What is Silicon Carbon Composite Anode Material?
The silicon carbon composite anode material can form a binary alloy with lithium and has a theoretical high capacity of 4200 mAh/g. It also has a low lithium voltage drop platform (less than 0.5VvsLi/Li+), and low reactivity with electrolyte. It is a very promising lithium-ion battery anode material.
Silicon Carbon Composite Anode Material Types
Types of silicon carbon composite anode material structures include the following:
1) Walnut Structure
The silicon carbon composite anode material like a walnut structure is a three-dimensionally connected pore network set by forming the silicon particles into a porous structure and then filling the porous silicon with carbon material. This nano-microstructure exhibits excellent electrochemical performance.
At a current density of 1 A/g, a reversible capacity of 1,459 mAh/g can still be maintained after 200 cycles of charge and discharge. At a current density of 12.8 A/g, there is still a reversible capacity of 700 mAh/g.
2) Core-Shell Structure
The core-shell structure is a common type of composite, where the carbon material is wrapped in the outer layer of silicon particles to form a composite material. After carbon is coated on the surface of the silicon material, the conductivity of the composite material can be enhanced.
The carbon material has a certain toughness to avoid agglomeration between the silicon particles and the volume expansion of the lithium during the lithium absorption and release process. At the same time, a SEI film is formed on the surface of the carbon material, which suppresses erosion by the anode material from the electrolyte and thereby increases the cycle life and improves the rate performance.
Compared with the silicon carbon composite anode material of the walnut structure, the silicon carbon composite anode material of the core-shell structure contains more silicon, which greatly increases the space for intercalation of lithium. In addition, the phenomenon of swelling and crushing of silicon particles is also reduced a lot.
The carbon-encapsulation of silicon material by constructing a core-shell structure helps improve the cycle stability of the material. However, when the pyrolytic carbon in the silicon-carbon core-shell structure is clad on the surface of the silicon particles without voids, the volume effect of the lithiation process of the silicon core will be too large, which will cause the entire core-shell particles to swell and even cause the surface carbon layer to break up. As a result, the composite structure collapses and the cycle stability rapidly decreases.
In order to solve this problem, some researchers have begun to design the double-shell structure by strengthening the mechanical properties of the shell. Firstly, SiO2 is coated on the surface of silicon particles, and then a layer of carbon material is further coated on the surface of the composite material, which can effectively relieve structural changes of the composite material and improve the cycle life of the lithium battery.
3) Triple Embedded Composite Structure
Embedded silicon-carbon structures are often found on new silicon-carbon composites such as silicon/CNT and silicon/graphene composites.
First, a carbon film is coated on the silicon particles, and then carbon nanotubes are attached to the surface, and these materials are then caused to have a spherical shape. The surface of the silicon particles is covered with a carbon film. The thickness of the film is nanoscale (10-20 nm), and carbon nanotubes adhere to the film.
This way, the carbon nanotubes are filled between the silicon particles, which not only plays a conductive role, but also plays a role in absorbing the volume expansion of the silicon particles. Finally, the composites of silicon and carbon with these nanotubes adhering to the carbon nanotubes are manufactured into a pellet. These pellets have a particle size of about 10 μm.
4) Ternary Coating Filling Structure
The Institute of Physics and Chemistry at the Chinese Academy of Sciences has developed a silicon-carbon composite like a watermelon structure. Nano-silicon and graphite are combined (adulterated) together, and then a layer of carbon material is wrapped on the outer layer to form a silicon-carbon composite material similar to a watermelon-like structure. The structure can effectively reduce the volume change and particle fragmentation under the high-pressure electrode density. Based on practical considerations, the prepared silicon carbon anode has an appropriate reversible capacity of 620 mA h/g, and shows a cycle stability of over 500 cycles at a higher surface capacity (2.54 mAh/cm2).
Silicon carbon composite anode material is prepared by ball milling, pyrolysis, chemical vapor deposition, sputter deposition, and evaporation. Therefore, the structure of silicon-carbon materials made is various, but they are designed based on the idea of improving the capacity of lithium batteries and reducing the disadvantages of swelling and smashing of silicon particles. Today there are four common silicon carbon anode material production methods, high temperature pyrolysis, sol gel method, ball milling, and chemical deposition method.
Table 1: Silicon carbon composite anode material production methods
|Production Method||Brief Introduction||Characteristics|
|high temperature pyrolysis||A carbon-rich polymer or organic material is used as raw material, and carbon particles are obtained through high-temperature pyrolysis and composited with silicon to form a negative electrode material.||Have high capacity amount|
|sol gel method||The silicon salt is a precursor, which is hydrolytically condensed in the liquid phase, added with a carbon source, and then dried and sintered to prepare a silicon/carbon anode material.||High degree of crystallinity, granularity evenly distributed|
|ball milling||Use a ball mill to crush the silica into a nano granular carbon composite||Easy production method|
|Chemical deposition method||Under a set temperature and pressure gas phase reaction can form silica to precipitate||High cash costs, not suitable for chemical production|
Global Market Analysis
Global lithium battery anode material production is mainly from China and Japan. These two countries account for more than 95% of global anode material sales. In 2015, the leaders in the market of anode materials were Hitachi Chemicals (31% of the market) with BTR Energy following in second place (19%) and Nippon Carbon in third place (7%). Other producers of anode material include Mitsubishi Chemical, LS Mtron Carbonics, ShanshanTech, and Tokai Carbon.
