Battery current collector – importance in battery

Batteries play an important role in our daily life. Have you ever wondered what components make up lithium ion

battery structure?

How can they work properly? This article will introduce the battery current collector, which is an important part of the battery.

What is battery current collector?

Battery 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 beneficial to reduce the internal resistance of the lithium-ion battery and improve the coulombic efficiency, cycle stability and rate performance of the battery.

Battery current collectors are usually foils of aluminum and copper, which account for more than 15% of the battery’s weight. For example, copper is easily oxidized at a higher potential and is suitable for use as a negative electrode current collector.

While aluminum is used as a negative electrode current collector because the corrosion problem is more serious and is suitable for use as a positive electrode current collector.

In principle, an ideal current collector for lithium-ion batteries should meet the following conditions: (1) high electrical conductivity; (2) good chemical and electrochemical stability; (3) high mechanical strength; (4) compatibility with electrode active materials Good compatibility and binding force; (5) cheap and easy to obtain; (6) light in quality.

However, in the actual application process, there are still some problems with different battery current collector materials, so they cannot fully meet the above multi-scale requirements.

The development trend of battery current collector

The current development trend of battery current collectors is mainly thinner and lighter. In the entire lithium-ion battery, the battery current collector occupies a large weight component, but it does not provide energy density by itself. Light and thin is the demand for high-energy-density lithium-ion batteries.

At present, the thickness of aluminum foil is usually 10um, and the minimum can reach 8um; the thickness of copper foil is usually 6um, and the minimum can reach 4.5um; in terms of mass ratio, copper foil accounts for about 9%, and aluminum foil accounts for about 7%.

In terms of cost ratio, taking the ternary lithium battery as an example, the cost of aluminum foil accounts for 1.3%, and the cost of copper foil accounts for 7.8%;
Taking lithium iron phosphate as an example, the cost of aluminum foil accounts for 1.7%, and the cost of copper foil accounts for nearly 10%;

Thinning of the battery current collector mainly brings about:
① Reduce the material cost of the battery;
② The energy density of the battery can be increased by thinning and weight reduction. Compared with the 8um lithium battery copper foil, the use of 6um/4.5um lithium battery copper foil can increase the energy density of the lithium battery by 5%/9% respectively.

Production of battery current collectors

Production of Copper Foil:

There are two production methods of copper foil: one is rolled copper foil, and the other is electrolytic copper foil. Calendered copper foil is made by repeated heating and rolling of copper plate. The copper foil obtained by this method has good elasticity and folding resistance, and the surface is smooth.

Electrolytic copper foil is to electrolyze Cu+ in copper sulfate solution onto steel or a roller, and then wind up the copper foil. The copper foil obtained by this method has different roughness on both sides. The side close to the cathode roller is smooth, and the other side is rough. Matte. At present, electrolytic copper foil occupies most of the market.

Compared with rolled copper foil, electrolytic copper foil is relatively simple to prepare and low in cost, so most lithium ion batteries use electrolytic copper foil as the negative electrode substrate.

In the preparation process of electrolytic copper foil, it is necessary to control the temperature, concentration, additive content of the electrolyte and the flow rate of the electrolyte when the foil is produced. These factors will affect the purity, thickness, weight, strength and elongation of copper foil.

The finished copper foil needs to be cleaned subsequently to remove residual electrolyte on the surface, surface anti-oxidation treatment and drying treatment. The degree of these surface treatments directly affects the surface finish and surface tension of the copper foil.

After the surface treatment, the slitting is completed and cut into the size required by the lithium battery manufacturer. For the development of copper foil current collectors in the future, the most intuitive way is to cooperate with copper foil suppliers to develop high-strength, high-elongation copper foil, or develop thinner copper foil or use copper mesh to increase battery energy density.

Production of aluminum foil:

Aluminum foil preparation process: the aluminum foil blank is prepared to the required thickness after multiple heat treatment and rolling, the aluminum foil is surface treated after finishing rolling, and finally cut into the size required by the manufacturer.

Generally, lithium battery manufacturers use aluminum ingots provided by upstream suppliers for rolling, and achieve the expected strength and elongation of the aluminum foil by controlling the rolling pressure and heat treatment process.

For aluminum foil, its excellent performance is mainly achieved through surface treatment, such as roughening treatment, cleaning treatment, etc. Recently, carbon-coated aluminum foil has been studied more, that is, a thin layer of conductive carbon is coated on the surface of normal aluminum foil to optimize battery performance.

Some special battery current collectors

Special battery current collectors are mainly for lithium batteries with special performance requirements, or current collectors that have not been developed for higher performance materials. For example, motorcycle battery pack need to meet the demand for higher energy density.

Some time ago, relevant experts proposed “no negative electrode battery”, which actually imagined that the negative electrode copper foil was replaced by metal lithium, and the lithium layer was very thin; at the same time, some experts proposed to replace the higher-priced copper with cheap iron.

However, this information was quickly questioned after it was released: From the perspective of lithium layer and iron foil materials themselves, the performance cannot meet the high performance required by lithium batteries.

From the perspective of manufacturing processes and equipment, it is difficult to meet the production requirements. The cost issue is also basically difficult to solve.

There are also R&D teams that do not use other materials to replace the copper foil of the negative electrode substrate, but start from the copper foil itself and process it accordingly to improve performance and optimize the performance of lithium batteries.

The most intuitive way is to cooperate with copper foil suppliers to develop high-strength and high-elongation copper foil, and develop thinner copper foil to improve battery energy density; in addition, copper mesh is used to increase battery energy density.