Correlation Between Particle Size and Mechanical Properties of Silicon-Carbon Anode Materials

1. Introduction

To simultaneously optimize electrode compaction density, slurry processability, conductive network continuity, electrode sheet resistance, electrolyte pore distribution, and the balance between cycle life and fast-charging performance, battery material manufacturers typically prepare silicon-carbon particles across multiple particle size grades — combining large and small particles, or blending silicon-carbon materials with graphite of different particle sizes. The differences in bulk compressive resistance among different particle size grades directly affect roll-press fracture rate and charge/discharge pulverization degree during cycling. This study uses the IEST SPFT2000 Single Particle Force Tester to systematically measure single-particle compression tests across multiple particle size grades for silicon-carbon samples prepared by the same process, establishing the intrinsic relationship between particle size, crushing force, and crushing strength. Simultaneously, hard carbon and resin carbon were tested to establish the particle size — mechanical property relationship across three anode material types, providing experimental data for anode powder granulation, particle size grading control, and formulation blending design.

2. Equipment and Method

2.1 Test Equipment: IEST SPFT2000 Single Particle Force Tester

All single particle mechanical property tests were performed using the IEST SPFT2000 Single Particle Force Tester (Figure 1a), a dedicated instrument for single particle mechanical testing of battery active materials in accordance with Chinese national standard GB/T 43091-2023. The system integrates a high-precision load cell, motorized compression stage, and optical bottom-view imaging system that enables particle centering and diameter verification before each test.

Figure 1. IEST SPFT2000 Single Particle Force Tester: (a) instrument; (b) compression test mode — particle positioned between indenters, load and displacement recorded simultaneously to derive crushing force and crushing strength; (c) optical bottom-view imaging for particle centering and size verification before each test.

2.2 Test Samples

• Silicon-carbon materials: two silicon-carbon samples prepared by the same process, designated SC1 and SC2. Each material was separated into two particle size grades: 4.5 ± 0.5 µm and 8.5 ± 0.5 µm.
• Hard carbon materials: two hard carbon samples prepared by the same process, designated HC1 and HC2. Each separated into 4.5 ± 0.5 µm and 8.5 ± 0.5 µm grades.
• Resin carbon: one resin carbon sample designated RC, separated into 8.5 ± 0.5 µm and 15 ± 0.5 µm grades.

2.3 Test Procedure

Sample solutions were dispersed uniformly and individually dropcast onto glass slides. Five particles were tested per particle size grade for each material. The optical imaging system confirmed particle diameter and centering before each compression test. Crushing force (N) and crushing strength (MPa) were calculated according to the formula in Equation (1).

The compressive strength of the powder is calculated according to Equation (1):

\[p_c = \alpha \times 1000 \times \frac{F_k}{\pi \cdot d^2}\]

where:

\( p_c \) — compressive strength, in MPa;

\( \alpha \) — calculation coefficient, taken as 2.48;

\( F_k \) — crushing force, in mN;

\( d \) — particle size (diameter), in \(\mu\)m.

2.1 For the Same Material: Crushing Force Increases with Particle Size

Figure 3 shows the crushing force comparison across three anode material types at different particle sizes. The result is consistent across all materials: crushing force increases with increasing particle size for silicon-carbon SC and hard carbon HC (8.5 µm particles show significantly higher crushing force than 4.5 µm), and for resin carbon RC (15 µm particles show significantly higher crushing force than 8.5 µm).

Figure 2. Crushing force vs. particle size for SC1, SC2 (silicon-carbon), HC1, HC2 (hard carbon), and RC (resin carbon). Crushing force increases with particle size across all three material types: larger particles require significantly higher total force to fracture, because larger cross-sectional area and volume increase effective load-bearing capacity.

Mechanical mechanism: as particle size increases, the particle’s effective load-bearing cross-sectional area and total volume both increase. The critical external force needed to initiate fracture therefore rises proportionally. This explains why large-particle anode materials can withstand higher total roll-press forces during calendering — though this does not imply they are intrinsically stronger per unit area (see Section 2.2).

2.2 For the Same Material: Crushing Strength Decreases with Increasing Particle Size

Crushing strength represents compressive resistance per unit cross-sectional area — a size-normalized, intrinsic measure of particle mechanical performance. Figure 4 shows that crushing strength follows the opposite trend from crushing force: for all three anode material types, larger particle size corresponds to lower crushing strength. Silicon-carbon SC and hard carbon HC both show higher mean crushing strength at 4.5 µm compared to 8.5 µm; resin carbon RC shows higher crushing strength at 8.5 µm compared to 15 µm.

