Silicon-based anode refers to battery anode materials with silicon as the main active material, and its theoretical capacity far exceeds that of traditional graphite anode materials. Traditional lithium-ion battery anode materials are mainly graphite, which has a low theoretical capacity of only 372mAh/g, while the theoretical capacity of silicon-based anode is as high as 4200mAh/g, which is about ten times that of graphite, and has broad market prospects!
Lithium battery negative electrode materials are mainly divided into carbon-based materials and non-carbon-based materials. Carbon-based materials include natural graphite negative electrode, artificial graphite negative electrode, mesophase carbon microbeads (MCMB), soft carbon (such as coke) negative electrode, hard carbon negative electrode, carbon nanotubes, graphene, carbon fiber, etc. Non-carbon-based materials are mainly divided into silicon-based and its composite materials, nitride negative electrode, tin-based materials, lithium titanate, alloy materials, etc.
The raw materials of silicon-based negative electrodes are mainly composed of silicon materials and graphite. Silicon, as a negative electrode material for lithium-ion batteries, has outstanding advantages. First, silicon alloys with lithium at room temperature, and the theoretical specific capacity is as high as 4200mAh/g, which is more than ten times that of current graphite negative electrode materials; second, compared with graphite, silicon is abundant in the earth’s crust and widely distributed, accounting for 25.8% of the mass of the earth’s crust, and is the second most abundant element in the earth’s crust; third, silicon has a slightly higher potential platform than graphite (about 0.4 V, Li/Li+), there is no hidden danger of lithium precipitation, and it is safe; fourth, the low-temperature performance of silicon-based negative electrode materials is better than that of graphite; fifth, it can provide channels for lithium ion embedding and extraction from all directions, and has excellent fast charging performance. Silicon negative electrodes are expected to become an ideal substitute for graphite negative electrodes.
Silicon-carbon negative electrode solution: Whether it is silicon-carbon or silicon-oxygen, there are still some problems to be solved in industrialization. From the perspective of solutions, the expansion rate problem of silicon-carbon is generally solved by carbon coating technology of nano-silicon, while the silicon-oxygen negative electrode improves the initial efficiency by pre-magnesium or pre-lithium.
Carbon-coated nano-silicon is a structure with nano-silicon as the raw material and a carbon layer coated on the surface. The main principles and functions include:
(1) Carbon coating can protect silicon, thereby avoiding direct contact between the electrode and the electrolyte and inhibiting the excessive growth of the SEI film;
(2) Carbon materials have good conductivity and can build a continuous conductive network on the silicon surface to reduce the internal resistance of the battery;
(3) Carbon materials have strong mechanical properties and can buffer the stress changes caused by the volume expansion of silicon, thereby maintaining the integrity of the electrode structure.
The size of silicon particles is the key. The larger the particle size, the lower the cost, but the cycle performance may be poor. The volume expansion of large-sized silicon negative electrode particles will cause cracks inside the composite material, destroy the continuity of electronic conduction, and reduce performance. In theory, the smaller the silicon grains, the better the cycle performance. The preparation of nano silicon is divided into different technical routes, including grinding or vapor deposition, and vapor deposition is divided into PVD and CVD.
Grinding: The particle size of the traditional physical grinding method is about 100nm, which is far from meeting the particle size requirements of the silicon negative electrode. A new grinding process is needed to grind and crush large particles of silicon from top to bottom to continuously reduce its particle size. The current grinding cost per ton is 200,000 yuan/ton, which is the lowest cost solution for nano silicon and the current mainstream solution. It needs to use high-energy ball milling and other technical improvements. The disadvantage is that the product has a large particle size, which is easy to introduce impurities, resulting in low purity and the shape and particle size distribution of the particles cannot be controlled.