Compaction Density Measurement of Powder Materials-Pressurizing, Unpressurizing & Bouncing

1. Preface

Battery powder materials are critical to lithium-ion battery performance, with resistivity and compaction density serving as key quality metrics under various pressure conditions. The powder compaction process involves complex physical interactions—particle movement, rearrangement, deformation, and densification—leading to the formation of a tightly packed structure.

Initially, particles are loosely stacked with large interstitial pores. As pressure increases, particles slide, rotate, and rearrange, rapidly reducing volume and increasing density. Under moderate pressure, elastic deformation occurs at contact points, further reducing gaps without permanent particle deformation. At higher pressures, particles may undergo plastic deformation or brittle fracture, resulting in cold welding, mechanical interlocking, or size reduction, significantly increasing density toward the material’s theoretical limit.

Beyond compression, the depressurization phase is equally vital. When pressure is released, elastic recovery causes rebound, affecting finalcompact density (also referred to as press density). This rebound behavior offers insights into internal stress, strain hardening, and fracture characteristics.

In this study, we evaluate the compaction density of NCM, LCO, and LFP cathode materials during both pressurization and depressurization using IEST’s PRCD series equipment, providing new perspectives on assessing mechanical properties in battery powder materials.

2. Experimental Program

The powder compaction tests were conducted using the IEST PRCD series equipment (see Figure 1), which enables precise measurement of compact density and resistivity under controlled pressure cycles. The testing involves:

• Applying graduated pressure up to a predefined maximum.

• Maintaining pressure for a set duration.

• Releasing pressure to observe rebound behavior.

This method allows for evaluating the press density during loading and unloading phases, capturing elastic recovery and plastic deformation effects.

Figure 1.Schematic diagram of PRCD series powder resistance & compaction density tester

3. Results and Discussion

The compact density of a material is influenced by particle size, morphology, and mechanical properties. As illustrated in Figure 2, the press density increases during compression due to particle rearrangement and deformation, but decreases upon pressure release due to elastic rebound.

Figure 2. Variation of Sample Compaction Density with Pressure

Five samples with distinct particle size distributions (see Table 1) were tested, including:

• Polycrystalline hollow spheres (Sample 1)

• Polycrystalline solid spheres (Sample 2)

• Single-crystal spheres (Sample 3)

Results indicate that:

• Under pressure, compact density values followed the order: LCO > NCM > LFP.

• Upon pressure release, rebound behavior varied significantly. Sample 1 (hollow sphere) showed minimal rebound due to irreversible deformation, whereas Samples 2 and 3 exhibited higher rebound, indicating greater internal stress.

These differences underscore how material microstructure—such as hollow vs. solid architecture and crystalline form—affects powder compaction and post-compression recovery.

Table 1. Particle Size Distribution of Test Samples

The compaction process occurs in three primary stages:

1. Particle Rearrangement: At low pressure, particles slide and reorient.

2. Elastic-Plastic Deformation: Increased pressure causes deformation at contact points, reducing voids.

3. Particle Fracturing: At high pressure, brittle materials may fracture, further increasing density.

Material properties such as Young’s modulus, hardness, and particle shape directly influence these stages. Additives and lubricants also affect inter-particle friction and compaction behavior.

Figure 3. Schematic diagram of changes in compaction density for pressurization, depressurization and rebound compaction

4. Mechanistic Interpretation

• Particle mechanics: Young’s modulus, yield strength, fracture toughness and surface roughness control whether compaction results in elastic recovery or permanent rearrangement.

• Geometric factors: Particle size distribution and shape (spherical vs. angular) affect packing efficiency; well-graded mixes fill voids better and show higher compact density with lower rebound.

• Process implications: Press density alone can be misleading for downstream electrode manufacturing—rebound determines the final electrode thickness and porosity after calendering/assembly.

5. Implications for Battery Electrode Manufacturing

The rebound behavior of battery powder materials post-compaction is critical in electrode calendering. Excessive rebound can affect electrode thickness and uniformity, influencing cell assembly and final energy density. By analyzing press density changes during unloading, manufacturers can better predict material behavior during electrode rolling and prevent issues such as delamination or poor conductivity.

Moreover, understanding these properties at the powder level allows early-stage screening of materials, reducing dependency on later-stage electrode tests. Complementary tools, such as single-particle mechanical testers (e.g., IEST’s SPFT series), can further enhance understanding from individual particles to bulk powder performance.

6. Conclusion

This study demonstrates the importance of evaluating battery powder materials under both pressurized and unpressurized conditions to capture full powder compaction behavior. The rebound compact density offers valuable insight into internal stresses and material mechanical properties, which vary significantly with particle morphology and structure.

For accurate press density measurement, it is essential to control factors such as mold size, sample amount, and pressure profile, and to specify whether values are recorded under load or after rebound. These considerations ensure consistent quality assessment and help bridge powder-level properties to electrode-level performance.

7. References

[1] B K K A ,  A S A ,  A H N , et al. Internal resistance mapping preparation to optimize electrode thickness and density using symmetric cell for high-performance lithium-ion batteries and capacitors[J]. Journal of Power Sources, 2018, 396:207-212.

[2] Yang Shaobin, Liang Zheng. Lithium-ion Battery Manufacturing Process Principles and Applications.