What is pre-lithiation?

For a full battery, the SEI film formed at the negative electrode interface will consume the lithium ions deintercalated from the positive electrode and reduce the capacity of the battery. If we can find another lithium source outside the positive electrode material, so that the formation of the SEI film consumes the lithium ions of the external lithium source, so that the lithium ions deintercalated from the positive electrode will not be wasted in the formation process, and finally the full battery can be improved. capacity. This process of providing an external lithium source is pre-lithiation.

Knowledge window: The core of pre-lithiation is to find an external lithium source, so that the whole battery can be converted into lithium ions that consume the lithium ions provided by the external lithium source instead of the lithium ions that are deintercalated by the positive electrode, so as to retain the lithium ions deintercalated by the positive electrode to the greatest extent. And increase the full battery capacity.

Several pre-lithiation methods are introduced below to give you a better understanding of this technology.

1. The negative electrode formation method in advance

When the full battery is formed, the lithium ions deintercalated from the positive electrode will be consumed. If we can make the negative electrode separate, and then assemble it with the positive electrode after the negative electrode forms an SEI film, it can avoid the loss of lithium ions on the positive electrode and greatly improve the overall performance. The first efficiency and capacity of the battery. Undoubtedly, the key step here is the separate formation of the negative electrode, the negative electrode sheet and the lithium sheet are immersed in the electrolyte, and connected to an external circuit for charging. In this way, it can be ensured that the lithium ions consumed during formation come from metal lithium sheets rather than the positive electrode. After the negative electrode sheet is formed, it is assembled with the positive electrode sheet, and the cell does not need to be formed again, so that the lithium ions of the positive electrode will not be lost due to the formation of the SEI film on the negative electrode, and the capacity will be significantly improved.

The advantage of this pre-lithiation method is that it can simulate the normalization process to the maximum extent, and at the same time ensure that the formation effect of the SEI film is similar to that of the full battery. However, the two processes of the preformation of negative electrode sheets and the assembly of positive and negative electrode sheets are too difficult to operate.

2. Negative electrode spraying lithium powder method

Since it is difficult to use the negative electrode sheet to form lithium supplementation alone, people have thought of the lithium supplementation method of directly spraying lithium powder on the negative electrode electrode sheet.

First, a stable metal lithium powder particle should be produced. The inner layer of the particle is metal lithium, and the outer layer is a protective layer with good lithium ion conductivity and electronic conductivity. In the pre-lithiation process, the lithium powder is first dispersed in an organic solvent, and then the dispersion is sprayed on the negative electrode sheet, and then the residual organic solvent on the negative electrode sheet is dried, thus obtaining a negative electrode sheet with completed pre-lithiation. Subsequent assembly work is consistent with the normal process.

During the formation, the lithium powder sprayed on the negative electrode will be consumed in the formation of the SEI film, so as to maximize the retention of lithium ions deintercalated from the positive electrode and improve the capacity of the full battery.

The disadvantage of using this pre-lithiation method is that it is difficult to guarantee the safety, and the cost of material and equipment transformation is high.

3. Negative three-layer electrode method

Due to the limitations of equipment and processes, high-cost transformation for the purpose of pre-lithiation is not a priority for battery factories. If pre-lithiation can be completed in a way that battery factories are familiar with, the popularization will be greatly enhanced. The three-layer electrode method mentioned below makes the operation of the battery factory simpler. The core of the three-layer electrode method lies in the treatment of the copper foil. Compared with the normal copper foil, the copper foil of the three-layer electrode method is coated with the metal lithium powder required for the later formation. In order to protect the lithium powder from reacting with the air, it is coated with A protective layer is applied; the negative electrode is directly coated on the protective layer. After the cell is filled with liquid, the protective layer will dissolve in the electrolyte, so that the metal lithium is in contact with the negative electrode, and the lithium ions consumed by the formation of the SEI film during formation are supplemented by the metal lithium powder. The picture of the electrode after charging is as follows: This method does not have strict requirements on the processing conditions of the battery factory, but the stability of the protective layer at the pole piece rewinding and unwinding, rolling, cutting and other stations is a great challenge to the research and development of electrode materials It is also difficult to ensure the adhesion of the negative electrode material after the metal lithium powder disappears.

3. Cathode Li-rich Material Method

The small partners who work in the enterprise must have deeply experienced that even things that can be successful under laboratory conditions are likely to be difficult to move to the large-scale production of enterprises. The transformation cost of equipment, the cost of mass input of materials, and the control cost of the processing environment may all become fatal injuries that cannot be promoted by new technologies. For an industry where the process and equipment of lithium battery are basically mature, the pre-lithiation solution that enterprises prefer will definitely be a method that can be directly promoted without making too many on-site changes or even taking it over. The cathode lithium-rich material method just meets the needs of battery factories in this regard.

When the first effect of the negative electrode is lower than that of the positive electrode, too many lithium ions will be lost to the negative electrode during formation, resulting in that the effective space of the positive electrode cannot be underfilled by lithium ions after discharge, resulting in a waste of the positive electrode lithium intercalation space. If a small amount of high-gram capacity lithiated material is added to the positive electrode, it can not only provide more lithium ions for the formation of the SEI film during chemical synthesis, but also do not have to worry that the lithiated material cannot intercalate lithium again during discharge (because the Consuming all the lithium ions provided by the lithium-rich material), isn't it the best of both worlds?

At present, a typical lithiated material is Li5FeO4, which has a gram capacity of up to 700mAh/g. Each molecule can release four Li+ during formation. The equation is as follows:

Li5FeO4→4Li++4e-+LiFeO2+O2

The above reaction is not as reversible as the delithiation of the positive electrode for Li-ion batteries, since the O2 generated in the reaction is discharged out of the battery along with the degassing. However, due to the high gram capacity of lithium-rich materials, adding a small proportion of them into the positive electrode can supplement enough extra lithium ions for negative polarization. Therefore, as long as the stability of the product LiFeO2 in the electrolyte is ensured, it can improve the full battery. role of capacity.

In the specific implementation of this plan, if the process of compounding and coating Li5FeO4 with the positive active material is explored, it will allow the battery factory to complete the pre-lithiation without any equipment modification, which seems foolish. This type of operation is often the favorite of enterprises.

The various methods of pre-lithiation are introduced here. It should be noted that the various pre-lithiation methods mentioned above are aimed at the full battery whose negative electrode first efficiency is lower than that of the positive electrode. . For batteries with a lower positive first effect, the above method is basically useless, because the first effect of the full battery is limited by the fact that there is no longer enough space for lithium insertion after the positive electrode is charged, even if the external lithium is supplemented, it cannot be Embedding the positive electrode and thus has no effect.