The operating principle of lithium-ion batteries is based on the embedding and de-embedding process of lithium ions between positive and negative electrode materials. In the charging state, lithium ions are de-embedded from the positive electrode material and migrate to the negative electrode through the electrolyte solution, where the negative electrode material is in a lithium ion-rich state. Conversely, during the discharge process, lithium ions are de-embedded from the negative electrode material and migrate back to the positive electrode. The capacity of the battery is mainly determined by the number of reversibly embedded and de-embedded lithium ions, and the performance parameters of the battery will change under different charging and discharging conditions or during the cycle. So, where does the lithium go?

Lithium ions inside the battery are mainly stored in the following places:

(1) Active lithium that moves and deintercalates between the positive and negative electrodes;

(2) Unusable lithium that is embedded in the negative electrode layer and cannot be removed;

(3) SEI film formed on the surface of negative electrode particles;

(4) Lithium precipitated on the surface of negative electrode coating;

(5) Lithium solvated in the electrolyte;

(6) Lithium fluoride formed by decomposition of electrolyte;

(7) CEI film formed on the surface of positive electrode particles;

(8) Unusable lithium that cannot be removed from the inside of positive electrode materials.

During the cycle of lithium-ion batteries, or under different charge and discharge rates and temperature conditions, the distribution of lithium inside the battery changes. Researchers at the University of Münster in Germany studied this.

During the study, a T-shaped battery structure was first constructed. After the battery has been cycled or aged, it is disassembled and the following steps are performed:

.The positive and negative electrodes are first cleaned with DMC, and then dissolved in water through microwave reaction. After the cleaning solution and dissolving solution are diluted with distilled water, the lithium content is determined using ICP technology.

.The diaphragm is soaked in hydrochloric acid, and the soaking solution is diluted and then tested for ICP lithium content.

.The nylon membrane used in battery assembly is washed and diluted with distilled water, and then tested for ICP lithium content.

.The other main components of the battery are also tested for ICP lithium content after being washed with distilled water.

In addition, as a control, the researchers tested the lithium content of the battery that was cycled once at a 0.1C rate, assuming that a solid electrolyte interface (SEI) and a positive electrode electrolyte interface (CEI) film were formed on the electrode surface. The original negative electrode sheet does not contain lithium, so the lithium content of the negative electrode sheet of the control battery is regarded as the lithium content that forms the SEI film. The lithium content of the positive electrode sheet of the control battery minus the lithium content of the original electrode sheet is regarded as the additional lithium content that forms the CEI film. The accuracy of all test results is ±7%.