Correlation Analysis Between Roll Compaction Density and Lithium Electrode Resistance

10/10/2024
Electrode Characterization Case

1. Preface

The electrode resistance represents the electronic conductivity of the electrode, and this parameter is closely related to the compaction density, porosity, and formulation of the electrode. When the electrode is prepared with different roll pressure, the surface roughness and compaction density of the electrode will be changed, which has a certain positive effect on enhancing the contact conductivity between the active particles in the electrode as well as on enhancing the volumetric energy density of the core. However, in the process of roll compaction, the particles in the electrode, in addition to longitudinal extrusion, are also subjected to lateral shear, which will cause the particles to be rearranged. When testing the resistance of the electrode, the upper and lower electrodes are perpendicular to the surface of the electrode, and the applied electrical signal runs vertically from the surface of the electrode to the other side of the electrode, which measures the electrical conductivity of the particles themselves on the one hand, and the contact resistance between the particles and the particles on the other hand. Therefore, when the electrode is rolled, whether the value of the electrode resistance decreases as expected with the increase of the roll compaction density, this paper tested and verified for the anode and cathode electrodes of different systems.

2. Experimental Equipment and Test Methods

2.1 Experimental instrument

Battery Electrode Resistance Tester: BER1300 (IEST), electrode diameter 14mm, can be applied to the pressure of 5~60MPa.

Figure 1. (a) BER1300 appearance; (b) BER1300 structure

2.2 Sample preparation

Single-sided electrodes were prepared with the slurry ratios of cathode electrode powder: SP: CMC=90:5:5, anode electrode powder : SP : PVDF=96.5:1.5:2, and after coating and drying, the electrodes were rolled with different pressures by a roller press to prepare electrodes with different roll compaction density.

2.3 Test method

the electrodes to be tested before and after roller pressing were cut into rectangular sizes of about 5cm×10cm, placed on the sample stage, set the parameters of test pressure and holding time on the MRMS software, and started the test, and the software automatically read the data of the thickness of the electrodes, electrical resistance, resistivity, conductivity, and so on.

3. Data Analysis

The resistance test was conducted on the single-sided anode and cathode electrode sheets before rolling and after rolling at different pressures, and the data results are shown in Figure 2. From the trend of the results, the graphite electrode has been showing an increase in resistivity as the roll compaction density increases, and it only slightly decreases when it reaches a roll compaction density of 1.63g/cm³. The compaction density of the ternary NCM electrode is 1.60g/cm³ before rolling, and the corresponding resistivity is relatively small. Once rolled, the resistivity shows a trend of first rising and then falling. The resistivity trend of lithium cobalt oxide LCO and lithium iron phosphate LFP electrode sheets is similar to that of ternary, but the resistivity corresponding to the first roll compaction density point before and after rolling of the LCO electrode sheet is not much different.

The electrons inside the battery electrode coating are mainly conducted through solid powder particles. The resistance specifically includes the conductivity of the active particles and the conductive agent particles themselves, which is related to the structure and morphology of the material; in addition, it also includes the contact resistance between solid particles, between active particles, between conductive agent particles, and between active particles and conductive agent particles. For the anode, the electronic conductivity of the active material is much lower than that of the conductive agent particles, and the conductivity of the active particles can be almost ignored. The graphite cathode electrode itself also has good conductivity, and the active particles and the conductive agent are the main electronic conduction paths. For the contact resistance between particles, this is related to factors such as the contact area and interface state between the particles. Rolling will hardly change the resistivity of the active material and the conductive agent itself, but the rearrangement of the particles will cause the contact area and interface state of the particles to change, thereby affecting the interface resistance. In addition, during the electrode resistance test, the resistance tested includes not only the resistance of the electrode coating, but also the interface resistance between the coating and the current collector, the contact resistance between the probe and the coating, etc. It is generally believed that rolling will increase the compaction density of the coating, increase the contact area between the particles, and thus increase the conductivity. However, the actual test results are more complicated. Next, the reasons for the change trend of the electrode resistance will be analyzed by electron microscopy and surface roughness testing methods.

