Regardless of whether a full cell or a half cell, the performance of the first charge and discharge will be significantly different from the subsequent cycles. The description parameter for this characteristic is the first efficiency.
The discussion of the first efficiency should be divided into half cells and full cells; and a major improvement scheme for the first efficiency: pre-lithiation.

After the positive electrode material half-cell (the positive electrode material is the positive electrode, and the metal lithium sheet is the negative electrode) is completed, it first goes through a charge-discharge cycle: during the charging process, lithium ions are deintercalated from the positive electrode and precipitated on the negative electrode metal lithium sheet. ; During discharge, the metal lithium sheet loses electrons to form lithium ions and passes through the electrolyte, and then intercalates into the positive electrode.

Let's take the lithium cobalt oxide half-cell data as an example. The first charging capacity of the half-cell is slightly higher than the first discharging capacity. That is to say, 100% of the lithium ions deintercalated from the positive electrode during charging do not return to the positive electrode during discharging. 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 < first charge capacity". The first 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 does the lost capacity 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 cathode material structure changes due to delithiation, thereby reducing the position of lithium intercalation in the material, and lithium ions cannot It is all embedded back into the positive electrode during the first discharge, resulting in a loss of capacity.

Like the cathode material half-cell, the anode material half-cell is also affected by the first efficiency. Taking the graphite material half-cell as an example, the graphite material has a higher lithium ion deintercalation and insertion potential, so it is the positive electrode, and the metal lithium sheet is the negative electrode. During the first cycle, lithium ions must first lose electrons from the lithium sheet (negative electrode) and then intercalate. Graphite (positive electrode), so the half-cell is first discharged and then charged. The first charge capacity of the half-cell is significantly lower than the first discharge capacity, that is to say, after the lithium ions come to the graphite layer during the discharge process, they are not 100% deintercalated from the graphite during subsequent charging. Where is the lost lithium ions consumed during this period? I believe that friends with a certain theoretical basis can think of this reason: when the graphite half-cell is discharged for the first time, the lithium ions will form an SEI film on the graphite surface before being embedded in the graphite, and the lithium ions dedicated to the SEI film cannot be recovered during subsequent charging. 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 has changed after the first delithiation, resulting in the failure of 100% lithium ion intercalation. For carbon-based anodes, the first efficiency is mainly caused by the formation of SEI.
For the currently commonly used graphite or mesophase anode materials, the first efficiency is generally between 90 and 92%. For lithium titanate, a material that hardly forms an SEI film, the first efficiency will be significantly improved, about 97%. In addition, for the current emerging silicon carbon anode materials, since the first efficiency of silicon carbon anode is only 50%, the first efficiency of silicon carbon anode will gradually decrease with the increase of silicon content.

Conclusion The first efficiency of the half-cell has introduced so much to you. Regardless of the performance of the half-cell, what we care most about is the performance of the full-cell. When the positive and negative materials with the first efficiency characteristics are combined into a full battery, what kind of characteristics will they show? The answer will be revealed in the next article.