The current collector is one of the indispensable components in lithium-ion batteries. It can not only carry the active material, but also collect and output the current generated by the electrode active material, which is conducive to reducing the internal resistance of lithium-ion batteries, improving the coulombic efficiency, cycle stability, and rate performance of the battery.
Lithium ion battery current collector
In principle, an ideal lithium-ion battery current collector should meet the following conditions: (1) high conductivity; (2) Good chemical and electrochemical stability; (3) High mechanical strength; (4) Good compatibility and binding strength with electrode active materials; (5) Cheap and easy to obtain; (6) Lightweight.
However, in practical applications, different current collector materials still have various problems, so they cannot fully meet the multi-scale requirements mentioned above. Copper is prone to oxidation at higher potentials and is suitable for use as a negative electrode current collector; Aluminum, as a negative electrode current collector, has a more serious corrosion problem and is suitable for use as a positive electrode current collector. At present, materials that can be used as current collectors for lithium-ion batteries include metal conductor materials such as copper, aluminum, nickel, and stainless steel, semiconductor materials such as carbon, and composite materials.
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1.1 Copper current collector
Copper is an excellent metal conductor with conductivity second only to silver, and has many advantages such as abundant resources, low cost and easy availability, and good ductility. However, considering that copper is prone to oxidation at higher potentials, it is often used as a current collector for negative electrode active materials such as graphite, silicon, tin, and cobalt tin alloys. Common copper collectors include copper foil, foam copper, copper mesh and three-dimensional nano copper array collector.
1.1.1 Copper foil current collector
According to the production process of copper foil, it can be further divided into rolled copper foil and electrolytic copper foil. Compared with electrolytic copper foil, rolled copper foil has higher conductivity and better extension effect. Lithium ion batteries with low requirements for curvature can choose electrolytic copper foil as the negative electrode current collector. Research has shown that increasing the roughness of the copper foil surface is beneficial for improving the bonding strength between the current collector and the active material, reducing the contact resistance between the active material and the current collector, and correspondingly improving the rate discharge performance and cycle stability of the battery.
1.1.2 foam copper collector
Foam copper is a kind of three-dimensional network material similar to sponge, which has many advantages such as light weight, high strength and toughness, and large specific surface area. Although silicon and tin negative electrode active materials have high theoretical specific capacity and are considered to be one of the promising negative electrode active materials for lithium-ion batteries, they also have disadvantages such as large volume changes and pulverization during cyclic charging/discharging, which seriously affect battery performance. The research shows that the foam copper collector can inhibit the volume change of the active materials of silicon and tin anode during the charge and discharge process, slow down the pulverization phenomenon, and thus improve the battery performance.
1.2 Aluminum current collector
Although the conductivity of aluminum metal is lower than that of copper, the quality of aluminum wire is only half that of copper wire when transporting the same amount of electricity. Undoubtedly, using aluminum current collectors can help improve the energy density of lithium-ion batteries. In addition, compared to copper, aluminum is cheaper in price. During the charging/discharging process of lithium-ion batteries, a dense oxide film is formed on the surface of the aluminum foil current collector, which improves the corrosion resistance of the aluminum foil and is often used as the current collector for the positive electrode in lithium-ion batteries.
Like copper foil current collectors, surface treatment can also improve the surface properties of aluminum foil. After direct current etching, a honeycomb structure will form on the surface of the aluminum foil, which is more tightly bound to the positive electrode active material and improves the electrochemical performance of lithium-ion batteries. However, in fact, aluminum current collectors often suffer from severe corrosion due to the destruction of surface passivation films, leading to a decrease in the performance of lithium-ion batteries. Therefore, in order to improve the corrosion resistance of etched aluminum foil, it is necessary to optimize its surface and form a more stable passivation film.
1.3 Nickel current collector
Relatively speaking, nickel is a base metal with a relatively low price, good conductivity, and stability in acidic and alkaline solutions. Therefore, nickel can be used as both a positive electrode current collector and a negative electrode current collector. There are both positive electrode active materials such as lithium iron phosphate and negative electrode active materials such as nickel oxide, sulfur, and carbon silicon composite materials that match it.
The shape of nickel collector usually includes foam nickel and nickel foil. Because foam nickel has developed channels and large contact area with active substance, the contact resistance between active substance and collector is reduced. When using nickel foil as the electrode current collector, as the number of charge/discharge cycles increases, the active material is prone to detachment, which affects the battery performance. Similarly, the surface pretreatment process is also applicable to nickel foil current collectors. After etching the surface of the nickel foil current collector, the bonding strength between the active substance and the current collector is significantly enhanced.
