Copper Foam: The Importance of Current Collectors in Lithium-Ion Batteries

Current collectors are one of the indispensable components of lithium-ion batteries. They not only support the active material but also collect and transmit the current generated by the Electrode active material, which helps reduce the internal resistance of lithium-ion batteries and improves their coulombic efficiency, cycling stability, and rate capability.
In principle, an ideal lithium-ion Battery current collector should meet the following conditions:
(1) High electrical conductivity;
(2) Good chemical and electrochemical stability;
(3) High mechanical strength;
(4) Good compatibility and binding strength with the electrode active material;
(5) Low cost and easy availability;
(6) Lightweight.
However, in practical applications, different current collector materials still face various issues and therefore cannot fully meet the above multi-scale requirements. For example, copper is prone to oxidation at higher potentials and is suitable for use as a negative electrode current collector; conversely, aluminum suffers from severe corrosion issues when used as a negative electrode current collector and is therefore more suitable for positive electrode applications. Currently, materials available for use as lithium-ion battery current collectors include metallic conductors such as copper, aluminum, nickel, and stainless steel; semiconductor materials such as carbon; and composite materials.
2.1 Copper Current Collectors
Copper is an excellent metallic conductor with electrical conductivity second only to silver. It offers numerous advantages, including abundant resources, low cost, and good ductility. However, given that copper is prone to oxidation at higher potentials, it is commonly used as a current collector for anode active materials such as graphite, silicon, tin, and cobalt-tin alloys. Common copper current collectors include copper foil, copper foam, copper mesh, and three-dimensional nano-copper array current collectors.
2.1.1 Copper Foil Current Collectors
Based on the manufacturing process, copper foil can be further classified into rolled copper foil and electrolytic copper foil. Compared to electrolytic copper foil, rolled copper foil offers higher electrical conductivity and better elongation. For lithium-ion batteries with low bending requirements, electrolytic copper foil can be selected as the anode current collector. Studies have shown that increasing the surface roughness of copper foil helps improve the bonding strength between the current collector and the active material, reduces the contact resistance between the active material and the current collector, and consequently enhances the battery’s rate discharge performance and cycling stability.
2.1.2 Foamed Copper Current Collector
Foamed copper is a three-dimensional mesh material similar to a sponge, offering numerous advantages such as light weight, high strength and toughness, and a large specific surface area. Although silicon and tin anode active materials possess high theoretical specific capacities and are considered to be among the most promising anode active materials for lithium-ion batteries, they suffer from drawbacks such as significant volume changes and pulverization during the charge/discharge cycle, which severely impair battery performance. Research indicates that foamed copper current collectors can suppress the volume changes of silicon and tin anode active materials during charging and discharging, slow down their pulverization, and thereby improve battery performance.
2.2 Aluminum Current Collectors
Although the electrical conductivity of metallic aluminum is lower than that of copper, aluminum wires require only half the mass of copper wires to carry the same electrical charge. Undoubtedly, the use of aluminum current collectors helps improve the energy density of lithium-ion batteries. Furthermore, aluminum is more cost-effective than copper. During the charging and discharging of lithium-ion batteries, a dense oxide film forms on the surface of the aluminum foil current collector, enhancing its corrosion resistance. It is commonly used as the current collector for the cathode in lithium-ion batteries. Similar to copper foil current collectors, surface treatment can also improve the surface properties of aluminum foil. After DC etching, a honeycomb-like structure forms on the surface of the aluminum foil, allowing for a tighter bond with the cathode active material and improving the electrochemical performance of the lithium-ion battery. However, in reality,
aluminum current collectors often suffer from severe corrosion due to the breakdown of the surface passivation film, which consequently reduces the performance of lithium-ion batteries. Therefore, to improve the corrosion resistance of etched aluminum foil, its surface must undergo optimization to form a more stable passivation film.

