The current collector in lithium-ion batteries is an indispensable key component. Its main functions are to support the electrode active material, gather and output the current generated by the active material, deliver electrode current into the active material, and distribute the current uniformly.

Supporting electrode active materials:

The current collector is typically made into foil sheets (such as aluminum foil or copper foil), which physically support the electrode slurry (composed of active material, conductive agent, binder, etc.) coated on its surface, forming structurally stable electrode sheets (cathode or anode). It establishes a low-resistance electrical connection between the active material layer (such as lithium cobalt oxide, lithium iron phosphate, graphite, etc.) and the external circuit.

Gathering and delivering current:

This is the most basic and important function of the current collector. Acting as a highway for electron flow, it effectively collects the current (electrons) generated by the internal chemical reactions of the battery and conducts it to the external electrode (cathode or anode tab), or distributes the incoming current (during charging) evenly to the electrode active material. In this way, it provides a low-resistance electrical connection between the active material layer and the external circuit.

Uniform current distribution:

An excellent current collector has high conductivity, ensuring that current is uniformly distributed across the electrode surface, thereby reducing internal current density differences and uneven potential distribution. Uniform current distribution helps the active material be more fully utilized, reduces local overcharge/over-discharge, and improves the battery’s rate capability (charge/discharge speed), cycle life, and energy efficiency.

Therefore, the requirements for current collectors mainly include: high purity, high electrical conductivity, good chemical and electrochemical stability, high mechanical strength, and strong adhesion with the electrode active material.

In lithium-ion batteries, copper foil and aluminum foil are typically used as current collectors. Because copper foil is easily oxidized at higher potentials, it is mainly used for the anode. Aluminum foil, on the other hand, suffers from severe corrosion issues at low potentials and is mainly used for the cathode. If the composition of the current collector is impure, the resulting oxide film on the surface may be non-dense, leading to pitting corrosion or even the formation of Li–Al alloys.

Both copper and aluminum form oxide films on their surfaces:

• The oxide layer on copper is semiconductive, allowing electron conduction, but if the oxide layer becomes too thick, impedance increases significantly.

• The oxide layer on aluminum is insulating. When very thin, it can conduct electrons through the tunneling effect, but once thickened, its conductivity becomes extremely poor.

Although the current collector does not directly participate in the energy storage reaction, it plays a crucial role in determining the internal resistance, rate capability, cycle life, safety, and energy density of the battery. Key design parameters include the choice of material (aluminum foil/copper foil), thickness, surface treatment (such as roughening or carbon coating), and mechanical properties.