Tab metal coatings are of vital importance to the performance and longevity of tabs and thus, Li-ion pouch cells. Coatings protect the vulnerable conductive metal from the corrosive environment within the battery cell. It is crucial that a suitable coating is chosen for a cell to maintain its functional properties.

One of the primary purposes of coatings being applied to the underlying base metal is to improve the resistance to the hydrofluoric acid containing electrolyte; it acts as a barrier between the corrosive electrolyte and the less corrosion resistant metal substrate. The coating preserves the underlying metal and enhances the sealant adhesion, prolonging the service life of the cell. The coating should be selected based on the cell chemistry, and the polymer adhesion requirements.

The choice of coating has the possibility to reduce the electrical conductivity of the tab surface via several mechanisms, potentially in a detrimental manner. Poor electrical conductivity can increase the resistive heating in the cell, leading to an overheating scenario, where damage occurs to the components, or worse – thermal runaway. Good tab conductivity will improve cell efficiency through charging and discharging cycles. It is important for the coating material to be sufficiently electrically conductive so as not to impact cell performance. At ABM, extensive selection criteria and tests are performed to determine the viability of new coatings.

Coatings that are not adhered well to the metal substrate may detach during operation like in a vehicle, for example. In the event of swelling the coating may have a lower adhesion strength to the metal substrate than the polymer sealant and can also detach. Such scenarios could leak electrolyte, causing harm to nearby people, other components, and the environment. The coating must remain adhered to the tab under mechanical loads such as bending; some battery manufacturers have the preference to bend the tab to position it better for welding. Suitable coatings can also enhance the adhesion of the polymer sealant to the surface of the tab.

Coatings that are too thick can cause issues when welding the electrode foil stacks to the tabs. If the coating is too thick, then the weld joint will not be formed well due to it interfering with the transmission of ultrasonic vibrations. Changes in the absorptivity of the tab surface, as a result of the coating material, may cause thermal damage to the coating or adjacent components or result in a poorly formed weld when laser welding. It is also important that the chosen material coating is able to form a compatible bond with the electrode material to prevent detachment. Coatings should be of even thickness, otherwise the weld quality will be inconsistent and may fail. Coatings must also not introduce sharp burrs because the introduction of protruding metal fragments may pierce the encapsulating pouch film, causing an electrical short or leak of electrolyte.

In summary there are a multitude of parameters that can be affected by the type of coating applied to a metal tab lead. Finding a suitable coating that can address all the requirements of cell performance is a great challenge; at ABM we have the facilities and team capable of assessing materials against the crucial parameters. If your cell has specific requirements, visit www.avocetbattery.com to learn more about our products, services, and how we can assist your battery needs.