How much do you know about the low-temperature technology of lithium-ion batteries? (3)-Material improvement

by:CTECHi     2021-07-08

Before mentioning the improvement of materials, let us first review the charging process of lithium-ion batteries. There are four steps in total:

1) Lithium ions are deintercalated from the positive electrode particles and enter the electrolyte Inside

2) The transfer of lithium ions in the electrolyte

3) The lithium ions pass through the SEI membrane to contact the negative electrode

4) The lithium ions are in the negative electrode The embedding and diffusion of

and the content of material improvement to be discussed next in this article is developed from the above four points one by one.

——Lithium ions are deintercalated from the positive electrode particles and enter the electrolyte

This is the journey of lithium ion movement during the charging process, and it is also the least resistance of the four steps , The easiest step to complete. The resistance of lithium ion cathodes to deintercalation mainly depends on the structure of the cathode material. Lithium cobalt oxide has a layered structure. Lithium ions can be freely deintercalated and intercalated in four directions: front, back, left, and right. Therefore, it has a good performance even at low temperatures. The molecular structure of lithium cobalt oxide is shown as follows:

Compared with the layered structure of lithium cobalt oxide, lithium iron phosphate has an olivine structure. In this structure, PO4 limits the volume change of the crystal structure, so lithium ions are intercalated and The resistance of deintercalation is greater, and the relative low temperature performance is not as good as that of lithium cobalt oxide.

In addition, for active material particles, the smaller the particle, the shorter the lithium ion migration path. At room temperature, due to the rapid diffusion of lithium ions, the effect of large and small particles on the capacity is not obvious, but at low temperatures, the advantages of small particle materials will begin to appear. The comparison results of the capacity of particles of the same material size at different temperatures are as follows:

——The transfer of lithium ions in the electrolyte

Lithium ions are deintercalated from the positive electrode with the least hindrance and happily come to the electrolyte. In the electrolyte, the degree of obstruction depends on the ionic conductivity of the electrolyte at low temperatures. In order to ensure the low temperature performance of the electrolyte, the content of the high melting point solvent EC (melting point 39~40℃) needs to be reduced, generally 15%~25% is appropriate. Some low melting point PC (melting point -48.8℃) can be added, but film-forming additives should be added at the same time to avoid the peeling of the graphite layer caused by PC. The schematic diagram is as follows:

High ions Conductivity is the standard configuration of low-temperature electrolyte, but high ionic conductivity at room temperature does not necessarily mean better low-temperature performance. The key to the problem is to ensure the ionic conductivity at low temperatures. The ionic conductivity is determined by the dielectric constant and viscosity. The dielectric constant refers to the amount of Li+ in the free state under the same lithium salt concentration. Naturally, the more the better; the viscosity refers to the resistance to the Li+ transfer Naturally, the smaller the better.

The ionic conductivity of the electrolyte at low temperature is an important factor affecting the low-temperature performance of the electrolyte. The ionic conductivity is determined by the dielectric constant and the viscosity. The greater the dielectric constant, the lower the viscosity. The higher the ion conductivity.

——Lithium ions pass through the SEI film to contact the negative electrode

Lithium ions pass through the electrolyte to the surface of the negative electrode, but first pass the SEI film, which is hindered here. The size depends on the impedance of lithium ions passing through the SEI film. We hope that the impedance of the SEI film will not be too large at low temperatures.

So how can I improve the impedance of the SEI film? It is nothing more than changing the composition and thickness of the SEI film, and the method of operation has to return to the electrolyte: by adding the film-forming additives of the electrolyte to reduce the impedance of the SEI film. Common additives are sulfites (ES/PS), vinylene carbonate (VC) and sulfone-based compounds.

——The insertion and diffusion of lithium ions in the negative electrode

After a long journey, lithium ions finally came to the final destination of charging: the negative electrode. But don’t worry, lithium ions will encounter the greatest resistance during the entire charging process when they are inserted into the negative electrode. Lithium ions that cannot overcome the resistance can only be sacrificed heroically on the surface of the negative electrode to form a single element of lithium that can never be reused. (That is, the low-temperature charging lithium precipitation mentioned above).

The improvement of lithium ion insertion and diffusion in the negative electrode mainly includes carbon coating, surface oxidation, doping or coating of other elements. For example, carbon with a larger spacing can be coated on the graphite surface to reduce the resistance during lithium ion intercalation, and the graphite surface can be doped or coated with tin to improve low-temperature performance.

After taking everyone to complete the lithium-ion wandering tetralogy of the charging process, let us go back and think about it: In the four stages, which stage of the lithium-ion encounters the greatest resistance? The answer is obvious: The process of inserting lithium ions into the negative electrode; this is determined by the characteristics of the negative electrode material. During the charging process, the four steps mentioned above happen at the same time, and the slowest step determines the final speed of the charging reaction.

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