The electrode peel strength of lithium ion batteries has a significant impact on the performance, so it is necessary to explore the factors affecting the peel strength. This paper mainly discusses the influence of coating process and slurry.
Changes in the internal structure of the electrode during drying
In the initial stage of drying, the components are uniformly distributed in the electrode. During the drying process, due to the evaporation of the solvent, the electrode film shrinks, a solubility gradient exists at the gas-liquid interface, and a meniscus is formed on the surface of the consolidated particle layer. The solvent migrates between the graphite particles to the surface by capillary force and is accompanied by the expansion of the particle layer. The particle size of the SBR binder and carbon black is at least 100 times smaller than that of graphite. The graphite particle spacing is sufficient to allow the SBR and carbon black to flow freely. The solvent flow will migrate with the polymer and small particles, enriching them in the upper layer of the electrode. , when the solvent is completely evaporated, the electrode film shrinkage stops.
The drying process is mainly divided into the following three stages:
(1) The solvent content at the air-membrane interface decreases continuously until the critical concentration is reached.
(2) From this critical point, the solvent concentration at the air-membrane interface remains constant,
(3) When the solvent is further evaporated, the solvent solubility at the air-film interface begins to decrease until the film is formed.
Effect of Baking Rate on Peel Strength
From the analysis of the drying process, during the drying process of the pole piece, the SBR binder and carbon black migrate to the surface with the evaporation of the solvent, so how does the baking rate affect the distribution of the internal components of the electrode?
The figure below shows the distribution of binder concentration from the current collector to the gas-liquid interface under different baking rates. It can be seen from the figure that in the case of low baking rate (LDR), the binder distribution at different positions is relatively uniform , and in the case of high bake rate (HDR), the binder concentration from the current collector to the gas-liquid interface is continuously increasing, and the binder distribution is very small at the current collector and dressing area, which will directly lead to a decrease in peel strength. , and the electrode impedance will also increase accordingly.
Improvement of coating process
When designing high energy density batteries, the electrode thickness is generally large. According to the electrode characteristics of the baking process, Darjen Liu et al. proposed a new type of double-layer coating, and designed two slurries with different binder contents. The binder content of the upper coating was lower than that of the lower coating. Increase the binder content of the undercoat to compensate for the effect of binder migration to the surface during drying.
The figure below shows different coating schemes, in which 80°C and 150°C represent the baking temperature respectively, 3.0wt%, 3.5wt%, 4.0wt%, 4.5wt% and 5.0wt% respectively represent the paste coating with different binder contents. Floor.
Effect of Coating Area Density on Peel Strength
If the fixation speed of the particles in the electrode structure is faster than the compensation flow speed of the solvent to the electrode surface, the migration phenomenon of the binder will be greatly reduced.
For low areal density electrodes, the electrode impedance is basically not affected by temperature, mainly because the total amount of solvent is low and the electrode structure cures quickly, and the binder does not migrate significantly.
The high areal density electrode shows completely different characteristics. The constant-speed evaporation stage is very long, which provides sufficient time for the migration of the binder. As the temperature increases, the migration of the binder and carbon black intensifies, resulting in a change in peel strength. poor, and the electrode impedance will also increase.
At 80°C, the peel strengths of different areal densities have little difference.
When it reaches 110 °C, the migration speed will increase, and the areal density will increase to 5.4mg/cm2, then the drying time will increase, which will provide sufficient migration time for the binder. When the areal density further increases to 12mg/cm2 cm2, the peel strength will decrease by 52%.
When the temperature increases to 130 °C, the migration phenomenon will be further intensified, resulting in poor peel strength.
Effect of Slurry on Peel Strength
The decrease of solid content will lead to the decrease of peel strength, mainly because after the decrease of solid content, it will take longer time or higher temperature for drying, which will lead to intensified migration of the binder, resulting in poorer peel strength. . Increasing the proportion of binder can also improve peel strength.