Figure 2. Trend of the resistance of the anode and cathode electrodes before rolling and after rolling at different pressures

Cross-sectional SEM observation of graphite electrodes with three different compaction densities shows that the graphite lamellar structure, which was originally cross-aligned, tends to be parallel-aligned with the increase of roll pressure. As for the graphite material, its crystal structure is composed of carbon hexagonal lamellae arranged in parallel, divided into planar and end faces, in which most of the lithium ions are embedded from the end faces into the graphite interlayers. Moreover, the three electrons between the carbon atoms in the graphite interlayer are bonded by covalent bonds with SP2 hybridization, and the remaining one π-electron is free to move so as to have a good electronic conductivity, but with significant anisotropy, there is a good electronic conductivity along the direction of the plane, and the perpendicular direction of the plane is poor in the conductivity of the electrons. Therefore, when the graphite electrode is rolled, more planes are parallel to the surface of the electrode, which causes the current applied perpendicular to the electrode during the electrode resistance test to be more difficult to penetrate the electrode coating in the longitudinal direction, and therefore the resistance increases with the increase of the roll compaction density. On the other hand, as the roll compaction density increases, the contact between the graphite particles and the conductive agent particles becomes more confined, which in turn decreases the resistance, and the two oppose each other in affecting the wafer resistance. Therefore, the effect of the actual roll compaction process on the resistance of the electrode is very complex, and needs to be analyzed in conjunction with the specific material morphology characteristics, the microstructure of the electrode. Testing the resistance of the cathode electrode can, on the one hand, analyze the electron transport characteristics in combination with the microstructure, and on the other hand, the resistance test can characterize the uniformity of the resistance at different positions of the same group of electrodes to assess the uniformity of the electrode.

Figure 3. Cross-section of graphite electrodes with different compaction densities

Figure 4. Graphite crystal structure and morphology

For three sets of anode electrodes, the measured resistance of the electrodes after a smaller pressure roll pressure is larger than the electrode resistance before the roll pressure. As described above, the electrode plate electron conductivity is mainly formed by the electron conduction between conductive agent particles, in slurry preparation, conductive agent evenly dispersed in the solvent, formed a better interconnected 3 d electron transmission network, the subsequent coating and drying process, conductive agent 3 D network structure maintains good connectivity, although poor contact between active particles, but the conductive agent network makes the electrode has good electronic conductivity, electrode low resistance. The conductive agent electronic transmission network is broken at lower roll pressure, resulting in increased electrode resistance. Combined with three different roll pressure of ternary electrode surface topography map and surface carbon distribution diagram can be inferred that after roll pressure, coated on the surface of active particles of conductive carbon molecules may not roll pressure lateral shear force extrusion, disconnected between the conductive agent particles, cannot conduction electron, so the resistance will be compared before the roller plate resistance increases.

Moreover, this may also be related to the constantly decreasing roughness of the polar sheet surface, as shown in Figure 5. Because the electrode resistance measured by the electrode resistance test principle of the upper and lower probes inevitably contains the contact resistance between the upper and lower electrodes and the electrode surface, if the roughness of the electrode surface becomes smaller, the contact between the test electrode and the electrode surface becomes worse, and the contact resistance becomes large, thus the measured total resistance becomes large.

As the roll pressure increases further, the active particles are further compacted, and the conductive carbon particles contact each other closely, forming a reconnected 3D network, so the polar resistance is reduced. Therefore, the anode electrode roll pressure needs to ensure the larger roll pressure, otherwise the slight roll pressure will destroy the electronic conductive network, increase the electrode resistance, is not conducive to the performance of the battery performance. In addition, when testing the electrode plate resistance of the anode electrode plate, the surface state of the electrode plate should be kept as far as possible, so as to compare the influence of different process formulas on the electrode plate resistance can be used, and the electrode plate coating uniformity can also be evaluated by testing the resistance uniformity of different positions in the same group of electrodes.

Figure 5. Trend of the surface roughness of the three anode electrode

Figure 6. Surface electron micrographs and carbon distribution of three ternary electrodes

3. Summary

In this paper, the resistance of anode and cathode electrode with different roll compaction density was characterized, and it was found that the resistance of anode and cathode electrodes changed with the increase of rolling densification. The maximum value is related to the orientation of the electrode, while the resistance of the anode electrode first increases and then decreases with the rolling, which is related to the electron transport path of the three-dimensional network of the conductive agent of the electrode and the roughness of the surface. Therefore, when using the electrode resistance method to evaluate the difference in the conductivity of the anode and cathode electrodes and the uniformity of conduction, special attention should be paid to the orientation of the active particles and the consistency of the surface state of the electrodes.

4. References

[1] Henrik Lyder Andersen, Lisa Djuandhi, Uttam Mittal, Neeraj Sharma, Strategies for the Analysis of Graphite Electrode Function, Adv. Energy Mater., 2021, DOI:10.1002/aenm.202102693

[2] Hiroki Kondo et al. Influence of the Active Material on the Electronic Conductivity of the Positive Electrode in Lithium-Ion Batteries[J]. Journal of the Electrochemical Society, 2019,166 (8) A1285-A1290.

[3] B.G. Westphal et al. Influence of high intensive dry mixing and calendering on relative electrode resistivity determined via an advanced two point approach[J]. Journal of Energy Storage 2017, 11, 76–85.

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