1.Preface

The silicon-carbon anode prepared by chemical vapor deposition (CVD), also known as vapor-deposited silicon anode, is a silicon-based anode material obtained through the chemical vapor deposition (CVD) process. The core of this preparation method involves storing silicon in a porous carbon framework. Silane gas is introduced into the pores of porous carbon particles, and high-temperature pyrolysis causes the gas to deposit, forming silicon nanoparticles dispersed within the pores of the porous carbon. This method enables molecular-scale control over the prepared nanomaterials, resulting in good product morphology. Additionally, the deposited silicon-carbon material has a uniform composition and a relatively dense structure. The internal voids of the porous carbon act as a buffer for volume expansion, leading to low expansion rates and excellent cycling performance.

In the industry today, the term “vapor-deposited silicon-carbon anode” usually refers to the technology developed by the American company Group14. In April 2021, Group14 announced that its flagship product, the silicon-carbon composite anode material SCC55™ (with a carbon-silicon ratio of 55:45), had begun commercial production at the world’s first-of-its-kind BAM (Battery Active Materials) factory. SCC55™ is a stable silicon-carbon composite anode with a capacity five times that of graphite anode materials and can provide 50% higher energy density than traditional graphite. Its unique hard carbon-based scaffold maintains silicon in its most ideal form: amorphous, nano-sized, and carbon-encapsulated. Moreover, SCC55™ is fully compatible with graphite, and even at a 20% blend, SCC55™ can increase energy density by 30% over 1000 cycles.

The carbon framework in this new silicon-carbon material not only has a low production cost but also possesses a good lithium storage capacity. Additionally, the low density and lightweight nature of the carbon framework contribute to the material’s high energy density. Furthermore, the chemical vapor deposition (CVD) process for depositing silicon has a short production flow, requires minimal equipment, and theoretically has a low cost. Performance test results show that the porous silicon-carbon prepared by the chemical vapor deposition (CVD) method exhibits excellent performance in multiple dimensions, including specific capacity, initial efficiency, cycle life, and rate capability. In production, the porous silicon-carbon anode can reduce the need for prelithiation and premagnesiation, offering significant cost reduction potential compared to the silicon-oxygen route. Therefore, whether evaluated based on the current achieved performance, cost, and product stability, or the future potential of the technological route, vapor-deposited silicon-carbon is considered the most advantageous direction among the three technological routes.

Against this backdrop, downstream battery manufacturers and automotive companies have shown a keen interest in the porous silicon-carbon technology route, which theoretically offers significant cost reduction potential while maintaining performance advantages. At the request of cell manufacturing clients, domestic companies with years of experience in silicon-oxygen materials and leading enterprises in silicon-carbon produced by milling are transitioning to and investing in the vapor-deposited silicon-carbon technology route. Additionally, silicon-based anode startups with agile market intelligence are also beginning to shift their focus and develop vapor-deposited silicon-carbon.

In vapor-deposited silicon-carbon anodes, the properties of the porous carbon directly determine the product characteristics. Porous carbon materials, which feature various pore structures, high chemical stability, high conductivity, high specific surface area, and a rich, adjustable porous structure, show great potential in energy storage, conversion, catalysis, and adsorption separation. Their application in silicon-carbon anodes has driven the development of the porous carbon industry. The quality of the carbon framework directly affects the mass production capability of the product. Different porous carbons need to be matched with different graphites to achieve optimal performance in battery cells. The requirements for pore size, pore volume, and porosity of the carbon framework vary significantly under different scenarios, resulting in substantial performance differences. Specialized evaluation techniques are required to support the development process.

2. Test Equipment

Figure 1 shows the Powder Resistivity & Compaction Density Instrument (PRCD3100) developed by IEST. This device can simultaneously measure the resistivity, conductivity, and compaction density of powder samples while applying different pressures (up to 5T). It assists researchers in studying the effects of varying pressure on the electrical and mechanical properties of powder samples.

