In-depth interpretation of Li-ion battery first efficiency

by:CTECHi     2021-07-30

After the cathode material half-cell (the cathode material is the positive electrode and the metal lithium sheet is the negative electrode) is completed, it first undergoes a charge-discharge cycle: during the charging process, lithium ions are deintercalated from the positive electrode and precipitated in the negative electrode On the metal lithium sheet; during discharge, the metal lithium sheet loses electrons to form lithium ions and pass through the electrolyte before being embedded in the positive electrode.

We use the lithium cobalt oxide half-cell data as an example to create its first charge and discharge curve, as shown in the figure below:

From the figure above, we can It can be seen that the first charge capacity of the half-cell is slightly higher than the first discharge capacity, that is, 100% of the lithium ions extracted from the positive electrode during charging do not return to the positive electrode during discharge. The first discharge capacity/first charge capacity is the first efficiency of this half-cell.

Not only lithium cobalt oxide, but other common cathode materials such as ternary, lithium iron phosphate, etc. half-cells also have the phenomenon of 'first discharge capacity

From the graphs above, it can be seen that the first-time efficiency of ternary is the lowest , Generally 85~88%; lithium cobalt oxide is second, generally 94~96%; lithium iron phosphate is slightly higher than lithium cobalt oxide, 95%~97%.

So where did the capacity lost during the first charge and discharge go? For the cathode material half-cell, the capacity loss is mainly caused by the change of the material structure after the first discharge: after the first discharge, the structure of the cathode material changes due to delithiation, thereby reducing the places where lithium can be inserted in the material, and the lithium ions cannot be All are inserted back to the positive electrode during the first discharge, which causes a loss of capacity.

Like the cathode material half-cell, the anode material half-cell will also be affected by the initial efficiency. Taking the graphite material half-cell as an example, the lithium ion deintercalation and intercalation potential of graphite material is higher, so it is the positive electrode, and the metal lithium sheet is the negative electrode. During the first cycle, lithium ions must lose electrons from the lithium sheet (negative electrode) and then be inserted. Graphite (positive electrode), so the half-cell is first discharged and then recharged. The first discharge and charge curve of the graphite material half-cell is as follows:

It can be seen from the above figure that the first charge capacity of the half-cell is significantly lower than the first discharge capacity, which means that the lithium ion is After the graphite layer was reached during the discharge process, it was not 100% deintercalated from the graphite during subsequent charging. Where is the lithium ion lost during this period consumed? I believe that a small partner with a certain theoretical basis can think of this reason: when the graphite half-cell is discharged for the first time, before the lithium ions are inserted into the graphite, the SEI film will be formed on the graphite surface. To the negative electrode of the lithium sheet, resulting in the first discharge capacity of the graphite half-cell> the first charge capacity.

For cathode materials such as lithium cobalt oxide and ternary, the main reason for the first efficiency is that the structure of the material changes after the first delithiation, which causes the lithium ions to fail to be 100% inserted. For carbon anodes, the first efficiency is mainly caused by the formation of SEI.

For the currently commonly used graphite or mesophase anode materials, the initial efficiency is generally between 90% and 92%. For lithium titanate, a material that hardly forms an SEI film, the efficiency will increase significantly for the first time, about 97%. In addition, for the current emerging silicon-carbon anode materials, since the first-time efficiency of the silicon anode is only 50%, the first-time efficiency of the silicon-carbon anode will gradually decrease as the silicon content increases.

Source: Zhixing Lithium Battery

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