Analysis on the mechanism and suppression of gas expansion in lithium titanate batteries

Lithium titanate space group belongs to Fd3m, spinel structure, because of its unique three-dimensional lithium ion diffusion channel, has the advantages of excellent power characteristics and high and low temperature performance. At the same time, the lithium titanate crystal structure can maintain a highly stable volume change of less than 1 % in the lithium ion deembedding cycle, which lays the foundation for lithium titanate to become an important negative electrode material.

Lithium titanate(Li4 Ti5O12 commonly known as LTO) space group belongs to Fd3m, spinel structure, because of its unique three-dimensional lithium ion diffusion channel, has the advantages of excellent power characteristics and high and low temperature performance. At the same time, the lithium titanate crystal structure can maintain a highly stable volume change of less than 1 % in the lithium ion deembedding cycle, which lays the foundation for lithium titanate to become an important negative electrode material.

More importantly, it eliminates the safety hazards of batteries and is called the safest lithium battery negative electrode 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 negative materials, lithium titanate has a high potential of 1.55 V vsLi + / Li, a theoretical capacity of 175mAh/g, an open circuit voltage of 2.4 V, and a low energy density and voltage platform.

Analysis of gas expansion mechanism and inhibition in lithium titanate batteries

Lithium titanate batteries have the advantages of high safety, high recharging, and long cycle life. However, when lithium titanate is used as a negative electrode, the battery has serious gas expansion during the charging and discharging cycle, and it is even more serious at high temperatures. Although the study of lithium titanate battery gas expansion has never stopped, including carbon coating modification, hybrid, nanometer, etc., the gas expansion problem has not yet been completely solved, which hinders the market promotion of lithium titanate batteries.

Lithium titanate battery flatulence mechanism

The academic community believes that the reason why lithium titanate / NCM batteries are more severe than graphite-NCM is that lithium titanate can not form a SEI membrane on its surface like a graphite negative system battery, inhibiting its reaction with the electrolyte. During the process of charging and discharging, the electrolyte is always in direct contact with the surface of Li4 Ti5O12, resulting in continuous reduction and decomposition of the electrolyte on the surface of the Li4 Ti5O12 material, which may be the root cause of the gas expansion of the Li4 Ti5O12 battery.

The main components of the gas are H2, CO2, CO, CH4, C2H6, C2H4, C3H8 and so on. When lithium titanate is immersed in the electrolyte alone, only CO2 is produced. After it is prepared into a battery with a NCM material, the resulting gas includes H2, CO2, CO, and a small amount of gaseous hydrocarbons, and is used as a battery. H2 is produced only when circulating and discharging, and the content of H2 in the gas produced 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 balance in the electrolyte:

PF5 is a strong acid that easily causes the decomposition of carbonates, and the amount of PF5 increases with increasing temperature. PF5 helps the electrolyte decompose to produce CO2, CO and CxHy gases. According to relevant studies, H2 is produced from trace water in the electrolyte, but the water content in the electrolyte is generally 20 & amp; Times; Around 10 -- 6, the contribution to H2 production is very low. Wukai of Shanghai Jiaotong University used graphit/NCM111 as a battery in the experiment. The conclusion was that the source of H2 was the decomposition of carbonates under Gaodianya.

Lithium titanate battery flatulence suppression

At present, there are three main solutions to inhibit gas expansion in lithium titanate batteries. The first is the processing modification of LTO negative electrode materials, including improved preparation methods and surface modification. Second, develop electrolytes that match the negative electrode of LTO, including additives and solvent systems; Third, improve battery technology.

(1) Improve the purity of raw materials and avoid the introduction of impurities in the manufacturing process. Imurity particles not only catalyze the classification of electrolytes to produce gas, but also greatly reduce the performance, cycle life, and safety of lithium batteries. Therefore, the introduction of impurities in batteries must be reduced as much as possible.

(2) The surface of lithium titanate is covered with carbon nanoparticles. The apparent reason for the formation of gas by the negative pole LTO is that the SEI membrane forms slowly and less, resulting in the phenomenon of flatulence with its lifetime. The study found that the establishment of an isolation layer between the interface between lithium titanate and electrolytes(such as the construction of a nanocarbon coating on the surface of lithium titanate(LTO/C), The solid electrolyte interface(SEI) membrane formed on the co-coating layer, on the one hand, reduces the contact area of the LTO material with the electrolyte and prevents the generation of gas.

On the other hand, carbon itself can produce a SEI membrane to make up for the lack of LTO, but 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 and large-scale application and development of high-energy lithium titanate power cells.

(3) Improve the functionality of electrolytes. For the development of new electrolytes, many patents tend to use additives to promote the formation of dense SEI membranes on the surface of LTO to inhibit the occurrence of LTO and electrolyte interface side reactions. Certain electrolyte additives, such as fluorinated carbonates and phosphates, are conducive to the formation of stable SEI membranes on positive surfaces, reducing the dissolution of metal ions on positive surfaces, thereby reducing gas production.

Membrane-forming additives can also inhibit the amount of gas produced, The added membrane additives are lithium borate salt, Dingerjing or adienitrile, R-CO-CH = N2 structure compounds(where R is an alkyl or phenyl of C1 to C8), cyclic phosphate esters, phenyl derivatives, phenylacetylene derivatives, LiF additives, etc.. These film forming additives are conducive to the formation of SEI membranes on the surface of LTO, which to some extent inhibits the occurrence of flatulence.

(4) Positive polar surface coating. Covering a stable compound on a positive surface, such as aluminum oxide, can effectively inhibit the dissolution of metal ions. However, the overcomplicated coating will inhibit the deembedding of lithium ions and affect the electrochemical properties of the materials.

(5) Improve the battery production process. Battery production, to control the environmental humidity, operation process water introduction. From the reason why the gas is produced, it can be seen that the water in the air reacts with the positive material to form lithium carbonate and accelerate the electrolyte decomposition to produce carbon dioxide. In addition, the lithium titanate material itself has a strong water absorption(which needs to be operated in a dry chamber). After the negative electrode absorbs water, it reacts with the PF5 generated by the reversible decomposition of the electrolyte to produce H2, so strict water control is essential.

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