Lithium titanate space group belongs to Fd3m, spinel structure, because of its unique three-dimensional lithium ion diffusion channel, it has the advantages of excellent power characteristics and good high and low temperature performance. At the same time, the crystal structure of lithium titanate can maintain a high degree of stability and the volume change is less than 1% during the lithium ion deintercalation cycle, which lays the foundation for lithium titanate to become an important negative electrode material. Lithium titanate (Li4Ti5O12 commonly known as LTO) space group belongs to Fd3m, spinel structure, because of its unique three-dimensional lithium ion diffusion channel, it has the advantages of excellent power characteristics and good high and low temperature performance. At the same time, the crystal structure of lithium titanate can maintain a high degree of stability and the volume change is less than 1% during the lithium ion deintercalation cycle, which lays the foundation for lithium titanate to become an important negative electrode material.
More importantly, it eliminates the potential safety hazard of the battery and is known as the safest lithium battery anode material. The physical structure of lithium titanate is suitable as a negative electrode material for lithium batteries, so what are its electrochemical properties? Compared with carbon anode materials, lithium titanate has a higher potential of 1.55VvsLi+/Li, a theoretical capacity of 175mAh/g, an open circuit voltage of 2.4V, and a lower energy density and voltage platform.
Lithium titanate batteries have the advantages of high safety, high-rate charging, and long cycle life. However, when lithium titanate is used as the negative electrode, the battery has serious flatulence during the charge-discharge cycle, and it is even more serious at high temperatures. Although the research on the flatulence of lithium titanate batteries has never stopped, including carbon coating modification, hybridization, nanonization, etc., the problem of flatulence has not been completely solved, which hinders the market promotion of lithium titanate batteries.
1. Gas mechanism of lithium titanate battery
The academic community believes that the reason why lithium titanate/NCM batteries are more gassy than graphite/NCM is that lithium titanate cannot form an SEI film on the surface like graphite anode system batteries, inhibiting its reaction with the electrolyte. During the charge and discharge process, the electrolyte is always in direct contact with the surface of Li4Ti5O12, resulting in continuous reduction and decomposition of the electrolyte on the surface of the Li4Ti5O12 material, which may be the root cause of the flatulence of the Li4Ti5O12 battery.
The main components of gas are H2, CO2, CO, CH4, C2H6, C2H4, C3H8, etc. When lithium titanate is immersed in the electrolyte alone, only CO2 is produced. After it is prepared into a battery with NCM materials, the gas produced includes H2, CO2, CO and a small amount of gaseous hydrocarbons. After the battery is made, only in the cycle When charging and discharging, H2 will be generated, and the content of H2 in the gas generated at the same time exceeds 50%. This indicates that H2 and CO gases will be generated during the charging and discharging process.
LiPF6 has the following equilibrium in the electrolyte:
PF5 is a strong acid, which can easily cause the decomposition of carbonates, and the amount of PF5 increases with the increase of temperature. PF5 helps the electrolyte to decompose and produce CO2, CO and CxHy gas. According to related research, the production of H2 comes from trace water in the electrolyte, but the water content in the electrolyte is generally about 20×10–6, which contributes very little to the production of H2. Wu Kai of Shanghai Jiaotong University chose graphite/NCM111 as the battery in the experiment, and concluded that the source of H2 is the decomposition of carbonate under high voltage.
2. Lithium titanate battery flatulence suppression
At present, there are three main solutions to suppress the flatulence of lithium titanate batteries. First, the processing and modification of LTO anode materials, including improving the preparation method and surface modification, etc.; second, developing an electrolyte that matches the LTO anode, including additives 1. Solvent system; 3. Improve battery technology.
(1) Improve the purity of raw materials and avoid the introduction of impurities in the manufacturing process. Impurity particles will not only catalyze the grading of the electrolyte to generate gas, but will also greatly reduce the performance, cycle life and safety of the lithium battery, so the introduction of impurities in the battery must be reduced as much as possible.
(2) The surface of lithium titanate is covered with nano-carbon particles. The apparent reason for the formation of gas in the negative LTO is that the formation of the SEI film is slower and less, resulting in flatulence that accompanies it throughout its life. Studies have found that establishing an insulating layer between the interface of lithium titanate and the electrolyte (such as constructing a nano-carbon coating layer on the surface of lithium titanate (LTO/C), synergistically forming a solid electrolyte interface (SEI) film on the coating layer On the one hand, the contact area between the LTO material and the electrolyte is reduced to prevent the generation of gas.
On the other hand, carbon itself can produce SEI film to make up for the deficiency of LTO, and can also enhance the conductivity of LTO materials. The above research results are of great significance to solve the gas production behavior of lithium titanate batteries, and promote the design, large-scale application and development of high-energy lithium titanate power batteries.
(3) Improve electrolyte functionality. For the development of new electrolytes, many patents tend to use additives to promote the formation of a dense SEI film on the surface of LTO to suppress the occurrence of side reactions at the interface between LTO and electrolyte. Certain electrolyte additives, such as fluorinated carbonates and phosphates, are beneficial to form a stable SEI film on the cathode surface and reduce the dissolution of metal ions on the cathode surface, thereby reducing gas generation.
Film-forming additives can also suppress gas production. The added film-forming additives include lithium borate salts, succinonitrile or adiponitrile, and compounds with the structure R-CO-CH=N2 (wherein R is an alkyl or phenyl group of C1-C8 ), cyclic phosphate esters, phenyl derivatives, phenylacetylene derivatives, LiF additives, etc. These film-forming additives are all conducive to the formation of SEI film on the surface of LTO, which inhibits the occurrence of flatulence to a certain extent.
(4) Positive electrode surface coating. Covering the surface of the positive electrode with a stable compound, such as alumina, can effectively inhibit the dissolution of metal ions. However, an overly complex coating layer will inhibit the deintercalation of lithium ions and affect the electrochemical performance of the material.
(5) Improve the battery production process. During battery production, it is necessary to control the humidity of the environment and the introduction of moisture during operation. It can be seen from the cause of the gas that the moisture in the air will react with the positive electrode material to form lithium carbonate and accelerate the decomposition of the electrolyte to generate carbon dioxide. In addition, the lithium titanate material itself has strong water absorption (it needs to be operated in a dry room). After the negative electrode sheet absorbs water, it will react with the PF5 produced by the reversible decomposition of the electrolyte to generate H2, so strict water control is very important. .