Editor's Note
Combining the advantages and disadvantages of carbon and silicon materials, these two are often used together to maximize their benefits. Composite materials can generally be classified into two types based on the type of carbon material: conventional silicon-carbon composites and new silicon-carbon composites.
Lithium-ion batteries are widely used in computers, mobile phones, electric vehicles, and other portable electronic devices due to their high energy density, high open-circuit voltage, and long cycle life. Currently, lithium batteries are highly commercialized. As one of the four main components (positive electrode material, negative electrode material, separator, and electrolyte) of a lithium battery, the performance of the anode material plays a crucial role in determining the overall battery performance. At present, most lithium battery manufacturers use graphite as the anode material, which is a type of carbon-based anode material, including both artificial and natural graphite.
Figure 1. Types of Lithium Battery Anode Materials
Graphite is considered an ideal anode material for lithium batteries because of its good cycle stability, excellent electrical conductivity, and layered structure that allows for efficient lithium intercalation. However, as performance demands for lithium batteries continue to rise, the limitations of graphite as a negative electrode material have become more apparent. For example, its low specific capacity (around 372 mAh/g), tendency for structural degradation after many cycles, and limited potential for increasing energy density have led researchers to seek alternative materials.
Silicon has attracted significant attention as a promising anode material due to its high theoretical capacity (4200 mAh/g), low lithiation voltage (below 0.5 V vs. Li/Li+), low reactivity with electrolytes, abundant availability, and low cost. However, silicon also faces major challenges when used as a standalone anode material.
Figure 2. Structural Comparison Between Graphite and Silicon
During charging, lithium ions intercalate into the silicon lattice, causing a volume expansion of approximately 300%. During discharging, the lithium ions are extracted, leaving large voids. This volume change leads to several issues, such as delamination of the anode from the current collector, electrochemical corrosion, and reduced battery safety and lifespan. Additionally, repeated expansion and contraction cause the SEI layer to break down and reform, increasing lithium ion consumption and reducing overall capacity.
To overcome these challenges, researchers have combined silicon with carbon materials, creating silicon-carbon composites that leverage the strengths of both. These composites are typically categorized into two types: conventional silicon-carbon composites (e.g., silicon-graphite, MCMB, and carbon black) and new silicon-carbon composites (e.g., silicon-carbon nanotubes and graphene).
The composite methods and structures of silicon-carbon materials include:
First, the walnut structure. In this design, silicon particles are made porous and filled with carbon, forming a three-dimensional network that enhances electrochemical performance. A study by Professor Ci Lijie from Shandong University demonstrated the effectiveness of a walnut-like porous silicon/reduced graphene oxide (P-Si/rGO) material, which showed excellent cycling stability and high capacity.
Second, the core-shell structure. In this configuration, carbon is coated around silicon particles, improving conductivity, preventing aggregation, and protecting the silicon from structural damage during charge-discharge cycles. Some researchers have further improved this by introducing a double-shell structure, where SiOâ‚‚ is first coated on the silicon surface before a carbon layer is added, effectively mitigating structural changes and enhancing cycle life.
Third, the ternary embedded structure. This involves embedding silicon within carbon nanotubes or graphene, allowing for better volume expansion control and enhanced conductivity. These structures help maintain the integrity of the anode over multiple cycles.
Fourth, the watermelon-like structure. Developed by the Institute of Physics and Chemistry, Chinese Academy of Sciences, this design features a multi-layered structure that effectively reduces volume expansion and particle fragmentation, leading to improved cycle stability and high-rate performance.
The preparation methods for silicon-carbon composites include ball milling, high-temperature cracking, chemical vapor deposition, sputtering, and evaporation. These techniques result in various structures designed to enhance battery capacity while minimizing the drawbacks of silicon.
In terms of market development, domestic companies like Shanhan, Jiangxi Zijing, and Shenzhen Beitui have already started producing silicon-carbon anode materials. Major battery manufacturers such as Guoxuan Hi-Tech, BYD, CATL, and others are actively developing and testing silicon-carbon anode systems. Internationally, Tesla has incorporated silicon-based materials into its Model 3, achieving a battery capacity above 550 mAh/g and an energy density of up to 300 Wh/kg.
Companies like GS Yuasa in Japan and Hitachi Maxell have also introduced silicon-based anode materials, with applications in electric vehicles and high-current batteries. The production and application of silicon-carbon anodes are accelerating, and it is expected that these materials will see significant growth in the lithium battery market in the coming years.
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