By Mark Plantier, vp-Marketing, Enercon Industries Corp.
The global demand for high-performing lithium-ion (Li-ion) batteries is projected to hit $129.3 billion by 2027, at a CAGR of 18% from 2020 to 2027 [1]. Critical to supporting this growth is the implementation of efficient technologies that can produce the film and coatings required for these products. Many of the mission-critical film substrates used in this market require surface treatment. Surface treatment enables proper coating adhesion which allows the batteries to perform as designed. This article will review important criteria for successful surface treating that enables adhesion without damaging the substrates.
Anatomy of the lithium-ion battery
Lithium-ion batteries have four main components: the cathode, anode, electrolyte and separator film (see Figure 1). The cathode determines the capacity of the battery. The anode enables the passing of currents to an external circuit. The electrolyte facilitates the movement of ions. The separator film provides safety as it prevents contact between the cathode and anode while allowing ions to pass through it.
FIGURE 1. The basic components of a lithium-ion battery
Within these components, various polymer films and foils are used. The cathode features aluminum, the anode uses copper, and the separator is comprised of an engineered, porous polymer film. These substrates are coated, and the performance of the battery is heavily dependent on the quality of the coatings.
Surface treating is used to prepare the substrates for bonding by removing contamination, increasing surface energy and enabling wettability. It is critical that surface treating achieves the desired benefit without damaging the films so the applied coatings function as they are intended, without compromising battery performance or safety.
Surface-treatment technology options
The three main categories of in-line surface treating are corona, plasma and flame. Each method can be highly effective at achieving the results of cleaning organics from the substrate surface and activating surfaces for adhesion. Determining which technology is best for a given application requires a careful review of the proposed application details. It is best practice to always install the surface treater immediately prior to the coating process.
Flame: Flame treaters produce an intense blue flame when flammable gas and atmospheric air are combined and combusted. Treated surfaces are made polar as species in the flame plasma affect the electron distribution and density on the surface. This polarization is made through oxidation. In addition, functional groups are deposited on the surface. Generally speaking, flame is considered for high-speed applications and used more for aluminum and copper foils than for polymer films. Thin-gauge foils are not a good candidate for flame treating due to the exposure to heat.
Plasma: In-line atmospheric plasma technologies introduce gas chemistries into the treatment process. Carrier (inactive) and reactive gas molecules are diffused toward the surface of the material under the influence of electric and/or magnetic fields. This technology is often recognized as a gentler treatment than flame because its discharge has a lower temperature.
While plasma treaters are relatively easy to use, they are more complex than a corona treater. Sophisticated gas-control architecture is required to ensure consistent treatment, and there are additional physical, dielectric design considerations as well. It’s worth noting that there is also an additional operating cost for gas consumption with plasma treaters.
Plasma treaters offer an excellent technology application when desired results cannot be achieved by other surface-treatment methods.
Corona: Corona is essentially the ionization of air. When a substrate passes through a field of corona treatment, the primary goal is to improve surface wettability. Corona is popular because of lower system and operating costs when compared to plasma and flame technologies. Corona can be created in a variety of ways depending on the dielectric materials used in the system design. The properties and combination of these different dielectrics will produce subtle differences in the corona, which can make or break the success of an application.
Typical corona treaters produce a highly filamentary discharge (see Figure 2) For many applications, this is very effective, but the inconsistency of the corona and the harshness of the discharge is not ideal for many substrates.
FIGURE 2. Typical highly filamentary corona discharge
High-Definition Corona treatment
Reducing the harshness of a filamentary discharge is important for applications where pinholing and unevenness of treatment are unacceptable. Ceramic electrode systems with a bare ground roll generally produces less of these filamentary discharges; however, they may be limited in the amount of treatment they can provide and be subject to other application issues such as the oxidation of the ground roll.
Many corona-treater manufacturers offer a conductive, ceramic ground-roll coating that protects the ground roll from oxidation. However, the dielectric properties of all ceramic coatings are not the same. When combined with optimized ceramic electrodes, the result is a homogenous smooth corona that meets the sweet spot of all application requirements. It provides effective surface treatment, efficient power requirements, protects against oxidation, and offers assurance against pinholing, backside treatment and film wrinkling.
This firm optimizes its ceramic electrodes, its proprietary ceramic ground roll, and airflow to create this “soft corona,” also known as High-Definition Corona (see Figure 3). This treatment not only looks better than traditional corona, but it produces better and more efficient treatment results than other dielectric designs.
FIGURE 3. High-Definition Corona discharge is smoother, more homogenous.
Use on battery-separator materials
We have found that this dual-dielectric, corona system design does an excellent job of treating battery-separator materials. In fact, it’s been confirmed by users that such a dual-dielectric system enhances the properties of the separator material far better than their “conventional” corona treater. In this application, because the separator material is porous, a corona system using a bare metal or conductive-surface ground roll will result in a disproportional amount of energy dispersed into the porous portion of the substrate. This untimatey leads to inadequate treatment of the substrate overall; whereas, the High-Definition or dual-dielectric system will disperse the energy evenly in the air gap, and the substrate will be afforded the full treatment from the corona.
One experienced user of these technologies stated that the High-Definition Corona system also yielded better treatment results than a plasma system they had previously employed. It’s important to note the improved efficiencies gained with this technology. Table 1 shows improvement in substrate wettability (measured in dynes) and is achieved with less power (measured in watt density) than comparable corona configurations.
High-Definition Corona is ideal for polymer battery-separator films and also is effective on aluminum and copper. As each application is different, the best way to determine which technology is best fs to schedule a lab trial. Many corona-treater manufacturers offer this service. However, it should be noted that only this firm offers a full complement of corona, plasma and flame technologies for comparison.
Manufacturing environments
Dry-room production areas are used for lithium-ion battery manufacturing to ensure very low levels of humidity. These rooms also need to support cleanliness to avoid contamination during manufacturing. This is a benefit to operating surface-treating equipment. Humidity, as well as dirt and debris, can have negative effects on surface treating. High levels of humidity are known to lead to high-voltage arcing. This firm’s corona treaters use a high volume of exhaust air, which helps keep the electrodes and ground rolls cool. If corona treaters are required in areas near solvents, a purged system can be designed to operate safely in those environments.
Conclusion
The race is on as manufacturers of lithium-ion batteries look to improve battery performance, keep pace with demand, and increase efficiencies – all while reducing scrap. Critical to their success is enabling high-quality, consistent adhesion between high-performance substrates and coatings. The key to optimizing these bonding applications is to work with your suppliers on closely evaluating flame, plasma and variations of corona treaters to determine the perfect balance of economy, efficiency and performance.
References
1. Valuates Reports, Lithium-Ion Battery Market Report
Mark Plantier, vp-Marketing for Enercon Industries Corp. (Menomonee Falls, WI), has 30 years of experience in the packaging and converting industries. He has authored and published dozens of technical articles, contributed to numerous technical papers, and produced a series of highly informative Webinars with insights on new technologies and applications. Mark earned a Bachelor of Science in Marketing Management from Siena College. He can be reached at 262-255-6070, email: mplantier@enerconmail.com, www.enerconind.com
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