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Elementary analysis: Defects in coating process of lithium battery electrode

2020-05-28 16:42:19

The manufacturing of lithium-ion battery is a process closely linked by multiple process steps, mainly including the electrode manufacturing process, battery assembly process, as well as liquid filling, precharge, formation, aging process, etc.


Cell electrode coating is a process that evenly agitated slurry is evenly coated on the collector and the organic solvent in the slurry is dried.

锂电池制造工艺流程

 

Figure 1. Lithium battery manufacturing process


The manufacturing process of battery electrode includes five procedures: slurry preparation, slurry coating, electrode rolling, electrode cutting and electrode drying.


In the process of battery manufacturing, there will be more or less waste of resources in each process, and the reasons are various, such as staff error, equipment error, environment and other factors. In order to ensure that the product yield is high enough, Must try best to ensure that every step of the process is qualified in order to get high enough product yield.


In the process of battery production, there are some defects in the electrode coating process. Reducing these defects, improving the coating quality and yield, and reducing the production cost are the main tasks of the industry professionals.


In the process of electrode coating, the common problems include raw material pollution, unstable coating process, nonstandard operation, wrong drying procedure setting, etc., and these problems will cause more or less defects, such as point defects, thick edge defects and so on.


The spot defects mainly come from bubbles and foreign matters in the slurry. Bubbles can come from incomplete defoaming during mixing, feeding or coating process, and foreign matters mainly come from operation errors or environmental problems.


In the process of electrode coating, bubbles inside the paste are sprayed on the electrode, and when dried in the oven, bubbles break and form white round spots on the electrode. However, the coating of these active substances is thin, which is easy to cause micro short circuit in the process of battery charging and discharging.


In addition, when there are foreign matter in the electrode sheet, the coating area around the particles is a low surface tension area, and the liquid film migrates to the surrounding area in an emission manner, forming a point like defect.


In order to prevent the occurrence of such defects, we can control the operating environment, optimize the slurry mixing, control the coating speed, andmake the substrate clean.


Thick edge defect refers to the fact that the thick edge bears more pressure in the rolling process, which not only causes the difference in the transverse density of the electrode piece, but also causes the active material particles at the thick edge to be crushed.


After being pressed, the plates with thick edge defects will be warped seriously, which will have a great impact on the subsequent slitting and winding process.


After the active material particles at the thick edge are crushed, the transmission path of lithium ion and electron becomes farther during the charging and discharging process, which will lead to the increase of internal resistance and the deepening of polarization of the battery, and affect the service life and safety of the battery.


In addition, lithium evolution and micro short circuit are also very harmful to the performance of the battery.


The main reason for the thick edge is that the slurry moves to the uncoated part of the electrode edge driven by the surface tension of the slurry, and the thick edge is formed after drying.


电池极片干燥过程示意图.png

 

Figure 2. Drying process of battery electrode


The results show that the coating speed has no significant effect on the edge width and height, the edge gradient increases with the increase of coating speed, and the edge effect can be reduced by reducing the gap ratio.


In addition, the related gap coating research shows that the thick edge can be reduced by adjusting the coating gap and pressure pre adjustment, and the thick edge can also be reduced to a certain extent by adding surfactant to reduce the slurry surface tension.


The electrode coating equipment is mainly composed of winding unit, feeding unit, tension control system, coating head, oven and other parts.


Electrode coating can be divided into transfer coating and extrusion coating, and both have advantages and disadvantages.


The results show that the coating speed has no significant effect on the edge width and height, the edge gradient increases with the increase of coating speed, and the edge effect can be reduced by reducing the gap ratio.


In addition, the related gap coating research shows that the thick edge can be reduced by adjusting the coating gap and pressure pre adjustment, and the thick edge can also be reduced to a certain extent by adding surfactant to reduce the slurry surface tension.

转移式涂布的优点是对浆料粘度的要求不高,比较容易调节涂布参数,且没有堵料。

转移式涂布的不足之处在于,对动力电池来说涂布精度较差,无法保证极片的一致性。浆料在辊间暴露于空气中,对浆料的性质有影响。

挤出式涂布是指上料系统将涂料输送给螺杆泵,再将浆料动力输送至挤出头中,通过挤出形式将浆料制成液膜后涂布至移动的集流体上,经过干燥后形成质地均匀的涂层,如图3所示。

挤出式涂布工艺.png

图3. 挤出式涂布工艺

挤出式涂布的优点是,涂膜后极片非常均匀且精度较高,涂层边缘平整度高,密闭操作系统,不受异物影响,适合量产。

挤出式涂布的不足之处在于,设备精度要求较高,维护保养要求也高,浆料粘度范围要求也高,变换规格时需要更换新的垫片。

极片涂布工艺中,浆料涂覆是继制备浆料完成后的一道工序,主要目的是为了将稳定性好、粘度好、流动性好的浆料均匀地涂覆在正负极集流体上。

从电池寿命来讲,浆料涂覆前后差异大、极片混入粉尘、极片左右厚度不均匀等等,都关系到电池电化学性能的优劣。

从电池的容量来讲,在涂布过程中,若极片前、中、后三段位置正负极浆料涂层厚度不一致,就容易引起电池容量过低、过高,在电池循环过程中形成析锂,影响电池寿命。

从电池性能的一致性来讲,电池厂比较忌讳的是电池的容量差异、循环寿命差异较大,所以在极片涂布过程中要保证极片前后参数一致。

从电池的安全性来讲,极片涂布之前要做好5S工作,确保涂布过程中没有颗粒、杂物、粉尘等混入极片中,如果混入杂物会引起电池内部微短路,严重时导致电池起火爆炸。

操作实践表明,挤压式涂布因为具有高精度、涂布均匀、适合较大宽度涂布等优点,被广泛应用于动力电池领域,且逐渐取代适用于中试线的转移式涂布机。

根据市场发展来看,未来涂布工艺可能会朝着高设备性能、高稼动率、在线测厚控制精度、提高干燥效率等方向发展。


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