intensive process. You can imagine that the costs of operating a dry environment depend primarily on the volume of the room. The use of customised minienvironments can significantly reduce the air volumes required and therefore the amount of energy needed to dry the air without having to change the requirements for the process environment. This can also increase process reliability and minimise the impact of personnel on the process. In addition, the automation and interlinking of the individual process stations must be promoted. In dry room environments, just as in cleanroom environments, people are the greatest risk of contamination. Moisture should also be considered a contamination factor here.
Advantages of a Minienvironment
- The purified air in conventional cleanrooms often does not reach the process due to the required separation of the machine from the surrounding room (protective housing). In contrast, the filtered air is brought directly to the process in the minienvironment system.
- Only the machine and the process inside are supplied with clean and dry air and not a large unused volume.
- Safe control of cleanliness and dryness in and around the process through containment and separation of the process from the human contamination source. Cross-contamination of neighbouring machines and personnel cannot occur.
- The purity that can be achieved on the product is higher than it would be possible in a cleanroom with a human contamination source.
- In addition, savings are realised in production by eliminating the changing procedure and employee training.
- Simplifying the construction and approval of the production environment by eliminating the cleanroom in a hall, as this is integrated into the machine and not part of the building. This means that additional authorisations and fire protection requirements may not be necessary.
- Reduction in space requirements for the technology needed for a cleanroom.
- Reduction of the air volumes required to comply with the cleanliness classes and dryness requirements by reducing the volume to be protected.
- Significant reduction in the energy required to generate ultrapure and dry air - by reducing the air volumes required and by eliminating air distribution networks, shut-off and control elements, etc. (reduction in pressure losses) - The comparison of a centralised ultrapure air supply with H14 HEPA filter outlets with a fan-filter-module solution shows an electrical energy saving of approx. 60 %.
- Maintenance costs are reduced.
- Improved control and adaptation of the flow conditions within the production environment to protect the product and adapt to the individual production steps.
- Ability to expand and scale as well as increased flexibility: Adapting a cleanroom that has already been built to new machines can be very time- and cost-intensive (example: the cleanroom is 2 metres too short). With a centralised air supply, expansion is often associated with the replacement or additional installation of a ventilation unit.
- The minienvironment/ machine housing is a component of the machine and is simply taken with it if it is moved to other production areas, which would either not be possible with a clean or drying room or would only be possible with greater effort.
- Changes to the machine (additional process stations, a modified process sequence, etc.) can be realised with little effort by modifying and/or enlarging the machine housing.
- The investment costs, running costs and delivery times for the machine enclosure are significantly lower than for a clean or drying room.
- For maintenance, only the corresponding minienvironment with the individual process machine needs to be taken out of operation and not the entire drying room.
- The use of minienvironments also increases the pressure to introduce automation, which also reduces the influence of harmful environmental conditions on the products and accelerates the process speed of a complete production line.
- If continuous maintenance cleaning of the process machine is required, this can be carried out using conventional cleaning technologies from cleanroom technology, as the required dryness of the air only needs to be restored in the small volume of the minienvironment and not in a large drying room. Furthermore, this cleaning work has no impact on neighbouring processes.
Summary
In the production of battery cells, as in the semiconductor industry, there will be no getting round the use of minienvironments. In addition to the numerous technical advantages, the energy savings for generating the required purity and drying parameters play a decisive role.
Literature:
[1] ARDMAC; Dry Cleanroom Requirements; https://www.ardmac.com/dry-cleanroom-requirements/, 08.02.2023
[2] I300I Factory Guideline Compliance: Factory Integration Maturity Assessment (FIMA) for 300 mm Production Equipment: Version 4.01; International SEMATECH Technology Transfer # 98023468C-TR,29. Februar 2000
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