18650 battery 3500mah

Briefly describe the application of nanotechnology in 18650 battery 3500mah

As an efficient energy storage component, lithium-ion batteries have been widely used in the field of consumer electronics. From mobile phones to laptops, lithium-ion batteries can be found. The brilliant achievements of lithium-ion batteries are due to their ultra-high energy storage density and good safety performance. With the continuous development of technology, the energy density and power density of lithium-ion batteries are also constantly improving, and nanotechnology has made an indelible contribution. Speaking of the application of nanotechnology in lithium-ion batteries, the first thing that comes to my mind is LiFePO4. Due to its poor conductivity, LiFePO4 has been prepared into nanoparticles to improve its conductivity, which greatly improves the electrochemical properties of LiFePO4. In addition, silicon anode is also a beneficiary of nanotechnology. Nano silicon particles can well inhibit the volume expansion of Si during the lithium insertion process and improve the cycle performance of Si materials. Recently, Jun Lu of Argonne National Laboratory in the United States published an article in Nature nanotechnology magazine, summarizing and reviewing the application of nanotechnology in lithium-ion batteries.

Positive electrode materials

1. LiFePO4 materials

LiFePO4 materials have good thermal stability and low cost, which has attracted widespread attention. However, due to the unique covalent bond structure inside LiFePO4 materials, the electronic conductivity of LFP materials is very low, which limits its high-rate charge and discharge performance. For this reason, people make LFP materials into nanoparticles and use conductive materials (such as carbon), conductive polymers and metals to coat them. In addition, people have found that by adding high-valent metal cations into nano LFP particles using a non-stoichiometric solid solution doping method, the electronic conductivity of LFP nanoparticles can be increased by 108, so that LFP materials can be charged and discharged within 3 minutes, which is particularly important for electric vehicles.

Figure a below shows the crystal structure of LFP crystals in the (010) direction. The "PO6" octahedrons in the crystal are connected together by sharing O atoms. This connection method also leads to low electronic conductivity of the material. In addition, another problem that affects the performance of LFP materials is the Fe occupancy problem. In the 1D direction, Li+ has a high diffusion coefficient, but part of Fe occupies the position of Li, which affects the diffusion rate of Li in the (001) direction, resulting in large polarization of the material and poor rate performance.

2. Inhibit the decomposition of LiMn2O4 materials

LMO materials have three-dimensional Li+ diffusion channels, so they have a high ion diffusion coefficient, but Mn3+ will be formed under low SoC conditions. Due to the existence of the Jonh-Teller effect, the LMO structure is unstable, and part of the Mn element dissolves into the electrolyte and eventually deposits on the surface of the negative electrode, destroying the structure of the SEI film. At present, one solution is to add some low-valent main group metal ions, such as Li, to LMO to replace part of Mn, thereby increasing the valence state of the Mn element under low SoC and reducing Mn3+. Another solution is to coat the surface of the LMO material particles with a layer of oxides and fluorides with a thickness of 10-20nm, such as ZrO2, TiO2 and SiO2.

3. Inhibit the chemical activity of NMC

NMC materials, especially high-nickel NMC materials, have a specific capacity of up to 200 mAh/g and have excellent cycle performance. However, NMC materials are very easy to oxidize the electrolyte when charged. Therefore, in actual production, we do not want to make NMC materials into nanoparticles, but we can inhibit the chemical activity of NMC by nano-coating.

In order to inhibit the reaction activity of high-nickel NMC materials and electrolytes, people try to use nanoparticles to coat the materials to avoid direct contact between material particles and electrolytes, thereby greatly improving the cycle life of the materials, as shown in Figures a and b below. Atomic layer deposition is also an important method to protect NMC materials. Studies have shown that 3 to 5 atomic layer depositions can obtain the best performance NMC materials. However, due to the lack of acidic functional groups on the surface of NMC materials, it is difficult to effectively perform atomic layer deposition.

Negative electrode materials

1. Protection of graphite materials

Graphite materials have low lithium insertion voltage (0.15-0.25VvsLi+/Li), which is very suitable as the negative electrode material of lithium-ion batteries, but graphite materials also have some disadvantages. Graphite after lithium insertion has strong reactivity and will react with organic electrolytes, causing graphite flakes to fall off and electrolyte decomposition. Although the SEI film can inhibit the decomposition of the electrolyte, the SEI film cannot 100% protect the graphite negative electrode. At present, common graphite surface protection methods include surface oxidation and nano-coating technology.

Nano-coating technology includes three categories: amorphous carbon, metal and metal oxide. Among them, amorphous carbon is mainly obtained by vacuum chemical deposition CVD method, which is low-cost and suitable for large-scale production. Metal and metal oxide nano-coatings are mainly obtained by wet chemical methods (electroplating), which can well protect graphite and prevent electrolyte decomposition.

2. Improve the rate performance of lithium titanate LTO and TiO2 materials

LTO (Li4Ti5O12) materials are highly safe, do not generate stress during Li insertion and deinsertion, have a high lithium insertion potential, and will not cause decomposition of the electrolyte. It is a very excellent negative electrode material, but LTO materials still face the following problems: 1) Low specific capacity, with a theoretical specific capacity of only 175mAh/g; 2) Low electronic and ionic conductivity. At present, nanotechnology is mainly used in LTO in the following three aspects: 1) Particle nano-sizing; 2) Nano-coating technology; 3) Composite of LTO nano-materials and conductive materials. Nano-sizing of LTO materials can effectively reduce the diffusion distance of Li+ and increase the contact area between LTO and the electrolyte. Nano-coating technology can enhance the charge exchange between LTO and the electrolyte and improve the rate performance. Several common nano-coating technologies are shown in the figure below, where Figure a shows the composite structure material of nano-TiO2 and porous carbon material. Figure b shows how to prepare LTO+CMK-3 mesoporous carbon composite materials.

3. Improve the energy density of silicon negative electrode

Si material has a theoretical specific capacity of 3572mAh/g, which is much higher than graphite material, so it has attracted widespread attention. However, Si will produce up to 300% volume expansion in the process of lithium insertion and removal, causing particle breakage and active material shedding. In order to overcome this shortcoming, people make Si materials into nanoparticles to relieve the mechanical stress generated by the expansion of Si particles. At present, other Si nanostructures include 1D nanowires, which can form good contact with the current collector and electrolyte and leave enough space for Si expansion. Therefore, the reversible specific capacity of this material is as high as 2000mAh/g, and has good cycle performance.

Application of nanotechnology in Li-S batteries

Li-S batteries have high energy density and low cost, and are very promising next-generation energy storage batteries. However, the main problems currently faced by Li-S batteries are low S conductivity and dissolution of lithium insertion products. In order to solve this problem, people have adopted a variety of composite nanomaterial technologies. For example, by combining S with porous hollow carbon or metal oxide nanoparticles, the stability of S can be significantly improved and the cycle performance of the electrode can be improved. In addition, the composite of S and graphene materials can also significantly improve the cycle performance of the S negative electrode.

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