The relationship between battery formation parameters and chemical process

The formation process of the SEI film in the formation process of the lithium ion battery specifically includes the following four steps:

Step 1: electrons are transferred from the inside of the current collector-conductive agent-graphite particles to point A where the SEI film is to be formed;

Step 2: The solvated lithium ion is diffused from the positive electrode to the point B of the surface layer of the SEI film being formed under the encapsulation of the solvent;

Step 3: The electrons at point A diffuse to point B through electron tunneling;

Step 4: The electrons transitioning to point B react with lithium salt, solvated lithium ion, film former, etc., and the SEI film is continuously formed on the surface of the original SEI film, so that the thickness of the SEI film of the graphite particle surface is continuously increased, and finally a complete SEI is formed membrane.

It can be seen that the overall reaction process formed by SEI can be specifically decomposed into the above four step-by-step reactions, and the four step-by-step reaction processes determine the film formation process of the entire SEI film.

Step 1: Electrons are internally transferred from the current collector-conductive agent-graphite particles to point A where the SEI film is to be formed.

The number of electrons reaching point A will be determined by the uniformity of the current and current used in the formation between the positive and negative electrodes: the larger the formation current, the larger the current passing through the electrode pad a point; When the unevenness is at a point close to each other (a), the current is larger; when the current at the point a of the electrode increases, the current of the active material particles passing through the point a will be larger, that is, the number of electrons reaching the point A per unit time will increase. Therefore, the film formation reaction occurring at point A will be changed (as described in the above article: a large amount of electrons are concentrated on the surface of the graphite particles, which is more likely to undergo a two-electron reaction process with the film former and lithium ions).

Step 2: The solvated lithium ion is diffused from the positive electrode to the B point of the surface layer of the SEI film being formed under the encapsulation of the solvent: when the composition of the electrolyte is constant, the temperature is raised, the viscosity of the electrolyte is lowered, and the film is formed. The transport resistance of the solvated lithium ion in the electrolyte will decrease; at the same time, the conductivity of the electrolyte will increase when the temperature rises (as shown in the figure below, the viscosity and conductivity of a certain electrolyte at different temperatures), The above process will cause more film-forming agent and solvated lithium ions to reach point B on the surface of the active material particles per unit time, thus affecting the film-forming reaction process at point B (as described in the previous article: relatively less The electrons (because the film former, more solvated lithium ions) accumulate on the surface of the graphite particles, which is more likely to undergo a one-electron reaction with the film former and lithium ions).

Step 3: The electrons at point A diffuse to point B through electron tunneling; the speed of this process must be related to the structure and composition of the formed SEI film: the denser the SEI film, the higher the proportion of organic components, and the effect of blocking electrons, the stronger the resistance, the greater the resistance of the electrons through the same distance. The thickness of the SEI film formed at this time will be smaller, and the lower the total amount of irreversible reactions, the higher the first efficiency of the battery.

Step 4: The electrons transitioning to point B react with lithium salt, solvated lithium ion, film former, etc., and the SEI film is continuously formed on the surface of the original SEI film, so that the thickness of the SEI film of the graphite particle surface is continuously increased, and finally a complete SEI is formed membrane. The secondary process is the free collision and the combined reaction process. The higher the temperature, the faster the molecular motion, the higher the probability of collision, and the higher the reaction speed, the smaller the resistance of this step.

The above analysis details the specific effects of various process parameters on the formation process and SEI film formation:

1. The formation current and the uniformity of current distribution on the electrode sheet will affect the specific composition of the SEI film;

2. The formation temperature will have an effect on the structure and composition of the SEI film.

So, how should we choose to process the process parameters?

1. The first is the formation temperature:

The formation temperature mainly affects the chemical conversion effect by affecting the viscosity and conductivity of the electrolyte and the ion diffusion rate of the electrode material. Generally, the higher the formation temperature, the lower the viscosity of the electrolyte, the higher the conductivity of the electrolyte, and the faster the ion diffusion rate of the electrode material, therefore the smaller the polarization the better the formation effect. However, when the temperature is too high, the structure of the formed SEI film will be destroyed and the composition thereof will be changed. At the same time, the electrolyte is an organic solvent solution, and the excessive temperature will accelerate the volatilization rate of the low-boiling component in the electrolyte, affecting Turn into effects. Therefore, the optional formation temperature is recommended to be RT ~ 90 ° C; high temperature formation (45 ~ 70 ° C) can be used if laboratory conditions permit.

2. Secondly, the distance between the positive and negative electrodes and the distance consistency

For the button battery, the factors affecting the distance between the positive and negative electrodes and the distance consistency are mainly the selection of the button battery and the button battery assembly.