Figure 1. Schematic Diagram of the Powder Resistivity & Compaction Density Instrument (PRCD3100) and Different Modes of Mechanical Performance Testing

Figure 2 shows the Electrode Resistivity Instrument (BER2500) independently developed by IEST. The electrode sample has a diameter of 14mm and can be subjected to pressures ranging from 5 to 60 MPa. This device can simultaneously measure the resistance, resistivity, conductivity, and compaction density of the electrode sheets. The appearance and structure of the equipment are shown in Figure 2.

Figure 2. Appearance and Structural Diagram of the Electrode Resistivity Instrument (BER2500)

3. Data Analysis

This paper primarily compares the compaction density and rebound characteristics of vapor-deposited silicon-carbon (PSC), milled silicon-carbon (SiC), and conventional artificial graphite (GR) at the powder level. It also prepares silicon-carbon materials with capacities of 450 mAh/g and 550 mAh/g from the same vapor-deposited silicon powder. Using the same process and formulation, the silicon-carbon and graphite materials are made into electrode sheets to study the differences in mechanical properties between the silicon-carbon and graphite electrode sheets.

First, the compaction density of vapor-deposited silicon-carbon powder (PSC), milled silicon-carbon powder (SiC), and artificial graphite powder (GR) was tested under pressures ranging from 10 to 350 MPa, as shown in Figure 3. At the same pressure, the compaction density of the graphite material was significantly higher than that of the two silicon-based materials. Among the silicon-based materials, the compaction density of vapor-deposited silicon-carbon powder was higher than that of the milled silicon-carbon, which is primarily due to the differences in their microstructures.

Figure 4. Thickness rebound curves of the three powder samples

In terms of powder compression performance, the industry mainly focuses on the rebound characteristics after compression. At the same pressure or compaction density, a smaller thickness rebound indicates that the material can provide a higher volumetric energy density and better forming ability in the initial roll pressing. As shown in Figure 4, the thickness rebound capability of the graphite material is significantly higher than that of the silicon-based materials. Among the two silicon-based materials, vapor-deposited silicon-carbon powder exhibits a smaller rebound characteristic compared to milled silicon-carbon powder.

A comprehensive analysis of the mechanical properties of vapor-deposited silicon-carbon (PSC), milled silicon-carbon (SiC), and conventional artificial graphite (GR) powders reveals that graphite materials exhibit higher compaction density and greater thickness rebound characteristics. Among the two silicon-based materials, vapor-deposited silicon-carbon powder shows higher compaction density and smaller thickness rebound. This is linked to its microstructure: vapor-deposited silicon-carbon uses a porous carbon framework to deposit silicon. During compression, the pores in the porous carbon are more easily compressed and less likely to rebound after decompression, which is why its microstructure enables it to achieve higher compaction density and smaller thickness rebound.

Figure 5. Thickness rebound curves of the electrode

Figure 5 shows the thickness rebound curves of electrode sheets made from graphite material, and vapor-deposited silicon-carbon materials with capacities of 450 mAh/g (PSC-450) and 550 mAh/g (PSC-550). Similar to the powder results, the thickness rebound of the graphite electrode sheets is significantly higher than that of the vapor-deposited silicon-carbon materials at the same pressure. Among PSC-450 and PSC-550, PSC-450 exhibits greater thickness rebound than PSC-550 due to the addition of more graphite material during the mixing process.

4. Summary

Among vapor-deposited silicon-carbon, milled silicon-carbon, and conventional artificial graphite, the graphite material exhibits higher compaction density and greater thickness rebound characteristics. Among the silicon-based materials, vapor-deposited silicon-carbon powder has a higher compaction density and smaller thickness rebound capability. This is mainly due to the porous carbon framework in vapor-deposited silicon-carbon, where the pores are more easily compressed and less likely to rebound after decompression, resulting in higher compaction density and smaller thickness rebound for the vapor-deposited silicon-carbon material. The rebound performance of the electrode sheets mirrors that of the powders, providing additional evidence of the reliability of the powder results and their relevance to electrode sheets. Thus, these experimental results offer new insights into improving the compaction density of vapor-deposited silicon-carbon materials and provide a reliable validation method for related material companies studying the mechanical properties of vapor-deposited silicon-carbon materials.