The tab is a metal conductor that is drawn from the positive and negative collectors of the battery and is connected to the battery case (cylindrical/square) or to an external module structure (soft pack), and the current must flow through the tab to connect to the outside of the battery.
Usually, we divide the battery into positive and negative poles. The battery tab is the metal conductor that leads the positive and negative electrodes from the core. In layman’s terms, the positive and negative ears of the battery are the contact points when charging and discharging are performed. This contact point is not the copper piece we see on the exterior of the battery, but a connection inside the battery. There are three kinds of materials for the battery tab. The positive electrode of the battery uses aluminum (Al) material, the negative electrode uses nickel (Ni) material, and the negative electrode also has copper-nickel plating (Ni-Cu) material, and they are all made of a composite of two parts: the film and the metal strip.
The positive collector is aluminum and the negative collector is copper. The positive tab is aluminum and we will solder it to the aluminum on the cap to form the positive electrode. The negative terminal tab is nickel and we spot weld it to the copper collector. But we generally use nickel-plated copper for a negative cap on the lid.
Lithium cathode voltage is high, it will oxidize the common metals under this voltage. And then it will generate metal ions and dissolve them. Aluminum, on the other hand, generates a passivation film (aluminum oxide/aluminum fluoride) in the electrolyte, which can prevent the oxidation and dissolution of aluminum. Therefore, we generally use aluminum to make the cathode collector and tab of lithium-ion.
Why do you use aluminum for the positive electrode and nickel for the negative electrode of lithium batteries?
1. Both are conductive, soft, easy to stick together, cheap, and also form an oxide film on the surface.
2. The oxide of copper/nickel is made of semiconductor, which has the property of conductivity, and the thickness of the oxide is too thick, so it has high impedance, while the oxide of aluminum is an insulating material, it can’t conduct electricity, but it is thin, it can use its channeling effect to conduct, if the oxide layer is too thick, its conductivity will be reduced, and it will even lead to insulation. Usually, the collector should be surface cleaned to remove dirt and remove the thicker oxide layer before use.
3. With higher positive potential and thicker aluminum film, it can effectively prevent the oxidation of the collector. And copper/nickel oxide layer is relatively loose, in order to avoid oxidation, preferably at low potential, and Li is not easy to form an embedded lithium alloy with Cu / Ni at low potential, but if copper/nickel is heavily oxidized, the embedded lithium reaction between Li / and Cu / Ni, but at a lower potential, aluminum foil can not be used as a negative material, will lead to the alloying of Li Ai.
4. The composition of the concentrate is very demanding. The impure composition of aluminum will cause the film to be non-dense. This leads to pitting, and more seriously, the surface layer of aluminum is destroyed, thus forming LiAi alloy.
One more question is, can we use the positive aluminum tab for the negative terminal?
The same reason why we can not use aluminum foil for the positive electrode. With high anodic potential, the aluminum surface can form a dense oxide film. In the case of high potential is not easy to corrode. If used in the negative electrode, at low potentials, the lithium-embedded reaction will occur, generating an aluminum-lithium alloy. So we can not use it in the negative electrode, which is only from the electrochemical point of view.
So why use nickel or copper plating for the negative electrode tabs of lithium batteries instead of copper?
1. Mainly because of copper-copper welding. Copper is not corrosion-resistant compared to nickel. So generally do not use copper as the pole tabs.
2. Copper welding nickel conductivity and thermal conductivity are not too bad changes.
3. Regular manufacturers tab to go through electrolyte immersion, bending test, high and low-temperature thermal shock test, and coating corrosion resistance test, based on these tests so generally do not use copper.