The reason for the lithium battery anode electrode sticking to the roller

During the process of rolling and pressing the anode electrode materials, the problem of sticking to the roller is often encountered. The sticking of the anode electrode materials to the roller not only wastes working hours and affects work efficiency, but also may render the electrode unusable, resulting in economic losses. Therefore, it is very important for lithium battery production and manufacturing to analyze the reasons for the sticking of the anode electrode to the roller and understand the problems. Researchers have summarized and analyzed the reasons for the sticking of anode electrode materials to the roller in practice, mainly including eight aspects. Let's take a look at them below.

1.       The surface of the roller axis of the rolling mill is not cleaned properly. Because the surface of the roller axis is coated with a protective layer when the equipment is not in use, it needs to be cleaned before use. If the surface of the roller axis is not clean when rolling the anode electrode sheets, it is easy to stick to the roller. Some lithium battery companies separate and use equipment for different systems and materials of positive (oil-based) and anode (water-based) electrodes to avoid mutual pollution. However, there are also special cases where positive and anode electrode sheets share the same rolling mill, and even the coating machine is shared by both. Frequent replacement of positive and anode electrode sheets can lead to cross-contamination and easy sticking to the roller.

2.       The anode electrode sheets are not fully dried. If the oven temperature is not high enough or the running speed is too fast during coating, the electrode sheets may not reach the drying standard. When the sheets are being rolled, if they still contain a certain amount of moisture, the binder cannot fully exert its ability to bond various substances. The adhesion between the anode electrode graphite, copper foil, and binder is weak, and it is easy for the sheets to stick to the roller during the deformation process of rolling. A piece of electrode sheet can be taken for weighing and then placed in the oven for a period of time for baking and then weighed again. The difference in weight can be used to determine whether the drying of the electrode sheets during coating is satisfactory.

3.       The temperature of the oven is too high, and the negative electrode is too dry. If the baking temperature is too high, the solvent will evaporate too fast, and the binder will volatilize and adhere to the surface of the electrode, forming a micro-structure of the electrode with a stepwise increase in binder concentration from the foil to the surface of the electrode. During rolling, the surface negative electrode adhesion force is greater than the adhesion force between the foil and the negative electrode material, which is prone to the phenomenon of sticking to the roller, resulting in particles falling off the roller and onto the electrode. If there is linear sticky material on the electrode during rolling, it is necessary to check the electrode drying oven of the coating machine. It is possible that there are screws on the fixed air nozzle that are not screwed on or tightened. Different settings of the exhaust frequency of the oven during coating will affect the evaporation rate of the solvent on the electrode and the distribution of the binder, which will also affect the rolling of the electrode.

4.       If the binder content in the slurry is too low, it can lead to insufficient adhesion between the active materials and insufficient adhesion with the foil. Generally, SBR and CMC are used synergistically as binders for graphite anode electrodes, with CMC as a thickening agent and SBR as a binder. When the content of CMC and SBR is too low, the graphite and carbon black in the anode electrode cannot be evenly dispersed in the slurry, leading to agglomeration. SBR cannot evenly cover the surface of the graphite particles, resulting in poor adhesion between the graphite particles in the electrode sheets, as well as between the graphite particles and the foil. When rolling, the graphite particles tend to detach and adhere to other objects immediately. When using aqueous anode electrode slurry, the ratio of CMC to SBR can be considered, as too little can lead to poor adhesion.

5.       If the ratio of SBR in the mixing process is not suitable, it can lead to sticking of the roller during the rolling of the electrode sheets. SBR used in lithium batteries is a water-based binder of styrene-butadiene rubber latex. It is produced by emulsion polymerization and copolymerization of styrene and butadiene monomers using water as the medium and adding emulsifiers and initiators. It is an aqueous emulsion with a solid content of about 50%. SBR is a substance that coexists with hydrophilic and hydrophobic properties. The aqueous groups bind to the surface groups of the foil to form adhesion, while the oily chain segments bind to the anode electrode graphite to form adhesion, achieving the effect of bonding. If SBR floats and appears blue during slurry preparation, it will cause uneven distribution of SBR concentration after coating, resulting in poor adhesion between the active material and the foil. This can easily lead to sticking of the roller during rolling.

6.       The type of anode electrode material can affect the rolling process. Compared to natural graphite, artificial graphite particles have rougher surfaces, more pointed and inverted corner morphologies, and more irregular structures, making them more difficult to wet. When artificial graphite is not fully dispersed, it is more prone to agglomeration. This agglomeration is gelatinous, can pass through screens, and the agglomerated areas of the coated electrode sheets are denser, making it easy to over-press and form point-like sticking and material shedding during rolling.

7.       The mixing process can also affect the rolling of the electrode sheets. Artificial graphite is difficult to disperse, and during conventional wet slurry preparation, long-term high-speed stirring can easily break the CMC molecular chain, resulting in poor dispersion effect. Simply increasing the line speed and the proportion of CMC often does not yield good results.

8.       When preparing the artificial graphite anode electrode mixture, both CMC and SBR are used as additives to ensure the stability and coating performance of the slurry. Different types of CMC can also affect the microstructure of the electrode sheets and cause sticking of the roller during rolling. This is because insoluble substances in CMC can form particle agglomerates with small particles of graphite and Super-P, making it difficult for the slurry to pass through screens. This can lead to multiple blockages in the screen mesh, coating scratches, and sticking of the roller during rolling

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