In the above half-cell article, the reasons for the first efficiency of the positive and negative electrode material half-cells were introduced. In fact, in the final analysis, the first efficiency is a type of capacity loss. When the positive and negative materials with the first efficiency at the same time form a full battery, what will the first efficiency look like? What is the impact of the first efficiency on the full cell? Is there a way to improve it? That's what this article is about.

Talking about the first efficiency of the full battery, we must start with its test method. After the battery is filled with liquid, it needs to go through the two charging and discharging processes of formation and capacity separation. Generally speaking, the first step of formation and capacity separation is the charging process. The sum of the two capacities is the first time the full battery is charged. The second step of the capacity distribution step is generally to discharge from a fully charged state to an empty charge, so the capacity of this step is the discharge capacity of the full battery. Combining the two, the algorithm for the first efficiency of the full cell is obtained:

The first efficiency of the whole battery = the discharge capacity of the second step of dividing the capacity / (the charging capacity of the formation + the charging capacity of the first step of dividing the capacity)

After understanding the test method, let's take a look at what determines the first efficiency of the full cell. To clarify this question, let us first assume a full battery like this: the positive electrode of the battery uses a ternary material with a first-time efficiency of 88%, and the negative electrode uses a graphite material with a first-time efficiency of 92%. What will be the first efficiency for this full cell?

After the first charge and discharge of 100 positive active lithium ions, only 88 can continue to be recycled. The reasons for the loss of 12 lithium ions are the loss of 8 lithium ions in the first effect of the negative electrode, and the lack of space for lithium insertion due to the first effect of the positive electrode, and 4 lithium ions remain in the negative electrode and cannot return to the positive electrode.

The conclusion is now obvious: when the positive first effect is 88% and the negative first effect is 92%, the first effect of the full cell is 88%, which is equal to the lower positive electrode. When the first effect of the negative electrode is lower, such as the lithium cobalt oxide positive electrode and the graphite negative electrode, the first effect of the whole battery is equal to that of the negative electrode with a lower first effect.

Knowledge window: The first-time efficiency of the full cell is equal to the lower first-time efficiency of the anode and cathode materials.

For the full battery, the formation factor of the first efficiency is also related to the side reactions of the first charge and discharge and the voltage range of the first charge and discharge (starting at 0V for charging and ending at 2.5~3.0V for discharge). However, these factors do not significantly affect the above conclusions.

In the last article, I introduced the first efficiency values of different types of materials. In addition to the influence of material types, what other factors will affect the first efficiency of full cells?

The first is the influence of the specific surface area of the graphite negative electrode. When the specific surface area of the graphite negative electrode is larger, the area for forming the SEI film will be larger, which will consume more lithium ions and reduce the first efficiency of the whole battery (the premise is that the first efficiency of the negative electrode is lower than positive).

Another factor that affects the first efficiency is the chemical charging system. When the SEI film formed by chemical formation is thinner and denser, the consumption of lithium ions in this process can be reduced and the first-time efficiency can be improved. In order to ensure the formation effect of the SEI film, the formation charge capacity is an important consideration.

In addition, an excessively large negative electrode excess requires the formation of more SEI films, thus reducing the first efficiency to a certain extent. The addition of PC solvent to the electrolyte will widen the operating temperature window of the lithium-ion battery, but if there is no corresponding film-forming addition to protect the negative electrode, it is easy to cause PC to peel off the graphite negative electrode and reduce the first time of the whole battery. efficiency.

Next, let’s make a summary of this article. First, I will introduce the calculation method of the first efficiency of the full battery, which is divided capacity discharge capacity / (formed charge capacity + divided capacity first step charge capacity). In addition, we know the determinant of the first effect of the whole battery: the first effect of the whole battery is equal to the lower of the first effect of the positive and negative electrodes. Finally, several factors affecting the first efficiency of the full battery are introduced.

Epilogue

In order to improve the product capacity, we must hope that the first efficiency of the full battery is as high as possible, so can the first efficiency of the full battery be significantly improved by relying on the several first efficiency influencing factors mentioned in the above article? The answer is no. The above improvement measures will either have little effect or will cause other negative effects. In short, they are not an effective way to improve the efficiency of the first time.

So what are the practical ways to improve first-time efficiency?
The answer lies in the last article of this series - pre-lithiation. Let's all look forward to it~~