The Main Factors Affecting The Compaction Density of Cathode Electrode Sheet

1. Preface

In the manufacturing process of lithium batteries, compaction density significantly influences battery performance. Generally, compaction density is closely related to the specific capacity, efficiency, internal resistance, and cycling performance of the battery. Identifying the optimal compaction density is crucial for battery design.

Within the allowable range of materials, a higher compaction density usually results in a higher battery capacity. Thus, compaction density is often considered a reference indicator of material energy density. However, excessively pursuing high compaction density can adversely affect the specific capacity and cycling performance of the battery.

As compaction density increases, the degree of compression between material particles also rises, reducing the porosity of the electrode. This decreases the electrode’s ability to absorb the electrolyte, making it harder for the electrolyte to penetrate. The direct consequence is lower specific capacity utilization, poor liquid retention, increased polarization during cycling, and a noticeable rise in internal resistance.

Therefore, an appropriate cathode compaction density can increase the discharge capacity, reduce internal resistance, minimize polarization losses, extend the battery’s cycle life, and improve the overall efficiency of lithium-ion batteries. If the compaction density is too high or too low, it will negatively impact the intercalation and deintercalation of lithium ions. So, what are the main factors affecting the compaction density of cathode electrode sheet?

Figure 1. Electrode Sheet Layer

2. True Density of Materials

The true density and achievable compaction density of several commercial cathode materials are shown in the table (with the NCM111 material selected for comparison). It is evident that the true densities of these materials follow a consistent pattern: Lithium Cobalt Oxide (LCO) > Ternary Material (NCM111) > Lithium Manganese Oxide (LMO) > Lithium Iron Phosphate (LFP). It should be noted that the true density of different compositions of ternary materials varies with their specific composition.

Table 1: True Density and Compaction Density Range of Several Commercial Cathode Materials

3. Material Morphology

The difference in true density between NCM111 and LCO is minimal, as shown in the table, where the true density difference is only 0.3 g·cm^-3. However, the compaction density of NCM111 is 0.5 g·cm^-3 lower than that of LCO, or even higher. Several factors contribute to this outcome, but the primary reason is the difference in morphology between LCO and ternary materials.

Commercially available LCO is composed of large, single-crystal primary particles, while ternary materials are fine secondary aggregates of small single crystals, as shown in the figures. The secondary spheres formed by the aggregation of hundreds of nanometer-sized primary particles contain many voids. When these spheres are formed into electrodes, additional voids are present between the spheres, further reducing the compaction density of ternary materials.

Figure 2. SEM Images of Lithium Cobalt Oxide and Ternary Materials

4. Particle Size Distribution

When spheres of equal diameter are packed, significant voids are left between them. If there are no appropriately sized smaller particles to fill these voids, the packing density will be low. Therefore, a suitable particle size distribution can enhance compaction density, whereas an unsuitable distribution can significantly reduce it.

5. Electrode Process

The areal density of the electrode, the amount of binder, and the conductive agent used will all affect the compaction density. The true density of common conductive agents and binders is shown in Table 2. From the table, it is clear that…

Table 2: True Density of Common Conductive Agents and Binders

While the true density of materials influences compaction density, it cannot be altered. However, by comparing compaction density and true density, it is evident that there is considerable room for improvement in the compaction density of ternary materials.

6. IEST Powder Resistivity & Compaction Density Measurement System (PRCD3100) Recommended

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Application:

• Lithium (sodium) positive and negative electrode powders (LCO/NCM/LFP/Graphite, etc.),
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• Solid electrolyte powders
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