1) Selection of button battery: button battery pack should include negative shell, spring piece, gasket, lithium sheet, separator, research electrode, cathode shell and other components; in order to ensure the shortest distance between positive and negative sheets, you need to choose Thinner isolating film (thin separator has a shorter ion transmission distance, but it is easier to short-circuit. The industry's solution is to apply ceramic coating treatment and PVDF treatment on the surface of the separator substrate to solve the short circuit between the positive and negative electrodes. Problem; however, the adhesion between the treatment layer and the separator substrate is limited. It is recommended to use it directly without ultrasonic cleaning of the separator. Generally, the industry does not need to ultrasonically clean the separator when assembling the battery. The industry has used 7um or even thinner separators, and then processes ceramics, PVDF, etc. on its surface. In order to make the distance between the positive and negative electrodes uniform, it is necessary to use spring plates for stress buffering and leveling, including flat gaskets, flat lithium sheets, and flat research electrodes.

2) Button battery assembly: It is recommended to use double gaskets for power assembly (as shown in the figure). Because the gasket is flat and has high rigidity and is not easily deformed, the electrode is placed between two gaskets. Effectively flatten the electrode, so that the distance between the positive and negative electrodes is high. (Dr. Ke is strongly recommended to the general powder to buy the battery pack for the Klud. The group purchase price is 0.6 yuan/set (including the battery case + spring piece +) A piece of gasket)).

3. Finally, the formation process

The formation process of a lithium ion battery mainly refers to the activation process of the battery, specifically: a process in which a film forming additive forms a solid electrolyte membrane on the surface of the electrode active material particles under a certain current. The process parameters affecting the whole process are mainly the formation current and the formation cut-off voltage, which are specifically controlled by the chemical forming equipment (chemical forming machine).

1) Chemicalization: In the above article, Dr. Ke has already shared with you that when a large current is used, a two-electron reaction is more likely to occur, that is, two electrons can participate in the reaction at the same time, and it is easier to generate an inorganic lithium salt component, and At this time, the SEI membrane molecules are more likely to be cluttered, and the structure is more loose, the corresponding thickness is larger, and the irreversible reaction is more; when the small current is formed, the single electron reaction is more likely to occur, that is, the reaction can be performed only by one electron participation, correspondingly, the organic lithium salt component is more easily formed, and at this time, the SEI membrane molecules are more easily ordered and stacked and the structure is more dense the corresponding thickness is smaller, and the irreversible reaction is less.

2) Formation of cut-off voltage: the formation process is the process of forming a SEI film on the surface of the active material particles by the film-forming additive, and the process of forming the SEI film is an irreversible reaction process, so it is only necessary to set the formation cut-off voltage to complete the reaction of the film-forming additive. Above the complete potential, the film formation reaction can be sufficiently carried out. However, the film formation potential of general film-forming additives (such as VC, FEC) is lower than 3.0V (potential to lithium metal), but due to the existence of polarization, it is necessary to ensure that the formation is fully carried out, for the study of lithium ion battery cathode material can be Set the cut-off potential to be between 3.5V and 3.8V (that is, when it is formed, only a small current is required to charge to 3.5V, then the battery can be charged and discharged using normal charge and discharge current). For the study of lithium ion battery anode materials the formation cut-off potential can be set to about 0.3V.

By optimizing the above-described formation current and the formation of the cutoff voltage, not only a battery having better performance can be obtained, but also the formation time can be greatly shortened, the possession of the chemical conversion equipment can be reduced, and the research cost can be reduced. But for a new system, how to determine whether the size of the formation current used is optimal, and how to choose the cut-off voltage? Dr. Kolu Deke will introduce a simple and effective verification method: trial and error.

Taking lithium cobaltate/graphite system as an example use VC and FEC as additives, select 0.02C for cross-flow charging until the battery is fully charged (4.2V), calculate the value of dQ/dV, and make dQ/dV-V. The graph, as shown in the figure below, the first reaction peak position (~2.8V) is the potential for the film-forming additive to decompose into a film, and the cut-off potential is ~2.92V; then the 0.03C current is selected for parallel sample formation. The first reaction peak position (~2.8V) is the potential at which the film-forming additive decomposes into a film, and the cut-off potential is ~2.94V. Finally, the 0.05C current is selected for parallel sample formation and the first reaction peak position is obtained (~2.9).V) is the potential at which the film-forming additive decomposes into a film, and the cut-off potential is ~3.16V. It can be seen from the above results that when the 0.02C current is formed into 0.03C, the film formation reaction seals overlap and does not have a large effect on the film formation. However, when 0.05C is formed, the peak position is obviously shifted to the right, and the polarization is increased. Large, will have an impact on the formation effect, so from the time of saving formation, and the angle analysis of the cell with excellent performance, the selection of 0.03C current is a better result. When the formation of different formation currents is used, the film formation reaction cut-off potentials are 2.92V, 2.94V, and 3.16V, respectively, and from the viewpoint of sufficient reaction, the formation of the cut-off potential can be selected to be 3.5V. That is, the chemical conversion process is completed by charging 0.03C constant current to 3.5V.

The above analysis, for the general section of the powder to elaborate the lithium ion battery research, the formation of the chemical process:

1. Formation temperature: RT ~ 90 ° C; preferably 45 ~ 70 ° C;

2. The distance between the positive and negative electrodes and the distance consistency: the thin diaphragm and double gasket are selected to be assembled and deducted;

3. Formation process: trial and error method to optimize the formation of current and formation deadline.

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