The Impact of Formation Process Parameters on Battery Performance

A key factor affecting the performance of lithium-ion batteries is the solid electrolyte interphase (SEI) film formed on the negative electrode surface by the decomposition of the electrolyte. The SEI film is formed during the first charge-discharge cycle of the battery formation process. A stable SEI film protects the negative electrode from being consumed during subsequent electrolyte decomposition and prevents graphite shedding. Therefore, battery formation equipment is a crucial machine in the lithium-ion battery manufacturing process.

The formation process involves subjecting a qualified battery, after electrolyte injection and settling, to its first charge-discharge cycle, forming the SEI film on the negative electrode surface. The battery formation process mainly includes four parts: open-end charging (pre-charge or venting), closed-end charging, closed-end aging, and closed-end discharging. Different formation processes result in different SEI film states, and these different SEI film states have different impacts on battery performance. Therefore, different formation processes have different effects on lithium-ion battery performance. These differences mainly include variations in formation charge-discharge current, formation charge-discharge time, formation charge-discharge cutoff voltage, and formation aging time and temperature. Battery performance mainly includes cycle performance, voltage, internal resistance, and high-temperature storage performance.

1. The Impact of Formation Charge/Discharge Current on Battery Performance

The formation charge/discharge current mainly includes the first part (open-circuit charging, or venting) current, the second part (closed-circuit charging), and the fourth part (closed-circuit discharging).

The first part, open-circuit formation (pre-charging or venting), mainly involves low-current charging to form a stable and dense SEI film, allowing gases generated by the reaction of additives in the electrolyte to escape, thus reducing the impact on battery cycle performance and rate performance. Furthermore, the type and quantity of electrolyte additives, reaction potential, and time all affect the required charging rate. Therefore, this stage primarily uses a stepped charging mode, i.e., a low-current charge in the first step, with subsequent steps increasing the current based on the previous step.

The second part, closed-circuit formation, mainly involves increasing the charging current based on the first part. In the first part, some additives in the electrolyte have already reacted, and a dense SEI film has formed. However, an excessively dense SEI film can affect lithium-ion transport during the reaction process. Therefore, the current needs to be gradually increased to allow the formed SEI film to transition from dense to porous. Increasing the charging current can shorten battery charging time and improve production efficiency. However, excessive charging current can cause the battery temperature to rise, damaging the SEI film and causing it to dissolve and reform. This leads to battery capacity decay, poor cycle performance, and even safety accidents.

The fourth part, closed-end discharge, involves the first discharge of a fully charged battery, completing the entire battery activation process. Before discharge, the SEI film on the negative electrode surface is basically formed, so the discharge current for this part can be equal to or slightly greater than the charging current in the second part. However, the current should not be too high, as this will lead to severe battery polarization and excessively rapid temperature rise. Additionally, to ensure battery consistency, a small-current discharge should be performed after the large-current discharge.

2. The Impact of Formation Charge-Discharge Time on Battery Performance

The formation charge-discharge time mainly includes the first part, open-end charging (pre-charge or venting) time, the second part, closed-end charging time, and the fourth part, closed-end discharge time.

The first part, open-end charging (pre-charge or venting) time, is a small-current charging time and should not be too long, because prolonged small-current charging will increase the impedance of the formed SEI film and increase the battery's internal resistance. By studying the impact of formation charging time on battery performance in lithium iron phosphate cathode and graphite anode power batteries, it was found that appropriately reducing the formation time under the same charging current is beneficial for the formation of the SEI film on the surface of the battery anode. The anode surface using this charging method is smooth, effectively improving battery internal resistance, cycle performance, and high-temperature storage performance.

The second part, closed-end charging time, without voltage limitations, leads to overcharging if charged for too long, while short charging times result in incomplete activation of the active materials in the battery's internal electrodes, leading to an incomplete and less dense SEI film, affecting battery performance. Therefore, this part of the charging time should be controlled in conjunction with the charging cut-off voltage.

The fourth part, closed-end discharge time, is related to the battery's depth of discharge. Without a discharge cut-off voltage limitation, the longer the battery discharge time, the deeper the discharge, leading to over-discharge and a shortened lifespan.

3. Impact of Formation Charge/Discharge Cut-off Voltage on Battery Performance

The first part, open-end charging (pre-formation) cut-off voltage, is the cut-off voltage after pre-charging. The purpose of pre-formation is to remove impurities and form the SEI film. Impurities include moisture, trace elements, and trace amounts of metallic impurities. The formation cutoff voltage affects the reaction pathway of SEI film formation.

The second part concerns the closed-end charging cutoff voltage, which is the voltage at which the battery is fully charged. Excessive voltage leads to overcharging, causing excess lithium ions to be released from the positive electrode active material and deposited on the negative electrode surface, forming lithium dendrites. Overcharging also causes the positive electrode to decompose, releasing oxygen, which is a catalyst for electrolyte decomposition. Furthermore, the electrolyte solvent reacts with the active lithium deposited on the negative electrode surface, resulting in the loss of positive electrode active material and battery capacity decay.

The fourth part concerns the closed-end discharge cutoff voltage, which is the control voltage for the battery's first complete discharge. Insufficient voltage leads to over-discharge, corrosion of the negative electrode current collector, and destruction and decomposition of the SEI film on the negative electrode surface. The reconstituted SEI film has poor performance, increasing battery impedance and polarization at the end of charge and discharge, resulting in reduced charge and discharge efficiency and poorer cycle performance. Experimental studies on the thermal performance of SONY 18650 lithium-ion batteries under overcharge and over-discharge conditions revealed that the battery voltage drops rapidly during the over-discharge phase, and the battery surface temperature rises continuously to 41°C. After approximately 250 seconds, the battery voltage and current drop to almost 0V and 0mA, respectively. This is a self-protection mechanism of the battery to prevent over-discharge and overheating.

4. Effects of Aging Time and Temperature on Battery Performance

Aging time is the interval between the first charge and the first discharge. After the first full charge, lithium-ion batteries require a certain resting time to remove internal polarization, which significantly affects the battery's capacity and impedance. Studies using 18650 lithium-ion batteries to investigate the effect of resting time on the cycle performance of lithium-ion batteries showed a significant impact. Batteries with resting times ≤2h showed no significant difference in cycle performance and impedance compared to those without resting time.

The effect of temperature on battery performance is mainly manifested in the accelerated decomposition of electrolyte and additives, thickening of the SEI film on the negative electrode surface, and increased internal resistance of the battery as temperature increases. The main component of lithium-ion battery electrolyte is LiPF6. At excessively high temperatures, LiPF6 undergoes thermal decomposition, generating PF5. PF5 further reacts with water in the electrolyte to form HF. HF is a significant cause of iron dissolution in the cathode material.

To improve the high-temperature cycle performance of lithium-ion batteries, methylene disulfonate (MMDS) is added to the electrolyte. MMDS significantly improves the battery's cycle performance at both room temperature and high temperatures, and the cycle stability increases with increasing additive dosage. However, this additive is temperature-sensitive; high-temperature use and storage can cause an increase in its color and acidity, affecting battery performance. Therefore, the storage temperature of the electrolyte, the post-filling settling temperature, and the battery degassing and formation temperature must be strictly controlled to prevent MMDS failure.