2.3 Nickel Current Collectors
Relatively speaking, nickel is a base metal that is inexpensive, has good electrical conductivity, and is relatively stable in acidic and alkaline solutions. Therefore, nickel can serve as both a cathode current collector and an anode current collector. It is compatible with cathode active materials such as lithium iron phosphate, as well as anode active materials such as nickel oxide, sulfur, and silicon-carbon composites.
Nickel current collectors typically come in two forms: nickel foam and nickel foil. Due to the highly developed pore structure of nickel foam, it offers a large contact area with the active material, thereby reducing the contact resistance between the active material and the current collector. However, when nickel foil is used as the electrode current collector, the active material tends to peel off as the number of charge/discharge cycles increases, which affects battery performance. Similarly, surface pretreatment processes are also applicable to nickel foil current collectors. For example, after etching the surface of a nickel foil current collector, the bonding strength between the active material and the current collector
is significantly enhanced.
2.4 Stainless Steel Current Collectors
Stainless steel refers to alloy steel containing elements such as nickel, molybdenum, titanium, niobium, copper, and iron. It possesses good electrical conductivity and stability, and can withstand chemical erosion from weak corrosive media such as air, steam, and water, as well as strong corrosive media such as acids, alkalis, and salts. A passivation film also forms easily on the surface of stainless steel, protecting it from corrosion. Additionally, stainless steel can be processed into thinner sheets than copper, offering advantages such as low cost, simple manufacturing processes, and suitability for mass production. Stainless steel can serve as a current collector for either the anode or cathode; common types of stainless steel current collectors include stainless steel mesh and porous stainless steel.
2.4.1 Stainless Steel Mesh Current Collectors
Stainless steel mesh has a dense structure. When used as a current collector, its surface is enveloped by the electrode active material, resulting in minimal direct contact with the electrolyte. This minimizes the likelihood of side reactions, thereby helping to improve the battery’s cycling performance.
2.4.2 Porous Stainless Steel Current Collectors
To fully utilize the active material and increase the electrode’s discharge specific capacity, a simple and effective method is to employ porous current collectors.
2.4.3 Carbon Current Collectors
Using carbon materials as current collectors for the cathode or anode avoids corrosion of metal current collectors by the electrolyte. Additionally, carbon offers advantages such as abundant resources, ease of processing, low electrical resistivity, environmental safety, and low cost. Carbon fiber cloth, with its inherent flexibility, conductivity, and electrochemical stability, can be used as a current collector for flexible lithium-ion batteries. Carbon nanotubes represent another form of carbon current collector; compared to metal current collectors, their distinct advantages lie in their lightweight nature and the ability to significantly increase the battery’s energy density.
2.4.4 Composite Current Collectors
In addition to single-material current collectors such as copper, aluminum, nickel, stainless steel, and carbon current collectors, which have received widespread attention, composite current collectors have also attracted the research interest of scholars in recent years, including conductive resins, carbon-coated aluminum foil, and titanium-nickel shape memory alloys.
2.4.5 Conductive Resin Current Collectors
Polyethylene (PE) and phenolic resin (PF) current collectors are composed of conductive fillers combined with a polymer resin matrix. Using PE and PF as matrix materials, these are uniformly mixed with conductive fillers (graphite, carbon black) to prepare composite current collectors, and their physicochemical properties are studied. Graphene is a unique, novel two-dimensional carbon functional material formed by sp²-hybridized carbon atoms. It possesses numerous advantages, including ultra-high electrical conductivity, specific surface area, and mechanical strength. It can serve as both a substitute for graphite as an anode active material in lithium-ion batteries and as a current collector material.
2.4.6 Titanium-Nickel Shape Memory Alloy Current Collectors
Titanium-nickel shape memory alloy is a binary alloy composed of nickel and titanium that can transform between two different crystalline phases in response to changes in external temperature or applied pressure. By altering its own phase state, titanium-nickel shape memory alloy can suppress volume changes in the active material during charging and discharging, thereby improving the battery’s cycle life.