1.Background

The IEST Single Particle Force Testing System (SPFT) is specifically designed for lithium battery materials to evaluate the mechanical properties of individual particles, such as crush force and crush strength. This system utilizes high-precision displacement control and pressure measurement to collect stress-strain curves from the loading of a probe onto single particles, analyzing the crush force at points of curve discontinuity. During testing, operators can observe particle morphology changes throughout the compression process using an optical microscope.

Upon its launch, the SPFT device quickly garnered widespread attention in the field of lithium battery material research and development. Numerous companies and research institutions have shown strong interest, actively inquiring about the device details and market conditions. Within just 4 months, over 70 entities have submitted 380+ samples for professional single particle mechanical performance testing using SPFT.

Based on extensive sample testing with SPFT, we have collected compression data from different types of lithium battery materials. Combining this data with relevant discussions in literature, we have compiled a set of curve models for the compression of single particles of lithium battery materials. This model not only describes the stress-strain relationship during particle compression but also reflects information on particle deformation mechanisms, fracture behaviors, and more.

2. Curve Models for Single Particle Compression

Figure 1. (a) SPFT Device; (b) Test Mode with Controlled Displacement; (c) Bottom View of the Optical System

SPFT offers multiple testing modes, allowing operators to choose the appropriate mode based on sample type or specific testing requirements. Typically, we employ a controlled displacement mode to test stress (Figure 1b), where the probe compresses particles at a constant displacement rate. This mode measures the stress variation during particle compression. The displacement rate is typically set at 1 μm/s with displacement stability maintained within ±0.01 μm. The instrument automatically stops testing and saves data when the probe reaches the maximum displacement or set pressure limit.

Of particular note, SPFT utilizes a bottom optical system for imaging, allowing the testing software to synchronously record the entire process of particle compression, deformation, and fracturing (Figure 1c). Observing these changes provides crucial insights and guidance for analyzing stress-strain curves in single particle compression. This capability forms the core basis for establishing the models of single particle compression curves in this study.

Figures A/B/C/D depict the behaviors and characteristics of lithium battery material particles under continuous compression, with detailed explanations and analyses provided in the following sections for each figure.

Figure 2. Analysis of Single Particle Compression Curves of Lithium Battery Materials (A-D)

3.Analysis of Single Particle Compression Curve Models

Figure A represents the compression curve of an ideal particle: initially, the particle undergoes elastic-plastic deformation when compressed, exhibiting some degree of elastic behavior. When the particle reaches fracture, known as the crush point, the corresponding stress value is termed the crush force, indicating the stress at which the particle fractures or fails. Subsequently, as the particle breaks and releases most of its internal stresses, the stress rapidly decreases until the probe compresses the particle and substrate together. At this point, the curve rises again in a certain pattern, corresponding to the probe pressing against the substrate. Figure (a) shows the single particle compression curves of various particles of a ternary material we tested, which closely resemble the behaviors and characteristics shown in Figure 1.

Figure 3, A, a. Single Particle Compression Curves of Various Particles of a Ternary Material

Figure B reflects the characteristics of larger particles such as ternary or secondary coated graphite particles. Multiple plateau stages appear at the end of the compression curve, which correspond to secondary fracture or sliding of the particles. For instance, after the primary fracture of a large ternary particle, as the probe continues to compress, the already fractured particle (which retains some structural integrity) may undergo secondary or multiple fractures. Therefore, particle fracture may occur in stages rather than all at once, with each stage of damage leaving a plateau on the curve. Figure (b) shows the single particle compression curves of various particles of a ternary material we tested, which closely resemble the behaviors and characteristics shown in Figure B.

Figure 4, B, b. Single Particle Compression Curves of Large Particles of a Ternary Material

Furthermore, the shape and compression displacement from zero to crush point marked in Figure B can provide characteristics of material brittleness. Under the same crush force conditions, a steeper curve indicates a quicker transition from elasticity to failure; a shorter compression displacement suggests greater brittleness as the material cannot withstand significant deformation before fracturing. The energy absorbed from zero to crush point, calculated as the area under the curve by integration, reflects the total energy input experienced by the particle before fracture, known as fracture energy or crush energy. Similarly, a smaller fracture energy indicates greater brittleness of the material.

Figure C illustrates the multi-level small steps (also known as “ripples” or “serrated” changes) observed in compression curves of some negative electrode particles. These phenomena involve situations where displacement increases within a specific small range while stress remains constant. These occurrences occur initially during particle compression, where optical microscopy observations show no significant deformation or fracture of the particle, but rather relate to discontinuities in the particle’s structure before fracture. Structural discontinuities in particles refer to features such as dislocations, defects, microcracks, voids, inhomogeneities, or phase interfaces within the particle material. These subtle features manifest as small steps in the compression curve. Powder that has undergone high pressure and powder particles scraped off the electrode after long cycles tend to exhibit similar phenomena during compression testing, with potentially more numerous, larger, or longer steps. These step-like changes provide crucial information for analyzing the failure mechanisms and internal structures of particle materials, aiding in understanding failure modes and improving their mechanical performance.

Figure 5,C and C are the single particle compression curves of a ternary material powder after compaction at different pressures

Figure C shows the single particle compression curves of powder particles from the same ternary positive electrode material tested after compaction at different pressures. It is important to note that all tested particles were structurally intact, showed no signs of fracture under optical microscopy, and were approximately 18 μm in size. Comparing the compression curves of particles compacted at 100 MPa and 300 MPa, particles subjected to higher pressures exhibit phenomena similar to those seen in Figures 2 and 3. Key observations include:

(1) Decreased Elastic Modulus: Higher pressures lead to densification of particle internal structure or microcrack formation, resulting in a reduced elastic modulus of the particles, evident in the initial linear stage of the stress-strain curve.

(2) Decreased Crush Force: Higher pressures may introduce microcracks or other forms of damage, reducing the material’s strength and stability, causing particles to fracture at lower stress levels.

(3) Steps/Plateaus in Compression Curves: Internal defects in particles expand or new cracks form during compression, resulting in steps/plateaus in the compression curves, and the particle fracture process becomes more irregular.

Figure D presents more complex compression curves observed in specific particles due to surface structural differences or irregularities. These curves exhibit both pronounced bends or large steps at the front end and bends or plateaus at the rear end. Additionally, the curve from the crush point to the substrate shows nonlinear attenuation. Such mechanical behaviors indicate a non-linear stress-strain relationship in the material, often reflecting significant variations between particles of the same material. Figure (d) shows the single particle compression curves of powder particles from two silicon-carbon materials we tested, closely resembling the behaviors and characteristics shown in Figure D.

Figure 6, D, d. Single Particle Compression Curves of Powder Particles from Two Silicon-Carbon Materials

4. Conclusion

Through analyzing the single particle compression characteristics of lithium battery materials, one can gain a deeper understanding of the mechanical response of these particles to external forces, including crush force, brittleness characteristics, internal microstructure, and more. This information is crucial for the development of lithium battery materials, micro-scale simulation modeling, and optimization of battery performance. IEST will continue to deepen its research on single particle compression and welcomes continued attention to SPFT, which will continue to support the advancement of lithium battery research and development efforts.。