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Why Do Lithium Iron Phosphate Batteries Use Aluminum Foil for The Positive Electrode And Copper Foil for The Negative Electrode?

Views: 2     Author: Site Editor     Publish Time: 2024-04-17      Origin: Site


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Why do lithium iron phosphate batteries use aluminum foil for the positive electrode and copper foil for the negative electrode? The current collector used to store positive and negative electrode materials is also an important component of lithium iron phosphate batteries. Its main function is to collect the current generated by the battery's active substances, in order to form a larger current output. Why do lithium iron phosphate batteries use aluminum foil for the positive electrode and copper foil for the negative electrode?

Why do lithium iron phosphate batteries use aluminum foil for the positive electrode and copper foil for the negative electrode?

For lithium-ion batteries, the commonly used positive current collector is aluminum foil, and the negative current collector is copper foil. To ensure the stability of the current collector inside the battery, the purity of both is required to be above 98%. With the continuous development of lithium battery technology, whether it is used for digital products or electric vehicles, we all hope that the energy density of the battery is as high as possible, and the weight of the battery is becoming lighter and lighter. In terms of the current collector, the most important thing is to reduce the thickness and weight of the current collector, intuitively reducing the volume and weight of the battery. There are three reasons why aluminum foil is used for the positive electrode of lithium iron phosphate batteries, while copper foil is used for the negative electrode:

1、 Copper aluminum foil has good conductivity, soft texture, and low price.

We all know that the working principle of lithium iron phosphate batteries is an electrochemical device that converts chemical energy into electrical energy. In this process, we need a medium to transfer the electrical energy converted from chemical energy, which requires conductive materials.

Among ordinary materials, metal materials have the best conductivity, while among metal materials, copper foil and aluminum foil are inexpensive and have good conductivity. Meanwhile, in lithium batteries, we mainly have two processing methods: winding and stacking. Compared to winding, the electrodes used for preparing batteries need to have a certain degree of flexibility to ensure that they do not experience brittle fracture and other issues during winding. Among metal materials, copper aluminum foil is also a relatively soft metal. Finally, the cost of preparing lithium batteries should be considered. Relatively speaking, copper and aluminum foil are relatively cheap, and the world is rich in copper and aluminum resources.

2、 Copper aluminum foil is also relatively stable in the air.

Aluminum easily reacts with oxygen in the air, forming a dense oxide film on the surface layer of aluminum to prevent further reaction. This thin oxide film also has a certain protective effect on aluminum in the electrolyte. Copper itself is relatively stable in air and generally does not react in dry air.

3、 The positive and negative electrode potentials of lithium batteries determine the use of aluminum foil for the positive electrode and copper foil for the negative electrode.

The positive electrode potential is high, and copper foil is easily oxidized at high potential, while aluminum has a high oxidation potential. The surface of the aluminum foil has a dense oxide film, which also provides good protection for the internal aluminum. Both are used as fluid collectors because they have good conductivity, a relatively soft texture (which may also be beneficial for bonding), and are relatively common and inexpensive. At the same time, both surfaces can form a layer of oxide protective film.

Copper foil and aluminum foil have advantages such as good conductivity, formed oxide protective film, soft texture that is conducive to bonding, mature manufacturing technology, and relatively low price, so they are selected as the main materials for lithium battery current collectors. The positive electrode potential of lithium iron phosphate batteries is high, and the oxide layer of aluminum foil is relatively dense, which can prevent the oxidation of the current collector. However, copper will undergo lithium insertion reaction at high potential, so it is not suitable to be used as the positive electrode current collector. Aluminum foil is generally used as the positive electrode current collector; The potential of the negative electrode is low, and aluminum foil is prone to forming aluminum lithium alloy at low potential. The negative electrode current collector generally uses copper foil, and there is no interchangeability between copper foil and aluminum foil.

Thickness requirements for copper aluminum foil used in lithium iron phosphate batteries

With the rapid development of lithium iron phosphate batteries in recent years, the development of current collectors for lithium batteries has also been rapid. The positive electrode aluminum foil has decreased from 16um in previous years to 14um, and then to 12um. Nowadays, many lithium battery manufacturers have mass produced and used 10um aluminum foil, even using 8um. The use of copper foil for the negative electrode, due to its good flexibility, has reduced its thickness from 12um to 10um, and then to 8um. Currently, a large number of battery manufacturers use 6um in mass production, and some manufacturers are developing 5um/4um which may be used.

Due to the high purity requirements of copper aluminum foil used in lithium iron phosphate batteries, the density of the material is basically at the same level. As the development thickness decreases, the surface density also decreases accordingly, and the weight of the battery naturally becomes smaller, which meets our needs for lithium batteries.

Surface roughness requirements for copper aluminum foil used in lithium batteries

For the current collector, in addition to its thickness and weight having an impact on lithium iron phosphate batteries, the surface performance of the current collector also has a significant impact on the production and performance of the battery. Especially for the negative current collector, due to defects in preparation technology, copper foils on the market are mainly single-sided, double-sided, and double-sided roughened varieties. This two sided structure asymmetry leads to asymmetric contact resistance between the coatings on both sides of the negative electrode, which in turn prevents the uniform release of the negative electrode capacity on both sides; Meanwhile, the asymmetry on both sides also leads to inconsistent bonding strength of the negative electrode coating, resulting in a serious imbalance in the charging and discharging cycle life of the two negative electrode coatings, which in turn accelerates the capacity degradation of lithium batteries.

What are the pollution problems in the production of lithium batteries?

Firstly, the pollution caused by the cleaning of positive and negative electrode pulping equipment and coating equipment during the production process includes substances such as positive and negative electrode materials, NMP, glue, etc. Some companies have poor control (maliciously speculate not to control), resulting in the leakage of toxic wastewater. The cleaning and injection of electrolyte during the production process of lithium batteries may cause water containing electrolyte to flow into the sewer. In general, technical researchers are likely to directly discharge water into the sewer.

Secondly, NMP recycling is not thorough.

Thirdly, there are indeed very few lithium batteries circulating in the market that are recycled. Firstly, lithium batteries themselves have low pollution, and people do not attach enough importance to them; Secondly, the revenue from lithium battery recycling is low. As a result, lithium batteries do not have the same large-scale recycling applications as lead-acid batteries, but are more like nickel hydrogen batteries, with only a few slogans such as "do not dispose of batteries casually".

The above is why lithium iron phosphate batteries use aluminum foil for the positive electrode and copper foil for the negative electrode. The positive and negative electrode materials of lithium batteries have a significant impact on the battery, so everyone must choose carefully.

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