Figure 3. Crushing strength vs. particle size for SC1, SC2, HC1, HC2, and RC. Crushing strength decreases with increasing particle size across all material types. At 8.5 µm: SC2 = 1,632 MPa; RC = 968 MPa; SC1 = 339 MPa — confirming that intrinsic microstructure (carbonization quality, defect density) sets the absolute crushing strength ceiling, with particle size modulating performance within that range.

Why do smaller particles have higher crushing strength? During granulation and sintering of anode materials, smaller particles achieve higher densification. Smaller particle volume means shorter diffusion distances during sintering, more complete pore elimination, fewer residual internal micropores, fewer primary cracks, and lower structural defect density overall. Because crack propagation and fracture initiation in ceramic-like particles originate at internal defects, a particle with fewer and smaller defects requires higher stress to initiate fracture — resulting in higher crushing strength per unit area.

This relationship has a direct practical implication for long-cycle battery formulation design: small-particle anode materials buffer the internal stress generated by lithiation volume expansion more effectively, and are more resistant to the particle fracture and pulverization that causes capacity fade during cycling. This is precisely why long-cycle power battery formulations preferentially incorporate small-particle-grade materials — not because they withstand higher total force, but because they have higher intrinsic per-unit-area compressive resistance that preserves particle integrity during repeated lithiation stress.

2.3 At Equal Particle Size, Intrinsic Material Microstructure Determines the Crushing Strength Ceiling

Comparing crushing strength at the same particle size (8.5 µm) across the three material types reveals that intrinsic microstructure — not particle size — is the dominant factor setting the upper limit of particle mechanical performance:

• SC2 (silicon-carbon): 1,632 MPa — the highest among all tested materials at 8.5 µm, exceeding both hard carbon grades.
• Resin carbon (RC): 968 MPa — intermediate strength at 8.5 µm.
• SC1 (silicon-carbon): 339 MPa — substantially lower than SC2 despite identical particle size and the same stated process, indicating that the specific carbonization conditions, silicon dispersion state, or precursor quality within the “same process” range produce significantly different microstructural outcomes.
• HC1 and HC2 (hard carbon): both fall between RC and SC1 in crushing strength at 8.5 µm.

The SC1 vs. SC2 comparison at the same particle size is particularly instructive: two silicon-carbon materials described as “same process” differ in crushing strength by nearly 5× (339 MPa vs. 1,632 MPa). This confirms that particle size is a secondary modulator of crushing strength — the primary determinant is the intrinsic microstructure established during material synthesis, including carbonization degree, carbon shell continuity, internal porosity, and primary defect density. Particle size can only adjust crushing strength within the range defined by the material’s fixed microstructural properties; it cannot overcome a fundamentally weak microstructure.

This finding has direct relevance to anode particle size optimization for cycling stability: selecting a smaller particle size grade of a microstructurally inferior material may improve its crushing strength relative to its own larger grade, but will not match the performance of a microstructurally superior material at the same particle size. Effective formulation design must address both dimensions — material microstructure and particle size grading — rather than optimizing either in isolation.

3. Conclusions

Using the IEST SPFT2000 Single Particle Force Tester (GB/T 43091-2023), this study systematically establishes the particle size — mechanical property relationships for silicon-carbon, hard carbon, and resin carbon anode materials:

• Crushing force increases with particle size (positive correlation): larger particles have greater effective load-bearing cross-sectional area and volume, requiring higher total external force for fracture. This trend is consistent across silicon-carbon (4.5 µm vs. 8.5 µm), hard carbon (4.5 µm vs. 8.5 µm), and resin carbon (8.5 µm vs. 15 µm).
• Crushing strength decreases with particle size (inverse correlation): smaller particles densify more completely during sintering, leaving fewer internal defects and microcracks, resulting in higher per-unit-area compressive resistance. Small-particle anode materials better resist lithiation-induced internal stress and particle pulverization during cycling — explaining why long-cycle power battery formulations preferentially use small-particle-grade materials.
• Intrinsic material microstructure sets the crushing strength ceiling: at equal particle size (8.5 µm), SC2 achieves 1,632 MPa, resin carbon RC 968 MPa, and SC1 only 339 MPa — confirming that carbonization quality and microstructural density are the primary determinants of particle mechanical performance, with particle size modulating strength within material-defined limits.

Single particle mechanical testing using the SPFT2000 provides a quantitative, standardized method to characterize the particle size — mechanical property relationship in anode powders. This data directly guides anode powder granulation process control, particle size grading classification, and multi-grade formulation blending design — offering a low-cost route to simultaneously improving anode material processability and long-cycle stability. Note: the trends reported here apply to materials prepared by the same process; special formulations or atypical manufacturing processes may produce deviations from these general rules.

4. References

[1] GB/T 43091-2023. Powder Crushing Strength Test Method [S]. China National Standard, 2023.

5. FAQs: Anode Particle Size and Mechanical Properties