Foil, as a current collector, is an important production material besides the four main materials of lithium battery (Cathode material, cathode material, electrolyte, diaphragm) . To increase the capacity of the battery, it is necessary to increase the amount of positive and negative active materials, so that the increase of positive and negative coating can only reduce the thickness of separator and current collector. Of course, is not the thinner the better, too thin collector easy to break, preheat shrinkage fast.

Today we mainly talk about the negative current collector-copper foil manufacturing process. Have studied the chemistry the small partner all to know, here most main craft is the electrolysis. Electrolytic copper foil, from the electrolyte of copper ions in the smooth rotation of stainless steel plate (generally titanium plate) on the round cathode drum deposition, copper foil close to the surface of the cathode drum called smooth surface, while the other side is called hair surface. In the early years, we used more than 7 m of copper foil, but now, 6 m have been mass production, even 5 m have begun to trial. But the domestic general foil factory, is not the sole production of lithium foil, there are electronic foil, that is, for computers, PCB copper foil.

1. Dissolve copper

As the name suggests, first buy back the raw materials. The raw materials here are mainly copper wires. The larger the surface area of the copper, the better. There should be smaller gaps between the copper materials to increase the reaction area, the treated copper wire, which is 95% or more, is placed in a tank of sulfuric acid and heated, of course, to which oxygen is added, usually Compressed air, which we call a blast, to facilitate the oxidative reaction, to promote the dissolution of copper. Here we mainly look at the copper content of the material, reaction rate and the total surface area of copper in the trough, the larger the surface area, the faster the reaction rate. Secondly, it is related to the air volume, the air volume increases and the reaction speed also speeds up.

2. Filter

The copper wire after the copper pot has dissolved into copper sulfate liquid, of course, there will be some impurities in the copper wire, and we lithium battery factory is very nasty to impurities, in addition to the finished product testing will require impurities. So when the copper is dissolved, we filter out the liquid copper sulfate, filtering out the impurities and some large undissolved substances. The filtration is divided into multi-stage, there are primary filtration, small Mesh size, large aperture, mainly to intercept the large material; diatomite filtration, adsorption of some impurities; and precision filtration, Mesh size, small aperture. From before we know, copper wire dissolved in sulfuric acid, so our filter screen material, need to choose acid-resistant corrosion-resistant material, such as resin, polyester fiber, etc. .

Since the temperature is too high to transfer the copper sulfate solution directly to the back, the temperature needs to be lowered. In addition, some additives need to be added. The additives are also the core of a copper foil enterprise, in particular, the type and proportion of additives. Liquid cooling is generally double-layer pipeline heat exchange, internal flow of copper sulfate solution, external flow of cooling water.

4. Foil passivation

After cooling the copper sulfate liquid to the most critical step, the green foil. The so-called Green Foil, is electrolytic copper reduction into copper process. There's a cathode roller, an anode holder, an electrolyzer. Cathode roll is generally titanium roll, smooth surface, roughness to meet the requirements; anode base is generally pure copper plate; copper sulfate as electrolyte. Under the action of direct current, the cation moves to the cathode and the anion moves to the anode. Cu 2 + can be reduced to Cu 2 + on the cathode, and the CU 2 + can be crystallized on the cathode roller to form the foil. Impurities such as iron and zinc, which are more active than copper, are dissolved with copper into ions (Zn and Fe) , which are less likely to precipitate out than copper ions, therefore, the ion precipitation on the cathode can be avoided by properly adjusting the potential difference during electrolysis. Less reactive impurities such as gold and silver than copper are deposited at the bottom of the cell. The response is as follows.

(1) Anodic reaction: Cu -- 2E -= Cu2Me -- 2E -= Me2H2O -- 2E -= 2H 1/2O2SO4 2- -- 2E -= SO3 1/2O2

In the formula, Me represents more electronegative metals such As FE, NI, PB, As and SB, which dissolve from the anode into the solution. The reaction of H2o and So42-to lose electrons does not occur under normal conditions because the potential of H2o is more positive than that of copper. The noble metal's potential is corrected and does not dissolve into the anode mud.

(2)Cathode reaction: Cu22E = Cu2H 2E - = H2Me2E - = Me

In these reactions, metal ions with standard potential higher than copper can be reduced on the Cathode, but they do not dissolve on the anode, so only copper ion reduction is the main reaction. Copper Ions to the electrons in the cathode after reduction, then the formation of copper foil, exposed to the air of copper foil will be oxidized, so will add passivation solution. In general, copper foil is passivated with chromate (or zinc salt) solution (that is, anti-oxidation treatment) , which makes the surface of copper foil form a complex film layer with chromium (or chromium-zinc) as the main body, so that the copper foil will not change color because of direct contact with air, at the same time, the heat resistance of copper foil is improved. Here the thickness of the copper foil is controlled by the cathode current density and the speed of the cathode roller.

5. Cut

Foil passivated by green foil, usually in large rolls of more than one meter. According to the needs of each lithium battery factory, cut into the desired length, at the same time, just raw foil for testing, testing whether the surface is smooth, whether there is a dent, whether there is a bulge, and so on, for unqualified foil recycling.

6. Test items

The main test items are thickness, surface density, tensile strength/elongation, roughness, anti-oxidation, brightness and so on. Whether you want 5M, 6m, or 7M, each has its own requirements, the test is not difficult, micrometer can test; surface density test can choose a square or round piece of copper foil, weighing, and then calculate; Tensile strength/elongation can be tested under normal testing conditions using a tensile machine, like we use more than 7M copper foil, the tensile strength should reach 400N/m2, the elongation is more than 4% ; roughness can be measured using a roughness tester, foil is divided into rough surface and smooth surface, there will be some differences; wettability, with a Dain pen can be used to test surface tension, wettability, Dain pen simple and efficient; anti-oxidation performance is mostly baked, and then watch its color change.

Summary above, electrolytic copper foil production process is simple, the main process has three: Solution Foil, surface treatment and product cutting. The production process seems to be simple, but it is a set of electronics, machinery, electrochemical as one, and the production environment requirements are particularly strict a production process. Therefore, the electrolytic copper foil industry does not have a set of standard and universal production equipment and technology, each manufacturer has its own magic power. This is also an important bottleneck that affects the domestic electrolytic copper foil production capacity and quality improvement. In addition, the requirements of customers vary, it also leads to waste of copper foil, which is a kind of intermediate product in manufacturing. The ratio of one input to one output is less than 80% . In the future, now the main line of manufacturers towards ultra-thin and long winding direction in the optimization upgrade, I believe that the industry will be standardized, standardized guidance, and better.

Source: LITHIUMAN Jiangzicai public

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