The steps of the process of lithium-ion battery formation

The formation process of the SEI membrane during the formation of lithium ion batteries includes the following four steps:

Step 1: The electrons are transmitted internally from the collector-conductor-graphite particles to point A to form the SEI membrane;

Step 2: Solvent lithium ions spread from the positive pole to the B point on the surface of the emerging SEI membrane under the coating of the solvent;

Step 3: The electrons at point A spread to point B through the electronic tunneling effect;

Step 4: The electrons that transition to point B react with lithium salts, soluble lithium ions, and film forming agents, and continue to form a SEI membrane on the surface of the original SEI membrane, resulting in an increase in the thickness of the SEI membrane on the surface of the graphite particle, eventually forming a complete SEI membrane.

It can be seen that the overall reaction process formed by SEI can be specifically described by the above four step-by-step reactions. The four step-by-step reaction processes determine the membrane formation process of the entire SEI membrane.

Step 1: Electrons are transmitted internally from the collector-conductor-graphite particles to point A to form the SEI membrane.

The number of electrons reaching point A will be determined by the uniformity of the current and current used in the conversion between the positive and negative poles: the greater the chemical current, the greater the current passing through the electrode point a; When the positive and negative electrode plates are uneven, the near point(a) has a larger current; When the current at point a of the electrode increases, the current passing through the particles of the active substance at point a will be greater, that is, the number of electrons reaching point A will increase per unit time. Therefore, the membrane reaction at point A will change(as mentioned in the previous article: that is, a large number of electrons are concentrated on the surface of graphite particles, and it is easier to react with the film forming agent and lithium ion in a dual-electron reaction process).

Step 2: Solvent lithium ions spread from the positive electrode to the B-point on the surface of the emerging SEI membrane under the encapsulation of the solvent: When the electrolyte composition is unchanged, the temperature rises and the electrolyte viscosity decreases. Membrane forming agent, solvent lithium ion transmission resistance in the electrolyte will be reduced; At the same time, when the temperature rises, the conductivity of the electrolyte will increase(as shown in the figure below, the viscosity and conductivity of a certain electrolyte at different temperatures), and the above process will cause the unit time, More film forming agents and lithium-ion solvents reach point B on the surface of the active substance particles, thereby affecting the membrane reaction process of point B(as mentioned in the previous article: that is, relatively few electrons(because of the film forming agent at this time, Solvent lithium ions are more concentrated on the surface of graphite particles, It is easier to react with membrane forming agents and lithium ions in a single electron reaction process).

Step 3: The electrons at point A spread to point B through the electronic tunneling effect; The speed of this process must be related to the structure and composition of the SEI membrane that has been formed: the denser the SEI membrane and the higher the proportion of organic components, the stronger the effect of blocking electrons, and the greater the resistance of electrons through the same distance. The thickness of the SEI membrane formed at this time will be smaller, and the lower the total amount of irreversible reaction, the higher the initial efficiency of the battery.

Step 4: The electrons that transition to point B react with lithium salts, soluble lithium ions, and film forming agents, and continue to form a SEI membrane on the surface of the original SEI membrane, resulting in an increase in the thickness of the SEI membrane on the surface of the graphite particle, eventually forming a complete SEI membrane. The secondary process is a free collision and a combined reaction process. The higher the temperature, the faster the molecular motion, the higher the probability of collision, the higher the reaction speed, and the less resistance the step.

In this paper, the effects of various process parameters on the process of chemical formation and SEI membrane formation are described in detail.

1. The current size and the uniformity of the current distribution on the electrode sheet will affect the specific components of the SEI membrane;

2. The temperature will affect the structure and composition of the SEI membrane.

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