graphite electrode for ladle furnace

Research status and prospects of long-life graphite electrode for ladle furnace for lithium-ion batteries

In 1991, the Japanese manufacturer of Sony launched the first commercial lithium-ion battery. So far, the cathode materials of commercial lithium-ion batteries are lithium-containing transition metal oxides such as layered lithium cobalt oxide () and ternary nickel cobalt. Aluminum (?y?2), ternary nickel manganese cobalt (?y?2), olivine structure lithium iron phosphate (4) and spinel structure lithium manganate (). The anode material mainly uses layered graphite. With the emergence of markets such as portable electronic products and electric vehicles, there is a huge demand for advanced lithium-ion batteries, including high-energy density lithium batteries. Silicon anode is a potential anode material. Its specific capacity is as high as ·h/g at room temperature. When Li+ is inserted and extracted into silicon anode material, about 300% volume expansion occurs in silicon, resulting in high resistance and Low conductivity. Although the theoretical specific capacity of graphite is low (LiC6 is ·h/g), graphite is widely used as the main negative electrode material in LIBS due to its excellent characteristics, such as light weight, low potential, high conductivity, and long life.

In the field of electric vehicles, there are more stringent requirements for the service life of lithium-ion batteries. The goal of the American Advanced Battery Council () in the Free Vehicle Research Initiative is: require 42 V battery systems and hybrid electric vehicles (HEV) calendar life 15 years; electric vehicles (EV) 10 years. In terms of cycle life, it is required to have a life of up to 1000 times under 80% depth of discharge (DOD). Mainstream electric vehicles at home and abroad, such as Weilai, BYD, and Tesla, all use lithium-ion batteries, but these batteries will have certain failures during use and transportation. These failures will affect the battery life and even cause safety problems. For example, the American Tesla S electric car caught fire, the South Korean Samsung mobile phone battery caught fire and exploded, and the lithium battery energy storage system caught fire and exploded, which affected the promotion of new energy technology to a certain extent. The failure phenomenon in lithium-ion batteries is caused by the interaction of complex physical and chemical mechanisms. A correct understanding of the failure mechanism plays an important role in the improvement of lithium-ion battery performance and technological upgrading.

There are extensive studies on the failure mechanism of lithium-ion batteries at home and abroad, including possible failures of positive and negative materials, current collectors, electrolytes, and diaphragms. The ultimate goal is to improve the performance of lithium-ion batteries through the development and modification of battery materials. Service life, power density, volumetric energy density, etc. Graphite is currently the main commercial lithium-ion battery anode material. By extending the life of graphite anode, it can increase the cycle life of chemical energy storage batteries and reduce the cost of lithium-ion batteries, which is of great significance to the promotion of new energy technologies. This article first summarizes the failure mechanism of the graphite electrode for ladle furnace material, and then according to the failure mechanism of the graphite electrode for ladle furnace, extends the service life of the graphite electrode for ladle furnace from two aspects: material design and electrode design, and finally points out the long-life graphite The development trend of electrode for ladle furnace.

1. Failure mechanism of graphite electrode for ladle furnace

The negative electrode material of commercial lithium-ion batteries is usually graphite, and the electrolyte used is usually a liquid organic electrolyte. As shown in Figure 1, the stable voltage window of an ordinary liquid organic electrolyte is 0.8-4.5 V, and the graphite negative electrode works at a voltage of about 0.05 V. , Beyond the stable voltage window of the electrolyte. Therefore, theoretically, the graphite negative electrode of a lithium-ion battery is thermodynamically unstable. However, during the first charge and discharge of a lithium-ion battery, various substances in the electrolyte undergo a reduction reaction on the surface of the graphite anode/electrolyte, thereby forming a passivation protective layer, which is usually called a solid electrolyte interface film (SEI). The SEI layer is a good Li+ conductor, but it is an insulator for electron flow. The existence of this film separates graphite from the electrolyte and limits the further decomposition of the electrolyte. Therefore, lithium-ion batteries with graphite as the negative electrode can be recycled. Use and keep it stable.

A good SEI layer is of great significance for improving the service life of the graphite electrode for ladle furnace. However, the SEI film generated in the actual battery environment is not perfect. Not only the unsolvated lithium ions can pass, but the solvated cations, Electrons, anions, solvents and solutes can also pass. During the process of lithium intercalation, the graphite particles will have a small volume expansion. At this time, the SEI layer on the surface of the graphite particles will break, resulting in a new SEI layer, which consumes electrolyte and increases internal resistance. In severe cases, thermal runaway will result. The graphite anode becomes invalid due to aging.

Graphite has a layered structure. In the original state, the distance between layers is 0.34 nm. During the charging and discharging process of the graphite negative electrode, the layer distance of graphite is expanded to accommodate Li+. When the lithium intercalation is completed, the layer distance is expanded to 0.37 nm. Due to the intercalation of lithium ions, volume expansion occurs (about 10% or less depending on the material). The original graphite particles have no cracks and voids, but after 200 cycles of 1 C rate, cracks parallel to the current collector are generated. The expansion of these cracks will cause the cracking and shedding of graphite particles. During the cycle, the solvated lithium ions and organic solvents are embedded between the graphite layers. These organic solvents undergo oxidation-reduction reactions between the graphite layers to produce gas. There is a further expansion of the damage to the graphite particles, which causes the graphite particles to break and fall off.

Lithium metal has been widely used in early lithium batteries and new battery systems, such as lithium-air batteries and lithium-sulfur batteries. Due to the continuous dissolution and deposition of metallic lithium on the lithium metal electrode, there is dendritic lithium deposition on the lithium electrode, and the deposited lithium dendrites will cause an internal short circuit of the battery, reducing the service life and safety. The working potential of the graphite negative electrode is close to that of metal lithium, so in some cases (low temperature, high charging rate, relatively high state of charge), it is easy to deposit lithium on the graphite negative electrode, which affects the service life of the graphite negative electrode and the entire battery. Use performance. The contact between the current collector and the electrolyte is corroded, and the corrosion products with poor electronic conductivity will cause over-potential, and cause uneven current and potential distribution, and finally cause lithium evolution. The presence of corrosion products also causes poor contact between the current collector and the graphite negative electrode and affects the service life of the graphite negative electrode.

In summary, the main failure mechanisms of lithium for graphite anodes of lithium-ion batteries are: excessive growth of the SEI layer; fragmentation of graphite particles; lithium deposition; and current collector corrosion.

2 Long life graphite electrode for ladle furnace

At present, there is a systematic understanding of the internal mechanism of lithium-ion battery failure. The study of failure mechanism provides theoretical support for prolonging the service life of lithium-ion batteries. The reasons for the failure of lithium-ion batteries include the deterioration of the positive and negative electrodes, diaphragm, and electrolyte. The failure mechanism of lithium-ion batteries is shown in Figure 2. Prolong the service life of lithium-ion batteries by suppressing or reducing these side reactions that cause failure, including thermal management systems, electrode material modification, new electrolytes, electrode design, etc. This section mainly introduces the research progress of long-life graphite electrode for ladle furnace from two aspects: material design and electrode design.