Nickel oxide has the advantages of stable structure, low price, and high theoretical specific capacity, making it a widely used negative electrode active material for lithium-ion batteries. Based on this, a layer of nickel oxide was in-situ grown on the surface of foam nickel by solid phase oxidation method, and nickel oxide anode with foam nickel as collector was prepared. Compared with nickel foil/nickel oxide negative electrode, the specific capacity of the first discharge of foam nickel/nickel oxide negative electrode increased significantly. The reason is that compared with two-dimensional current collectors, three-dimensional structured current collectors reduce interface polarization phenomena and improve the charge/discharge cycle stability of batteries.
Lithium iron phosphate is considered as an ideal positive active material for power lithium ion batteries because of its good safety and wide source of raw materials. Coating it on the surface of foam nickel collector can increase the contact area between LiFePO4 and foam nickel, reduce the current density of interface reaction, and thus improve the rate discharge performance of LiFePO4.
1.4 Stainless steel current collector
Stainless steel refers to alloy steel containing elements such as nickel, molybdenum, titanium, niobium, copper, and iron. It has good conductivity and stability, and can withstand chemical corrosion from weak corrosive media such as air, steam, and water, as well as strong corrosive media such as acid, alkali, and salt. Stainless steel surface is also prone to forming a passivation film, which can protect its surface from corrosion. At the same time, stainless steel can be processed thinner than copper, with advantages such as low cost, simple process, and large-scale production. Stainless steel can be used as a positive or negative current collector, and common types of stainless steel current collectors include stainless steel mesh and porous stainless steel.
1.4.1 Stainless steel mesh collector fluid
The texture of stainless steel mesh is dense. When used as a current collector, its surface is wrapped by electrode active materials and does not come into direct contact with the electrolyte, making it less prone to side reactions and improving the cycling performance of the battery.
1.4.2 Porous stainless steel current collector
A simple and effective method to fully utilize active materials and improve the discharge specific capacity of electrodes is to use porous current collectors.
1.5 Carbon current collector
When using carbon materials as positive or negative electrode current collectors, it can avoid the corrosion of electrolyte on metal current collectors, and it has the advantages of abundant resources, easy processing, low resistivity, no harm to the environment, and low price.
Carbon fiber cloth can be used as a current collector for flexible lithium-ion batteries due to its excellent flexibility, conductivity, and electrochemical stability. Carbon nanotubes are another form of carbon current collector, which have obvious advantages over metal current collectors in terms of lightweight and can significantly improve the energy density of batteries.
1.6 Composite current collector
In addition to single collectors such as copper collectors, aluminum collectors, nickel collectors, stainless steel collectors, and carbon collectors, composite collectors have also attracted the research interest of scholars in recent years, such as conductive resins, carbon coated aluminum foils, and titanium nickel shape memory alloys.
1.6.1 Conductive resin current collector
Polyethylene (PE) and phenolic resin (PF) current collectors are composed of conductive fillers and a polymer resin matrix. A composite current collector was prepared by uniformly mixing PE and PF as matrix materials with conductive fillers (graphite, carbon black), and their physical and chemical properties were studied. Graphene is a unique and novel two-dimensional carbon functional material formed by sp2 hybridization of carbon atoms. It has many advantages such as ultra-high conductivity, specific surface area, and mechanical strength. It can replace graphite as the negative electrode active material of lithium-ion batteries or as a current collector material.
1.6.2 Titanium nickel shape memory alloy current collector
Titanium nickel shape memory alloy is a binary alloy composed of nickel and titanium, which can transform between two different crystal phases with changes in external temperature or pressure. Titanium nickel shape memory alloy can suppress the volume change of active substances during charging and discharging by changing its own phase state, thereby improving the cycle life of batteries.
1.6.3 Carbon coated aluminum foil current collector
Carbon coated/aluminum foil current collector refers to a composite current collector in which a carbon containing composite layer is coated on the surface of an aluminum foil. Among them, the carbon containing layer is composed of carbon fibers and conductive carbon black particles treated with dispersants, which can be tightly combined with aluminum foil to improve the conductivity and corrosion resistance of the electrode.
The current collector is one of the indispensable and important components in lithium-ion batteries, with multiple functions of carrying electrode active materials and collecting output current. The performance of current collectors prepared from different materials and production processes varies, and their impact on lithium-ion batteries is also different.