Revealing the "little devil of thick edges" in negative electrode coating of lithium batteries
Hello, all you battery manufacturing experts! Today, we are going to discuss a significant challenge in lithium battery production - the "thick edge" occurrence in negative electrode coating. This "thick edge" is like a mischievous imp, constantly making trouble during the production of lithium batteries and severely impacting the quality and production efficiency. Now, what exactly is this "thick edge"? Don't be impatient. Let me tell you in detail.
Ⅰ.The peculiar manifestations of the “thick edge” phenomenon
1.The thick edge adventure at start and end points
At the outset of this coating adventure, the slurry is like a drowsy little lazybones and has not entered a stable flowing state yet. Consequently, it accumulates like a small mound at the starting point, and the coating thickness instantly thickens. When the adventure is coming to an end, the slurry supply is abruptly halted. But it still has the little tail of flow inertia. It piles up once again at the end point, and so the thick edge comes into being.
2.The thick edge crescent show on both sides
During the fascinating journey of coating, the slurry at the edges is rather unruly. It is subjected to a different shear force from the internal slurry, which easily leads to uneven flow. Consequently, the edges on both sides of the coating thicken, resembling a lovely crescent. Nevertheless, this thick edge is not merely endearing in appearance. It will cause the coating thickness to be uneven and also have an impact on the electrochemical performance and safety of the battery.
So, how can we judge if the coating thickness is uniform? Well, at this moment, our professional thickness measuring instrument needs to take the stage! Use it to measure the coating at different locations and then compare the differences in the measurement results. If the difference is minor, it indicates that the coating thickness is relatively even, and you can give yourself a pat on the back and celebrate. If the difference is significant, then you must be on high alert. It means there is an issue of uneven thickness. Quickly find the “little devil of thick edges”.
Ⅱ.The causes of the birth of the “little devil of thick edges
1.Slurry fluid characteristics - the source of magic for the little devil
（1）The viscosity and surface tension of the slurry: If the viscosity of the slurry becomes like a big fatty or the surface tension turns into a strongman, then its fluidity will deteriorate. During coating, it is prone to accumulate. Just like the little devil has cast a spell, and the thick edge phenomenon occurs.
（2）Thixotropy of slurry: Some slurries have a magical ability. When there is shear force, the viscosity becomes lower. When there is no shear force, the viscosity returns to its original high level. This characteristic causes the viscosity of the slurry to change at the starting and ending points of coating. It's just like the little devil performing magic, which then leads to thick edges.
2.Coating process parameters exceeding the coating window range - the stage for the little devil to make trouble
（1)Slurry volume flow: If the slurry volume flow becomes like a flood, too much slurry will be coated in unit time. At the starting point, ending point, and edges, it cannot be distributed evenly in time. Thus, the little devil of thick edges happily appears.
（2）Coating speed: If the coating speed turns into a speedster, the slurry will not have enough time to spread evenly on the coating surface. Especially in the edge areas, it is very easy for the little devil of thick edges to take over.
（3）Slit gap: If the slit gap is set unreasonably, either becoming too large like a big fatty or too small like a skinny person, it will affect the uniformity of slurry outflow. Then the little devil of thick edges will take the opportunity to cause trouble.
Besides slurry characteristics being the magic source of the “little devil of thick edges”, other elements such as equipment precision and stability and environmental factors can also provide a stage for it to cause chaos. Equipment with precision deviations or unstable operation may lead to inaccurate control of coating parameters. Changes in temperature and humidity can affect slurry fluidity and drying speed, influencing coating process parameters and giving the “little devil” a chance.
3.Improper temperature control during the baking process - heating assistance for the little devil
（1）Excessive baking temperature: If the temperature turns into a raging furnace, the binder in the coating will rapidly volatilize or migrate. Just like the little devil being heated up, it causes changes in the fluidity of the slurry at the edges and forms thick edges.
（2）Uneven temperature distribution: During baking, if the temperature turns into a naughty imp and is not evenly distributed, the drying speed of the coating in different areas will be different, and the little devil of thick edges may also take the opportunity to emerge.
Ⅲ.The consequences of the “little devil of thick edges” causing trouble
1.Great damage to the battery manufacturing process
（1）Increase the difficulty of subsequent processes: Uneven coating thickness will turn the subsequent processes such as rolling and slitting into big challenges. It's just like the little devil has set up obstacles for the production process, affecting both production efficiency and product quality.
（2）Affect electrode consistency: The thick edge phenomenon will make the thickness differences of electrodes in the same batch very large. It's just like the little devil has made the electrodes uneven, reducing the consistency of the electrodes and thus affecting the overall performance of the battery.
2.Great impact on battery performance
（1）Reduce battery capacity: The thick edge area has a relatively high content of active materials, but this is like a trap set by the little devil. Because the electrode is uneven, the current distribution inside the battery is uneven, the charge and discharge performance is affected, and the battery capacity is reduced.
（2）Affect battery cycle life: Uneven coating thickness will cause uneven volume changes of the electrode during the charge and discharge process. It's just like the little devil is causing damage and is likely to cause damage to the electrode structure, thereby shortening the cycle life of the battery.