China Anode Material Market
In 2016 China anode material annual production exceeded 122,500 tonnes, up 68.3% on the previous year. It is estimated that by 2020 China’s anode material production output will reach up to 295,000 tonnes per annum.
In the years to come the PRC anode industry is expected to have a CAGR of 30-35%, keeping constant growth. In 2016 anode material production value in China reached more than 6.64 billion CNY (A$1.28 billion) in value, up 64%.
China has already established a full anode material production industry chain in three regions: River Pearl Delta, Yangtze River Delta and Central China (Hunan and Henan). These three regions have a high degree of concentration, where it accounts for more than 80% of national anode material output.
Rapid growth in Anode Material Production is from:
- Domestic Electric Vehicle production output exceeding 50%, bringing great demand for anode material, specifically artificial graphite anode material;
- China digital lithium-ion battery market is rocketing;
- BTR, ShanShan, Zichen are all increasing anode output, especially Zichen where its exports have increased more than export output from both South Korea and Japan. In 2016 China’s anode material export will reach 30% of global output;
- In 2016-Q3, domestic electric vehicles are optimistic for the second half, anode material is a key component material in battery enterprises.
As the application and development of silicon carbon composite anode material is still in its early stage, only a few battery manufacturers worldwide apply silicon carbon composite anode material to commercial products. At present, the silicon carbon composite anode material accounts for a very low proportion in the entire lithium battery anode material market.
Silicon Carbon Composite Anode Material - Commercial Analysis
In 2015 the silicon carbon composite anode material market share was approximately 0.76% and is forecast to be 15% in 2020. Silicon carbon composite anode material usage in 2015 was 836 tonnes and is forecast to be 40,915 tonnes in 2020.
Table 2: Silicon Carbon Composite Anode Material Market Share Predictions
|Silicon Carbon Composite Anode Material Market Share Forecast|
|Anode Material Usage （10,000t)||11||12.7||15||18.7||22.6||29.3|
|Silicon Carbon Composite Anode Material Market Infiltration (%)||0.76%||1%||2%||5%||9.5%||15%|
|Silicon Carbon Composite Anode Material Usage (tonnes)||836||1,272||3,001||9,346||21,480||40,915|
|Market Share (CNY billion)||0.84||1.02||1.55||2.34||3.22||4.39|
Several companies in China have invested in developing large-scale silicon carbon composite anode material production lines for lithium-ion battery commercial applications. Details are given in Table 4 below:
Table 3: List of silicon carbon composite anode material producers in China
|Company Name||Product Guidance|
|Ningbo Shanshan Co., Ltd||In 2017 expected to complete silicon carbon anode material production line of 4,000tpa.|
|Hefei Guoxuan High-Tech Power Energy Co., Ltd||In November 2016 announced it would build a 5,000tpa silicon based anode material production line.|
|BTR New Energy Materials Inc.||Silicon carbon anode material production line of 1,000tpa.|
|Shanghai Putailai New Energy Technology Co., Ltd||Invested 5 billion CNY to build an anode production line, which includes silicon carbon composite material products.|
|Jiangxi Zichen Technology Co., Ltd||20,000tpa anode material production line, among which includes silicon carbon anode material product lines.|
|Tianjin Lishen Battery Joint Stock Co., Ltd||Key battery products include silicon carbon composite material.|
|Pride Power Co., Ltd||Entering the silicon carbon anode material market|
|Shenzhen Sinuo Industry Co., Ltd||Silicon composite production line of 3,000tpa, and by through implementing expansion, plans to increase annual production output to 10,000tpa|
|Huzhou Chuangya Battery Power Materials Co., Ltd||Can produce a silicon carbon material product reaching 600mAh/g, in the latter half of 2017 it went into pilot production|
|BYD Auto Co., Ltd||2018 planned ternary material battery energy density was 240Wh/kg, the company plans to increase to 300Wh/g by 2020. It uses SiO or nano silicon in its anode material.|
Future Research Focus Points
- Improve the dispersion of silicon nanoparticles and form an effective composite structure with carbon materials.
- The mechanism of the composite of silicon and carbon materials and the mechanism of lithium insertion in different microstructures should be studied to investigate the influence of different microstructures on the electrochemical performance, such as the influence of specific surface area on the formation of SEI films, and the carbon content and structure in different composite systems.
- Simplify and optimise the material preparation process, using cost-effective, short-cycle raw materials and preparation methods.
- Explore adhesives and electrolytes that better match the performance of silicon carbon composite anode material.
- Under the condition of maintaining the cycle stability, the large-current charge-discharge performance of silicon-carbon materials to be improved, which is significant for power-type batteries.
- Material selection and process must consider safety and environmental protection, and develop in the direction of green, environmental protection, and recycling.