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.