In-depth! Detailed explanation of lithium-ion battery formation technology

In-depth! Detailed explanation of lithium-ion battery formation technology

Lithium-ion battery production requires formation to achieve electrode wetting and full activation of electrode materials. During the first charge, as lithium ions are embedded in the negative electrode, the electrolyte components undergo a reduction reaction at the negative electrode to form a stable solid electrolyte interface film (SEI film) to prevent irreversible consumption of electrolyte and lithium ions in subsequent cycles.

Therefore, this technology is of extraordinary significance to battery performance. The effect of formation directly affects the subsequent performance of lithium-ion batteries, including storage performance, cycle life, rate performance and safety. This article focuses on the technical parameters/methods of formation and its impact on battery performance.

1. Temperature

The formation and aging temperatures play a decisive role in the characteristics of the electrode SEI film. Regarding the formation temperature, the industry has two opposing research results. One side believes that high-temperature formation has serious capacity loss. As the formation temperature rises, the capacity loss of the graphite negative electrode is attributed to the increased decomposition of the electrolyte components. The capacity loss of the NCM positive electrode decreases due to accelerated kinetics, but the capacity loss caused by low-temperature formation can be partially recovered during the subsequent normal temperature cycle. Therefore, high-temperature formation has no advantage for the full battery, and both the positive and negative electrodes show serious lithium loss and a decrease in the cycle stability of the graphite electrode.

 

The other side believes that after high-temperature formation, the battery has a higher discharge capacity and better capacity retention rate. By exploring the cycle performance of LiNi1/3Co1/3Mn1/3O2/artificial graphite batteries at different formation temperatures (25 ℃ and 45 ℃), the results show that the irreversible capacity loss dropped from 18.4% at 25 ℃ to 10.5% at 45 ℃. High-temperature formation at 45 ℃ is conducive to reducing SEI film impedance and irreversible capacity loss, and the irreversible capacity loss under 1.077 mA/cm2 high current formation conditions is only 12.8%. Also with a higher transmission rate under high-temperature formation, a more uniform SEI film can be formed on the graphite negative electrode.

However, for the aging process, it is generally believed that the most suitable temperature is determined by the previous chemical formation conditions. For example, if the chemical formation is at room temperature, a 5°C aging temperature can obtain a long cycle performance; while when the chemical formation is at a low temperature of 5°C, a high temperature aging of 45°C can obtain the best cycle performance.

2. External mechanical pressure

The application of external mechanical pressure to lithium-ion batteries has both advantages and disadvantages in existing literature reports. Advantages include better electrode contact, less lithium deposition, and less gas generation and distribution. Disadvantages include the possibility of graphite expansion due to lower mechanical pressure and deformation caused by uneven pore closure of the separator under higher pressure, which hinders the internal dynamics of the battery.

As shown in the figure below, when the external mechanical pressure increases from 0.05 kN to 1.70 kN, the constant voltage charging stage time is significantly reduced, while the constant current charging and discharging stage time is not much different. The entire process can save 14.7% of the formation time by increasing the external mechanical pressure. The article also proves that high external mechanical pressure has more potential to reduce battery formation time than high ambient temperature, so the possibility of saving battery costs is also greater. In addition, when high temperature and high mechanical pressure are combined, the battery temperature rises, which can inhibit the battery exothermic reaction.

 

3. Charge and discharge current

With the continuous exploration and understanding of the SEI film of graphite negative electrode, its composition, structure and importance to the electrode have gradually become clear. EIS (electrochemical impedance spectroscopy) research found that the impedance of the SEI film of graphite negative electrode reached the maximum in the voltage range of 0.8-0.3V, and the SEI film was completely formed during the first lithium insertion process. The SEI film is divided into two layers, the inner layer is mainly composed of inorganic substances, including Li2CO3, Li2O, LiF, etc.; the outer layer is mainly organic products, such as alkyl lithium oxide (ROLi) and alkyl lithium carbonate (ROCO2Li).

The reaction on the electrode surface is a competitive process of passivation film formation and charge transfer. Due to the diffusion speed and migration number of different ions, the main electrochemical reactions occurring at different current densities are different, and the resulting SEI film also has different properties. Therefore, controlling the formation current density is particularly important for obtaining a uniform, dense and thin SEI film. Compared with high current density, the SEI film formed during the formation process at low current density contains more organic lithium salts and less inorganic lithium salts, which can wrap the graphite electrode more evenly and protect the electrode material well.

4. Charge and discharge voltage

The choice of voltage range also affects the formation of SEI film. When the charge cut-off voltage is 3.6V, the SEI film is not fully formed, resulting in poor cycle performance; when the charge cut-off voltage is 3.7V, the SEI film is fully formed, and increasing the cut-off voltage will not improve the electrochemical performance of the battery.

Therefore, 3.7 V is the most suitable charging cut-off voltage for LiCoO2/C batteries. Similarly, when NMC/graphite or Li/NMC batteries are charged to 3.6 V or 3.7 V (50% SOC), the capacity difference in the subsequent full charge cycle is not obvious compared with the formation charge cut-off voltage of 4.2 V.

5. Charge state

Battery state of charge (SOC) is also often optimized as an important formation technology parameter. It is closely related to the charge and discharge voltage in the previous section. For example, when optimizing the charge cutoff voltage of ordinary LiCoO2 or NMC batteries, charging to 3.6-3.7 V represents a charge close to the general battery (i.e., 50% SOC), which is an ideal choice in terms of performance and formation time. Different states of charge will cause different degrees of reaction during the aging process, affecting the properties of the SEI film and thus the battery performance. The 25% state of charge shows a large impedance before and after battery aging, and the capacity retention rate is low. The formation method corresponding to the best performance is to charge to 100% SOC first and then discharge 25% SOC, that is, the battery maintains a 75% state of charge. Combined with aging at room temperature, the highest first discharge capacity and capacity retention rate will be obtained.

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