Joining, protection, conduction of heat: the range of tasks for adhesives and sealants in lithium-ion batteries is wide. New areas of application are opening up for dosing technology, but there are also challenges.
Lithium-ion batteries must have high energy density and be shockproof and as light as possible. On the outside, they must also be protected against dirt, dust and moisture. Adhesives and sealants play a significant role in meeting these requirements. They not only connect the individual components of a battery – sometimes structurally, sometimes elastically. They also ensure that the temperature of the sensitive battery cells does not exceed 80 °C during charging and operation, they prevent short circuits between the cells and seal the battery.
Battery cells for modern lithium-ion batteries are combined into modules and then inserted into the battery housing. The houses are mostly made of aluminum these days. Inside is the cooling system: aluminum profiles arranged as a shelf through which coolant flows, between which the battery modules are embedded.
Anti-collision adhesives stiffen the battery housings
The authors around Sergio Grunder of DowDuPont summarize the types and tasks of adhesives and sealants in battery applications in the journal Adhäsion 1-2/19. The range includes both high-strength and hardened adhesives and sealants, which are also thermally conductive and/or electrically insulating, depending on where they are used.
High-strength adhesives with elastic moduli of over 1000 MPa are used in parallel with mechanical point joining processes to construct the house. They stiffen the housing and must not only withstand crashes and dynamic loads, but also seal the battery against heat, moisture and corrosive and liquid media. These requirements are met by two-component (2-K) adhesives based on epoxy resin. Where there are no structural loads on the housing, one-component (1-component) silane-modified polymer adhesives are also used, which also adhere to unpainted aluminum substrates and resist severe corrosion and temperature changes.
Delivery technology faces challenges
Special 2-component polyurethane adhesives were developed for heat management in the battery. For example, structural 2-component adhesives with thermal conductivities of more than 1 W/mK connect the cooling system profiles to the housing. Battery modules are thermally coupled to the cooling system using sealants with thermal conductivities well above 2 W/mK in some cases. The bird masses withstand temperatures between -40 and 80 °C, dampen vibrations during operation and also compensate for relative movements between the individual components.
The different requirements and areas of application for adhesives and sealants in the battery open up numerous new areas of application for manufacturers of dosing systems. In the article Liming, sealing and potting in battery production in the adhesion 7-8/22, the writing team around Carolin Gachstetter from the company bdtronic does not hide the challenges associated with it. One concerns the fixation of the battery cells when assembling the modules. To do this, the two components of the electrically insulating glue must be mixed in a ratio of 100:5, where the glue consists of a high and a low viscosity component. The potting material and the dosing process must be precisely adapted to the battery cells during the process, so that no air pockets form between the cells during the potting process.
Mastering abrasive thermal pastes
The completed modules are later inserted into the battery housing shell and connected to the cooling system frame. According to the authors, the complex geometry of the cells requires a measuring line with numerous corners and changes of direction. The dosing capacity and the speed of the axis movement must be linked and controlled in such a way that the glue is applied evenly and with the shortest possible cycle time.
According to the authors, dosing of the thermally conductive pastes is particularly challenging. The so-called gap fillers are heavily filled with abrasive substances and require correspondingly hard-wearing dosing systems. The material must also be transported with as little pressure as possible so that the thermal paste does not fill up. According to the authors, advanced cavity pumps have proven themselves up to this task.
To ensure that threads do not form when the application is interrupted, the servo-controlled dosing pump, the application robotics and the dosing contour must be precisely matched to each other. The same applies to robot-assisted application of liquid seals for house bonding. According to the authors, the start and stop points of the adhesive beads are particularly susceptible to leaks and errors.
Too much thermal paste is wasted
Daniel Bös and Fabian Schaaf of Atlas Copco focus on material waste when applying thermally conductive pastes in their contribution Intelligent Thermal Management in the Battery Joining Process in Adhesion 7-7/22. The authors estimate that up to 15 kg of heat-conducting media are installed in a vehicle, and the costs are calculated at around 10 euros/kg. According to the authors, manufacturers often use too much material when dosing the pastes between the cell module and the battery compartment – the gaps have high tolerances of between 0.5 and 3 mm – to avoid the risk of insufficient filling and air pockets.
In their article, the authors therefore present a dosing system with coupled image processing. During the process, 3D sensors first measure the common surfaces of the battery module and the battery compartment. An algorithm then calculates the optimal amount of material for the respective application and sends a corresponding set of parameters to the system control. According to the authors, this saves weight and up to 20% in material costs.
According to the authors, however, the loss of material when changing the container is also problematic. Due to the high material density, the barrels are often only half full, and conventional pumps are not able to empty a barrel completely. With a 200-liter container with a capacity of 150 liters, about 3 to 6 liters remain unused each time the barrel is changed. With a new generation of pumps, which largely automates the barrel change and no longer requires manual venting and rinsing of the barrel, the material loss during barrel change can be reduced to 1 l according to the authors.