（3）Increase safety hazards: The heat accumulation in the thick edge area may be more than in other areas. It's just like the little devil is secretly setting fires. In extreme cases, it may cause safety issues such as thermal runaway.
Ⅳ.The magic secret book to defeat the “little devil of thick edges
1.Optimize coating process parameters - put a tight hoop on the little devil
（1）Precisely control slurry volume flow rate: By optimizing the feeding system, invite the two masters of high-precision metering pumps and flow controllers. Ensure that the slurry volume flow rate is stable and accurate, and matches the coating speed and slit gap, leaving the little devil with nowhere to escape.
（2）Adjust coating speed: According to the characteristics of the slurry and the capacity of the equipment, select an appropriate coating speed. Generally speaking, a slower coating speed helps the slurry spread evenly. It's like slowing down the little devil. However, production efficiency must also be considered. We can't let the little devil be too leisurely.
（3）Optimize the slit gap: Through experiments and simulations, find the optimal slit gap. It's like creating a suitable cage for the little devil. Let the slurry flow out evenly from the slit and form a coating with uniform thickness.
2.Improve slurry preparation - weaken the magic of the little devil
（1）Optimize the slurry formula: Select the proper ratio of binder, conductive agent, and active material. It's like making adjustments to the magic formula of the little devil to improve the fluidity and stability of the slurry.
（2）Enhance the uniformity of slurry dispersion: Employ advanced dispersion equipment and techniques, such as high-speed stirring and ultrasonic dispersion which are powerful tools, to ensure that the particles in the slurry are evenly dispersed. Don't let the magic of the little devil prevail and prevent the occurrence of agglomeration.
（3）Remove bubbles: During the slurry preparation process, take effective defoaming measures, such as vacuum defoaming. It's like popping the magic bubbles of the little devil to reduce the impact of bubbles during the coating process.
3.Control the baking process - cool down the little devil
（1）Optimize the baking temperature curve: According to the slurry composition and coating thickness, formulate a reasonable baking temperature curve. It's like setting temperature rules for the little devil. Don't let the temperature be too high or too low, nor can it change too quickly.
（2）Guarantee uniform temperature distribution: Enhance the structure and heating mode of the baking equipment. For instance, employing hot air circulation or infrared heating is like upgrading the oven for the little devil to ensure the uniformity of temperature distribution of the coating during the baking process.
The “little devil of thick edges” in lithium battery negative electrode coating is cunning and relates to slurry fluid characteristics, coating process parameters and the baking process. But don't be afraid. By analyzing its causes and impacts and using magic handbooks like optimizing process parameters, improving slurry preparation and controlling baking, we can defeat it and reduce thick edge phenomena, improving battery performance and consistency. Let's work together to make superb lithium batteries.
Ⅰ.The peculiar manifestations of the “thick edge” phenomenon
1.The thick edge adventure at start and end points
At the outset of this coating adventure, the slurry is like a drowsy little lazybones and has not entered a stable flowing state yet. Consequently, it accumulates like a small mound at the starting point, and the coating thickness instantly thickens. When the adventure is coming to an end, the slurry supply is abruptly halted. But it still has the little tail of flow inertia. It piles up once again at the end point, and so the thick edge comes into being.
2.The thick edge crescent show on both sides
During the fascinating journey of coating, the slurry at the edges is rather unruly. It is subjected to a different shear force from the internal slurry, which easily leads to uneven flow. Consequently, the edges on both sides of the coating thicken, resembling a lovely crescent. Nevertheless, this thick edge is not merely endearing in appearance. It will cause the coating thickness to be uneven and also have an impact on the electrochemical performance and safety of the battery.
So, how can we judge if the coating thickness is uniform? Well, at this moment, our professional thickness measuring instrument needs to take the stage! Use it to measure the coating at different locations and then compare the differences in the measurement results. If the difference is minor, it indicates that the coating thickness is relatively even, and you can give yourself a pat on the back and celebrate. If the difference is significant, then you must be on high alert. It means there is an issue of uneven thickness. Quickly find the “little devil of thick edges”.
Ⅱ.The causes of the birth of the “little devil of thick edges
1.Slurry fluid characteristics - the source of magic for the little devil
（1）The viscosity and surface tension of the slurry: If the viscosity of the slurry becomes like a big fatty or the surface tension turns into a strongman, then its fluidity will deteriorate. During coating, it is prone to accumulate. Just like the little devil has cast a spell, and the thick edge phenomenon occurs.
（2）Thixotropy of slurry: Some slurries have a magical ability. When there is shear force, the viscosity becomes lower. When there is no shear force, the viscosity returns to its original high level. This characteristic causes the viscosity of the slurry to change at the starting and ending points of coating. It's just like the little devil performing magic, which then leads to thick edges.
2.Coating process parameters exceeding the coating window range - the stage for the little devil to make trouble
（1)Slurry volume flow: If the slurry volume flow becomes like a flood, too much slurry will be coated in unit time. At the starting point, ending point, and edges, it cannot be distributed evenly in time. Thus, the little devil of thick edges happily appears.
