The latest progress in suppressing flatulence in lithium titanate batteries

Lithium titanate (Li4Ti5O12 commonly known as LTO) space group belongs to the 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 high stability during the lithium ion deintercalation cycle and the volume change is less than 1%, which lays the foundation for lithium titanate to become an important negative electrode material. More importantly, it eliminates the hidden dangers of battery safety and is known as the safest negative electrode material for lithium batteries. The physical structure of lithium titanate is suitable as a negative electrode material for lithium batteries, so what are its electrochemical characteristics? Compared with carbon anode materials, lithium titanate has a higher potential of 1.55V vs Li+/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, long cycle life, etc. However, when lithium titanate is used as the negative electrode, the battery will have serious flatulence during the charge and discharge cycle, which is more serious at high temperatures. Although the research on the flatulence of lithium titanate batteries has never stopped, including carbon coating modification, hybridization, nanometerization, etc., the flatulence problem has not been completely solved, which hinders the market promotion of lithium titanate batteries.

1. Flatulence mechanism of lithium titanate battery

The academic community believes that the reason why the flatulence of lithium titanate/NCM battery is more serious than that of graphite/NCM is that lithium titanate cannot form an SEI film on its surface like a graphite anode system battery to inhibit its reaction with the electrolyte. During the charging and discharging 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 Li4Ti5O12 battery flatulence.

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 made into a battery with NCM materials, the gases produced include H2, CO2, CO and a small amount of gaseous hydrocarbons. During charging and discharging, H2 is produced, and the content of H2 in the gas produced at the same time exceeds 50%. This indicates that H2 and CO gas will be generated during the charge and discharge process.

LiPF6 has the following balance in the electrolyte:

PF5 is a strong acid, which easily causes the decomposition of carbonates, and the amount of PF5 increases with the increase of temperature. PF5 helps the electrolyte to decompose to produce CO2, CO and CxHy gas. According to related studies, the generation 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 used graphite/NCM111 as the battery in his experiment and concluded that the source of H2 is the decomposition of carbonate under high voltage.

2. Inhibition of flatulence in lithium titanate batteries

At present, there are mainly three solutions to inhibit the flatulence of lithium titanate batteries. First, the processing and modification of LTO anode materials, including improved preparation methods and surface modification, etc.; second, the development of electrolytes that match LTO anodes, including additives , Solvent system; third, improve battery technology.

(1) Improve the purity of raw materials and avoid the introduction of impurities during the manufacturing process. The impurity particles will not only catalyze the classification of the electrolyte to generate gas, but also greatly reduce the performance, cycle life and safety of the lithium battery. Therefore, the introduction of impurities in the battery must be minimized.

(2) The surface of lithium titanate is covered with nano carbon particles. The apparent reason for the formation of gas in the negative electrode LTO is that the formation of the SEI film is slower and less, which leads to the phenomenon of flatulence accompanying its life. The study found that an insulating layer was established between the lithium titanate and the electrolyte interface (such as building a nano-carbon coating layer on the lithium titanate surface (LTO/C), and the solid electrolyte interface (SEI) film formed 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, the carbon itself can produce an SEI film to make up for the lack of LTO, and at the same time, it can also enhance the conductivity of the LTO material. The above research results can solve the problem of lithium titanate battery production. The gas behavior is of great significance, and promotes the design and large-scale application and development of high-energy lithium titanate power batteries.

(3) Improve the functionality of electrolyte. 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 the LTO to suppress the occurrence of side reactions at the interface between the LTO and the electrolyte. Certain electrolyte additives, such as fluorinated carbonates and phosphates, are conducive to the formation of a stable SEI film on the positive electrode surface, reducing the dissolution of metal ions on the positive electrode surface, thereby reducing gas generation. Film-forming additives can also inhibit gas production. The added film-forming additives include lithium borate, succinonitrile or adiponitrile, and compounds with the structure of R-CO-CH=N2 (where R is C1-C8 alkyl or phenyl ), cyclic phosphate, 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, and inhibit 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 battery production technology. When the battery is produced, it is necessary to control the environmental humidity and the introduction of moisture during the operation. It can be known 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 extremely strong water absorption (it needs to be operated in a dry room). After the negative pole piece absorbs moisture, it will react with PF5 produced by the reversible decomposition of the electrolyte to form H2, so strict moisture control is essential .