The two most important indicators for lithium-ion batteries are energy density and power density. Energy density refers to the amount of energy stored per unit volume or weight of lithium-ion batteries. Power density refers to the unit weight or volume that can be output. The power size. We hope that lithium-ion batteries have a high energy density on the power battery, let us have a higher cruising range, and we also hope that the power battery has a higher power density, to meet our driving force in the fierce driving Output. But in the design and production of lithium-ion batteries, these two indicators are precisely contradictory. In general, in order to increase energy density, we need to increase the amount of electrode coating, and increase the proportion of active substances, which leads to power performance. In order to increase the power density, we need to reduce the amount of coating and increase the proportion of conductive agents. Therefore, it becomes very difficult to achieve a balance between the two.
Recently, Kazuaki Kisu (first author) and Etsuro Iwama (corresponding author), Katsuhiko Naoi (corresponding author), Tokyo University of Agriculture and Technology, Japan, analyzed the important indicators in the production of lithium-ion batteries - compaction density and electrode thickness for lithium-ion battery power. The effect of performance on the analysis shows that for the NCM material, the minimum electrode resistance value can be obtained when the electrode thickness is 70 μm and the compaction density is 2.9 g/cm 3, so as to ensure high energy density while ensuring excellent battery performance. Magnification performance.
In order to eliminate the influence of the reference electrode on the test results, Kazuaki Kisu adopted a symmetrical battery structure (as shown in the above figure a), that is, the positive and negative electrodes are the same electrode, and the metal Li is inserted between the two electrodes. The way of the electrode adjusts the SoC of the two electrodes, and then the metal Li electrode is removed in a dry environment, and then the battery is subjected to an AC impedance test.
Figure b above shows the EIS results for 0% SoC. We can see a line with a slope of 45 degrees in the high frequency region, which represents the diffusion resistance of Li+ in the electrode. Figure c above shows the diffusion resistance of Li+. The relationship between the thickness of the electrode and the thickness of the electrode shows a linear correlation between the diffusion resistance Rion of Li+ and the thickness of the electrode. Figure D above shows the EIS spectrum of a 50% SoC electrode. The semicircle in the high frequency region represents the electrode. The charge-exchange impedance RCT, we can note from the figure that the charge exchange impedance and the electrode thickness have a negative correlation, that is, the thicker the electrode thickness, the smaller the charge exchange impedance.
Compaction density is an important parameter in the production of lithium-ion batteries. In order to increase energy density, we generally hope to increase the compaction density as much as possible. The figure above shows 2.7, 2.9, and 3.4 g/in case of constant control thickness. The density of the inner pores of the electrode under the compaction density of cm3 can be seen from the figure. With the gradual increase of the compaction density, the size of the micropores inside the electrode is also gradually reduced.
Kazuaki Kisu deduced the relationship between the micropore diameter in the electrode and compaction density, and obtained the relationship between the micropore radius of the electrode and compaction density, as shown below.
Based on the relationship between Rion and electrode thickness and number of micropores, we can further deduce the relationship between Rion and electrode compaction density, as shown below, from which it can be seen that Rion and compaction density are not simple The linear relationship, but when the compaction density is close to the true density of the active material will lead to a sharp increase in Rion.
The following figure shows the EIS results of different densified electrodes. As can be seen from figure b below, the correlation between Rion and compaction density is weak before compaction density reaches 3.0 g/cm3, with compaction. As the density increases, Rion only has a slight increase, but after the compaction density exceeds 3.g/cm3, Rion rapidly increases, which is consistent with our previous prediction. From the perspective of the charge exchange resistance RCT in Figure C below, With the increase of compaction density, the charge exchange impedance actually decreases to some extent. Kazuaki Kisu thinks this is mainly because the higher compaction density means that the coating amount per unit area increases when the thickness does not change. As a result, the contact area between the active material and the electrolyte increases, resulting in a decrease in the charge exchange resistance RCT.
Kazuaki Kisu believes that the compaction density of lithium-ion batteries is often an important factor in increasing the impedance of lithium-ion batteries, because at lower densities, it tends to cause contact between the active material particles, the active material, and the positive Al foil. The impedance increases. The following figure shows the relationship between the contact resistance of the electrode and the compaction density of the electrode under different compaction densities obtained from the EIS test results. From the test results, the contact resistance Rcon of the electrode is gradually improved as the compaction density is gradually increased. Reduce quickly.
According to the above experimental data and formulas, Kazuaki Kisu obtained the relationship between the electrode's total impedance and the electrode thickness and compaction density (as shown in the figure below). In the following figure a, the X-axis is the electrode thickness, and the Y-axis is the electrode compaction. Density, the color in the figure represents the total impedance of the electrode, blue represents the low impedance, and red represents the high impedance. From the figure we can see that when the thickness of the electrode is 70um and the compacted density is 2.9g/cm3, we can Get the lowest electrode resistance (including Rion, RCT and Rcon), which we can also see from the following figures b and c. When the compaction density is too high or low, it will lead to the increase of internal resistance and polarization of the battery. .
Kazuaki Kisu's work allowed us to have a deep understanding of the impact of the two important parameters, compaction density and coating thickness, on the total electrode impedance. In particular, the author obtained the electrode impedance and compaction density and coating thickness at the end of the paper. The relationship diagram has important guiding significance for the design of lithium-ion batteries.