（2）Coating speed: If the coating speed turns into a speedster, the slurry will not have enough time to spread evenly on the coating surface. Especially in the edge areas, it is very easy for the little devil of thick edges to take over.
（3）Slit gap: If the slit gap is set unreasonably, either becoming too large like a big fatty or too small like a skinny person, it will affect the uniformity of slurry outflow. Then the little devil of thick edges will take the opportunity to cause trouble.
Besides slurry characteristics being the magic source of the “little devil of thick edges”, other elements such as equipment precision and stability and environmental factors can also provide a stage for it to cause chaos. Equipment with precision deviations or unstable operation may lead to inaccurate control of coating parameters. Changes in temperature and humidity can affect slurry fluidity and drying speed, influencing coating process parameters and giving the “little devil” a chance.
3.Improper temperature control during the baking process - heating assistance for the little devil
（1）Excessive baking temperature: If the temperature turns into a raging furnace, the binder in the coating will rapidly volatilize or migrate. Just like the little devil being heated up, it causes changes in the fluidity of the slurry at the edges and forms thick edges.
（2）Uneven temperature distribution: During baking, if the temperature turns into a naughty imp and is not evenly distributed, the drying speed of the coating in different areas will be different, and the little devil of thick edges may also take the opportunity to emerge.
Ⅲ.The consequences of the “little devil of thick edges” causing trouble
1.Great damage to the battery manufacturing process
（1）Increase the difficulty of subsequent processes: Uneven coating thickness will turn the subsequent processes such as rolling and slitting into big challenges. It's just like the little devil has set up obstacles for the production process, affecting both production efficiency and product quality.
（2）Affect electrode consistency: The thick edge phenomenon will make the thickness differences of electrodes in the same batch very large. It's just like the little devil has made the electrodes uneven, reducing the consistency of the electrodes and thus affecting the overall performance of the battery.
2.Great impact on battery performance
（1）Reduce battery capacity: The thick edge area has a relatively high content of active materials, but this is like a trap set by the little devil. Because the electrode is uneven, the current distribution inside the battery is uneven, the charge and discharge performance is affected, and the battery capacity is reduced.
（2）Affect battery cycle life: Uneven coating thickness will cause uneven volume changes of the electrode during the charge and discharge process. It's just like the little devil is causing damage and is likely to cause damage to the electrode structure, thereby shortening the cycle life of the battery.
（3）Increase safety hazards: The heat accumulation in the thick edge area may be more than in other areas. It's just like the little devil is secretly setting fires. In extreme cases, it may cause safety issues such as thermal runaway.
Ⅳ.The magic secret book to defeat the “little devil of thick edges
1.Optimize coating process parameters - put a tight hoop on the little devil
（1）Precisely control slurry volume flow rate: By optimizing the feeding system, invite the two masters of high-precision metering pumps and flow controllers. Ensure that the slurry volume flow rate is stable and accurate, and matches the coating speed and slit gap, leaving the little devil with nowhere to escape.
（2）Adjust coating speed: According to the characteristics of the slurry and the capacity of the equipment, select an appropriate coating speed. Generally speaking, a slower coating speed helps the slurry spread evenly. It's like slowing down the little devil. However, production efficiency must also be considered. We can't let the little devil be too leisurely.
（3）Optimize the slit gap: Through experiments and simulations, find the optimal slit gap. It's like creating a suitable cage for the little devil. Let the slurry flow out evenly from the slit and form a coating with uniform thickness.
2.Improve slurry preparation - weaken the magic of the little devil
（1）Optimize the slurry formula: Select the proper ratio of binder, conductive agent, and active material. It's like making adjustments to the magic formula of the little devil to improve the fluidity and stability of the slurry.
（2）Enhance the uniformity of slurry dispersion: Employ advanced dispersion equipment and techniques, such as high-speed stirring and ultrasonic dispersion which are powerful tools, to ensure that the particles in the slurry are evenly dispersed. Don't let the magic of the little devil prevail and prevent the occurrence of agglomeration.
（3）Remove bubbles: During the slurry preparation process, take effective defoaming measures, such as vacuum defoaming. It's like popping the magic bubbles of the little devil to reduce the impact of bubbles during the coating process.
3.Control the baking process - cool down the little devil
（1）Optimize the baking temperature curve: According to the slurry composition and coating thickness, formulate a reasonable baking temperature curve. It's like setting temperature rules for the little devil. Don't let the temperature be too high or too low, nor can it change too quickly.
（2）Guarantee uniform temperature distribution: Enhance the structure and heating mode of the baking equipment. For instance, employing hot air circulation or infrared heating is like upgrading the oven for the little devil to ensure the uniformity of temperature distribution of the coating during the baking process.
The “little devil of thick edges” in lithium battery negative electrode coating is cunning and relates to slurry fluid characteristics, coating process parameters and the baking process. But don't be afraid. By analyzing its causes and impacts and using magic handbooks like optimizing process parameters, improving slurry preparation and controlling baking, we can defeat it and reduce thick edge phenomena, improving battery performance and consistency. Let's work together to make superb lithium batteries.
